Retracted Article: Facile preparation of bithiazole-based material for inkjet printed light-emitting electrochemical cell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

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.     

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

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.     

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.     

Assembly and functionalization of supramolecular polymers from DNA-conjugated squaraine oligomers

DNA conjugated oligomers of organic molecules are candidates for applications in the materials and medical sciences, in diagnostics, in optical devices, for delivery or for the design of complex molecular architectures. Herein, we describe the synthesis and properties of DNA-conjugated squaraine (Sq) oligomers. The oligomers self-assemble into supramolecular polymers that are amenable to further functionalization via DNA hybridization, as shown by the attachment of gold nanoparticles (AuNPs).

The assembly of supramolecular polymers of DNA-linked squaraine oligomers and their subsequent derivatization is described.

While synthetic supramolecular polymers (SPs) were primarily designed to assemble in organic solvents, similar behaviour was later also demonstrated in an aqueous environment.^1,2 The change of solvent polarity offers possibilities to design and develop additional types of supramolecular systems and materials with diverse properties.^3,4 The tuning of such properties, however, remains challenging due to difficulties in the predictability of the self-assembly behaviour of new types of building blocks. These non-covalent interactions, however, are responsible for the dynamic nature of the supramolecular assemblies and, thus, for their structural homogeneity and integrity.^5,6 Growing structural and functional complexity is of particular importance once applications beyond a simple assembly–disassembly process are becoming a focus of interest.⁷ This affects areas like the development of responsive materials,⁸ drug delivery,⁹ molecular sensors¹⁰ and signalling cascades.¹¹ Ultimately, the design of building blocks should aim at predictability of the supramolecular interactions¹² and, at the same time, enable further functionalization of the formed SPs. One very robust way of addressing these prerequisites relies on the use of DNA as the addressable molecular building block.^13,14 Materials based on DNA base-pairing and strand hybridization show great promise for the assembly of defined nanostructures and elaborated molecular systems.^15–17 Increasing efforts are put into the synthesis and construction of hybrid materials, e.g. DNA-functionalized polymers that exhibit diverse properties in hydrogels,¹⁸ responsive drug delivery systems¹⁹ or optoelectronic devices.²⁰Recently, we reported on DNA-grafted supramolecular polymers²¹ by combining oligoarenotides²² and oligonucleotides. These hybrid building blocks allow the formation of supramolecular ribbons,^21,23 nanosheets²⁴ and vesicular constructs.^25,26 Typically, these systems show a high level of reversibility by either thermal denaturation or strand displacement reactions and they respond to external factors (e.g. temperature changes). In addition, the aforementioned supramolecular polymers are amenable to further modification, e.g. for the assembly of light-harvesting photonic wires.²⁷ Both, the ability to control the precise arrangement of functional building blocks and the process of supramolecular assembly of the oligomers, demonstrate the potential of DNA-functionalized materials, as neither pure synthetic supramolecular polymers nor DNA-only based systems can easily attain similar properties.^28,29Long-wavelength, water-compatible supramolecular assemblies are especially attractive for possible applications in the medical sciences and as optoelectronic devices.^30–32 Squaraine-based systems therefore represent ideal candidates for the creation of such “long-wavelength” supramolecular materials.^33–35 Squaraine dyes exhibit molar absorptivities of up to 260 000 M⁻¹ cm⁻¹,³⁶ they absorb and emit light in the long-wavelength region of the visible spectrum and possess the well-documented property of forming aggregates in aqueous solutions.^37,38 Squaraine assemblies designed for biomedical applications are primarily based on copolymer systems. For example, hydrophobic phospholipid bilayers of liposomes (nanovesicles) with embedded squaraine dyes are promising structures for near-infrared fluorescence and photoacoustic tomography imaging.³⁹ Here, we report the preparation of squaraine–DNA block co-oligomers, their assembly into SPs and their further derivatization with AuNPs. †).^36,40 The conjugates contain a varying number of squaraine units. As described below, this number determines whether defined SPs are formed. Formation of the SPs was followed by atomic force microscopy (AFM) and/or transmission electron microscopy (TEM). After supramolecular polymerization, complementary DNA strands modified with gold nanoparticles (AuNPs) were hybridized onto the DNA-modified SPs, as illustrated in Scheme 1. Since AuNPs are easily monitored by TEM imaging, they provide a straightforward tool to visualize the specific functionalization of the SPs via hybridization.⁴¹Sequences of squaraine–DNA oligomers and AuNP-modified DNA strands. The molecular structure of the squaraine derivative (Sq) is shown in Scheme 1

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.     

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.     

A concise and sequential synthesis of the nitroimidazooxazole based drug,Delamanid and related compounds

A concise, protection-group free and sequential route has been developed for the synthesis of the nitroimidazole based FDA-approved multi-drug resistant anti-tuberculosis drug, Delamanid and anti-leishmanial lead candidate VL-2098. The synthesis required chiral epoxides (11 and 17) as key intermediates. The chiral epoxide 11 was synthesised by sequential reaction cascades viz., allylation, selective N-arylation, Mitsunobu etherification, Sharpless asymmetric dihydroxylation and epoxidation, which do not require any special/dry reaction conditions. The steps involved towards the synthesis of epoxide also worked nicely in gram scales. After the synthesis of epoxide 11, the synthesis of Delamanid was achieved by reaction with 2-bromo-4-nitroimidazole 12 with an overall yield of 27%. Similarly, anti-leishmanial lead candidate VL-2098 was also synthesized in an overall yield of 36%.

A concise, protection-group free and sequential route has been developed for the synthesis of the nitroimidazole based FDA-approved multi-drug resistant anti-tuberculosis drug, Delamanid and anti-leishmanial lead candidate VL-2098.

Imidazoles are present in a wide variety of biologically relevant molecules exhibiting diverse pharmaceutical properties. Specifically, nitroimidazole containing compounds are active therapeutic agents against a wide variety of protozoan, bacterial (anaerobic) and leishmanial infections of humans and animals.¹ The notable examples of drugs are fexindazole,² metronidazole,³ benznidazole,⁴ tinidazole,⁵etc. Nitroimidazole was also well explored in the area of tuberculosis (TB) drug discovery (the structure of nitroimidazole containing drugs and lead compounds shown in Fig. 1).^6–9 CGI-17341 (I, developed by Ciba-Geigy, India) represents one of the very earliest outcomes but was not continued because of mutagenicity.⁶ Continued efforts to overcome the mutagenic liability led to the discovery of Delamanid (II, OPC-67683, a blockbuster against multi-drug resistant-TB, approved in 2014 by the EU)⁷ and Pretomanid^8a (III, PA-824, against multi-drug-resistant-MTB, approved in 2019 by the USFDA).¹⁰ Our involvement in the area of the TB drug discovery program and nitroimidazole chemistry^8b,c motivated us to develop a new concise and improved strategy towards the synthesis of Delamanid. The first synthesis of Delamanid was done by Tsubouchi et al. in 2004 (Otsuka Pharmaceutical Ltd. WO2004033463A1) which involved 16 steps (details shown in Fig. S1 of ESI^†).^11,12 The synthesis involves protection–deprotection strategy and made the process lengthy. Later on, in 2011, Otsuka had developed another concise route for Delamanid with improved yield (upper half of Fig. 2, details shown in Fig. S2 of ESI^†). This method involved the Sharpless epoxidation of 2-methyl allylalcohol followed by ring opening with 4-bromophenol. Then coupling with 4(4-trifluoromethoxy phenyl)piperidine fragment under palladium catalyzed conditions, which in turn was synthesized in 2–5 steps.¹³ Considering the importance of Delamanid, It could be better if more concise route was developed. In this regard, here we devised a route which involves sequential addition of fragments which not only avoid protection and deprotection but also provides the advantages of avoidance of dry conditions and costly intermediates. The lower half of Fig. 2 represents the strategy of the present method used for the synthesis of Delamanid, which involve the sequential coupling with following cascade viz., allylation, selective N-arylation, Mitsunobu ether formation, Sharpless dihydroxylation, epoxidation, ring opening and cyclization using inexpensive and easily available starting materials.Open in a separate windowFig. 1Nitroimidazole containing drugs and leads.Open in a separate windowFig. 2Previous and current approaches.The present synthesis started with 2-methylallyl chloride as first starting material and its selection is because of following reasons (i) as protecting group, (ii) inexpensive (33$ per 100 mL, Sigma) in comparison to 2-methylallyl alcohol (766$ per 100 mL, Sigma) which is used in earlier reported method and (iii) provide double bond functionality to generate epoxide via Sharpless dihydroxylation approach, which operates under open atmosphere conditions and avoids the anhydrous and dry environment as required for Sharpless epoxidation, used in the previous reported methods.^7,11b,12     

Hyperpolarization study on remdesivir with its biological reaction monitoring via signal amplification by reversible exchange

Our experiments indicate hyperpolarized proton signals in the entire structure of remdesivir are obtained due to a long-distance polarization transfer by para-hydrogen. SABRE-based biological real-time reaction monitoring, by using a protein enzyme under mild conditions is carried out. It represents the first successful para-hydrogen based hyperpolarization application in biological reaction monitoring.

Hyperpolarized proton signals in the entire structure of remdesivir are obtained due to a long-distance polarization transfer by para-hydrogen. Biological real-time reaction monitoring, by using a protein enzyme under mild conditions is carried out.

Nuclear magnetic resonance is a versatile and powerful analytical method for real-time monitoring of significant bio-catalyzed reactions. However, even though NMR has great potential as a reaction monitoring system, providing much structural information, it has not been studied in depth due to low sensitivity. To enhance the sensitivity, the hyperpolarization technique has been suggested which has long been acknowledged as a breakthrough in reaction monitoring by NMR.There are major hyperpolarization techniques, generating non-Boltzmann population distribution for hyperpolarized signal to noise such as dynamic nuclear polarization,^1–5 spin-exchange optical pumping,^6–8para-hydrogen induced polarization (PHIP),^9–11 and signal amplification by reversible exchange (SABRE).^12–14 In the case of para-hydrogen-based SABRE, the substrate and the para-hydrogen bind to a catalyzing metal complex together, thus allowing polarization to be transferred to the substrate through scalar coupling. To achieve better enhancement, the iridium N-heterocyclic carbene complex¹⁵ exhibits the highest polarization transfer efficiency, which delivering an 8100-fold enhancement in ¹H NMR signal amplification relative to non-hyperpolarized pyridine. Additionally, enhancements can be increased by employing the bulky electron-donating phosphines of the Crabtree catalyst.¹⁶ Most iridium N-heterocyclic carbene catalysts are produced as [Ir(H)₂(NHC)(substrate)₃]Cl while they are capable of delivering various NMR signal gains such as ¹H,^17–1913C,^9,20,2115N,^22–2419F,^25,2631P.²⁷ Recently, a published work has described the extension of the SABRE substrate scope to include a wide range of common drugs such as tuberculosis drugs,²⁸ antifungal,²⁹ and antibiotic agents.^30,31 It is mainly N-heterocyclic compounds with low molecular weight. Hyperpolarization experiments using large molecular weight COVID-19 drug candidates have rarely been reported.³² Here, we extend the current scope of biologically relevant SABRE substrates to remdesivir and monitor its enzymatic hydrolysis.SABRE has been widely considered as a potentially promising reaction monitoring tool for the real-time reaction via hyperpolarization.^33,34 After its successful application on the amide-coupling reaction monitoring, it could not be applied to the biological reaction monitoring due to the solvent used for the reaction, mild biological reaction conditions, and mostly, small molecule polarization capacity. Therefore, its direct application on the biological reaction could open up new possibilities in the real-time reaction monitoring via hyperpolarization.To overcome this COVID-19 pandemic, many researchers have engaged in drug development including drug repurposing. Recently, remdesivir has received emergency use authorization from the FDA, as the first organic medicine to treat COVID-19 around the world. Its prodrug form has been controversial as its intact form is not detectable even after two hours post-injection and as its efficiency for all clinical treatment should also be catalyzed by several key biological reactions including esterase hydrolysis reaction.^35–37 Therefore, further research on its molecular level of understanding is still required for enhanced drug development. Along with that, repurposing nucleoside analogue drugs have been considered as the attractive future drug candidates to overcome this pandemic and beyond. They target the RNA polymerase and prevent viral RNA synthesis in a broad spectrum of RNA viruses, including human coronaviruses.^38,39 Thus, to develop the most appropriate repurposing nucleoside analogue drug candidates for COVID-19, in-depth timely research on its pharmacokinetics and pharmacodynamics at the molecular level is also essential to overcome this pandemic and its consequent recurrences.In our previous study, we reported the hyperpolarization on the several anti-viral drug candidates of COVID-19.³² However, these drug candidates have been reported rather ineffective in treating COVID-19.⁴⁰ Furthermore, even though those unprecedented hyperpolarization on the large drug molecules are new, their polarizations are only done in methanol solvent, which is not applicable in a biological form.³² Our research results indicate successful hyperpolarization of remdesivir via SABRE under mild conditions and hyperpolarization performed in DMSO, a more non-toxic solvent for in vitro application. To provide a more clinical perspective of using this technique, its biological reaction by enzyme was successfully monitored by signal enhancement via SABRE. Moreover, to widen its future applications, we added one more Ir-catalyst by matching external magnetic field condition for efficient polarization transfer to remdesivir. Our findings will expand newly applicable research areas, not only in biological reaction monitoring via NMR, but also in other biomedicine research, in order to cope with dreadful diseases in the future.Remdesivir structure has several potential key polarization sources for SABRE: nitrile, amine, and triazine. However, its complex structure and large molecular weight have been considered as the limiting factors for hyperpolarization. Fig. 1 depicts the normal signal and its hyperpolarized signal after SABRE using IMes-Ir-catalyst, which shows the different extent of hyperpolarization of remdesivir. Among the amplified proton signals, proton 6 represents the highest hyperpolarization attached to the 5′-carbon of the nucleoside. However, its polarization indicates no major difference compared to those protons in the whole structure.Open in a separate windowFig. 1Remdesivir molecular structure and its normal ¹H NMR signal in the methanol-d₄ solvent (black spectrum). Hyperpolarized signals from remdesivir after SABRE in the presence of 130 G external magnetic field in the methanol-d₄ solvent (red spectrum).Therefore, we can anticipate its polarization transfer is mostly from the SABRE-Relay^41–43 or SPINOE,^44–46 which are discussed more on conclusion. To optimize hyperpolarization, the external magnetic field is changed, and its polarization is maximized at around 130 G.However, no major difference was noted in the extent of hyperpolarization in different magnetic fields, which could have been due to Ir-catalyst''s fast exchange. Furthermore, its polarization transfer could be from other factors such as SPINOE, other than from the Zeeman effect and J-coupling matching condition. Referring to a recent study on the SABRE hyperpolarization difference between Ir-catalysts,⁴⁷ we tested the SABRE with Crabtree''s-Ir-catalyst, which indicates the different polarization number. Interestingly, its polarization confirmed that the number of hyperpolarization using the Crabtree''s-Ir-catalyst was slightly higher than IMes-Ir-catalyst. The hyperpolarization patterns in the different magnetic field also present no major changes from the different Ir-catalyst (Fig. 2 and S1^† for structures of Ir-catalysts). Remdesivir is considered to be one of the most important treatments amid this pandemic due to its usage in blocking viral RNA production, leading to an additional study on hyperpolarization in which a more biological solvent has been conducted. Interestingly, SABRE in the CD₃SOCD₃ with IMes-Ir-catalyst shows the highest polarization efficiency with approximately 17-fold enhanced signal (Fig. 3). DMSO has various biological impacts, such as the ability to increase the skin penetration of chemicals. It can pass through biological membranes, including human skin, probably by changing lipid packing structure and producing breaks in the bilayer.^48,49 This result observed in the current research opens a new possibility for applying the in vitro experiment with biological tests since DMSO has been widely used in the in vitro drug test.^50,51 Its polarization result of remdesivir among the Ir-catalyst led to higher IMes-Ir-catalyst than Crabtree''s-Ir-catalyst, different from the CD₃OD. This indicates the polarization transfer mechanism with chelating structure is not solely dependent on the solvent or catalyst. Furthermore, the polarization trend in its structure is the highest in the proton of 9, followed by the proton of 5, which bonded with 5′-carbon of the nucleoside. This different trend in each Ir-catalyst indicates that the polarization transfer efficiency varies depending on solvents in different external magnetic fields and different solvent systems.Open in a separate windowFig. 2Signal amplification value (SE) of individual protons from hyperpolarized remdesivir using IMes-Ir-catalyst and Crabree''s-Ir-catalyst.Open in a separate windowFig. 3(a) ¹H spectrum of remdesivir before (black spectrum) and after SABRE (red spectrum) in the DMSO-d₆; (b) signal amplification value (SE) of individual protons from hyperpolarized remdesivir using IMes-Ir-catalyst and Crabtree''s-Ir-catalyst.Remdesivir activates analogue, inhibits RNA-dependent RNA polymerase, and prevents viral RNA synthesis as a phosphoramidate prodrug.⁵² The activation pathway of remdesivir has been proposed to have four steps: (1) cell entrance; (2) enzymatic elimination of masking group with 2-ethylbutyl ester and phenoxy; (3) phosphorylation; and (4) incorporation into COVID-19 RNA.⁵³ The masking group of remdesivir performs increasing hydrophobicity to facilitate cellular entry.⁵⁴ Also its inventor found that the proton of 7 was shifted to 3 to 4 ppm when the masking group of remdesivir was removed.^55,56 Monitoring hydrolysis of a 2-ethylbutyl ester by esterase confirmed the proton of 7* in 3.2 ppm via hyperpolarization (Fig. 4a). The splitting pattern of 7* is controversial because of its doublet instead of quartet. It attributes to several factors. Because of the 5-membered ring intermediate from remdesivir, the splitting pattern of 7* could be affected. The initial metabolism of remdesivir produces an intermediate of cyclopentane containing 7* is made.⁵⁷ Therefore, stereochemical relations in the equilibrium among protons in 5-membered rings cannot be determined by simply measuring coupling constants, except in cases where the substitution pattern of the specific ring system has been carefully investigated. It could be covered by a water peak close to 7*. Another possibility is that hyperpolarized proton 8 of the 2-ethyl butyl group that is released by enzymatic hydrolysis can be observed. However, the cleavage of the phenoxy group is hard to prove via spectra due to little variation in the signal of aromatic region (Fig. S2^†). Its normal cleaved remdesivir signal after the same reaction time and condition was not shown in a scan and its maximum polarization was calculated by ∼22 enhancement after comparing with the multiple scan average (Fig. 4b and S3^†). To the best of our knowledge, this is the first study that conducts hyperpolarization reaction monitoring via SABRE in biological condition is conducted for the first time. Furthermore, its NMR-based reaction monitoring on a biologically important prodrug suggests that significantly wide applications in biomedical research. Its application can be significantly widened in the biomedical researches.Open in a separate windowFig. 4(a) ¹H spectra of enzymatic hydrolysis monitoring of remdesivir; elimination of 2-ethylbutyl ester group; (b) ¹H spectra of remdesivir; enzymatic hydrolysis after 120 min (black spectrum) and amplified through hyperpolarization (red spectrum) at the same time.     

Separating graphene quantum dots by lateral size through gel column chromatography

Graphene quantum dots (GQDs) prepared through photo-Fenton reaction of graphene oxide are separated via gel column chromatography. The as-separated GQDs were selectively introduced into the active layer of organic solar cells and achieved an enhancement of power conversion efficiency (PCE).

GQDs prepared through a photo-Fenton reaction were separated into eight groups with different sizes and fluorescent colors via gel column chromatography.

Graphene quantum dots (GQDs) show potential applications in photovoltaic devices, bio-probers, sensors, and catalysts.^1–6 As the properties of GQDs can be affected severely by their lateral sizes and size distributions,^7,8 to acquire GQDs with controlled size and narrow distribution is prerequisite. However, GQDs prepared directly by the methods developed so far usually assume wide size distribution which limits somehow the practical applications of GQDs.^2,9–12Recently, several protocols have been developed for post separation of GQDs, such as dialysis,¹³ ultrafiltration,¹⁴ gel electrophoresis,⁸ reverse micelle methods,¹⁵ column chromatography on silica¹⁶ or Sephadex G-25 gel,¹⁷ chromatographic separation,¹⁸ and size-selective precipitation,^19,20 but can''t satisfy the bulk production. For examples, Kim et al. successfully obtained GQDs with different sizes using dialysis bags with different interception molecular weights and a 20 nm nanoporous membrane, but with an unacceptable yield.¹³ Fuyuno et al. obtained the GQDs with different fluorescence by the size-exclusion high performance liquid chromatography (HPLC).¹⁸ Jiang et al. separated the single atomic layered GQDs from reaction mixture containing double multilayer allotropes successfully through a Sephadex G-25 gel.¹⁷ In our previous work, the GQDs generated through the photo-Fenton reaction of graphene oxide (GO) have been sorted into three categories with different fluorescence by gel electrophoresis.^2,8,20 Nevertheless, it is still challenging to obtain high quality GQDs with controlled size and size distribution which can satisfy the practical applications.Herein, we describe an efficient GQDs separation procedure via Sephadex G25 gel column. The GQDs prepared through photo-Fenton reaction of GO with wide size distribution are separated into eight groups of GQDs with different size and fluorescent colours.² The size and morphology of as-obtained GQDs were characterized by atomic force microscopy (AFM) and transmission electron microscopes (TEM) measurements. The optoelectronic properties of the GQDs were studied by photoluminescence (PL) and UV-vis absorption spectroscopy techniques. The results showed that this separating technique is very beneficial for obtaining high quality GQDs with a variety of specific sizes and properties. Finally, the as-separated GQDs were introduced into the inverted hybrid solar cells based on the poly(3-hexylthiophene) (P3HT) and poly(3-hexylthiophene)/(6,6)-phenyl-C61 butyric acid methylester (PCBM), and it is found that the solar cell containing the separated GQDs showed a higher performance than that with the raw GQDs, which verified the importance of the size separation for GQDs.The raw GQDs used in the work are first characterized using atomic force microscopy imaging. As shown in Fig. S1a and b,^† their sizes are ranged from 2 to 40 nm with obviously large size distribution, that is further confirmed by PL spectrum and image (Fig. S1c,^† and the inset). Sephadex G25 gel column, one of common size exclusion column, is widely used to purify or separate protein or peptide.²¹ Here, GQDs are separated through Sephadex G25 gel column by size and the as-separated GQDs are named as GQDs 1–8 according to the collection order. The yield of GQDs is close to 80% with this separating technique. Actually, unlike other separating methods such as multiple dialysis¹³ and ultrafiltration,¹⁴ there is almost no significant loss of GQDs in our separation process. Taking the well dispersibility of as-prepared GQDs in water into consideration, we selected water as developing solvent in this work. Their morphologies, size and size distributions are revealed by AFM imaging and the results are shown in Fig. 1. The average size of GQDs 1–8 (calibrated with the parameters of AFM tip deconvolution⁸) are of 27.5, 23.5, 15.5, 12.0, 8.5, 6.3, 5.2 and 3.0 nm, respectively, with narrow size distributions (see the as-inset histograms in Fig. 1). As shown in Fig. S2,^† the sizes of GQDs 1–8 are also measured by TEM imaging, which are in agreement with the AFM images.Open in a separate windowFig. 1Tapping mode AFM images (height) of the separated GQDs samples along the collecting order (a) GQDs 1, (b) GQDs 2, (c) GQDs 3, (d) GQDs 4, (e) GQDs 5, (f) GQDs 6, (g) GQDs 7, (h) GQDs 8. The insets are the histograms of the size.The PL and UV-vis spectra of the GQDs can reflect the size difference, too.^7,8,13,18 The top row of Fig. 2a shows the optical images of the GQDs 1–8 acquired under a daylight lamp, and they were all transparent. The bottom row of Fig. 2a shows the optical images of the corresponding GQDs 1–8 observed under a UV irradiation (302 nm), illustrating that the GQDs 1–8 have fluorescence properties, which are red, orange, yellow, green, cyan, light blue, blue, and purple, respectively. In contrast to the raw GQDs, the result indicates that the GQDs are successfully separated by size through Sephadex G-25 gel column. This should be beneficial for further exploring the relationship between the size and properties of GQDs. As shown in Fig. 2b, the PL spectra of GQDs 1–8 match well with the fluorescence photos. The peak wavelengths of their PL are 587, 565, 554, 483, 462, 452, 385, 384 nm, correspondingly.Open in a separate windowFig. 2(a) The top row is the optical images aqueous suspensions of GQDs 1–8 obtained under daylight lamp; the bottom row is the optical images of the aqueous suspensions GQDs 1–8 acquired under UV irradiation (302 nm). (GQDs-1/red, GQDs-2/orange, GQDs-3/yellow, GQDs-4/green, GQDs-5/cyan, GQDs-6/light blue, GQDs-7/blue, GQDs-8/purple). (b) PL spectra of GQDs 1–8 (the excitation wavelength is 340 nm). (c) UV-vis absorption spectra of GQDs 1–8 (the spectra were normalized at 200 nm for comparison).As shown in Fig. 2c, with the size decreasing, the absorption onset of the GQDs blue-shifted gradually. The absorption around 225 nm corresponding to the π → π* transition of sp² domains in GQDs, and the absorption in the range of 275–325 nm from the n → π* transition of C O groups at the edge of GQDs are also observed clearly, which is similar to the literature.⁸ More specifically, the absorption peak in the range of 275–325 nm becomes more and more obvious with the GQDs size decreasing, indicating the density of carboxylic groups at the edge of GQDs increases from GQDs-1/red to GQDs-8/purple. The reason is that the number of GQD carboxylic groups is directly proportional to its lateral size and the area of GQDs is proportional to the square of its size, as a result the small sized GQDs have higher density of carboxylic groups than the large sized GQDs and present the obvious peak around 275–325 nm.⁸The PL spectra of the as-separated GQDs are shown in Fig. 3 and Fig. S3.^† Comparably, the PL intensities of GQDs 3, GQDs 4, GQDs 5, GQDs 6 (Fig. 3) are stronger than those of others (Fig. S3^†). The PL emission peaks of the GQDs 3 and GQDs 4 shift more obviously than that of GQDs 5 and GQDs 6 with the increase of the excitation wavelength, implying the size distributions of GQDs 3 and GQDs 4 are worse than those of GQDs 5 and GQDs 6. For GQDs 3, as shown in Fig. 3a, there are two peaks in the emission spectra with the excitation wavelength of 360, 380, 400 nm. The left peak is attributed to the π* → n transition of carbonyl or carboxylic and the right peak is attributed to the sp² domains in carbon skeleton. For GQDs 4, as displayed in Fig. 3b, with the excitation wavelength increasing from 300 to 400 nm, the main contribution for the PL is still the sp² domains in carbon skeleton. With the excitation wavelength of 380, 400 nm, two slight shoulders occur in the emission spectra, the left is attributed to the π* → n transition of carbonyl or carboxylic, too. With the decreasing of GQDs size, the PL intensity from the sp² domains gets weak, but the one of π* → n transition increases. The emission peaks of GQDs 5 and GQDs 6 shift slightly (Fig. 3c and d), which is mainly attributed to the π* → n transition of carbonyl or carboxylic, but partly from the sp² domains in carbon skeleton.^8,22Open in a separate windowFig. 3The fluorescence emission spectra of GQDs-3/yellow samples with excitation wavelengths from 360 nm to 520 nm (a), GQDs-4/green with excitation wavelengths from 300 nm to 400 nm (b), GQDs-5/cyan with excitation wavelengths from 280 nm to 380 nm (c), and GQDs-6/light blue samples with excitation wavelengths from 280 nm to 360 nm (d).The PL quantum yields (QYs) of raw and separated GQDs are measured using quinine sulfate as a reference (QY = 57.7%),^3,23 and are summarized in Table S1.^† The QYs of the raw GQDs and GQDs 1–8 are 0.99, 0.611, 0.758, 2.592, 5.905, 1.816, 0.486, 0.259, and 0.199%, respectively. Obviously, the QYs of GQDs 3, GQDs 4, and GQDs 5 are much higher than those of others, but the QYs of GQDs 1, GQDs 2, GQDs 6, GQDs 7, and GQDs 8 are much lower than that of the raw GQDs. This may be resulted from the comprehensive factors from the quantum confinement effect, and the functional groups on the edge of GQDs.In fact, the size and surface functionality of the raw GQDs are the key factors dominating Sephadex G25 gel column separation efficiency. By simply varying the photo-Fenton reaction time, different raw GQDs are prepared. Accordingly, as shown in Fig. S4,^† GQDs assuming different fluorescent colours can be obtained. When the photo-Fenton reaction time was 90 minutes, big sized GQDs with yellow and orange fluorescence can be rarely obtained. Only blue and cyan fluorescence GQDs could be separated using G25 gel chromatography (Fig. S5^†). It can be concluded that the separating extent is seriously depended on the size and surface functionality of GQDs as-obtained via photo-Fenton reaction. Similarly, only the GQDs with blue fluorescence could be separated from the raw material of GQDs prepared by hydrothermal method⁴ using G25 gel chromatography. Recently, various GQDs prepared by reported methods are mono-fluorescence such as blue or green and they are not suitable for the suggested separating technique.^24–26 Thus, the suggested separating technique is not universal for GQDs obtained via different preparing methods.In order to explore the advantages of the as-separated GQDs, raw GQDs and GQDs 4 with the highest QY are used as additivity for the electron acceptor material PCBM, and inverted structure organic ternary hybrid solar cells (Ag/MoO₃/P3HT:PCBM:GQDs/ZnO/ITO) were assembled. The photovoltaic performances of as-fabricated solar cells were characterized, and the results are depicted in Fig. 4 and Table S2.^† The power conversion efficiencies (PCEs) of the solar cells containing raw GQDs and GQDs 4 are of 3.46% and 3.91%, respectively, which are higher than that of the control group (3.07%). Further, the performance of the solar cell with GQDs 4 is even better than that with raw GQDs, which means that the size and size distribution are crucial to the optoelectronic performances. However, the detailed mechanism of the photovoltaic performances of the as-assembled inverted structure organic ternary hybrid solar cells are not clear for us at moment, and will be further addressed in our coming work.Open in a separate windowFig. 4 J–V characteristics of the solar cells based on P3HT:PCBM:GQDs active layers with different GQDs.     

Synthesis of novel benzopyran-connected pyrimidine and pyrazole derivatives via a green method using Cu(ii)-tyrosinase enzyme catalyst as potential larvicidal,antifeedant activities

A series of benzopyran-connected pyrimidine (1a–g) and benzopyran-connected pyrazole (2a–i) derivatives were synthesized via Biginelli reaction using a green chemistry approach. Cu(ii)-tyrosinase was used as a catalyst in the synthesis of compounds 1a–g and 2a–ivia the Biginelli reaction. The as-synthesized compounds were characterized by IR, ¹H NMR, ¹³C NMR, mass spectroscopy, and elemental analysis. The as-synthesized compounds were screened for larvicidal and antifeedant activities. The larvicidal activity was evaluated using the mosquito species Culex quinquefasciatus, and the antifeedant activity was evaluated using the fishes of Oreochromis mossambicus. The compounds 2a–i demonstrated lethal effects, killing 50% of second instar mosquito larvae when their LD₅₀ values were 44.17, 34.96, 45.29, 45.28, 75.96, and 28.99 μg mL⁻¹, respectively. Molecular docking studies were used for analysis based on the binding ability of an odorant binding protein (OBP) of Culex quinquefasciatus with compound 2h (binding energy = −6.12 kcal mol⁻¹) and compound 1g (binding energy = −5.79 kcal mol⁻¹). Therefore, the proposed target compounds were synthesized via a green method using Cu(ii)-enzyme as a catalyst to give high yield (94%). In biological screening, benzopyran-connected pyrazole (2h) was highly active compared with benzopyran-connected pyrimidine (1a–g) series in terms of larivicidal activity.

Cu(ii)-tyrosinase catalytic help with the synthesis of benzopyran-connected pyrimidine and pyrazole derivatives and their larvicidal activity.

Benzopyrans (coumarins) are an important group of naturally occurring compounds widely distributed in the plant kingdom and have been produced synthetically for many years for commercial uses.¹ In addition, these core compounds are used as fragrant additives in food and cosmetics.² The commercial applications of coumarins include dispersed fluorescent brightening agents and as dyes for tuning lasers.³ Some important biologically active natural benzopyran (coumarin) derivatives are shown in Fig. 1. Mosquitoes are the vectors for a large number of human pathogens compared to other groups of arthropods.⁴ Their uncontrollable breeding poses a serious threat to the modern humanity. Every year, more than 500 million people are severely affected by malaria. The mosquito larvicide is an insecticide that is specially targeted against the larval life stage of a mosquito. Particularly, the compound bergapten (Fig. 1), which shows the standard of larivicidal activity,⁵ is commercially available, and it was used as a control in this study for larvicidal screening. Moreover, the antifeedant screening defense mechanism makes it a potential candidate for the development of eco-friendly ichthyocides. Coumarin derivatives exhibit a remarkably broad spectrum of biological activities, including antibacterial,^6,7 antifungal,^8–10 anticoagulant,¹¹ anti-inflammatory,¹² antitumor,^13,14 and anti-HIV.¹⁵Open in a separate windowFig. 1Biologically active natural benzopyran compound.Coumarin and its derivatives can be synthesized by various methods, which include the Perkin,¹⁶ Knoevenagel,¹⁷ Wittig,¹⁸ Pechmann,¹⁹ and Reformatsky reactions.Among these reactions, the Pechmann reaction is the most widely used method for the preparation of substituted coumarins since it proceeds from very simple starting materials and gives good yields of variously substituted coumarins. For example, coumarins can be prepared by using various reagents, such as H₂SO₄, POCl₃,²⁰ AlCl₃,²¹ cation exchange resins, trifluoroacetic acid,²² montmorillonite clay,²³ solid acid catalysts,²⁴ W/ZrO₂ solid acid catalyst,²⁵ chloroaluminate ionic liquid,²⁶ and Nafion-H catalyst.²⁷Keeping the above literature observations, coumarin derivatives 1a–g and 2a–i are usually prepared with the conventional method involving CuCl₂·2H₂O catalysis with using HCl additive. This reduces the yield and also increases the reaction time. To overcome this drawback, we used mushroom tyrosinase as a catalyst without any additive, a reaction condition not reported previously. The as-synthesized compounds were used for the biological screening of larvicidal and antifeedant activities (marine fish). In addition, in this study, we considered the molecular docking studies study based on previous studies for performing the binding ability of hydroxy-2-methyl-4H-pyran-4-one (the root extract of Senecio laetus Edgew) with the odorant binding protein (OBP) of Culex quinquefasciatus.²⁸     

Dual emission from nanoconfined R-phycoerythrin fluorescent proteins for white light emission diodes

A facile strategy to encapsulate R-phycoerythrin (R-PE) proteins and CdSe_xS_1−x/ZnS quantum dots (QDs) in ZIF-8 thin films is developed through a one-pot solid-confinement conversion process. The resultant R-PE/CdSe_xS_1−x/ZnS@ZIF-8 thin film exhibits high-quality white light emission and good thermal stability up to 80 °C.

The nanoconfined R-phycoerythrin protein in ZIF-8 shows dual color emissions and exhibits high-quality white light emission and good thermal stability.

White light-emitting diodes (WLEDs) have been deemed as a promising alternative to incandescent light bulbs as well as fluorescent lamps due to their high efficiency, long lifetime, and environmental friendliness.^1–3 In general, there are two main strategies to fabricate WLEDs.⁴ One is to combine individual green, red and blue LEDs. Nevertheless, this suffers from high costs, electrochemical corrosion-induced degradation and poor color stability, which restrict their further applications.^5,6 The other strategy commercially accepted is the integration of a blue- or UV-LED chip with color-conversion phosphors based on an additive color mixing principle. However, the different degradation rates between the individual chips and phosphors results in a relatively low efficiency, chromatic aberration and complicated fabrication processes.^7,8 Therefore, white light-emitting materials, which can be prepared from a single phase, are thus highly preferred. Furthermore, down-conversion materials, as explained before, must also satisfy the high luminous efficacy of radiation, thermal stability, absorption of the LED wavelength and photo-stability.⁹Fluorescent proteins (FPs)¹⁰ with a complicated polypeptide structure^11,12 have many remarkable luminescence properties, such as a narrow emission line width, good photo-stability and outstanding photon flux saturation.¹³ Ever since the discovery of FPs in 1962,¹⁴ they have been utilized for live-cell imaging,^15,16 protein labelling¹⁷ and environmental biosensors.^18,19 Lately, these versatile molecules have been applied in lighting devices.^3,20–22 Notably, FPs have been found to be a promising candidate toward eco-friendly white lighting sources, since the disposal or recycling of FPs causes negligible environmental negative effects. However, their poor thermal stability and requirements for an aqueous environment strongly restrict their applications in WLEDs.²³ To tackle these issues, Costa and co-workers reported a novel method to achieve FP-based WLEDs with a novel coating system using green, blue and red fluorescent protein-based rubber materials.³ Very recently, they demonstrated FP-based WLEDs with a micro-patterned single layer by means of a 3D plotting technique, realizing x/y color coordinates of 0.33/0.33 and a CCT of 5500 K.²¹ In contrast, Nizamoglu and co-workers directly integrated green and red fluorescent proteins on blue chips which led to a cold white light with a CCT of 8400 K.²² Significant achievements have been made in FP-based WLEDs to obtain high-quality white light, however two or three colors of fluorescent proteins are frequently used following a complicated preparation process.R-phycoerythrin (R-PE) is a kind of fluorescent protein, which carries two types of chromophores including phycoerythrobilin (PEB) and phycourobilin (PUB).^24–27 Generally, the UV-vis absorption peak of R-PE at 498 nm is attributed to PUB and those at 540 nm as well as 560 nm are attributed to PEB. In pH 6.8 and 0.1 M phosphate buffer, R-PE emitted strong fluorescence at 578 nm due to the energy transfer between PEB and PUB.^28,29 If the energy transfer between PEB and PUB is impeded,²⁶ it is possible to produce dual color emissions from R-PE proteins.Not only the fluorescent proteins but also the matrix of the FP composite membranes determine the resulting optical performance and thermal stability. Therefore, a matrix with high thermal stability and good transparency, and one that can have FPs easily and uniformly introduced into it is critical for FP-based WLEDs. Metal–organic frameworks (MOFs),^30,31 which are self-assembled from organic ligands and metal ions, have become highly promising organic–inorganic hybrid materials due to their permanent porosity, tuneable structures and diverse properties.^32–34 The porous structures of MOFs make them an ideal support for protein encapsulation. Among the thousands of kinds of MOF, ZIF-8 with well-defined cavities (11.6 Å) and a hydrophobic surface stands out due to its excellent thermal stability (up to 823 K), good hydrothermal and chemical stability, and high transparency within the visible range,^35–38 which make it a promising support for R-PE to fabricate WLEDs. However, MOF crystals applied in lighting devices are usually in the form of powders, which require an extra complicated process to coat them on a blue- or UV-LED chip.^5,39,40Herein, following a similar strategy, we synthesized film-like WLEDs by encapsulating R-PE proteins and blue-emitting (B) semiconductor QDs into a ZIF-8 thin film (R-PE/QDs@ZIF-8) at room temperature through a very simple one-pot solid-confinement conversion process.^41,42 The nanofibrous zinc hydroxide nanostrands (ZHNs) not only serve as a zinc source but also firmly confine R-PE and the blue QDs during ZIF-8 growth by reacting with methyl-imidazole (Hmim). As a result, R-PE confined in ZIF-8 dominantly emits green (518 nm) and red light (650 nm). The single orange light (578 nm) of the original R-PE was dramatically suppressed. This is probably because the polypeptide structures of R-PE are pulled apart due to electrostatic-assisted assembly along the ZHNs, and the formation of PEB–Zn and PUB–Zn complexes.^{26,27,29,43–45} The resultant R-PE/QDs@ZIF-8 thin film exhibits a high photoluminescence quantum yield (PLQY) of 29.8% as R-PE and the QDs are well isolated in the ZIF-8 thin film, which restricts aggregation-caused PL quenching.^4,5 What’s more, the prepared thin film was processed into a high-quality warm-white LED with ideal CIE coordinates of (0.34, 0.34), a high CRI value of 85 and a moderate CCT value of 4955 K. This process provides a new method to manipulate the fluorescence colors and enhance the thermal stability of FP based-thin films for WLEDs.The R-PE@ZIF-8 thin film was synthesized through a solid confinement conversion process by in situ encapsulation of R-PE into the ZIF-8 thin film at room temperature (Scheme 1). The zinc hydroxide nanostrands (ZHNs) were synthesized according to our previous procedure.^4,41,42 Then, a different volume of R-PE aqueous dispersion (24.4 μg mL⁻¹) was added into 10 mL ZHN solution. The highly positively charged ZHNs⁴⁶ composed of hexagonal clusters of [Zn₆₁(OH)₁₁₆(H₂O)_n]⁶⁺ could attract R-PE by electrostatic interaction and assemble into the R-PE/ZHNs composite nanofiber dispersions. During this process, R-PE proteins were assembled along the ZHNs, which was similar to the mixture of cadmium hydroxide nanostrands and horse spleen ferritin.⁴⁷ After filtering the R-PE/ZHNs composite nanofiber dispersion onto a polycarbonate (PC) membrane with a pore size of 200 nm, R-PE was evenly embedded in the synthesised R-PE/ZHNs composite thin film. Afterwards, this R-PE/ZHNs composite thin film was transferred onto a quartz plate by being carefully peeled off in ethanol, and immersed into a 25 mM methyl-imidazole (Hmim) ethanol/water solution (volume ratio 1 : 4) for 24 hours. During the ZIF-8 growth process, the top ZHN layer was first converted into a ZIF-8 layer. With an increase in time, the ZIF-8 layer became dense and continuous, which prohibited the release of R-PE proteins. Eventually, a very nice R-PE@ZIF-8 thin film was obtained (Fig. 1) and the R-PE proteins were encapsulated into the ZIF-8 crystal matrix (see details in the Experimental section in the ESI^†).Open in a separate windowScheme 1Schematic illustration of the synthesis process of the R-PE/QDs@ZIF-8 thin films on quartz.Open in a separate windowFig. 1Structure characterization and fluorescent properties. (a) Surface and (b) cross-section SEM images of the R-PE@ZIF-8 thin (S-2) film; (c) XRD patterns of ZIF-8 and the R-PE@ZIF-8 (S-2) thin film; (d) TEM and (e) S element and (f) Zn element mapping of one crystal scraped from the R-PE@ZIF-8 (S-2) thin film; (g) PL spectra of the R-PE@ZIF-8 (S-2) thin film and the same amount of R-PE aqueous solution excited at 405 nm.In order to investigate the effect of R-PE concentration on the structure and optical properties of the prepared thin films, R-PE@ZIF-8 thin films with different R-PE amounts of 6.5, 10 and 13.5 wt% were prepared and were respectively named S-1, S-2 and S-3 (Table S1^†). It is clear that all of the R-PE@ZIF-8 thin films were well-intergrown and continuous with thicknesses of approximately 400 nm (Fig. 1a and b and S1^†). Additionally, the representative ZIF-8 crystal phase was formed, as proven by the XRD results (Fig. 1c and S1d^†). This indicates that the incorporation of R-PE proteins has no effect on the phase of the ZIF-8 crystals. However, the grain size of the ZIF-8 crystals in the R-PE@ZIF-8 thin film decreased with the increment of the incorporated R-PE amount (Fig. S1a–c^†). This might mean that the polar functional groups of the R-PE proteins may easily complex with zinc ions and provide a nucleation site for ZIF-8 growth. Therefore, the more protein included, the more nucleation sites provided, resulting in a smaller size of ZIF-8 crystal.The TEM element mapping images (Fig. 1d–f) show that the sulfur element from the thio-ether bonds of the R-PE proteins is evenly distributed in the ZIF-8 crystals.²⁹ This indicates that the R-PE proteins are uniformly encapsulated and well distributed in the ZIF-8 thin film. As expected, when the R-PE content in the ZIF-8 thin films increased from 6.5 wt% (S-1) to 10 wt% (S-2), the PL intensity of the prepared films rose accordingly (Fig. S1e^†). However, when the content of R-PE further increased to 13.5 wt% (S-3), the PL intensity did not increase but was slightly weaker than that of S-2 R-PE@ZIF-8 with an R-PE content of 10 wt%. This means that if the content of R-PE is too high in the ZIF-8 matrix, this will cause aggregation-induced PL quenching.^4,5 Furthermore, the PL intensities of all of the R-PE@ZIF-8 thin films were far stronger than that of a drop-casted R-PE thin film on quartz with the same amount (0.0927 mg) of R-PE encapsulated in S-2 with the same diameter due to the aggregation-caused PL quenching in the R-PE casted film.^4,5 In addition, ZIF-8 itself had no obvious fluorescence emission in the emitting range of the R-PE proteins (Fig. S1e^†).It is interesting that, after confining the R-PE proteins in the ZIF-8 matrix, two new fluorescence emission peaks at 518 nm (green) and 650 nm (red) in the visible region appeared, and the emitting peak at 578 nm (orange) was significantly suppressed. Two other weaker peaks at 468 nm and 603 nm are in the alignment with that of the pure R-PE dilute solution (Fig. 1g). The UV-vis absorption spectra of the R-PE@ZIF-8 thin film (S-2) and the pure R-PE solution (Fig. S1f^†) clearly present a significant red-shift of the absorption peaks at 498 nm to 509 nm and 565 nm to 587 nm, respectively. The reason for this phenomenon is probably the interaction between the Zn ions and the chromophores of the R-PE proteins. It has been proven that after the formation of complexes PEB–Zn and PUB–Zn, with a tetrahedral structure of which the Zn ion is located at the center, the energy of the π* orbital decreased, and the π → π* and n → π* absorptions were redshifted.^{26,27,29,43–45} The absorption wavelength maximum of PUB was shifted to 509 nm, which is consistent with that of PUB–Zn salts reported in the literature (509 nm). The absorption wavelength maximum of PEB shifted to 597 nm and is also close to that of PEB–Zn salts reported in the literature (583 nm),⁴³ indicating the formation of complexes PEB–Zn and PUB–Zn. The absorption peak of PUB–Zn is due to the green fluorescence emission (518 nm) from the R-PE@ZIF-8 thin films. It has been reported that the vanish of around 510 nm emission but the enhancement of 578 nm emission of the R-PE solution is due to the energy transfer from PUB to PEB.^28,29,47 Therefore, the maximum emission peak of the R-PE@ZIF-8 thin films at 518 nm was probably because ZIF-8 pulls the polypeptide structure of the R-PE molecules apart, and the energy transfer between PEB and PUB was thus blocked.²⁶ Furthermore, the rigid structure of the R-PE proteins became loose during this process, so that it was much easier for the Zn ions to get into the proteins and react with the chromophores including PEB and PUB. The absorption peak of PEB–Zn resulted in the emission peak of the R-PE@ZIF-8 thin films at 650 nm through a red-shift.^27,43 Besides, different local nano-environments could affect the fluorescence emission characteristics of the R-PE molecules. This might be another reason for the red-shift.⁴⁸ Remarkably, the PLQY of R-PE@ZIF-8 (S-2) with 10 wt% R-PE was close to 20%, which was higher than the 11.8% of the same amount of R-PE aqueous dispersion (Table S2^†), on account of the well-isolated distribution of the R-PE molecules in the ZIF-8 matrix and the blocking of energy transfer between PEB and PUB. Now, the only orange light-emitting R-PE can emit nice green and red light simultaneously with a high PLQY, which is desirable for building red, green and blue based WLEDs by combining a blue light source.^7,8To investigate whether the metal in the MOFs could affect the emission properties of the composite films, R-PE@HKUST-1 membranes with different R-PE amounts of 3, 7 and 12 wt% were prepared and respectively named S-4, S-5 and S-6 (see details in the Experimental section in the ESI^†). The surface SEM images of S-4 and S-6 (Fig. S2a and b^†) show that all of the membranes are compact and continuous. The cross-sectional SEM image of S-4 (Fig. S2c^†) shows that the R-PE@HKUST-1 thin film was well-intergrown with a thickness of approximately 4 μm. In addition, the XRD patterns of the S-4 and S-6 (Fig. S2d^†) thin films were in accordance with those of pristine HKUST-1, implying that the incorporation of R-PE does not affect the crystalline phase of HKUST-1. Similar to R-PE@ZIF-8, with the increment of the R-PE content in the composite films, the PL intensities of R-PE@HKUST-1 increased first and then decreased (Fig. S3a^†). This might be because if the R-PE content is too high in the HKUST-1 matrix, this will cause aggregation-induced PL quenching.^4,5 Interestingly, after encapsulating R-PE into the HKUST-1 crystals, a new fluorescence emission peak at 515 nm in the visible region appeared and the emission peak of R-PE at 578 nm was significantly suppressed, which is different from R-PE@ZIF-8 (Fig. S3b^†). Therefore, the metal in the MOFs could affect the emission properties of the composite thin films. To obtain the FP-WLEDs, we chose R-PE@ZIF-8 which could emit dual color fluorescence and ZIF-8 has relatively high transparency in the visible range.Before applying the R-PE@ZIF-8 thin films in the WLED field, their thermal stability should be taken into consideration.^1,4,6,11 As mentioned before, ZIF-8 with a hydrophobic surface exhibits excellent thermal and hydrothermal stability, and is a suitable host for FP-based WLEDs.^35–38 Therefore, the optical thermal stability of R-PE encapsulated in ZIF-8 is important. As is well known, fluorescent proteins are not stable at high temperatures.^49,50 This was proven when we heated the R-PE dilute solution at 80 °C for different durations. As shown in Fig. S4d,^† the fluorescence of R-PE at 578 nm in aqueous buffer solution degraded quickly with an increase in duration. After 1 hour, it had already degraded basically, and the characteristic peaks completely disappeared. Nevertheless, after encapsulating the R-PE molecules into ZIF-8, Fig. S4c^† shows that the degradation of the PL intensity of R-PE@ZIF-8 (S-2) at 518 nm is only 0.92% after treatment at 80 °C for 3 hours in air, and 6.5% for 10 hours. This means that the encapsulation of the R-PE proteins into ZIF-8 could significantly enhance their optical stability. In addition, after thermal treatment, the crystal structure of R-PE@ZIF-8 (S-2) was reserved (Fig. S4a and b^†). It has been mentioned that the poor thermostability of FPs limits their wide applications in the lighting field. Fortunately, this problem was resolved when we encapsulated the R-PE molecules into the ZIF-8 crystals.Although the fluorescence emission of the R-PE@ZIF-8 thin films included nice green and red light, they could not emit white light on account of lacking blue light. With this in mind, blue CdSe_xS_1−x/ZnS QDs with fluorescence emission at 458 nm excited under 405 nm were added into the R-PE@ZIF-8 thin films through a similar process (Scheme 1 and more details are in the Experiment section in the ESI^†). Typically, the highly positive ZHN solution (10 mL) was mixed with a certain volume of R-PE aqueous dispersion (0.0244 mg mL⁻¹) and negatively charged QD (0.1 mg mL⁻¹) dilute solution. The R-PE molecules and QDs subsequently assembled onto the surface of the ZHNs to form R-PE/QDs/ZHNs composite nanofibers by electrostatic interactions. Then an R-PE/QDs/ZHNs composite film was formed by filtering the R-PE/QDs/ZHNs composite nanofibers on a PC membrane with a pore size of 200 nm. After transferring the composite nanofibrous thin film from the PC to a quartz substrate, it reacted with imidazole to form an R-PE/QDs@ZIF-8 thin film on quartz.To obtain white light-emitting thin films, we prepared R-PE/QDs@ZIF-8 thin films with different CdSe_xS_1−x/ZnS QD amounts of 3, 5, 10 and 13 wt% but kept the R-PE content as 10 wt%, and these were respectively named S-7, S-8, S-9 and S-10. It was obvious that the R-PE/QDs@ZIF-8 thin film (S-9) was continuous and well-intergrown (Fig. 2a and S5^†). Furthermore, the cross-section element mapping images show that the blue QDs are distributed homogeneously in the thin film with a thickness of 500 nm (Fig. 2b). In addition, the TEM images (Fig. 2c and h) and element mapping (Fig. 2d–g) further confirmed that the QDs were incorporated into the ZIF-8 crystals. The high-resolution TEM (HRTEM) image (Fig. 2i) indicated that the QDs with a diameter of approximately 4 nm were evenly encapsulated in the ZIF-8 crystals. Not surprisingly, a ZIF-8 crystal phase was obtained, as confirmed by the XRD results (Fig. S6^†).Open in a separate windowFig. 2Characterization of the R-PE/QDs@ZIF-8 thin film (S-9). (a) Surface and (b) cross-section SEM images; (c) TEM image and the corresponding element mapping for (d) Zn, (e) Cd, (f) N and (g) S, respectively; (h) high magnification and (i) HRTEM images. The insets in (b) are the corresponding element mapping of S, Zn, and Cd.Obviously, upon excitation at 405 nm, the R-PE/QDs@ZIF-8 thin film (S-9) emitted blue, green, and red light at 450 nm, 518 nm, 603 nm and 650 nm, respectively (Fig. 3a). The reasons for the appearance of green and red fluorescence emission peaks have been explained before. In addition, the characteristic emission at 458 nm was consistent with the PL emission of the blue QD dilute solution. Notably, the addition of QDs further enhanced the fluorescence emission of R-PE through some uncertain energy transfer, and this function would be strengthened by increasing the QD content (Fig. 3b). Compared to the same amount of the R-PE/QDs dilute solution, the R-PE/QDs@ZIF-8 thin film exhibited a higher PLQY up to 29.8% (Table S2^†), indicating that the QDs and R-PE molecules were well isolated and evenly encapsulated in the ZIF-8 thin films to decrease the aggregation-caused PL quenching.Open in a separate windowFig. 3Optical properties of the R-PE/QDs@ZIF-8 thin films. (a) PL spectra of the R-PE/QDs@ZIF-8 thin film (S-9), the R-PE solution, and the QD solution with the same amount excited at 405 nm. (b) PL spectra of the R-PE/QDs@ZIF-8 thin films (S-7 to S-10) with different amounts of R-PE and QDs excited at 405 nm. (c) CIE coordinates of the R-PE/QDs@ZIF-8 thin films (S-7 to S-14) with different amounts of R-PE and QDs excited at 405 nm. The inset photo in (c) is a photograph of the white light-emitting LEDs assembled from the R-PE/QDs@ZIF-8 thin film (S-9) with UV-violet (405 nm) LED curing chips at the on state. (d) Emission colors in the CIE 1931 chromaticity diagram of the R-PE/QDs@ZIF-8 thin film (S-9) excited at 405 nm.After carefully optimizing the concentration of the QDs, a R-PE/QDs@ZIF-8 thin film with an R-PE content of 10 wt% and a blue QD content of 10 wt% was prepared and named S-9. Under 405 nm excitation, the fluorescence emission of this thin film could cover the whole visible spectrum, leading to white light emission (Fig. 3b and d). For application as a WLED, we placed R-PE/QDs@ZIF-8 (S-9) prepared on quartz directly upon a UV-violet (405 nm) LED chip array. As a result, the Commission Internationale de I’E’clairage (CIE) coordinates of the R-PE/QDs@ZIF-8 thin film were (0.34, 0.34), which are very close to the ideal white-light emission (0.33, 0.33). By putting the resultant film into an integrating sphere, the absolute quantum yield (QY) of S-6 was measured as 29.8%, which is much higher than the 11.1% of the R-PE/QDs dilute solution with the same amount (Table S2^†). Besides, the correlated color temperature (CCT) was 4955 K, and the color rendering index (CRI) was approximately 85. Compared to the solar spectrum at 5000 K (Fig. S7^†), the fluorescence emission spectrum of the R-PE/QDs@ZIF-8 thin film (S-9) was basically consistent with sunlight, which is suitable for human eyes. Based on these results, we concluded that the R-PE/QDs@ZIF-8 thin film could emit high-quality white light, and might find a potential application in practical lighting devices.In conclusion, we have demonstrated a one-pot solid-confinement conversion process to encapsulate R-PE molecules and blue quantum dots into ZIF-8 crystals for the design of high-quality white-emitting thin films. The R-PE molecules and QDs are well isolated and evenly distributed in the ZIF-8 crystals, leading to a higher PLQY by suppressing aggregation-caused PL quenching. Interestingly, the chromophores of R-PE could form complexes with Zn ions. Meanwhile, the crystal growth process might block the energy transfer between these two types of chromophore, so that R-PE could emit green and red colors in the visible region, replacing the single orange color from its solution. Furthermore, the thermal stability of R-PE embedded in the ZIF-8 films at high temperatures has been improved significantly. Upon excited at 405 nm, the resultant R-PE/QDs@ZIF-8 thin film emits high-quality warm white light with CIE coordinates of (0.34, 0.34), a CRI of 85, and a CCT of approximately 4955 K, thus demonstrating its promising applicability for color-conversion based warm W-LEDs. This solid-confinement conversion process could also be used to encapsulate other fluorescent proteins or phosphorous molecules in metal–organic frameworks to realize high-quality white-light emission for applications in lighting devices.     

Thermally induced fragmentation of nanoscale calcite

Calcite nanorods ∼50 nm wide are thermally separated into nanoblocks. The fragmentation is ascribed to the ion diffusion on metastable crystal surfaces at temperatures (∼400 °C) much lower than the melting point. The presence of water molecules enhances the surface diffusion and induces deformation of the nanorods even at ∼60 °C.

Calcite nanorods ∼50 nm wide are thermally separated into nanoblocks.

Calcium carbonate is a common industrial material that is used as a micrometric filler for papers, rubbers, plastics, and inks. The shape and size of micrometric grains are important parameters that affect the physical and chemical properties of composite materials.^1–5 In recent years, nanometric particles of calcium carbonate have attracted much attention as basal building blocks of biogenic minerals^6–10 and functional materials with high biocompatibility and low environmental load.^11–14 Since various properties are influenced by the miniaturization of crystal grains below 100 nm, characterization of the carbonate particles is necessary for application in practical fields. However, their properties, including the thermal characteristics of nanometric calcium carbonate in the nanometric region, have not been sufficiently clarified because calcium carbonate is easily decomposed above 550 °C.^15,16 In the present work, we studied thermally induced deformation on nanometric calcite at temperatures lower than the melting point (1597 °C at 3 GPa)¹⁷ of the bulky crystal.Since the melting temperature of metals decreases when their size is decreased below ∼50 nm,^18,19 metallic nanowires fragment into nanospheres at temperatures much below the melting point of bulk metal.^20–22 The fragmentation is ascribed to the Rayleigh instability that is known for liquid. These results suggest that the surface diffusion of ions and atoms occurs easily on the nanometric particles when the surface instability is increased. The cleavage of solid nanowires has been observed only for metallic phases. In the current work, we found the morphological transformation of ion crystal nanorods into faceted nanograins through the surface diffusion at relatively low temperatures.The preparation of bulky calcium carbonate consisting of nanocrystals is required for reinforcing materials^23–25 and as a precursor of biomedical materials.^26,27 In general, however, fabricating large, bulky bodies through conventional sintering techniques is difficult because calcium carbonate is thermally decomposed into calcium oxide and carbon dioxide. Several methods, such as sintering with a flux^16,28 or in a carbon dioxide atmosphere²⁹ and hot-pressing under hydrothermal conditions^30,31 were developed to prepare bulky calcium carbonate materials. On the other hand, the thermal behaviors of pure calcium carbonate have not been sufficiently studied at temperatures below the decomposition and melting points.In nature, bulky calcium carbonate crystals are commonly produced as biominerals, such as shells, eggshells, sea urchins, and foraminiferal skeletons, under mild conditions.^{3,9,10,32–34} The bulky biogenic bodies are composed of nanometric grains 10–100 nm in size that are arranged in the same crystallographic direction.³ The formation of bridged architectures through oriented attachment is generally related to the ion diffusion on specific surfaces at relatively low temperatures. A detailed study on the stability of nanometric surfaces at relatively low temperatures is needed to understand the morphological change of calcium carbonate crystals.In the present report, we discuss the morphological change of calcite nanocrystals below the decomposition and melting temperatures by observing metastable crystal surfaces in the nanometer-scale range. Calcite nanorods elongated in the c direction were utilized as a typical nanometric shape covered with metastable surfaces. Here, thermally induced fragmentation was studied with and without water vapor. The surface diffusion was found to occur on the metastable surface at around 400 °C under a dry condition and at around 60 °C with water vapor. Our findings are important for clarification of the surface property of nanometric calcium carbonate and for the fabrication of bulky bodies through the attachment of nanocrystals.Single-crystal calcite nanorods elongated in the c direction were utilized in the present study (Fig. 1 and S1^†). Calcite nanorods up to ∼500 nm were formed through the combination of the carbonation of calcium hydroxide and the subsequent oriented attachment of resultant calcite nanoblocks ∼50 nm in diameter by stirring. The detailed mechanism was described in our previous study.³⁵ As shown in Fig. S1,^† the calcite nanorods were covered with metastable surfaces having a curvature. Moreover, we observed depressed parts originating from the oriented attachment of the original calcite grains. As shown in the Fig. S2,^† the XRD peaks of the nanorods were found to shift to higher angles than those of standard X-ray diffraction data (ICDD 00-005-0586). Thus, the crystal lattice of the nanorods was suggested to be stressed with the coverage of irregular surfaces.Open in a separate windowFig. 1SEM (a) and TEM (b and c₁), HRTEM (c₂) images, and the FFT pattern (c₃) of the lattice in (c₂) of calcite nanorods in aqueous dispersions at pH 12 and at 25 °C with stirring. (c) Reprinted from ref. 35 published by The Royal Society of Chemistry.As shown in Fig. 2, the calcite nanorods were deposited on a silicon substrate for clear observation of the morphological change. We redispersed the calcite nanorods in ethanol and evaporated the dispersion medium to deposit them on the substrate at 25 °C (Fig. 2a and S3a^†).³⁶ The nanorods were arranged on the solid surface through evaporation-driven self-assembly. Specifically, monolayers of the calcite nanorods were obtained by adding poly(acrylic acid) (PAA, MW: 5000 gmol⁻¹) to the ethanol dispersion. The dispersibility of the nanorods in ethanol was improved by the modifying agent. The organic components of the PAA-modified nanorods were removed through oxidation in air at around 300 °C (Fig. S4^†).Open in a separate windowFig. 2SEM images and schematic illustrations of calcite nanorods deposited on a silicon substrate before treatment (a) and heated at 400 °C for 1 (b) and 2 h (c). The PAA-modified calcite nanorods were used to obtain the monolayers (b and c). We used bare nanorods for take the images of original nanorods because the definite surfaces were not observed due to the presence of PAA (a).The calcite nanorods were deposited on the solid surface to study the morphological change when heated in air. Obvious changes were not found in the shape of the nanorods upon heating to a temperature below 350 °C in air for 24 h (Fig. S2^†). On the other hand, we observed significant deformation upon heating to 400 °C (Fig. 2). The depressed parts on the side surfaces enlarged in 1 h. The fragmentation of the nanorods was finally induced after treatment for 2 h. Calcite grains were formed by the thermally induced cleavage (Fig. 2c). The average size of the cleaved grains was ∼100 nm, which was larger than the average width of the nanorods, ∼50 nm. As shown in Fig. 3, we observed the definite surfaces covering nanograins. Most of the definite facets were assigned to the (104) of calcite by FFT analysis of the lattice fringes of nanograins in the HRTEM images. Some (012) planes were found in the nanograins. On the other hand, the surfaces of the deformed nanorods during fragmentation were curved and irregular. According to XRD patterns, the crystal phase was not changed with the fragmentation (Fig. S2^†). The diffraction peaks were shifted to the standard values and sharpened with the treatments. This suggests that the formation of the stable faces with the fragmentation is associated with the lattice relaxation.Open in a separate windowFig. 3TEM (a), HRTEM (b), and FFT images of nanocrystals before and after heating at 500 °C.We observed the stability of rhombohedral grains that were covered with the stable {104} planes. As shown in Fig. 4, the deformation of the rhombohedral grains was not observed at 400 °C in air for 6 h. These results indicate that the ion diffusion is not induced drastically on the stable surfaces at a temperature lower than the decomposition temperature.Open in a separate windowFig. 4SEM (a_1,2 and b_1,2), TEM (a₃), and SAED (a₄) images of calcite nanoblocks before treatment (a) and heated to 400 °C for 6 h (b). (a₃) A schematic illustration of a calcite rhombohedron covered with {104} faces.The fragmentation of the calcite nanorods was enhanced in the presence of water vapor. As shown in Fig. 5a, we found cleavage of the nanorods even at 60 °C in a closed vessel containing water. The formation of unifaceted rhombohedral nanoblocks with {104} faces was clearly observed at 100 °C for 24 h (Fig. 5b–d). Since the morphological change was similar to that under a dry condition, the ion diffusion on the surface is deduced to be assisted by adsorbed water molecules. The X-ray diffraction signals shifted to the standard values with the fragmentation (Fig. S2^†). Thus, the stable faces were formed with the lattice relaxation with the exposure to water vapor.Open in a separate windowFig. 5SEM (a and b), TEM (c), and SAED (d) images of calcite nanorods deposited on a silicon substrate subjected to high humidity at 60 °C (a) and 100 °C (b) for 24 h. (c) A schematic illustration of a calcite rhombohedron covered with {104} faces.The ion diffusion at a relatively low temperature, below the decomposition temperature, has not been reported for calcium carbonate. In the present work, however, we found the fragmentation of calcite nanorods at around 400 °C under a dry condition and at around 60 °C with water molecules. These results suggest that the ion diffusion occurs on the nanoscale calcite crystals. On the other hand, rhombohedral calcite grains covered with the stable {104} planes were not deformed at those temperatures. Thus, the diffusion at low temperatures is induced only on metastable surfaces that are exhibited on the nanoscale calcite. Moreover, the presence of water molecules enhances the ion diffusion on the metastable surfaces.The fragmentation of the calcite nanorods can be explained by Rayleigh instability. The cleavage by Rayleigh instability is ascribed to the enlargement of tiny perturbations on cylindrical liquids,³⁷ polymers, and metals. In general, the cylindrical bodies evolve into several spheres to decrease the total surface energy. Recently, Rayleigh instability was applied to the thermally induced fragmentation of metal^20–22 and organic³⁸ nanowires. The breakup phenomena were attributed to surface oscillations due to the high surface energy induced by increased surface-to-volume ratios.³⁹ Thus, metal nanowires are cleaved and form isotropic nanoblocks at temperatures well below the melting point. In the present work, we found fragmentation of the calcite nanorods at relatively low temperatures. The ion diffusion is induced on the metastable surfaces that are exhibited on nanoscale crystals. The depressed parts exist as perturbations on the side faces of the original nanorods. The cleavage occurs through enlargement of the depressed parts and formation of the stable faces to reduce the surface energy and relax the lattice strain.Polyhedral grains covered with flat planes were formed instead of spherical particles by the fragmentation of calcite nanorods. Formation of the stable {104} plane is achieved to reduce the surface energy. The {012} plane of calcite is not stable under the ambient temperature. However, the surface energy of {012} decreases with increasing temperature.⁴⁰ Thus, the facets are deduced to also be formed at temperatures around 400 °C.     

Transition metals enhance prebiotic depsipeptide oligomerization reactions involving histidine

Biochemistry exhibits an intense dependence on metals. Here we show that during dry-down reactions, zinc and a few other transition metals increase the yield of long histidine-containing depsipeptides, which contain both ester and amide linkages. Our results suggest that interactions of proto-peptides with metal ions influenced early chemical evolution.

Transition metals enhance prebiotic proto-peptide oligomerization reactions through direct association with histidine.

Around half of all known proteins associate with metal ions or organometallic cofactors.^1–4 Metals stabilize protein structure (e.g., zinc fingers) and mediate catalysis (nitrogenases), electron transfer (cytochromes), ligand transport (hemoglobin), and signalling (calmodulin). Metals stabilize nucleic acids; around 500 divalent metal cations associate with a single ribosome.⁵ The dependence of biochemistry on metals appears ancient and rooted in chemical evolution, preceding emergence of life on Earth.^6,7 Long-standing questions about the origins of life might be answered by understanding how metals interact with ancestral biopolymers. For example, metals could have affected oligomerization reactions of early proto-polymers, and could have conferred stability and function, in analogy with their roles in extant biochemistry.Depsipeptides, containing mixtures of ester and amide linkages, are plausible ancestors of polypeptides.^8–14 During drying of hydroxy acids and amino acids at mild temperatures (i.e., 65 °C) ester bonds form first and are converted to peptide bonds by ester–amide exchange. Hydroxy acids and amino acids are thought to have been abundant on the prebiotic earth.^15–19Here we tested the hypothesis that interaction with Zn²⁺ or other metal ions with the amino acid histidine (His) might affect rates and extents of depsipeptide formation in dry-down reactions. We observed that Zn²⁺ and other selected transition metals increase yields of long His-containing depsipeptides. The increase in yield is specific for imidazole-containing amino and hydroxy acids and metals that interact directly with them.We previously showed that His is readily incorporated into depsipeptides during dry-down reactions under mildly acidic conditions (pH ∼3) at 85 °C.¹² These conditions are known generally to promote oligomerization in mixtures of hydroxy acids and amino acids into depsipeptides.^8–11 Here, mixtures of glycolic acid (glc) and l-His were reacted under these known reaction conditions in the presence or absence of various metals.Addition of Zn²⁺ led to an increase in the average lengths of His-containing depsipeptides. Depsipeptide oligomers are readily detected by high-performance liquid chromatography (HPLC) using a C18 column (Fig. 1a). Longer oligomers exhibit longer retention times²⁰ allowing us to compare various dry-down reactions. The yield of products with a higher retention times on HPLC is increased following addition of Zn²⁺ to the reaction mixture (Fig. 1a). ¹H NMR analysis confirmed that the distribution of oligomeric species changes with addition of Zn²⁺ to the dry-down reaction (Fig. 1b–d). MS analysis verified the increased abundance of longer His-containing depsipeptides following dry-down with Zn²⁺ (Fig. S1^†). On the other hand, Zn²⁺ decreases the extent of conversion of His into oligomers. While 42% of His monomer converted into oligomers in the absence of Zn²⁺, only 22% of His monomer converted into oligomers in its presence (Fig. 1b–d). Oligomerization is indicated by shifts in the β-proton resonance envelope centered at ∼3.37 ppm and in the imidazole proton envelopes at ∼7.40 ppm and ∼8.68 ppm (Fig. 1b–d). Extent of conversion to oligomers was calculated based on the integrals of residual nonreacted imidazole proton resonances (Fig. 1b–d).Open in a separate windowFig. 1Zinc increases the yield of long His-containing depsipeptides in dry-down reactions. (a) His monomer was dried with glycolic acid (glc) at a 1 : 1 molar ratio at 85 °C for seven days in the presence or absence of Zn²⁺, at a 1 : 1 molar ratio (His : Zn²⁺). Analysis of samples via C18-HPLC showed a dramatic increase in the yield of longer oligomers in the presence of Zn²⁺. (b and d) ¹H NMR spectrum of a mixture of glc and His in D₂O before (b) and after (c and d) dry-down at 85 °C for seven days in the absence (c) or presence (d) of 1 eq. of Zn²⁺. (e) A possible coordination complex between Zn²⁺ and two His monomers.The observed increase in depsipeptide length upon addition of Zn²⁺ might result from direct association between Zn²⁺ and His monomers (Fig. 1e)²¹ because the effect is Zn²⁺ dose-dependent and is maximal at a 1 : 1 molar ratio of Zn²⁺ to His (Fig. S2^†). Specific interaction of Zn²⁺ with His monomer has been reported previously.²² The increased production of long His-containing depsipeptides with increasing Zn²⁺ is reversed when the number of Zn²⁺ equivalents exceeds that of His. Three equivalents of Zn²⁺ completely inhibited oligomerization reactions (Fig. S3^†). The effect of Zn²⁺ on reactivity of His during the dry-down conditions might arise from either a locked chelate conformation or increased electrophilicity of the carbonyl. Importantly, Zn²⁺ did not cause polymerization of His in the absence of glycolic acid (Fig. S4^†).The effect of Zn²⁺ on depsipeptide formation is not a generic effect and is specific to His. We dried-down mixtures of glc with either alanine (Ala) or lysine (Lys). The addition of Zn²⁺ to a 1 : 1 molar ratio with these amino acids does not increase the yield of products in any length range. In fact, Zn²⁺ inhibited depsipeptide formation for both Ala and Lys (Fig. 2).Open in a separate windowFig. 2Zinc does not increase extent of polymerization or length of oligomers of Ala- or Lys-containing depsipeptides in dry-down reactions. Alanine (a) or lysine (b) were dried with glc at a 1 : 1 molar ratio, for one week at 85 °C under unbuffered conditions, in the presence or absence of Zn²⁺, at a 1 : 1 molar ratio of amino acid to Zn²⁺. The addition of Zn²⁺ hindered depsipeptide formation for both amino acids tested, as opposed to the reverse trend observed with His.In addition to Zn²⁺, we investigated the effects of Na⁺, K⁺, Ca²⁺, Mg²⁺, Cu²⁺, and Co²⁺ on the oligomerization of His in depsipeptides. We dried-down mixtures of glc and His in the presence or absence of various metals (1 : 1 molar ratio of M⁺ or M²⁺ to amino acid). Analysis by HPLC showed that Zn²⁺, Cu²⁺, and Co²⁺, but not Na⁺, K⁺, or Mg²⁺, increased the production of longer His-containing depsipeptides (Fig. S5 and S6^†). Ca²⁺ decreased the production of His-containing depsipeptides (Fig. S5^†).Circular dichroism spectroscopy (CD) supported our hypothesis that enhancement of oligomerization of His in the presence of Zn²⁺ results from direct association between monomeric His and Zn²⁺. We added various concentrations of Zn²⁺ to monomeric glc plus His. The CD spectrum inverts at equal molar ratio of Zn²⁺ and His (Fig. 3a). We attribute the inversion to formation of His–Zn²⁺ complex in concert with a change in the conformation of His. An example of a possible complex is shown in Fig. 1e.Open in a separate windowFig. 3Circular dichroism analysis confirms that zinc and several other transition metals interact with His monomer. Glycolic acid (glc) and His were mixed, at a 5 : 1 molar ratio, in 50 mM Tris buffer (pH 7.2). Circular dichroism (CD) spectra were collected for the mixture in the absence or presence of 1 eq. (referring to the amount of His monomer) of (a) Zn²⁺ or (b) various other metals. CD spectra showed a clear shift in the signal of His following addition of Zn²⁺, Ni²⁺, Cu²⁺, and Co²⁺, but not for the other metals tested.CD spectra of glc plus His only report conformational changes of His because glc is achiral. The conformational change upon Zn²⁺ binding is dose dependent; the change in the CD spectra increases with increasing concentrations of Zn²⁺ (Fig. S7^†). Addition of Zn²⁺ to the dry-down product mixture of His-containing depsipeptides resulted in far more subtle changes in the CD spectra, which appears to arise from binding of Zn²⁺ to small amounts of remnant His monomer that was not converted into polymers during the dry-down reaction (Fig. S8^†).¹² In accordance with the observed species-specific impact of metals on dry-down reactions (Fig. S5^†), the inversion of the CD signal of His monomer was also observed for Co²⁺, Cu²⁺, and Ni²⁺, but not for Na⁺, K⁺, Li⁺, Mg²⁺, or Ca²⁺. Thus, it appears that low-lying d-orbitals of metals are important for interaction with His. The differences in the electron configuration between the different metals affect their metal coordination properties. Transition metals are more electronegative and have more oxidation states than alkaline and alkaline-Earth metals, and their valence electrons in the d-shell tend to promote stable coordination complexes. By contrast, Zn²⁺ inhibited oligomerization of Ala or Lys in dry-down reactions. This inhibition is consistent with recent thermodynamic calculations²³ that examined effects of metals on the monomer–oligomer equilibria of glycine. Metals shift the equilibria toward the monomer, particularly at neutral and alkaline pH.²³To determine if oligomerization of imidazole-containing monomers other than His is promoted by Zn²⁺, we dried l-β-imidazole lactic acid (the hydroxy acid analog of His, herein termed his) with glc for one week at 85 °C. The reaction produced polyesters, co-polymers of his and glc (Fig. S9^†). Addition of Zn²⁺ to his and glc dry-down reaction mixtures increased the yield of longer polyester oligomers (Fig. 4a). ¹H NMR analysis of these polyesters indicate that Zn²⁺ did not change the extent of conversion of his monomer into oligomers: 39% of his converted into oligomers in the absence of Zn²⁺ and 38% of his converted into oligomers in its presence (Fig. S10^†). Therefore, Zn²⁺ does not increase the overall oligomeric yield but rather the distribution of product oligomers, increasing the yield of longer oligomers (Fig. 4a and Fig. S10^†). These results imply that a terminal alcohol can support a chelation complex with Zn²⁺, in analogy with the suggested chelation complex of Zn²⁺ by His (compare Fig. 4b to Fig. 1e).Open in a separate windowFig. 4Zinc increases lengths of his-containing polyesters in dry-down reactions. (a) l-β-Imidazole lactic acid (His) was dried with glycolic acid (glc) at a 1 : 1 molar ratio at 85 °C for seven days in the presence or absence of Zn²⁺, at a 1 : 1 molar ratio of his to Zn²⁺. Analysis of samples via C18-HPLC showed a dramatic increase in the yield of longer oligomers in the presence of Zn²⁺. (b) Possible coordination complex between Zn²⁺ and his monomers.Several distinct non-exclusive mechanisms can explain why Zn²⁺ promotes formation of longer His-containing depsipeptides. Dry-down reactions are conducted under mildly acidic conditions (pH ∼ 3), in which the imidazole side chain (pK_a of ∼6) and the α-amino group (pK_a of ∼9) of monomeric His are protonated and the carboxylic acid (pK_a of ∼2) is deprotonated. Deprotonation of α-amino group would be promoted by His coordination of Zn²⁺ (Fig. 1), favoring an intermediate in formation of depsipeptides through ester–amide exchange. Zn²⁺ coordination by His might also lock His in a specific reactive conformation and/or increase the electrophilicity of the His carbonyl group. Zn²⁺ is expected to pull electron density from His and expose the carbonyl to nucleophilic attack (Fig. 1e). Dehydration would promote His coordination with Zn²⁺ by depleting competing water molecules.^21,24 In principle, it is possible that a complex is formed in which glc and His simultaneously chelate Zn²⁺, or in which His and glc-based oligomers chelate Zn²⁺ such that a favored configuration for a nucleophilic attack is reached.The effects of metals on oligomerization of amino acids by methods other than dry-down reactions has been investigated previously.^23,25–30 Concentrated sodium chloride (>3 M) promotes oligomerization in the presence of Cu²⁺, to increase the yield of glycine (Gly)- and Ala-based peptides.²⁵ Various metals can affect oligomerization of chemically activated amino acids (N-carboxyanhydrides).^31,32 Chemical activation studies focused on Gly, the simplest and most reactive amino acid, but resulted in only low yields of very short peptides.^{23,25–28,33–39} It has been proposed that minerals might catalyze dry-down oligomerization of amino acids.^33–39 McKee et al. observed that silica hinders the amidation of Gly in the presence of lactic acid, the hydroxy acid analog of Ala.⁴⁰ However, silica did lead to an enrichment of amide bonds over ester bonds.⁴⁰His may not be a prebiotic amino acid. It has been proposed that the prebiotic chemical ancestor of His might be imidazole-4-acetaldehyde,^41–44 which is produced by Strecker synthesis.^45,46 The His-containing depsipeptides produced here do not appear to bind to Zn²⁺ (Fig. S8^†). This absence of chelation is consistent with the low number and density of His side chains, and the absence of backbone amines at ester linkages. Longer depsipeptides with greater number and density of His residues may bind Zn²⁺ and might lead to emergence of small metalloenzymes.The importance of small proto-metalloproteins on the prebiotic Earth is supported by the cooperative interactions of metals and proteins in extant biology. Mulkidjanian proposed the zinc world theory, according to which the first metabolism was driven by zinc sulfide minerals that catalyzed photochemical reactions.^47–49 Primordial cooperation may have existed between metals and proto-peptides prior to the emergence of coded protein synthesis.^50–53 For instance, amyloidogenic heptapeptides can function as Zn²⁺-dependent esterases.⁵¹ Zn²⁺ promotes fibril formation by His-containing peptides, acting as a catalytic cofactor. Short peptides with acidic residues, such as aspartic acid and glutamic acid,^52,53 could have protected short RNA molecules against Mg²⁺-induced degradation.^54,55 Coordination of metal ions induces peptide conformational changes and supramolecular assembly.^56–60 In addition to effects on peptide self-assembly and function, metal–peptide interactions are utilized for fabrication of nanofiber materials for various applications, including three-dimensional cell culture and tissue engineering.^56,61,62It is generally accepted that Zn²⁺ concentration has remained fairly constant in seawater through time, whereas the concentrations of Co²⁺ and Ni²⁺ were higher in earlier stages of Earth history than in modern seawater.^63–72 If accumulation of metals occurred at shallow lakes or similar environments that were subjected to dry-wet cycling, they might have affected distribution of polymers that formed within these environments in a specific manner. Our results suggest that the close relationship between metals and biopolymers has roots in prebiotic chemistry and shaped their co-evolution.     

Rapid detection of donor-dependent photocatalytic hydrogen evolution by NMR spectroscopy

Understanding molecular processes at nanoparticle surfaces is essential for designing active photocatalytic materials. Here, we utilize nuclear magnetic resonance (NMR) spectroscopy to track photocatalytic hydrogen evolution using donor molecules and water isotopologues. Pt–TiO₂ catalysts were prepared and used for isotopic hydrogen evolution reactions using alcohols as electron donors. ¹H NMR monitoring revealed that evolution of the H₂ and HD species is accompanied by the oxidation of donor molecules. The isotopic selectivity in the hydrogen evolution reaction gives rise to formal overpotential. Based on a comparison of the rates of hydrogen evolution and donor oxidation, we propose the use of ethanol as an efficient electron donor for the hydrogen evolution reaction without re-oxidation of radical intermediates.

Isotopic molecule processes at photocatalytic hydrogen evolution reactions observed by NMR clarify the importance of the choice of electron donors for efficient chemical energy conversions at electrified interfaces.

The conversion of light energy to chemical energy requires a combination of electronic excitation and sequential electron transfer.^1–3 Efficient electronic excitation is achieved by choosing materials with suitable optical properties, while efficient electron transfer can be achieved by rational design of catalytically active surface sites.⁴ To achieve high catalytic performance, an understanding of the molecular processes occurring at the catalyst surface is required.Photocatalytic hydrogen evolution is accompanied by oxidation of the electron donor. Most studies on this reaction have been conducted using in-line mass spectrometry measurements⁵ or oxygen-quenching methods.⁶ However, monitoring the whole reaction cycle using one methodology remains challenging.Nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for observing chemical reactions. This method is mainly used to confirm small-molecule conversions in organic synthesis. However, NMR spectroscopy can also be used to gain information of nanoparticle surfaces⁷ or even for the detection of photocatalytic reactions.^8–10 Furthermore, NMR spectroscopy can be used to determine the nuclear spin states of product molecules.¹¹ Nevertheless, there are very few reported studies on the in situ observation of photocatalytic hydrogen evolution using NMR spectroscopy.Accordingly, in the present study, we utilized NMR spectroscopy to observe the photocatalytic hydrogen evolution reaction. We employed Pt–TiO₂, which is frequently used for the photocatalytic hydrogen evolution reaction, as a model catalyst for this study. NMR spectroscopy enabled sub-micromole-scale detection of reaction products within one minute. We investigated the dependence of isotopic hydrogen evolution reactions on the donor molecules. The effects of efficient donors on the photocatalytic hydrogen evolution reaction are discussed.Pt–TiO₂ nanoparticles were prepared by a typical chemical reduction method (see ESI^†). The morphologies of TiO₂ and Pt–TiO₂ nanoparticles were characterized by transmission electron microscopy, as shown in Fig. S1 and S2.^† The average size of the Pt nanoparticles is approximately 5 nm. The average size of the TiO₂ nanoparticles is within the range 20–30 nm. Typically, 5 mg of catalysts and 0.6 mL of reaction mixture were introduced to an NMR tube under Ar for observation of the photocatalytic hydrogen evolution reaction by NMR spectroscopy. Fig. 1a shows the ¹H NMR spectra of Pt–TiO₂/2-propanol/D₂O before and after light irradiation. Before light irradiation, three peaks are observed (Fig. 1a, black). The single peak observed at 4.81 ppm is assigned to HDO.¹²¹H signals from the methine and methyl groups of 2-propanol are observed at 4.01 and 1.20 ppm, respectively.Open in a separate windowFig. 1 ¹H NMR spectra of Pt–TiO₂/2-propanol/D₂O. The black lines are the ¹H NMR spectra before light irradiation. The red lines are the ¹H NMR spectra after light irradiation for 15 min. (a) Full spectrum. (b) Enlargement of the oxidation product. (c) Enlargement of the area for the H₂ and HD species.After light irradiation, additional species are observed (Fig. 1a, red). We confirmed the photocatalytic response of the Pt–TiO₂ nanoparticles from ON–OFF experiments (Fig. S3^†). The oxidation product is acetone resulting from two-electron and two-proton oxidation of 2-propanol. The signal at 2.25 ppm is assigned to the methyl group in the acetone (Fig. 1b).¹² Hydrogen evolution is observed as a reduction reaction. Four peaks are observed between 4.5 and 4.7 ppm (Fig. 1c). The single peak at 4.63 ppm can be assigned to H₂ dissolved in the solvent.¹³ Other peaks at 4.66, 4.59, and 4.52 ppm are assigned to the HD.^14–16 The observed coupling constant for HD is 43 Hz, which is a typical value for HD.^14–16 The difference in the chemical shifts of H₂ and HD is due to variation of the nuclear magnetic screening constants with interatomic separation as a consequence of the zero-point energy in vibration.^17,18Importantly, NMR spectroscopy can detect H₂ and HD species from the photocatalytic hydrogen evolution reaction. The observed peak splitting of the three peaks is due to the heteronuclear coupling between hydrogen and deuterium atoms.^14–16 The observed chemical shift for H₂ in methanol/D₂O is 4.56 ppm. The observed chemical shifts of HD in methanol/D₂O are 4.60, 4.53, and 4.46 ppm (Fig. S4^†). These values are similar to those for 2-propanol/D₂O. The observed chemical shift of H₂ in ethanol/D₂O is 4.61 ppm. Those for HD in ethanol/D₂O are 4.65, 4.57, and 4.50 ppm (Fig. S5^†). The slight shift in the H₂ and HD signals is due to the difference in the shielding effect depending on the solvation environment.^19–21 The coupling constant between hydrogen and deuterium in HD is 43 Hz, and it is 43 Hz in methanol/D₂O and ethanol/D₂O. The similarity in the coupling constants for the different solvents indicates that the chemical bonding between hydrogen and deuterium is consistent.^14–16 Interestingly, the fullwidth at half maximum (FWHM) values for the H₂ and HD signals are dependent on the solvent. The FWHM values for the H₂ signal are 1.48, 2.24, and 3.51 Hz in methanol/D₂O, ethanol/D₂O, and 2-propanol/D₂O, respectively. The FWHM values for the HD signal are 1.59, 2.00, and 3.32 Hz for methanol/D₂O, ethanol/D₂O, and 2-propanol/D₂O, respectively. H₂ and HD show similar FWHM values in the same solvent. However, the FWHM value is solvent-dependent. In general, a wider peak indicates lower mobility.¹⁶ Therefore, it is expected that 2-propanol induces lower mobility for the hydrogen, probably because of the rotation or diffusional freedom of hydrogen molecules. The solvation environment of hydrogen influences the molecular mobility of hydrogen species in the photocatalytic hydrogen evolution reaction.Oxidation products of the donor molecules are observed in the NMR spectra, as shown in Fig. S4 and S5.^† The number of product molecules is quantified on the basis of the hydrogen atoms in the alkyl chain groups in 2-propanol, ethanol, and methanol as reactants. For methanol, the signals for methylene glycol, 1-methoxymethanol, and methyl formate are observed as shown in Fig. S4.^† For ethanol, acetaldehyde and acetic acid are observed as the products, as shown in Fig. S5.^† As described above, the oxidation product of 2-propanol is limited to acetone. This is due to the unstable intermediate formed in the oxidation of 2-propanol.²² Conversely, the reaction products of methanol^23–26 and ethanol²⁷ are complicated owing to the sequential oxidation and/or hydration reactions.We evaluated the isotopic selectivity of the hydrogen evolution reaction depending on the donor molecules. Fig. 2 shows the typical isotopic selectivity of the hydrogen evolution reaction. The amounts of H₂ and HD were quantified from the NMR spectra. The HD/H₂ ratios were calculated to be 4.1, 3.4, and 1.9 for 2-propanol, ethanol, and methanol, respectively, where the mixture ratio of D₂O and alcohol is 1 : 1. In both cases, attenuation of the hydrogen evolution reaction is specifically observed for methanol. This is probably due to poisoning of the Pt surface with carbon monoxide molecules evolved from the oxidation of methanol at the TiO₂ surface.²⁸Open in a separate windowFig. 2Isotopic selectivity for HD (black) and H₂ (red) from the photocatalytic hydrogen evolution reaction using a 1 : 1 mixture of D₂O and the corresponding alcohol upon light irradiation for 15 min.H₂ is classified as o-H₂ or p-H₂ depending on the nuclear spin isomer.^29,30o-H₂ is observable and p-H₂ is not by NMR because of the Zeeman splitting of the nucleus spin momentum. Because of the spin statistic, the ratio of o-H₂ and p-H₂ is 3 : 1.^29,30 D₂ is not included in the observation because of the low sensitivity to D atoms, even in ²H NMR spectroscopy measurements. Similarly, we observed an increase in the oxidation products of methanol and ethanol. Importantly, the selectivity for the oxidation and hydrogen evolution reaction were continuously monitored, as shown in Fig. S6–S11.^† In addition, the maximum concentration of H₂ in this photocatalytic reaction is approximately 1 mmol L⁻¹, which is below the solubility limits of water and alcohol.^31–34 These results suggest that a robust photocatalytic process continues throughout the catalytic cycle.Isotopic hydrogen evolution provides information about the reaction mechanism at the metal surface.^35–39 The reaction follows an electrochemical adsorption and desorption cycle. The adsorption of atomic hydrogen from the proton donor (Volmer step)⁴⁰ is followed by either desorption via recombination of adsorbed hydrogens (Tafel step)⁴¹ or desorption of atomic hydrogen with a proton donor (Heyrovsky step).⁴² The enrichment of hydrogen over deuterium is observed for the Heyrovsky, Tafel, and Volmer step sequence.^40–42 Generally, the Tafel step is rate-limiting in the hydrogen evolution process for Pt surfaces. Therefore, isotopic selectivity is not dependent on electrochemical potential.As shown in Fig. 2, the isotopic selectivity is similar for the reactions using 2-propanol and ethanol. This suggests that the formal potential of the hydrogen evolution reaction is similar for these two conditions. Additionally, we evaluated self-diffusion of water molecules and each alcohol molecule as shown in Table S1.^† We determined diffusion coefficients for the alcohols and HDO. These results suggest that the diffusion of the reactant in the hydrogen evolution reaction is not the rate-determining step in the photocatalytic reaction cycle.⁴³Interestingly, the efficiency of the multi-electron transfer is dependent on the donor molecule. Fig. S12^† shows the time-course of the oxidation and reduction reactions obtained by accounting for the half-reaction. Linearity in the time-course plot is observed, indicating stable photocatalysis. Therefore, the reaction rate was calculated from the slope of each reaction. Fig. 3 and S12^† show the rates of oxidation and reduction obtained by accounting for the number of electrons in the half-reaction, defined as r_ox and r_red. For a 3 : 1 ratio of D₂O and alcohol, r_red is nominally low. This is probably due to the small number of donor molecules in the catalytic reaction. Importantly, r_red shows the highest value of 0.26 μmol min⁻¹ for the combination of 2-propanol/D₂O (1 : 1). This value is comparable with that for ethanol/D₂O (1 : 1), which is 0.22 μmol min⁻¹. Conversely, the r_ox values for 2-propanol and ethanol are not comparable. Indeed, r_ox for 2-propanol is seven times higher than that for ethanol.Open in a separate windowFig. 3Rates of the oxidation reaction (black) and hydrogen evolution reaction (red) using a 1 : 1 mixture of D₂O and the corresponding alcohol.Finally, we discuss the effect of donor molecules on the efficiency of the photocatalytic hydrogen evolution reaction. The stability of the radical derived from the alcohol plays an important role in the reaction efficiency. 2-Propanol is oxidized to the tertiary carbocation radical intermediate, which is consumed by spontaneous oxidation at the TiO₂ surface (Fig. 4a).^44–47 For ethanol (Fig. 4b), the oxidized carbocation radical species is expected to be unstable compared with that for 2-propanol. Therefore, the rate of the hydrogen evolution reaction is comparable with the rate of the oxidation reaction. For methanol (Fig. 4c), the carbon monoxide evolved is expected to attenuate the hydrogen evolution reaction.²⁸ Thus, the efficiency of the redox reaction can be evaluated from the NMR spectroscopy results.Open in a separate windowFig. 4Schematic representations of photocatalytic hydrogen evolution reactions over Pt–TiO₂ using (a) 2-propanol, (b) ethanol, and (c) methanol.In conclusion, we used NMR spectroscopy to track the photocatalytic hydrogen evolution reaction using Pt–TiO₂ as a model catalyst. We performed rapid detection of dissolved hydrogen molecules in the solvent and the oxidized product at the sub-micromole scale by ¹H NMR. The method is useful for observation of the dynamic state of molecules in solution and product-based determination of the reaction mechanism. This method is also applicable to the screening of photocatalysts under given conditions. In addition, we found that an efficient multi-electron-transfer photocatalytic reaction is possible using ethanol as the donor molecule. This study demonstrates the utility of NMR for the clarification of the hydrogen evolution reaction mechanism as a means to evaluate potential catalysts, from organic molecular catalysts to inorganic nanocrystals.     

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).     

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.     

Protein-activated transformation of silver nanoparticles into blue and red-emitting nanoclusters

Proteins are very effective capping agents to synthesize biocompatible metal nanomaterials in situ. Reduction of metal salts in the presence of a protein generates very different types of nanomaterials (nanoparticles or nanoclusters) at different pH. Can a simple pH jump trigger a transformation between the nanomaterials? This has been realized through the conversion of silver nanoparticles (AgNPs) into highly fluorescent silver nanoclusters (AgNCs) via a pH-induced activation with bovine serum albumin (BSA) capping. The BSA-capped AgNPs, stable at neutral pH, undergo rapid dissolution upon a pH jump to 11.5, followed by the generation of blue-emitting Ag₈NCs under prolonged incubation (∼9 days). The AgNPs can be transformed quickly (within 1 hour) into red-emitting Ag₁₃NCs by adding sodium borohydride during the dissolution period. The BSA-capping exerts both oxidizing and reducing properties in the basic solution; it first oxidizes AgNPs into Ag⁺ and then reduces the Ag⁺ ions into AgNCs.

Protein capping can trigger nanoparticle to nanocluster transformation at elevated pH.

Noble metal nanomaterials, especially silver (Ag) and gold (Au), have witnessed exceptional research exploration in the last couple of decades from both fundamental and application perspectives.¹ These nanomaterials mainly exist in two distinct size regimes with unique optical characteristics. Ultra-small nanoclusters (NCs) (size typically <3 nm) contain only a handful of atoms (few to hundred), while relatively large nanoparticles (NPs) may comprise thousands of atoms. NPs may display strong extinction (absorption or scattering) spectra in the UV-vis region but are generally non-fluorescent.² In contrast, metal nanoclusters (MNCs) exhibit bright emission but not so noteworthy absorption spectra.^3,4 The distinct optical characteristics of the two nanomaterials have been exploited in various applications. For example, metal nanoparticles (MNPs) are extensively used in photothermal therapy⁵ and imaging,⁶ while NCs are more suited in fluorescence imagining⁷ and sensing⁸ applications. A facile transformation between the two nanomaterials could enable us to combine the complementary optical properties in a single system. Moreover, the kinetics of transformation can provide insights on various intermediate processes like dissolution, etching and digestive ripening etc.^9–11Silver nanoparticles (AgNPs) and nanoclusters (AgNCs) are of particular interest, as it not only possess the intriguing physicochemical properties of MNPs and MNCs, but also feature unique properties pertaining to silver.^3,12,13 For example, metallic silver has been well known for its capability to prevent infection since the ancient times, while recent studies revealed that ultrasmall AgNCs exhibit even superior antibacterial properties towards a broad spectrum of bacteria.^13,14 Moreover, due to superior plasmonic properties and bright fluorescence, AgNPs and AgNCs are preferred over other metal nanomaterials.^15,16The fluorescence properties of AgNCs mainly be attributed to the quantum confinement effect or surface ligand effect.¹⁷ The strong fluorescence generally arises from the electronic transition between occupied d band and states above the Fermi level (sp bands) or the electronic transition between highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).¹⁸ Several reviews have been devoted for the fundamental understanding of the fluorescence origin of AgNCs.^17,19 Recently, it was demonstrated that aggregation-induced emission (AIE) may also contribute to the luminescence pathway of MNCs.^19,20 The origin of AIE from MNCs could be attributed to the restriction of intramolecular vibration and rotation of ligand on the surface of MNCs after aggregation, which facilitates the radiative energy relaxation via inhibiting of non-radiative relaxations.^21,22Protein capping is quite common for obtaining both NPs^23,24 and NCs.^25–29 Serum proteins, bovine serum albumin (BSA) and human serum albumin (HSA) are the most popular among trials with different proteins.^26–29 BSA is a large protein which provides steric stabilization to the MNCs with its various functional group like –OH, –NH₂, –COOH, –SH.^25,30 The disulfide bond of BSA may have strong interaction with the MNCs where sulfur may be covalently bonded to the MNCs core.^24,31 The nanomaterials are synthesized within the protein template at very different pHs. AgNPs are obtained from the reduction of silver salts at neutral pH (6–8),²⁴ whereas the same process at a higher pH (>11) leads to AgNCs.^30,32 The protein capping itself may reduce Ag⁺; AgNCs are formed without any external reducing agent.^25,33 However, an external reducing agent may change the nature and kinetics of the NCs significantly.³⁰Thus, the influence of pH on the protein structure may govern the selective synthesis of AgNPs or AgNCs. BSA can achieve several conformations – N (native), B (basic), A (aged) and U (unfolded) as the pH of the medium gradually changes from neutral to highly alkaline.^34,35 It may be possible that a specific type of nanomaterial is stable within a particular conformation dictated by the pH of the medium. Hence, by simply changing the pH, we may expect a significant modulation of the morphology of the nanomaterial. Herein, we applied this concept to show an effortless transformation from AgNP to AgNC. Although BSA template is exceptionally popular in the preparation of both AgNPs and AgNCs, however, to the best of our knowledge, no report is available on the conversion from AgNP to AgNC within the protein capping.The BSA-capped AgNPs (BSA-AgNPs) were first synthesized at a neutral pH (pH = 6) using sodium borohydride reduction (see ESI^†). The AgNPs show a sharp surface plasmon resonance (SPR) band at 415 nm (Fig. 1a) and have uniform diameters of 12.5 ± 1.5 nm (Fig. 1b). The AgNPs are quite stable at this pH with no apparent change in the SPR band even after 15 days (Fig. S1^†).Open in a separate windowFig. 1(a) UV-vis spectrum and (b) TEM image of BSA-protected AgNPs synthesized at pH 6. The insets show the appearance of the AgNP solution under regular and UV light (left panel), and size distribution histogram (right panel).However, when the BSA-AgNPs were treated with NaOH to elevate the pH to 11.5, we observed a remarkable decrease in the SPR band at 415 nm and a color change from dark to light brown within 2 h of the pH jump (Fig. 2a). The observations indicate the dissolution of AgNPs, which was further confirmed from the TEM images taken quickly (∼10 min) after the NaOH treatment (Fig. S2^†). Heterogeneous distribution of AgNPs was obtained with sizes varying from 2.6 nm to 17 nm, which is in sharp contrast to the uniform AgNPs before the addition NaOH (cf.Fig. 1b). Upon further incubation (6 h), the light brown color gradually faded to light yellow with a further decrease in the SPR band absorbance (Fig. S3^†).Open in a separate windowFig. 2Time evaluation of the (a) UV-visible and (b) emission spectra (λ_ex = 370 nm) of BSA-capped AgNPs after enhancement of the pH from 6 to 11.5 (by addition of NaOH at t = 0). The inset shows the snapshot of the final blue-emitting AgNC solution under normal and UV lights. (c) TEM of the blue-emitting AgNCs along with the HRTEM image and the analyzed size distribution in the inset. (d) MALDI-mass spectra of the BSA protein and BSA-capped blue-emitting AgNCs.Interestingly, the solution also develops distinct fluorescence with a maximum at ∼460 nm after the addition of NaOH (Fig. 2b). The fluorescence intensity gradually grows up upon incubation, and finally, an intense blue fluorescence was developed within ∼9 days. The final NaOH-treated AgNP solution appears to be light yellow under normal light and blue-fluorescent when viewed under a hand-held UV lamp (Fig. 2b, inset). The blue-emitting AgNCs exhibit a single band excitation spectrum with a maximum at 372 nm (Fig. S4^†).TEM image of the optimized NCs (after 9 days incubation at 37 °C) exclusively reveals uniform AgNCs of ∼2.10 ± 0.28 nm diameter without any trace of large NPs (Fig. 2c). The mass of the BSA-capped AgNCs (67 375 Da) was shifted by 845 Da from that of native BSA (66 530 Da) (Fig. 2d). Thus, the new species should correspond to Ag₈ cluster. The characteristics of the blue-AgNCs were quite similar to the human serum albumin (HSA)-protected blue-AgNCs, directly prepared from silver salt.³³ However, the formation time of those AgNCs was significantly less (∼10 h) than the present method (∼9 days).³³ Thus, the initial dissolution process, although quite fast, may have a crucial role in the kinetics of the protein-protected NCs. When we performed a similar pH jump experiment on a citrate-stabilized AgNP,³⁶ the extinction spectrum of the AgNPs showed much less variation compared to the BSA-AgNPs. Instead of a strong decrease, the SRP band showed a red-shift with an extended tail indicating aggregation rather than dissolution of NPs (Fig. S5^†).Furthermore, a red-emitting cluster was generated when an external reducing agent, sodium borohydride (NaBH₄), was added during the dissolution process. NaBH₄ was added after ∼11 min of the NaOH addition when the SPR band of BSA-AgNP was already decreased by half (Fig. 3a). The SPR band (λ_max = 415 nm) of AgNP continues to diminish similarly before and after the addition of NaBH₄ (Fig. S3^†). Thus, NaBH₄ may not have any significant effect on the dissolution process of AgNP. However, it has a strong impact on the modulation of the fluorescence; a new fluorescence band was developed at ∼650 nm within a much shorter duration (1 h) (Fig. 3b). The solution exhibits a bright-red fluorescence under a UV lamp (Fig. 3b, inset) with a quantum yield of 3.5%.Open in a separate windowFig. 3Early time evolution of (a) UV-visible and (b) emission spectra (λ_ex = 370 nm) of the BSA-protected AgNPs upon subsequent treatments with NaOH (pH 11.5) and NaBH₄ at t = 0 and 11 min, respectively. The decrease of the SPR band at 415 nm and a concomitant increase of the fluorescence band at ∼650 nm indicates dissolution of the AgNPs and formation of the red-emitting cluster. The inset shows the visuals of the AgNCs formed after 1 h under normal light and UV light. (c) TEM of the red-emitting AgNCs along with the HRTEM image and the analyzed size distribution in the inset. (d) MALDI-mass spectra of the BSA protein and BSA-capped red-emitting AgNCs.TEM measurements of the red-emitting species show homogeneous distribution of AgNCs with ∼2.25 ± 0.25 nm diameter (Fig. 3c). MALDI-mass experiment further assigned the red-emitting species as Ag₁₃ cluster (Fig. 3d). The excitation spectrum (λ_em = 650 nm) displays two distinct peaks at 370 nm and 470 nm, which match closely to the reported excitation peaks of the Ag_13–15 clusters within BSA/HSA capping (Fig. S4^†).^30,32,33 Moreover, the fluorescence decay of the red-emitting-AgNCs converted from AgNP almost matches with those prepared directly from AgNO₃; both display very similar average lifetimes (0.95 ns vs. 0.89 ns) (Fig. S6 and Table S1^†).³⁷Another important observation is that the red-emitting AgNCs have only transient stability at 37 °C. With further incubation, the absorbance at ∼470 nm (characteristic excitation peak of the red-emitting cluster) reduces and the absorbance at 370 nm (excitation peak of the blue-emitting cluster) increases simultaneously (Fig. 4a). The red-emission at 650 nm also decreases gradually with a concomitant increase of a blue emission band at 465 nm (Fig. 4b). Thus, both absorption and emission measurements clearly indicate transformation of red- to blue-emitting clusters which takes up to ∼15 days for completion. The solution finally becomes light yellow and exhibits a bright blue fluorescence under UV light similar to the blue-emitting cluster obtained earlier from the AgNP in the absence of NaBH₄. Interestingly, other characteristics of the regenerated blue-emitting AgNCs (converted from Ag₁₃NCs) also match quite nicely with the directly prepared blue-AgNCs (converted from AgNPs in the absence of NaBH₄). The size of this blue cluster was 2.04 ± 0.12 nm, which is similar to the previously obtained direct blue-emitting cluster (2.10 ± 0.28) (Fig. 4c). Furthermore, MALDI-mass measurement reveals that both the blue-emitting clusters may have the same composition, Ag₈ (Fig. 4d). In addition to this, the average lifetime (0.53 ns) of the blue-emitting AgNCs synthesized from AgNP agrees well to the average lifetime (0.40 ns) of the blue-emitting AgNCs converted from the red-emitting AgNCs (Fig. S7 and Table S2^†). However, the quantum yield (23%) of blue-emitting AgNCs, converted from red-emitting AgNCs, was higher than the quantum yield (18%) of the blue-emitting AgNCs, converted from AgNPs. Since, the emission characteristics of the blue and the red-emitting clusters nearly matches with earlier report, we expect that silver may be present in the zero oxidation state as determined in those studies.^30,31Open in a separate windowFig. 4Transformation of red-emitting to blue-emitting cluster: (a) UV-visible and (b) emission spectra (λ_ex = 370 nm) showing transformation of the BSA-protected red-emitting Ag₁₃NCs (obtained at 1 h) to blue-emitting AgNC upon prolonged incubation. Red and blue arrows respectively denote the decrease/increase of the red/blue cluster absorbance and emission intensity with time. The inset (b) shows a magnified wavelength region in 580–720 nm of the emission spectra. (c) TEM image of the blue-emitting silver nanocluster while its inset shows HRTEM image with size histogram of corresponding silver nanocluster. (d) MALDI-mass spectra of BSA and BSA-containing blue-emitting silver nanocluster synthesized from Ag₁₃NCs.Moreover, the atomic composition of the NCs can be also be estimated from the Jellium model using the equation^38,39E_em = E_Fermi/N^0.33where E_Fermi is the Fermi energy of the metal (Ag), E_em is the emission energy of the MNCs and N is the number of atoms constituting a MNC. Using the model equation, the number of silver atoms for the blue-emitting AgNCs can be predicted as 8.45 (∼8) Ag atoms which is a good agreement with our MALDI data (8 Ag atoms). However, the theoretical calculation estimated as N ∼ 24 for the red-emitting AgNCs, which is not in agreement with the MALDI data (13 Ag atoms). This is because of the well-known deviation of the Jellium model for higher number of Ag atoms in AgNCs because of increase in the electronic screening effects and the harmonic distortion in the potential energy well.¹⁹Although the red-emitting cluster is not very stable at the experimental condition (pH 11.5, 37 °C), it may be easily stabilized by lowering the temperature or pH. The fluorescence intensity of the red-emitting and blue-emitting cluster kept at 4 °C, was almost preserved for more than 15 days (Fig. S8^†). On the other hand, lowering the pH to 6, also inhibits the red to blue-cluster transformation (Fig. S9^†). The observations indicate that the red-blue transformation has a moderate activation barrier and the conversion may be governed by the change in the structure of the protein in the alkaline condition. Acidification of the solution can stop the transformation of the protein conformation and inhibits the process.From these observations, we may conclude that the conversion from NPs to NCs occurs in two steps. First, a rapid dissolution of AgNP occurs in the alkaline medium. The kinetics of the dissolution process can be monitored through a time-dependent decrease of the SPR band and the time constant was found to be ∼13 min (Fig. S10^†). Dissolution of AgNPs is an important issue and assumed to be the leading cause of toxicity of AgNPs in biological mediums.⁴⁰ The dissolution is commonly favored at a low pH but drastically inhibited at high pH.⁴¹ The swift dissolution of the BSA-protected AgNP observed here at a high pH (11.5) is unprecedented. Thus, the BSA capping may have an active role in the dissolution process. We comprehend that the oxidation power of protein may be activated in the basic medium.Organothiols (R-SH) are known to promote dissolution of AgNPs; R-SH progressively reacts with Ag atoms to form RS-Ag complex.⁴² Since cysteine is also an organothiol, it is expected to play an essential role in the dissolution of AgNPs. Gondikas et al. showed that excess cysteine could favor the dissolution process of AgNPs, whereas another amino acid, serine (S–H bond is replaced by O–H bond), has no effect.⁴³ Zang and coworkers showed that only the isolated or reduced cysteine in a protein has a dominant role in the dissolution of NPs.⁴⁴ Although BSA contains as many as 35 cysteine residues; 34 of them are involved in S–S bond formation and only a single cysteine is present in free form (S–H). Hence, the dissolution of AgNPs at neutral pH may be negligible.Most proteins rich in sulfur-containing residues (cysteine and methionine) may degrade in alkaline solution. Florence reported that about 5 of 17 S–S bridges in BSA may be cleaved in the presence of 0.2 M NaOH.⁴⁵ Thus, at higher pH, some disulfide bonds may be cleaved and more cysteine residues may participate in the dissolution of BSA-capped AgNPs.In the second step, Ag⁺ ions generated from the dissolution of AgNPs, can be reduced either by the protein capping itself or by an external reducing agent to form NCs (Scheme 1). The tyrosine residues may be responsible for the reduction of the metal ions to NCs.^25,33 At a pH, higher than the pKa (10.46) of tyrosine, the reduction capability of tyrosine is enhanced by deprotonation of the phenolic group.^25,33,46 Moreover, the addition of a strong reducing agent (e.g., NaBH₄) may lead to a faster reduction, which favors quicker nucleation and growth of Ag atoms forming the bigger NCs (Ag₁₃NCs). However, the large Ag₁₃NCs may not be adequately stabilized by the protein conformation at that condition and hence may transform into the more stable blue-emitting Ag₈NCs.Open in a separate windowScheme 1Schematic representation of the transformation of the BSA-capped AgNPs to blue- and red-emitting AgNCs.The conformation change of the protein capping during the conversion was also supported by the circular dichroism (CD) measurements (Fig. S11^†). The formation of AgNPs results in a negligible change in the protein conformation (Table S3^†). However, the formation of red Ag₁₃ cluster results in a substantial modification in the BSA conformation. The α helix content reduces from 57% to 49%, whereas coil randomness increases from 17% to 21% without a major change in the β sheet. Interestingly, blue-emitting Ag₈ cluster perturbed the conformation of the BSA to a much larger extent (Table S3^†). As the cysteine disulfide bond has a direct role on maintaining the folded conformation of BSA, its breaking may change the protein conformation. The addition of NaOH induces breaking of S–S bond, which leads to formation of AgNCs with subsequent change in protein secondary structure.In conclusion, we report an unprecedented fast dissolution of AgNPs through activation of the protein (BSA) capping by elevating the pH of the medium to 11.5. At higher pH, the disulfide bonds may be cleaved, and the free cysteine may activate the dissolution process. The protein capping also plays a crucial role in the formation of fluorescent nanocluster after the completion of the dissolution process. Thus, we explored multiple roles of the BSA capping – (1) a stable capping agent at neutral pH to stabilize the AgNPs (2) activates the dissolution process probably via oxidative dissolution of the AgNPs (3) adsorbing the nascent silver ions within its scaffold and (4) finally reducing them to fluorescent nanocluster.