Insight into trifluoromethylation – experimental electron density for Togni reagent I

3,3-Dimethyl-1-(trifluoromethyl)-1,3-dihydro-1-λ3,2-benziodoxole represents a popular reagent for trifluoromethylation. The σ hole on the hypervalent iodine atom in this “Togni reagent” is crucial for adduct formation between the reagent and a nucleophilic substrate. The electronic situation may be probed by high resolution X-ray diffraction: the experimental charge density thus derived shows that the short intermolecular contact of 3.0 Å between the iodine and a neighbouring oxygen atom is associated with a local charge depletion on the heavy halogen in the direction of the nucleophile and visible polarization of the O valence shell towards the iodine atom. In agreement with the expectation for λ3-iodanes, the intermolecular O⋯I–C_aryl halogen bond deviates significantly from linearity.

The experimentally observed electron density for the “Togni reagent” explains the interaction of the hypervalent iodine atom with a nucleophile.

A “halogen bond” denotes a short contact between a nucleophile acting as electron density donor and a (mostly heavy) halogen atom as electrophile;^1,2 halogen bonds are a special of σ hole interactions.^3–5 Such interactions do not only play an important role in crystal engineering;^6–11 rather, the concept of a nucleophile approaching the σ hole of a neighbouring atom may also prove helpful for understanding chemical reactivity.The title compound provides an example for such a σ hole based reactivity: 3,3-dimethyl-1-(trifluoromethyl)-1,3-dihydro-1-λ3,2-benziodoxole, 1, (Scheme 1) commonly known as “Togni reagent I”, is used for the electrophilic transfer of a trifluoromethyl group by reductive elimination. The original articles in which the application of 1¹² and other closely related “Togni reagents”¹³ were communicated have been and still are highly cited. Trifluoromethylation is not the only application for hypervalent iodine compounds; they have also been used as alkynylating¹⁴ or azide transfer reagents.^15,16 The syntheses of hypervalent iodanes and their application in organofluorine chemistry have been reviewed,¹⁷ and a special issues of the Journal of Organic Chemistry has been dedicated to Hypervalent Iodine Catalysis and Reagents.¹⁸ Recently, Pietrasiak and Togni have expanded the concept of hypervalent reagents to tellurium.¹⁹Open in a separate windowScheme 1Lewis structure of the Togni reagent, 1.Results from theory link σ hole interactions and chemical properties and indicate that the electron density distribution associated with the hypervalent iodine atom in 1 is essential for the reactivity of the molecule in trifluoromethylation.²⁰ Lüthi and coworkers have studied solvent effects and shown that activation entropy and volume play relevant roles for assigning the correct reaction mechanism to trifluoromethylation via1. These authors have confirmed the dominant role of reductive elimination and hence the relevance of the σ hole interaction for the reactivity of 1 in solution by ab initio molecular dynamics (AIMD) simulations.^21,22 An experimental approach to the electron density may complement theoretical calculations: low temperature X-ray diffraction data of sufficient resolution allow to obtain the experimental charge density and associate it with intra- and inter-molecular interactions.^23–25 Such advanced structure models based on aspherical scattering factors have also been applied in the study of halogen bonds.^26–30 In this contribution, we provide direct experimental information for the electronic situation in Togni reagent I, 1; in particular, we analyze the charge distribution around the hypervalent iodine atom.Excellent single crystals of the title compound were grown by sublimation.^§ The so-called independent atom model (IAM), i.e. the structure model based on conventional spherical scattering factors for neutral atoms, confirms the solid state structure reported by the original authors,¹² albeit with increased accuracy. As depicted in Fig. 1, two molecules of 1 interact via a crystallographic center of inversion. The pair of short intermolecular O⋯I contacts thus generated can be perceived as red areas on the interaction-sensitive Hirshfeld surface.³¹Open in a separate windowFig. 1Two neighbouring molecules of 1, related by a crystallographic center of inversion. The short intermolecular I⋯O contacts show up in red on the Hirshfeld surface³² enclosing the left molecule. (90% probability ellipsoids, H atoms omitted, symmetry operator 1 − x, 1 − y, 1 − z).The high resolution of our diffraction data ^¶ for 1 allowed an atom-centered multipole refinement^33,34 and thus an improved model for the experimental electron density which takes features of chemical bonding and lone pairs into account. Fig. 2 shows the deformation density, i.e. the difference electron density between this advanced multipole model and the IAM in the same orientation as Fig. 1.^|| The orientation of an oxygen lone pair (blue arrow) pointing towards the σ hole of the heavy halogen in the inversion-related molecule and the region of positive charge at this iodine atom (red arrow) are clearly visible. Single-bonded terminal halides are associated with one σ hole opposite to the only σ bond, thus resulting in a linear arrangement about the halogen atom. Different geometries and potentially more than a single σ hole are to be expected for λ3-iodanes such as our target molecule, and as a tendency, the resulting halogen bonds are expected to be weaker than those subtended by single-bonded iodine atoms.³⁵ In agreement with these theoretical considerations, the closest I⋯O contacts in 1 amount to 2.9822(9) Å. This distance is significantly shorter than the sum of the van-der-Waals radii (I, 1.98 Å; O, 1.52 Å (ref. 36)) but cannot compete with the shortest halogen bonds between iodine and oxygen^37,38 or iodine and nitrogen.^39–41Fig. 1 and ​and22 show that the C_aryl–I⋯O contacts are not linear; they subtend an angle C10–I1⋯O1′ of 141.23(3)° at the iodine atom. On the basis of theoretical calculations, Kirshenboim and Kozuch³⁵ have suggested that the split σ holes should be situated in the plane of the three substituents of the hypervalent atom and that halogen and covalent bonds should be coplanar. Fig. 3 shows that the C_aryl–I⋯O interaction in 1 closely matches this expectation, with the next oxygen neighbour O1′ only 0.47 Å out of the least-squares plane through the heavy halogen I1 and its three covalently bonded partners C1, O1 and C10.Open in a separate windowFig. 2Deformation density for the pair of neighbouring molecules in 1; the dashed blue and red arrows indicate regions of opposite charge. (Contour interval 0.10e Å⁻³; blue lines positive, red lines negative, green lines zero contours, symmetry operator 1 − x, 1 − y, 1 − z).Open in a separate windowFig. 3A molecule of 1, shown [platon] along O1⋯C1, and its halogen-bonded neighbour O1ⁱ. Symmetry operator 1 − x, 1 − y, 1 − z.The Laplacian, the scalar derivative of the gradient vector field of the electron density, emphasizes local charge accumulations and depletions and it allows to assess the character of intra- and inter-molecular interactions. A detailed analysis of all bonds in 1 according to Bader''s Atoms In Molecules theory⁴² is provided in the ESI.^‡ We here only mention that the electron density in the bond critical point (bcp) of the short intermolecular I⋯O contact amounts to 0.102(5)e Å⁻³; we are not aware of charge density studies on λ3-iodanes, but both the electron density and its small positive Laplacian match values experimentally observed for halogen bonds involving O and terminal I in the same distance range.⁴³The crystal structure of 1 necessarily implies additional contacts beyond the short halogen bond shown in Fig. 1 and ​and2.2. The shortest among these secondary interactions is depicted in Fig. 4: it involved a non-classical C–H⋯F contact with a H⋯F distance of 2.55 Å.Open in a separate windowFig. 4C–H⋯F contact in 1; additional information has been compiled in the ESI.^‡ Symmetry operators i = 1 − x, 1 − y, 1 − z; ii = 1 − x, 1 − y, −z.The topological analysis of the experimental charge density reveals that this non-classical C–H⋯F hydrogen bond and all other secondary contacts are only associated with very small electron densities in the bcps. Table S8 in the ESI^‡ provides a summary of this analysis and confirms that the I⋯O halogen bond discussed in Fig. 1 and ​and22 represents by far the most relevant intermolecular interaction.The relevance of this halogen bond extends beyond the crystal structure of 1: Insight into the spatial disposition of electrophilic and nucleophilic regions and hence into the expected reactivity of a molecule may be gained from another electron-density derived property, the electrostatic potential (ESP). The ESP for the pair of interacting molecules in 1 is depicted in Fig. 5.Open in a separate windowFig. 5Electrostatic potential for a pair of molecules in 1 mapped on an electron density isosurface (ρ = 0.5e Å⁻³; program MoleCoolQt^44,45). Fig. 5 underlines the complementary electrostatic interactions between the positively charged iodine and the negatively charged oxygen atoms. One can easily imagine to “replace” the inversion-related partner molecule in crystalline 1 by an incoming nucleophile.The ESP tentatively obtained for a single molecule in the structure of 1 did not differ significantly from that derived for the inversion-related pair (Fig. 5), and even the results from theoretical calculations in the gas phase for an isolated molecule²⁰ are in good qualitative agreement with our ESP derived from the crystal structure. In the absence of very short contacts, polarization by neighbouring molecules only has a minor influence on the ESP. The experimentally observed electron density matches the proven reactivity for the title compound, and we consider it rewarding to extend our charge density studies on related hypervalent reagents.     

Observation of the generation of peroxynitrite in mouse liver after acetaminophen overdose with a boronate-based ratiometric fluorescence probe

A ratiometric fluorescent probe, BTPB, for the selective monitoring of hepatic peroxynitrite (ONOO⁻) in situ after acetaminophen (APAP) overdose has been developed. Our study provided direct evidence for supporting the generation of ONOO⁻ in APAP-induced liver injury. This new probe will be a useful tool for studying the roles of ONOO⁻in vivo.

A ratiometric fluorescent probe, BTPB, for the selective monitoring of hepatic peroxynitrite in situ after acetaminophen overdose has been developed.

Acetaminophen (APAP) is a safe and effective antipyretic/analgesic drug when used at recommended doses, and is one of the most commonly used drugs in the US and throughout the West.¹ However, APAP overdose can cause severe acute liver injury with the potential to progress to liver failure. It is responsible for almost 80 000 emergency department visits in the US each year,² as well as 26 000 hospitalizations and nearly 500 deaths.³ In fact, APAP hepatotoxicity is one of the most common etiologies of acute liver failure.⁴Despite substantial progress in understanding the mechanism of APAP-induced liver injury during the last four decades, many details are still unknown. At therapeutic levels, about 50–70% of the administered APAP is glucuronidated, and 25–35% undergoes sulfation, while only 5–10% is converted to the reactive electrophilic intermediate N-acetyl-p-benzoquinone imine (NAPQI) by cytochrome P450 enzymes.⁵ NAPQI can be detoxified by reaction with the cysteine sulfhydryl group on glutathione (GSH). Once the administration of APAP overdose, NAPQI can deplete GSH stores, and bind to protein sulfhydryl groups, especially mitochondrial proteins, which triggers mitochondrial oxidant stress. The amplification of mitochondrial oxidative stress further leads to the opening of the mitochondrial membrane permeability transition (MPT) pore, then the rupture of the outer mitochondrial membrane. As a result, there is extensive release of proteins from the intermembrane space, including endonuclease G and apoptosis-inducing factor, both of which can translocate to the nucleus because of their nuclear localization sequences. This then results in DNA fragmentation and, finally, oncotic necrosis.⁶During resulting in MPT, it was recognized that the strong oxidant and nitrating peroxynitrite (ONOO⁻), formed from superoxide (O₂˙⁻) and nitric oxide (NO), is being generated in hepatocytes and a critical mediator in the injury mechanism. The generation of ONOO⁻ was only demonstrated by immunohistochemical staining for 3-nitrotyrosine protein adducts.^7,8 The well-known nitration mechanism involves the initial one-electron oxidation of tyrosine to yield tyrosyl radical followed by a diffusion-controlled reaction with nitrogen dioxide (˙NO₂) to yield 3-nitrotyrosine.^9,10 However, in biological systems, ˙NO₂ arises from a variety of sources, except most notably the decomposition of ONOO⁻, but the hemeperoxidase-dependent oxidation of nitrite,^10,11 which is a ONOO⁻-independent mechanism. Actually, protein 3-nitrotyrosine, which was initially conceived as a specific footprint of ONOO⁻, typically represents a fairly stable end product that is generated by the action of NO-derived oxidants.¹² It seems insufficient for the generation of ONOO⁻ after the administration of APAP overdose only according to the observation of the formation of protein 3-nitrotyrosine. Therefore, there is an urgent need to provide the direct evidence for supporting the generation of ONOO⁻ after APAP overdose.The lifetime of ONOO⁻ in biological systems is limited to only a few milliseconds, which makes it impossible to quantitate ONOO⁻ in processed cell or tissue samples directly.¹³ Fluorescence based techniques are advantageous for detection of ONOO⁻. In accordance with the multiplicity of ONOO⁻ biochemistry,¹⁴ the trifluorocarbonyl (HKGreen 1-3)^15–17 and phenyl boronic esters (CBA)¹⁸ based fluorescent probes, which can directly detect OONO⁻, have been reported, mainly for in vitro applications. However, these probes are not suitable for biological studies in vivo because of their high molecular weight, bearing easily metabolized lactone group, such as HKGreen 1-3, or a low response signal, such as CBA. Chemiluminescence (CL) probes^19,20 can distinguish ONOO⁻ from a variety of other ROS owing to the radicals from the decomposition of ONOO⁻, such as ONOOH, ˙OH and O₂˙⁻, not directly to the reaction with ONOO⁻. Additionally, CL probes were not used to detect OONO⁻in vivo.In this regard, to provide direct and strong evidence for the formation of OONO⁻ in liver after APAP overdose, we developed a novel and low background phenyl boronic acid-containing fluorogenic probe for sensitive and selective detecting of OONO⁻. This probe was introduced into the mice administrated APAP overdose by heart perfusion to monitor in situ the formation of OONO⁻ in liver.The new fluorescent probe was designed by the direct conjugation of an boronic acid reaction group with a (phenyl-2-yl)benzothiazol scaffold, serving as a fluorophore. Phenyl boronic acid group was selected because the group can react with OONO⁻ directly, rapidly and stoichiometrically,²¹ which is more suitable to detect OONO⁻ than other groups that react with those secondary radicals derived from OONO⁻, including HONOO, ONOOCO₂⁻, or metal–OONO⁻ complexes. Moreover, phenyl boronic acid group was recently reported to be highly reactive to OONO⁻ nearly a million times faster than H₂O₂ and two hundred times faster than ClO⁻.^22,23 The (phenyl-2-yl)benzothiazol scaffold was selected because the probe designed reacts with OONO⁻ to yield corresponding hydroxyl derivatives 2-(benzothiazol-2-yl)phenol, which possesses the excellent photophysical properties, such as high photostabilities and high fluorescence quantum yields, of members of this family.^24,25 The boronic acid group is converted to the hydroxyl by the reaction with OONO⁻, with the conversions offering ratiometric responses because 2-(benzothiazol-2-yl) phenylboronic acid (BTPB) and 2-(benzothiazol-2-yl) phenol (BTP) fluoresce at different wavelengths, which makes this probe sensitive. More importantly, benzothiazol group is metabolically stable and a usual drug fragments,^26,27 and this probe has a low molecular weight and appropriate water solubility, which makes this probe suitable to use in vivo. The design rationale is depicted in Fig. 1, and illustrated as follows.Open in a separate windowFig. 1Structure of BTPB and detection of peroxynitrite by oxidation of BTPB.Probe BTPB was prepared using the one-step reaction shown in ESI.^†28 2-(Benzothiazol-2-yl) phenol, BTP, the expected product resulting from reaction of BTPB with ONOO⁻, was also prepared utilizing a previously described method.²⁹ The structures of BTPB and BTP were confirmed by ¹H NMR, ¹³C NMR, and HRMS spectra (Fig. S1–S6^†), which match those of literatures.^28,29Firstly, the spectroscopic response of BTPB to ONOO⁻ was investigated in an abiotic system. We examined the spectroscopic properties of BTPB (1.0 μM) towards ONOO⁻ (1 equiv.) in 100.0 mM PBS buffer with 1.0 mM cetyltrimethylammonium bromide (CTAB) at pH 7.4 at 25.0 ± 0.1 °C. 50 μL of BTPB (1.0 mM) was added to ONOO⁻ (1.0 equiv.), then vigorously stirred for 30 seconds. After dilution to 1.0 μM with 100.0 mM PBS buffer 1.0 mM CTAB, the mixture was vigorously stirred for 1 min before measurement. The absorption and fluorescence spectra of BTPB before and after reaction with ONOO⁻ are shown in Fig. 2. As is seen, BTPB shows a major UV-absorption centered at 296 nm, but its reaction with ONOO⁻ produces two absorption peak at 290 nm and 338 nm. Moreover, BTPB itself displays a low fluorescence with the maximum emission peak at 372 nm, whereas the reaction with ONOO⁻ leads to a fluorescence enhancement with the maximum emission peak at 462 nm.Open in a separate windowFig. 2Absorption spectra (a) and fluorescence spectra (b) of BTPB (1.0 μm) before and after reaction with ONOO⁻ (1.0 μm). Spectra were obtained in 100.0 mm phosphate buffer (pH 7.4) with 1.0 mm cetyltrimethylammonium bromide (CTAB) under vigorous stirring for 1 min after incubation of the probe with ONOO⁻ for 30 seconds at 25.0 °C, λ_ex = 296 nm.The fluorescence response of BTPB to varied concentrations of ONOO⁻ (0–1.0 μM) was examined. When the emission wavelength was set to 372 nm, upon addition of ONOO⁻, the main excitation peak of BTPB at 296 nm decreases with a concurrent increase in excitation peaks at 296 nm, 332 nm and 386 nm when the emission wavelength was 462 nm (Fig. 3a and b). Thus, the ratiometric feature in the spectra was obtained at an excitation wavelength of 296 nm.Open in a separate windowFig. 3(a) Fluorescence excitation spectra of BTPB (1.0 μm) to OONO⁻ (0–1 equiv.), λ_em = 372 nm. (b) Fluorescence excitation spectra of BTPB (1.0 μm) to OONO⁻ (0–1 equiv.), λ_em = 462 nm. (c) Fluorescence emission spectra of BTPB (1.0 μm) to OONO⁻ (0–1 equiv.), λ_ex = 296 nm. (d) Linear relationship of emission intensity ratio (F₄₆₂/F₃₇₂) to OONO⁻ (0–1.0 μm). All spectra were obtained in 100.0 mm phosphate buffer (pH 7.4) with 1.0 mm cetyltrimethylammonium bromide (CTAB) under vigorous stirring for 1 min after incubation of the probe with ONOO⁻ for 30 seconds at 25.0 °C.As expected, in the absence of ONOO⁻, BTPB (1.0 μM) in 100.0 mM PBS buffer with 1.0 mM CTAB at pH 7.4 showed a very poor fluorescence at 372 nm (ϕ_F = 0.05); upon addition of ONOO⁻ (1.0 μM), a dramatic fluorescence enhancement was observed immediately at 462 nm (ϕ_F = 0.33). The ratio of emission intensities (F₄₆₂/F₃₇₂) upon excitation at 296 nm varies from 0.41 in the absence of ONOO⁻ to 26.45 after reaction with ONOO⁻, a ca. 65-fold emission ratio change (Fig. 3c), which is more than the response signal of CBA.¹⁸ As shown in Fig. 3d, The ratio of emission intensities (F₄₆₂/F₃₇₂) exhibits a good linear relationship in the concentration range of 0–1.0 μM ONOO⁻ with an equation of F = 27.27 × [ONOO⁻] (μM) + 0.41 (R² = 0.988), where F represents the ratio of fluorescence intensity of the reaction solution (F₄₆₂/F₃₇₂). The detection limit was calculated to be 12.3 nM ONOO⁻. These results revealed that BTPB could potentially test ONOO⁻ in abiotic systems, which is similar with physiological conditions, both qualitatively and quantitatively.It is known that CTAB is a cationic surfactant and forms spherical micelles within the concentration range 0.9–100 mM in water. Both BTPB and BTP are low soluble in water; to apply them efficiently in an aqueous environment, CTAB was introduced into the aqueous phase. Thus BTPB molecules could be encapsulated in micelles, which makes more intense emission of BTPB in CTAB-added PBS than that in PBS without CTAB (Fig. S7^†). The formation of micelles was also confirmed by means of scanning electron microscopy (Fig. S8^†).The selectivity of BTPB (1.0 μM) toward ONOO⁻ was determined by measuring fluorescence changes that take place in the presence of various ROS and RNS including superoxide (O₂˙⁻), hypochlorite (ClO⁻), hydroxyl radicals (˙OH), hydrogen peroxide (H₂O₂), single oxygen (¹O₂), tert-butoxyl radical (˙O^tBu), nitrite (NO₂⁻) and nitric oxide (NO) in phosphate buffer (100 mM, pH 7.4, 1.0 mM CTAB) at 25 °C. As shown in Fig. 4, only ONOO⁻ promotes a dramatic enhancement in fluorescence intensity at 462 nm (F₄₆₂/F₃₇₂ = 26.5). In contrast, no changes in emission intensities occur when the probe was incubated with O₂˙⁻, H₂O₂, ˙OH, ¹O₂, ˙O^tBu, NO₂⁻ and NO. ClO⁻ induces a small fluorescence response (F₄₆₂/F₃₇₂ = 1.2). Taken together, these results established that BTPB is sensitive enough and also considerably selective toward ONOO⁻ over other ROS or RNS under the conditions of low concentration (1.0 μM) and short reaction time (1.0 min). The observations also corroborate the previous finding that boronate-containing small molecules react with ONOO⁻ much faster than with H₂O₂ or ClO⁻.^22,23Open in a separate windowFig. 4(a) Fluorescence emission spectra of BTPB (1.0 μm) to various ROS and RNS (1 equiv. ONOO⁻ and 10 equiv. for other ROS and RNS). (1) Blank; (2) ONOO⁻; (3) ˙OH; (4) ClO⁻; (5) H₂O₂; (6) NO₂⁻; (7) NO; (8) O₂˙⁻; (9) ¹O₂; (10) ˙O^tBu. (b) Emission intensity ratio (F₄₆₂/F₃₇₂) of various ROS and RNS.Our approach to develop a ONOO⁻ specific probe relies on the use of oxidation of phenyl boronic acid group of BTPB to hydroxyl group of BTP. Subsequently, to confirm this reaction, the process taking place between BTPB and ONOO⁻ was monitored by using HPLC. HPLC analysis of the mixture generated from the reaction of ONOO⁻ with BTPB showed that in the presence of various concentrations of ONOO⁻, the intensity of the peak of BTPB (eluted at 2.0 min) gradually decreased, in the meanwhile that of the peak of oxidation products BTP eluted at 5.5 min emerged and gradually increased (Fig. S9^†). The results confirmed that the fluorescent product was indeed BTP, resulting from the ONOO⁻ induced oxidation reaction of BTPB. The ratiometric fluorescence mechanism is concerned with internal charge transfer (ICT). The boron atom in boronic acid group is sp² hybridized and possesses fiercely electron-withdrawing character due to the availability of its vacant p orbitals, forming the ICT state with benzothiazol moity as the electron-donating group. After the reaction with ONOO⁻, the boronic acid group is converted to the electron-donating hydroxyl, which interferes internal charge transfer of BTPB molecule and results in fluorescent emission enhancement. The red shifts in emission maxima could be attributed to the lower-energy emission of the phenolate anion of BTP in PBS at pH 7.4, which matches the peak emission of the deprotonated phenolate anion of benzothiazol derivative in aqueous solution.^30,31MTT assays (Fig. S10^†) revealed that BTPB of concentrations below 40 μM does not exhibit obvious cytotoxicity living HepG2 cells. Finally, encouraged by the above promising results, especially fast response rate (within 30 seconds), excellent sensitivity (detection limit = 12.3 nM) and high selectivity, BTPB was employed for confocal imaging of ONOO⁻ in the liver tissue sample. ONOO⁻ formation has long been implicated in APAP-induced liver injury on the basis of indirect detection, i.e., immunostaining of protein nitration, which, however, cannot be used as unique evidence for ONOO⁻ formation. To address this question, the mice administrated APAP overdose (400 mg kg⁻¹) were employed to monitor ONOO⁻ formation in situ. Six hours after administrated APAP overdose, living heart tissues were perfused with BTPB (1.0 μM) in saline for 3 min under anesthesia. Mouse livers were then excised and cryosectioned at 10 μm intervals for confocal imaging and hematoxylin and eosin (HE) staining. The mice without APAP administration were perfused with BTPB as the control.As shown in Fig. 5, HE staining of liver sections indicate the substantial centrilobular necrosis 6 hours after APAP administration, but no necrosis is observed in the control group without APAP administration, suggesting that APAP overdose induces liver injury, which is consistent with those studies.^32,33 Then liver sections were monitored using a confocal microscope over the wavelength range of 417–477 nm with an excitation wavelength at 408 nm. Without APAP administration no fluorescence in the liver sections was observed, while strong blue fluorescence in the centrilobular regions was imaged after treatment with overdose APAP, which correlated well with the areas of necrosis. This result indicates the generation of ONOO⁻ during APAP-induced liver injury, which is the first direct evidence provided by our study. Also, these observations suggest that BTPB has satisfactory cell permeability and is well-retained during histological sample preparation and sensitive enough to visualize endogenous ONOO⁻ in live tissues.Open in a separate windowFig. 5Hematoxylin and eosin staining of liver sections for hepatic necrosis (a–d) and confocal microscopic fluorescence images of liver sections for detecting of ONOO⁻ generation via BTPB (e–h) in controls and 6 h after 400 mg kg⁻¹ APAP. (a) Control, ×100. (b) Control, ×200. (c) 6 h after 400 mg kg⁻¹ APAP, ×100. (d) 6 h after 400 mg kg⁻¹ APAP, ×200. (e) Control, ×100. (f) 6 h after 400 mg kg⁻¹ APAP, ×100. (g) 6 h after 400 mg kg⁻¹ APAP, ×200. (h) 6 h after 400 mg kg⁻¹ APAP, ×400.     

Leucomethylene blue probe detects a broad spectrum of reactive oxygen and nitrogen species

Reactive oxygen and nitrogen species (ROS, RNS) are ubiquitous in biology with a variety of physiological and pathological functions. Here we describe a broad spectrum ROS/RNS detecting fluorogenic probe with red fluorescence emission and up to 100-fold gain. Hence these modified probes are useful for in vivo non-invasive quantification of ROS/RNS.

A broad spectrum ROS/RNS sensing butylated phenol tethered leucomethylene blue is presented. This probe detects a variety of ROS/RNS, with up to 100-fold gain in fluorescence in the red range, is suitable for microscopic and macroscopic in vivo fluorescence imaging.

Reactive oxygen species (ROS: O₂˙⁻, H₂O₂, HO˙, OCl⁻, and HO₂˙) and reactive nitrogen species (RNS: ONOO⁻, NO˙, NO₃⁻) are classes of short-lived molecules produced in biological environments, e.g. in cellular metabolism, and in neutrophil actions.^1,2 They function as messengers, pathogen neutralizers, and play key roles in inducing inflammation,^3,4 and cancers.^5–7 Therefore, early non-invasive detection of ROS/RNS enable the onset of diseases. Several strategies, methods and probes are employed currently for the detection of ROS and RNS.^2,8–10 Electron paramagnetic resonance (EPR)¹¹ and fluorescence-based techniques are prevalent,¹² followed by photometric, chemiluminescent and electrochemical methods. In all these techniques, the applied probes change their properties post-reaction (or trapping) with ROS/RNS, and enable the detection.Typically, many of above probes detect only one of the particular ROS/RNS species, and “overlook” those formed in earlier or subsequent metabolic steps, e.g. in EPR only radical species, in fluorescence and colorimetric techniques one type of ROS/RNS species. However, in biological systems the primarily produced ROS species (e.g. O₂˙⁻) undergoes a variety of dissociation pathways forming secondary ROS species (see Fig. 1) and cause oxidative burden.¹³ Thus, to estimate an overall oxidative stress, it is necessary to accurately assess the pathogenic levels production of all ROS/RNS species using a single probe.Open in a separate windowFig. 1Schematic overview for generation of a variety of ROS/RNS species in biological conditions and their interconversions.Highly studied ROS detecting fluorogenic probes, 2′-7′-dichlorodihydrofluorescein (DCFH), its diacetate (DCFH-DA),¹⁴ and dihydrorhodamine (DHR),¹⁵ produce O₂˙⁻ from O₂ after reaction with one-electron-oxidizing ROS/RNS species, via the radical ion (DCFH˙⁻ or DHR˙⁻). This catalytic formation of O₂˙⁻, and its further conversion to other reactive species leads to an inaccurate detection of cellular ROS.¹⁶ Further, the absorption and emissions wavelengths are incompatible for in vivo use.¹⁷ It is optimal to have probes exhibiting optical properties in red-to-near infrared (NIR) range, where tissue auto fluorescence and phototoxicity are minimal. Towards such red-to-NIR emitting probes, previous studies employed reduced hydrocyanines, which undergo HO˙ and O₂˙⁻ mediated oxidation yielding red-to-NIR emitting cyanine dyes.¹⁸ However the polymethine chain in cyanine itself is sensitive to oxidative cleavage with ROS species,^19,20 thus could lead to underestimation of ROS. Thus, new probes are of interest, which show high stability towards ROS, simultaneously exhibit fluorescence in red-to-NIR range.Methylene blue (MB) dye has attracted our attention, as it exhibits absorption and emission in the biocompatible region, at 665 nm and 686 nm, respectively.²¹ Further, MB core is chemically modifiable,^22,23 and FDA-approved for therapeutic use,^24,25 and has been applied in a variety of applications, in disinfecting blood,²⁶ in treatment a variety of cancers,²⁷ as an antidote for methemoglobinemia²⁸ and in the photodynamic inactivation of bacteria,²⁵ funguses²⁹ and viruses.^30,31 These biological uses coupled with in vivo suitable optical properties make MB an attractive core the detection of ROS/RNS.Recently, a reduced leucomethylene blue (LMB) was applied to selectively detect HOCl by formylating 10-N site (N¹⁰-CHO) of LMB.³² In another study, the N¹⁰-site was modified with an enzyme³³ or light cleavable synthon,³⁴ which upon applying the respective trigger, yielded LMB and spontaneously oxidized to MB. We therefore are interested in modifying the LMB for detection of a broad spectrum of ROS/RNS species.In our previous studies, we have identified that 2,6-di-tert-butyl phenol (BHP) moiety substituted dyes like porphyrins,³⁵ BODIPYs³⁶ reacted with a variety of ROS/RNS species, undergoing changes in their optical properties. The reactivity of BHP towards a variety of ROS/RNS is similar to that of the antioxidant 2,6-di-tert-butyl hydroxy toluene (BHT, a food additive),³⁷ as both share the same reactive site. Extending this strategy to LMB, we conceived to prepare a N¹⁰-BHP appended LMB (BHP-LMB; 1, 2) for the ROS detection. As the BHP unit is hydrophobic, we approached a modification of LMB with polar groups like 1-butanesulfonate. Here we report the synthesis, and ROS detection characteristics as well as imaging applications of both constructs.Towards the synthesis of the target BHP-LMB 1, we employed a Buchwald–Hartwig C–N coupling, using reduced LMB and 4-iodo-2,6-tert-butylphenol (6). This method is appealing for its simplicity, as the C–N coupling chemistry has been extensively studied.³⁸ The C–N coupling reaction was performed in toluene with Pd₂(dba)₃ catalyst at 120 °C under nitrogen atmosphere (Scheme 1A). The BHP-LMB 1 could be isolated in 10% yield after column chromatography. Low yield was due to solubility of LMB in toluene, further sensitivity towards dissolved oxygen. Thus for purification of the coupling product 1, N₂-gas was employed in chromatography, and the dye 1 could be obtained as pale-turquoise coloured solid after removal of the solvent. The HPLC and NMR showed >98% purity. The air-oxidation can be prevented by using reducing agents like citric acid, especially for spectroscopic measurements.Open in a separate windowScheme 1(A) Synthesis of hydrophobic BHP-LMB (1), (B) bis-sultone added hydrophilic BHP-LMB (2). (a) K₂CO₃, Na₂S₂O₄ in DCM/H₂O at 40 °C under N₂, (60%) (b) Pd₂(dba)₃, DPPF, 6, NaO^tBu in toluene at 110 °C under N₂, (10%, air sensitive) (c) NaIO₄ in DCM/AcOH, BHP, at 40 °C (34%); (d) BH₃·SMe₂ in toluene at 110 °C (87%); (e) 1,4-butanesultone, DIPEA in MeCN at 80 °C under N₂ (15%).For the hydrophilic 1-butanesulfonylated BHP-LMB 2, we have developed a second generation multi-step synthetic route starting from phenothiazine (PTZ). Towards 2, we have first prepared a 2,8-diaminoacyl phenothiazine derivative (PTZ-NHAc, 3) from 3,7-dinitro-10H-phenothiazine-5-oxide (see ESI^†) as reported previously.³⁹ Then, for introducing the BHP unit at 3, a convenient metal-free dehydrogenative amination method was applied based on a recently published route (Scheme 1B).⁴⁰ An advantage of this route is also the high stability of 3 and 4 towards oxygen. The following reduction of the acetyl groups in 4 was achieved using BH₃·SMe₂ complex, obtaining mono ethyl BHP-PTZ derivative (5), which sets the stage for the final reaction towards the desired product 2. Treating 5 with 1,4-butanesultone under basic conditions, by nucleophilic ring opening of the sultone, yielded 1-butanesulfonate attached BHP-LMB (2), along with mono sultone addition product. The hydrophilic product 2 was isolated by preparative RP-HPLC (reverse phase, with CH₃CN/H₂O gradient) as white solid after lyophilisation.Having obtained two BHP-tethered probes 1 & 2, we performed a screening with a variety of ROS/RNS by optical methods (Fig. 2; Fig. S1 and S2 in ESI^† for titrations). The absorption (UV-vis) spectra of compounds were obtained, 1 in DMSO and of 2 in water; both BHP-LMBs showed a similar reactivity towards ROS, yielding MB as the final compound, though some peculiarities were observed. The O₂˙⁻ was found to be highly aggressive especially in DMSO with 1, compared to 2 in water. In DMSO, the O₂˙⁻ reaction yielded MB instantly, that underwent decomposition in the presence of excess O₂˙⁻. We investigated this reaction by NMR spectroscopy to identify any intermediates towards MB, using 1 titration with O₂˙⁻ and MB with O₂˙⁻, but found no stable species (Fig. S5^†). However, during the treatment with OH˙ and ^tBuO˙ species, it was found that 2 in water gave a persistent far-red absorbing species which was not observed in DMSO with 1 (Fig. S3^†versus Fig. S1^†). Similar spectral features were observed with H₂O₂ and tBuOOH towards 2 in H₂O albeit with a lower intensity, but not with 1 in DMSO. This is presumably, a relatively stable BHP-radical appended LMB formed in water but not in DMSO (Fig. S3^†). For anionic ROS species, O₂˙⁻, ONOO⁻ and OCl⁻, the absorption and fluorescence emission are consistent in both solvents with a small discrepancy in the gain ratio. This can be attributed to a different rate of reaction for formation of MB. In treating 1 with RNS species, NO₂⁻ and NO₃⁻ in DMSO, resulted MB (Fig. S2^†). These optical characteristics confirm that 1 and 2 exhibit a good sensitivity towards most of the ROS/RNS species and are thus suitable for estimating overall oxidative stress.Open in a separate windowFig. 2Optical properties of MB-BHT (25 nmol mL⁻¹) after treating sufficient amount of ROS (>5 equiv.). Fluorescence emission (concentration 12 nmol mL⁻¹) at the maximum of 689 nm for excitation at 645 nm (†: due to fast decomposition of the formed MB).Mechanistic investigations into the formation of intermediates were conducted by HPLC and NMR (Fig. 3). Here, 1 was treated with ONOO⁻, OCl⁻, O₂˙⁻ as they were found to be consistent in both solvents. Adding ONOO⁻ and OCl⁻ in excess to 1 gave a clean MB product without any trace of other species (Fig. 3A). Treatment of 1 with a minimal amount of O₂˙⁻ showed existence of both 1 and MB traces in HPLC. For NMR titrations, to a DMSO-d₆ solution of 1, ROS species in D₂O were added. For addition of O₂˙⁻, the chemical shifts corresponding to MB are downfield shifted slightly compared to ONOO⁻, OCl⁻. Furthermore, the NMR signals of the cleaved BHP unit were upfield shifted for O₂˙⁻ than ONOO⁻, OCl⁻ addition (Fig. 3B). This could indicate that the formed by-product, ^tBu-hydroquinol, might have been further oxidized to a ^tBu-quinone with O₂˙⁻, but with ONOO⁻, OCl⁻ persistent ^tBu-hydroquinol. Nevertheless, as no other intermediate species were detected in HPLC, and all ROS produced MB as the final compound, 1 could be suitable for detecting all the ROS species.Open in a separate windowFig. 3(A) HPLC chromatograms (detection at 254 nm; along with MB and neat 1) and (B) NMR-spectra (in DMSO-d₆) for addition of O₂˙⁻, ONOO⁻, OCl⁻ species to 1 in D₂O/H₂O to the DMSO-d₆.Having characterized the ROS reactions of 1 and 2 in the formation of MB, we focused on imaging the cellular produced ROS by confocal laser scanning microscopy. For this, we chose the established ROS-producing macrophage cell line J774.A1, which express ROS when treated with lipopolysaccharide (LPS) and interferon-γ (IFN-γ).The cells attached to coverslips were treated with LPS and IFN-γ, incubated for 24 hours to induce ROS. To these activated cells in 1 mL medium, 10 nmol of BHP-LMB 1 or 2 was added as DMSO or phosphate buffered saline (PBS) solution, respectively, and incubated for 6 h. The cells were fixated with 4% formalin, washed PBS and mounted on glass slides with Mowiol for microscopy. The membrane labelling WGA-488 probe was used additionally for reference. For control cells, no ROS induction was carried out with LPS/IFN-γ (Fig. 4, top). In addition, the hydrophobic 1 incorporated controlled cells, after formalin fixation, were treated with ONOO⁻ for 30 min, and washed with PBS (Fig. 4, bottom) to characterize the activation of intracellularly accumulated 1. This confirmed intra-cellularization of 1 and trapping the cellular produced ROS (Fig. 4, middle panel) or the added ROS by BHP-LMB 1 yielding red-emitting MB. A similar procedure was performed with 2, and microscopy imaging confirmed that there was no fluorescence (Fig. S4^†) indicating the dye 2 was not intracellularized.Open in a separate windowFig. 4Confocal microscopy images using 1 and J774.A1 cells for the detection of ROS; top: without ROS induction; middle: ROS expression induced by LPS/IFN-γ; bottom with ONOO⁻ addition to the fixed cells (conc: 10 nmol mL⁻¹ of 1, 6 h incubation; green: membrane labelling probe WGA-488 (exc. 488 nm), red: 1 after ROS reaction (exc. 660 nm).In summary, here we described broad-spectrum ROS/RNS detecting probes based on 2,6-di-tert-butyl phenol (BHP) appended leuco-methylene blue (BHP-LMBs, 1, 2). Hydrophobic 1 and hydrophilic 2 are sensitive towards a variety of ROS and RNS, producing the red fluorescent MB. In vitro titration with a variety of ROS showed formation of MB from 1 in DMSO, and also a persistent far red-absorbing radical species from 2 in H₂O. The NMR and HPLC analysis of 1 in DMSO with O₂˙⁻, ONOO⁻ and OCl⁻ showed no stable intermediates en route to MB. Then 1 was employed in detecting cellular produced ROS by fluorescence microscopy, which confirmed its intracellularization and suitability for detecting cellular generated ROS. The hydrophilic 2 found to be not intracellularized, and hence it may be used for extracellularly diffused ROS detection. These results suggest the suitability of 1, 2 in broad spectrum in vivo detection of ROS, which will be explored in future studies.     

Synthesis of a metal–organic framework by plasma in liquid to increase reduced metal ions and enhance water stability

Synthesis of a metal–organic framework by plasma in liquid was demonstrated with HKUST-1 as an example. HKUST-1 synthesized by this method contains a higher amount of monovalent copper ions than that synthesized by other conventional methods. The enhanced water stability was also confirmed.

Plasma in liquid provides a method for the synthesis of HKUST-1 with increased reduced metal ions and high water stability.

Metal–organic frameworks (MOFs) are a class of hybrid materials composed of organic linkers and metal nodes. They have high surface areas with tunable pore size and functionality.^1,2 Because of these features, MOFs have potential applications in the fields of gas adsorption/storage,^3,4 catalysis,^5,6 drug delivery,^7,8 and fabrication of luminescent materials.^9,10 Many methods have been proposed for synthesizing MOFs; these include solvothermal methods,^11,12 microwave-assisted methods,^13,14 ultrasonic-assisted methods,^15,16 mechanochemical methods,¹⁷ and electrosynthesis methods.¹⁸ Different synthesis methods result in the different crystallinity and size. Further, the synthesis method can influence the physicochemical and semiconductor properties.¹⁹Recently, a method for synthesizing MOFs in liquid contacting gas-phase dielectric barrier discharges has been reported.^20,21 However, MOFs synthesized by plasma have not been sufficiently characterized yet, thereby demanding further research. Plasma is a unique reaction field and has the advantage of being able to reduce and functionalize species using electrons and radicals. These characteristics have been widely utilized even in liquid phase for the synthesis of various materials such as carbon materials^22,23 and metal nanoparticles^24,25 and for introducing functional groups in materials.²⁶ Using plasma, it may be possible to synthesize functional MOFs with reduced metal ions or additional functional groups. Plasma in liquid generally have higher density of electrons and radicals than plasma in the gas phase,²⁷ and the influence of reactive species can be observed more prominently.In this study, we focused on [Cu₃(BTC)₂]_n (BTC = 1,3,5-benzenetricarboxylate). This is known as HKUST-1 and is one of the most widely researched MOFs. This MOF is assembled from copper nodes and BTC, with each copper coordinated with four oxygen atoms. HKUST-1 is a promising candidate for applications such as H₂ and SO₂ storage.^28,29 However, the crystal structure of HKUST-1 is changed by water molecules, and its surface area decreases.^30,31 To ensure the practical applications of HKUST-1, it is important to increase its resistance to water. To improve its stability to water, strategies such as incorporation of other materials, including graphite oxide³² and carboxyl-functionalized attapulgite,³³ have been developed. In the post-processing using plasma, the adsorption of water molecules on HKUST-1 is inhibited by introducing hydrophobic groups with perfluorohexane.³⁴ In another method, the adsorption of water molecules on unsaturated metal sites was prevented by irradiating the HKUST-1 with oxygen plasma.³⁵ In addition, to improve the water tolerance, reduction of Cu(ii) ions in HKUST-1 was also proposed.³⁶ However, in the above-mentioned methods, mixing of other materials or post-treatment steps was essential.In this work, we synthesized HKUST-1 containing Cu(i) by plasma in liquid; the method involved in situ plasma treatment during the synthesis activated also by plasma. Its resistance to water was compared with HKUST-1 synthesized by conventional methods such as heating or addition of triethylamine at room temperature.HKUST-1 was synthesized using the plasma generated by applying a bipolar pulse voltage to the electrode in an ethanol–water solution containing copper nitrate trihydrate and BTC (Fig. S1 and S2^†). Hereafter in this paper, the thus-synthesized HKUST-1 will be referred to as PL-HKUST-1. For comparison, HKUST-1 was synthesized by the conventional heating method and by the addition of triethylamine at room temperature,¹⁹ and the HKUST-1 formed are referred to as CH-HKUST-1 and RT-HKUST-1, respectively. The details of the synthetic methods are described in the ESI.^†All the samples were characterized by X-ray diffraction (XRD) and thermogravimetric analysis. The XRD patterns of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 are shown in Fig. 1. In all the cases, the characteristic diffraction peaks of HKUST-1 were identified, confirming the phase-pure formation of HKUST-1 samples irrespective of the synthetic method.^37,38 Thermogravimetric analysis of HKUST-1 revealed that the thermal stability of PL-HKUST-1 was similar to those of CH-HKUST-1 and RT-HKUST-1 (Fig. S3^†).Open in a separate windowFig. 1XRD patterns of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 before and after immersion in water. Fig. 2 shows the fluorescence (FL) spectra of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 upon excitation at 310 nm at room temperature. Note that a broad peak around 520 nm was observed only for PL-HKUST-1 and not for CH-HKUST-1 and RT-HKUST-1. Such a behavior has been previously reported for Cu(i) complexes and is attributed to the metal-to-ligand charge transfer (MLCT) from Cu(i) to an empty antibonding π* orbital of the ligand.^39–42 Thus, the FL spectrum indicates that a certain amount of Cu(i) is formed in PL-HKUST-1, which highly contrasted to CH-HKUST-1 and RT-HKUST-1.Open in a separate windowFig. 2FL spectra of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 (λ_ex = 310 nm).The existence of Cu(i) in PL-HKUST-1 was also confirmed by X-ray photoelectron spectroscopy (XPS). The XPS spectrum of PL-HKUST-1 showed peaks at 932.2 and 934.6 eV, assignable to Cu(i) and Cu(ii), respectively (Fig. S4^†).^43,44 Although the reduction to Cu(i) inevitably proceeded upon X-ray irradiation,⁴⁵ the concentration of Cu(i) in PL-HKUST-1 was apparently higher than those in CH-HKUST-1 and RT-HKUST-1, which was consistent with the FL spectra. The plasma has ample free electrons and ions, which can reduce metal ions via electrochemical reactions.⁴⁶ The temperature of the plasma in liquid could be as high as 1000–7000 K,²⁷ and the reduction of Cu(ii) to Cu(i) by heat might occur.⁴⁷To examine the water stability of the MOFs, HKUST-1 was immersed in water at room temperature for 12 h. The XRD patterns of the samples after immersion in water were compared with those of the as-synthesized samples. The water treatment resulted in the appearance of new peaks at 2θ ≈ 9.4°–9.7°, 10.1°, 11.1°, 17.3°, and 19.6° for CH-HKUST-1 and RT-HKUST-1 (Fig. 1). Notably, the XRD patterns of PL-HKUST-1 before and after immersion in water were identical, with no detectable changes.The effect of water treatment on the morphology of HKUST-1 was also investigated using scanning electron microscopy (SEM). Particles with octahedral shape were obtained by the plasma in liquid method, similar to those obtained by the conventional heating method (Fig. 3). After immersion in water, the formation of particles with a thread-like morphology was observed for CH-HKUST-1 and RT-HKUST-1, suggesting the decomposition of HKUST-1.^30,48 In contrast, although PL-HKUST-1 contained a portion of the etched corners, the particle shape and size remained unchanged after water treatment.Open in a separate windowFig. 3SEM images of (a and b) CH-HKUST-1, (c and d) RT-HKUST-1, and (e and f) PL-HKUST-1 before and after immersion in water.Fourier transform infrared (FTIR) spectra of HKUST-1 before and after the water treatment are shown in Fig. 4. All as-synthesized samples showed the typical characteristic peaks of HKUST-1. The bands from 1300 to 1700 cm⁻¹ are associated with the carboxylate group of the BTC ligand.⁴⁹ The two peaks in the range 1371–1448 cm⁻¹ correspond to the symmetric stretching vibrations of the carboxylate group.⁵⁰ The bands at 730 and 758 cm⁻¹ correspond to the in-plane C–H bending mode.⁴⁹ Peaks observed below 600 cm⁻¹ correspond to the bonds involving copper ions.⁴⁹ After immersion in water, the FTIR spectrum of PL-HKUST-1 did not show any significant change whereas those of CH-HKUST-1 and RT-HKUST-1 showed shifts in the peaks and formation of new peaks. When exposed to water, peaks corresponding to COO vibrations were observed for CH-HKUST-1 and RT-HKUST-1 between 1200 and 1700 cm⁻¹ due to a change in the environments of the linker BTC.⁴⁸ The peak around 1640 cm⁻¹ shifted to longer wavelengths, and a new peak appeared around 1570 cm⁻¹, which is consistent with previous research.³⁸ The Raman spectra of CH-HKUST-1 and RT-HKUST-1 also showed the structural change in the coordination bonding upon the water treatment (Fig. S5^†).⁵¹Open in a separate windowFig. 4FTIR spectra of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 before and after immersion in water. Fig. 5). The BET surface areas of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 were 888, 574, and 739 m² g⁻¹, respectively. After immersion in water, the amount adsorbed by CH-HKUST-1 and RT-HKUST-1 was lowered compared to that before immersion, and their BET surface areas were 407 and 351 m² g⁻¹, respectively. By contrast, in the case of PL-HKUST-1, the adsorption amount remained high even after immersion in water, and its BET surface area was 811 m² g⁻¹.Brunauer–Emmett–Teller surface areas of CH-HKUST-1, RT-HKUST-1, and PL-HKUST-1 before and after immersion in water

Retracted Article: Facile preparation of bithiazole-based material for inkjet printed light-emitting electrochemical cell

Light-emitting electrochemical cell of bithiazole-based material was fabricated by solution processing rendered high external quantum efficiency over 12.8% and luminance of 1.8 10⁴ cd m⁻².

Light-emitting electrochemical cell of bithiazole-based material was fabricated by solution processing rendered high external quantum efficiency over 12.8% and luminance of 1.8 10⁴ cd m^−2.

It is well known that inkjet printing works as precise and versatile patterning method for printed electronics.^1,2 As for its advantages, it is already being exploited widespread for printing electronics.^3,4 Those merits are easily processed from solutions and conveniently used for air-stable electrodes.^5,6Light-emitting electrochemical cells (LECs) have emerged as an active layer,^7,8 arousing tremendous attention over the years.^9–11 Nevertheless, LECs are simpler than organic light-emitting diodes (OLEDs).^12,13 Due to its single layer architecture, low fabrication price and operating voltages, LECs are considered as a promising, next-generation, emissive thin-film technology.^14–16The most efficient device used to date for LECs are biscyclometalated iridium(iii) (Ir) complexes, because they have highly efficient and stable devices spanning the whole visible range.^17–19 However, avoiding the use of Ir is strongly desired because of its high cost and limited supply.^20–22 Till now, thiazole has worked as active component in a LECs system with a long-lived charge-separation molecule, without additional ions in its active layer.^23–26 As for its advantages, it can reduce recombination of charge carriers and facilitate carrier transfer.^27–29 More and more importance has been attached to thiazole compounds, especially bithiazole-based ones.^30–33 Therefore, these have aroused interest in the syntheses of bithiazoles.^34–36Based on our previous research,^37–41 the use of bithiazole ligands already explored by our group for photocatalytic technology has the advantage of an easy transformation of neutral complexes into charged ones by substitution on the N-atom of the thiazole moiety.^42,43 Furthermore, we demonstrate the fabrication of LEC devices by a combination of inkjet printing and spin coating, on A4 paper substrates for low cost, disposable and flexible conductive pattern.^44,45 Their corresponding material characteristics were closely investigated, a detailed report of the material properties of the resulting coatings, and an envisaged proof of concept application are disclosed in this paper. These fully printed devices demonstrate the potential upscaling of the fabrication of optoelectronic devices.Herein we report the design for a range of bithiazole derivatives 1a–1c (Fig. 1, their synthesis is shown in Scheme S1^†), and explore their properties. First of all, the UV-visible diffuse-reflectance (UV-vis DRS) spectra of those specimens at room temperature are displayed in Fig. 2 and the data are collected in Table S1.^† This enhancement was ascribed to the increase of aromatic rings, which has an intense absorption in the visible-light region. And their bandgap energies (E_g) are between 2.97–3.01 eV (Table S1^†), and are estimated from these absorption spectra according to the Kubelka–Munk method. Expanding the conjugated system and electron density with these donor groups, it can lead to a larger bathochromic shift of the absorption maximum.^46,47Open in a separate windowFig. 1Chemical structures of compounds 1a–1c described in this work.Open in a separate windowFig. 2UV-vis DRS spectra of 1a–1c.All of the three compounds are photoluminescent at room temperature, the relevant fluorescence peak maximum ranges from 360 to 398 nm (Fig. 3). They emit blue light when they are being excited. In agreement with Fig. 3, the presence of electron-releasing groups would shift the emission maximum. Among them, the most electron-donating carbazole unit would give the most red-shifted peak at 398 nm for 1c (Table S1^†). The PL quantum yield (ϕ_PL) studies show that both 1a and 1b are as high as 62 and 78%, while another one is 85% (Table S1^†). The data for their fluorescent quantum yields largely depends on interaction effect with molecules 1a–1c in the crystal packing. In this context, π–π intermolecular interactions can inhibit the fluorescence.⁴⁸Open in a separate windowFig. 3Typical PL spectra of 1a–1c.It may indicate that enlarging the conjugation length and electron density with these donor groups plays an important role in increasing the ϕ_PL.⁴⁹ Moreover, those with high ϕ_PL may be suitable for application as efficient light-emitting material in LECs.⁵⁰In addition, time-resolved measurements of the donor lifetime (τ) in 1a–1c were carried out, and their corresponding values are given in Table S1.^† The bithiazole derivatives 1a–1c showed long-lived singlet fluorescence lifetimes (τ_F), ranging between 1.04 and 9.54 ns (Table S1^†). This was on average more than double the fluorescence lifetime of the bithiazole starting material.⁵¹ These three samples possess relatively high decay time, as a result of their inhomogeneity.⁵² Compound 1c exhibited the longest fluorescence lifetime decay τ_F = 9.54 ns. The sensitivity of the emission to the polarity of the solvent is beneficial for an intramolecular charge transfer (ICT)-like emission.⁵³ This difference probably accounts for the high concentration of donor units in the bithiazole backbone, low rates of intersystem crossing to reactive triplet states.^48,54Consequently, the composite 1c was the best performing LEC and chosen for the below test. The EL spectra obtained for both devices were almost identical, with a main peak at 498 nm and other peak at 504 nm wavelength as illustrated in Fig. 4a. It may account for trapping, cavity, and self-absorption effects from within the LEC device 1c multilayer-structure. The spin-coated emission is quantified by the commission Internationale de L’Eclairage (CIE) coordinates of (0.28, 0.42), and a colour rendering index (CRI) of 65 (Table S2^†). While inkjet-printed light emission with CIE coordinates of (0.34, 0.43) and a CRI value of 83 was achieved for device 1c (Table S2^†). It exhibits good colour rendering indices (CRI > 70), this device exhibited a warm-white appearance, and the colour rendering is considered sufficient for indoor lighting applications.Open in a separate windowFig. 4Comparison of device characteristics with spin-cast and inkjet printer emitting layer for 1c: (a) electroluminescent spectra, (b) luminance vs. voltage, (c) current density vs. voltage, (d) efficiency vs. luminance behaviour.The time-dependent brightness and current density under constant biases of 2.9–3.3 V for device 1c are shown in Fig. 4b and c. Interestingly, the device with the inkjet-printed emitting layer (EML) produced a light output of around 3600 cd m⁻², whereas 4600 cd m⁻² was achieved with the spin-coated EML. In other words, the device with the inkjet printed EML obtained about 78.3% of brightness compared with the reference device. The differences between the two devices regarding current efficiency (J) and power efficiency (PE) were rather closed, as listed in the Table S2.^† A luminous efficiency much larger than 10 cd A⁻¹ was achieved for the inkjet-printed EML, which tended to be 73.9%, and is similar to the spin-coated emitting layer.The performance gap between the devices (Fig. 4d) can be attributed to differences in the height and surface roughness of the emitting layer.⁵⁵ Lateral sizing histograms (Fig. S11^†) show that the spin-coated EML possessed a thickness of 35 nm and a very smooth surface. The inferior performance of the inkjet-printed EML is caused by a less homogeneous surface morphology and an overall fatter layer.⁵⁶ The depth of the inkjet-printed EML ranged from 25 nm (pixel centre) to 35 nm (pixel edges). As a consequence, it is an inkjet deposition process and the related evaporation dynamics reduces the light output and efficiency, but also postpones the stability of the device in the system.^10,57We then shifted our attention to the turn-on kinetics, efficiency and long-term stability of the LEC device for 1c; the typical evolution of current density and light emission is presented in Fig. 5a and b, showing a slow turn-on followed by a progressive decay over time.Open in a separate windowFig. 5(a) Current density (closed symbols) and brightness (open symbols) versus time at 3 V for a device with the representative sample 1c; (b) EQE versus time at an applied voltage of 3 V.The build-up of the light output is synchronous with that of the current density. This time-delayed response is one of the striking features of the operation of an electrochemical cell and reflects the mechanism of device operation.²² The champion device in this set exhibited a peak power efficiency (PE) of 17.4 lm W⁻¹ at a luminance of 18 000 cd m⁻². Meanwhile, the current density began to decrease, possibly due to the electrochemical oxidation or electro corrosion that occurred on component 1c through accumulated electroinduced holes, and then it kept up a durative datum.⁵⁸ Apparently it exhibited outstanding long-term operation stability beyond 40 h (Fig. 5b).The time-dependent external quantum efficiency (EQE) of LEC for the representative substrate 1c are shown in Fig. 5b. It also exhibited similar temporal tendency in device efficiency. When a bias was applied on the LECs, the EQE quickly increased since balanced carrier injection was achieved by the formation of the doped layers.⁵⁹ After attaining the crest value, the device current was still rising while the EQE reduced little by little. It implied that both growing the doped layers and weakening of the EQE was resulting from exciton quenching near the bithiazlole core in persistently extended doped layers.⁶⁰ Doping-induced self-absorption was rather adaptable to the temporal roll-off in device efficiency. This configuration shows the best performance (EQE = 12.8%) compared to those devices prepared with the single compounds, which is given in Table S3.^† The reason may be that the steady-state recombination zone in 1c device was close to the central active layer.⁶¹ Besides it contains fluorenes on its side chains, constructing plane structure, benefiting from good electron injection abilities, leading to enhancing the EQE itself.⁶²The fabrication of inkjet-printed bithiazole interdigitated electrode (IDE) is illustrated in Fig. 6. All the prepared varieties of bithiazole-based ink have been found to be highly stable with nil or miniscule precipitation in over 180 days being stored on the shelf (Fig. 6a). After HCl treatment of the paper substrate, the prepared inks have filled within ink cartridges of a low-cost desktop printer Canon iP1188 (Fig. 6b). Fig. 6c shows the photograph of inkjet-printed conductive patterns.Open in a separate windowFig. 6(a) The representative ink based on sample 1c was stable for 180 days; (b) electrode printing using Canon iP1188 printer; (c) scale showing size of printed electrodes; (d) interdigitated electrode (inset shows small size printed pattern).In addition, the most attractive prospect of the bilayer device structure at this stage is the possibility for patterned emission for the creation of a static display. Fig. 7 presents a photograph of a small portion of a larger static display, with a resolution of 170 PPI, which repeatedly exhibits a message in the form of the word “LEC”. As shown in Fig. 7, the pixel array produced are fairly luminous. The EL spectra were collected from each pixel on the substrate, the emission peak wavelength was about 510 nm and the full width at half maximum was about 170 nm. The low variation in emission intensity in the different pixels implies a small variation in thickness of the solution-coated layers. More specifically, if we assume a minimum diameter of 20 μm for an inkjetted electrolyte droplet, and a smallest inter-droplet distance of 10 μm, we attain a pitch of 30 μm, which corresponds to a high display resolution of 850 PPI. Finally, we draw attention to the herein presented static-display LEC that comprises solely air-stabile materials, and that we routinely fabricate the bilayer stack under ambient atmosphere.Open in a separate windowFig. 7(a) The patterned light emission from a bilayer LEC, with the emission pattern defined by the selected positions of the ink jetted electrolyte droplets. (b) The droplet diameter and pitch were 50 and 150 μm, respectively, and the device was driven at V = 3 V. The scale bar measures 300 μm.     

Electrochemical preparation system for unique mesoporous hemisphere gold nanoparticles using block copolymer micelles

Gold nanoparticles (AuNPs) are widely used in various applications, such as biological delivery, catalysis, and others. In this report, we present a novel synthetic method to prepare mesoporous hemisphere gold nanoparticles (MHAuNPs) via electrochemical reduction reaction with the aid of polymeric micelle assembly as a pore-directing agent.

Mesoporous hemisphere Au nanoparticles using self-assembled micelles, for the first time, are demonstrated by using electrochemical reduction on a Ti substrate.

Gold (Au) is one of the most stable and versatile elements utilized in various fields, including catalysis, optics, and industrial purposes. Consequently, various shapes and sizes of AuNPs have been intensively studied to improve the performance of Au in different applications.^1–6 Previously, nanoporous or dendritic metal nanostructures, including Au nanostructures, have been synthesized by employing different reagents and conditions such as SH-terminated amphiphilic surfactant,⁷ pH controlling,⁸ and hard-templates.^9,10 The reported porous and dendritic Au nanostructures possess high surface areas and rich active sites, which in turn lead to highly enhanced catalytic activities.Recently, a soft-template method using self-assembled micelles or lyotropic liquid crystals as pore-directing agents has allowed the successful synthesis of mesoporous nanoparticles^11–13 and films^14–17 with different metal compositions. The metals with mesoporous structures demonstrate superior catalytic activity per weight or surface area over their nonporous bulk forms. Previously, our group reported a several-fold increase in the catalytic activity of mesoporous metals in reactions such as the methanol oxidation reaction (MOR),^14,15 ethanol oxidation reaction (EOR),^13,15–17 and nitric oxide reduction¹² as compared to their bulk nanoparticles and films. Such improvement in the catalytic activity of mesoporous structures is mainly attributed to their significantly larger surface areas, more exposed catalytically active sites, and increased durability against aggregation.Interestingly, nanoporous or mesoporous Au structures had been successfully synthesized by using a dealloying method¹⁸ and a hard templating method.⁹ Such methods, however, are a little complicated, and pore-directing templates often remain within the pores, thus leading to severe contamination. Using a thiol group is an alternative way to synthesize mesoporous Au nanospheres.⁷ A significant drawback of using a thiol group, however, is its strong chemical bonding with Au, thus becoming unable to be removed. The synthesis of mesoporous structures using self-assembled polymeric micelles as soft-templates, on the other hand, is a more facile method with fewer synthetic steps, and it is also known to be free of contaminations within the pores. Although a soft-templating method using polymeric micelles has been utilized for the preparation of mesoporous Au and Au-based alloy films towards surface-enhanced Raman scattering (SERS) signals,¹⁹ glucose sensing,^20,21 and MOR,²² the obtained morphologies have been limited to only films.Despite such apparent benefits arising from mesoporous structures and their synthesis using soft-templates, the synthesis of mesoporous AuNPs using soft-templates has not been achieved yet. It is mainly due to the physical and chemical properties of Au which make it extremely hard to form mesoporous structures. Herein, we adopt an electrochemical approach and the soft-template method to synthesize MHAuNPs successfully. As discussed above, we expect MHAuNPs to be highly efficient in various applications in medical diagnosis,²³ optical sensing,²⁴etc.In this report, MHAuNPs with different shapes and sizes are for the first time reported by changing various electrochemical deposition conditions such as applied potentials between electrodes and deposition times. Scheme 1 shows the schematic illustrations of the entire process of precursor preparation (Scheme 1a) and the MHAuNPs fabrication process (Scheme 1b), including the deposition and the detachment of the nanoparticles. The characterization methods implemented in this paper are mentioned in ESI.^†Open in a separate windowScheme 1(a) The process of Au precursor solution preparation and (b) fabricating MHAuNPs by electrochemical reduction.In a typical experiment, a p-doped silicon (Si) wafer was cleaned by using acetone, isopropyl alcohol, and deionized water (DIW) with sonication for 5 minutes, followed by nitrogen (N₂) gas blowing to dry the Si wafer. After the wet cleaning process, the Si surface was treated by oxygen (O₂) plasma for 5 minutes (Oxford Instruments PlasmaPro 80 Reactive Ion Etcher) to remove residual organic impurities. Then, 10 nm of titanium (Ti) layer and 100 nm of Au layer were deposited sequentially by electron beam evaporation (Temescal FC-2000 e-beam evaporator) at 10⁻⁶ torr. Commercially available Au etchant (Sigma-Aldrich) was used to etch the Au film to expose the Ti area (the left image in Scheme 1b). During etching, about 20 percent of Au area was left to be connected to the electrochemical work station, as drawn in Scheme 1b. In preparation of the Au precursor solution, 5 mg of poly(styrene)-block-poly(ethylene oxide) (PS-b-PEO, the number of average molecular weight (M_w) for each block is 18 000 for PS and 7500 for PEO, respectively) was mixed in 1.5 ml of tetrahydrofuran (THF) followed by stirring at 300 rpm for 8 hours. Then, 0.75 ml of ethanol, 0.5 ml of HAuCl₄ aqueous solution (40 mM), and 1.25 ml of DIW were added sequentially. The solution was stirred for another 30 minutes at 200 rpm. The existing block copolymer micelles can be confirmed by TEM observation, and the average diameter is 25 nm, as shown in Fig. S1.^† For the electrochemical deposition, an electrochemical workstation (CH Instruments Inc. 660e) with three electrode system was used to deposit MHAuNPs on the Ti/Si substrate. After the deposition, the particles were carefully washed by chloroform, followed by a rinse using DIW to remove the residual micelles completely. To detach and collect MHAuNPs from the Ti/Si substrate, the substrate was soaked in ethanol and strongly sonicated for a few minutes (Scheme 1b).Fig. S2^† shows the details of the growth mechanism of MHAuNPs by different deposition times. At the initial stage (Fig. S2a^†), small nanoparticles are generated by reducing Au ions in the precursor solution throughout the substrate. Then, the seed starts growing and forming MHAuNPs as the deposition time increases (Fig. S2b–e^†). This similar growth mechanism is the same as the previous report.¹⁹ The high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM) image (Fig. S2f^†) shows the mesopores inside the MHAuNPs are homogeneously generated. As-obtained MHAuNPs consist of a pure Au element without any impurities, as shown in Fig. S3.^† Fig. 1 and S4^† show scanning electron microscope (SEM) images of MHAuNPs deposited at different voltages from −0.2 V to −0.9 V vs. Ag/AgCl at high magnification and low magnification, respectively. Different deposition voltages lead to significant changes in the particle sizes but slight differences in the particle shapes. The size distributions of MHAuNPs and the plots of the average diameters of MHAuNPs by different deposition voltages are described in Fig. 2. The distribution graphs show the large sizes of particles, such as more than 1 μm in diameter, when the high voltage (−0.2 V vs. Ag/AgCl) is applied (Fig. 2a). The distribution becomes narrower upon the lower applied voltage. The average diameter-applied voltage plots in Fig. 2b show that the average particle size decreases from around 1.1 μm at −0.2 V to about 300 nm at −0.9 V. Thus when the lower deposition voltages are applied (i.e., the deposition rate is higher) (Fig. 1g–h), the smaller particles with a higher degree of size uniformity are obtained. The opposite trend is observed at higher deposition voltages (i.e., the deposition rate is lower) (Fig. 1a and b), at which the particles become larger and their size uniformity decreases. This trend is because the higher voltage allows only a limited number of seed particles to be deposited on the Ti/Si substrate, and each seed individually grows with no additional seed formation. Whereas the lower voltage can allow a higher number of seeds, leading to a uniform supply of electrons from the working Ti/Si electrode (Fig. S5^†). In addition, the lower deposition voltages make the particle shape more hemispherical in Fig. 1f–h.Open in a separate windowFig. 1The SEM images of MHAuNPs electrochemically deposited at (a) −0.2 V, (b) −0.3 V, (c) −0.4 V, (d) −0.5 V, (e) −0.6 V, (f) −0.7 V, (g) −0.8 V, and (h) −0.9 V for 500 s. The scale bars indicate 200 nm.Open in a separate windowFig. 2(a) Size distributions of MHAuNPs generated by different voltages and (b) the average diameter–the applied voltage plots. Fig. 3 shows the SEM images of MHAuNPs deposited at −0.2 V and for different deposition times from 250 s to 1000 s. Although longer deposition time does not change the number of MHAuNPs, it leads to the growth of MHAuNPs in lateral and vertical directions. Although the MHAuNPs grow more than about two or three times larger at long deposition time, the mesoporous formation does not seem to be changed, as shown in insets in Fig. 3. This point indicates that the deposition time is not the main factor affecting the formation of mesoporous structures as well as the number of particles (seeds), but it affects the sizes of particles.Open in a separate windowFig. 3The SEM images of MHAuNPs deposited at −0.2 V (vs. Ag/AgCl) for (a) 250 s, (b) 500 s, and (c) 1000 s. The scale bars indicate 10 μm. The insets in each figure are magnified SEM images of each condition (The scale bars in insets indicate 500 nm).In this report, 10 nm Ti layer on Si wafer plays an important key role in the formation of MHAuNPs, as previously mentioned in the experimental procedure. The use of the Ti substrate with low conductivity (ca. 2.38 × 10⁶ S m⁻¹), which is about only 5.8% in comparison with that of Au (ca. 4.10 × 10⁷ S m⁻¹), is not common in the electrochemical plating research field.^25–30 Most of the papers on mesoporous metal structures synthesized by electrochemical deposition have utilized Au or Pt substrates due to its chemical stability and high electrical conductivity.^14–17 Fig. S6^† shows the amperometry (i–t) curves during the deposition of MHAuNPs (black dots) on a Ti/Si substrate and mesoporous gold films (red dots) on an Au substrate at the same deposition condition. As shown in Fig. S6,^† around 1/7 times less current flows on the Ti/Si substrate throughout the deposition time. This low current density on the Ti/Si substrate is one of the factors for fabricating MHAuNPs. Low current density causes the formation of a few particles (i.e., seeds) at the initial stage of the deposition and leads to seed growth in a few places, as explained in Fig. S2.^† Furthermore, the use of Ti/Si substrates affects the bottom parts of MHAuNPs to become an arch shape. Only edges of MHAuNPs attach onto the Ti/Si substrates, as shown in Fig. 4. This attachment is because the interaction between the deposited MHAuNPs and the Ti substrate surface (probably, the Ti surface can be partially oxidized, forming TiO_x) is very weak. Therefore, the deposited MHAuNPs can be easily detached from the Ti/Si substrates by sonicating the substrates in solvents (Scheme 1b). The collected MHAuNPs in a solvent are obtained as colloidal particles as shown in Fig. S7.^† Such interesting hemispherical mesoporous nanoparticles have advantages to electrocatalytic activities in comparison to spherical mesoporous metals.³¹ The method using a Ti/Si substrate as a working electrode can be repeatedly implemented with one substrate and without change of the precursor solution, thus it can be effective for mass production in the future.Open in a separate windowFig. 4The SEM image of MHAuNPs deposited at −0.6 V. The arrow shows that the bottom of the MHAuNPs is an arch.Finally, surface-enhanced Raman scattering (SERS) effects on MHAuNPs were investigated by using an adsorbate called rhodamine 6G (R6G), as shown in Fig. S8.^† The resulting MHAuNPs at all conditions (−0.3 V, −0.6 V, and −0.9 V) show substantially strong SERS intensity (Fig. S8a^†), while Ti/Si and Si substrates without MHAuNPs show noise level of intensity. To further investigate enhancement factor (EF) and limit of detection (LoD), various concentrations of R6G with MHAuNPs fabricated at −0.9 V were used for the SERS studies (Fig. S8b^†). The main peak of SERS is 1363 cm⁻¹, and it disappears from less than 10⁻⁶ M concentration, while the 1183 cm⁻¹ peak still exists at 10⁻⁸ M (Fig. S8c^†). The maximum EFs at 1363 cm⁻¹ (10⁻⁶ M) and 1183 cm⁻¹ (10⁻⁸ M) are 1.5 × 10⁴ and 3.1 × 10⁶, respectively (Fig. S8d^†). Transmission electron microscope (TEM) images in Fig. S9^† show the detailed particle structures and the electron diffraction (ED) pattern confirmed the crystal structure is the face-center cubic (FCC) structure. The sharp surface structures and the pores on MHAuNPs provide abundant hot spots that have been reported as the origin that enhances SERS intensity owing to the plasmon resonances.^19,32 Besides, the high density of small-sized MHAuNPs (Fig. S5 and S10^†) boosted higher SERS intensity.In conclusion, we have synthesized MHAuNPs by using 10 nm Ti-coated Si substrates as a working electrode on a Si wafer and electrochemical deposition using self-assembled polymeric micelles as pore-directing agents. The low current generates Au seeds at only a few places, and it acts as the points that MHAuNPs start growing. The particle shapes and sizes can be controlled by changed applied voltages and deposition times. The lower voltages make small particles and the great hemispherical AuNPs with mesoporous architecture. The long-time deposition does not affect any mesoporous formation, but the particle shape and size. Besides, the low affinity between Au and Ti (probably, oxidized layer) results in the arch on the bottom of MHAuNPs, which helps the particles detached from the substrates easily. These results indicate that different thicknesses and compositions of working electrodes can provide different metal deposition phenomena, which can bring out unique shaped particles with mesoporous architectures in the future.     

Preparation of solution processed photodetectors comprised of two-dimensional tin(ii) sulfide nanosheet thin films assembled via the Langmuir–Blodgett method

We report the manufacture of fully solution processed photodetectors based on two-dimensional tin(ii) sulfide assembled via the Langmuir–Blodgett method. The method we propose can coat a variety of substrates including paper, Si/SiO₂ and flexible polymer allowing for a potentially wide range of applications in future optoelectronic devices.

Norton et al. report the manufacture of fully solution processed photodetectors based on two-dimensional tin(ii) sulfide assembled via the Langmuir–Blodgett method.

Two-dimensional (2D) materials are condensed matter solids formed of crystalline atomic layers held together via weak van der Waals forces.¹ They have a wide range of applications including use as channel materials in transistors,² absorber layers in solar cells,³ light emission,⁴ energy storage⁵ and drug delivery⁶ among others. 2D materials often have different properties from their bulk counterparts such as increased strength⁷ and electrical conductivity.⁸ 2D semiconductors may exhibit a change in electronic states from confinement in 1D.⁹ Thin films are often required for the creation of devices from nanomaterials for practical applications and can often be made into flexible devices such as thin film solar cells¹⁰ or photodetectors.^11,12 Thin film solar cells in particular have several advantages over conventional solar cells including lower materials consumption and are lightweight, yet have the potential for high power conversion efficiency.¹⁰Many of the two-dimensional materials produced thus far have been derived from mechanical exfoliation, where Scotch tape or an equivalent is manually used to remove single crystalline layers from a bulk van der Waals solid followed by transfer to a substrate. Whilst this method in general produces extremely high quality crystalline atomic layers,¹³ and is therefore often used to produce prototype devices, it inherently lacks scalabilty. In order to address the problem of mass manufacture of two dimensional materials, liquid phase exfoliation (LPE) was introduced as a cost effective method for producing two dimensional nanomaterials¹⁴ with the possibility of 100 L scales being produced and production rates up to 5.3 g h⁻¹ demonstrated by Coleman et al. with both NMP and aqueous surfactant solutions utilised.¹⁵ This method also does not require the high temperatures needed for methods such as CVD¹⁶ or transfer between the growth and final substrates. Liquid phase exfoliated nanomaterials are also directly processable from solution.¹⁵ Furthermore, LPE has been shown to be effective for the production of a wide range of 2D materials such as graphene,¹⁵ transition metal dichalcogenides¹⁷ and monochalcogenides such as SnSe.¹⁸Tin(ii) sulfide (SnS) is a van der Waals solid with a puckered ab structure consisting of alternating Sn and S atoms, and is isostructural and isoelectronic with black phosphorus.¹⁹ The bulk material has attracted interest due to its indirect band gap energy of 1.07 eV,²⁰ similar to bulk silicon at 1.14 eV. This band gap energy for SnS is useful for applications such as photodetection²¹ and due to its higher theoretical Shockley–Queisser efficiency limit (24%) for solar cells.²² The liquid phase exfoliation method established by Coleman et al. enables nanosheets to be separated from the bulk into solution utilising matching surface energies of the material and solvent.²³ Liquid phase exfoliation of SnS was first reported by Lewis et al. it was established that as layer number reduced, band gap energy increased, and by tuning layer number the onset of photon absorption can be tuned over the near infrared²³ to visible range.²⁴ Overall, LPE is capable of creating large quantities of nanosheets, with potential for industrial scale production. Liquid phase exfoliated SnS has, for example, recently been used in the creation of photoelectrochemical systems with strong stability under both acidic and alkali conditions.²⁵ Many of the functional devices produced thus far have been derived from micromechanical exfoliation and manual nanomanipulation. A far more elegant solution to producing functional devices is to assemble them from solution, for example Kelly et al. recently reported a transistor based on exfoliated WSe₂ nanosheets.²The Langmuir–Blodgett method involves the use of a trough with a layer of water and controllable barriers to compress the film. Nanomaterials in solution are added to the surface of the water and spread evenly to reduce their surface energy,²⁶ often by using a low surface tension spreading solvent such as chloroform.²⁷ The surface pressure is measured as the film is compressed with the substrate being withdrawn when the film becomes solid.²⁸ The Langmuir–Blodgett method has the advantages of large area deposition and improved control of the film at the nanoscale in comparison to vacuum filtration as well as the advantage of requiring no volatile solvents in comparison to liquid–liquid assembly methods. The use of movable barriers also allows for greater film compression.²⁶This method has been used to assemble large scale films of exfoliated MoS₂ by Zhang et al. MoS₂ was exfoliated using n-butyl lithium followed by solvent exchange. MoS₂ was deposited onto the water surface using a 1 : 1 mix of DMF and dichloroethane. Substrates up to 130 cm² were coated with a surface coverage of 85–95%.²⁶ Collapse mechanisms of MoS₂ Langmuir films have also been studied²⁹ alongside MoS₂ deposition on the surface of water with an upper hexane layer.³⁰ Graphene films have also been prepared using the Langmuir–Blodgett method.³¹ The Langmuir–Blodgett method has been used for the assembly of organo-clay hybrid films via the coating of octadecylammonium chloride in a 4 : 1 chloroform : ethanol solution onto a 2D nanoclay liquid phase exfoliated film using an electrospray method.³² A solvent mix of chloroform and NMP has also been utilised for the deposition of nanosheet films.³³ Recently the Langmuir–Blodget method has been used for the assembly of unmodified clay nanosheets,³⁴ Ti₃C₂Tx MXene nanosheet films for the removal of Cr(vi) and methyl orange from an aqueous environment³⁵ as well as for the growth of rGO wrapped nanostructures for use in electrocatalysts.³⁶Given the chemical similarity of the basal planes of inorganic 2D materials, we hypothesised that the assembly of group IV–VI nanomaterials such as SnS should also be possible at the air water interface. Due to their interesting semiconducting and properties described, it should also be possible to produce prototype optoelectronic devices from a fully solution processed pathway. In this paper we now communicate a methodology to assemble thin films comprised of 2D SnS nanosheets using the Langmuir–Blodgett technique (Scheme 1a). We report the use of these films in simple photodetectors. This represents a scalable methodology to produce fully solution processed devices based on 2D materials.Open in a separate windowScheme 1Preparation of SnS nanosheet thin films via the Langmuir–Blodgett method. (a) Cartoon of Langmuir–Blodgett film preparation. (b) Image of Langmuir–Blodgett trough with compressed SnS film. (c) Surface pressure profile during film compression. (d) Image of sample prepared on Si/SiO₂ substrate with edges masked (scale bar 1.5 cm). Scheme 1(a) shows the step by step process of film preparation. Firstly, bulk SnS is broken down by liquid phase exfoliation from the bulk material to produce a stable dispersion of crystalline nanosheets. Characterisation of the exfoliated nanomaterials was undertaken using atomic force and electron microscopy yielding average sheet dimensions of 23.9 nm height × 224 nm longest side length (Fig. S1^†). The nanosheets were then deposited onto the water air interface. The film is then compressed whilst an immersed substrate is withdrawn, leading to the creation of a densely packed nanosheet film. Scheme 1(b) shows that SnS can be successfully deposited on the water–air interface via the addition of chloroform as a spreading solvent, as shown previously with other Langmuir based films.²⁷Scheme 1(a) shows a z-type deposition of SnS as the hydrophilic glass and Si with a 300 nm oxide layer is withdrawn through the film at 1 atm pressure. The film compression occurred at a rate of 5.88 cm² s⁻¹. No further treatments were performed to change the hydrophilicity of the substrates, the oxide layer present was sufficient to provide hydrophilicity to the substrate.³⁷Scheme 1(c) shows a gradual increase in surface pressure as the area was decreased from 1175 cm² to 298 cm² before a sharp increase in pressure, indicating the film has reached full compression. The sharp increase in surface pressure during compression is common in Langmuir–Blodgett assembled films of nanomaterials.³⁸ In response to compression the surface pressure profile in Scheme 1(c) rises rapidly until it reaches a maximum due to the size of the sheets and the potential difficulty in sliding over each other compared to polymers or smaller nanomaterials. Scheme 1(d) shows that the film is capable of being coated onto Si/SiO₂ with a mask defining the areas covered.We characterised the resulting structural and electronic properties of the thin film of SnS nanosheets deposited via the Langmuir–Blodgett method using a range of techniques. Fig. 1(a) shows a height profile AFM image of a film edge with an average on-film roughness (R_a) of 31.9 nm and an average film thickness of 78.6 nm (Fig S3^† provides an additional film profile). Previous work on Langmuir–Blodgett deposition has produced thinner films. The use of high centrifugation speeds yielded 7 nm thick films for a single deposition³¹ whilst the use of lithium ion intercalation before exfoliation enabled film thicknesses of under 2 nm per layer to be realised.²⁶ The average film thickness is above the average sheet thickness, suggesting that the film is made up of overlapping flake multilayers. However, the thickness of the films is significantly lower than those grown via chemical bath deposition (e.g. 290 nm (ref. 39)) indicating that thinner films can be produced compared to chemical bath methods, and potentially at a much lower cost than methods such as CVD. Images of the film morphology in plan view SEM (Fig. 1(b)) suggest no notable alignment of the nanosheets in the lateral dimension as the film is formed and deposited (see Fig S4^† for statistical analysis of sheet angle measurement). The coverage of the film is 94.6% as determined by image thresholding using imagej software to determine the area left uncovered. This gives a coverage of 0.0142 gm⁻² as calculated from average thickness, SnS density and % coverage of the substrate. Preliminary SEM results also suggest that the Langmuir–Blodgett method is effective at coating SnS onto a variety of substrates including polyolefin films (Parafilm®), aluminium foil and paper (Fig S6^†). We also probed the structure of the thin films by powder X-ray diffraction (XRD). After exfoliation and film assembly, the diffraction peak associated with the (400) of SnS is still the most intense reflection but is characterised by a much larger FWHM compared to that of bulk SnS under the same recording conditions (0.442° ± 18.5% compared to 0.175° ± 5%). This indicates a successful breakdown of the crystal structure and thinning of the material in the (400) plane during exfoliation due to the reduction in long range order⁴⁰ (reflections for bulk SnS are assigned to orthorhombic SnS and indexed in Fig S2^†). The lack of any additional peaks indicates that there has not been any significant degradation of the material to the corresponding oxide which is in agreement with previous works.^24,25 The reflections at 88° and 94° are unlikely to be from crystalline silicon⁴¹ due to the thick oxide layer and low angle of incidence used. We tentatively ascribe these peaks to the 3,0,−3 and 3,2,4 peaks for SnS.⁴¹ However a confident assignment of this reflection requires further studies.Open in a separate windowFig. 1Structural characterisation of SnS nanosheet thin films assembled by the Langmuir-Bllodgett method. (a) AFM image of LB assembled SnS film edge. Inset film profile, scale bar = 10 μm. (b) SEM image of LB assembled film on Si/SiO₂ at 3 kV using secondary electron imaging, scale bar = 1 μm. (c) XRD pattern of coated film and bulk SnS powder, (additional peaks labelled in Fig. S2^†). (d) Raman spectra and for bulk and Langmuir–Blodgett assembled SnS nanosheets. (e) UV-Vis spectra of SnS suspension and deposited SnS film on glass (f) Tauc plot of SnS solution and film.We also characterised the optical properties of the nanosheet thin films using Raman and UV-Vis-NIR absorption spectroscopy. No shifts in the Raman peak positions B₃g, Ag and B₃u from bulk SnS to Langmuir–Blodgett film were observed. The broad feature at around 300 cm⁻¹ for the LB film may potentially be due to SnS₂ and Sn₂S₃ impurities.⁴² It is predicted that due to the lower density compared to SnS⁴³ the impurities may increase in concentration compared to the bulk after centrifugation. These impurities may have significant effects on the efficiency of the devices produced.⁴⁴A shift in peak positions is typically observed in nanomaterials which exhibit quantum confinement,⁴⁵ this occurs at 14 nm for SnS.⁴⁶Fig. 1(e) shows a UV-Vis spectra from which the absorption coefficients at fixed wavelengths may be obtained, for 350 nm, 405 nm, 450 nm, 500 nm, 600 nm and 800 nm the values obtained were: 2.26 × 10⁵ cm⁻¹, 2.21 × 10⁵ cm⁻¹, 2.16 × 10⁵ cm⁻¹, 2.04 × 10⁵ cm⁻¹, 1.67 × 10⁵ cm⁻¹ and 1.05 × 10⁵ cm⁻¹ respectively, this matches well to the absorption coefficients of SnS in literature (greater than 10⁴ cm⁻¹).⁴⁷ It also suggests there may be a greater response at shorter wavelengths. Fig. 1(f) shows a band gap of 0.92 eV for the exfoliated SnS in NMP which is below the expected value of 1.07 eV (ref. 20) although lies within the reasonable error introduced by the use of Tauc plots.⁴⁸ The band gap also matches well with SnS exfoliated in NMP in previous work.²⁴ The band gap of the film appears to change from nanosheet suspension to film in 1(f). This has been observed previously for Langmuir–Blodgett⁴⁹ and other deposited films. It has also been observed that apparent decreases in band gap may occur due to the presence of scattering artefacts within films of nanoscale objects.⁵⁰We then produced simple prototype photodetectors via the printing of Ag nanoparticles to form interdigitated electrodes on top of the SnS nanosheet film. Additionally, SnS films were deposited onto lithographically defined Au interdigitated electrodes for characterisation and referencing to the printed devices.Previously SnS photodetectors have been created via methods such as electron beam deposition,⁵¹ thermal evaporation⁵² and chemical bath deposition.⁵³ The Langmuir–Blodgett method allows SnS to be directly processed into a film from a liquid phase exfoliated solution, allowing them to be produced cheaply and with the potential for scalability.Inset to Fig. 2(a) is an image of an interdigitated Ag electrode SnS photodetector device with an area of 6.4 × 10⁻⁵ m². The electrodes can be clearly identified with an average spacing of 99 μm, and an average RMS edge roughness value of 1.89 μm (determined for individual contact lines using the imageJ ‘analyze_stripes’ plugin⁵⁴ (Fig S7^†)). Fig. 2(a) shows an increase in the slope of the I–V curve in the third quadrant indicating a reduction in resistance under 1 sun illumination (1000 W m⁻²) with the AM1.5 spectrum. No short circuit current under illumination was observed indicating that the device functions as a photoconductor. The non-linear response upon negative biasing is due to initial trap filling which once equilibrium has been reached results in linear device operation. Previously it has been shown that silver diffusion into SnS has an interstitial doping effect, neutralising defect states and lowering the film resistivity.^55,56 It is also possible that the Ag ink morphology and the concentration of nanoparticles in the ink may play an effect on the device properties.⁵⁷ A resistivity of 2.85 × 10⁶ Ω sq⁻¹ was obtained for the device which is significantly higher than SnS films prepared by physical vapour deposition (250 Ω sq⁻¹),⁵⁸ likely due to poor carrier mobility between flakes.Open in a separate windowFig. 2(a) IV curves of printed contacts SnS device under darkness and AM1.5 illumination with inset photograph of pseudo Langmuir–Blodgett device with printed Ag contacts scale bar 5 mm. (b) Device under +40 V bias under fixed darkness/illumination cycle. Fig. 2(b) indicates that a clear response is present under illumination when an external bias is applied (giving a field strength of 0.4 V μm⁻¹). Closer inspection shows a fast and slow decay component following the illumination being blocked. This biexponential decay indicates the capture of trapped carriers and the presence of trap states within the device.^59,60 This again supports the photoconductive nature of the device operation with a rise time of ∼0.22 s and a fall time of ∼2.83 s,⁶¹ both being longer than the shutter closing/opening time of 3.7 ms (which was considered negligible). The rise time is the time taken to get from 10% to 90% of the light current with the fall time being the time taken from 90% of the light current to 10%.Previous work performed by Jiang et al. has shown a slow fall time in Ag/SnS photoconductor devices arising from carrier trapping.⁶² Similarly, in our devices the large rise time may also be due to the presence of a high trap density which must be filled upon light exposure.The mean dark current is 2.78 × 10⁻¹⁰ A with a standard deviation of 2.02 × 10⁻¹¹ A. The mean light current was found to be 3.92 × 10⁻¹⁰ A with a standard deviation of 4.03 × 10⁻¹¹ A. A poor signal to noise ratio appears to be present within the device, possibly due to the large number of SnS nanosheets involved in charge carrier transit, leading to a low signal, hence a low signal to noise ratio. The noise could be reduced via surface passivation⁶³ or the use of a diode like structure to reduce leakage current under reverse bias.⁶⁴ A low responsivity of 2.00 × 10⁻⁹ A W⁻¹ ± 1.5 × 10⁻¹⁰ A W⁻¹ was found for energies above the band gap energy of 0.6 eV for the deposited film.The low responsivity may be due to poor bridging between individual SnS nanosheets and the poor transport of holes between adjacent flakes (hopping) relative to the higher mobility within each flake.⁶⁵ There are potentially hundreds of nanosheets between the contacts as determined by the average length obtained (Fig S1^†). To confirm that the optical response was due to the presence of the SnS a reference device was tested (without SnS deposition, Fig S8^†) with no photoresponse observed. Despite the low responsivity, it is notable that the SnS devices fabricated are one of the few examples of a thin film photodetector device based on 2D materials requiring only solution processing at ambient temperature and atmospheric pressure.To demonstrate that the observed behaviour originates from the photoresponse of the SnS flakes a second device was fabricated by pseudo Langmuir–Blodgett deposition on to lithographically defined Au interdigitated electrodes (15 μm separation) on fused silica (inset Fig. 3(b)). This enabled us to remove any effect of photoinduced Ag migration from the observed behaviour as well as eliminating the issue of potential printing irregularities. Fig. 3(a) shows that the devices display a similar photoresponse to the devices with printed Ag electrodes when exposed to modulated AM1.5 illumination. The dark current remains similar at ∼0.3 nA, though during illumination the current is higher (0.7 nA vs. 0.4 nA). This increase directly correlates to the higher electric field strength (0.66 V μm⁻¹vs. 0.4 V μm⁻¹) between the interdigitated electrodes. The responsivity of the device was determined to be 1.79 × 10⁻⁸ A W⁻¹, with a photoresponse rise and fall time of 0.77 s and 0.85 s respectively. The responsivity is lower than for photodetectors prepared by Guo et al.⁶⁶ Improvements to the device to improve the responsivity could include methods to improve the lateral size of nanosheets such as intercalation.⁶⁷ Other routes to improve the device may include doping^68,69 or a change in architecture to a phototransistor type device.⁷⁰ The removal of potential SnS₂ and Sn₂S₃ impurities via methods such as annealing at 500 °C, 500 mbar pressure under argon or the use of higher quality starting material may also be a key route to improve the efficiency of the device.⁴²Open in a separate windowFig. 3(a) Device under 30 s off, 30 s on solar simulator illumination at 1 sun and 10 V bias (b) IV curves under darkness and 350 nm illumination with inset optical microscopy image of contacts (c) monochromatic illumination responses under 10 V bias mapped onto UV-Vis transmission spectra (d) device response under fixed 10 V bias under 350 nm and 405 nm monochromatic illumination.It is also noticeable that the level of noise present in Fig. 3(a) is reduced compared to that in Fig. 2(b), indicating that the Ag electrodes themselves (in addition to the SnS sheets) also affect the performance.When exciting using AM1.5 illumination it is possible that thermal effects may be present which could give rise to the observed behaviour.In order to demonstrate a true photoresponse monochromatic illumination was used to determine if illumination energies above the band gap generated a photocurrent response in the device. Fig. 3(b) shows a small response under 350 nm (3.54 eV) illumination. (IV curves for other wavelengths are available in Fig. S9^†). Fig. 3(c) shows an increased response for 350 nm wavelength as determined via the IV curves. This increased response is likely due to increased absorption as shown in the UV-Vis spectra (Fig. 1e), the signal at longer wavelengths is difficult to observe due to the low responsivity. A higher response at lower wavelength has been observed previously for SnS.⁵³ Fig. 3(d) shows that an increase in current is present under 350 nm and 405 nm illumination which can be cycled on and off. A rise and fall time of 1.09 of 1.44 seconds respectively was observed for 405 nm illumination. A light/dark current ratio of 1.03 was obtained under 405 nm. To account for noise the on and off section had their current averaged using origin software. A drift in current during measurement was observed, this was considered as the reason for the significant difference between the dark current for 350 nm and 405 nm. To further reduce noise surface passivation may also be used to improve the device properties.⁶³ Alternatively, an increase in bias voltage or an increase in monochromatic illumination intensity may improve the signal: noise ratio though may risk damage to the device. A magnified off/on cycle for 405 nm is shown in Fig. S10.^†In conclusion, we report here a methodology for the assembly of 2D SnS nanosheets into thin films using the Langmuir–Blodgett method, and the testing of the films as prototype all-solution processed photodetectors. Tin(ii) sulfide was successfully exfoliated with an average sheet thickness of 33 nm with the average longest side length of 224 nm. A nanosheet based film was coated onto a variety of substrates via the Langmuir–Blodgett method with the addition of chloroform as a spreading solvent. The films were found to be polycrystalline with an average thickness of 78.6 nm with a high surface coverage up to 94.6% for an Si/SiO₂ substrate. The films were found to be semiconductive with the ability to respond to light under bias as shown by AM1.5 and monochromatic illumination. Proof-of-concept photodetectors have been successfully produced. It was also confirmed that the response was due to the photoresponse as opposed to a heating effect. This deposition method could potentially be used to create a variety of SnS films using different exfoliated nanosheet sizes separated via cascade centrifugation as well as the potential for future flexible photodetector devices. Despite the low responsivity, large rise and fall times further work could allow the gain to be optimised. We also note that the use of the Langmuir–Blodgett trough is an easily scalable technology and could provide coatings over very large area substrates not only for photodetectors but for other devices such as thin film solar cells.     

Photocatalytic degradation of organic pollutants through conjugated poly(azomethine) networks based on terthiophene–naphthalimide assemblies

A conjugated poly(azomethine) network based on ambipolar terthiophene–naphthalimide assemblies has been synthesized and its electrochemical and UV-vis absorption properties have been investigated. The network has been found to be a promising candidate for the photocatalytic degradation of organic pollutants in aqueous media.

A conjugated two-dimensional poly(azomethine) network based on ambipolar terthiophene–naphthalimide assemblies has been synthesized and its electrochemical and UV-vis absorption properties have been investigated.

Due to the rapid growth of urbanization and intensive industrialization, pollution has evolved into a serious concern that produces a great negative impact on human health and the environment.^1,2 Therefore, many efforts are currently devoted to addressing environmental remediation through the degradation and removal of hazardous contaminants.^3–5 In this regard, photocatalysis has been identified as a suitable approach for environmental remediation given that it is an energy efficient technique that does not require chemical input and does not produce sludge residue.⁶ In recent years, organic semiconducting polymers have evolved into a new type of metal-free and heterogeneous photocatalyst suitable for solar-energy utilization.⁷ The modularity of organic polymers allows the efficient tunning of their electronic and optical properties by bottom-up organic synthesis through the choice of suitable monomeric building blocks.^8–10 Within this context, there is a growing demand for new organic polymeric semiconductors carefully designed to have suitable energy levels of the frontier orbitals, an appropriate bandgap and good intrinsic charge mobility.¹¹For the design of suitable polymeric semiconductors for photocatalysis, it is not only important that the photocatalysts absorb light in the visible light range but also an efficient dissociation of the photogenerated charge carriers is required. The combination of electron-poor acceptor (A) and electron-rich donor (D) moieties in the polymer structure may prevent a fast recombination process following photoexcitation.¹² In addition, it has been found that polymers networks bearing conjugated moieties may exhibit π-stacked columns that can facilitate charge transport.¹³In this respect, molecular and polymeric materials based on the combination of oligothiophene^14,15 and naphthalimide moieties^16,17 connected through conjugated linkers have shown to be very effective in order to efficiently tune their frontier orbital levels and produce tunable organic semiconductors with good charge transport properties.^18–24 As an example, in Fig. 1 is depicted the structure of NIP-3T, an ambipolar organic semiconductor, for which the one-electron HOMO–LUMO excitation consists of the displacement of the electron density from the HOMO, primarily localized on the oligothiophene fragment, to the LUMO, localized on the naphthalimide unit.²⁵Open in a separate windowFig. 1(a) Monomer containing an electron donor terthiophene system directly conjugated with an electron acceptor naphthalimide moiety through a conjugated pyrazine linker (NIP-3T). (b) HOMO and (c) LUMO computed orbital topologies for NIP-3T.²⁵Among conjugated polymers, poly(azomethine)s have found application as organic semiconductors in heterogeneous photocatalysis because of their π-conjugated system and suitable band levels matching the redox window of water.²⁶ The incorporation of D–A monomeric assemblies into poly(azomethine) networks represents an efficient strategy to obtain ambipolar polymeric networks with tunable frontier orbital levels for photocatalytic applications. Thus, in this communication we report the synthesis of a novel donor–acceptor poly(azomethine) network (NIP3T-ANW, Scheme 1) based on NIP-3T monomers. The potential of this system as photodegrading agent for the elimination of contaminant organic dyes in aqueous media is also explored.Open in a separate windowScheme 1Schematic representation of the synthesis of NIP3T-ANW.The synthesis of the macromolecular poly(azomethine) network NIP3T-ANW is acomplished through Schiff-base reactions between trigonal monomers endowed with amine functionalities (TAPB,²⁷Scheme 1) and linear naphthalimide–thiophene-based monomers endowed with complementary aldehyde functional groups (NIP3T2CHO,²⁴Scheme 1). Typically, both monomers were dissolved in an o-dichlorobenzene/n-butanol/acetic acid (1 : 1 : 0.1) mixture, which was then heated at 120 °C under solvothermal reaction conditions for 72 h. A black solid was obtained which was insoluble in common solvents such as water, acetone, THF, toluene or chlorinated solvents like dichloromethane or chloroform. The obtained solid was washed several times with THF to remove the starting materials and low-molecular weight by-products. After drying under vacuum, a black solid was obtained. The yield, as determined by weight, was 98%.To investigate the chemical nature of the material, as well as to determine the conversion of the functional groups after the reaction, we have employed attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy (Fig. 2). The bands arising from the NH₂ stretching (3000–3400 cm⁻¹) and NH₂ deformation (1650 cm⁻¹) vibrations of the primary amine group of TAPB and the signals from the aldehyde groups of NIP3T2CHO around 2870 (C–H stretching) and 1663 cm⁻¹ (C O stretching) are virtually absent in the NIP3T-ANW spectrum. In addition, a prominent new band is found at 1573 cm⁻¹, which can be assigned to the C N stretching vibration of the imine linkages within the newly formed poly(azomethine) network.^28–30Open in a separate windowFig. 2(a) IR spectra of NIP3T2CHO (blue), TAPB (red) and NIP3T-ANW (black). (b) Solid-state ¹³C CP-MAS NMR spectrum of NIP3T-ANW.Solid-state ¹³C cross-polarization magic angle spinning NMR (¹³C CP-MAS NMR) spectrum (Fig. 2) reveals the characteristic imide signals of the 1,8-naphthalimide moiety at 164.4 ppm, as well as the signal corresponding to the imine carbon at 154 ppm and a signal at 148 ppm which can be assigned to the aromatic carbon neighbouring the nitrogen of the C N group. The absence of the sp² carbons from the NIP3T2CHO ²⁴ aldehyde functionalities above 180 ppm satisfactorily confirms the condensation between the aldehyde and the amine derivatives.Due to the rigidity and geometry of the building blocks, the imine linkers could be ideally generated in such a way that result in a canonical layered hexagonal structure³¹ as predicted by theoretical calculations (Fig. S1 and S2^†). However, in the actual framework, X-ray diffraction (XRD) measurements indicate that the material is mainly amorphous with only some ordered regions, as indicated by the good agreement between the weak and broad diffraction peaks observed at 2θ values larger than 3 and those predicted by calculations for the ideal canonical layered hexagonal structure (Fig. S3^†). In this regard, NIP3T-ANW was submitted to an exfoliation process following a previously described protocol³² for the exfoliation of two-dimensional polymers (see ESI^† for details). The exfoliated material was analysed by dynamic light scattering (DLS) showing a monomodal size distribution of ca. 400 nm (Fig. S4^†) and transmission electron microscopy (TEM) reveals a sheet-like structural aspect (Fig. S5^†).Thermogravimetric analysis of the poly(azomethine) network NIP3T-ANW shows that the degradation starts at around 450 °C and only 40% weight loss is observed at 700 °C (Fig. S6^†). This thermal stability is significantly higher than that observed for NIP3T (Fig. 1), the analogous molecular system based on terthiophene connected with naphthalimide through pyrazine for which the degradation starts at 200 °C (Fig. S6^†).^22,25In photocatalysis mediated by semiconductors, electron–hole pairs (excitons) are generated after light absorption and afterwards dissociate into free charge carriers that can be utilized for redox reactions^33,34 such as CO₂ fixation,³⁵ water splitting³⁶ or organic mineralization.³⁷ Some of the crucial factors that make the photocatalytic process favourable are the levels of conduction and valence as well as the width of the band gap.^38–40 Thus, in order to characterize these parameters, the electrochemical and optical properties of NIP3T-ANW have been analysed.The electrochemical properties of NIP3T and NIP3T-ANW were studied by cyclic voltammetry (Fig. 3). Both materials show ambipolar redox behaviour in which the reversible reduction processes are characteristic of the naphthalimide unit, while the oxidation processes can be ascribed to the conjugated oligothiophene moiety.^19–22,24 For NIP3T-ANW, the first reversible reduction wave (−1.29 V) is shifted to less negative values in comparison with NIP-3T (−1.41 V). On the other hand, the first oxidation half wave potential for NIP3T is observed at +0.41 V and for NIP3T-ANW at +0.44 V. These shifts agree with the electron acceptor ability of the imine linker.Open in a separate windowFig. 3(a) The UV-vis DRS spectra of NIP3T and NIP3T-ANW. (b) Cyclic voltammetry of the NIP3T monomer and the corresponding polymer.The absorption spectrum of NIP3T-ANW as determined by UV-vis diffuse reflectance spectroscopy (UV-vis DRS, Fig. 3) shows a strong absorption in all the UV-vis range, extending even to the near infrared. This broad absorption is red-shifted in comparison with the one observed for NIP3T (Fig. 3), which reflects the formation of the new polymer network with an extended conjugation through the alpha positions of the terthiophenes.Using the corresponding cut-off wavelengths, the optical band gaps E_g found for NIP3T and NIP3T-ANW are 1.59 and 1.42 eV respectively. This optical result suggests that the incorporation of the NIP3T core into an extended conjugated system efficiently harvests photons from the visible range, even extending into the near IR region.To shed some light into the degree of crystallinity of the synthesized NIP3T-ANW network, we have carried out a battery of density functional theory (DFT)-based calculations with the QUANTUM ESPRESSO plane-wave DFT simulation code⁴¹ (see details in ESI^†). We have considered periodic boundary conditions to obtain a fully-relaxed ground-state crystal structure. Optimization of the cell-shape and size, simultaneously to the relaxation of the structure, reveals a hexagonal 2D lattice with an optimized parameter of 48.46 Å. Different interlayer stacking fashions have been tested, with only one yielding a good agreement with the experimental diffractogram from 2θ values >3°. The most favourable stacking predicted by theory consists in an intermediate configuration between the perfectly eclipsed and staggered configurations, with an interlayer distance of 3.42 Å, and permits an adequate accommodation of the layers profiting adjacent pores. Details on the structure can be found in the ESI.^†Additionally, we have computed the electronic band diagram of the obtained crystal structure along the high-symmetry k-path Γ → K → M → Γ, revealing a wide-gap (1.91 eV) semiconducting character, with rather dispersive valence and conduction bands, mainly resembling the molecular HOMO and LUMO of the molecular building blocks (see Fig. S7^†). Besides, computed time-dependent DFT (TDDFT) UV-vis spectrum manifests an excellent agreement with the experimental UV-vis spectra (Fig. S8^†). A broad and pronounced peak-feature is obtained between 600 and 800 nm, centred at around 720 nm (1.7 eV), which agrees with the optical gap of 1.6 eV found for NIP3T from Fig. 3a. This feature corresponds to electronic transitions between the valence and conduction bands, with an energy difference of around 0.2 eV between the optical and the electronic gap, which indicates that charge relaxation in excited states is not much significative. The good agreement between theoretical predictions on the canonically periodic computed system and the experimental evidences seems to justify the presence of some high-crystallinity regions from the synthesis.The band gap of a semiconductor material and the reduction and oxidation potentials are key parameters which determine its light-harvesting properties and types of reaction that can be conducted and therefore the overall photo-catalytic activity. A shift in the adsorption edge of a semiconductor towards longer wavelengths implies a narrower band gap and the efficient harvesting of a wider photons range.⁴²NIP3T-ANW seems to be an appealing material to be utilized as photocatalyst given (i) the optimal light harvesting properties as shown by the optical characterization, (ii) the efficient generation of electron–hole pairs owing to the insertion of terthiophene moieties, and (iii) the right energy band positions for the material.⁷ We therefore evaluated the photocatalytic activity of NIP3T-ANW under white light for the degradation of a model organic pollutant (Rhodamine B dye, RhB) in aqueous solution.⁴³ In the absence of catalyst, RhB remains stable in solution under illumination (Fig. 4a and S9^†). However, in presence of NIP3T-ANW nearly 90% of RhB in an aqueous solution is degraded after 120 min, showing the enhanced catalytic activity of the material (Fig. 4a and S10^†). Furthermore, a good stability is shown upon 4 straight catalytic cycles (Fig. 4b, S11 and S12^†). In contrast, in the presence of the NIP3T moiety, only a 55% degradation of RhB is observed in the same timeframe (Fig. 4a and S11^†).Open in a separate windowFig. 4RhB degradation curves. (a) Comparison between the degradation effect of NIP3T, NIP3T-ANW and without catalyst. (b) NIP3T-ANW stability after four recycling cycles.In the photocatalytic degradation of organic pollutants, they are typically broken down through the attack of superoxide and hydroxyl species, formed when atmospheric oxygen reacts with photogenerated electrons or when water or OH ions are oxidized by holes, respectively.⁴⁴ Additionally, electron–hole pairs (or excitons) can directly reduce or oxidize organic pollutants in aqueous environments. Consequently, the evaluation of the photodegradation mechanism of organic pollutants, despite challenging, can provide meaningful insights about the nature of a semiconductor photocatalyst.⁴⁵ With the aim of evaluating the photodegradation mechanism we performed the measurements in the presence of different scavengers, namely an aqueous solution of AgNO₃ (100 mg L⁻¹), which captures photogenerated electrons, or triethanolamine (TEOA), which traps photogenerated holes (Fig. S13^†).^46,47 In the presence of Ag⁺ we could observe that the photocatalytic efficiency is enhanced, while the addition of TEOA quenched the performance, therefore suggesting that holes are the active specie in the photodegradation mechanism (Fig. S14^†).In summary, we have presented an approach towards the incorporation of D–A π-conjugated monomeric assemblies into poly(azomethine) networks to yield a purely organic semiconductor for the photocatalytic degradation of organic pollutants in aqueous media. The poly(azomethine) network benefits from a straightforward poly(condensation) approach which favourably competes with the elaborate high-temperature protocols applied for the preparation of inorganic materials. This work enriches the family of donor–acceptor organic semiconductor networks and, given its modular nature, paves the way for the development of a promising family of materials for photocatalytic applications.     

Facile one-pot solvothermal preparation of two-dimensional Ni-based metal–organic framework microsheets as a high-performance supercapacitor material

We report a facile one-pot solvothermal way to prepare two-dimensional Ni-based metal–organic framework microsheets (Ni-MOFms) using only Ni precursor and ligand without any surfactant. The Ni-MOFms exhibit good specific capacities (91.4 and 60.0 C g⁻¹ at 2 and 10 A g⁻¹, respectively) and long-term stability in 5000 cycles when used for a supercapacitor electrode.

Two-dimensional Ni-based metal–organic framework microsheets (Ni-MOFms) were synthesized via a facial one-pot solvothermal approach and exhibited good specific capacities and excellent long-term stability when used for a supercapacitor electrode.

With the continuous growth of energy demand worldwide, high-performance, environmental-friendly, and low-cost energy storage devices have attracted extensive research interest.^1–3 Among them, supercapacitors are considered most promising because of their high power density, long lifespan, and fast charging/discharging speed.^4–6 To date, numerous materials have been explored for fabricating supercapacitors. Carbon materials have been usually used for electrical double-layer capacitors (EDLCs), including carbon fibers, carbon nanotubes, carbon spheres, carbon aerogels, and graphene,^7–12 while conducting redox polymers and transition metal oxides/hydroxides are widely explored as active materials for pseudocapacitance and battery-type electrodes.^13–16Metal–organic frameworks (MOFs), a porous crystalline material composed of metal nodes and organic linkers, have been widely applied in versatile fields including chemical sensors, catalysis, separation, biomedicine, and gained more and more attention in the area of energy storage.^17–25 Recently, two-dimensional (2D) MOFs have aroused great interest as a new kind of 2D materials.^26,27 Compared with traditional bulk MOFs, 2D MOFs possess distinctive properties, such as short ion transport distances, abundant active sites, and high aspect ratios, making them exhibit better performance than their bulk counterparts.^28–32 Bottom-up methods are generally adopted to prepare 2D MOFs with the addition of surfactants to control the growth of MOFs in a specific direction.^33–35 However, the use of surfactants inevitably blocks part of the active sites at the expense of the performance of materials. Therefore, it is highly necessary to explore and develop a direct solvothermal synthesis of 2D MOFs with the advantages of additive-free, simple operation, and easy scale-up.Herein, we report a facile one-pot solvothermal method to synthesize 2D Ni-based MOF microsheets (denoted as Ni-MOFms) by treating nickel chloride hexahydrate (NiCl₂·6H₂O, the metal precursor) together with the trimesic acid (H₃BTC, the ligand) in a mixed solvent of N,N-dimethylformamide (DMF), ethanol (EtOH) and H₂O. During the whole preparation process, only Ni precursor and the ligand are used while no surfactant is added. When used as active materials for a supercapacitor electrode, the obtained Ni-MOFms displayed excellent reversibility and rate performance. It also exhibited specific capacities of 91.4 and 60.0 C g⁻¹ at 2 and 10 A g⁻¹, respectively. Besides, they showed a good cycling performance in 5000 cycles with about 70% of the specific capacity and almost 100% of the coulombic efficiency maintained.Morphologies of the Ni-MOFms were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. 1a and b, the Ni-MOFms were successfully fabricated via the facile one-pot solvothermal method with varying lateral sizes on the micron scale. Energy dispersive spectroscopy (EDS) mapping indicated that the obtained microsheets were mainly composed of C, O, and Ni. A trace amount of N was also observed, which could be attributed to the residual DMF in the mixed solvent (Fig. 1c). These elements were uniformly distributed throughout the whole microsheet. To measure the exact thickness of the Ni-MOFms, atomic force microscopy (AFM) was used. Fig. 1d showed that the thickness of the microsheet was about 58 nm. Considering the large lateral size, even such thickness could produce a relatively high aspect ratio, which is beneficial to the electrochemical performance.Open in a separate windowFig. 1(a) TEM image, (b) SEM image, (c) EDS mapping, and (d) AFM image and the corresponding height profile of the Ni-MOFms.The composition information of the Ni-MOFms was analyzed by X-ray diffraction (XRD) and the resulting diffraction pattern was shown in Fig. 2a. It was clear that the sample was a crystalline material. However, the exact structure was difficult to determine because no matching MOF structure has been found. Therefore, the structure of the Ni-MOFms was further confirmed by Fourier transform infrared spectroscopy (FT-IR). As shown in Fig. 2b, there was a sharp peak at 1721 cm⁻¹ for H₃BTC, which could be ascribed to the stretching vibration of C O in the nonionized carboxyl group.³⁶ For the Ni-MOFms, the peak at this location disappeared while four new peaks appeared. Bands at 1634 and 1557 cm⁻¹ were related to the asymmetric stretching vibration of carboxylate ions (–COO⁻) and peaks at 1433 and 1371 cm⁻¹ were the characteristic peaks of the symmetric stretching vibration of –COO⁻.^37,38 All these changes indicate that the ligand interacted well with the metal precursor.Open in a separate windowFig. 2(a) XRD pattern of the Ni-MOFms. (b) FT-IR spectra of H₃BTC and the Ni-MOFms.The chemical status and surface composition of the Ni-MOFms were further examined by X-ray photoelectron spectroscopy (XPS). From Fig. S1a^† we could see that the Ni-MOFms were composed of C, O, Ni, and N, which was consistent with the result of EDS mapping. High-resolution spectra of C 1s, Ni 2p, O 1s, and N 1s were shown in Fig. S1b–e.^† Characteristic peaks of C 1s at 288.27, 286.50, 285.85, and 284.80 eV were related to O C–OH, C–O, C–C, and C C, respectively, suggesting the presence of H₃BTC (Fig. S1b^†).³⁹ The Ni 2p spectrum showed two peaks at 873.32 and 855.77 eV, which could be ascribed to Ni 2p_1/2 and Ni 2p_3/2, respectively, together with two satellite peaks at 879.26 and 861.05 eV, verifying the existence of Ni²⁺ (Fig. S1c^†).⁴⁰ In the O 1s region, bands positioned at 532.94 and 531.40 eV could be ascribed to the adsorbed H₂O molecules on the surface of Ni-MOFms and typical metal–oxygen bonds, respectively, further corroborating the coordination between H₃BTC and Ni²⁺ (Fig. S1d^†).³⁹ Finally, the high-resolution spectrum of N 1s was also analyzed (Fig. S1e^†). There were two main peaks at 400.18 and 402.21 eV that could be ascribed to neutral amine and charged nitrogen, respectively,⁴¹ further proving the residual DMF on the Ni-MOFms surface.To explore the crucial factors in the formation process of the Ni-MOFms, the reaction time and temperature, the solvent, the ligand addition amount, and the ligand type were studied. As shown in Fig. S2,^† different crystalline materials were obtained at different reaction times. With the increase of reaction time, the material gradually changed from sphere to sheet. The reaction temperature is another crucial factor. At 120 °C, the material was amorphous and spherical. When the temperature rose, the crystal formed and appeared as microsheets (Fig. S3^†). The effect of solvent was illustrated in Fig. S4.^† Microsheets could not be synthesized in DMF or DMF with a small amount of EtOH. In the mixed solvent of DMF and H₂O, crystals could be prepared, indicating the vital role of H₂O. However, spheres existed in the products. Only when a mixture of DMF, EtOH, and H₂O with a certain proportion was used as the solvent, the Ni-MOFms could be obtained. Furthermore, we investigated the effect of the ligand addition amount. From Fig. 1 and S5^† we can see that the Ni-MOFms crystals formed when the molar ratio of Ni precursor and H₃BTC was 1 : 2 (Fig. 1). We speculated that ligands could simultaneously act as regulators to adjust the morphology of materials, avoiding the use of additional surfactants. When the ligand was replaced with 2-methylimidazole (2-MI) or terephthalic acid (H₂BDC), flower-like crystals rather than microsheets were obtained (Fig. S6^†), indicating the importance of the ligand type. Taking the above factors into account, we could finally determine the suitable conditions for preparing the Ni-MOFms (see the experimental section in ESI^†).The potential application of the Ni-MOFms in supercapacitors was first tested by cyclic voltammetry (CV) in 3 M KOH between 0 and 0.4 V (vs. saturated calomel electrode, SCE). As can be seen from Fig. 3a, all CV curves had similar shapes and the peak currents improved gradually as the scan rate increased, suggesting the good capacitive behavior of the Ni-MOFms electrode.⁴² When the scan rate was as high as 150 mV s⁻¹, redox peaks could still be observed, which indicated the excellent rate performance and kinetic reversibility.⁴³ Besides, as the scan rate went up from 10 to 150 mV s⁻¹, the reduction and oxidation peaks moved towards negative and positive potential, respectively, demonstrating the electrode polarization at large scan rates.⁴⁴Open in a separate windowFig. 3Electrochemical measurements of the Ni-MOFms. (a) CV curves at different scan rates. (b) GCD curves at various current densities and (c) corresponding specific capacities. (d) The EIS Nyquist plot at the bias potential of 0.4 V and the equivalent circuit model with the fitted plots (the red dots).The galvanostatic charge–discharge (GCD) behavior was further investigated to assess the coulombic efficiency and the specific capacity of the Ni-MOFms (see the ESI^† for detailed calculation method).^45,46 As shown in Fig. 3b, the shape of GCD curves was highly symmetric during charging and discharging, indicating that the coulombic efficiency of Ni-MOFms was almost 100% at various current densities. The specific capacities of 91.4, 78.4, 71.4, 64.0, and 60.0 C g⁻¹ were achieved at current densities of 2, 4, 6, 8, and 10 A g⁻¹ (Fig. 3c), respectively, demonstrating the excellent rate capability with about 65.6% of the specific capacity maintained from 2 to 10 A g⁻¹. The specific capacity at 2 A g⁻¹ was comparable with or even superior to that of some MOF materials reported in the literatures (Table S1^†).^47–50The kinetics of the electroanalytical process was then investigated by electrochemical impedance spectroscopy (EIS). Fig. 3d showed the Nyquist plot of Ni-MOFms from 0.01 to 100000 Hz and the corresponding equivalent circuit model (inset) with the fitted plots. CPE was the constant phase element related to the double layer capacity.⁵¹ The equivalent series resistance was denoted by R_s and its value obtained from the x-axis intercept was about 2.1 Ω, indicating the low resistance of the solution.⁴³R_ct represented the charge-transfer resistance at the interface of the electrode and electrolyte.⁵² For Ni-MOFms, the value of R_ct was up to 147.1 Ω, which could be attributed to the poor conductivity of MOF materials.The long-term stability of Ni-MOFms was also explored by charging–discharging at 10 A g⁻¹ for 5000 consecutive cycles. From Fig. 4 we could see that the specific capacity retention remained about 70% after 5000 cycles and the coulombic efficiency was maintained at almost 100% throughout the whole process. Furthermore, the inset in Fig. 4 exhibited that the GCD curves of the last 10 cycles were the same as the first 10 cycles, indicating excellent cycling stability.Open in a separate windowFig. 4Cycle property of Ni-MOFms at 10 A g⁻¹. Inset: GCD curves of the first 10 cycles (left) and the last 10 cycles (right).     

Size effect studies in catalysis: a simple surfactant-free synthesis of sub 3 nm Pd nanocatalysts supported on carbon

Supported Pd nanoparticles are prepared under ambient conditions via a surfactant-free synthesis. Pd(NO₃)₂ is reduced in the presence of a carbon support in alkaline methanol to obtain sub 3 nm nanoparticles. The preparation method is relevant to the study of size effects in catalytic reactions like ethanol electro-oxidation.

A simple surfactant-free synthesis of sub 3 nm carbon-supported Pd nanocatalysts is introduced to study size effects in catalysis.

A key achievement in the design of catalytic materials is to optimise the use of resources. This can be done by designing nanomaterials with high surface area due to their nanometre scale. A second achievement is to control and improve catalytic activity, stability and selectivity. These properties are also strongly influenced by size.^1–3 To investigate ‘size effects’ it is then important to develop synthesis routes that ensure well-defined particle size distribution, especially towards smaller sizes (1–10 nm).Metal nanoparticles are widely studied catalysts. In several wet chemical syntheses, NP size can be controlled using surfactants. These additives are, however, undesirable for many applications^4,5 since they can block active sites and impair the catalytic activity. They need to be removed in ‘activation’ steps which can negatively alter the physical and catalytic properties of the as-produced NPs. Surfactant-free syntheses are well suited to design catalysts with optimal catalytic activity⁶ but their widespread use is limited by a challenging size control.³Palladium (Pd) NPs are important catalysts for a range of chemical transformations like selective hydrogenation reactions and energy applications.^7–9 It is however challenging to obtain sub 3 nm Pd NPs, in particular without using surfactants.² Surfactant-free syntheses are nevertheless attracting a growing interest due to the need for catalysts with higher performances.^10–14Promising surfactant-free syntheses of Pd NPs were recently reported.^8,15 The NPs obtained in these approaches are in the size range of 1–2 nm and show enhanced activity for acetylene hydrogenation⁸ and dehydrogenation of formic acid.¹⁵ Enhanced properties are attributed to the absence of capping agents leading to readily active Pd NPs. The reported syntheses consist in mixing palladium acetate, Pd(OAc)₂, in methanol and the reduction of the metal complex to NPs occurs at room temperature. The synthesis is better controlled in anhydrous conditions to achieve a fast reaction in ca. 1 hour. Another drawback is that the synthesis must be stopped to avoid overgrowth of the particles. Therefore, a support material needs to be added after the synthesis has been initiated and no simple control over the NP size is achieved.^8,15In this communication a more straightforward surfactant-free synthesis leading to sub 3 nm carbon-supported Pd NPs in alkaline methanol at ambient conditions is presented. A solution of Pd(OAc)₂ in methanol undergoes a colour change from orange to dark, indicative of a reduction to metallic Pd, after ca. a day. However, only ca. 1 hour is needed with Pd(NO₃)₂, Fig. 1 and UV-vis data in Fig. S1.^† The fast reduction of the Pd(NO₃)₂ complex in non-anhydrous conditions is a first benefit of the synthesis presented as compared to previous approaches.Open in a separate windowFig. 1Pictures of 4 mM Pd metal complexes in methanol without or with a base (as indicated).For particle suspensions prepared with Pd(OAc)₂ or Pd(NO₃)₂ the NPs agglomerate and quickly sediment leading to large ‘flake-like’ materials. When the reduction of Pd(NO₃)₂ in methanol is performed in presence of a carbon support and after reduction the solution is centrifuged and washed in methanol, a clear supernatant is observed indicating that no significant amount of NPs are left in methanol. Transmission electron microscope (TEM) analysis confirms that NPs are formed and well-dispersed on the carbon support surface and no unsupported NPs are observed, Fig. 2a. Likely, the reduction of the NPs proceeds directly on the carbon support. However, the size of the NPs is in the range 5–25 nm, which is still a relatively large particle size and broad size distribution.Open in a separate windowFig. 2TEM micrographs of Pd NPs obtained by stirring 4 mM Pd(NO₃)₂ in methanol and a carbon support for 3 hours, (a) without NaOH and (b) with 20 mM NaOH. Size distribution histograms are reported in Fig. S4.^† The same samples after electrochemical treatments are characterised in (c) and (d) respectively. Size distribution histograms are reported in Fig. S7.^†Assuming a ‘nucleation and growth’ mechanism, the NPs should become larger over time.¹⁶ But the reaction is so fast that by stopping the reaction before completion, size control is not achieved and unreacted precious metal is observed, Fig. S2.^† To achieve a finer size control and more efficient use of the Pd resources, a base was added to the reaction mixture, e.g. NaOH.³ In alkaline media, the formation of Pd NPs is slower; it takes ca. 60 minutes to observe a dark colour for a 5 mM Pd(NO₃)₂ solution with a base/Pd molar ratio of 10 in absence of a support, Fig. 1.Also in alkaline methanol, the NPs agglomerate over time in absence of a support material. However, if the alkaline solution of Pd(NO₃)₂ is left to stir in presence of a carbon support the desired result is achieved, i.e. Pd NPs with a significantly smaller size and size distribution of ca. 2.5 ± 1.0 nm, Fig. 2b. The NPs homogeneously cover the carbon support and no unsupported NPs are observed by TEM suggesting that the NPs nucleate directly on the carbon surface. Furthermore, the supernatant after centrifugation is clear, indicating an efficient conversion of the Pd(NO₃)₂ complex to NPs, Fig. S3.^† Furthermore, there is no need for an extra reducing agent as in other approaches, for instance in alkaline aqueous solutions.⁹The benefits of surfactant-free syntheses of Pd NPs for achieving improved catalytic activity have been demonstrated for heterogeneous catalysis.^8,15 Surfactant-free syntheses are also well suited for electrochemical applications where fully accessible surfaces are required for fast and efficient electron transfer. Several reactions for energy conversion benefit from Pd NPs. An example is the electro-oxidation of alcohols,⁷ in particular ethanol¹⁷ (see also Table S1^†).Previous studies optimised the activity of Pd electrocatalysts by alloying,^18–20 by using different supports^17,21–23 or crystal structures.^24,25 Investigating NPs with a diameter less than 3 nm was challenging.^2,26,27 The surfactant-free synthesis method presented here allows to further study the size effect on Pd NPs supported on carbon (Pd/C) for electrocatalytic reactions.In Fig. 3, results for ethanol electro-oxidation in 1 M ethanol solution mixed with 1 M KOH aqueous electrolyte are reported based on cyclic voltammetry (CV) and chronoamperometry (CA) with Pd/C catalysts exhibiting 2 significantly different size distributions. The electrode preparation, the measurement procedure and the sequence of electrochemical treatments are detailed in the ESI.^† In order to highlight size effects, we compare geometric and Pd mass normalized currents (Fig. 3a and c) as well as the oxidation currents normalized to the Pd surface area (Fig. 3b).Open in a separate windowFig. 3Electrochemical characterisation of carbon supported Pd NPs with 5–25 nm (grey) and 2.5 nm (dark) size in 1 M KOH + 1 M ethanol aqueous electrolyte. (a) 2^nd CV before chronoamperometry (CA), (b) current normalised by the electrochemically active surface area of Pd, (c) CA recorded at 0.71 V vs. RHE after 50 cycles between 0.27 and 1.27 V.It is clearly seen that based on the geometric current density, the smaller Pd NPs exhibit significantly higher currents for ethanol oxidation than the larger NPs. To differentiate if this observation is a sole consequence of the different surface area, the electrochemically active surface area (ECSA) has been estimated based on “blank” CVs (without ethanol) recorded between 0.27 and 1.27 V vs. RHE in pure 1 M KOH aqueous electrolyte and integrating the area of the reduction peak at ca. 0.68 V, Fig. S5.^† As conversion factor, 424 μC cm⁻² was used.²⁸Using this method, the smaller NPs with a size around 2.5 nm exhibit an ECSA of 92 m² g⁻¹ whereas the larger NPs with a size in the range 5–25 nm exhibit an ECSA of 47 m² g⁻¹, consistent with a larger size. Normalising the ethanol electro-oxidation to these ECSA values instead of the geometric surface area, Fig. 3b, still indicates a size effect. It is clearly seen that the smaller Pd NPs exhibit higher surface specific ethanol oxidation currents, in particular at low electrode potentials. Furthermore, a clear difference in the peak ratios in the CVs is observed. The ratio in current density of the forward anodic peak (j_f, around 0.9 V) and the backward cathodic peak (j_b, around 0.7 V vs. SCE) is around one for the smaller NPs, whereas it is about 0.5 for the larger NPs. The forward scan corresponds to the oxidation of chemisorbed species from ethanol adsorption. The backward scan is related to the removal of carbonaceous species not fully oxidised in the forward scan. The higher j_f/j_b ratio therefore confirms that the smaller NPs are more active for ethanol electro-oxidation and less prone to poisoning, e.g. by formation of carbonaceous species that accumulate on the catalyst surface.^29,30 This observation is further supported by a chronoamperometry (CA) experiment, Fig. 3c, at 0.71 V performed after continuous cycling (50 cycles between 0.27 and 1.27 V at a scan rate of 50 mV s⁻¹). In the CA testing of the thus aged catalysts at 0.71 V, the ethanol oxidation current on the two catalysts starts at around the same values, however, its decay rate is significantly different. The Pd mass related oxidation currents for the smaller NPs are after 30 minutes almost twice as high (ca. 200 A g_Pd⁻¹) as for the larger ones (ca. 130 A g_Pd⁻¹), confirming that the small Pd NPs are less prone to poisoning. In particular a factor up to 4 in the Pd mass related ethanol oxidation currents after 1800 s of continuous operation is achieved compared to a recently characterised commercial Pd catalyst on carbon,²⁰ Table S1.^† Despite different testing procedure reported in the literature, it can be concluded from these investigations that the surfactant-free synthesis presented shows promising properties for electrocatalytic ethanol oxidation even after extended cycling.The extended cycling, however, has different consequences for the two catalysts. For the small (2.5 nm) NPs of the Pd/C catalyst, a massive particle loss, but only moderate particle growth is observed as highlighted in Fig. 2 (see also Fig. S6^†). TEM micrographs of the two Pd/C samples recorded before and after the complete testing (CVs and CAs, for details see Fig. S7^†) show that the catalyst with small Pd NPs exhibits a pronounced particle loss as well as a particle growth to ca. 6 nm probably due to sintering. By comparison, for the Pd/C catalyst with the large Pd NPs, no significant influence of the testing on particle size or particle density is apparent.     

Mesoporous-silica-coated palladium-nanocubes as recyclable nanocatalyst in C–C-coupling reaction – a green approach

We report the straightforward design of a recyclable palladium-core–silica-shell nanocatalyst showing an excellent balance between sufficient stability and permeability. The overall process – design, catalysis and purification – is characterized by its sustainability and simplicity accompanied by a great recycling potential and ultra high yields in C–C-coupling reactions.

A green approach: in a single-step coating process a mesoporous silica shell was tailored onto palladium-nanocubes. Along with a PEG-matrix this core–shell-nanocatalyst could be recovered after C–C-coupling reactions and reused without any significant decrease in product yield.

In contrast to bulk materials, metal-nanocyrstals (NC) possess unique physical and chemical properties. Both, nano-scale and geometry can dictate their optical, electronic and catalytic behavior.^1–3 Consequently, nanomaterials have been implemented already in various fields like medicine,⁴ sensing,⁵ nano-electronics⁶ and organic synthesis.⁷ Smart strategies for a sustainable usage of limited resources such as precious metals and hydrocarbons are inevitable due to the consistently growing population and economy.^8–10 Generally, catalysis gives rise to novel and energy-saving synthetic routes. However, homogeneous catalysis is not widely used in industrial processes, due to the need of mostly toxic ligands and the costly purification along with a restricted reusability potential.^11–13 Heterogeneous catalysis based on the utilization of metal nanocrystals overcomes most of these limitations. Caused by its high surface-to-volume ratio, catalytic activities are drastically increased in comparison to bulk materials. However, the most common disadvantage in NC-based catalysis lies in the occurrence of aggregates during the reaction, which leads to a decrease of the catalytic active surface. Several studies were already presented in literature to delay that phenomenon using micelle-like- or core–shell-nanostructures as potential nanocatalytic systems.^14–20 However, these surfactants or shells are either potentially harmful for the environment or very step-inefficient to produce. Other approaches used the deposition of small NCs in a mesoporous support which offers a large active surface area.^21–24 In this context, it is crucial to find an adequate balance between permeability for small organic molecules and the overall stability of the nanocatalyst. Overcoming these obstacles can contribute to a sustainable supply of drugs and other organic substances.In the present work palladium-nanocubes (Pd-NCubes) were fabricated in aqueous solution using cetyltrimethyl-ammoniumbromide (CTAB) as surfactant. The procedure is adapted from a previously published study.^25,26 The respective transmission electron microscopy (TEM) image and the selected area electron diffraction (SAED) pattern of the as-obtained Pd-NCubes are depicted in Fig. S1.^† The SAED confirms the single-crystallinity of Pd-NCubes bound by {100}-facets. TE micrographs of Pd-NCubes revealed an average edge-length of (18 ± 2) nm (for histogram see Fig. S2^†). The formation of polyhedra and nanorods was found to be less than 1%.To the best of our knowledge, there is no procedure reported that showed the direct fabrication of a mesoporous silica (mSi) shell tailored on Pd-NCs. However, Matsuura et al. demonstrated a single-step coating approach of CTAB-capped gold-nanorods and CdSe/ZnS quantum dots obtaining a mesoporous silica shell.²⁷ The pores were determined to be 4 nm in width with 2 nm thick walls. Since the Pd-NCubes are covered by CTAB, already no surfactant exchange is necessary. Consequently, this procedure could be directly transferred to the as-obtained Pd-NCubes (∼10¹⁵ particles per L) of this study using tetraethyl orthosilicate (TEOS) as silica precursor in an alkaline solution. Here, CTAB serves as organic template for the formation of the mesoporous silica shell.TE micrographs of individual Pd-mSi-nanohybrids are depicted in Fig. 1 showing a spherical silica coating with a thickness of (17 ± 2) nm (for histogram see Fig. S3^†). The porosity is essential to ensure that vacant coordination sites on the palladium-core are present and accessible for catalysis. In contrast to other multistep approaches, pores are formed in situ with no additional etching step necessary.^28,29 This avoids the usage of harmful etching agents such as fluorides or ammonia.³⁰ The silica shell then served as platform for further surface modification using two different PEG-silanes (M_n = 5000 g mol⁻¹ and M_n = 20 000 g mol⁻¹). TE microscopy did not reveal any changes in the structure of the PEG functionalized Pd-mSi-nanohybrids opposed to the unfunctionalized nanocatalyst, since the contrast of polymer is too low (see Fig. S4^†). However, dynamic light scattering (DLS) measurements in diluted aqueous solutions proved an increased hydrodynamic radius with increasing molecular weight of the PEG-chain grafted onto the silica shell (see Fig. 2). These results provide evidence that only individual nanostructures are formed while no larger aggregates are present.Open in a separate windowFig. 1Exemplary TE micrographs of Pd-mSi-nanohybrids.Open in a separate windowFig. 2DLS results along the different stages of the hierarchal fabrication process of the nanocatalyst.The successful functionalization of the Pd-mSi-nanohybrid with PEG provides the dispersibility for the overall nanocatalyst in a PEG matrix. Due to its lack of toxicity and its simple recovery, caused by its melting point at ∼50 °C, PEG is considered as a “green” reaction medium.³¹ It has already been shown that PEG can act as suitable solvent for both homogeneous and heterogeneous catalysis.^32–34 Consequently, further experiments were performed using only the Pd-mSi-nanohybrid functionalized with PEG-silane having an average molecular weight of M_n = 5000 g mol⁻¹ (PEG-5k). For catalytic reactions, Pd-mSi-PEG-5k was dispersed in a PEG matrix (PEG-2000, M_n = 2000 g mol⁻¹) and charged into a Teflon centrifuge tube. Using only one tube for the reaction and the product separation avoids an additional transfer step and prevents any loss of the nanocatalyst between reaction cycles (see recycling Scheme 1). Here, the C–C-coupling between ethyl acrylate and p-iodoanisole served as model Heck-reaction to prove the catalytic activity of the designed catalyst (4.4 mol% overall Pd conc. equal to ca. 0.3 mol% surface-available Pd; conc. is determined by ICP-MS measurements, see Table S1^†). Sodium phosphate was used as base providing the largest product yield when compared to other bases, such as Na₂CO₃, K₂CO₃ and K₃PO₄. Once the catalysis was performed and the PEG-2000 was cooled down, diethyl ether was added to extract the product and separated from the reaction medium via centrifugation.Open in a separate windowScheme 1Recycling process of the Pd-mSi-PEG-5k-nanocatalyst and PEG-2000 as solvent after the Heck-reaction between ethyl acrylate and p-iodoanisole to form ethyl p-methoxycinnamate.To exclude any suspended compounds from the desired product, the mixture was passed through a PTFE-filter. Et₂O was removed in vacuo without the need of column chromatography. Comparing the ¹H-NMR spectra of the as-obtained product with the educts indicate a yield of 98% with only small amounts of PEG-2000 present (identified by the signal at ∼3.6 ppm, < 1 weight-%). The results show that both educts and the base Na₃PO₄ are able to diffuse through the mesoporous silica shell to the palladium core (see Fig. 3). A detailed ¹H-NMR signal assignment of the product is given in Fig. S5.^†Open in a separate windowFig. 3 ¹H-NMR spectra of the educts ethyl acrylate (top) and p-iodoanisole (center) and the product ethyl p-methoxycinnamate (bottom).After recovering the reaction mixture containing Pd-mSi-PEG-5k and PEG-2000, seven further Heck-reaction-cycles were conducted under the same conditions. Results obtained from ¹H-NMR and gravimetry indicate no significant decrease in catalytic activity (see ¹H-NMR spectra in Fig. 4). Along the eight Heck-reactions, product yields were determined between 94% and 99%. Only small traces of p-iodoanisole (identified by the signal at ∼6.75 ppm) were still present while ethyl acrylate could be fully removed in vacuo. The yields obtained after each cycle are displayed in Fig. 5. ICP-MS measurements of the catalysis product were performed to determine the palladium leaching out of the catalytic system. The results are displayed in Table S1^† indicating that leaching is strongly suppressed since the overall Pd-content in the product ranges from 0.3–5.7 ng, only. This corresponds to 0.002–0.044 ppm palladium with respect to the product mass. The data are in good agreement with the high product yields along the eight Heck-reactions. To trace the evolution of the nanocatalyst along the cycles, TE micrographs were taken after the 1^st and the 8^th Heck-cycle (see Fig. 6). It can be seen that the cubical structure of the palladium vanishes during the first reaction (left TEM image). Inside the silica shell spherical palladium particles were formed. An explanation for this rearrangement lies in the suggested Heck-mechanism.³⁵ Here, a Pd²⁺-species forms after the oxidative addition of the p-iodoanisole which can desorb from the Pd-NCube. Once the reductive elimination of the product occurs the Pd⁰-species is generated again that can re-deposit on the palladium-core.²¹ Since the spherical geometry possesses the lowest free surface energy, globules were eventually formed.³ The mesoporous silica shell is not affected significantly by the catalysis and the rearrangement of the palladium. The porosity appears to stay intact, enabling the penetration of further small organic molecules. Control experiments were performed to validate whether these observations are based on either a thermally or a chemically induced rearrangement process of the palladium core. Therefore, only the nanocatalyst was dispersed in PEG-2000 and heated at 110 °C for 24 h without any conducted catalysis reaction. TEM results showed no changes in the structure of neither the palladium core nor the silica shell (see Fig. S6^†). After the 8^th reaction, no core–shell-nanostructures could be detected anymore via TEM. Only small randomly shaped Pd-nanoparticles (≤20 nm) were found indicating a slow leaching of the palladium out of the silica shell. However, no larger aggregates were formed (see right TE micrograph in Fig. 6) which explains the continuously high catalytic activity.Open in a separate windowFig. 4 ¹H-NMR spectra of the product ethyl p-methoxycinnamate after the 1^st Heck-reaction (bottom) up to the 8^th reaction (top).Open in a separate windowFig. 5Conversions of ethyl p-methoxycinnamate obtained via NMR and gravimetry after each respective Heck-reaction (1–8).Open in a separate windowFig. 6TE micrographs of the Pd-mSi-PEG-5k-nanocatalyst taken after the 1^st (left) and the 8^th Heck-reaction (right).     

Coumarin-based fluorescent ‘AND’ logic gate probes for the detection of homocysteine and a chosen biological analyte

With this research we set out to develop a number of coumarin-based ‘AND’ logic fluorescence probes that were capable of detecting a chosen analyte in the presence of HCys. Probe JEG-CAB was constructed by attaching the ONOO⁻ reactive unit, benzyl boronate ester, to a HCys/Cys reactive fluorescent probe, CAH. Similarly, the core unit CAH was functionalised with the nitroreductase (NTR) reactive p-nitrobenzyl unit to produce probe JEG-CAN. Both, JEG-CAB and JEG-CAN exhibited a significant fluorescence increase when exposed to either HCys and ONOO⁻ (JEG-CAB) or HCys and NTR (JEG-CAN) thus demonstrating their effectiveness to function as AND logic gates for HCys and a chosen analyte.

With this research we set out to develop of a number of coumarin-based ‘AND’ logic fluorescence probes that were capable of detecting a chosen analyte in the presence of HCys.

Homocysteine (HCys) is a non-proteinogenic amino acid, formed from the de-methylation of methionine,¹ which is then converted into cysteine (Cys) via a vitamin B₆ cofactor. Typical physiological concentrations of HCys range between 5–15 μmol L⁻¹.² However, elevated levels of HCys (>15 μmol L⁻¹), which is known as hyperhomocysteinemia (hHCys),³ have been associated with pregnancy disorders, Alzheimer''s disease, cardiovascular disease and neurodegenerative diseases (NDs).^4–6 It is believed that the main cause of HCys induced toxicity is through the non-enzymatic modification of proteins. This is achieved through irreversible covalent attachment of the predominant metabolite of HCys, homocysteine thiolactone (HTL), to lysine residues; a phenomenon known as ‘protein N-homocysteinylation’ that results in the loss of a proteins structural integrity leading to loss of enzymatic function and aggregation.⁷A number of fluorescent sensors have been developed for the detection of HCys to help improve our understanding of its role in biological systems.^8–11 However, these fluorescent probes have focused on the detection of a single biomarker (HCys), however, processes associated with HCys induced toxicity often involve more than one biochemical species. For example, it has been reported that peroxynitrite (ONOO⁻) and nitric oxide (NO˙) play a significant role in HCys-mediated apoptosis in trigeminal sensory neurons¹² and HCys has been reported to induce cardiomyocytes cell death through the generation of ONOO⁻.¹³ The production of ONOO⁻ is believed to be the result of an increased production of superoxide (O₂˙⁻) by HCys activating the enzyme NADPH oxidase.^14–16 This increased production of O₂˙⁻ leads to a reduction in the bioavailability of NO˙ by increasing the formation of ONOO⁻ (NO˙ + O₂˙⁻ → ONOO⁻).¹⁷ The reported ONOO⁻ concentrations in vivo are believed to be approximately 50 μM but, higher concentrations of 500 μM have been found in macrophages.^18,19 Furthermore, hypoxia has been reported to facilitate HCys production in vitamin-deficient diets²⁰ where hypoxia leads to an upregulation of nitroreductase (NTR) – a reductive enzyme upregulated in cells under hypoxic stress.^21,22 Therefore, the development of tools that enable an understanding of the relationship of HCys with these biologically important species would be highly desirable.To achieve this, a number of fluorescent probes have been developed that are capable of detecting two or more analytes.²³ These include AND logic gate based-fluorescence probes, which require both analytes to work in tandem to produce a measurable optical output.^24–28 In our group, we have developed several ‘AND’ reaction-based probes for the detection of ROS/RNS and a second analyte.^29–32 These ‘AND’ logic scaffolds have been used to detect two analytes within the same biological system.^24,33Owing to the pathological role of HCys, we set out to develop the first example of a fluorescent probe for the detection of HCys and biological related analyte. Aiming towards that target, we became interested in a previously reported coumarin-based fluorescent probe developed by Hong et al.CAH, with a salicylaldehyde (Fig. 1).³⁴ Salicylaldehyde is a known reactive unit towards HCys/Cys, therefore we believed CAH could be used as a scaffold for the development of ‘AND’-based systems for the detection of HCys/Cys and a second analyte.³⁴ In the presence of HCys, CAH exhibited a ‘turn-on’ fluorescence response which is attributed to the nucleophilic nature of the nitrogen and sulfur atoms resulting in thiazine ring formation (Scheme S1, Fig. S1 and S2^†).^34–36Open in a separate windowFig. 1(a) CAH – a core fluorescent unit that enables the synthesis of ‘AND’ based fluorescent probe for the detection of HCys/Cys and a second analyte. (b) JEG-CAB enables the detection of HCys/Cys and (ROS/RNS) while (c) JEG-CAN enables the detection of HCys/Cys and NTR.We believed that CAH was a useful core unit that can be used to introduce the chosen reactive chemical trigger on the phenol for the detection of the corresponding analyte with HCys/Cys. Owing to the relationship between HCys and ONOO⁻/NTR, we set out on the development of a HCys AND ONOO⁻ probe and a HCys AND NTR probe.Therefore, we set out to prepare JEG-CAB and JEG-CAN, which are able to detect HCys/Cys and peroxynitrite (ONOO⁻) or nitroreductase (NTR), respectively (Scheme 1). For JEG-CAB, a benzyl boronate ester was introduced as a ONOO⁻ reactive unit.³⁷ For JEG-CAN, a p-nitrobenzyl group was installed as it is known to be an effective substrate for NTR.^38–40Open in a separate windowScheme 1Synthesis of target probe JEG-CAB and JEG-CAN.To afford CAH, compound 2 was synthesized by refluxing umbelliferone and acetic anhydride at 140 °C. Compound 2 was then dissolved in trifluoroacetic acid at 0 °C followed by the addition of hexamethylenetetramine (HMTA). The mixture was heated to reflux overnight and the solvent was then removed. The intermediate was then hydrolyzed in H₂O for 30 min at 60 °C. Upon isolating CAH, it was then alkylated using 4-bromomethylphenylboronic acid pinacol ester and K₂CO₃ in DMF at r.t. to afford JEG-CAB in 51% yield. JEG-CAN was produced by alkylating CAH using 4-nitrobenzyl bromide and K₂CO₃ in DMF at r.t. to give 49% yield (Scheme 1).We then evaluated the ability of JEG-CAB to detect ONOO⁻ ‘AND’ HCys in PBS buffer solution (10 mM, pH 7.40). The maximum absorption of JEG-CAB at 336 nm shifted to 323 nm with the addition of HCys and then slightly shifted to 328 nm following the addition of ONOO⁻ (Fig. S3^†). As shown in Fig. 2, JEG-CAB was initially non-fluorescent, but the addition of HCys (1 mM) led to a small increase in fluorescence intensity, the subsequent additions of ONOO⁻ (0–24 μM) led to a significant increase in fluorescence intensity (>17-fold, see Fig. S5^†). These results demonstrated the requirement for both ONOO⁻ ‘AND’ HCys to obtain a significant turn ‘‘on’’ fluorescence response.Open in a separate windowFig. 2Fluorescence spectra of JEG-CAB (15 μM) with addition of HCys (1 mM) and incubated for 40 min then measured. Followed by incremental additions of ONOO⁻ (0–24 μM). The data was obtained in PBS buffer solution (pH 7.40, 10 mM) at 25 °C. λ_ex = 371 (bandwidth 20) nm. Dashed line represents JEG-CAB and Hcys addition only. Blue line represents highest intensity after addition of ONOO⁻.The addition of HCys and ONOO⁻ were then performed in reverse where JEG-CAB exhibited a negligible increase in fluorescence intensity upon addition of ONOO⁻ (16 μM). However, in a similar manner to that shown in Fig. 2, a large increase in fluorescence intensity was produced after the subsequent addition of HCys (0–5.5 mM) (Fig. 3 and S6^†). LC-MS experiments were carried out to ascertain the reaction mechanism and the results confirmed the sequential formation of the thiazine ring in the presence of HCys followed by boronate ester cleavage in the presence of ONOO⁻ or vice versa (Scheme S2 and Fig. S19–S21^†).Open in a separate windowFig. 3Fluorescence spectra of JEG-CAB (15 μM) with addition of ONOO⁻ (16 μM) and followed by incremental additions of HCys (0–5.5 mM) measurements were taken after 40 min of both additions. The data was obtained in PBS buffer solution (pH 7.40, 10 mM) at 25 °C. λ_ex = 371 (bandwidth 20) nm. Dashed line represents JEG-CAB and ONOO⁻ addition only. Blue line represents highest intensity after addition of HCys.As expected, probe JEG-CAB was shown to have excellent selectivity with ONOO⁻ against other ROS in the presence of HCys (1 mM) (Fig. S9 and S10^†). Furthermore, JEG-CAB exhibited a high degree of selectivity towards a series of amino acids where only HCys and Cys led to a fluorescence response in the presence of ONOO⁻. This is due to the formation of stable five or six-membered thiazine rings (Fig. S7 and S8^†).³⁴We then evaluated the changes in the fluorescence of JEG-CAN with both HCys and NTR in PBS buffer solution (10 mM, pH 7.40, containing 1% DMSO). As shown in Fig. 4, addition of HCys led to a small increase in fluorescence intensity. However, subsequent addition of NTR (4 μg mL⁻¹) led to a large time dependant increase in fluorescence intensity. To ensure both analytes were required, NTR and NADPH was kept constant (4 μg mL⁻¹ and 400 μM respectively) resulting in a 3.4 fold fluorescence increase (Fig. 5). We attribute the large initial increase to background fluorescence of NADPH.⁴¹ NTR then facilitates reduction of the nitro group of JEG-CAN releasing the core probe CAHvia a fragmentation cascade (Scheme S3^†).^38,42 Subsequent addition of HCys (2.0 mM) led to a 2 fold increase in fluorescence intensity. Again, LC-MS experiments confirmed the proposed reaction mechanism (Fig. S22^†).Open in a separate windowFig. 4Fluorescence spectra of JEG-CAN (15 μM) with initial addition of HCys (2 mM) and incubated for 60 min. Followed by addition of nitroreductase (4 μg mL⁻¹) and NADPH (400 μM) and measured over 90 min in PBS buffer solution (pH = 7.40, 10 mM, containing 1% DMSO). λ_ex = 363 nm. Ex slit: 5 nm and em slit: 5 nm. Dashed line represents JEG-CAN and HCys addition only. Blue line represents highest intensity after addition of NTR.Open in a separate windowFig. 5Fluorescence spectra of JEG-CAN (15 μM) with initial addition of nitroreductase (4 μg mL⁻¹) and NADPH (400 μM) and incubated for 60 min. Followed by addition HCys (2 mM) and measured over 90 min in PBS buffer solution (pH = 7.40, 10 mM, containing 1% DMSO). λ_ex = 363 nm. Ex slit: 5 nm and em slit: 5 nm. Dashed line represents JEG-CAN and NTR addition only. Blue line represents highest intensity after addition of HCys.Kinetic studies for JEG-CAN with both NTR and HCys were carried out (Fig. S11–S18^†) where it is clear that JEG-CAN exhibits a dose dependant fluorescence increase in response of both HCys and NTR.Unfortunately, the probes failed to give good data in cells, which could be due to their short excitation wavelengths or the extremely low intracellular HCys concentrations (5–15 μM). We are now pursuing the development ‘AND’ logic fluorescence probes with longer excitation and emission wavelengths suitable for in vitro and in vivo applications.In summary, we have developed two coumarin-based ‘AND’ logic fluorescence probes (JEG-CAB and JEG-CAN) for the detection of HCys/Cys and ONOO⁻ or NTR, respectively. CAH is a useful platform that enables easy modification for the development of ‘AND’-based fluorescent probes for the detection of HCys/Cys and a second analyte. Both JEG-CAB and JEG-CAN were shown to be ‘AND’-based fluorescent probes.     

Complement activation by gold nanoparticles passivated with polyelectrolyte ligands

Gold nanoparticles passivated by polyelectrolyte ligands are widely used to confer stability and biofunctionality. While nanoparticles and polyelectrolytes have been reported as activators, their ability to activate the complement system as hybrid polyelectrolyte-coated nanoparticles is poorly characterized. Here, we found that gold nanoparticles passivated by common polyelectrolytes activated the system differently. The surface area of AuNPs appeared to be a major determinant of complement activation level as it determined the amount of adsorbed polyelectrolytes. Although a moderate negative correlation between AuNP surface hydrophilicity and their activation level was observed, the surface charge and functional group of polyelectrolyte ligands also influenced the final complement activation level.

We reported that the surface area and hydrophilicity of polyelectrolyte-coated gold nanoparticles influence their complement activation, a biological response not well understood to date.

Nanomaterials may elicit various biological responses upon contact with blood, of which activation of the complement system in innate immunity is one of the earliest.^1–3 The complement system is a collection of over 40 soluble and membrane-bound proteins, which acts in any of three distinct enzymatic cascades: classical, lectin, and alternative leading to the formation of a C3 convertase complex, and its accompanying range of biological responses, including inflammation, opsonization and cytolysis.^2,4While these responses could lead to undesirable physiological responses from over-activation and rapid clearance of nanomaterials from circulation, the intricate link between complement activation and adaptive immunity^5–7 also presents opportunities for exploitation in immune-related applications such as vaccines development. Therefore, complement activation by nanomaterials in biomedical applications has attracted great attention recently.^3,8–10We previously reported complement activation by gold nanoparticles (AuNPs) of different shapes in their as-synthesized citrate and CTAB coatings.¹¹ Polyelectrolyte ligands are also widely used in the preparation of AuNPs not only to stabilize AuNPs against aggregation, but to also enhance their solubility and confer additional surface functionalities.^12–14 These polyelectrolytes with different functional groups have been reported as complement activators in both their particulate and planar surface-immobilized forms.^15–21 However, the ability of polymer-passivated AuNPs to activate the complement system and the underlying mechanism remain poorly understood, although our previous study demonstrated that passivating the surface of AuNPs by poly(ethylene glycol) (PEG) modulated the activation level.¹¹Herein, we stabilized different shapes of AuNPs with different polyelectrolytes and examined their levels of complement activation and underlying mechanism. This would provide rational guidelines on the use of polyelectrolytes to modulate complement activation. All polyelectrolytes used in this study are hydrophilic and widely used as delivery platforms^5,22 and surface passivating ligands of nanomaterials²³ (Scheme 1).Open in a separate windowScheme 1Schematic illustration of gold nanoparticles (AuNPs) core, polyelectrolytes and methods used in this study. AuNPs with spherical shape of 20 nm (Au20) and 40 nm (Au40) as well as rod-shape (AuNR) were used as AuNP cores. These AuNPs were passivated with various polyelectrolyte ligands, examined for their hydrophilicities and levels of complement activation.Spherical AuNPs of two diameters, 20 nm (Au20) and 40 nm (Au40), were synthesized by well-known citrate reduction method.²⁴ AuNPs of rod-like shape (AuNR) were synthesized by hexadecyltrimethylammonium bromide (CTAB)-mediated method²⁵ with the CTAB ligands subsequently replaced by citrate using previously established protocol.²⁶ All AuNPs were then passivated by different polyelectrolytes following previously established layer-by-layer protocols (see Experimental section, ESI^†). With this library of AuNPs, we sought to examine the effects of size and shape of AuNP core as well as polyelectrolyte shell on the level of complement activation.The TEM images showed that Au20 and Au40 were spherical and homogenous while AuNR was rod-shaped with dimensions of approximately 40 × 10 nm (Fig. 1a–c). Hydrodynamic diameters, D_h, (Fig. 1d) were consistent with sizes of 20.2, 41.8, and 10.6 × 40.2 nm obtained from TEM images for Au20, Au40, and AuNR respectively (Fig. S1b–e, ESI^†). The UV-Vis absorption spectra showed surface plasmon resonance (SPR) peaks of 523 nm and 530 nm for Au20 and Au40, respectively while AuNR had transverse peak of 509 nm and longitudinal peak of 800 nm (Fig. S1a, ESI^†).^27,28 All citrate-capped Au20, Au40 and AuNRs had similar zeta potentials of −30 mV, which became nearly neutral after passivated with poly(vinyl alcohol) (ζ_AuNP-PVA ≈ −5 mV) and polyamidoamine (ζ_AuNP-PAMAM ≈ −10 mV) (Fig. 1e). In contrast, surface passivation with poly(acrylic acid) (AuNP-PAA), poly(styrenesulfonate) (AuNP-PSS), and heparin (AuNP-Heparin) conferred AuNPs a more negative zeta potential of ≈−40 mV (Fig. 1e), while coating with poly(ethyleneimine) (AuNP-PEI) conferred AuNPs a positive surface charge of ≈+40 mV. The presence of polyelectrolytes on AuNPs was further confirmed by the increase in D_h of polyelectrolyte-passivated AuNPs (Fig. 1d).Open in a separate windowFig. 1Physical properties of library of AuNPs. TEM images of (a) citrate-capped Au20, (b) Au40, and (c) AuNR. (d) Hydrodynamic diameter, D_h, and (e) zeta potential, ζ, of citrate-capped AuNPs and polyelectrolyte-passivated AuNPs. Each data point represents mean ± standard deviation (SD) of triplicate experiments. Scale bar in the TEM images represents 50 nm.When these polyelectrolyte-passivated AuNPs were incubated in normal human serum, the generation of SC5b-9 as an endpoint biomarker showed complement activation regardless of the activation pathway.^3,4,8 Here, elevated levels of SC5b-9 were detected in all AuNPs-treated sera (Fig. 2), indicating activation of the complement system. Prior to surface passivation by polyelectrolytes, we observed an expected complement activation by AuNP-citrate, although the level was significantly lower than our positive control zymosan (a well-known complement activator derived from the wall of yeast cell). Also, Au40-citrate induced more SC5b-9 than Au20-citrate and AuNR-citrate (Fig. 2).Open in a separate windowFig. 2Detection of endpoint product of complement activation, SC5b-9, using ELISA kit. 1× PBS and zymosan (10 mg ml⁻¹) were used as negative (−) and positive (+) controls, respectively. Each data point represents the mean ± standard deviation of triplicate experiments.The level of complement activation was dependent on both the AuNP core and polyelectrolyte ligand. Amongst the polyelectrolytes, we observed the highest level of complement activation from PEI, comparable or even higher than zymosan (Fig. 2). This agreed with previously published results where positively charged polymers carrying primary amino groups were shown to interact strongly with complement proteins to activate the complement system.^18,21Between the three AuNP cores, Au40-PEI had the highest level of complement activation ([SC5b-9] = 1.80 μg ml⁻¹), followed by Au20-PEI ([SC5b-9] = 1.29 μg ml⁻¹) and AuNR-PEI ([SC5b-9] = 1.16 μg ml⁻¹), which had comparable levels of complement activation (Fig. 2). The same trend was true not only for AuNPs-PEI but also other polyelectrolyte-passivated AuNPs. While complement activation by nanomaterials have been shown to depend on their sizes and shapes,^8,29 the differences observed here were more likely attributed to the amount of PEI adsorbed on the surface of AuNPs, which was in turn dictated by their surface area, since PEI by itself has been shown to activate the complement system in concentration-dependent manner.¹⁸ Here, Au20 and AuNR had comparable surface areas (1256 and 1413 nm², respectively) and hence similar levels of complement activation, while Au40 had the largest surface area (5024 nm²), thus accounting for the highest level of complement activation (Fig. 2).Unlike PEI, AuNPs passivated by both PVA and PAA induced the lowest levels of SC5b-9 (Fig. 2). While PVA is widely known as a biocompatible ligand on AuNRs,²³ its complement activation was not totally avoided probably due to its nucleophilic hydroxyl groups,^16,17 similar to PEI with nucleophilic amine groups. However, its complement activation level was much lower, and was likely due to its near neutral surface charge which did not promote interaction with many negatively charged complement proteins unlike the positively charged PEI.Similarly, the highly negative charge AuNP-PAA due to high density of carboxyl groups did not promote their interaction with negatively charged complement proteins, thus inducing an equally low level of complement activation as AuNP-PVA. Therefore, PAA is one of the most widely used water-soluble polyelectrolyte, superabsorbent polymer as well as food additive. Nonetheless, PAA conjugated to IgG has been found to specifically interact with positively charged C1q complement protein to activate the classical pathway.^30,31 Hence, AuNP-PAA has been found to promote inflammation via activation with fibrinogen.³² In this study, the AuNP-PAA could have activated the complement system via the classical pathway due to their interactions with C1q complement protein as reported.^30,31Owing to its nature as an anticoagulant, heparin has been reported to minimize complement activation.^33–35 However, we found that AuNP-heparin still activated the complement system although their level of complement activation was comparable to PVA and PAA, and much lower than PEI, PSS and PAMAM.Interestingly, despite the presence of nucleophilic amine groups on PAMAM similar to PEI, AuNP-PAMAM activated the complement system at much lower levels compared to AuNP-PEI. PAMAM dendrimers were known to be generation-dependent complement activators with stronger complement activation observed in higher generations.¹⁵ Here, we used PAMAM of generation 2.0 with aminoethanol surface and reasoned that coupling of amine with hydroxyl group helped to reduce activation level of AuNP-PAMAM. In fact, the activation level of AuNP-PAMAM was between that of AuNP-PVA with hydroxyl groups, and AuNP-PEI with amine groups. Furthermore, since primary amine groups adsorbed more C3b, a major component of complement proteins in serum, than secondary or tertiary amino groups,²⁰ the lack of primary amine group in PAMAM also explained its weaker complement activation than PEI which possesses a mixture of primary, secondary, and tertiary amine groups.The amount of SC5b-9 induced by AuNP-PSS was comparable to that of AuNP-PAMAM (Fig. 2). PSS is a widely used polyelectrolyte building block in layer-by-layer assembly,³⁶ and we used it as an intermediate ligand to prepare AuNR-citrate (Fig. S2, ESI^†). PSS by itself interacted with complement proteins of the classical pathway to activate the complement system.¹⁹ Our results not only confirmed complement activation by PSS passivation but also highlight the potential side effects of block-copolymer containing PSS commonly used as drug delivery platform in activating the complement system.Since the hydrophilicity of nanoparticles has been shown to modulate non-specific protein adsorption³⁷ and dictate immune response,³⁸ we further examined for possible correlation between the hydrophilicity of polyelectrolyte-passivated AuNPs and their level of complement activation. We measured relative hydrophilicity of polyelectrolyte-passivated AuNPs by dye absorption using a hydrophilic dye Nile Blue which interacted with a hydrophilic moiety (see Experimental section, ESI). On mixing the dye with AuNPs passivated with different polyelectrolyte ligands, we determined the partitioning quotient (PQ) as the ratio of dye bound on nanoparticles surface to the amount of free dye. In a plot of PQ versus surface area of nanoparticles, the slope of this linear regression line represented relative surface hydrophilicity.³⁹We observed different levels of hydrophilic dye adsorption on all polyelectrolyte-passivated AuNPs after 3 h incubation, indicating differences in their hydrophilicities (Fig. S8–10, ESI^†). AuNPs-PEI was the least hydrophilic (Fig. 3) as determined from the smallest slope of the linear regression line (Table S1, ESI^†), while AuNPs passivated with PVA, PAA, and heparin are amongst the most hydrophilic as given by the larger slope of the PQ vs. nanoparticle surface area plot (Fig. 3 and Table S1, ESI^†).Open in a separate windowFig. 3Negative correlation between surface hydrophilicity and complement activation by polyelectrolyte-passivated (a) Au20, (b) Au40 and (c) AuNR. X scale bar was plotted as ln of the value of relative hydrophilicity.We also observed a negative correlation between surface hydrophilicity and complement activation level across all AuNPs regardless of shape or size (Fig. 3). However, this correlation was only moderate with Pearson correlation coefficient, r = −0.2730, −0.4101, and −0.5489 for Au20, Au40 and AuNR, respectively (Fig. 3). Since Moyano et al. reported a positive correlation between nanoparticle surface hydrophobicity and their immune response previously,³⁸ the similarity in our observations suggested that the complement system could be a potential mediator between surface hydrophilicity/hydrophobicity and downstream immune response. Nonetheless, our results also demonstrated the complexity of complement activation by engineered nanomaterials, as it was not dependent solely on any one physical property such as shape, size, surface charge, surface functional group or surface hydrophilicity, but the interplay of these properties to influence the complement proteins adsorption.In summary, AuNPs passivated with different polyelectrolyte ligands activated the complement system at different levels, as characterized by the presence of endpoint product of complement activation, SC5b-9. The surface area of AuNPs appeared as a major determinant of complement activation level as it determined the amount of adsorbed polyelectrolytes. Although a moderate negative correlation between AuNPs surface hydrophilicity and their activation level was observed, the surface charge and functional group of polyelectrolyte ligands also influenced the final complement activation level. These findings provide new insights to rational selection and design guidelines for the use of polyelectrolytes to either suppress complement activation and downstream immune response for nanoparticulate drug delivery systems or to enhance complement activation and immune response for vaccine development.     

Renewable juglone nanowires with size-dependent charge storage properties

Inspired by the biological metabolic process, some biomolecules with reversible redox functional groups have been used as promising electrode materials for rechargeable batteries, supercapacitors and other charge-storage devices. Although these biomolecule-based electrode materials possess remarkable beneficial properties, their controllable synthesis and morphology-related properties have been rarely studied. Herein, one dimensional nanostructures based on juglone biomolecules have been successfully fabricated by an antisolvent crystallization and self-assembly method. Moreover, the size effect on their electrochemical charge-storage properties has been investigated. It reveals that the diameters of the one dimensional nanostructure determine their electron/ion transport properties, and the juglone nanowires achieve a higher specific capacitance and rate capability. This work will promote the development of environmentally friendly and high-efficiency energy storage electrode materials.

Renewable juglone nanowires have been successfully fabricated, and their size effect on electrochemical charge-storage properties has been investigated.

Currently, the development of high-performance electrochemical energy-storage materials and devices is attracting intensive interest. Conventional electrode materials involving transition metal compounds,^1–4 elementary substances,^5–8 and conductive polymers,^9–12 with superior charge storage properties have been widely investigated. However, the poor biocompatibility, rising prices and depletion issues limit their sustainable applications due to their intrinsic material properties.¹³ Thus, exploring naturally abundant and renewable charge-storage materials with promising electrochemical performance is of great significance.In the biological system, its metabolic process mainly relies on ions transport and energy exchanges of redox-active biomolecules with special functional groups such as carbonyl groups, carboxyl groups, and pteridine centres.¹⁴ Due to their abundance, sustainability, environmental benignity, these renewable and nature-derivable biomolecules with well-defined charge-storage behaviors are ideal alternatives to conventional electrode materials for the next-generation green energy-storage devices.^15–17 For instance, biomolecules such as lignin,¹⁸ melanin,¹⁹ riboflavin,²⁰ juglone²¹ and humic acid²² have been demonstrated as promising electrode materials for the rechargeable batteries, supercapacitors and other charge-storage devices. Although these biomolecule-based electrode materials possess remarkable beneficial properties, they are still confronted with several serious problems of poor conductivity and high electrochemical reaction impedance.²³ For some conventional inorganic and organic active electrode materials, decreasing their size has been demonstrated to be effective strategies to enhance the electrochemical reaction kinetics by exposing more active sites to electrolytes and conductive agent.^8,24,25 These results have strongly motivated us to develop biomolecule-based nanostructures, and investigated their size-correlative charge storage behavior.^26,27Herein, one dimensional (1D) nanostructures based on juglone, a renewable redox-active biomolecule which can be derived from matured fruits of black walnut and the green peel of juglandaceae, have been successfully fabricated by an antisolvent crystallization and self-assembly method.^28–30 The size effect on the electrochemical charge-storage properties of these biomolecule-based 1D nanostructures have been investigated. It reveals that the electronic/ionic transport properties and charge-storage performance can be modulated by the size of self-assembled 1D nanostructures, and the samples with smaller diameter realize the higher specific capacitance and rate capability. Our work will provide insights for the development of high-performance biomolecule-based green energy-storage materials and devices.Juglone, also called 5-hydroxy-1,4-naphthalenedione, is a nature-derivable biomolecule, and displays a well-defined redox behavior in acetonitrile due to its quinone groups (Fig. 1a and b).³¹ As organic molecule inherently, it is soluble in organic solvent and difficult to dissolve in water, so its nano-architectures could be condonably fabricated by an antisolvent crystallization strategy (Fig. 1c) and would carry out stably for charge storage in an aqueous electrolyte.^29,30 Firstly, the juglone-biomolecule-based 1D nanostructures with different size were synthesized. The juglone micropillars with a mean diameter around 12 μm were prepared by directly recrystallizing a water/acetonitrile mixed solution of juglone at room temperature (Fig. 2a and d). Compared to commercially available raw juglone materials (Fig. S1^†), the juglone micropillars could increase its charge storage performance, but its relative large size would still confine its contact with electrolyte, and thus remarkably reduce the reaction kinetics.³² To further improve the potential reaction kinetics, the juglone microwires with an average diameter about 1 μm (Fig. 2b and e) and juglone nanowires with a mean diameter about 550 nm (Fig. 2c and f) were fabricated at room temperature using the reprecipitation method.³³ In the specific synthesis process, a high-concentration juglone acetonitrile solution is injected into water, which is a poor solvent for juglone molecules. Under stirring, juglone started to crystallize within a few seconds owing to its poor solubility in the water/acetonitrile mixture solvent and self-assembled into 1D nanostructures, and this procedure could be resulted from the π–π interaction.²⁹ It can be found that the diameter of 1D juglone materials is adjustable by tuning the concentration of juglone in acetonitrile, and the higher concentration of juglone acetonitrile solution yields the smaller size of 1D juglone materials. The insert panels show the percentage of juglone micropillar, microwire, nanowire with different diameter coverage (Fig. 2d–f).Open in a separate windowFig. 1(a) Chemical structural formula of juglone biomolecules which can be derived from the bark of black walnuts. (b) Juglone redox activity verified in a mixed solution of acetonitrile/deionized water by a three-electrode system using Pt foils as both the counter and working electrodes, Ag/AgCl as the reference electrode, and 2.3 M H₂SO₄ as the electrolyte. (c) Schematically illustration of the fabrication of the juglone nanowire/microwire.Open in a separate windowFig. 2SEM images of juglone 1D nanostructures at different magnification. (a and d) Juglone micropillar, (b and e) juglone microwire, (c and f) juglone nanowire. The insert images in panels (d–f) are corresponding mathematical statistic results of juglone samples with different diameter.Generally, the covalent bond, hydrogen bond, van der Waals forces, electrostatic forces, surface tension forces/dewetting, and π–π stacking interactions are considered as the effective factors in the self-assembly procedure of organic compounds.^34–36 The juglone biomolecule has a α-naphthol backbone with two carbonyl groups, which may induce the molecules self-assembly by the π–π interactions.^29,37–39 Fourier Transform Infrared Spectroscopy (FTIR) of such samples was utilized to examine the ingredient of these samples. As shown in Fig. S2,^† all the samples show similar characteristic peaks, the peak at 1640 cm⁻¹ is attributed to the stretching vibration of carbonyl groups, which is the main reversible redox center presented in juglone molecules. Meanwhile, Raman spectra also corroborated the results (Fig. S4^†). The crystal structures of these samples are further characterized by X-ray diffraction (XRD) as shown in Fig. S3.^† No impurity peak is observed in these patterns, demonstrating that these samples have the same crystal structure. And the peak intensity of juglone nanowire is stronger than juglone microwire and micropillar, suggesting that juglone nanowires have high crystallinity and relatively uniform crystal size.³⁰To investigate the redox behavior of 1D juglone micro/nanostructures, the cyclic voltammetry (CV) measurements were firstly performed (Fig. 3a), and the result reveals that these samples show a superior reversible redox performance, implying a potential application as energy storage materials. Meanwhile, compared with the CV curves of raw juglone, juglone micropillar and juglone microwire, the juglone nanowire exhibits the strongest redox peak, suggesting that the juglone nanowire-based electrode has a better charge storage behavior. To further evaluate the electrochemical performance of these samples, galvanostatic charge–discharge (GCD) measurements were carried out (Fig. 3b). First, juglone nanowire shows a higher specific capacity than that of microwire and micropillar. Besides, the plot of juglone 1D-based electrodes exhibit symmetric triangular shape with one pair of charge–discharge voltage plateau, and the voltage plateau of juglone micropillar, microwire and nanowire appears at 0.28 V, 0.26 V, 0.22 V, which is roughly consistent with the results of CV curves. Furthermore, it further reveals that the 1D nanostructure with smaller size exhibits higher specific capacity during the scan rates increasing from 10 to 200 mV s⁻¹ (Fig. 3c). In detail, at a low scan rate of 10 mV s⁻¹, the specific capacity delivered by juglone nanowire, juglone microwire, juglone micropillar are 389 F g⁻¹, 375 F g⁻¹, 116 F g⁻¹, respectively, and these samples possess the specific capacities of 232 F g⁻¹, 205 F g⁻¹, 80 F g⁻¹, when the scan rate are enhanced to 200 mV s⁻¹. The general perception is that nanomaterials with higher specific surface areas usually possess better electrochemical performance.⁴⁰ Obviously, the higher specific capacitance and superior rate capability are achieved by juglone nanowire electrode.Open in a separate windowFig. 3Electrochemical performance of the juglone samples with different diameter. (a) CV curves of juglone micropillar, microwire, nanowire electrodes in the potential range of −0.1 to 0.7 V (vs. Ag/AgCl) with a scan of 50 mV s⁻¹, (b) galvanostatic charge–discharge curves of juglone electrodes with different diameter at 2 A g⁻¹. (c) Statistical study of the rate performance conducted at each scan rate based on the cyclic voltammetry capacity of five electrodes as one batch. (d) Impedance phase angle as a function of frequency for juglone micropillar, microwire, nanowire electrodes. Inset is the Nyquist plot of them. (e) Correlation between the diameter and the specific capacity of the juglone samples at various scan rates.Then, the reaction kinetics of juglone with different sizes were investigated by electrochemical impedance spectroscopy (EIS) (Fig. 3d and S5^†). The crooked curve at high frequency denotes the interface resistance, involving contact and charge transfer resistances, while the low-frequency line represents ion diffusion resistance. The interface resistances of juglone nanowire, microwire, micropillar, raw material electrodes are 3.45 Ω, 3.80 Ω, 3.95 Ω, 4.02 Ω. The plot of the phase angle against the frequency reveals the characteristic frequency f₀ at the phase angle of −45° is 7.46 Hz for juglone nanowire-based electrode, and it is relatively higher than that for juglone microwire (7.36 Hz), micropillar (7.25 Hz) and raw material (6.48 Hz). Accordingly, the time constant t₀ (t₀ = 1/f₀) that represents the minimum time to discharge ≥50% of all the energy from the electrode was 0.134 s for juglone nanowire, whereas 0.136 s, 0.138 s, 0.154 s were required for juglone microwire, micropillar and raw material. The low charge transfer resistance, and short time constant validated the excellent charge and discharge capability of the juglone nanowire-based electrode.⁴¹In addition, for solid-state diffusion of H⁺ in electrode materials, the mean diffusion time is proportional to the square of the diffusion path length according to the following equation:t ≈ L²/D_H⁺1where L is the diffusion length and D_H⁺ the diffusion constant.^42,43 It has been found that the diffusion pathway will be shortening by nanostructuring of electrode materials when the diffusion constant D is same,⁴⁴ indicating that the smaller size nanostructures of juglone facilitate rapid ion/electron transport. Thus, 1D juglone nanowires possess higher specific capacitance and show slower capacity decay during the increased scan rates (Fig. 3e).Furthermore, the CV and GCD for juglone nanowire-based electrodes were tested under various scan rates and current densities. The CV profiles show that the redox activity can be maintained very well when increasing the scan rate from 10 to 100 mV s⁻¹ (Fig. 4a). Moreover, the GCD curves collected at different current densities exhibit symmetric triangular shape with one pair of charge–discharge voltage plateau, which is similar to the results of CV test (Fig. 4b and c). And the specific capacity is 342 F g⁻¹ at a current density of 2 A g⁻¹, it is approximately equivalent to the 345 F g⁻¹ at a scan rate of 20 mV s⁻¹. Next, the specific capacitance of our juglone nanowire is further compared to those of the reported conventional electrode materials.^45–51 As shown in Fig. 4d, the capacity of N/rGO (reduced graphene oxide doping with nitrogen) is 138 F g⁻¹, and our juglone nanowire have about 2-fold higher specific capacity than N/rGO, 1.5-fold higher than Fe₃O₄/rGO (154 F g⁻¹).Open in a separate windowFig. 4(a) CV curves of the juglone nanowire electrode at different scan rates from 10 mV s⁻¹ to 100 mV s⁻¹. (b) Charge and discharge curves in the current density range from 2 A g⁻¹ to 10 A g⁻¹. (c) Various specific capacity of the juglone nanowire electrode when increasing the scan rate from 10 mV s⁻¹ to 200 mV s⁻¹. (d) Specific capacity of juglone nanowire in comparison with different materials.     

A novel fluorescent probe for imaging the process of HOCl oxidation and Cys/Hcy reduction in living cells

A new on-off-on fluorescent probe, CMOS, based on coumarin was developed to detect the process of hypochlorous acid (HOCl) oxidative stress and cysteine/homocysteine (Cys/Hcy) reduction. The probe exhibited a fast response, good sensitivity and selectivity. Moreover, it was applied for monitoring the redox process in living cells.

A new on–off–on fluorescent probe, CMOS, was designed and applied to detect the process of HOCl oxidation and Cys/Hcy reduction.

Reactive oxygen species (ROS) are indispensable products and are closely connected to various physiological processes and diseases.¹ For instance, endogenous hypochlorous acid (HOCl) as one of the most important ROS, which is mainly produced from the reaction of hydrogen peroxide with chloride catalyzed by myeloperoxidase (MPO), is a potent weapon against invading pathogens of the immune system.^2,3 However, excess production of HOCl may also give rise to oxidative damage via oxidizing or chlorinating the biomolecules.⁴ The imbalance of cellular homostasis will cause a serious pathogenic mechanism in numerous diseases, including neurodegenerative disorders,⁵ renal diseases,⁶ cardiovascular disease,⁷ and even cancer.⁸ Fortunately, cells possess an elaborate antioxidant defense system to cope with the oxidative stress.⁹ Therefore, it is necessary and urgent to study the redox process between ROS and antioxidants biosystems.Fluorescence imaging has been regarded as a powerful visual methodology for researching various biological components as its advantages of high sensitivity, good selectivity, little invasiveness and real-time detection.^10,11 To date, amounts of small molecular fluorescent probes have been reported for detection and visualization of HOCl in vivo and in vitro.^12–22,29 The designed strategies of HOCl sensitive probes are based on various HOCl-reactive functional groups, such as p-methoxyphenol,¹³p-alkoxyaniline,¹⁴ dibenzoyl-hydrazine,¹⁵ selenide,¹⁶ thioether,¹⁷ oxime,¹⁸ hydrazide,¹⁹ hydrazone.²⁰ But, many of these probes display a delayed response time and low sensitivity. And, only few fluorescent probes can be applied for investigating the changes of intracellular redox status.²¹ Besides, it''s worth noting that most of the redox fluorescent probes rely on the organoselenium compounds.²² Even though these probes are well applied for detection of cellular redox changes, excessive organic selenium is harmful to organisms and the synthesis of organoselenium compounds is high requirement and costly. Additionally, almost all the reports have only investigated the reduction effects of glutathione (GSH) as an antioxidant in the redox events. While, there are the other two important biothiols, cysteine (Cys) and homocysteine (Hcy), which not only present vital antioxidants, but also are tightly related to a wide variety of pathological effects in biosystem, such as slowed growth, liver damage, skin lesions,²³ cardiovascular,²⁴ and Alzheimer''s diseases.²⁵ However, the fluorescent probes for specially studying internal redox changes between HOCl and Cys/Hcy are rarely reported. In this respect, a novel redox-responsive fluorescent probe, CMOS, was designed and synthesized in this work, and we hope that it can be a potential tool for studying their biological relevance in living cells.Based on literature research, the aldehyde group has excellent selectivity in identification of Cys/Hcy, and the thiol atom in methionine can be easily oxidized to sulfoxide and sulfone by HOCl.^26,27 Considering these two points, we utilized 2-mercaptoethanol to protect the 3-aldehyde of 7-diethylamino-coumarin as the recognition part of HOCl, meaning that two kinds of potential recognition moieties are merged into one site. Fluorescent probe CMOS can be easily synthesized by the acetal reaction in one step (Scheme S1^†). A control molecule CMOS-2 was also prepared by 3-acetyl-7-diethylaminocoumarin (CMAC) similarly. The structure of all these compounds have been convinced by ¹H NMR, ¹³C NMR, and HR-MS (see ESI^†).As shown in Scheme 1a, we estimated that both CMOS and CMOS-2 can be rapidly oxidized in the appearance of HOCl. The oxidation product CMCHO of CMOS, which has the aldehyde moiety, can further react with Cys/Hcy to obtain the final product CMCys and CMHcy, respectively. In contrast, the oxidation product CMAC of CMOS-2 cannot combine with Cys/Hcy or other biothiols anymore (Scheme 1b).Open in a separate windowScheme 1Proposed reaction mechanism of CMOS and CMOS-2 to HOCl and Cys/Hcy.In order to confirm our design concept, the basic photo-physical characteristics of CMOS, CMCHO, CMOS-2 and CMAC were tested (Table S1, Fig. S1^†). Under the excitation wavelength 405 nm, CMOS and CMOS-2 exhibited strong fluorescence centred at 480 nm in PBS buffer solution, while the fluorescence of CMCHO and CMAC was weak around this band. The emission properties of CMOS and CMCHO were also investigated at the excitation wavelength 448 nm under the same experimental conditions as well (Fig. S2^†). After careful consideration, we chose 405 nm as the excitation wavelength in the follow-up experiments in vitro and in vivo.Next, the sensitivity of CMOS and CMOS-2 to HOCl and Cys/Hcy were investigated. As we expected, both the CMOS and CMOS-2 exhibited good response to HOCl. The fluorescence intensity of CMOS and CMOS-2 decreased gradually with addition of NaOCl (Fig. 1a, S3a^†), indicating that the fluorescence was switched off obviously in the presence of HOCl. The variation of intensity displayed good linearity with concentration of HOCl in the range of 0–20 μM (R² = 0.993, Fig. S4^†), and the detection limit of CMOS to HOCl was calculated to be 21 nM (S/N = 3). Subsequently, when Cys/Hcy was added to the final solution in Fig. 1a, the fluorescence intensity increased gradually within 180 min (Fig. 1b, S5^†). However, the fluorescence cannot be recovered by addition thiols to the CMOS-2 solution with excess HOCl (Fig. S3b^†). These results indicate that the probe CMOS can response to HOCl and Cys/Hcy in a fluorescence on-off-on manner, and can be used for monitoring the redox process with high sensitivity.Open in a separate windowFig. 1(a) Fluorescence responses of CMOS (2 μM) to different concentrations of NaOCl (0–200 μM). (b) Fluorescence responses of the CMOS solution (2 μM) with HOCl (200 μM) to Cys/Hcy (5 mM). (20 mM PBS buffer/CH₃CN, 7 : 3, v/v, pH = 7.4, λ_ex = 405 nm).To further identify the recognizing mechanism of probe CMOS, high performance liquid chromatography (HPLC) and mass spectral (MS) analysis were used to detect the redox process. Initially, probe CMOS displayed a single peak with a retention time at 3.7 min (Fig. 2a, S6^†) while reference compound CMCHO produced a single peak with a retention time at 2.5 min (Fig. 2b, S7^†). Upon the addition of HOCl to the solution of CMOS, the peak at 3.7 min weakened while 2.5 min and 2.2 min appeared (Fig. 2c). According to corresponding mass spectra, the new main peak at 2.5 min is related to compound CMCHO (Fig. S8^†). The other new peak of 2.2 min corresponds to the compound C3, which can be predicted as an intermediate in the oxidation process (Fig. S8^†).²⁸ The addition of Cys to the solution of CMCHO also caused a new peak with a retention time at 2.1 min, which has been confirmed to be the thioacetal product CMCys (Fig. S9^†). The possible sensing mechanism is depicted in Fig. S10.^†Open in a separate windowFig. 2The reversed-phase HPLC with absorption (400 nm) detection. (a) 10 μM CMOS. (b) 10 μM CMCHO. (c) 10 μM CMOS in the presence of 50 μM HOCl for 30 s. (d) 10 μM CMCHO in the presence of 1 mM Cys for 30 min. (Eluent: CH₃CN containing 0.5% CH₃COOH; 100% CH₃CN (0–7 min), 0.5 ml min⁻¹, 25 °C; injection volume, 5.0 μL).To study the selectivity of CMOS towards HOCl, we performed fluorescence response to different reactive oxygen species (ROS), reactive nitrogen species (RNS) and reactive sulfur species (RSS). As shown in Fig. 3a, CMOS exhibited significant change of fluorescence intensity only in the presence of HOCl, while other ROS and RNS, such as singlet oxygen (¹O₂), hydrogen peroxide (H₂O₂), hydroxyl radical (HO·), superoxide anion (O₂⁻), nitric oxide (NO), tert-butylhydroperoxide (t-BuOOH) and tert-butoxy radical (t-BuOO·) had no obvious fluorescence emission changes. Additionally, RSS which are abundant in biological samples, showed no influence in this process under the identical condition. The detection of reducing process was also investigated. As displayed in Fig. 3b, only cysteine and homocysteine induced excellent fluorescence recovery towards other reducing materials, such as RSS and various amino acids. Furthermore, the selectivity of CMOS-2 was also studied in the same condition. As expected, CMOS-2 could selectively detect HOCl, and not alter fluorescence intensity under various kinds of biothiols (Fig. S11^†). Therefore, our design strategy for the on–off–on probe is confirmed by results obtained above, with which CMOS can be utilized for detecting the redox process between HOCl and Cys/Hcy with high selectivity.Open in a separate windowFig. 3(a) Fluorescence response of CMOS (2 μM) to different ROS, RNS and RSS (200 μM). Bars represent emission intensity ratios before (F₀) and after (F₁) addition of each analytes. (a) HOCl; (b) KO₂; (c) H₂O₂; (d) ¹O₂; (e) HO·; (f) t-BuOOH; (g) t-BuOO·; (h) NO₂⁻; (i) NO₃⁻; (j) NO; (k) GSH; (l) Cys; (m) Hcy; (n) Na₂S; (o) Na₂S₂O₃; (p) Na₂S₂O₈; (q) NaSCN; (r) DTT; (s) Na₂SO₃. (b) Fluorescence response of the solution added HOCl in (a) to different RSS and amino acids. Bars represent emission intensity ratios before (F₂) and after (F₃) addition of each analytes (5 mM). (a) Cys; (b) Hcy; (c) Na₂S; (d) Na₂S₂O₃; (e) Na₂S₂O₈; (f) NaSCN; (g) DTT; (h) Na₂SO₃; (i) Ala; (j) Glu; (k) Gly; (l) His; (m) Ile; (n) Leu; (o) Met; (p) Phe; (q) Pro; (r) Ser; (s) Trp; (t) Vc; (u) GSH. (20 mM PBS buffer/CH₃CN, 7 : 3, v/v, pH = 7.4, λ_ex/λ_em = 405/480 nm).Subsequently, the influence of pH on probe CMOS was measured. The fluorescence intensity of CMOS and CMCHO perform no significant variances in wide pH ranges (pH = 4–11, Fig. S12a^†). Fluorescence intensity changes could be observed immediately when HOCl was added into the solution of probe CMOS, especially in alkaline condition (Fig. 4a). Considering the pK_a of HOCl is 7.6,²⁹CMOS is responsive to both HOCl and OCl⁻. Alkaline condition was also benefit for the fluorescence recovery of CMOS from Cys/Hcy (Fig. S12b^†). It is reasonable to consider that thiol atom displays higher nucleophilicity in alkaline condition. From the stop-flow test, the UV-visible absorbance of probe CMOS sharply decreased at the wavelength of 400 nm (Fig. 4b). The response time was within 10 s and the kinetic of the reaction was fitted to a single exponential function (k_obs = 0.67 s⁻¹). The ability of instantaneous response is extremely necessary to intracellular HOCl detection.Open in a separate windowFig. 4(a) Fluorescence responses of CMOS to HOCl under different pH values. Squares represent emission intensity ratios after (F₁) and before (F₀) addition of 200 μM HOCl (λ_ex/λ_em = 405/480 nm). (b) Time-dependent changes in the absorption intensity of CMOS (1 μM) before and after addition of HOCl. (20 mM PBS buffer/CH₃CN, 7 : 3, v/v, pH = 7.4, λ_abs = 400 nm).With these data in hand, we next applied CMOS for fluorescence imaging of the redox changes with HOCl and Cys/Hcy in living cells. After incubation with 5 μM CMOS at 37 °C for 30 min, intense fluorescence was observed of the SKVO-3 cells in the optical window 425–525 nm (Fig. 5a and d), indicating the probe can easily penetrate into cells. Treating the cells with 100 μM NaOCl led to remarkable fluorescence quenching as the probe sensed the HOCl-induced oxidative stress (Fig. 5b and e). After 3 min, the cells were washed with PBS buffer three times, and added 5 mM Cys/Hcy for 1 h, respectively. Then the fluorescence was recovered obviously (Fig. 5c and f). Experimental results clearly declare that the probe CMOS was successfully used to detect the process of HOCl oxidative stress and Cys/Hcy reducing repair in living cells.Open in a separate windowFig. 5Fluorescence imaging of the process of HOCl oxidative stress and thiols repair in CMOS-labeled SKVO-3 cells. Fluorescence images of SKVO-3 cells loaded with 5 μM CMOS at 37 °C for 30 min (a and d). Dye-loaded cells treated with 100 μM NaOCl at 25 °C for 3 min (b and e). Dye-loaded, NaOCl-treated cell incubated with 5 mM Cys (c), 5 mM Hcy (f) for 1 h. Emission intensities were collected in an optical window 425–525 nm, λ_ex = 405 nm, intensity bar: 0–3900.     

Colour-tunable ultralong organic phosphorescence upon temperature stimulus

A new class of single-component molecular crystal with colour-tunable ultralong organic phosphorescence (UOP) was designed and synthesized through alkyl chain engineering. Forming a more rigid environment at 77 K, the colour-tunable UOP from yellow-white to blue-green is achieved through dual-emission of crystal and amorphous states.

A new class of single-component molecular crystal with colour-tunable ultralong organic phosphorescence (UOP) was designed and synthesized through alkyl chain engineering.

Ultralong phosphorescence, a kind of phosphorescence that can be observed by the naked eye after removing the excitation source, has received great attention in the fields of sensing,¹ displays,² imaging,³ anti-counterfeiting⁴ and so on during the past years. Unfortunately, it is limited to inorganic materials because of the weak spin–orbit coupling (SOC) and strong non-radiative transition of pure organic materials.⁵ Compared with inorganic materials, however, organic materials have some excellent merits, such as inexpensive cost, relative safety to the environment, and soft preparation conditions.⁶ The realization of UOP becomes very significant. As mentioned above, UOP can be achieved by promoting intersystem crossing (ISC) through enhancing SOC and suppressing non-radiative transitions. In view of these factors, various strategies such as polymerization,⁷ H-aggregation,⁸ crystallization,⁹ host–guest doping,¹⁰ and freezing conditions¹¹ have been explored to achieve UOP with the unremitting efforts of scientific researchers.Intelligent-response organic luminescent materials can change their luminescent properties such as colour, lifetime, intensity, etc. after being stimulated by the external factor, such as mechanical forces, temperature, pH, light, solvent, etc., which have caught great attraction. The sensitivity of luminescent molecule to excitation wavelength, temperature or oxygen can be applied in sensors, optical recording and so on.^12–14However, the luminescence of these materials with intelligent-response are mostly limited to fluorescence or room temperature phosphorescence (RTP) with relatively short lifetimes.^15–18 Few samples display persistent luminescent feature.¹⁹ Most organic compounds can only tune their afterglow properties by changing molecular side groups or multi-component doping.^20–23 Therefore, the development of single component UOP materials with intelligent response remains a challenge. Inspired by the alkyl-chain engineering²⁴ and freezing conditions, herein we speculate that temperature response UOP might be induced by controlling the activity of the alkyl chain to regulate non-radiative transition rate. By means of reducing the temperature to restrict the molecular motions at amorphous state, colour-tunable UOP with temperature-response can be realized by dual-emission of molecules at both amorphous and crystalline states.In our previous study, MCzT crystals showed yellow UOP at RT.²⁵ The crystals showed the same UOP at 77 K with that at RT (Fig. 1a). Here, 9-(2-((4,6-dimethoxy-1,3,5-triazin-2-yl)oxy)ethyl)-9H-carbazole (MTOD) was designed and via alkyl chain attaching carbazole with a triazine core. The target molecule was characterized by ¹H NMR and ¹³C NMR (Fig. S1–S3^†). Its melting point reaches 130 °C. (Fig. S4^†) MTOD achieved a lifetime of up to 860 ms under ambient conditions. Surprisingly, the UOP changed from yellow-white to blue-green after the removal of the UV-lamp for several seconds at 77 K (Fig. 1b), demonstrating colour-tunable property of UOP with a low temperature stimulus.Open in a separate windowFig. 1Molecular structures and UOP photographs of MCzT (a) and MTOD (b).In order to explore the reasons for the colour-tunable UOP of MTOD at low temperature. The photophysical properties of MTOD in the crystal state were first investigated under ambient conditions. As shown in Fig. 2a, the photoluminescence (PL) spectrum of MTOD shows two main emission peaks at 375 and 413 nm and a shoulder at 435 nm. From the lifetime decay profiles (Fig. S5a^†), it was confirmed that they were all assigned to the fluorescence. Notably, the crystals of MTOD presented yellow afterglow after turning off the UV lamp. From the delayed phosphorescence spectrum, the main emission of MTOD was located at 414, 556 and 600 nm with lifetimes of 681.91, 860.56 and 860.59 ms, respectively (Fig. 2b and S5b^†). Among these, the emission around 414 nm is assigned to triplet–triplet annihilation (TTA) fluorescence originating larger π–π overlaps of carbazole groups from the crystal data (Fig. S6^†).²⁶ Remaining two emission peaks are attributed to UOP emission. From the phosphorescence excitation-emission spectra of MTOD, UOP can be efficiently excited from 260 to 380 nm, with the optimal excitation at 360 nm (Fig. S7^†).Open in a separate windowFig. 2Photophysical properties of MTOD at room temperature and at 77 K. (a) Steady-state photoluminescence (PL, blue dashed line) and phosphorescence (red solid line) spectra at RT and 77 K. Inset: photographs taken after removing excitation. (b) Time-resolved phosphorescence decay of the emission bands at 556 and 600 nm at room temperature, respectively. (c) Time-resolved phosphorescence decay of the emission bands at 478, 564 and 612 nm at 77 K, respectively. (d) PL (blue dashed line) and phosphorescence (red solid line) spectra at molten state.Subsequently, we measured the PL and phosphorescence spectra of MTOD at 77 K (Fig. 2a). The steady-state PL spectrum showed four peaks at 361, 376, 410 and 436 nm, with little change compared with its corresponding spectrum at RT. However, a new phosphorescence peak appears at about 480 nm with an intense emission than others, which displays an ultralong lifetime of 2.5 s. Obviously, the new emission peak plays an indispensable role in the colour-changed UOP at 77 K. The yellowish-white UOP observed by naked eyes at 77 K is generated by the combination of three phosphorescence peaks. However, as time goes by, only the blue-green afterglow at about 480 nm due to the shorter lifetime of long-wavelength phosphorescence can be observed.To find out the origin of this new peak at 480 nm, the PL and phosphorescence spectra of MTOD at molten state were measured as amorphous emission. As shown in Fig. 2d, the PL peak is located at 380 nm and the phosphorescence spectrum shows a broad band with a main peak at 484 nm. It is suggested that the new phosphorescence emission peak at 77 K may be attributed to amorphous state. Taken together, we deduce that the colour-tunable property of the crystal is possibly due to the presence of an amorphous state at low temperature.The X-ray single crystal diffraction of MTOD crystal was taken to explore the mechanism of UOP at room temperature. Abundant intermolecular and intramolecular interactions (Fig. 3a, S6, S8 and S9a^†) in the crystals strongly restrict the torsional molecular configuration. The dihedral angle between the triazine and carbazole groups is about 75° (Fig. S10^†). In the crystal of MTOD, the single molecule is limited by multiple intermolecular interactions, including C–H⋯N (2.681 Å), π-H⋯π (2.791, 2.876 Å), π-H⋯N (2.703, 2.711 Å) (Table S3^†). The rich intermolecular interactions are beneficial to limit molecular motions to suppress non-radiative transitions of excited molecules, leading to UOP. However, the amorphous molecules around the crystals with weak restriction displayed negligible phosphorescence at room temperature due to the strong motions of alkyl chains.Open in a separate windowFig. 3Intermolecular interactions (a) at room temperature and (b) at 100 K.Comparatively, the single crystal of MTOD molecule at 100 K was measured in order to explore the colour-changing mechanism of UOP at low temperature (Fig. 3b and S9b^†). By comparison, MTOD crystals exhibit more intermolecular interactions at 100 K and the distance become shorter. Molecular conformation of MTOD changed slightly, the dihedral angle between triazine and carbazole changed from 74.95° to 75.32° (Fig. S10^†). These increased interactions can constrain the molecules more effectively and the stronger restriction of alkyl chain and carbazole will further suppress non-radiative transitions, resulting in the much longer phosphorescence lifetime of over 1.0 s at low temperature. Compared with the molecules in the crystalline state, freezing condition can provide a more rigid environment to minimize the movement of the alkyl chain, greatly reducing the non-radiative transition rate at amorphous state, resulting in the lifetime of short wavelength phosphorescence at 478 nm up to 2.5 s.According to the above results, the photophysical process of colour-tunable phosphorescence can be described by Jablonski diagram as shown in Fig. 4. Upon photoexcitation, both electrons in amorphous and crystalline molecules transforms to lowest singlet states (S₁). Then, the electrons in S₁ would further transform to the lowest triplet (T₁) through ISC. At room temperature, amorphous molecules show strong molecular motions to facilitate the non-radiative transitions. However, crystalline molecules due to closely arrangement can exhibit phosphorescence through radiative decay. At 77 K, both excitons in amorphous and crystal states are dominated by radiative transitions, leading to colour-tunable UOP.Open in a separate windowFig. 4(a) Jablonski diagram of the relevant photophysical processes illustrating amorphous and crystalline UOP process at room temperature (top) and 77 K (bottom). (b) Phosphorescence spectra of MTOD at different temperature and (c) corresponding coordinates in CIE.In view of the interesting luminescent phenomenon, we have investigated a series of phosphorescent spectra of MTOD crystals at different temperatures ranged from 183 to 273 K. As shown in Fig. 4b, as the temperature increases, the phosphorescence intensity of the amorphous molecules gradually decreases. The colour variations of the MTOD crystals in response to the environmental temperatures are shown in the Commission International de l’Eclairage (CIE) coordinate diagram (Fig. 4c). As the temperature was gradually changed from 183 to 273 K, the UOP changed from green to yellow with good linearity of the CIE coordinates. This demonstrated that MTOD crystals may have potential in low temperature sensing.In conclusion, we synthesized a colour-tunable single-component UOP compound through alkyl chain engineering. Combined the spectral and single crystal analyses, it is indicated that colour-tunable UOP comes from dual-emission of molecules at amorphous and crystalline states. Low temperature provides better rigid effect on UOP of amorphous molecules than crystals, resulting in the UOP colour changed from yellow-white to blue-green. More interestingly, red-shifted UOP of MTOD crystals with the increase of temperature can be achieved, demonstrating its potential for temperature sensing. This study will provide a platform for the design of single-component UOP molecules with tunable colour emission and broadens its application field.     

Theoretical hydrogen bonding calculations and proton conduction for Eu(iii)-based metal–organic framework

A water-mediated proton-conducting Eu(iii)-MOF has been synthesized, which provides a stable proton transport channel that was confirmed by theoretical calculation. The investigation of proton conduction shows that the conductivity of Eu(iii)-MOF obtained at 353 K and 98% RH is 3.5 × 10⁻³ S cm⁻¹, comparable to most of the Ln(iii)-MOF based proton conductors.

A water-mediated proton-conducting Eu(iii)-MOF has been synthesized, which provides a stable proton transport channel that was confirmed by theoretical calculation.

In recent years, with the aggravation of environmental pollution and growing depletion of petroleum, coal and other traditional fossil energy, the demand to exploit alternative cleaner energy is increasingly urgent. Compared to the dispersion of several developed new energy sources, such as solar energy, wind, geothermal heat, and so on,¹ the proton exchange membrane fuel cell (PEMFC) is recognized as a promising energy conversion system.² As an important component in PEMFC, the proton exchange membrane (PEM) directly affects the transmission efficiency of protons between electrodes.³ Currently, Nafion has been widely used as a PEM in commerce, and shows a conductivity higher than 10⁻¹ S cm⁻¹.⁴ However, the large-scale applications of Nafion are limited due to their high costs, narrow working conditions (low temperature and high relative humidity), amorphous nature, etc.⁵ To overcome these limitations, several types of proton-conducting materials have been explored over the past decade.⁶ Among them, MOF materials were employed as ideal platforms to regulate proton conductivity owing to their high crystallinity, tunable structure and tailorable functionality. The crystallographically defined structure is also conductive to the deeply analysis of proton transport path and mechanism,⁷ furthermore, due to the visual structure of MOF, Density Functional Theory (DFT) is recently used to analyse the factors that affect proton conduction from a theoretical perspective, thus providing strong support for the experimental results.⁸ Multi-carboxylate ligands usually exhibit versatile coordination modes and strong complexing ability to metal ions. Moreover, the hydrophilic –COOH groups not only donate protons but also facilitate the formation of continuous hydrogen bond channel with water molecules. Simultaneously, selecting lanthanide metal ions as nodes in the construction of MOF, more water molecules tend to be bound by Ln(iii) ions, leading to an increase in the concentration of proton carrier, which would be beneficial for the effective proton transport. Therefore, the carboxylate-bridged Ln(iii)-MOF are good candidates for proton conduction.⁹ Currently, there are several proton conductive MOF materials, such as {H[(N(Me)₄)₂][Gd₃(NIPA)₆]}·3H₂O (σ = 7.17 × 10⁻² S cm⁻¹, 75 °C, 98% RH),¹⁰ Na₂[Eu(SBBA)₂(FA)]·0.375DMF·0.4H₂O (σ = 2.91 × 10⁻² S cm⁻¹, 90 °C, 90% RH)¹¹ and {[Tb₄(TTHA)₂(H₂O)₄]·7H₂O}_n (σ = 2.57 × 10⁻² S cm⁻¹, 60 °C, 98% RH)¹² that showing ultra-high conductivities (>10⁻² S cm⁻¹). These superprotonic conductors provided advantageous supports for the assembly strategies involved Ln(iii) ion and carboxylate ligand. In this work, 1,3,5-triazine-2,4,6-triamine hexaacetic acid (H₆TTHA) and Eu(NO₃)₃·6H₂O were assembled at 140 °C for 72 h through solvothermal reaction to afforded colourless crystals, namely {[Eu₂(TTHA)(H₂O)₄]·9H₂O}_n (1). This complex has been previously reported by Wu and co-workers.¹³ In their work, the thermal stability and fluorescence properties of 1 were mainly focused. Research suggested that the complex 1 maintained structural stability until 400 °C and demonstrated strong fluorescent emission with high quantum yields (Φ > 70%), treating as a good candidate for light applications. To the best of our knowledge, MOFs usually exhibit a variety of potential applications for their structural diversity.¹⁴ For different researchers, their concerns about the applications of MOF may vary, but it is the continuously exploration and excavation of different performance that will enrich their potentials and meet them in different fields of the applications. Through careful structural analysis, we found that there is a rare infinite water cluster ((H₂O)_n) existing in the crystal structure of 1 (Fig. S1^†), (H₂O)_n further interacts with –COO⁻ groups to form an abundant hydrogen bond network (Fig. S2 and Table S1^†). The stability of (H₂O)_n as well as more complex hydrogen bond formed between (H₂O)_n and –COO⁻ groups has been confirmed by the density functional theory (DFT) calculations. The advantageous structural features including high concentration of water molecules and stable hydration channel provide the possibility to realize high proton conductivity of 1. Therefore, the proton conductivities of 1 under varying conditions were investigated in detail.Complex 1 crystallizes in the monoclinic space group C2/c, with the asymmetric building unit composed of two Eu(iii) ions, one [TTHA]⁶⁻ anion, four coordination water molecules and nine lattice water molecules. The Eu(iii) atom is distorted enneahedron coordinated by seven carboxylate oxygen atoms and two water molecules (Fig. 1a and Table S2^†). The bond length of Eu–O is in the range of 2.374(5)–2.606(5) Å (Table S3^†), comparable to that of the Eu(iii) complex reported in the literature.¹⁵ The coordination mode of [TTHA]⁶⁻ can be described as μ₆-η²η¹η¹η¹η¹η¹η¹η¹η¹η¹η¹η². In the complex 1, the adjacent metal ions were connected through O–C–O and μ–O bridging, forming a dimer, [Eu1]₂. The dimer acts as a linker and connects with four [TTHA]⁶⁻ (Fig. 1b). Furthermore, the [TTHA]⁶⁻ anions coordinate with [Eu1]₂ through six flexible arms in different directions, leading to the formation of a three-dimensional network structure, where the cavities with regular size of 8.356 × 10.678 Ų are left (Fig. 1c and Fig. S3^†). The topological representation of the network of 1 was analysed by using TOPOS software.¹⁶ As shown in Fig. 1d, the Eu(iii) ions are connected to four [TTHA]⁶⁻, which can be considered as 4-connected nodes. And the [TTHA]⁶⁻ anions were also viewed as 4-connected nodes for their connections with four Eu(iii) ions. So, the whole 3D structure was described as a 4,4-c net with an extended Schläfli symbol of {4²,8⁴}.Open in a separate windowFig. 1Coordination mode of the [TTHA]⁶⁻ in 1 showing [EuO₉] enneahedron (a). The dimer, [Eu1]₂, formed by O–C–O and μ–O bridging, connects with four [TTHA]⁶⁻ (b). The 3D structure of 1 formed by the coordination of Eu(iii) and [TTHA]⁶⁻ as well as water molecules (c). Topological representation of the network of 1 (d). Symmetry codes (i: 1.5 − x, 1.5 − y, 1 − z; ii: 0.5 + x, 1.5 − y, −0.5 + z; iii: x, 2 − y, −0.5 + z; iv: 2 − x, 2 − y, 1 − z; v: 2 − x, y, 0.5 − z; vi: 1 − x, y, 1.5 − z; vii: 0.5 + x, 0.5 + y, z).In 1, the theoretical hydrogen bonding calculations of (H₂O)_n and complex cluster were performed using the Gaussian 09 program. All the structures were obtained from the analysis of XRD results and the hydrogen atoms are optimized. We calculated the energy at DFT level by means of B3LYP-D3.^17a As polarity of molecule has great influence on intermolecular hydrogen bonding,^17b hydrogen bond-forming orbitals require larger space occupation.^17c Thus, diffuse and polarization functions augmented split valence 6-311+G(d,p) basis set is used. The binding energy (E_binding) is calculated as the difference between the energy of hydrogen-bonded cluster and the summation of the energies of each component monomer: E_binding = E_tol − ∑N_iE_iE_tol and E_i are energy of hydrogen-bonded cluster and each individual component monomer, respectively. A hydrogen-bonded cluster is more stable if interaction energy is more negative compared to other hydrogen-bonded configurations. With the help of density functional theory (DFT), we calculate the binding energies (E_binding) to compare the stability of systems. The binding energy of water cluster and complex cluster is −619.65 and −710.34 kcal mol⁻¹ (Fig. 2), respectively, indicating the complex cluster system is more stable.Open in a separate windowFig. 2The structures of water cluster and complex cluster. Oxygen, hydrogen, carbon, nitrogen atoms are marked by red, white, cyan, blue, respectively.The PXRD patterns of 1 were shown in Fig. S4.^† It was found that the diffraction peaks of powder sample are in good agreement with the simulated data from single-crystal diffraction, showing the high purity of the synthesized sample. The IR spectrum of 1 exhibits a strong peak at 3422 cm⁻¹, which corresponds to the stretching vibration of water molecules.^18a The absorption peaks appeared at 1551 cm⁻¹ and 1400 cm⁻¹ are attributed to the antisymmetric stretching of –COO⁻ groups^18b (Fig. S5^†). The water adsorption property of 1 was investigated at 25 °C by DVS Intrinsic Plus. Before the measurement, the sample was treated under 0% RH for 6 h (Fig. S6^†). Water adsorption and desorption isotherms of the fully dehydrated sample were shown in Fig. S7.^† The adsorption process in the RH range of 0–95% can be divided into three stages. In the initial stage (0–10%), the adsorption of water molecules increased rapidly, which can be attributed to the hydrogen bond interaction between carboxylic acid oxygen atoms and water molecules. Then the water adsorption increased slowly at 10–70% RH, corresponding to the formation of water clusters. Another abrupt increase of water adsorption was found when the RH is above 70%, illustrating that enough energy is needed for the water clusters to exist in the cavity of the crystal.¹⁹ Clearly, large hysteresis was observed in the adsorption–desorption isotherms, this phenomenon was caused by the strong hydrophilic of –COO⁻ groups in 1.²⁰ Furthermore, the structural integrity of the sample after adsorption/desorption cycle was confirmed by PXRD (Fig. S4^†).Based on the previous structural analysis, the proton conduction of 1 was evaluated by the alternating-current (AC) impedance analyses. The Nyquist plots of 1 obtained at different temperature and relative humidity are shown in Fig. 3a and b and Fig. 3d. The resistance is estimated from the intercept of spikes or arcs on the Z′ axis, and the conductivity (σ) is calculated by the equation of σ = l/(A·R), where l, A and R represent the sample thickness, surface area and resistance, respectively. It was found that there are two different modes observed from the impedance spectroscopies under lower relative humidity (60–90% RH), a partial arc at high frequency component can be attributed to the grain interior contribution, while a characteristic spur at low frequency component illustrates that partial-blocking electrode response allows limited diffusion.²¹ So, the only spikes displayed in the Nyquist spectra at 98% RH and 293–353 K suggest that high temperature and high relative humidity are more favourable for the proton conduction. From the temperature-dependent measurements under 98% RH, significantly, the conductivity of 1 increases gradually from 1.34 × 10⁻⁴ S cm⁻¹ at 293 K to 3.5 × 10⁻³ S cm⁻¹ at 353 K (Fig. 3c and Table S4^†). The increasing conductivity can be attributed to the important role of water molecule. The high concentration of water molecules act as carriers and transmit in the form of H⁺(H₂O)_n, and the mobility of H⁺(H₂O)_n accelerates with the rising temperature. Moreover, the higher acidity of water molecules at higher temperature is more conducive to the improvement of proton conductivity. The relative humidity dependence measured at 298 K indicated that the conductivity of 1 presented significant positive correlations with the humidity changes. The conductivity is 1.42 × 10⁻⁵ S cm⁻¹ at 60% RH and increases to be 1.63 × 10⁻⁴ S cm⁻¹ at 98% RH (Fig. 3e and Table S5^†). This can be explained by the ability of (H₂O)_n to bind water molecules and strong hydrophilic of –COO⁻ group that has been confirmed by the water adsorption process, especially when the RH is above 60%. For water-mediated proton conductors, the lower RH usually results in the insufficient of transport media and further affects the diffusion of protons. At present, the theoretical simulations (e.g. aMS-EVB3)²² and activation energy (E_a)^23–27 are the main methods to analysis the proton conduction mechanism. Compared with the theoretical calculations, the judgment rule with E_a is more straightforward. Here, the E_a of 1 determined from the linear fit of ln(σT) vs. 1000/T is 0.44 eV (Fig. 3f), which reveals that the proton transfer in 1 follows a typical vehicle mechanism.¹² Further evaluate the long-term stability of 1, the time-dependent proton conductivity has been conducted, indicating negligible decline of proton conductivities even lasted 12 h (Fig. 4, S8 and Table S6^†). The sample of 1 after property measurements was collected and characterized by PXRD to examine any structural change, and the PXRD spectrum shows structural integrity even at high temperature and high relative humidity environment (Fig. S4^†). The long-term stable proton conductivities of 1 can be attributed to the robust hydrogen bonding channel that has been confirmed by the DFT calculations. In recent years, the proton conductive carboxylate-based MOF have been systematic reviewed by G. Li’ group,⁹ it was found that the complex 1 shows higher conductivity of 3.5 × 10⁻³ S cm⁻¹ under 353 K and 98% RH when compared to the Ln(iii)-MOF materials, such as [Me₂NH₂][Eu(ox)₂(H₂O)]·3H₂O (σ = 2.73 × 10⁻³ S cm⁻¹, 95% RH, 55 °C),²³ {[Gd(ma)(ox)(H₂O)]_n·3H₂O} (σ = 4.7 × 10⁻⁴ S cm⁻¹, 95% RH, 80 °C),²⁴ (N₂H₅)[Nd₂(ox)₄(N₂H₅)]·4H₂O (σ = 2.7 × 10⁻³ S cm⁻¹, 100% RH, 25 °C),²⁵ {[SmK(BPDSDC)(DMF)(H₂O)]·x(solvent)}_n (σ = 1.11 × 10⁻³ S cm⁻¹, 98% RH, 80 °C),²⁶ [Nd(mpca)₂Nd(H₂O)₆Mo(CN)₈]·nH₂O (σ = 2.8 × 10⁻³ S cm⁻¹, 98% RH, 80 °C),²⁷ MFM-550(M) and MFM-555(M) (M = La, Ce, Nd, Sm, Gd, Ho) (σ = 1.46 × 10⁻⁶ to 2.97 × 10⁻⁴ S cm⁻¹, 99% RH, 20 °C)²⁸ as well as other conductive materials showing lower conductivities in the range of 10⁻⁹ to 10⁻⁵ S cm⁻¹.⁹ However, the conductivity of 1 is inferior to those Ln(iii)-MOFs with conductivities higher than 10⁻² S cm⁻¹ × ^10–12 In recent years, another two H₆TTHA-derived MOF and CP, {[Tb₄(TTHA)₂(H₂O)₄]·7H₂O}_n¹² and {[Co₃(H₃TTHA)₂(4,4′-bipy)₅(H₂O)₈]·12H₂O}_n^19b have been previously reported by our group, which show highest proton conductivities of 2.57 × 10⁻² S cm⁻¹ at 60 °C and 8.79 × 10⁻⁴ S cm⁻¹ at 80 °C under 98% RH, respectively. The noticeable performance difference between these two complexes and 1 was analysed based on the visual structures. The higher conductivity of 1 when compared to the Co(ii) complex can be attributed to the concentration of water molecules, 23.25% for 1 and 15.92% for the Co(ii) complex. The high concentration of proton carrier in 1 promotes the transfer of protons. Although the water molecular concentration of 1 is higher than that of the Tb(iii) complex, however, the coordination numbers of Ln(iii) ions in the two compounds are different, eight for the Tb(iii) ion and nine for the Eu(iii) ion, respectively. The coordination sites are obviously not satiated, especially for the Tb(iii) complex, the lower coordination number may prone to chelate more water molecules under high relative humidity, leading to the formation of more consecutive hydration channel with TTHA⁶⁻ anions and water molecules, thus accelerating the proton transport. In contrast, the molecular structure of the Eu(iii) compound contains nearly a quarter of water molecules, these water molecules have almost filled the pores, so the smaller pore structure is difficult to accommodate more adsorbed water molecules.Open in a separate windowFig. 3Nyquist plots for proton conductivity of 1 (98% RH) at 293–313 K (a) and 318–353 K (b). Plot of log(σ) vs. T for 1 in the temperature range of 293–353 K (c). Plots of the impedance plane for 1 at different relative humidities and 298 K (d). Humidity dependence of the proton conductivity at 298 K (e). Arrhenius plot of 1 at 98% RH (f).Open in a separate windowFig. 4Time-dependent proton conductivity of 1 at 343 K and 98% RH.In conclusions, a water-mediated proton-conducting Eu(iii)-MOF has been synthesized, displaying a 3D network structure with high concentration water molecules and –COO⁻ groups as well as abundant H-bond networks. Interesting, there is an infinite water cluster of (H₂O)_n existing in the crystal structure of Eu(iii)-MOF, which is rare in the H₆TTHA-derived complexes and even other reported MOF/CPs. Based on this, the density functional theory was conducted to evaluate the stability of water cluster and complex cluster. As expected, the calculated binding energies indicate that the more stable system was formed by (H₂O)_n and –COO⁻ groups, which provides a favourable guarantee for proton conduction. The advantageous structural features of Eu(iii)-MOF result in the realization of comparable proton conductivity of 3.5 × 10⁻³ S cm⁻¹ at 353 K and 98% RH and long-term stability at least 12 h. Additionally, the factors affecting the electrical conductivity of several H₆TTHA-derived MOF/CPs have been compared and analysed from the visual structures, and the structure-activity relationship of such compounds was also summarized, which will provide guidance to design novel crystalline superprotonic conductors assembled from multi-carboxylate.     

Determination of berberine in Rhizoma coptidis using a β-cyclodextrin-sensitized fluorescence method

In this work, a method for the determination of berberine in Rhizoma coptidis using β-cyclodextrin-sensitized fluorescence technology is established. Berberine is the main extract of Rhizoma coptidis, a medicinal material, which causes an envelope reaction with β-cyclodextrin to generate fluorescence sensitization. In the environment of its own aqueous extract, with 0.0065 mol L⁻¹ of β-cyclodextrin, a fluorescence excitation wavelength (λ_ex) of 345 nm and an emission wavelength (λ_em) of 540 nm were selected to avoid interference from other distractors. The fluorescent sensor for the detection of berberine exhibits a low limit of detection (3.59 × 10⁻⁹ mol L⁻¹) and a wide linear range from 2.7 × 10⁻⁷ mol L⁻¹ to 2.7 × 10⁻⁶ mol L⁻¹. Our sensor can be also used to detect berberine in real medicinal materials. The content of berberine in Rhizoma coptidis medicinal material was found to be 7.60% using this method with an average recovery rate of 99.5%. The result obtained by thin-layer chromatography with fluorescence detection was 7.61%, which is consistent with the result from the β-cyclodextrin sensitized fluorescence method. This method is simple and environmentally friendly with high sensitivity and good selectivity and gives reliable results, which is promising for practical application.

In this work, a method for the determination of berberine in Rhizoma coptidis using β-cyclodextrin-sensitized fluorescence technology is established.

Rhizoma coptidis¹ is the dried rhizome of Coptis chinensis (Chuan Lian), Coptis chinensis (Ya Lian) or Yunlian (Chuan Lian), and is an important traditional Chinese medicine commonly used in clinics. Rhizoma coptidis has the functions of clearing heat and dampness, purging fire and detoxifying.² Its active ingredients are isoquinoline alkaloids,³ such as berberine hydrochloride (also known as berberine), coptisine hydrochloric (also known as coptisine), palmatine hydrochloride (also known as palmatine), and jatrorrhizine hydrochloride (also known as jatrorrhizine). Berberine is the main effective component of Rhizoma coptidis. Antibacterial,⁴ antiviral,⁵ and anti-diabetic⁶ effects of berberine have been reported, as well as its use for the treatment of liver cancer .⁷ Thus, the content of berberine is critical to the quality control of Rhizoma coptidis.At present, many methods are used for the determination of berberine in Rhizoma coptidis, such as UV-vis spectroscopy, capillary electrophoresis with field amplified sample stacking, high-performance liquid chromatography, high-performance liquid chromatography-mass spectrometry, near-infrared spectroscopy, proton nuclear magnetic resonance and nuclear magnetic resonance proton spectroscopy.^8–14 However, there are no relevant reports on the systematic study of fluorescence spectroscopy for berberine. Fluorescence analysis is the direct measurement of the secondary light emission of a substance. Fluorometric analysis has the advantages of high sensitivity and good selectivity compared to photometric methods and is simple, rapid and inexpensive compared to chromatographic methods. It is gradually receiving increasing attention for the analysis of proprietary Chinese medicines.^15–18Cyclodextrins (CDs), as shown in Fig. 1, are cyclic oligosaccharides consisting of (α-1,4)-linked α-d-glucopyranose units. The primary hydroxyl group extends at one end and the secondary hydroxyl group extends at the other end. Because the primary hydroxyl group can rotate freely to cover a part of the ring, while the secondary hydroxyl group is relatively rigid, CDs are not cylindrical but slightly tapered. The glycoside oxygen atom with an unbonded lone pair of electrons in the glucose group points to the center of the molecule, which has a high density. As a result of these factors, the center of the molecule is hydrophobic and the surface is hydrophilic. As shown in Fig. 1(b), it can match the size and cavity of the molecule, and the hydrophobic guest molecules form an inclusion complex.¹⁹ Cyclodextrin can form an inclusion complex with dapoxyl sodium sulfonate (DDS). A large fluorescence enhancement of DSS was observed upon formation of the inclusion complex.^20,21 The formation of inclusion complexes by cyclodextrin has the potential for tuning the optical properties of other chromophoric molecules by varying the cavity size of the macrocyclic host molecule.²²Open in a separate windowFig. 1Molecular structure (a) and molecular shape (b) of CDs.Berberine itself has weak fluorescence but produces stronger fluorescence when it forms an inclusion complex with other substances. Berberine can form an inclusion complex with cucurbit⁷ uril and produce a highly sensitive fluorescence response.²³ Berberine can form an inclusion complex with β-CD and a new spectrofluorimetric method for the determination of berberine in the presence of β-CD was developed. The linear range was 1.00–4.00 μg mL⁻¹ with a detection limit of 5.54 ng mL⁻¹.²⁴Both the collision between the fluorescent molecules and the quenchers around the fluorescent molecules can cause the inactivation of the fluorescent molecules and produce non-radiation. When the fluorescent molecule berberine enters the hydrophobic cavity of β-CD to form the inclusion compound, β-CD plays a shielding role, making berberine reduce the non-radiation deactivation process and quenching process, thus improving the fluorescence quantum yield.^25,26 It has been proved that both ends of the berberine molecule are matched with the pores of β-CD molecules, and 1 : 1 and 2 : 1 type inclusion compounds can be formed, as shown in Fig. 2.¹⁹ The experiment found that β-CD had no fluorescence enhancement effect on other isoquinoline alkaloids, so based on the sensitization and selectivity of β-CD for berberine, a new fluorescence analysis method for the determination of berberine content in the Chinese medicine Rhizoma coptidis was proposed.Open in a separate windowFig. 2Molecular structure of berberine (a) and models of the β-CD–BH inclusion mixtures: 1 : 1 inclusion mixture (b), 1 : 1 inclusion mixture (c), 2 : 1 inclusion mixture (d).In this work, a method for the determination of berberine in Rhizoma coptidis using β-cyclodextrin-sensitized fluorescence technology is established. Berberine is the main extract of Rhizoma coptidis medicinal material, which causes an envelope reaction with β-cyclodextrin to generate fluorescence sensitization. The fluorescent sensor for the detection of berberine exhibits a low limit of detection (1.33 ng mL⁻¹) and a wide linear range from 0.1 μg mL⁻¹ to 1.0 μg mL⁻¹. Our sensor can be also used to detect berberine in real medicinal materials. The content of berberine in Rhizoma coptidis medicinal material was found to be 7.60% with this method and the average recovery rate was 99.5%. The result obtained by thin-layer chromatography with fluorescence detection was 7.61%, which is consistent with the result from the β-cyclodextrin-sensitized fluorescence method.     

Rutin-modified silver nanoparticles as a chromogenic probe for the selective detection of Fe3+ in aqueous medium

The development of nanoprobes for selective detection of metal ions in solution has attracted great attention due to their impact on living organisms. As a contribution to this field, this paper reports the synthesis of silver nanoparticles modified with rutin in the presence of ascorbic acid and their successful use as a chromogenic probe for the selective detection of Fe³⁺ in aqueous solution. Limits of detection and quantification were found to be 17 nmol L⁻¹ and 56 nmol L⁻¹, respectively. The sensing ability is proposed to proceed via an iron-induced nanoparticle growth/aggregation mechanism. A practical approach using image analysis for quantification of Fe³⁺ is also described.

The use of rutin-modified silver nanoparticles for selective detection and sensitive quantification of Fe³⁺ in aqueous solution is described.

Metal ions are key species in nature due to their essential functions in living organisms.^1,2 On the other hand, heavy metals as well as essential metals at abnormally high levels are toxic.² Iron, for instance, in addition to its popular use in industry and construction, is essential to the human body and active in biological processes. Although the trivalent form of iron is particularly important for oxygen transport in blood and the mitochondrial respiratory chain, high levels of this cation are associated with important pathologies.^3,4 The detection of metal ions in aqueous solution is traditionally performed by methods including atomic absorption spectrometry,⁵ electrochemical measurements,^6,7 and inductively coupled plasma techniques,⁸ among others. However, these techniques have important drawbacks, notably the need for sophisticated instrumentation, in addition to being time-consuming and requiring laborious procedures. To overcome these issues, the development of chromogenic and fluorogenic chemosensors for the selective detection of metal-targets has attracted great attention, especially due to the possibility of fast, sensitive and non-expensive analysis.^9,10 In the last decade, nanoscaled materials have been reported as selective probes for metal ions, including Fe³⁺.^11–24Silver nanoparticles (AgNPs) are of particular interest because of the affordable price of starting materials, ease of controlling size and morphology, possibility to functionalize their surface with organic molecules, and optical properties that enable detection of a variety of analytes via simple UV-vis spectroscopy and digital image analysis. Furthermore, applications of AgNPs are also biotechnologically relevant due to the possibility of green synthetic protocols, including the use of plant extracts,²⁵ natural sources,²⁶ glycerol,²⁷ among others.Flavonoids are secondary metabolites naturally found in fruits and other vegetables with relevant roles due to their nutritional, pharmaceutical and medicinal properties.²⁸ Because of their adequate structural features, flavonoids are candidates to be employed in the synthesis of AgNPs.²⁶ Rutin (RU), a sugar-based flavonoid, may be employed as reducing agent in the synthesis of AgNPs along with a stabilizer such as polyvinylpyrrolidone (PVP)²⁶ or used as crude plant extract component.^29,30This paper reports the use of RU-modified AgNPs (RU-AgNPs) as a chromogenic probe for Fe³⁺ in aqueous medium in the presence of ascorbic acid (AA). Sensing ability of RU-AgNPs for the selective detection of Fe³⁺ toward other metal cations was investigated with UV-vis spectroscopy analysis. These data and transmission electronic microscopy (TEM) results allowed a mechanistic proposal involved in the selective detection of Fe³⁺ by RU-AgNPs. Furthermore, a practical approach based on correlation of images of solutions obtained with a conventional smartphone and chemometrics was employed for a simpler quantification of Fe³⁺ in aqueous medium.Initially, the order of reagent combination was investigated in the synthesis of RU-AgNPs. Concentration of RU ranged from 0.10 to 0.50 mmol L⁻¹, while concentrations of other components were fixed at 0.20 mmol L⁻¹ AgNO₃, 0.10 mmol L⁻¹ AA and 0.10 M NaOH. Water was used as solvent in all cases. Narrower surface plasmon resonance (SPR) bands were obtained from adding a solution of AA and NaOH to a solution containing RU and AgNO₃ (Fig. 1a) against the addition of RU, AA, and NaOH to AgNO₃ solution (Fig. S1a – ESI^†), or the addition of RU and NaOH to a solution of AA and AgNO₃ (Fig. S1b^†). RU-AgNPs obtained from the condition presented in Fig. 1a are small (4.1 nm average diameter) and considerably polydisperse (standard deviation of 4.7 nm), however, presenting only one population (Fig. 1b and S2^†). A study of the influence of RU concentration (0.10 to 0.50 mmol L⁻¹) on the stability of RU-AgNPs over time indicated that 0.10 mmol L⁻¹ RU generates more stable RU-AgNPs (Fig. S3^†). Next, a study on the influence of pH indicated that RU-AgNPs are only stable under strong alkaline conditions (pH 12.5 or higher) (Fig. S4^†).Open in a separate windowFig. 1(a) UV-vis analysis of RU-AgNPs under strong alkaline condition (pH > 12.5) based on the order of adding reagents (RU, 0.10 to 0.50 mmol L⁻¹; AgNO₃, 0.20 mmol L⁻¹; AA, 0.10 mmol L⁻¹; NaOH, 0.10 mmol L⁻¹); (b) TEM image of RU-AgNPs under the selected condition.The ability of RU-AgNPs to sense metal cations was investigated by both naked-eye and UV-vis spectroscopy analysis (Fig. 2). The separate addition of 10 μmol L⁻¹ of several metal cations (Fe³⁺, Co²⁺, Zn²⁺, Sr²⁺, Cu²⁺. Al³⁺, Ba²⁺, Cd²⁺, Pb²⁺, Ni²⁺, Mg²⁺, Hg²⁺, Cu⁺ and Cr³⁺) to solutions of RU-AgNPs (prepared according to Fig. 1a) indicated that only Fe³⁺ induces a significant colorimetric change in the final aspect of solution after 50 minutes (Fig. 2a).Open in a separate windowFig. 2Naked-eye (a) and UV-vis (b) analysis of RU-AgNPs in absence (control, C) and presence of 10 μmol L⁻¹ of selected metal cations (Fe³⁺, Co²⁺, Zn²⁺, Sr²⁺, Cu²⁺. Al³⁺, Ba²⁺, Cd²⁺, Pb²⁺, Ni²⁺, Mg²⁺, Hg²⁺, Cu⁺ and Cr³⁺) after 50 min; (c) calibration curve obtained by the addition of different amounts (0 to 10 μmol L⁻¹) of Fe³⁺ to solutions of RU-AgNPs; (d) TEM image of RU-AgNPs after addition of Fe³⁺ (10 μmol L⁻¹). In all experiments, RU-AgNPs were prepared in the presence of ascorbic acid.The results presented in Fig. 2a are consistent with the UV-vis spectroscopy analysis (Fig. 2b). Co²⁺ ions also induce some change in the system, however at a considerably smaller extension than Fe³⁺. In this study, AA played a crucial role in the selective detection of Fe³⁺ by the referred nanoprobe. In the absence of AA, Co²⁺ (mainly) as well as other cations induce stronger colorimetric and spectral changes (Fig. S5a and b,^† respectively) in the analysis of solutions of RU-AgNPs when compared to the system containing AA. This selectivity may arise from two possible reasons: (i) preservation of RU by avoiding its oxidation in the reduction of Ag⁺ ions; (ii) coordination of ascorbate anion to cations other than Fe³⁺.Interaction of RU-AgNPs and Fe³⁺ (10 μmol L⁻¹) stabilizes after approximately 40 minutes (Fig. S6^†). Next, a calibration curve was built from the direct relationship between the absorbance at 396 nm and the concentration of Fe³⁺ (Fig. 2c), presenting a good correlation (R² = 0.9929). The limits of detection and quantification were found to be 17 nmol L⁻¹ and 56 nmol L⁻¹, respectively, which is very satisfactory.^13,14 The influence of other cations in the detection of Fe³⁺ was investigated by UV-vis spectroscopy. Fig. S7^† clearly demonstrates that there are only small changes when a second cation (30 μmol L⁻¹) is added together with Fe³⁺ to the RU-AgNPs solution.Mechanistically, the detection of Fe³⁺ by RU-AgNPs in aqueous medium proceeds via a growth/aggregation-combined process. This proposal is first evidenced by UV-vis analysis due the suppression of SPR band (Fig. 2b), a behavior consistent with the literature.^31,32 Interestingly, TEM analysis clearly shows a growth in AgNPs size after addition of Fe³⁺ to the solution (Fig. 2d), resulting in a final single AgNPs population with average diameter of 14.7 nm ± 8.9 nm. Due to the strong alkaline medium, the main specie responsible for the behavior of NPs is likely to be Fe(OH)₃. A schematic illustration of the mechanism involving aggregation of RU-AgNPs induced by the addition of Fe³⁺ is presented in Fig. 3. AgNPs are initially formed by adsorption of anionic RU to the silver surface via deprotonated 5-hydroxychromen-4-one moiety. This is supported by the literature³³ and confirmed by alteration in the 1800–1500 cm⁻¹ region of the RU infrared spectra before and after coordination with silver (Fig. S9^†). Afterwards, the addition of Fe³⁺ induces the formation of a coordination complex through an anionic catechol group, in which at least 2 : 1 ligand–Fe³⁺ stoichiometry is required for an aggregated effect.Open in a separate windowFig. 3Mechanistic proposal for growth/aggregation of RU-AgNPs in the presence of Fe³⁺. Insert: binding model for RU-AgNPs.Due to increasing interest in image processing as an analytical tool for many purposes,^34,35 Multiple Linear Regression was employed to verify the capacity of the RU-AgNPs to probe Fe³⁺ standards at distinct concentrations, as presented in Fig. 4. The curve was obtained by plotting the color absorbances RGB-based values versus the concentrations of Fe³⁺ standards after RU-AgNPs interaction. Predicted iron is a vector based on RGB values that were then extracted from the filtered images and inserted in the equation described by Beer–Lambert law in order to generate the absorbances for the construction of the analytical curve. A linear behavior between the predicted response and the measured concentrations was observed (R² = 0.9806).Open in a separate windowFig. 4Calibration curve for Fe³⁺ analysis showing the predicted iron (RGB) vs. iron(iii) concentration (1 to 8 μmol L⁻¹). Adjusted R² = 0.9806.Regression coefficients of the calibration model (using the R, G and B channels simultaneously) obtained by MLR method are shown in eqn (1):[Fe³⁺] = 44.5R − 4.9G − 16.4B + 5.11where Fe³⁺ concentration is the dependent variable (Predicted Iron), 5.1 is the intercept (β₀), 44.5, −4.9 and −16.4 are the regression coefficients of the independent variables (R, G and B channels, respectively).It is possible to observe an interesting performance of the method using the three RGB color channels allied with MLR to quantify the Fe³⁺ content, which presented reasonable deviations in its responses considering that it is simple and low-cost. The good linearity is similar to other colorimetric methods such as sodium determination in seawater and coconut water (R² > 0.91) by Moraes and coauthors,³⁶ or iron(ii) in simulated seawater (R² = 0.9993) by Gasparotto et al.³⁷Second order regression was applied to the dataset to obtain a better adjustment, resulting in an adjusted R-squared of 0.9955. RGB values were then inserted in eqn (2) in order to generate the construction of the correlation:[Fe³⁺] = 14.4R + 30.7G − 23.4B − 398RG − 348RB − 0.8BG − 978R² + 695G² + 26.9B² + 4.52This paper reports the use of AgNPs functionalized with RU as nanoprobes for selective detection and sensitive quantification of Fe³⁺ in aqueous solution. The synthesis of RU-AgNPs is reproducible, easily performed and requires no stabilizer agent other than RU. AA has a crucial role in the selectivity by either the avoidance of oxidation of RU by silver and/or coordination of ascorbate with other cations. The literature brings relevant examples of chromogenic and fluorogenic chemosensors for selective detection of Fe³⁺ in solution. Many of these artificial organic receptors present high selectivity and relevant limits of detection, requiring, however, very specific reagents and laborious synthetic procedures.^38–41 Metal-based nanoparticles have emerged as potential probes for detection of Fe³⁺.^11–16 Although effective in Fe³⁺ sensing, the synthesis of these nanoprobes require some toxic reagents, such as NaBH₄ or PVP, use plant extracts, which may lead to some drawbacks, such as the understanding of the sensing mechanism. In contrast, our method is based on commercially available, nontoxic, low-cost reagents. Fe³⁺ sensing performed satisfactorily in the 1–10 μmol L⁻¹ range, and the limit of detection obtained with this method (17 nmol L⁻¹) is comparable to the most sensitive methods reported in literature. A mechanism for the detection of Fe³⁺ by RU-AgNPs involves a combined growth/aggregation of the NPs. There is a still limited number of nanoscaled systems reported as being selective and sensitive in the detection of Fe³⁺, which reinforces the relevance of the method reported herein. The linearity range obtained by both UV-vis spectroscopy and image analysis comprises the maximum of residual Fe³⁺ in drinking water according to the European and US legislations.^15,42     

Electrocatalytic oxidation of water by the immobilized [CuII(l-ala)(Phen)(H2O)]+ complex on a self-assembled NCS− modified gold electrode

Copper(ii) complex [Cu^II(l-ala)(Phen)(H₂O)]⁺ (l-ala = l-phenylalanine, phen = phenanthroline) was immobilized over a self-assembled NCS⁻ modified gold electrode for the electrocatalytic oxidation of water. This surface anchored molecular complex can catalyze water oxidation reaction at a remarkably low overpotential of 327 mV with a current density of 0.5 mA cm⁻² at neutral pH.

Copper(ii) complex [Cu^II(l-ala)(Phen)(H₂O)]⁺ (l-ala = l-phenylalanine, phen = phenanthroline) was immobilized over a self-assembled NCS⁻ modified gold electrode for the electrocatalytic oxidation of water.

Oxidation of water is the key step for natural or artificial photosynthesis processes. In green plants, oxidation of water occurs at the Mn₄CaO₅ active site of the oxygen-evolving enzyme of photosystem II.¹ Nature converts water to dioxygen very easily, but by artificial means this is highly challenging due to its highly demanding thermodynamic and kinetic nature.² To overcome these problems several water oxidation catalysts have been developed. Despite much progress on water-oxidation catalysts, major improvements are necessary in several areas like lowering of the overpotential, increasing catalyst stability, enhancing efficiency and reducing the overall cost. In recent years, various homogeneous catalysts and electrocatalysts for the water oxidation reaction based on earth abundant metals such as manganese,^3,4 iron,^5,6 cobalt,^7,8 nickel,^9,10 and copper^11,12 have been reported. Among them copper is an attractive choice due to its high abundance, rich coordination chemistry¹³ and vital role in bio-mimetic oxygen chemistry.¹⁴ Numerous copper complexes have been used as homogeneous water oxidation electrocatalysts. Mayer and co-workers reported for the first time a homogeneous copper catalyst [(bpy)Cu(μ-OH)]₂(OAc)₂ that could electrochemically oxidize water in basic media at an overpotential 750 mV with a turnover frequency of 100 s⁻¹.¹⁵ Several copper complexes of the polyamide ligand have been shown their catalytic water oxidation property.^16,17 F. Chen et al. reported a water soluble copper complex [L–Cu^II–OAc]⁻ (where OAc = acetate, L = N,N′-2,6-dimethylphenyl-2,6-pyridinedicarboxamidate) could electrocatalyze water oxidation to evolve O₂ in 0.1 M carbonate buffer (pH 10) with an onset overpotential of 650 mV.¹⁸ P. G. Barros and co-workers reported a copper(ii)-complex with redox non-innocent tetradentate amidate acyclic ligands that can electrocatalyze the water oxidation process at pH 11.5 with overpotential of 700 mV and as the electron-donating capacity at the aromatic ring increases, the overpotential notably reduced down to around 170 mV.¹⁹ Apart from these mononuclear copper catalysts which were operated in basic conditions, an oxidatively stable dinuclear copper-based catalyst, [Cu₂(BPMAN)(μ-OH)]³⁺ (BPMAN = 2,7-[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine) has been reported, which efficiently catalyzed water oxidation at a neutral pH without decomposition during long-term electrolysis.²⁰ On the other hand, several copper species such as Cu^II-Gly,²¹ Cu(ii)-1,2-ethylenediamine²² and [Cu^II(TPA)H₂O]²⁺ (ref. 23) have been used as precursors for active heterogeneous copper oxide formation through electrodeposition in alkaline media and catalyzed the water oxidation process. A. Prevedello et al. reported a copper(ii) species with tetraaza macrocyclic ligand that can acts as heterogeneous (active copper oxide layer) and homogeneous (molecular species) electrocatalyst in alkaline (pH = 9–12) and neutral media, respectively.²⁴ All of these known water oxidation catalysts have some practical problems, for example molecule based homogeneous catalysts are prone to decomposition under moderate applied potentials. To overcome these issues, some alternative strategies have been adopted such as grafting of molecular catalysts onto an electrode surface via covalent attachment or by anchoring on the functionalized self-assembled monolayer modified electrode. Placing the catalyst at an interface reduces the amount of catalyst needed and may enhance the rate. Few reports are available on surface anchored molecular complexes of Ru,²⁵ Ir,²⁶ V,²⁷ Ce²⁸ that could efficiently electrocatalyze the water oxidation reaction, are durable for a long time and have an overpotential that is quite low (250–350 mV) compared to the homogeneous system. Still earth abundant metal containing molecular complex modified electrode systems are scarce.In the present article we have fabricated a gold electrode using copper(ii)-complex, [Cu(l-phe)(Phen)(H₂O)](ClO₄) and NH₄SCN through two step self-assembly process (Scheme 1) and characterized by spectral, electrochemical and microscopic process. The modified electrode is stable and efficiently oxidizes water to oxygen in neutral pH medium.Open in a separate windowScheme 1Electrode modification using self-assembly process.The stepwise modification of the gold electrode was monitored by using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) (Fig. 1a and b). CV of 0.5 mM [Fe(CN)₆]^3−/4− (redox probe) in 0.1 M PBS (pH 7.0) at bare gold electrode shows a quasi-reversible couple (ΔE = 80 mV) with an anodic current density of 2.7 mA cm⁻². After modification with NCS⁻ on gold electrode surface, an irreversible redox couple for [Fe(CN)₆]^3−/4− was obtained with significant decrease (around 2.5 mA cm⁻²) of anodic current density. This result supports the formation of self-assembled layer of NCS⁻ which retards the electron transfer process between electrode and the probe molecule. After immobilization of [Cu(l-phe)(phen)(H₂O)]⁺ complex on NCS⁻ modified gold electrode, the anodic current density was around 0.9 mA cm⁻² indicates the electronic communication between Au electrode and [Fe(CN)₆]^3−/4− through Cu^II-complex and in-turn confirms the proper modification. EIS study clearly shows that the charge transfer resistance (R_ct) of Cu^II-complex–SCN–Au electrode is less than SCN–Au electrode and support the CV results.Open in a separate windowFig. 1Overlaid cyclic voltammogram (a) and Nyquist plot (−Z′′ versus Z′) obtained from EIS experiment (b) of 0.5 mM K₄[Fe(CN)₆] in 0.1 M PBS (pH 7.0) at bare Au (brown curve), NCS-Au (red curve) and [Cu(l-phe)(phen)(H₂O)]–SCN–Au (green curve).In order to confirm the fabrication of Cu^II-complex on SCN–Au modified electrode a comparative CV was taken for bare Au and [Cu(l-phe)(phen)(H₂O)]–SCN–Au in 0.1 M PBS (Fig. S1^†). An anodic peak at +0.84 V versus RHE was obtained and is due to the Cu^II/III oxidation couple^29–31 and proves the presence of Cu^II-complex over SCN–Au electrode.With varying concentration of Cu^II-complex from 1.0 mM to 5.0 mM both the anodic and cathodic current were increased (Fig. S2a^†). A plot of anodic current versus concentration of Cu^II-complex gives a linear regression equation I_pa (μA) = 0.221C (mM) + 1.511 (R² = 0.998) and is shown in Fig. S2b.^† This observation also certifies a successful immobilization of Cu^II-complex on self-assembled NCS⁻ modified gold electrode. From the Fig. S3,^† it can be seen that the current density increases linearly with the increasing square root of scan rate, following the linear regression equation, J (mA cm⁻²) = 0.0063 √ν (mV s⁻¹) + 0.0247 (R² = 0.996), thereby, indicating a diffusion controlled electron transfer process at [Cu(l-phe)(phen)(H₂O)]–SCN–Au modified electrode.³²The modification process was also confirmed by using FE-SEM and EDX analysis. FE-SEM image (Fig. S4a and b^†) shows the surface morphology of bare and copper complex modified gold electrode. Bare gold shows a smooth surface morphology whereas the Cu^II-complex–SCN–Au electrode shows nearly smooth surface and supports the film formation.The EDX analysis (Fig. S4c^†) and elemental mapping images (Fig. S5^†) confirms the presence of Cu, N, O and S elements and supports the proper immobilization of Cu^II-complex on self-assembled NCS⁻ modified gold electrode.For further confirmation, FTIR Spectra of SCN–Au and Cu^II-complex–SCN–Au electrodes were recorded in the frequency range 450–4000 cm⁻¹ (Fig. S6^†). A sharp and broad peak at 2059 cm⁻¹, a weak peak at 795 cm⁻¹ and a peak at 480 cm⁻¹ are assigned as the ν(CN), ν(CS) and δ(NCS), respectively and confirms that NCS⁻ adsorbed on Au electrode surface and coordinated through N atom.³³ After immobilization of Cu(ii) complex over SCN–Au electrode surface the characteristic peaks for CN, CS and NCS are shifted to 2136 cm⁻¹, 781 cm⁻¹ and 476 cm⁻¹, respectively and supports the bond formation between Au-NCS and Cu(ii)-complex. Coordination of sulphur with copper of Cu(ii)-complex is confirmed by the presence of new bands at around 575 cm⁻¹ which is also assignable to Cu–N bond stretching for the Cu(ii)-complex.³⁴ Another indication for (Cu-S) bond is the presence of weak band at 456 cm⁻¹.³⁵ IR bands at around 3053 cm⁻¹, 1653 cm⁻¹ and 1595 cm⁻¹ are due to of CH₂, COO⁻ and NH₂ group, respectively³⁶ and supports the presence of l-ala in the Cu(ii)-complex. The peaks for O–H, N–H and C–H are observed at 3434, 2937 and 3343 cm⁻¹, respectively. From the IR data it can be conclude that the gold electrode was properly modified with NCS⁻ and [Cu(l-phe)(phen)(H₂O)]⁺.The electrocatalytic activity of the bare and modified gold electrodes for water oxidation were investigated using linear sweep voltammetry (LSV) in 0.1 M PBS at pH 7.0 in the potential window +0.6 to +2.0 V versus RHE (Fig. 2a). Oxidation of water was observed at higher potential ∼ +1.83 V versus RHE with current density 0.02 mA cm⁻² and 0.01 mA cm⁻² at bare and NCS⁻ modified gold electrode, respectively. The anodic peak potential is shifted towards less positive potential +1.58 V versus RHE and at the same time current height is increased to 0.54 mA cm⁻² when Cu^II-complex–SCN–Au electrode was used as working electrode. These results establish the electrocatalytic activity of Cu^II-complex–SCN–Au modified electrode towards the oxidation of water.³⁷ The oxidation of water by the Cu^II-complex–SCN modified gold electrode shows remarkably low overpotential of around 327 mV at J = 0.5 mA cm⁻² and onset overpotential of around 120 mV (J = 0.1 mA cm⁻²) in neutral PBS and the obtained result is comparable or in some cases quite better than the reported homogeneous Cu(ii)-complex based systems or heterogeneous copper oxide films, copper foil etc. (Table S1^†).Open in a separate windowFig. 2Overlaid LSV obtained at bare Au, SCN–Au and Cu^II-complex-NCS-Au electrode in 0.1 M PBS solution (pH 7.0) (a), overlaid LSV obtained at Cu²⁺ ion–SCN–Au electrode and Cu^II-complex–SCN–Au electrode in 0.1 M PBS solution (pH 7.0) (b).The electrocatalytic activity towards water oxidation at Cu^II-complex and Cu²⁺ ion immobilized SCN–Au electrodes was studied and shown in Fig. 2b. The LSV result shows that the anodic peak current is quite high (∼0.32 mA cm⁻²) and peak potential is less positive (∼0.15 V) in case of Cu^II-complex–SCN–Au than Cu²⁺–SCN–Au electrode suggest that the electrocatalytic activity towards water oxidation is higher in case of Cu^II-complex than Cu²⁺-ion modified electrode.To confirm that the anodic peak at +1.58 V is solely due to the oxidation of water, LSV was performed using [Cu(l-phe)(phen)(H₂O)]–SCN–Au electrode in the potential range of +0.6 to +2.0 V versus RHE in ultrapure CH₃CN containing 0.1 M tetrabutylammonium perchlorate [Bu₄N][ClO₄] (pH = 7.0). No anodic peak was observed (Fig. S7a^†), but upon addition of water a distinguished oxidative peak was appeared at +1.58 V versus RHE which proves that the water oxidation reaction taking place at the Cu(ii)-complex modified electrode surface. It was also observed that with increasing water concentration (0.1–0.5 M) the anodic peak current was increased linearly (Fig. S7b^†) which also confirms that the peak appeared at +1.58 V versus RHE is only due to the oxidation of water.³⁸Linear sweep voltammetry was carried out at low scan rate 5 mV s⁻¹ in the applied potential range 260 mV to 280 mV in 0.1 M PBS at pH 7.0. The plot of logJ versus η (overpotential) produces a Tafel slope of 49 mV dec⁻¹ (Fig. S8^†) which indicates an excellent catalytic activity of the Cu^II-complex-NCS-Au electrode towards the oxidation of water.³⁹ Fig. 3a shows the LSV at different scan rate ranging from 20–100 mV using [Cu(l-phe)(phen)(H₂O)]–SCN–Au modified electrode in PBS solution (pH 7.0). A plot of normalized catalytic current (i/ν_1/2) versus scan rates (ν) (Fig. 3b) gives an inverse relationship. This result indicates that a rate-limiting chemical step taking place prior to quick electron transfer to the electrode.⁴⁰ It also confirms that the chemical rate determining step of the catalytic process is likely to be the O–O bond formation step.⁴¹Open in a separate windowFig. 3Overlaid LSV at Cu^II-complex–SCN–Au electrode in 0.1 M PBS (pH 7.0) with increasing scan rate in the range of 20–100 mV s⁻¹ (a). A plot of normalized catalytic current versus scan rates (b).Fig. S9^† illustrates the LSVs at Cu^II-complex–SCN–Au electrode with varying pH (6.0, 6.5, 7.0, 7.5, 8.0) of phosphate buffer solution. The anodic peak potentials were shifted towards less positive potential with increasing pH of the medium. The oxidation peak potential varies linearly with pH of the medium (Fig. S9b^†) and follows the linear regression equation E_pa (V) = −0.123pH + 2.502 (R² = 0.997).The slope of 0.123 V per pH shift indicating that 1e⁻/2H⁺ couple is involved in Cu^II-complex electrocatalyzed water oxidation reaction.⁴² The influence of pH on the peak current density (J) of water oxidation at the modified gold electrode (Fig. S10^†) revealed that the J values were increased linearly up to 7.0 and then slowly decreased and thereafter decreased sharply. This observation indicates that pH 7.0 is the most effective pH for the oxidation of water by the Cu^II-complex modified electrode.A plausible mechanism for the water oxidation over Cu^II-complex modified gold electrode is given in Scheme S1.^† In the proposed mechanism, the catalytically active [Cu^III(H₂O)]⁺ complex on the gold electrode surface is formed after anodic oxidation of [Cu^II(H₂O)] at +0.84 V versus RHE. Once formed, [Cu^III(H₂O)]⁺ oxidized H₂O to O₂ at +1.55 V versus RHE in neutral pH.^41–43The stability and oxygen generation capability of the Cu^II-complex–SCN–Au electrode was investigated using controlled potential electrolysis (CPE) at +1.58 V versus RHE using in 0.1 M PBS (pH 7.0) (Fig. 4a). The CPE experiment shows that the current density (J) rapidly declines to around 0.02 and 0.38 mA cm⁻² within 20 seconds for the bare and the Cu^II-complex–SCN modified gold electrode, respectively and thereafter the current remains stable over the entire period of electrolysis.⁴⁴ The obtained result agrees the high stability of the modified Au electrode during electrolysis. The stability of the complex modified electrode was also checked by using chronopotentiometry experiment for 60 minutes at a fixed current density of 0.51 mA cm⁻² (Fig. S11^†). A stable potential was obtained during two hour long electrolysis. This result also supports the high stability of Cu^II-complex–SCN modified electrode.Open in a separate windowFig. 4Bulk electrolysis with (green curve) and without (brown curve) the Cu^II-complex catalyst over the gold electrode in 0.1 M PBS at 1.58 V versus RHE (a). O₂ evolution during controlled potential electrolysis of water at Cu^II-complex–SCN–Au electrode. Dotted line denotes the theoretical O₂ evolution with 100% efficiency (b).During controlled potential electrolysis the generated oxygen in the head space of the gas tight electrochemical cell was monitored using a fluorescent probe. The concentration of the evolved oxygen was increased nearly linear fashion (Fig. 4b). It gives around 17 μmol of oxygen within 30 minutes of electrolysis with a Faraday efficiency of 96%. The theoretical yield of generated oxygen was calculated by assuming that the obtained current is due to four electron oxidation of water.⁴⁵The long term stability of the Cu(ii)-complex modified electrode was explored by LSV measurement in 0.1 M PBS at 15 days intervals (Fig. S12^†) and almost similar LSV responses were obtained after 15^th and 30^th day (relative standard deviation was 0.03%). Thus, it can be concluded that the electrocatalytic activity of the Cu^II-complex-NCS modified electrode does not suffer from dissolution and remained active.³⁷ For further confirmation of stability of the catalyst, long term bulk electrolysis experiment was performed at +1.58 V for around eight hours (Fig. S13^†) and a constant catalytic current was obtained over the entire period of electrolysis. This observation also establishes the robustness of the system. To check the surface morphology and durability of the modified electrode, FE-SEM and EDX analysis was done after the bulk electrolysis experiment (Fig. S14^†). No considerable change of surface morphology was observed by comparing with the SEM image of the electrode surface before electrolysis (Fig. S4b^†) and the presence of different elements such as Cu, N, O and S in the EDX spectrum (Fig. S14b^†) support the stability of the catalyst.In summary, [Cu(l-phe)(phen)(H₂O)]–SCN–Au electrode was developed, characterized and applied for the electrocatalytic oxidation of water in neutral pH. The newly developed electrode was able to oxidize water at an impressively low overpotential 327 mV with a current density of 0.54 mA cm⁻². Tafel slope of 49 mV dec⁻¹ indicates excellent catalytic activity of the Cu^II-complex towards water oxidation reaction. During 30 minutes of electrolysis, 17 μmol of oxygen was produced with a Faraday efficiency of 96%. The electrode material was chip and the modified electrode was highly stable, reusable and may help for the development of commercial water oxidation catalysts.