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.     

Highly dispersed Pt nanoclusters supported on zeolite-templated carbon for the oxygen reduction reaction

The formation of highly dispersed Pt nanoclusters supported on zeolite-templated carbon (PtNC/ZTC) by a facile electrochemical method as an electrocatalyst for the oxygen reduction reaction (ORR) is reported. The uniform micropores of ZTC serve as nanocages to stabilize the PtNCs with a sharp size distribution of 0.8–1.5 nm. The resultant PtNC/ZTC exhibits excellent catalytic activity for the ORR due to the small size of the Pt clusters and high accessibility of the active sites through the abundant micropores in ZTC.

Electrochemically synthesized highly dispersed Pt nanoclusters (PtNCs) stabilized by the nanocages of zeolite-templated carbon (ZTC) exhibit excellent electrocatalytic performance toward the oxygen reduction reaction.

Platinum (Pt) is currently considered one of the best electrocatalysts for the oxygen reduction reaction (ORR), which occurs at the cathode of a fuel cell and is the key process determining the overall performance.^1–5 However, the high cost and scarcity of Pt limit its wide commercialization in this field. According to the US Department of Energy, the total Pt loading is required to be below 0.125 mg cm⁻², in contrast to a presently used Pt loading of 0.4 mg cm⁻² or more for fuel cell application.⁴ Therefore, reducing the Pt loading without loss or with an improvement of the cathode performance has received significant interest in electrocatalytic research for fuel cell systems.^6–10 In this regard, reducing the size of Pt particles to a nanocluster scale (size < 2 nm) and maximizing the Pt dispersion may offer an efficient way to achieve maximum utilization of the Pt electrocatalyst with appropriate consumption.^4,11–15The size of nanomaterials generally plays a critical role in controlling the physical and chemical properties for catalytic applications.^16–20 With a decrease in the particle size to the nanoscale, quantum size effects are induced, which alter the surface energy of the material due to unsaturated coordination and change in the energy level of the d orbital of metal atoms, leading to spatial localization of the electrons.^17–20 This size-induced effect on the electronic structures at the active sites modifies the capability of binding the reactant molecules in catalytic reactions, thereby altering the activity of the nanocatalyst.²⁰ When the particle contains a few to several dozens of atoms with sizes, ranging from sub-nanometer to 2 nm often termed as nanocluster that bridges nanoparticle and a single atom.²¹ However, the Pt single atom is not an appropriate electrocatalyst for the ORR in a fuel cell system as the fast four-electron (4e⁻) pathway for the reduction of O₂ to H₂O requires at least two neighboring Pt atoms.^22,23 Anderson''s group demonstrated that the ratio between the production of H₂O (product of 4e⁻ process) and H₂O₂ (2e⁻) in the ORR strongly depends on the number of atoms in the Pt cluster. Typically, it requires more than 14 atoms in a Pt cluster to produce H₂O efficiently through the 4e⁻ pathway of the ORR.²⁴ Therefore, Pt nanoclusters having more than a dozen atoms have proven to be highly efficient ORR electrocatalysts for fuel cell systems.^13–15 Upon decreasing the size of the nanoparticles to a nanocluster, the electronic state and structure are known to be changed, leading to an increase of the catalytic activity in the ORR. Therefore, it is highly desirable to synthesize a Pt nanocluster-based material as an ORR electrocatalyst with high catalytic performance. To date, several synthesis strategies, such as wet-chemical, atomic-layer deposition, and photochemical methods, have been applied for the preparation of well-dispersed Pt nanoclusters on different types of support, such as dendrimer, metal oxide, and carbon materials.^{13–15,25–31}An alternative approach to synthesize Pt nanocluster (PtNC) is the encapsulation of the cluster within nanosized pores, for example, by utilizing microporous (diameters less than 2 nm) carbon materials.³² Among the microporous carbons, zeolite-templated carbon (ZTC) has been attractive for supporting Pt clusters due to its ordered microporous structure.^33–37 ZTC is a potentially promising material as catalyst support as it offers the advantages of extremely large surface area and high electrical conductivity of graphene-like carbon frameworks constituting a three-dimensional (3D) interconnected pore structure.³⁶ Moreover, the micropores of ZTC can serve as nanocages for stabilization of the Pt nanoclusters. Coker et al. used Pt²⁺ ion-exchanged zeolite as a carbon template to synthesize Pt nanoparticles in ZTC with size in a range of 1.3 to 2.0 nm.³³ Recently, atomically dispersed Pt ionic species was synthesized via a simple wet-impregnation method on ZTC containing a large amount of sulfur (17 wt%).²³ Itoi et al. synthesized PtNC consisting of 4–5 atoms and a single Pt atom in ZTC using the organoplatinum complex.³⁷ Although these methods produced Pt nanoclusters with narrow size distribution and atomic dispersion, they required multi-step processes and/or high-temperature treatment (>300 °C). High-temperature treatment often induces the sintering of nanoclusters to aggregated clusters. Therefore, it is highly desirable to develop a simple and low-cost method for the preparation of PtNC supported on ZTC (PtNC/ZTC) for use as an efficient ORR electrocatalyst. The electrochemical reduction approach offers an alternate and efficient route for the synthesis of PtNC in the micropores of ZTC. The electrochemical method is one of the popular ways to prepare electrocatalysts because it is a simple single-step procedure and ensures electrical contact between the nanoparticles and the support.^38,39Herein, we report a facile electrochemical method for the formation of PtNC with a narrow size range of 0.8–1.5 nm supported on ZTC. The resultant PtNC/ZTC shows higher electrocatalytic activities towards ORR compared to that of commercial Pt/C. Here, ZTC plays two important roles: (i) it provides nanocages to stabilize the PtNC and (ii) it accelerates the ORR activity by enhancing the accessibility of active sites through its abundant micropores. Fig. 1a shows a schematic representation of the typical electrochemical synthesis of PtNC/ZTC. In the first step, ZTC was impregnated with a Pt-precursor dissolved in a water–ethanol mixture. As ZTC possess ordered micropores (Fig. S1a^†) with high Brunauer–Emmett–Teller (BET) surface area of 3400 m² g⁻¹ (vide infra), the uniform adsorption and anchorage of PtCl₆²⁻ ions into the micropores of ZTC was favored. After impregnating and drying, the resultant ZTC–PtCl₆²⁻ was mixed with water–ethanol and Nafion to make the ink for the preparation of the electrode. Using the prepared electrode, a potential of 0.77 V vs. reversible hydrogen electrode (RHE) (Fig. 1b) was applied followed by potential cycling between 1.12 to −0.02 V vs. RHE until the cyclic voltammogram was stabilized. The Pt content of PtNC/ZTC was determined to be ∼10 wt% (Fig. S2^†) by thermogravimetric analysis (TGA). The obtained PtNC/ZTC was electrochemically characterized by cyclic voltammetry and electrochemical impedance spectroscopy. The cyclic voltammogram (Fig. 1c) after potential cycling in fresh KOH electrolyte shows the characteristic Pt peaks corresponding to hydrogen adsorption and desorption. The Nyquist plots (Fig. 1d) demonstrate that PtNC/ZTC has lower electrolyte resistance (42 Ω) than that of ZTC (70 Ω), implying an improvement in the conductivity of ZTC by the presence of PtNC. Due to the increase in the conductivity, PtNC/ZTC could facilitate the electron transfer more effectively than ZTC, enhancing its electrocatalytic activity.Open in a separate windowFig. 1(a) Illustration for the formation of PtNC/ZTC:Pt-precursor was impregnated into ZTC micropores, and then a potential (0.77 V vs. RHE) was exerted on the ZTC–PtCl₆²⁻ composite in a 0.1 M KOH solution to form PtNC/ZTC (b) Chronoamperometric response of ZTC–PtCl₆²⁻ at a constant potential of 0.77 V (vs. RHE) in 0.1 M KOH electrolyte. (c) Cyclic voltammogram of PtNC/ZTC in a fresh 0.1 M KOH at a scan rate of 20 mV s⁻¹. (d) Nyquist plots of ZTC and PtNC/ZTC in 0.1 M KOH. Fig. 2a and b show images from aberration-corrected scanning transmission electron microscope (STEM) with high-angle annular dark-field (HAADF). The HAADF-STEM images exhibit the typical morphology of the final product (PtNC/ZTC) after electrochemical reduction. As shown in Fig. 2a, it is very clear that isolated PtNCs are uniformly dispersed in ZTC. These PtNCs have a homogeneous distribution with a narrow size range (0.8–1.5 nm, Fig. 2b). On further magnification, the STEM image shows a cluster-like structure of Pt (Fig. 2c). The STEM image of selected PtNC (Fig. 2d) reveals that it consists of ∼20 atoms. The number of atom content in PtNC was further determined by matrix-assisted laser-desorption-ionization time-of-flight (MALDI-TOF) mass spectrometry using trans-2-[3-(4-test-butylphenyl)-2-methyl-2-propenylidene]malononitrile as the matrix.^40,41 As shown in Fig. S3,^† MALDI-TOF measurement produces a mass spectra with a predominant peak centered at ∼3700 Da corresponding to the Pt₁₉ cluster. The TEM image (Fig. S4 a and b^†) validates the formation of PtNC with an average size of 0.9 nm. In addition, the energy dispersive X-ray spectrometer (EDS) mapping images clearly shows the uniform dispersion of Pt nanocluster in ZTC (Fig. S4c^†). The X-ray powder diffraction (XRD) pattern (Fig. 2e) of PtNC/ZTC showed three broad peaks associated with small size metallic Pt corresponds to (111), (200), and (311) planes (Fig. 2e, inset), along with peaks of ZTC at 2θ = 7.8° and 14.9° corresponding to the ordered microporous structure. Along with the structural analysis, the porous texture of PtNC/ZTC was examined by Ar adsorption (Fig. 2f). PtNC/ZTC had a high BET surface area of 2360 m² g_ZTC⁻¹, which is 1.4 times lower than that of pristine ZTC (3400 m² g_ZTC⁻¹). The decrease in Ar adsorption capacity after the formation of PtNC in ZTC is interpreted as a result of the filling of ZTC micropores by PtNC. This micropore filling was confirmed in the pore size distributions of the pristine ZTC and the metal-loaded carbon (inset of Fig. 2f). The X-ray photoelectron spectroscopy (XPS) results reveal the signature of Pt in ZTC (Fig. S5^†). The elemental survey (Fig. S5a^†) shows the signature of C 1s, O 1s, F 1s (Nafion), and Pt 4f. The chemical nature of Pt in PtNC/ZTC was inspected by a detailed Pt 4f XPS analysis. The deconvoluted Pt 4f XPS spectra (Fig. S5b^†) reveals the presence of both metallic and ionic Pt species. The peaks observed at 71.0 (4f_7/2) and 74.2 (4f_5/2) eV correspond to metallic Pt whereas the other peaks positioned at 72.6 (4f_7/2) and 76.0 (4f_5/2) are attributed to Pt²⁺ and the peaks at 74.9 (4f_7/2) and 77.8 (4f_5/2) eV are attributed to Pt⁴⁺ originating from the surface oxidation of metallic Pt.⁴²Open in a separate windowFig. 2(a–d) Representative spherical aberration-corrected HAADF-STEM images of PtNC/ZTC at various magnifications. (e) XRD pattern of PtNC/ZTC and (f) Ar adsorption–desorption isotherms of ZTC and PtNC/ZTC. Inset in (e) shows a 30 times magnified high-angle region of XRD of PtNC/ZTC. Inset in (f) shows the pore size distributions of the ZTC and PtNC/ZTC.The formation of narrow sized PtNC by the electrochemical method can be ascribed to the stabilization of PtNC in the ZTC micropores, which serve as cages to impose a spatial limitation on the size of the Pt clusters. For comparison, Pt supported on ZTC was also prepared by the conventional incipient wetness impregnation and subsequent H₂-reduction at high temperature (300 °C). The Pt obtained by this incipient wetness impregnation method shows the formation of Pt nanoparticles on the exterior surface of ZTC (PtNP/ZTC) (Fig. S6^†). The formation of larger Pt nanoparticles is due to the sintering at high temperature, showing that even ZTC micropores could not prevent the aggregation of PtNCs at high temperatures. Fig. 3 shows the electrochemical ORR activity of PtNC/ZTC using linear sweep voltammetry (LSV) technique on a rotating disc electrode (RDE) in a 0.1 M KOH solution saturated with O₂ at a scan rate of 5 mV s⁻¹. The ORR activity of ZTC (without PtNC) was measured for comparison as well. As shown in Fig. 3a, PtNC/ZTC exhibited higher diffusion limiting current density and higher positive onset and half-wave potential compared to ZTC alone, indicating that PtNC is the active center for the ORR. To investigate the effect of the Pt loading amount on the ORR activity, PtNC/ZTC with various Pt loadings, 2–20 wt%, was used for the measurement of LSV at 1600 rpm. With an increase in Pt content, both the onset and half-wave potential shifted towards more positive potential up to 10 wt% loading of Pt (Fig. 3a and S7^†). Upon further increase of loading of Pt on ZTC to 20 wt%, both the onset and half-wave potential of PtNC/ZTC shifted towards less positive potential along with a slight decrease in the diffusion limiting current density (Fig. 3a). The decrease in the ORR activity of PtNC/ZTC at high loading of Pt (20 wt%) was attributed to the decrease in the electrochemically active surface area (Fig. S8^†) and decrease in the specific surface area (Fig. S9^†). The STEM image clearly shows that the aggregated Pt clusters were formed on the exterior surface of ZTC at 20 wt% loading of Pt (Fig. S10c^†), blocking the accessibility of active sites. Therefore, PtNC/ZTC with the optimum loading of 10 wt% of Pt leads to superior ORR activity with a high positive onset potential of 0.99 V, which is similar to commercial Tanaka Pt/C (Pt/C-TKK) (Fig. 3b), and a half-wave potential of 0.87 V, which is ∼10 mV more positive than that of commercial Pt/C-TKK (0.86 V) (Fig. 3b). Compared to the case of PtNC/ZTC, both the onset and half-wave potential of PtNP/ZTC prepared by the conventional incipient wetness impregnation and subsequent H₂-reduction with the same loading of Pt exhibited a less positive value (Fig. S11^†). The poorer activity of PtNP/ZTC is due to the blockage of active sites by larger PtNPs formed on the exterior surface of ZTC (Fig. S6^†).Open in a separate windowFig. 3(a) RDE ORR polarization curves of PtNC/ZTC with different mass loading of Pt. (b) Comparison of PtNC/ZTC (PtNC_10%/ZTC) with commercial Pt/C-TKK at the same loading of 40 μg_Pt cm⁻². (c) RDE ORR polarization curves of PtNC/ZTC at different rotation speeds. Inset in (c) shows the corresponding K–L plots at different potentials. (d) Represents the kinetic current density values of Pt/C-TKK and PtNC/ZTC at the potential of 0.8 V vs. RHE.To investigate the kinetics of the ORR activity of PtNC/ZTC, LSV measurements were performed with RDE at different rotating rates (Fig. 3c), and the kinetics was analyzed using a Koutecký–Levich (K–L) plot (Fig. 3c, inset). From Fig. 3c, it was observed that the current density increases with the increasing speed of rotation of the electrode, which is characteristic of a diffusion-controlled reaction. The corresponding linear K–L plots (Fig. 3c, inset) with a similar slope at different potentials reveal that the number of transferred electrons was ∼4, indicating that O₂ is directly reduced to OH⁻ and the ORR is dominated by the H₂O₂-free 4e⁻ pathway. To estimate the amount of produced peroxide ion, rotating ring-disc electrode (RRDE) measurement was performed and the produce peroxide ion calculated from RRDE curve was < 4% (Fig. S12^†). The kinetic current density (J_k) obtained from K–L plot at the potential of 0.8 V (Fig. 3d) for PtNC/ZTC (J_k = 50 mA cm⁻²) is 2.2 times higher than that of commercial Pt/C-TKK (J_k = 22 mA cm⁻²).As Pt-based electrocatalysts are known to be highly active in an acidic medium, the ORR activity of PtNC/ZTC in O₂-saturated 0.1 M HClO₄ was also evaluated by comparing it with that of commercial Pt/C-TKK with the same loading of Pt on the electrode surface using RDE at a scan rate of 5 mV s⁻¹. The PtNC/ZTC-based electrode exhibited ORR activity with an onset potential of 0.96 V (Fig. 4), which is close to that of Pt/C-TKK (0.98 V), and half-wave potentials of 0.84 V, which is 20 mV more positive than that of Pt/C-TKK (0.82 V). PtNC/ZTC showed a slightly higher diffusion-limiting current density of ∼5.9 mA cm⁻² (0.4–0.7 V) compared with that of the Pt/C-TKK catalyst (∼5.6 mA cm⁻²). The kinetics of the ORR in an acidic medium was further analyzed using RDE at different rotation rates (Fig. S13^†) and it was observed that the current density increases with the increasing speed of rotation of the electrode, as in the case of the alkaline medium. The number of electron involved and the amount of produced H₂O₂ estimated by RRDE measurement were ∼4 and < 5%, respectively (Fig. S14^†). The mass activity of PtNC/ZTC obtained using the mass transport corrected kinetic current at 0.8 V is 0.15 A mg⁻¹, which is 3.2 times higher than that of Pt/C-TKK (0.046 A mg⁻¹).Open in a separate windowFig. 4(a) RDE ORR polarization curves at 1600 rpm and (b) mass activity at 0.8 V of PtNC/ZTC and Pt/C-TKK in 0.1 M HClO₄.Furthermore, the methanol tolerance of PtNC/ZTC was assessed by intentionally adding methanol to the oxygen saturated electrolyte solution (both in alkaline and acidic media). The commercial Pt/C-TKK was used for comparison as well. The peak current densities for methanol oxidation with PtNC/ZTC were ∼2.8 and ∼3 times lower than that of Pt/C-TKK in alkaline (Fig. 5a) and acidic (Fig. 5b) media, respectively. These results indicate that PtNC/ZTC has much higher tolerance towards methanol than Pt/C-TKK does. This higher methanol tolerance of PtNC/ZTC can be attributed to the small size of the Pt cluster, which may not be sufficient to catalyze the oxidation of methanol efficiently, as the oxidation of methanol requires Pt ensemble sites.⁴³Open in a separate windowFig. 5ORR polarization curves of PtNC/ZTC and Pt/C-TKK in the absence (solid line) and presence (dotted line) of 0.1 M of CH₃OH at a rotation rate of 1600 rpm in (a) alkaline and (b) acid media.The durability of PtNC/ZTC was also investigated by the amperometric technique. The test was performed at a constant voltage of the half-wave potential in an O₂-saturated alkaline medium and at 0.7 V in an O₂-saturated acidic medium at a rotation rate of 1600 rpm (Fig. S15a and b^†). The durability of the PtNC/ZTC catalyst in the alkaline medium was higher than that of Pt/C-TKK, exhibiting a 30% decrease compared to a 40% decrease of Pt/C-TKK in 5.5 h of ORR operation (Fig. S15a^†). The higher durability of PtNC/ZTC compared to Pt/C-TKK in the alkaline medium may be due to the stabilization of PtNC by pore entrapment. In the acidic medium, however, PtNC/ZTC exhibited a 54% decrease in the initial current after 5.5 h of operation while a 33% decrease was observed in the case of Pt/C-TKK (Fig. S15b^†). The decrease in ORR activity in the acidic medium may be due to the leaching out of tiny Pt nanoclusters in acid electrolyte from the ZTC micropores. To understand the decrease in the ORR durability with time, STEM measurements of PtNC/ZTC after 5.5 h of ORR operation were performed. In the alkaline medium, the STEM image of post-ORR PtNC/ZTC shows a slight change in the size of PtNC (Fig. S15c^†) while the STEM image of PtNC/ZTC after ORR in the acidic medium exhibited sintering of PtNC into large particles with an average size of 30 nm (Fig. S15d^†), resulting in a decrease of the ORR activity. In the alkaline medium, the decrease in ORR activity with time may be due to the oxidation of the ZTC support in KOH.⁴⁴We attributed the excellent ORR activity of PtNC/ZTC to the interplay between the following: (1) the structure of the Pt cluster possessing a high ratio of surface atoms that benefits the surface reactions,^45–47 (2) the microporous 3D graphene-like structure of the ZTC support that enables easy access of O₂ and electrolyte molecules to the active sites,⁴⁸ and (3) the high conductivity and large accessible surface area of ZTC that facilitates the electron transfer.^49–51     

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.     

A stable tunnel-type NaGe3/2Mn1/2O4 anode for Na-ion batteries

Tunnel-type NaGe_3/2Mn_1/2O₄ was fabricated for anode of sodium ion batteries, delivering a discharge capacity of 200.32 mAh g⁻¹ and an ultra-low potential platform compared with that of pure Na₄Ge₉O₂₀ (NGO). The results of X-ray photoelectron spectroscopy (XPS) demonstrate that Ge redox occurs, and partial substitution of Mn effectively improves the Na-storage properties compared to those of NGO.

We investigated tunnel-type NaGe_3/2Mn_1/2O₄; the main structure is Na₄Ge₉O₂₀. NaGe_3/2Mn_1/2O₄ electrodes as anodes for sodium ions batteries deliver a discharge capacity of 200.32 mAh g⁻¹ and satisfactory capacity retention after 50 cycles.

In terms of the high abundance and ready availability of sodium, sodium-ion batteries (SIBs) have been generally regarded as a better alternative to lithium-ion batteries for power stations.^1–4 Hard carbon is widely recognized as one of the most attractive and ideal anode materials for SIBs.^5,6 However, the potential required for sodium ions to insert into hard carbon is very close to that for sodium plating, resulting in sodium dendrites, which raise safety concerns.^7,8 Moreover, the reaction of electrode materials with sodium through alloying or conversion mechanisms always results in serious volume changes in the process of sodium insertion and extraction.⁹ Therefore, insertion-type transition-metal oxides as anodes have attracted much attention owing to their suitable operating potentials and minor volume expansion.^10,11 Recently, embedded titanium/vanadium/molybdenum based oxides with layered structures have been studied as anode materials for SIBs,¹² such as layered Na₂Ti₃O₇,¹³ tunnel Na₂Ti₆O₁₃ (ref. 14) and spinel Li₄Ti₅O₁₂.¹⁵ In addition, post-spinel structured materials have been proposed, which show ultra-stable cycle performances via highly reversible sodium-ion insertion/desertion through large-size tunnels. Recently, in Zhou''s group, NaVSnO₄ (ref. 16) and NaV_1.25Ti_0.75O₄ (ref. 17) have been prepared and they have been shown to possess robust cycle lifetimes (more than 10 000 cycles) and discharge plateaus of 0.84 V and 0.7 V, respectively. Meanwhile, in our group, Na_0.76Mn_0.48Ti_0.44O₂ has been developed, which holds an initial discharge capacity of 103.4 mAh g⁻¹, shows a superb rate capability and retains 74.9% capacity after 600 cycles.¹⁸ The large radius of the redox active metal center could optimize the tunnel size and thus boosting the electrochemical performance. It is also a big challenge to find further suitable active centers for insertion-type transition metal oxides as anodes of SIBs. Besides, a host of published reports have said that germanium-based materials can be used as alloy anodes for SIBs with highly reversible sodium storage properties and satisfactory ionic/electronic conductivity.¹⁹ However, it is unclear to us whether Ge could act as an active center in a transition-metal oxide anodes.In this work, we fabricated a tunnel-type NaGe_3/2Mn_1/2O₄ (NGMO) material. When used as the anode of SIBs, it delivers a sustained discharge capacity of 200.32 mAh g⁻¹. Compared with NaVSnO₄ (ref. 16) and NaV_1.25Ti_0.75O₄,¹⁷ NGMO delivers a lower safety voltage of 0.36 V. Pure Na₄Ge₉O₂₀ (NGO) as a comparative sample, only exhibits a capacity of 24.8 mAh g⁻¹, which is far inferior to that of NGMO. During discharge and charge process, reversible redox reactions around Ge center occur, as confirmed by X-ray photoelectron spectroscopy (XPS) analysis. The introduction of Mn in the NGMO improves the reversibility of the Ge redox performance.The structure of NGMO was carefully characterized by XRD, and Rietveld refinement was performed as depicted in Fig. 1. The main Bragg peaks of NGMO could be assigned to space groups of P1(2) and I4₁/a(88), which were fitted to give lattice parameters of a = 10.56/15.04 Å, b = 11.18/15.04 Å, and c = 9.22/7.39 Å, and a volume of 811.2/1672.2 ų, respectively. Na₄Ge₉O₂₀ has a typical tunnel structure, which consists of polymerized Ge/MnO₄ tetrahedra connected with Ge/MnO₆ octahedra. Four Ge/MnO₆ octahedra are connected together by sharing edges to form a tetrameric (Ge/Mn)₄O₁₆ cluster. Each cluster is connected to six GeO₄ tetrahedra, and adjacent clusters are connected by GeO₄ tetrahedra. Na atoms are located in the channels and have elongated Na–O bonds.¹⁹ This highly stable crystal structure can effectively accelerate the migration of sodium ions.²⁰Fig. 2 shows the low and high magnification scanning electron microscopy (SEM) images of NGMO, which is composed of particles of different sizes from 1 to 3 μm; the larger particles are the result of sintering at high temperature. SEM images of NGO with different magnifications are given in Fig. S1,^† and show that the average particle size of NGO is 1 μm.Open in a separate windowFig. 1The Rietveld refinement spectra of NGMO.Open in a separate windowFig. 2(a) and (b) SEM images of NGMO at different magnifications.The morphology and fine structure were studied by transmission electron microscopy (TEM). Fig. 3a and b show the low magnification TEM images. It can be seen from the images that NGMO has an irregular sheet-like morphology with particle sizes from 250 nm to 2 μm. As shown in Fig. 3c, the lattice spacing of the (200) plane is 4.55 Å. In the SAED pattern of Fig. 3d, the red line corresponds to the (020) plane in NGMO, and the lattice spacing is 13.100 Å. These results clearly demonstrate that NGMO exhibits good crystallinity. The corresponding energy dispersive X-ray spectroscopy (EDS) results, and Raman and infrared spectra (IR) are provided in Table S1 and Fig. S2.^† The results indicate that the atomic ratio of Na : Ge : Mn is close to 1 : 1.5 : 0.5 and that there is little sodium loss. The Raman and IR peaks in the high frequency region are attributed to stretching vibrations of Ge–O–Ge and the peaks between 600 and 400 cm⁻¹ are attributed to the bending vibrations of Ge–O–Ge in NGMO.Open in a separate windowFig. 3(a) and (b) Low resolution TEM images, (c) a HRTEM image and (d) a SAED image of NGMO.Galvanostatic electrochemical measurements were evaluated in a voltage range of 0.05–2.0 V, with the current density of 20 mA g⁻¹. Fig. 4a and b show the discharge and charge profiles of NGMO and NGO, respectively. Because of the formation of a solid electrolyte interface (SEI) layer in the initial cycle, the electrochemical behaviour tends stabilize in the second cycle, so voltage profiles are given from the second cycle; the first cycles of the discharge–charge curves of NGMO and NGO materials are given in Fig. S3.^† It can be seen intuitively that both NGMO and NGO have low voltage platforms, while NGMO has the smaller polarization. In Fig. 4a, we notice a reversible voltage profile in the second cycle with discharge capacity of 200.32 mAh g⁻¹ for NGMO. Only NGMO has a flat potential platform and delivers an ultra-low plateau potential. It can be seen from Fig. 4b that NGO shows a capacity of 24.8 mAh g⁻¹, obvious polarization at the 20^th cycle and increased capacity due to the surface side-effect. Fig. 4c indicates that the capacity retention of the NGMO electrode after 50 cycles is 86.2%, which is superior to that of NGO; the coulombic efficiency of NGMO is also provided in Fig. S4.^† To further understand the redox reactions along with the discharge/charge process in NGMO, Fig. 4d displays the differential capacity versus voltage (dQ/dV) curve. The clear anodic peak at 0.33 V and cathodic peak at 0.81 V correspond well with the redox reactions of NGMO.Open in a separate windowFig. 4Electrochemical performance: (a) and (b) voltage profiles of NGMO and NGO, respectively; (c) cycling performance; (d) dQ/dV profile of NGMO. The current density was controlled at 20 mA g⁻¹ over a voltage range of 0.05–2.0 V.The electrochemical impedance spectra (EIS) of fresh and cycled electrodes of NGMO and NGO, with a frequency range of 0.01 Hz to 100 kHz, are shown in Fig. 5. From Fig. 5a, it is obvious that the charge-transfer resistance of the fresh NGMO electrode is lower than that of NGO. This indicates that the migration of charges in the NGMO material occurs more easily than in NGO, which also facilitates the shifting of ions on the surface and inside of the electrodes of NGMO. In Fig. 5b, NGMO electrode in its 10^th cycle exhibits a smaller charge-transfer resistance than both NGO and the NGMO fresh electrode, indicating that the surface of NGMO more readily forms a stable SEI film.¹⁸ The EIS results were fitted by the model shown in Fig. S5.^† The fitting results are provided in Table S2.^† The resistances of the fresh and cycled electrodes of NGMO and NGO are composed of an internal resistance (R_s), the resistance of the surface film (SEI) (R_f; a small semicircle in the high frequency region), the resistance of the charge transfer (R_ct; another opposite semicircle in the middle frequency region), and the Warburg resistance (W; an oblique line in the low frequency region).²¹ Both the fresh and cycled electrodes of NGMO deliver lower charge transfer resistance than NGO. Meanwhile, the transfer resistance of the cycled NGMO electrode is lower than that of the fresh electrode and its slope at low frequency is higher than that of the fresh one (Table S2^†). All these results show that NGMO has lower resistance and better electronic/ionic conductivity than NGO.Open in a separate windowFig. 5Nyquist plots of (a) fresh NGMO and NGO electrodes, and (b) NGMO and NGO electrodes after ten cycles.The evolution of the chemical valence states of the 150-times discharged electrodes were observed by XPS and SEM as provided in Fig. S6.^† It is generally clear that the electrode surface was covered with a thick SEI layer after discharging. Ar plasma etching was used to obtain the internal information. The Ge 3d core-level of the discharged NGMO electrode with and without etching is shown in Fig. S7.^† Before etching, the peaks of the Ge 3d core-level could be fitted to Ge¹⁺ and Ge²⁺.²² After etching, (Fig. S7c^†), the peaks at 30.8 eV and 30.2 eV were also associated with Ge¹⁺ and Ge²⁺, respectively. This result indicates that the valence of Ge decreases as a whole and that there is no obvious difference between the etched and non-etched samples. The reversible redox reactions of Ge remain stable even after cycling. Meanwhile, the Mn 2p core-level spectra are shown in Fig. S7d–f.^† For the Mn 2p core level, owing to the spin orbit coupling, the valence states of Mn comprise two couples including Mn³⁺ and Mn²⁺ (Fig. S7e and f^†). The binding energies of Mn³⁺ are 642.37 eV and 654.04 eV, and the binding energies of Mn²⁺ are 640.71 eV and 652.18 eV. Similarly, after discharging, the binding energies of Mn³⁺ are 642.40 eV and 654.06 eV, and those for Mn²⁺ are 640.69 eV and 652.20 eV, indicating that there are no changes in Mn binding energies before and after etching. This is in good agreement with results in previous reports.^23,24 All results also show that a thin SEI layer has been formed, favoring ions transfer on the repeatedly cycled electrode. It can be seen from the refined XRD results that NGMO consists of Ge⁴⁺, Mn²⁺ and Mn³⁺, and combined with XPS analysis, the results show that the valence states of Mn does not change for the discharged NGMO electrode. Ge displays electrochemical activity in NGMO, and Mn exhibits good chemical stability in the framework.     

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

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

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

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

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).     

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

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

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

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

An aluminium fluorosensor for the early detection of micro-level alcoholate corrosion

The detection of the dry alcoholate corrosion of aluminium is vital to design a corrosion resistive aluminium alloy for the storage and transportation of biofuel (methanol or ethanol). By synthesizing an Al³⁺ fluorescent probe operable in an alcoholic medium, we quantified the alcoholate corrosion in terms of the fluorometrically estimated soluble alkoxide (Al(OR)₃) generation under nitrogen atmosphere. With time, a linear increase in corrosion with specific aluminium dissolution rate constants ∼2.0 and 0.9 μg per day per cm² were estimated for aluminium and Al-7075 alloy, respectively. During open atmosphere monitoring, the adsorbed moisture converted small extent of Al(OR)₃ to the insoluble Al(OH)₃ at the alloy surface which retarded the alcoholate corrosion appreciably.

The monitoring of aluminium alcoholate corrosion using a fluorescent probe operable in alcohol.

Switching over from conventional fossil fuel to biofuel is of current interest owing to the maximum utilization of eco-friendly non-conventional energy.¹ Commercially produced less polluted biofuels such as methanol and ethanol, mixed with fossil fuels have an acceptable performance capacity for the gasoline engine.² Moreover, in comparison to the gasoline, methanol and ethanol have much higher octane rating or compression ratio to resist the knocking for better thermal efficiency.³ Since most of the fuel tanks/pipes are made of aluminium or its alloys owing to its high strength-to-density ratio, the aluminium corrosion due to the formation of alkoxide (alcoholate or dry corrosion) during storage or even transportation of such bio-alcohols may cause leakage in the fuel tanks and in worst cases enough threat is speculated for fire and explosion.⁴ Mechanical overloads, alloy impurities even at elevated temperatures are further contenders for accelerating the alcoholate corrosion.⁵ However, a prolong exposure to the moisture retards the alcoholate corrosion by forming a protective layer of hydrated aluminium oxide in the metallic surface but moisture impurity in the fuel may damage the gasoline engine.⁶ Hence, a maintenance optimization is crucial in critical engineering disasters by detecting alcoholate corrosion as in its nascent state with minimizing the chance of water contamination.^6,7Several electrochemical and mechanical methods have been exploited for decades to propose aluminium alcoholate and other corrosions;⁶ yet the early detection of the alcoholate corrosion is still a challenging task due to the lack of sensitive analytical methods.^6,8 Here, the fluorescence technique may act as a better alternative owing to its simplicity and high sensitivity.⁹ Till date, a large number of fluorescent probes for Al³⁺ have been exploited in the biological or environmental domain,¹⁰ but has never focused on alcoholate corrosion studies. Based on this requirement, we synthesized a fluorescent probe, namely HMBDC ((6Z)-6-(2-hydroxy-3-(hydroxymethyl)-5-methylbenzylideneamine)-2H-chromen-2-one), to detect alcoholate corrosion with μg-level detection ability along with its retarding signature in the presence of moisture in a judicious way. Such novel method may lead to an early detection of alcoholate corrosion in a simpler way.The non-fluorescent phenolic Schiff-base molecule containing a coumarin moiety (HMBDC) was prepared by condensing an equimolar mixture of 6-amino coumarin (6-ACO) and 2-hydroxy-3-(hydroxymethyl)-5-methylbenzaldehyde (HHMB) in dry ethanol (Scheme 1 and Fig. S1^†) (c.f. ESI^† for details). Among various organic solvents, the interaction of HMBDC with Al³⁺ was observed only in the alcoholic medium according to the UV-vis studies (Fig. S2^†). In methanol, the absorption intensity at ∼353 nm for HMBDC (5 μM) decreased gradually with the continuous addition of Al(NO₃)₃ until saturated at ∼8-equiv., giving rise to a new peak at ∼406 nm, where an isosbestic point at ∼384 nm assures the formation of Al³⁺/HMBDC complex (1) (Fig. 1A). Upon optimization of the complex formation affinity in various ethanol/methanol mixed media, highest reactivity with the lowest saturated Al³⁺ concentration (∼5 equiv.) compared to that obtained in pure methanol was observed in a 4 : 1 methanol/ethanol-mixed medium (Fig. S3^†). Most probably, more effective H-bonding interaction of the dimeric ethanol/methanol¹¹ with 1 induces greater complex (1) stability, although the complex formation reactivity was much less in pure ethanol compared to the methanol medium (Fig. 1 and S2^†).Open in a separate windowScheme 1Synthesis of HMBDC and its complexation with Al³⁺ in an alcohol solvent.Open in a separate windowFig. 1(A) UV-vis absorption and (B) fluorescence spectra of HMBDC (5 μM) in the presence (red) and absence (black) of increasing concentration of Al(NO₃)₃ (0–40 μM) in anhydrous methanol at 25 °C. The intensity changes with increasing Al³⁺ concentrations are indicated by arrows. (C) Al³⁺ concentration dependent relative increase in the fluorescence intensity with respect to its absence in methanol (red) or methanol/ethanol (4 : 1) (blue). (D) Fluorescence intensity ratios in the presence and absence of various ions or mixture of ions in the mixed solvent (25 μM each; blue) or methanol (40 μM each; other colors) or are shown by bar-diagram.In spite of the stronger H-bonding interactions of 1 with water compared to the methanol or ethanol, a complete dissociation of 1 in the presence of 20% (v/v) water in methanol (Fig. S4^†) suggests that, in addition to the solvent assisted H-bonded structural stability of 1, alcohol molecule may also participate in the coordination with Al³⁺ to form 1. Indeed, the possible methanol coordination is reflected in the ESI-MS⁺ analysis (Fig. S5B^†). In addition, the Job''s plots in the absorption studies showed that the HMBDC formed 1 : 1 stoichiometric complex with Al³⁺ (Fig. S6^†). To elucidate the probable structure of 1, we carried out the DFT-based theoretical calculation by considering the 1 : 1 stoichiometric Al³⁺/HMBDC complex with or without methanol coordination. A stable structure of 1 was obtained when the oxygen atom of the methanol molecule coordinates with Al³⁺ and other two coordination sites of Al³⁺ are occupied by the phenolic-oxygen and imine-nitrogen of HMBDC (Scheme 1, Fig. 2 and S7^†). Facile coordination of those hard donor sites of HMBDC towards harder Al³⁺ is susceptible towards alcohol assisted stabilization of 1. The UV-vis absorbance at ∼402 nm for 1 computed from the time-dependent DFT (TD-DFT) calculations in methanol medium, where the HOMO (90) → LUMO (92) excitation nicely matched with the experimental absorbance at ∼406 nm (Fig. 1 and ​and2).2). However, monitoring of the ¹H-NMR peak characterized for aldimine proton is a useful strategy to identify the bonding of the imine-N to Al³⁺.¹² We observed that the aldimine proton peak intensity for HMBDC in CD₃OD was quenched to a great extent with a considerable down-field shift from 8.80 to 8.88 ppm in the presence of Al³⁺ (Fig. S8^†); the down-field shift is expected owing to the imine-N and Al³⁺ coordination, but intensity quenching does not follow the previous trend in the aprotic polar medium.¹² The generation of a partial positive charge at the N-centre upon its binding with the Al³⁺ may enhance the acidity of the aldimine proton to become labile for participating in the H/D exchange in a protic medium (CD₃OD), as reported previously for other allied systems.¹³ These results strongly suggest the imine-N and Al³⁺ bonding in 1. On the other hand, Al³⁺ induced large decrease in the IR intensity at ∼3300 cm⁻¹ for phenolic-OH also supports the phenoxide coordination (Fig. S9^†).Open in a separate windowFig. 2Frontier molecular orbital profiles including various UV-vis absorption parameters of HMBDC (left panel) and HMBDC/Al³⁺ complex (right panel) based on TD-DFT (B3LYP/6-31G(d)).The electronic distribution in the molecular orbital diagram (MO) of the HMBDC evaluated from the DFT calculation showed an intra-molecular photo-induced electron transfer (PET) from coumarin to the HHMB moiety, which makes the HMBDC non-fluorescent (Fig. 2). Al³⁺ induced an instantaneous increase in the fluorescence intensity for HMBDC (5 μM) in the alcoholic medium (methanol/ethanol or their mixture) due to the formation of 1 (Fig. 1B and S10^†). A gradual fluorescence intensity increase at ∼506 nm (λ_ex = 406 nm) of ∼30-fold for 8 equiv. of Al³⁺ and ∼40-fold for 5 equiv. of Al³⁺ was observed in the methanol and 4 : 1 (v/v) methanol/ethanol medium, respectively (Fig. 1 and S10B^†). According to the HOMO and LUMO electronic distributions for 1 in the DFT studies, the PET process in HMBDC was highly restricted upon its binding with Al³⁺ in 1, causing for the large increase in the fluorescence intensity (Fig. 2). However, the better fluorescence response (lower intensity-saturated Al³⁺ concentration and larger intensity increase) in the mixed medium than pure methanol may be associated with greater stability of 1, as described in the previous section (Fig. S2^†). The fluorescence intensity increase remains invariant using other soluble Al(iii)-salts (Fig. 1D and S11^†), which eliminates the role of counter anions for the increasing intensity. To ascertain the Al³⁺ selectivity, we performed similar fluorescence studies with other potentially interfering cations but failed to produce any noticeable fluorescence (Fig. 1D and S12^†). However, a linear intensity increase with the increase in the concentration of Al³⁺ up to 6 equiv. in methanol and 4 equiv. in the 4 : 1 methanol/ethanol mixed medium can be useful for a ratiometric detection of unknown concentration of Al³⁺ (Fig. 1C), where the limit of detection¹⁴ (LOD) of Al³⁺ with HMBDC in the methanol medium was found to be ∼0.5 μM (c.f. details in ESI^†). Most importantly, HMBDC recognized Al³⁺ selectively from the mixture of various other cations, and also in presence of other soluble Al(iii) salts, particularly, aluminium alkoxide (ethoxide) with similar accuracy (Fig. 1D and S12^†). Therefore, the Al(iii) sensing ability for an alcoholate corrosion with an aluminium alloy must not be perturbed due to the interference of other leached cations.The dry alcoholate corrosion of aluminium or its alloy while forming soluble alkoxide (Al(OR)₃) can be detected upon incubation in an anhydrous alcoholic medium. However, under a condition of prolonged incubation, the contamination of trace amounts of moisture may also trigger the conversion of Al(OR)₃ to Al(OH)₃, followed by the hydrated alumina (Al₂O₃·xH₂O) coating on the metallic surface.⁶ The formation of hydrated alumina can also be possible via the decomposition of Al(OR)₃.⁶ To characterize the alcoholate corrosion as an exclusive process to the maximum limit, we minimized those wet-processes by allowing the corrosion under inert conditions. A previously grazed aluminium-sheet (dimension ∼3.5 × 1.5 × 0.2 cm³; surface area ∼12.5 cm²) was incubated for 18 days in 100 mL anhydrous methanol or methanol/ethanol (4 : 1) mixed solvent under nitrogen atmosphere by purging nitrogen every 24 h, where the small change in the solution volume if required was adjusted by injecting an appropriate amount of the nitrogen-saturated anhydrous solvent. The amount of Al(OR)₃ (R = -Me, -Et) generated in the medium was estimated by monitoring the HMBDC (5 μM) fluorescence. After 10-fold dilution of the medium with the parent solvent, the amount of Al(OR)₃ formed or the alcoholate corrosion was estimated in every 3 days interval according to the amount of Al³⁺ obtained from the time-dependent fluorescence responses (Fig. S13^†) as per the linear calibration plots in Fig. 1C multiplied by the dilution factor. A linear increase in the normalized fluorescence intensity from ∼3.5 to 16.8 and ∼7.3 to 36.1 was observed with an increase in the incubation time period from 3 day to 18 day for methanol and methanol/ethanol (4 : 1) media (Fig. 3A and S13^†), respectively, which correspond to the linear increase in the Al³⁺ amount in the medium from ∼3.2 to 16.6 μmol for either solvents (Fig. 3C). Indeed, the weight-loss of ∼0.47 mg i.e., ∼17.5 μmol was found to be closely similar with that of the increase in Al³⁺, revealing that not only the dry corrosion leads to the generation of Al³⁺ (Al(OR)₃) as the only product, but also HMBDC is highly effective for an accurate estimation of the alcoholate corrosion. In addition, the nice correlation between the weight-loss and Al(OR)₃ amount also reveals that the decomposition of alkoxide into insoluble alumina is negligibly small during the whole corrosion time-course.Open in a separate windowFig. 3(A and B) Extent of the fluorescence intensity increase due to corrosion-induced leached Al³⁺ (F(x)/F(0)) of HMBDC (5 μM) and (C and D) amount of Al³⁺ in the corrosion medium according to fluorescence response are plotted with various incubation times of pure aluminium sheet or its alloy (Al-7075) in different mediums/atmosphere conditions: nitrogen atmosphere in methanol (red) and methanol/ethanol (4 : 1) (blue); open atmosphere in methanol (green) and methanol/ethanol (4 : 1) (purple). The data at nitrogen conditions are only fitted linearly. (A and B) The fluorescence intensity of HMBDC (5 μM) were monitored after the 10-fold dilution of the corrosion medium with the same solvent. (C and D) The amount of Al³⁺ estimated as the amount obtained from the normalized intensity with comparing the linear plots in Fig. 1C multiplied by the dilution factor 10. The actual amount of alcoholate corrosions for the mixed medium under open atmosphere are depicted by solid circle (purple).However, under open atmospheric conditions maintained by air purging (average relative humidity ∼70%; average temperature 28 °C) in every 24 h interval while maintaining other similar experimental conditions and analysis protocol, the specific corrosion rate (∼2.0 μg per day per cm²) up to 12 days, was found to be closely similar to that detected under the nitrogen atmosphere (Fig. 3C and S13^†). The results also indicate that the early stage of the alcoholate corrosion process (at least up to 12 days) for pure aluminium is not affected significantly by the atmospheric moisture content, although the final corrosion amount after 18 days incubation in normal atmosphere was slightly lower (∼84%) for the mixed medium compared to that obtained for pure methanol (Fig. 3C). The decrease in the Al(OR)₃ amount can be affected by two processes: (a) Al(OR)₃ to insoluble Al(OH)₃ conversion due to the adsorbed moisture; (b) actual retardation of the corrosion rate due to the surface deposition of Al(OH)₃. The extent of the conversion of Al(OR)₃ to Al(OH)₃ in the corrosion medium under the open air condition can be assessed by estimating the fluorescence intensity at every 3 day time interval in the absence of aluminium sheet (from day-3 to day-18) with the addition of same amount of Al(OEt)₃ (3.2, 5.7, 8.0, 11.7, 14.2 and 16.4 μmol (final added amount) at day 0 (beginning of day 1), 3, 9, 12 and 15, respectively, in 100 mL mixed medium) as that of the alkoxide amount detected due to the corrosion under nitrogen condition (Fig. S14^†). In comparison to the actual added Al(OEt)₃, any decrease in the Al(OEt)₃ amount upon such incubation should be added with the corrosion induced formation of Al(OR)₃ amount under nitrogen condition for respective time interval to obtain the actual alcoholate corrosion. The actual corrosion was found to be slightly higher than that estimated from the corrosion-induced Al(OR)₃ formation (Fig. 3C, solid symbol). According to the LOD of Al³⁺, the detection of the alcoholate corrosion amount as minimum as ∼0.1 μg mL⁻¹ can be possible by monitoring the fluorescence response of HMBDC.Alcoholate corrosion in a widely used aluminium alloy, Al-7075 (composition: Al, 90%; Zn, 5.5%; Mg, 2.5%; Cu, 1.5 and Si, 0.5%) was also studied. The previously grazed alloy sheet with same dimension and surface area as that of the pure aluminium sheet was incubated in 100 mL anhydrous methanol or 4 : 1 methanol/ethanol under nitrogen as well as normal atmospheric conditions. The amount of the alcoholate corrosion in every 3 days interval up to 30 days was estimated by evaluating the fluorescence response of HMBDC (Fig. 3B and S15^†). In comparison to the pure aluminium sheet, the increase in corrosion from ∼1.5 to 4.0 μmol evaluated from the increase in the normalized fluorescence intensity (1.65 to 5.90 in methanol; 2.64 to 10.40 in methanol/ethanol (4 : 1) mixture) with the increase in the incubation time from day-3 to day-12 follows a similar linear relation regardless of the solvent compositions and atmospheric conditions (Fig. 3B and D), while the intrinsic rate of corrosion ∼0.95 μg per day per cm² was more than 2-fold slower (Fig. 3C and D). The lower rate constant value for the alloy compared to pure aluminium indicates that the contamination of other metals in the alloy resists the early stage alcoholate corrosion process. However, under normal atmospheric condition, the corrosion amount vs. time relation deviates from the linearity after 12 days. Importantly, after 30 days of incubation, a large reduction in the Al(OR)₃ amount from ∼11.38 to 6.64 μmol was estimated for the mixed medium, but the change was only from ∼13.20 to ∼12.33 μmol for pure methanol (Fig. 3D). By determining the hydration-induced conversion amount of Al(OR)₃ to Al(OH)₃ according to the procedure, as described before (Fig. S16^†), the actual alcoholate corrosion was found to decrease from ∼11.38 to 7.70 μmol by changing the condition from nitrogen to open atmosphere after 30 days (Fig. 3D, solid symbol). Our study reveals that in comparison to pure methanol, the formation of Al(OH)₃ under open atmospheric condition retards the alcoholate corrosion largely due to the presence of more hygroscopic ethanol.¹⁵ The deposition of Al₂O₃·xH₂O onto the alloy-surface is responsible for resisting the further alcoholate corrosion⁶ (Fig. 3D). In fact, the generation of more surface pits owing to the higher extent of the alcoholate corrosion in methanol over the mixed medium was also detected by naked eye (Fig. S17^†). The surface morphology in the SEM studies showed that the alloy surface was little bit smoother after the corrosion in the mixed medium (Fig. S18^†), justifying our proposition for the surface deposition of Al₂O₃·xH₂O. On the other hand, cyclic voltammetric studies in the corrosion medium exposed to normal atmospheric conditions identified an irreversible cathodic peak at ∼−0.7 V due to the formation of insoluble Al(OH)₃ in addition to the conversion from Al to Al³⁺, but such irreversible peak was not observed for the medium exposed to nitrogen (Fig. S19^†). Moreover, the formation of white gelatinous precipitate of Al(OH)₃ in the mixed medium was clearly visible by naked eye under normal atmospheric conditions (Fig. S17B^†). All those results strongly support that the initiation of the wet-process by forming Al(OH)₃ inhibits the alcoholate corrosion rate.In conclusion, a phenolic Schiff-base consisting of a coumarin unit as a fluorescent sensor for Al³⁺ operable only in the alcoholic medium is synthesized to monitor dry alcoholate corrosion. The photo-induced electron transfer process in the probe molecule exhibits Al³⁺ induced large increase of fluorescence intensity, lifted by its complexation with Al³⁺, which was further stabilized by the coordination and H-bonding interaction with the solvent molecule. The alcohol specific complex formation and subsequent fluorescence generation was suitably tuned to monitor the alcoholate corrosion by fluorometrically estimating aluminium alkoxide formation with a sensitivity of ∼10 μg L⁻¹. However, the simultaneous participation of small extent of the wet-process (Al(OR)₃ to Al(OH)₃ conversion) and its deposition in metal surface, particularly for the alloy, inhibits the dry alcoholate corrosion. The alloy specific detection of the early stage alcoholate corrosion is in progress to obtain suitable material useful as a biofuel container.     

2′-Deoxy-5-(hydroxymethyl)cytidine: estimation in human cancer cells with a simple chemosensor

A pyrrole-based rhodamine conjugate (CS-1) has been developed and characterized for the selective detection and quantification of 2′-deoxy-5-(hydroxymethyl)cytidine (5hmC) in human cancer cells with a simple chemosensing method.

A new chemosensor, CS-1, has been developed and characterized for the selective detection and quantification of 2′-deoxy-5-(hydroxymethyl)cytidine (5hmC) in human cancer cells.

2′-Deoxy-5-(hydroxymethyl)cytidine (5hmC) is found in both neuronal cells and embryonic stem cells. It is a modified pyrimidine and used to quantify DNA hydroxymethylation levels in biological samples^1–3 as it is capable of producing interstrand cross-links in double-stranded DNA. It is produced through an enzymatic pathway carried out by the Ten-Eleven Translocation (TET1, TET2, TET3) enzymes, iron and 2-oxoglutarate dependent dioxygenase.^4–7 In the DNA demethylation process, methylcytosine is converted to cytosine and generates 5hmC as an intermediate in the first step of this process which is then further oxidized to 5-formylcytosine (fC) and 5-carboxycytosine (caC) of very low levels compared to the cytosine level.⁸ Though the biological function of 5hmC in the mammalian genome is still not revealed, the presence of a hydroxymethyl group can regulate gene expression (switch ON & OFF). Reports say that in artificial DNA 5hmC is converted to unmodified cytosine when introduced into mammalian cells.^9,10Levels of 5hmC substantially vary in different tissues and cells. It is found to be highest in the brain, particularly in nervous system and in moderate percentage in liver, colon, rectum and kidney tissues, whereas it is relatively low in lung and very low in breast and placenta.^11,12 The percentage of 5hmC content is much less in cancer and tumor tissues compared to the healthy ones. The reason behind this loss is the absence of TET1, TET2, TET3, IDH1, or IDH2 mutations in most of the human cancer cells which means decrease of methylcytosine oxidation.^13–15 This loss of 5hmC in cancer cells is being used as a diagnostic tool for the detection of early-stage of malignant disease. Few analytical methods^16–19 such as glucosyltransferase assays, tungsten-based oxidation systems, and TET-assisted bisulfite sequencing (TAB-Seq) or oxidative bisulfite sequencing (oxBS-Seq) protocols are now developed to differentiate 5hmC from other nucleotide which are naturally occurred. There are also few methods such as liquid chromatography/tandem mass spectroscopy (LC/MS-MS), which determine the level of 5hmC in mammalian cancer cell.^20–22 However, these procedures are highly toxic and expensive due to requirement of catalyzation through enzymes or heavy metal ion and these techniques require expertise, facilities, much time and costs even beyond standard DNA sequencing. As a result, these detection techniques are currently inappropriate for the high-throughput screening of genome-wide 5hmC levels (performance comparison is shown in Table S1, ESI^†).Among all reputed methods fluorescence detection method using chemosensors is significantly important due to its indispensable role in medicinal and biological applications.^23–27 Chemosensors have been effectively explored to monitor biochemical processes and assays through in situ analysis in living systems and abiotic samples with much less time and cost.In this contribution we prepared and characterize (Scheme S1 and Fig. S1–S3, ESI^†) a pyrrole–rhodamine based chemosensor (CS-1) which shows efficient and selective fluorescence signal for 5hmC in aqueous medium (Scheme 1). A transparent single crystal of CS-1 (Fig. 1) was obtained by slow evaporation of the solvent from a solution of CS-1 in CH₃CN. It crystallizes as monoclinic with space group P2₁/n (Fig. S4 and Table S2, ESI^†).Open in a separate windowScheme 15hmC-induced FRET OFF–ON mechanism of the chemosensor CS-1.Open in a separate windowFig. 1ORTEP diagram of CS-1 (ellipsoids are drawn at 40% probability level).Spectrophotometric and spectrofluorimetric titrations were carried out to understand the CS-1–5hmC interaction with 1 : 1 binding stoichiometry (Fig. S5, ESI^†) upon adding varying concentrations of 5hmC to a fixed concentration of CS-1 (1 μM) in aqueous medium at neutral pH. Upon the addition of increasing concentrations of the 5hmC, a clear absorption band (K_a = 4.47 × 10⁵ M⁻¹, Fig. S6, ESI^†) appeared to be centered at 556 nm with increasing intensity (Fig. 2a). On the other hand, for the fluorescence emission spectra of CS-1 (Fig. 2b), upon irradiation at 325 nm, an emission maxima at 390 nm was observed, which was attributed to the fluorescence emission from the donor unit i.e. the pyrrole moiety of CS-1. When 5hmC were added, due to rhodamine moiety CS-1 showed a 95-fold increase in fluorescence at 565 nm (K_a = 4.61 × 10⁵ M⁻¹, Fig. S7, ESI^†) with the detection limit of 8 nM (Fig. S8, ESI^†). The binding of 5hmC induces opening of the spirolactam ring in CS-1, inducing a shift of the emission spectrum. Subsequently, increased overlap between the emission of the energy-donor (pyrrole) and the absorption of the energy-acceptor (rhodamine) greatly enhances the intramolecular FRET process,^28,29 producing an emission from the energy acceptor unit in CS-1.Open in a separate windowFig. 2(a) UV-vis absorption spectra of CS-1 (1 μM) upon gradual addition of 5hmC up to 1.2 equiv. in H₂O–CH₃CN (15 : 1, v/v) at neutral pH. (b) Fluorescence emission spectra of CS-1 (1 μM) upon addition of 1.2 equiv. of 5hmC in H₂O–CH₃CN (15 : 1, v/v) at neutral pH (λ_ex = 325 nm).In order to establish the sensing selectivity of the chemosensor CS-1, parallel experimentations were carried out with other pyrimidine/purine derivatives such as 5-methylcytosine, cytosine, cytidine, thymine, uracil, 5-hydroxymethyluracil, adenine and guanine. Comparing with other pyrimidine/purine derivatives the abrupt fluorescence enhancement was found upon addition of 5hmC to CS-1 while others do not make any fluorescence changes under UV lamp (Fig. 3, lower panel). Furthermore, the prominent color change from colorless to deep pink allows 5hmC to be detected by naked eye (Fig. 3, upper panel). The above observation shows consistency with the fluorescence titration experiments where no such binding of CS-1 with other pyrimidine/purine derivatives was found (Fig. S9, ESI^†).Open in a separate windowFig. 3Visible color (top) and fluorescence changes (bottom) of CS-1 (1 μM) in aqueous medium upon addition of 1.2 equiv. of various pyrimidine/purine derivatives (λ_ex = 325 nm) in H₂O–CH₃CN (15 : 1, v/v) at neutral pH.pH titration reveals that CS-1 becomes fluorescent below pH 5 due to the spirolactam ring opening of rhodamine. However, it is non-fluorescent at pH range of 5–13. Upon addition of 5hmC to CS-1 shows deep red fluorescence in the pH range of 5–8 (Fig. S10, ESI^†). Considering the biological application and the practical applicability of the chemosensor pH 7.4 has been preferred to accomplish all experiments successfully.In ¹H NMR titration (Fig. S11, ESI^†), the most interesting feature is the continuous downfield shift of aromatic protons on the pyrrole moiety of CS-1 upon gradual addition of 5hmC. This may be explained as the decrease in electron density of the pyrrole moiety upon binding with 5hmC through hydrogen bonding. Xanthene protons to be shifted downfield upon spirolactam ring opening indicates the probe to coordinate with 5hmC and electrons are accumulated around 5hmC. In ¹³C NMR titration the spiro cycle carbon peak at 65 ppm was shifted to 138 ppm along with a little downfield shift of the aromatic region of CS-1 (Fig. S12, ESI^†). This coordination led to the spiro cycle opening and changes to the absorption and emission spectra, further evident by mass spectrometry (Fig. S13, ESI^†), which corroborates the stronger interaction of CS-1 with 5hmC.The experimental findings were validated by density functional theory (DFT) calculations using the 6-31G+(d,p) method basis set implemented at Gaussian 09 program. Energy optimization calculations presented the conformational changes at the spirolactam position of CS-1 while 5hmC takes part to accommodate a probe molecule. After CS-1–5hmC complexation the energy is minimized by 19.45 kcal from the chemosensor CS-1, indicating a stable complex structure (Fig. 4 and Table S3, ESI^†). This theoretical study strongly correlates the experimental findings.Open in a separate windowFig. 4Energy diagram showing the energy differences between CS-1 and CS-1–5hmC complex.The desirable features of CS-1 such as high sensitivity and high selectivity at physiological pH encouraged us to further evaluate the potential of the chemosensor for imaging 5hmC in live cells (Fig. 5). A549 cells (Human cancer cell A549, ATCC no. CCL-185) treated with CS-1 (1 μM) exhibited weak fluorescence, whereas a deep red fluorescence signal was observed in the cells stained with CS-1 (1 μM) and 5hmC (10 μM), which is in good agreement with the FRET OFF–ON profile of the chemosensor CS-1 in presence of 5hmC, thus corroborating the in-solution observation (Fig. S14, ESI^†). Cytotoxicity assay measurement shows that the chemosensor CS-1 does not have any toxicity on the tested cells and CS-1–5hmC complex does not exert any significant adverse effect on cell viability at tested concentrations (Fig. S15, ESI^†). As far as we are aware, this is the first report where we are executing the possible use of the pyrrole–rhodamine based chemosensor for selective recognition of 5hmC in living cells. These findings open an avenue for future biomedical applications of the chemosensor to recognize 5hmC.Open in a separate windowFig. 5Confocal microscopic images of A549 cells treated with CS-1 and 5hmC. (a) Cells treated with only CS-1 at 1 μM concentration. (b) Bright field image of (a). (c) Cells treated with CS-1 and 5hmC at concentration 10 μM. (d) Bright field image of (c). All images were acquired with a 60× objective lens with the applied wavelengths: For (a) and (b), E_ex = 341 nm, E_em = 414 nm, filter used: DIDS; for (c) and (d) E_ex = 550 nm, E_em = 571 nm, filter used: Rhod-2.The concentration of 5hmC was also quantified from A549 human cancer cells. Lysate of 10⁷ A549 cells was added to 1 μM of CS-1 and the fluorescence signal was recorded. Presence of 5hmC in these cancer cells was detected with the help of CS-1–5hmC standard fluorescence curve (Fig. 6) using the selective detection ability of the chemosensor CS-1.Open in a separate windowFig. 6(a) Calibration curve obtained for the estimation of 5hmC. (b) Estimation of the concentration of 5hmC (red point) from the calibration curve.From the standard curve it was found that the concentration of 5hmC in the tested sample was 0.034 μM present in 16.7 mm³ A549 cell volume (†). Assay of 5hmC was further validated from multiple samples of A549 human cancer cells using CS-1. Increasing fold of fluorescence signals was also statistically validated after calculating the Z′ value (Table S5, ESI^†). All tested samples shows the Z′ score value more than 0.9, indicating an optimized and validated assay of 5hmC.Quantification of 5hmC in human cancer cell A549

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.     

Photoswitchable probe with distinctive characteristics for selective fluorescence imaging and long-term tracing

A photo-switchable and high-contrast bio-imaging indicator 4,4′-(1E,1′E)-(4,4′-(cyclopentene-1,2-diyl)bis(5-methylthiophene-4,2-diyl))bis(methan-1-yl-1-ylidene)bis(azan-1-yl-1-ylidene)bis(2-(benzo[d]thiazol-2-yl)phenol) (BMBT) has been demonstrated, by integrating photochromophore with excited-state intramolecular proton transfer (ESIPT) moiety. The ability of reversible emission switching enables arbitrarily selective labeling or concealing of cells simply by controlling light irradiation. Besides, when the emission was switched on, BMBT is demonstrated to exhibit unique characteristics of aggregation induced emission (AIE), providing a high on–off ratio for favorable bio-imaging. Thus, the non-labeling and easily-controlled selective imaging, as well as good biocompatibility indicates BMBT to be a favorable cell probe with great potentials for functional bio-imaging fluorophore.

In this work, a photoswitchable probe was synthesized by integrating a photochromophore with an excited-state intramolecular proton transfer (ESIPT) moiety. It was explored to be a favorable fluorophore for selective fluorescence imaging and long-term tracing.

Although there are many conventional fluorescent probes used for fluorescent imaging in the past few years,^1–5 such as rhodamine,⁶ cyanine dye,⁷ quantum dots,^8,9 and lanthanide probes,¹⁰ these fluorescent probes can only respond irreversibly to one event.^11,12 In comparison, photochromophores,¹³ which can reversibly response with UV and visible light,¹⁴ are more valuable fluorescent probes for regional optical marking of interested cells.^15,16 Because of their favourable characteristics, such as excellent thermal stability, good fatigue resistance and fast response time, diarylethenes derivatives have drawn wide spread concern of researchers.^17–20 Furthermore, the special optical properties of diarylethenes enable them to be suitable for long time and real time monitoring in bioimaging.The significance of targeted imaging of fluorescence probe for bio-samples in vitro or in vivo is well-known for their high sensitivity, non-invasiveness, real-time detection and especially selectivity.^21–24 However, the synthesis of these targeting materials is usually complicated. Moreover, for the same type of cells, no selectivity is demonstrated by these complex targeting agents.^21–24 Therefore, easy-prepared and easy-controlled non-labeling fluorescence imaging agents have always been pursued by both the industry and scientific communities. Among the above-mentioned photochromophores, diarylethenes are expected to be promising photochromic imaging candidates due to their favorable characteristics described in preceding paragraph. Prospectively, they can be used for selective long-term tracing if the photostability can be ensured.In our previous work, a series of AIE-active excited-state intramolecular proton transfer (ESIPT) complexes have been demonstrated to be good bio-imaging candidate with many advantages such as simple preparation, good biocompatibility, high quantum yields, fast cell staining as well as long-term anti-photobleaching.^25–27 Sometimes, ESIPT compounds exhibit dual emission, originated from keto and enol state, respectively. This caused extremely fast four-level photophysical cycle (E–E*–K*–K–E), mediated by intramolecular H-bonds immediately after photoexcitation, enables two emissions.^28,29 Herein, we have demonstrated a new type of multifunctional bio-imaging materials based on facile synthesis design concept, by introducing a photochromic diarylethene moiety to enable regional emission turn-on, and introducing an ESIPT moiety to allow good photo stability for long-term tracing. To the best of our knowledge, this is the first example with integrating abilities of non-labeling selectively long-term regional tracing.The new diarylethene derivative (BMBT, see the molecular structure in Scheme 1) has been synthesized (Fig. S1^†) via condensation reaction of 5-amino-2-(benzo[d]thiazol-2-yl)phenol with 4,4-(cyclopentene-1,2-yl)-bis(5-methyl-thiophene-2-formaldehyde), according to a previous reported procedure.^30,31 The molecular structures and purities were confirmed by ¹H NMR spectroscopy and mass spectroscopy (Fig. S6 and S7,^† see the synthesis details and full molecular characterizations in the ESI^†).Open in a separate windowScheme 1Structure and photochromic process of BMBT.The photoirradiation-induced changes in absorption and fluorescence spectra at room temperature are investigated in THF/DMEM mixture (1.0 × 10⁻⁵ mol L⁻¹). The open-ring isomer mainly exhibits two absorption peaked at 304 and 380 nm, respectively (Fig. S2^†). This is ascribed to the internal charge transfer and π–π transition of 2-(2′-hydroxy-phenyl)benzothiazole (HBT),^10,30 coupled with the CT inside the HBT unit from the hydroxyphenyl ring to the benzothiazole ring (Scheme 1). BMBT exhibits two emissions; one blue emission peak around 458 nm and a red emission peak around 600 nm, corresponding to the enol and keto emission, respectively (Fig. 1). Upon irradiation with ultraviolet light at 365 nm, the absorption band at longer wavelength region centered at 595 nm increased obviously with irradiation time (Fig. S2^†). This is caused by the formation of the closed-ring isomer (see the photochromic reaction in Scheme 1). Correspondingly, due to spectroscopic overlap between this longer-wavelength absorbance with the red emission ranged from 560–700 nm, the relative intensity of the red emission substantially decreases with the UV irradiation (Fig. 1), because of efficient energy transfer. Upon further visible light (λ = 520 nm) irradiation, the closed-ring isomer transfers back to the initial open-ring isomer, and thus the longer-wavelength absorption decreases and the red emission restores. This indicates good reversibility of the photochromic reaction.Open in a separate windowFig. 1Fluorescence emission changes of BMBT in THF/DMEM (1 : 200, vol : vol) mixture upon irradiation with 365 nm light (λ_ex = 385 nm) and visible light (λ_ex = 520 nm).Excitingly, BMBT exhibit prominent characteristics of Aggregation-Induced Emission (AIE).^31–33 The fluorescence intensity of BMBT in THF solution was relatively weak, while the powder or nanoparticles of the material dispersed in DMEM buffer exhibited a significantly enhanced emission (Fig. 2). When the DMEM buffer fraction was increased gradually from 0% to 20%, the fluorescence intensity only slightly enhanced. When the DMEM fraction was further increased to 40%, the emission exhibits a significant enhancement. The total intensity at the blue enol emission increased more than 10 times at 100% DMEM fraction as compared with that in THF solution. This AIE property enables high signal-to-noise ratios for favorable bioimaging (Fig. S3^†).Open in a separate windowFig. 2Fluorescence spectra of 1 × 10⁻⁵ M BMBT in the THF/DMEM mixture at different water fractions (λ_ex = 385 nm).Based on the good reversibility of light response and AIE characteristics, the practical application of the BMBT as bioprobe was further investigated. The biological imaging of BMBT was observed by using confocal laser scanning microscopy (CLSM). Blue luminescence in the cytoplasm of HeLa cells was observed after incubation with a THF/DMEM (1 : 200, vol : vol) solution of BMBT (20 μM) for 30 min at 37 °C (Fig. 2 inset). The overlay of luminescent images and bright-field images confirmed that BMBT was located mainly in the cytoplasm of cells rather than the membrane and nucleus (Fig. S3^†). Intense intracellular luminescence with a high signal-to-noise ratio (I₁/I₂ > 7) was detected between the cytoplasm (regions 3 and 1) and nucleus (region 2), also implying weak even few nuclear uptake of BMBT (Fig. S4^†). Besides, BMBT has a low cytotoxicity with the cellular viabilities estimated to be greater than 85% after 24 h incubation with the highest cultural concentration of 50 μM BMBT (Fig. S5^†).The luminescence switching of BMBT can also be achieved while alternating UV and visible light illumination in fixed HeLa cells. Cells (shown in red circle, Fig. 3a) were irradiated with 488 nm light (0.5 mW) for 3 min, the blue fluorescence of the irradiated cells was lamped off while the surrounding cells remained almost unchanged. Such fluorescence quenching is likely ascribed to the intramolecular fluorescence resonance energy transfer of BMBT, due to the intensified short-wavelength absorption band (270–450 nm) of the closed-open form of BMBT with the blue emission band (420–560 nm).³⁴ Upon irradiation with 405 nm light (1.25 mW) the fluorescence of all the selected cell was rapidly recovered within 1 min, caused by decrease of the relative intensity of the 270–450 nm absorption. The fluorescence can be repeatedly erased and recovered many rounds without significant fluorescence quenching, which was hardly achieved by conventional fluorophores (Fig. 3b).^35,36Open in a separate windowFig. 3(a) CLSM image (above) and the overlay image (bottom) of fixed HeLa cells incubated with 20 μM BMBT for 30 min at 37 °C (1) and (5) in original state; (2) and (6) irradiated by 488 nm light (0.5 mW) for a single cell; (3) and (7) all cells, and; (4) and (8) recovered by 405 nm light (1.25 mW). (b) Fluorescence switching of fixed HeLa cells by alternating UV (405 nm, 1.25 mW, 10 s/time) and visible (488 nm, 0.5 mW, 3 min/time) light illumination (λ_ex = 405 nm).This characteristic of selectively opto-marking or de-marking of cells may be used for non-invasive and dynamic tracing of the interested objects in vitro. That is, BMBT can arbitrarily opto-label or de-label interested cells without affecting cell proliferation. As shown in Fig. 4, the cell marked in the red circle was treated with visible light illumination (488 nm, 0.5 mW, 3 min), and its fluorescence was effectively quenched. Cells division of the remaining “bright” cells was observed under the microscope field of vision for a long-term tracing with time up to 36 h. Multiple new cells were produced, as indicated by upper white arrow. Even, as indicated by bottom white arrow, the tacked cells were observed to be doubled. The high brightness of the fluorescence in the proliferated cells indicated the good photo stability of BMBT. This indicated that BMBT can be used as a cell marker for arbitrarily selective erasing the fluorescence of the designated cell. It can also be used for selective lighting their emission by making them as the remaining bright cells after selective photo-erasing or selective photo-recovering after full erasing. This long-term tracking with non-labeling and selective optically marking or de-marking is seldom reported by other photochromophore-based bioimaging agents.³⁷ Herein, the excellent anti-photo bleaching characteristic in the long-term tracing is attributed to high photo stability of the HBT moiety.^26,27Open in a separate windowFig. 4Cells was treated with visible (488 nm, 0.5 mW, 3 min) light illumination for erasing the fluorescence. The remaining bright cells were incubated for another 36 hours, which were observed to be amplified normally (λ_ex = 405 nm).     

Ruthenium(iv) N-confused porphyrin μ-oxo-bridged dimers: acid-responsive molecular rotors

Ruthenium(iv) N-confused porphyrin μ-oxo-bridged complexes were synthesized via oxidative dimerization of a ruthenium(ii) N-confused porphyrin complex using 2,2,6,6-tetramethylpiperidine 1-oxyl. The deformed core planes in the dimers conferred a relatively high ring rotational barrier of ca. 16 kcal mol⁻¹. Rotation of the complexes was controlled by protonating the peripheral nitrogen.

Ring rotation of ruthenium(iv) N-confused porphyrin μ-oxo-dimer was controlled by protonation at the peripheral nitrogen moieties.

The design/creation/operation of molecular rotors, gears, and switches is an interesting topic in machinery nanotechnology, allowing the development of adaptive materials, switchable chiral sensors, stereo-controlled catalysts, etc.¹ The molecular approach to control nanoscale motions (e.g., rotation) is advantageous for realizing well-defined molecular devices or sets of practical supramolecular assembled systems.² As active components of molecular rotors, metal double-decker bisporphyrins and related macrocycles (e.g., phthalocyanines) are representative candidates that afford specific features, such as long-term stability, ease of fabrication, and visual responses that are detectable by microscopy (Fig. 1a).³ Bis-metal μ-oxo (or nitride)-bridged porphyrin complexes are also considered alternatives for modulating complexes'' rotational mode. However, there are few investigations concerning molecular rotation because of the limitation in synthesizing molecular rotamers comprising unsymmetrical core skeletons with diamagnetic and oxophilic metal ions (for which the rotational dynamics can be probed by NMR spectroscopy) (Fig. 1b).⁴Open in a separate windowFig. 1Chemical structures of (a) double-decker metal porphyrin complexes and (b) bis-metal μ-oxo-bridged porphyrin complex as representative motifs of molecular rotors (R¹ = R³ = p-methoxyphenyl, R² = R⁴ = 4-pyridyl). (c) N-confused porphyrin as a key rotor ligand and its (d) bis-iron μ-hydroxo-bridged porphyrin complex. (e) Schematic of the rotational mode of unsymmetrical N-confused porphyrin μ-oxo dimer complex. Sky-blue moiety in the planes is a confused pyrrole ring.To develop new rotors based on metal μ-oxo-bridged porphyrin dimers, we envisioned that a C_s-symmetric N-confused porphyrin (NCP) isomer possessing an inverted pyrrole ring in the macrocyclic scaffold could act as an alternative ring motif for the complexes (Fig. 1c).⁵ The NCPs bind various metal ions at the NNNC core, forming unique organometallic complexes.^5,6 For example, redox-active metal species such as iron facilitate the generation of metal–oxo linkages to form μ-oxo-bridged dimer complexes.⁷ In the case of iron-coordination of NCP, a unique bis-iron μ-hydroxo-bridged NCP dimer was developed, in which the structure was restricted by the coordination of peripheral sodium, as evident from X-ray crystallography (Fig. 1d).⁶We herein report the synthesis of novel ruthenium(iv) NCP μ-oxo-bridged dimers (Ru₂-2 and Ru₂-3) as a porphyrin-based molecular rotor. The ruthenium(ii) NCP complex (Ru-1) with an axial carbonyl and pyridine (=py) exhibits catalytic reactivity for alkene epoxidation (Scheme 1).⁸ The simultaneous formation of isomers can be anticipated owing to the two stereogenic centers in the NCP planes of Ru₂-2 and Ru₂-3, which differ in the relative position of the nitrogen atoms of the confused pyrrole rings in the scaffold. The Ru–μ-oxo dimers showed distinct ring rotation in solution when probed via NMR spectroscopy (Fig. 1e). Moreover, symmetry-dependent restriction of the rotation was achieved through protonation at the peripheral imino-nitrogen moieties in Ru₂-2 and Ru₂-3 using trifluoroacetic acid (TFA).Open in a separate windowScheme 1Synthesis of ruthenium(iv) NCP μ-oxo dimers, Ru₂-2 and Ru₂-3.Treatment of monomer Ru-1 with 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) in refluxing o-dichlorobenzene solution under argon atmosphere afforded two products with approximately the same yields (40% each) (Scheme 1). The fast atom bombardment mass spectra of the two products revealed identical parent ion peaks at 1474.2407 (M⁺) and 1474.2400 (M⁺), indicating dimeric ruthenium NCP structures (i.e., Ru₂-2 and Ru₂-3) bridged by an oxygen atom and two axial oxygen atoms (Fig. S1 in ESI^†). A characteristic broad feature tailing up to 900 nm was observed in the absorption spectra of both Ru₂-2 and Ru₂-3 (Fig. S2^†).The structures of Ru₂-2 and Ru₂-3 were unambiguously characterized by single-crystal X-ray diffraction analysis (Fig. 2 and Tables S1 and S2 in ESI^†). In both complexes, two ruthenium atoms coordinated at the NNNC core in the NCPs are bridged by an oxygen atom to form a sandwich-like structure. The characteristic metalaoxirane structure was found at the inner carbons of the confused pyrrole moieties in the scaffolds.^6,9 Such oxidative reactivity is a characteristic of Ru–NCP, which is in sharp contrast to the reactivity of the Ru^II(CO) azuliporphyrin with an NNNC core.¹⁰ Notably, the confused pyrrole rings in Ru₂-2 and Ru₂-3 are positioned opposite to the C17–Ru1–Ru2–C62(C61) torsion angle of 149.9(2)°, which is anticipated to induce molecular dipole moments. The strict structural difference between Ru₂-2 and Ru₂-3 is found in the position of the nitrogen atoms of the confused pyrrole rings; the former product (Ru₂-2) was eluted in the less polar fraction (R_f = 0.125) via silica gel chromatography using toluene/ethyl acetate (v/v = 19/1) and can be regarded as a syn conformer, whereas the polar fraction (R_f = 0.014) obtained under the same chromatographic conditions was eventually determined to be an anti-conformer (Ru₂-3).¹¹ The Ru1/2–O3 lengths for Ru₂-2 and Ru₂-3 are 1.792(3)/1.804(3) Å and 1.796(2)/1.800(2) Å, respectively, which are typical of μ-oxo-bridged ruthenium(iv) porphyrin complexes (e.g., [Ru^IV(TPP)(p-OC₆H₄CH₃)]₂–O: 1.789(11) Å).¹² The bond angles of Ru1–O3–Ru2 for Ru₂-2 and Ru₂-3 are almost linear 175.9(2)° and 176.1(2)°, respectively, similar to those of other reported ruthenium porphyrin μ-oxo dimer complexes (e.g., 177.8(7)°).¹² Due to the internal metal oxirane moieties, the confused pyrrole rings are inclined inward. Therefore, the overall geometries of the NCP planes are nonplanar with estimated mean plane deviation values (defined by 24 atoms of NCP skeleton) of 0.276 Å and 0.294 Å for Ru₂-2 and 0.286 Å and 0.292 Å for Ru₂-3.Open in a separate windowFig. 2Top (left) and side (right) views of crystal structures of (a) Ru₂-2 and (b) Ru₂-3 with thermal ellipsoids set at 50% probability level. Hydrogen atoms and solvent molecules excepting the H-atoms hydrogen-bonded to the peripheral nitrogen atoms were omitted for clarity.As anticipated above, the formal oxidation state of both ruthenium centers is expected to be +4 (d⁴) for the complexes. The X-ray photoelectron spectra of Ru₂-2 and Ru₂-3 revealed higher Ru (3p_3/2) binding energies of 462.2 and 462.1 eV, respectively, than that of the divalent ruthenium (d⁶) complex, Ru-1 (460.9 eV, Fig. S3 in ESI^†).The structures of Ru₂-2 and Ru₂-3 were further characterized by ¹H NMR spectroscopy in 1,1,2,2-tetrachloroethane-d₂ (TCE-d₂) (Fig. 3a and b). At 298 K, complicatedly overlapped signals were observed in the aromatic region (7–10 ppm). To distinguish the signals from the individual groups (β-pyrrole and meso-phenyl) in the complexes, we prepared the deuterated-phenyl derivatives (i.e., Ru₂-2D and Ru₂-3D) as shown in Scheme 1 (see, ESI^†). The ¹H NMR spectra of Ru₂-2D and Ru₂-3D (Fig. S4^†) show two singlets at 9.38 and 8.98 ppm for Ru₂-2 and 9.32 and 9.05 for Ru₂-3. The remaining six β-CHs are expected to have signals around 8 ppm for both complexes. This spectral feature suggests the existence of two possible rotational isomers in solution with an almost 1 : 1 molecular ratio. The 1D exchange nuclear Overhauser effect spectra revealed chemical exchange of the rotamers at 298 K (Fig. S5 and S6^†), which indicated that the NCP rings of Ru₂-2 and Ru₂-3 rotate slowly at 298 K. The rotational mode of the NCP rings was seemingly restricted at 243 K (Fig. S7 and S8^†). With increasing temperature, the signal of the set of α-pyrrole protons gradually disappeared and coalesced into a single peak at 9.15 ppm for Ru₂-2 and 9.27 ppm for Ru₂-3 at 408 K (Fig. 3c and d). Signals of the meso-phenyl protons also appeared with an averaged spectral feature in the region of 7.5–8.0 ppm (Fig. S9 and S10^†). These results suggest that the rotation rate of the NCP rings is faster than the NMR timescale above 353 K. Furthermore, for the ruthenium(iv) tetrakis(phenyl)-substituted porphyrin μ-oxo dimer (e.g., Ru(TPP)₂O) used as the reference complex, the inward and outward CH signals (i.e., o- and m-CHs) of the meso-phenyl rings did not coalesce even at 408 K in the variable temperature (VT) NMR spectra (Fig. S11^†). In this regard, the rotation of the bulky phenyl ring could be restricted by the spatially locked dimeric structure. The rotation mode of the NCP planes could be thus mutually un-synchronized with the rotational motion of the individual meso-phenyl rings.Open in a separate windowFig. 3 ¹H NMR spectra of (a) Ru₂-2 and (b) Ru₂-3 recorded in TCE-d₂ at 298 K. Partial ¹H NMR spectra (495 MHz, black line) of (c) Ru₂-2 and (d) Ru₂-3 in TCE-d₂ as a function of temperature and the line-shape simulated spectra for the compounds (red line).To gain further insight into the rotational barrier of the complexes, a line-shape analysis of the VT ¹H NMR spectra was conducted to estimate the rate constant for interconversion (k) in the specific temperature range. The free Gibbs''s energy (ΔG^‡ = ΔH^‡ − TΔS^‡) of Ru₂-2 and Ru₂-3 was determined to be (16.0 ± 0.8) kcal mol⁻¹ and (16.1 ± 0.4) kcal mol⁻¹, respectively, at 298 K, from the Eyring plots (Fig. S12^†). The rate constant (k < 200 s⁻¹) at 333 K indicates that the energy barrier of the ring rotations of Ru₂-2 and Ru₂-3 is more significant than that of the double-decker metal complexes (k = 760 s⁻¹ at 333 K).^3d Considering the structure of the complexes, the bent confused pyrrole moieties in the planes may play an essential role in the rotational motion beyond the barrier.Upon lowering the temperature, the ratio of the integrated α-CH signals for the rotamer mixture of Ru₂-2 changed to 0.85 (at 9.33 ppm) and 0.15 (at 8.93 ppm) (Fig. S13^†). The ratio of the two-rotamers of Ru₂-3 also changed when the temperature was further lowered (∼223 K), affording a thermodynamically stable conformer (Fig. S14^†). To understand the structure-dependent features of Ru₂-2 and Ru₂-3, we analyzed the thermodynamic energy profiles of the possible rotational isomers (e.g., Ru₂-2a–f and Ru₂-3a–c) by theoretical calculation with the B3LYP method (Tables S3 and S4^†). The most stable isomers (i.e., Ru₂-2c and Ru₂-3c) obtained in the optimization process are highly consistent with the crystal structures. Notably, the torsion angles of C17–Ru1–Ru2–C61(C62) in the structures of Ru₂-2e and Ru₂-3b are −102.8° and 98.5°, respectively, indicating stability (ΔE = +1.19 and +1.45 kcal mol⁻¹, respectively) relative to Ru₂-2c/-3c. Therefore, two energetically close conformers are present in the solution under the experimental conditions.The effect of protonation on the rotation of the rotamers was evaluated.¹³ Upon addition of trifluoroacetic acid (TFA), the absorption spectra of Ru₂-2 and Ru₂-3 gradually changed with isosbestic points (Fig. S15 and S16^†), reflecting protonation at the peripheral imino-nitrogen of the confused pyrrole moieties (Fig. 4a and b).¹⁴ In both complexes, a relatively large amount of TFA (at least 8 equiv) was necessary until spectral saturation (Fig. S15a and S16a). Strikingly, in the case of Ru₂-3, the relatively crowded imino-nitrogen site surrounded by meso-phenyl rings interfered with double protonation, as seen in the acid-spectral titration change after the addition of 8 equiv. of TFA (Fig. 4d, e, S15b and S16b^†). In the ¹H NMR spectrum, the resulting dication species of Ru₂-2 (i.e., Ru₂-2H₂²⁺) was well-resolved, demonstrating the interlocked single isomer at 298 K, as evident from the NMR spectral features (Fig. S17a^†). The VT NMR spectra revealed that ring rotation becomes feasible with increasing temperature due to the dissociation of the TFA molecule from the complex at equilibrium (Fig. S18^†). In sharp contrast, the ¹H NMR spectrum of the protonated species of the anti-symmetrical complex (i.e., Ru₂-3H⁺) exhibited a significantly broadened spectral feature at 298 K in TCE-d₂. The unsymmetrical spectral feature was observed at 243 K (Fig. S17b^†), which may be due to the above steric issue in the conformation of Ru₂-3. Therefore, the predominant formation of mono-protonated species was anticipated in Ru₂-3. Eventually, upon the addition of triethylamine as a base, the original spectral signal of the neutral species was reproduced for both complexes (Fig. S15c and S16c^†).Open in a separate windowFig. 4Symmetry-dependent acid-binding behaviors of Ru₂-2 and Ru₂-3. Schematic of acid-induced rotational control for (a) Ru₂-2 and (b) Ru₂-3. (c)–(e) Side-views of the protonation site.In summary, novel cofacial ruthenium(iv) NCP μ-oxo dimers, Ru₂-2 and Ru₂-3, were synthesized as motifs for molecular rotors. The resulting complexes, Ru₂-2 and Ru₂-3, are syn/anti-stereo-isomers having the nitrogen at different positions of the confused pyrrole rings. The symmetrical difference between the complexes leads to distinct rotational behavior of the Ru–NCP planes through the oxo-axis of the complex, as proven by VT ¹H NMR analysis. Additionally, considering the intrinsic basicity of the peripheral nitrogen sites, the rotational profile can be controlled by protonation. Further study on the cofacial metal–μ-oxo NCP dimers can enable applications in molecular machinery.     

Coumarin-based fluorescent probe for the rapid detection of peroxynitrite ‘AND’ biological thiols

A coumarin-based novel ‘AND’ logic fluorescent probe ROS-AHC has been developed for the simultaneous detection of ONOO⁻ and biological thiols. ROS-AHC was shown to exhibit only a very small fluorescence response upon addition of a single GSH or ONOO⁻ analyte. Exposure to both analytes, however, resulted in a significant fluorescence enhancement.

A coumarin-based novel ‘AND’ logic fluorescent probe ROS-AHC has been developed for the simultaneous detection of ONOO⁻ and biological thiols.

Peroxynitrite (ONOO⁻) is a short-lived reactive oxygen and reactive nitrogen species (ROS and RNS) produced intracellularly by the diffusion-controlled reaction of nitric oxide (NO˙) with superoxide (O₂˙⁻).^1–3 Despite playing a key role as a physiological regulator,⁴ it is commonly known for its high reactivity towards most types of biomolecules, causing deleterious effects and irreversible damage to proteins, nucleic acids, and cell membranes.^5,6 ONOO⁻ is therefore a central biological pathogenic factor in a variety of health conditions such as strokes, reperfusion injuries or inflammatory and neurodegenerative diseases (Parkinson''s disease, Alzheimer''s disease).^7–9 Conversely, biothiols such as glutathione and cysteine are endogenous reducing agents, playing a central role in the intracellular antioxidant defence systems.^10–12 Glutathione (GSH), in particular, is the most abundant biothiol in mammalian cells, and exists as both its reduced GSH form, and as the oxidised disulphide form GSSG.^13–15 Peroxynitrite and biothiols such as GSH are intimately linked, as abnormal levels of highly oxidising ONOO⁻ can perturb the delicate GSH/GSSG balance, causing irreversible damage to key processes such as mitochondrial respiration.¹⁶ Thus, abnormal levels of GSH are common in cells undergoing oxidative stress, in which the regulation of and interplay between ONOO⁻ and GSH is closely associated with physiological and pathological processes.^17,18 One such example is drug-induced liver injury (DILI), in which upregulation of ONOO⁻ occurs in hepatotoxicity. Treatment with GSH could be used to remediate this type of cell injury by depletion of ONOO⁻.^19–22One of our core research interests lies in the development of dual analyte chemosensors capable of detecting two distinct analytes such as biological reactive oxygen species and biothiols.^23–26 Although a wide range of single-analyte probes exist for the detection of ROS and thiols separately,^27–30 ‘AND’ logic sensors for their simultaneous detection are still rare.^31–33 We are therefore interested in developing such probes, containing two distinct sensing units, one for each analyte, working simultaneously or in tandem to elicit a fluorescence response.³⁴ This approach allows the monitoring of multiple biomolecular events and factors involved in specific disease pathologies, in order to achieve optimal predictive accuracy for diagnosis and prognostication.³⁵Using these principles, our group has recently focused on developing a range of ‘AND’ logic based sensors exploiting a variety of sensing units and mechanisms of fluorescence. Two such probes are shown below: GSH-ABAH (Fig. 1a), an ESIPT probe with a 4-amino-2-(benzo[d]thiazol-2-yl)phenol (ABAH) core, employing a maleic anhydride thiol-acceptor group;³¹ and JEG-CAB (Fig. 1b), a coumarin-based probe, this time with a salicylaldehyde homocysteine-reactive unit.²⁴ Both of these sensors employ a benzyl boronate ester as their peroxynitrite-reactive unit.Open in a separate windowFig. 1(a) GSH-ABAH, previously reported probe for simultaneous detection of ONOO⁻ and GSH. (b) JEG-CAB, previously reported probe for simultaneous detection of ONOO⁻ and GSH. (c) AHC – a core fluorescent unit that enables the synthesis of ‘AND’ based fluorescent probe for the detection of ONOO⁻ and GSH (d) ROS-AHC, a novel probe detailed in this work for simultaneous detection of ONOO⁻ and GSH.Herein, we set out to develop an ‘AND’ logic gate based fluorescence probe for simultaneous detection of ONOO⁻ and GSH. 3-Amino-7-hydroxy-2H-chromen-2-one (AHC) was chosen as a suitable coumarin fluorophore core for the development of an ‘AND’ logic based sensor, as its free phenol and amine functional groups provided a good opportunity for independent derivatization (Fig. 1).^36–39Previous literature reports show that protection of AHC with a maleic anhydride group results in quenching of the coumarin''s fluorescence intensity due to photoinduced electron transfer (PeT) processes. This fluorescence is rapidly restored in the presence of biological thiols, however, due to their fast addition to this functional group.⁴⁰ Therefore, we suggested that functionalization of the free phenol of this sensor using a benzyl boronic ester should further block the fluorescence, whilst serving as reporter unit for ONOO⁻. The greatly increased reactivity of peroxynitrite over other ROS towards boronate esters^41–43 should allow this functionality to act as a peroxynitrite-selective reporter, leading to an ‘AND’ logic based probe for the detection of ONOO⁻ and biological thiols (Fig. 1, Scheme 1). ROS-AHC was synthesized in 5 steps, starting with a 4-step synthesis of compound 1 adapted from literature procedures,^40,44 followed by the addition of the benzyl boronic pinacol ester (see Scheme S1 ESI^†).Open in a separate windowScheme 1Fluorescence ‘turn on’ mechanism of ROS-AHC in the presence of ONOO⁻ and GSH.The UV-Vis behaviour of ROS-AHC before and after exposure to both GSH and ONOO⁻ was evaluated in pH 7.40 buffer solution, showing a maximum absorption peak at 340 nm for both the unreacted probe and the probe following exposure to GSH, shifting to 350 nm with the addition of ONOO⁻ to the probe and 365 nm after sequential additions of GSH and ONOO⁻ to the probe (Fig. S1 ESI^†). Fluorescence experiments were then carried out. As expected, ROS-AHC was initially non-fluorescent, with a small fluorescence increase upon addition of ONOO⁻ (6 µM) (Fig. 2 and S2 ESI^†). Incremental additions of GSH (0–4.5 µM) resulted in a much larger increase in fluorescence intensity (>69-fold, see Fig. 2 and S3 ESI^†), demonstrating the need for both GSH and ONOO⁻ in order to achieve a significant ‘turn on’ fluorescence response.Open in a separate windowFig. 2Fluorescence spectra of ROS-AHC (5 µM) with addition of ONOO⁻ (6 µM), wait 5 min then incremental addition of GSH (0–4.5 µM), 5 min incubation before measurements in PBS buffer solution (10 mM, pH = 7.40). Fluorescence intensities were measured with λ_ex = 400 nm (bandwidth 8 nm). The green line represents the highest intensity after addition of GSH (4 µM).Similar fluorescence experiments were then carried out in reverse order, with the addition of GSH (6 µM) to ROS-AHC resulting in only a small increase in fluorescence intensity (Fig. 3 and S4 ESI^†). As before, incremental addition of the second analyte, in this case ONOO⁻ (0–5.5 µM), resulted in a large increase in fluorescence intensity (>46-fold, Fig. 3 and S5 ESI^†), confirming that ROS-AHC requires both GSH and ONOO⁻ for a full fluorescence ‘turn on’ response.Open in a separate windowFig. 3Fluorescence spectra of ROS-AHC (5 µM) with addition of GSH (6 µM), wait 5 min then incremental addition of ONOO⁻ (0–5.5 µM) with 5 min incubation before measurements in PBS buffer solution (10 mM, pH = 7.40). Fluorescence intensities were measured with λ_ex = 400 nm (bandwidth 8 nm). The orange line shows the highest intensity after addition of ONOO⁻ (5 µM).Subsequently, the selectivity of this probe towards both analytes was evaluated. A range of amino acids were evaluated (Fig. S6 ESI^†), with only thiol-containing analytes (glutathione, cysteine and homocysteine) eliciting significant fluorescence response, whilst non-thiol amino acids led to no changes in fluorescence intensity. A broad screen of ROS analytes was also carried out, demonstrating excellent selectivity for ONOO⁻, even over H₂O₂ (Fig. S7 ESI^†).The time-dependent response of ROS-AHC with both ONOO⁻ and GSH was also examined (Fig. S8 and S9 ESI^†). After initial addition of GSH or ONOO⁻ to the probe, subsequent addition of the second analyte triggered a rapid and significant increase in fluorescence, achieving maximum fluorescence intensity within 78 s in both cases. Furthermore, LC-MS experiments confirmed the formation of the suggested non-fluorescent intermediates, as well as the final fluorescent species shown in Scheme 1 (Fig S10, S11 and S12^†).In summary, we have developed a coumarin-based dual-analyte ‘AND’ logic fluorescent sensor, ROS-AHC, for the simultaneous detection of ONOO⁻ and biological thiols. ROS-AHC has shown high sensitivity and selectivity towards both ONOO⁻ and biological thiols.     

Ni(ii)-based coordination polymers for efficient electrocatalytic oxygen evolution reaction

The exploration of highly efficient, stable and cheap water oxidation electrocatalysts using earth-abundant elements is still a great challenge. Herein, alkaline-stable cationic Ni(ii) coordination polymers (Ni-CPs) were successfully obtained under hydrothermal conditions, which could stabilize the incorporation of Fe(iii) to form Fe-immobilized Fe@Ni-CPs. The newly developed Ni-based CPs were used for the first time as an effective electrocatalyst for the oxygen evolution reaction in strong alkaline media.

An alkaline-stable cationic Ni(ii) coordination polymer showed remarkable oxygen evolution reaction (OER) catalytic activity due to capturing Fe ions.

The oxygen evolution reaction (OER) plays a vital role in energy storage and conversion applications due to energy issues and the need for sustainable development.^1–4 Because of the sluggish kinetics of the OER, excellent electrocatalysts are required to work in acidic or strong alkaline environments.^5,6 Noble metal-based catalysts like IrO₂ and RuO₂ with high efficiency showed excellent OER catalytic activity.⁷ However, noble metal with high-cost and scarcity are impractical for scale-up applications. Currently, to substitute these precious metal-based materials, transition-metal-based (Fe, Co, Ni and so on) and metal-free (e.g., N, P and S) hybrid materials have been extensively developed.^8–10 For example, metal–organic frameworks (MOFs) such as ZIF-8/67 derived metal–carbon composite materials exhibit promising electrocatalytic performance.^11,12 Unfortunately, the pyrolysis process destroys the framework completely and causes agglomeration of metals, resulting in a decreased number of active sites. Therefore, to explore highly efficient and low-cost OER catalysts that can be directly used in the OER without calcination are desired, including complexes and MOFs.Recently, much of transition bimetallic materials showed excellent electrocatalytic activity.^13–15 However, a handful of examples such as Fe–Co-MOFs or Co–Ni-MOFs have been explored due to instability, poor conductivity and harsh synthesis conditions.^16–20 Notably, the disadvantages of MOFs have limited their usage in the potential OER. Therefore, it is urgent to develop cheap, stable and active OER catalysts to replace the precious metals. However, this is still a great challenge. Coordination polymers (CPs) with a low-dimensional framework similar to MOFs are constructed by metal ion and organic ligands with potential active sites and functional groups, exhibiting wide applications in sensing, photoluminescence and photocatalysis.^21–26 However, rare examples of CPs have been directly explored in the OER. For Fe/Ni-based bimetal electrocatalysts, a novel strategy involves doping Fe(iii) into a functional Ni-based CPs, which could enhance the electrocatalytic OER activities.Herein, we report the hydrothermal synthesis of a Ni-based CPs as a high-performance OER electrocatalyst in strong alkaline solutions (Scheme 1). The blue crystals of [Ni(bp)₃·(H₂O)₂]·(bp)·(ClO₄)₂ (bp = 4,4′-biprydine) were obtained upon the reaction of bp ligands with NiClO₄·6H₂O under hydrothermal systems. Chemical stability tests displayed that Ni-CPs could retain their original framework in water or even a strong alkaline (pH = 14) solution after 12 hours (Fig. S1^†), which is rarely reported for most transition metal CPs. The thermogravimetric analysis (TGA) showed that there is a significant change at about 110 °C due to the weight loss of the partial guest (Fig. S2^†). Interestingly, the cationic Ni-CPs successfully captured the Mohr''s salt (ammonium iron(ii) sulfate) by taking advantage of the post-synthetic strategy. The obtained Fe@Ni-CPs exhibited a high-efficient OER activity under strong alkaline conditions.Open in a separate windowScheme 1Illustration of the synthesis process for Fe doped Ni coordination polymers for the OER.Single-crystal X-ray diffraction analysis revealed that the Ni-CPs crystallized in the C2/c space group (Table S1^†). The obtained Ni coordination polymer was the isostructural compound reported by Talham,²⁷ but their packing modes were distinctly different (Fig. S3–S5^†). It also had a similar coordination environment to the railroad-like double chains synthesized by Yaghi.²⁸ The parallel chains were occupied by 4,4′-bpy, perchlorate and water molecules. In the Ni-CPs, most of the phenyl rings adopted the face-to-face mode. There were evident π⋯π interactions between the adjacent 4,4′-biprydine (Fig. 1a). In addition, there were strong intermolecular hydrogen bonds between 4,4′-bpy and perchlorate (strong Cl–O⋯C amongst adjacent layers) (Fig. 1a). The weak reaction increased the high density of the framework and protected the coordination bonds against external guest attacking. These chains and guests were further packed with a three-dimensional structure along the c-axis (Fig. 1b).Open in a separate windowFig. 1(a) The weak reaction between chains in Ni CPs, showing the Cl–O⋯C (green and black dashed line) and π⋯π interactions (pink dashed line) between the adjacent perchlorate and 4,4′-biprydine, respectively; (b) the packing view of Ni CPs along the c-axis. Cl in green, Ni in pale blue, N in blue, O in red and C in black. H atoms and partial guest molecules were omitted for clarity.The unique cationic Ni-CPs framework has the potential to immobilize some counterpart ions. To demonstrate this, Mohr''s salts were investigated. Most strikingly, the color slowly changed from blue to green in an aqueous solution, given by the optical image (Fig. 2a and b), which not only indicated that Ni-CPs captured Mohr''s salts via ion-exchange, but also suggested an alteration in the valence of Fe ions. It was possible that Fe²⁺ may have been further oxidized to Fe³⁺ under the O₂ and water environment when we prepared the Fe@Ni-CPs (4Fe²⁺ + 2H₂O + O₂ = 4Fe³⁺ + 4OH⁻). The PXRD pattern showed that the frameworks remained unchanged after doping with Fe ions (Fig. S1^†). From the transmission electron microscopy (TEM) images (Fig. 2c), after immobilization, the morphology of the Fe@Ni-CPs was still level and smooth; no ring-like patterns arose corresponding to the selected area for electron diffraction (SAED) (Fig. 2d), indicating that no bulk Fe particles formed during ion-exchange. This was further demonstrated using high resolution TEM (HRTEM) (Fig. 2e), in which there was no lattice fringe of crystallized Fe. The well distribution of C, N, O, Cl, Fe, and Ni in Fe@Ni-CPs was demonstrated by elemental mapping (Fig. 2f–l). Energy-dispersive X-ray spectroscopy (EDX) also agreed well with the above mapping data (Fig. S6^†). In addition, the Fe³⁺ uptake was 4.1 wt%, as determined by inductively coupled plasma atomic emission spectroscopy (ICP). These results showed Fe ions to have been successfully immobilized by the Ni-CPs.Open in a separate windowFig. 2(a–b) The optical image of Ni-CPs and Fe@Ni-CPs; (c) TEM images of Fe@ Ni-CPs; (d–e) the corresponding SAED and HRTEM pattern. (f–l) Element mapping of C, N, O, Cl, Fe, and Ni in Fe@Ni-CPs.The X-ray photoelectron spectroscopy (XPS) survey spectrum of the Fe@Ni-CPs also showed the presence of C, N, O, Cl, Fe, and Ni elements (Fig. 3a). The Fe 2p high resolution XPS spectrum exhibited peaks at 725 eV and 711 eV (Fig. 3b), further indicating the presence of the Fe³⁺ oxidation state. This could be explained by the transformations of Fe²⁺ to Fe³⁺ during the ion-exchange process. Similarly, in the Ni 2p spectra (Fig. 3c), two main peaks located at 855.8 eV and 873.5 eV can be ascribed to Ni²⁺ 2p_3/2 and Ni²⁺ 2p_1/2, respectively. These peaks are associated with two shakeup satellite peaks, indicating that Ni still remained in a divalent state. The Cl 2p and N 1s region could be corresponded to the ClO₄⁻ and biprydine, respectively (Fig. S7^†). The O 1s spectrum (Fig. 3d) was divided into two peaks at 531.7 eV and 533.2 eV, which could be assigned to the OH group from filled H₂O molecules and partial ClO₄⁻, respectively.Open in a separate windowFig. 3(a) XPS survey spectrum of the Ni-CPs; XPS spectra of the Ni-CPs in the (b) Fe 2p, (c) Ni 2p, and (d) O 1s regions.The above Ni-CPs with Fe doping encouraged us to investigate its electrocatalytic application in oxygen evolution reaction. To study the electrocatalytic activity of Fe@Ni-CPs for the OER, linear sweep voltammetry (LSV) was performed in a strong alkaline solution (pH = 14) for Fe@Ni-CPs@GC (fresh samples coated on glassy carbon electrode with Nafion binder). Fe@Ni-CPs@GC directly acted as working electrodes and showed good OER activity with an onset potential of 1.52 V (Fig. 4a), overpotential of 368 mV at 10 mA cm⁻², and a Tafel slope of 59.3 mV dec⁻¹ (Fig. 4b). These OER performances are close to some reported MOFs catalysts (Table S2^†) and even better than commercial benchmark OER catalysts like RuO₂ working at the same condition (Fig. S8^†). In contrast, the electrocatalytic OER activities of the pristine Ni-CPs@GC without Fe incorporation displayed much worse activity. The onset potential, overpotential (at 10 mA cm⁻²), and the Tafel slope reached 1.62 V, 458 mV, and 96.8 mV dec⁻¹, respectively (Fig. 4a and b). In addition, Fe@Ni-CPs showed a strong durability during the OER process. The chronoamperometric response of Fe@Ni-CPs displayed a slight anodic current attenuation within 12 h due to the peeling of samples during the evolution of a large amount of O₂ gas (Fig. S9^†). Furthermore, LSV of Fe@Ni-CPs showed negligible changes after OER tests for 12 h (Fig. S10^†). These results indicate that the Fe-doped Ni-CPs with more active sites could serve as an excellent candidate for OER in strong alkaline conditions.Open in a separate windowFig. 4(a) OER polarization curves and (b) Tafel plots of various electrocatalysts in a 1 M KOH aqueous solution; (c) linear relationship of the current density at 1.1 V (vs. RHE) vs. scan rates for Fe@Ni-CPs@GC and Ni-CPs@GC; (d) EIS of Fe@Ni-CPs and Ni-CPs electrode.When Fe(iii) was introduced, the resulting Fe@Ni-CPs catalysts greatly improved OER catalytic performance. There were dynamic collisions between Fe³⁺ ions and Ni-CPs, which allowed for more accessible catalytic active sites compared to the Ni-CPs. Particularly, Fe(iii) doping can contribute to the adsorption and reaction of OH⁻ groups in OER process.²⁹ As a result, Fe@Ni-CPs enhanced charge transfer under an apt electronic environment of the mixed Fe⋯Ni systems. In addition, electrochemical impedance spectrum and double-layer capacitance (C_dl) of the Fe@Ni-CP were also studied. The C_dl of Fe@Ni-CPs was confirmed to be 269.7 μF cm⁻² (Fig. 4c and S11^†), which is higher than that of Ni-CPs (C_dl = 174.2 μF cm⁻²) (Fig. 4c and S12^†). The semicircular diameter in EIS of Fe@Ni-PCP was smaller than that of Ni-CPS (Fig. 4d). These results further showed that Fe@Ni-CPs were more effective in enlarging the catalytically active surface area, conductivity and synergistic effects between Fe and Ni in comparison to Ni-CPs coated on electrodes.In conclusion, a new alkaline-stable cationic Ni(ii) coordinated polymers was synthesized under hydrothermal conditions. The Ni CPs could quickly interact with Mohr''s salt. Interestingly, the Ni CPs could act as a unique oxidation matrix to realize the transformation of Fe²⁺ to Fe³⁺ during the ion-exchange process. Furthermore, the resulting Fe@Ni-CPs electrode, for the first time, showed an excellent electrocatalytic activity for OER in strong alkaline media. This study provides a new avenue to explore stable coordinated polymers by incorporating the low-cost and high-activity transition metal, Fe, which will substitute the rare noble metals used in energy-related research.     

Development of an MRI contrast agent for both detection and inhibition of the amyloid-β fibrillation process

A curcumin derivative conjugated with Gd-DO3A (Gd-DO3A-Comp.B) was synthesised as an MRI contrast agent for detecting the amyloid-β (Aβ) fibrillation process. Gd-DO3A-Comp.B inhibited Aβ aggregation significantly and detected the fibril growth at 20 μM of Aβ with 10 μM of probe concentration by T₁-weighted MR imaging.

A curcumin derivative conjugated with Gd-DO3A (Gd-DO3A-Comp.B) was developed to significantly inhibit the amyloid-β (Aβ) aggregation and detect the fibril growth by T₁-weighted MR imaging.

A significant increase of Alzheimer''s disease (AD) patients urges the development of therapeutic and diagnostic technology.¹ As with the therapeutic development, diagnostic technology also faces several obstacles. To date, the definite diagnosis of AD relies on the histopathological data of post-mortem.^2,3 The non-invasive imaging technology targeting AD biomarkers such as amyloid β (Aβ) could provide phenotypical diagnostics, although the development of Aβ probes still remains challenging. Several contrast agents for single photon emission computed tomography (SPECT) and positron emission tomography (PET) such as Florbetapir-¹⁸F and Pittsburgh compound-B ([¹¹C]PiB) were developed as efficient tracers in mild cognitive impairment patients.^4,5 However, PET- and SPECT-based diagnostics require injection of radioactive probes, which cannot be measured frequently due to radiation exposure and limited availability of facilities. They also provide limited information on the anatomic profile of biomarkers due to their low spatial resolution and imprecise microscopic localization.⁶ In contrast, magnetic resonance imaging (MRI) contrast agents could quantify the Aβ accumulation in the anatomic brain image.⁷Several reported MRI contrast agents using gadolinium (Gd) complexes demonstrate potential use of Aβ detection. A clinically approved contrast agent, Gd(iii) diethylenetriaminepentaacetic acid (Gd-DTPA) complex accumulates in brain after opening the blood–brain barrier (BBB) by using mannitol and detects Aβ deposits in the mice AD-model.⁸ To improve the selectivity, Gd complexes were conjugated with compounds binding to Aβ such as Pittsburgh compound B (Gd-DO3A-PiB) which also serves as an approach for increasing MRI sensitivity.^9,10 An α,β-unsaturated ketone compound curcumin has been widely reported as an Aβ probe due to its ability to bind the hydrophobic site of Aβ.^11,12 Allen et al. firstly reported the direct conjugation of curcumin with Gd-DTPA which binds to Aβ with four times higher relaxivity than free Gd-DTPA.¹³ Furthermore, a polymalic acid-based nanoparticle covalently linked with curcumin and Gd-DOTA could also detect Aβ in human brain specimen by MRI.¹⁴ These previous studies demonstrate that the curcumin structure has significant potential for the development of MRI contrast agents for AD diagnosis.Previously, we reported a curcumin derivative, compound B, possesses 100-times stronger inhibitory activity of Aβ aggregation than curcumin on the basis of thioflavin T (ThT) competitive binding assay.^15,16 According to this result, we designed curcumin-based Gd probes for the detection and inhibition of Aβ (Fig. 1A–C). We hypothesized that these probes could accelerate proton longitudinal relaxation depending on the fibrillation stage of Aβ, because molecular tumbling rate of the Gd complexes becomes slower (Fig. 1A).¹⁷ As a result, the probes permit the detection of Aβ by longitudinal relaxation time (T₁)-weighted imaging. This mechanism could also be utilized to estimate the inhibitory activity of the probes by T₁-based analysis (Fig. 1B). The curcumin and compound B were directly conjugated with the macrocyclic DO3A ligand through the propylamine linker to obtain Gd-DO3A-Cur and Gd-DO3A-Comp.B, respectively (Fig. 1C).Open in a separate windowFig. 1(A) A probe concept that produces T₁ in a dependent manner of Aβ fibrillation process. (B) Inhibitor-based probes that cause moderate T₁ decreases due to inhibitory activity of fibrillation. (C) The chemical structures of the synthesized Gd probes for Aβ detection and inhibition.Gd-DO3A-Cur and Gd-DO3A-Comp.B were synthesized according to Scheme 1 (detail in Scheme S1, ESI^†). The compound 5a and 5b, which have asymmetric curcumin derivatives containing carboxylic acid group, were synthesized by three step reactions. Amide bond formation with DO3A(tBu)₃-propylamine ligand¹⁸ by condensation reaction afforded compound 7a and 7b. The tert-butyl groups were deprotected by trifluoroacetic acid producing compound 8a and 8b. The complexation was performed with GdCl₃·6H₂O by adjusting the reaction pH to 7, giving 43 and 41% yields of Gd-DO3A-Cur and Gd-DO3A-Comp.B, respectively. The T₁ relaxivities (r₁) of the curcumin-based Gd probes were estimated by T₁ measurement using a 1 tesla NMR relaxometry (Fig. S1, ESI^†). For the comparison, we synthesized Gd-DO3A-Chal which is a reported probe for Aβ.¹⁹ The r₁ of Gd-DO3A-Comp.B, Gd-DO3A-Cur, and Gd-DO3A-Chal were 7.1, 6.1 and 5.3 mM⁻¹ s⁻¹, respectively. These r₁ values are higher than that of clinically approved Gd-DOTA (3.9 mM⁻¹ s⁻¹).²⁰ The molecular weight of Gd-DO3A-Comp.B and Gd-DO3A-Cur is almost two times larger than that of Gd-DOTA. Because the r₁ increases approximately linearly with molecular weight in low magnetic field,¹⁷ the high r₁ values of Gd-DO3A-Comp.B and Gd-DO3A-Cur might be mainly attributed to their high rotational correlation time, rather than the high number of coordinated water molecules. The r₁ of Gd-DO3A-Chal was comparable to the value reported previously.¹⁹Open in a separate windowScheme 1Synthetic scheme of Gd-DO3A-Cur and Gd-DO3A-Comp.B. (a) B(OH)₃, morpholine, DMF, 100 °C, 10 min. (b) 3a/3b, B(OH)₃, morpholine, DMF, 100 °C, 10 min. (c) TFA, DCM. (d) DO3A(tBu)₃-propylamine ligand, PyBOP, HOBt, Et₃N, DMF. (e) 7a/7b, TFA, DCM. (f) GdCl₃·6H₂O, NaOH, H₂O.We evaluated the inhibitory effect of three probes toward Aβ aggregation by Congo red assay.²¹ After 24 h incubation of 20 μM Aβ with 10 μM probe, Gd-DO3A-Comp.B showed the lowest fluorescence intensity, indicating the strongest inhibitory activity followed by Gd-DO3A-Cur (Fig. 2A). As the comparison, the reported MRI agents, Gd-DO3A-Chal showed slight inhibitory activity. The inhibitory effect was further evaluated by transmission electron microscopy (TEM) with negative staining (Fig. 2B). In the absence of the probes, Aβ formed huge and massive fibril similar to the typical morphology of Aβ fibril.²² The TEM images of Aβ with Gd-DO3A-Comp.B showed the presence of white spheres below 10 nm, demonstrating that Gd-DO3A-Comp.B strongly inhibits Aβ aggregation. In fact, the fibril growth stopped at a stage of oligomer formation. Lower inhibitory activity of Gd-DO3A-Cur was also found to provide a shortened worm-like fibril, which is the typical morphology of Aβ exposed to curcumin.²³ In contrast, the small amount of white spheres and partial fibril disruption were found in the image of Aβ with Gd-DO3A-Chal. In comparison with a reported Gd-DTPA-curcumin possessing inhibitory activity starting at 50 μM, Gd-DO3A-Comp.B possessed stronger inhibition of Aβ aggregation at 10 μM.²⁴ The MTT assay using Neuro 2a cells showed that IC₅₀ of Gd-DO3A-Cur and Gd-DO3A-Comp.B. were more than 500 μM, indicating that these compounds did not possess significant cytotoxicity (Fig. S2, ESI^†).Open in a separate windowFig. 2Inhibitory effect of the Gd probes toward Aβ aggregation measured by Congo red assay (A) and negative staining TEM images (B). The Gd probes were co-incubated with monomeric Aβ for 24 h in PBS at pH 7.4. [Gd] = 10 μM, [Aβ] = 20 μM. Scale bars = 100 nm.To detect fibrillation process by NMR relaxometry, we measured T₁ of the probe mixture with Aβ which were pre-incubated for 1, 3, 6, 12, and 24 h to make it form the fibrils of different growth stages (Fig. 3A and B). The T₁ of Gd-DO3A-Comp.B solution decreased with pre-incubation time of Aβ, demonstrating that the Gd-DO3A-Comp.B can detect Aβ fibril depending on the growth stage (Fig. 3B). Lower T₁ involved with Aβ growth could be caused by the reduction in tumbling rate of the Gd complex.²⁵ We also co-incubated the probes with the Aβ monomer and monitored T₁ changes over the incubation time (Fig. 3A, B and S3, ESI^†). Interestingly, the Gd-DO3A-Comp.B did not cause significant T₁ decreases even after 24 hours co-incubation with Aβ monomers, demonstrating that Gd-DO3A-Comp.B has a strong inhibitory effect on fibril formation and the inhibition can be monitored by T₁ measurement (Fig. 3B). The inhibitory effect was consistent with the results of Congo red assay and TEM (Fig. 2). On the other hand, the time-dependent increases of T₁ were observed in Gd-DO3A-Chal and Gd-DO3A-Cur. This might be because these two probes were buried in the hydrophobic pocket as Aβ fibril grew up and fewer water molecules permitted access to the Gd ions. It is also possible that these probes have lower binding affinity, especially for matured fibril, and require higher concentrations to produce significant T₁ changes.²⁶ These probe did not produce the significant ΔT₁ between monomer and fibril samples (Fig. 3B and S3, ESI^†), although they showed little inhibition in Congo red assay and TEM (Fig. 2).Open in a separate windowFig. 3(A) Experimental design of T₁-based detection of Aβ fibrillation and inhibition by using the Gd probes. (B) T₁ changes of the Gd probe solutions with pre-incubated fibrils and monomers in PBS at pH 7.4 (mean ± SEM, n = 3). [Gd] = 10 μM, [Aβ] = 20 μM.The feasibility of the Gd probes was further evaluated by in vitro MRI measurement using a 1 tesla scanner. The T₁-weighted images showed that Gd-DO3A-Comp.B produced slight T₁ signal increases with Aβ monomers for 2 and 24 h (Fig. 4A and B). More significant signal increases were observed in the Gd-DO3A-Comp.B with Aβ fibril pre-incubated for 24 h (Fig. 4C). In contrast, Gd-DO3A-Chal and Gd-DO3A-Cur did not show significant signal changes in the presence of Aβ monomers or fibrils (Fig. 4A–C). These results were mostly consistent with the T₁ profile measured by NMR (Fig. 3). Compared to the previously reported Gd-DO3A-Chal that required 100 μM of the probe concentration to detect the equimolar Aβ,¹⁹ Gd-DO3A-Comp.B could detect five-times lower concentration of Aβ (20 μM) with ten-times lower probe concentration (10 μM). Therefore, Gd-DO3A-Comp.B could be promising to further develop highly sensitive diagnostic MRI contrast agents of AD.Open in a separate windowFig. 4 T ₁-weighted images of the Gd probe solutions in the presence of monomeric Aβ at 2 h incubation (A), monomeric Aβ at 24 h incubation (B), and Aβ fibrils pre-incubated for 24 h (C). Incubation was conducted in PBS at pH 7.4.In conclusion, we synthesized the curcumin-based Gd probes which enabled the detection and inhibition of Aβ fibril formation. Gd-DO3A-Comp.B allowed for the highly sensitive detection of Aβ fibril by the T₁ measurement. Moreover, the inhibitory activity could be estimated by T₁ measurement, because Gd-DO3A-Comp.B decreased T₁ depending on the growth stage of Aβ fibril formation. Such unique modality would be useful not only for the diagnostics but also for the direct evaluation of the therapeutic efficacy in vivo. For the future application, it would be important to combine with BBB penetration methods targeting the brain such as transient opening of the BBB using focused ultrasound or mannitol injection.^27,28     

A novel square-planar Pt(ii) complex as a monomeric and dimeric G-quadruplex DNA binder

A novel phenanthroimidazole ethylenediamine Pt(ii) complex with coumarin derivative (1) was synthesized and showed higher affinity, selectivity and thermal stabilization for mixed-type dimeric G-quadruplexes (G2T1) over monomeric G-quadruplexes (G1) and duplex DNA. Complex 1 could bind to G-quadruplexes via end-stacking and external-binding modes.

A phenanthroimidazole ethylenediamine Pt(ii) complex with coumarin derivative (1) showed high binding properties and thermal stabilization for dimeric quadruplexes G2T1.

G-quadruplex DNA, as a noncanonical secondary DNA structure, is formed by G-rich sequences widespread in biologically important regions of the human genome. Its stabilization at telomeric regions can inhibit telomerase activity and interfere with telomere biology, which makes it a potential target for the development of new anticancer therapies.^1,2 However, bioinformatic studies have shown that over 700, 000 DNA sequences within the human genome have potential to form G-quadruplex structures.³ So it is crucial to selectively bind the different sequences and conformations of G-quadruplexes. The ca. 200 bases of the single-stranded overhang of telomeric DNA can potentially fold into multimeric telomeric G-quadruplexes consisting of several consecutive G-quadruplex units linked by TTA spacers.^4,5 Moreover, multimeric G-quadruplexes could be formed by telomeric DNA and r(GGGGCC)_n RNA repeats, being relevant in amyotrophic lateral sclerosis.^6,7 Thus it is significant to design some binders for selectively binding and stabilizing multimeric G-quadruplexes.Many square-planar Pt(ii) complexes, such as square-planar platinum(ii) phenanthroline complexes, have been reported as good binders of G-quadruplexes for possessing a large electron deficient π-aromatic surface, positively charged substituents and a positively charged center.^8–12 Some Pt(ii) complexes have been modified with a pendant cyclic amine or pyridine side arm and exhibited high affinity for human telomeric G-quadruplexes.^8–10 Other structurally analogous Pt(ii) complexes, including ones with phenanthroimidazol,¹¹ dipyridophenazine,¹² and C-coordinated phenylpyridine ligands,¹² have exhibited considerably stronger interactions with G-quadruplex DNA for possessing an extended π-surface. Though a few small molecules have been studied to specifically bind multimeric G-quadruplex structures,^13–18 most square-planar Pt(ii) phenanthroline complexes have been discussed to selectively bind monomeric G-quadruplexes rather than multimeric G-quadruplexes.It has been reported that an excellent binder of monomeric G-quadruplexes, such as TMPyP4 and azatrux, could also show high binding properties toward multimeric G-quadruplexes, and even result in the significant differences of the binding properties toward multimeric G-quadruplexes and monomeric G-quadruplexes.¹⁹ With this thought in mind, together that the coumarin derivatives possess a π-surface and an amino substituent, we chose phenanthroimidazole ethylenediamine with coumarin derivative L previously reported²⁰ as a ligand, synthesized its Pt(ii) complex 1 (Scheme 1), and systematically studied its binding affinities, selectivities and thermal stabilization towards human telomeric dimeric quadruplexes G2T1 and monomeric quadruplexes G1.Open in a separate windowScheme 1Synthesis of complex 1. Reagents and conditions: (a) K₂PtCl₄, aqueous DMSO, 140 °C, 2 h; (b) ethylenediamine, EtOH, 80 °C, 12 h.Complex 1 was synthesized according to the synthetic route in Scheme 1. Phenanthroimidazole with coumarin derivative L reacted with K₂PtCl₄ in aqueous DMSO and got a red solid of Pt(ii) complex 2. Complex 2 reacted with ethylenediamine in ethanol and got the crude product, which was washed with CHCl₃ to afford complex 1 as a red solid in 39% yield. Complex 1 was fully characterized by ¹H NMR, MS (LR and HR), IR and elemental analysis (seeing ESI^†).^‡The binding effect of complex 1 on the structures of dimeric G-quadruplexes G2T1 was investigated by circular dichroism (CD) spectra. At first, addition of complex 1 led no significant changes in the ellipticity of antiparallel G2T1, and only induced minor changes in the negative ellipticity at 265 nm (Fig. S3^†). These results suggest that complex 1 brought about no structural changes and low binding affinity toward antiparallel G2T1. Subsequently, for mixed-type G2T1, addition of complex 1 led to the increasement of the band with maximum at 291 nm and the shoulder at 270 nm and the shift of the maximum band from 291 nm to 287 nm (Fig. 1a). These results show that complex 1 strongly bound with mixed-type G2T1.Open in a separate windowFig. 1(a) CD spectra of mixed-type G2T1 (3.0 μM) in the presence of complex 1: (1) 0 equiv.; (2) 2 equiv.; (3) 4 equiv. and (4) 8 equiv., respectively. (b) CD-melting profiles at 290 nm for mixed-type G2T1 (3.0 μM) with complex 1 (0, 12 and 24 μM, respectively). Values are the average ± SD of three independent measurements.Thermal stabilization of complex 1 towards mixed-type G2T1 was further assessed by CD-melting assays (Fig. 1b and S4^†). Complex 1 displayed a ΔT_m value being 9.0 °C at 4 : 1 complex-to-G2T1 ratio. And the values of ΔT_m increased with the increasing amounts of complex 1. A higher thermal stabilization (ΔT_m = 11.5 °C) was observed at 8 : 1 complex-to-G2T1 ratio, which suggests that complex 1 exhibited comparable thermal stabilization with those mixed-type G2T1 binders reported in the literature.^16,17,19,21 In contrast, complex 1 had negligible thermal stabilization towards monomeric quadruplexes G1 (ΔT_m = 1.2 °C, Fig. S5a^†) and double-stranded (ds) DNA (ΔT_m = −2.9 °C, Fig. S5b^†). These results show that complex 1 had the preferential thermal stabilization towards mixed-type G2T1 over G1 and ds DNA.Based on higher thermal stabilization of complex 1 towards mixed-type G2T1 over G1, the binding selectivity of complex 1 was further confirmed for mixed-type G2T1 by gel electrophoresis (Fig. 2). The gel reveals that addition of complex 1 to mixed-type G1 led to no appearance of any new band (lane 2–3). However, the presence of complex 1 increased the mobility of the mixed-type G2T1 (lane 5). These results suggest that complex 1 could form a compact complex with G2T1 rather than G1,^14,16 which was further verified by incubating complex 1 with a mixture of G1 and G2T1 and then analysing their gel electrophoresis. Obviously, a mixture of G1 and G2T1 in the absence of complex 1 gave the characteristic bands corresponding to intramolecular G1 and G2T1 (lane 6). When complex 1 added to a mixture of G1 and G2T1, a new band corresponding to the complex of G2T1 with complex 1 (G2T1 + 1) appeared and became more intense with the increasing amounts of complex 1 (lanes 7 to 9). However, no new band appeared corresponding to the complex of G1 and complex 1 (lanes 7 to 9). These results indicate that complex 1 had higher binding selectivity towards G2T1 over G1.Open in a separate windowFig. 2Native gel electrophoretic analysis of G1, G2T1 and their mixture in the presence of complex 1 in Tris–HCl buffer (10 mM, 100 mM KCl and pH 7.0). Lanes 1–3: G1 (16 μM) in the presence of complex 1 (0, 16 and 32 μM); lanes 4–5: G2T1 (8 μM) in the presence of complex 1 (0 and 8 μM); lanes 6–9: mixtures of G1 (16 μM) and G2T1 (8 μM) in the presence of complex 1 (0, 8, 16 and 32 μM, respectively); lane 10: DNA ladder.The binding affinities of complex 1 towards mixed-type G2T1 and G1 were determined by UV-Vis titrations (Fig. 3 and S6a^†). The gradual addition of G2T1 and G1 to complex 1 resulted in considerable hyperchromicity, a noticeable red-shift at ca. 449 nm (15 nm for G2T1 and 11 nm for G1) and the appearance of a new and strong absorbance peak at ca. 481 nm (Fig. 3a and S6a^†), which suggests that complex 1 could interact with G2T1 and G1. Then the data of UV-Vis titrations were also used to calculate the binding constant (K) of complex 1 and the number of binding sites towards G2T1 and G1 by Scatchard eqn (1a):²²r/C_f = nk − rK1ar = C_b/C_DNA1bC_b = C_t (A − A₀)/(A_max − A₀)1chere, C_t is the total complex concentration, C_b is bound complex concentration, C_f is free complex concentration, A and A_max are the observed and maximum absorption values of complex 1 at ca. 481 nm with addition of DNA, and A₀ is the absorption value of complex 1 at ca. 481 nm without addition of DNA. In eqn (1a), r represents the number of moles of bound complex per mole of DNA, D_f represents the concentration of unbound complex, K is the binding constant, and n is the number of complex-binding sites on the G-quadruplex. The plot of r/D_fversus r gives the binding constant. The results were presented in Fig. 3b and Fig. 3b, the Scatchard plots had no single linear relationship but two regression curves, which suggests the existence of two types of binding sites in the interaction of complex 1 and DNA. The binding stoichiometries of complex 1 were 1.0 : 1 for G2T1 and 1.1 : 1 for G1, respectively, when [1]/[DNA] was lower than 1.0. And the binding stoichiometries of complex 1 were 1.5 : 1 for G2T1 and 1.7 : 1 for G1, respectively, when [1]/[DNA] was higher than 1.0 (†). These results show that complex 1 had higher binding affinity towards G2T1 over G1 and CT DNA. The K_a value of complex 1 towards mixed-type G2T1 was (9.70 ± 0.26) μM⁻¹ (13,19,21,23Open in a separate windowFig. 3(a) UV-Vis titrations of 20 μM complex 1 in the presence of mixed-type G2T1 (from 0–25 μM). (b) Scatchard plots for complex 1 with G2T1 and G1. The absorbance values at ca. 481 nm were used to construct the Scatchard plots. Values are the average ± SD of three independent measurements.Binding parameters obtained from UV-Vis titrations^a

Green synthesis of crystalline bismuth nanoparticles using lemon juice

Lemon juice effectively served as a reducing and capping agent for an easy, cost-effective, and green synthesis of crystalline bismuth nanoparticles (BiNPs) in basic aqueous media. Spherical BiNPs with a rhombohedral crystalline structure are capped by phytochemicals and stably dispersible in aqueous media. The BiNPs effectively catalyze the reduction of 4-nitrophenol to 4-aminophenol by NaBH₄.

Lemon juice effectively served as a reducing and capping agent for an easy, cost-effective, and green synthesis of crystalline bismuth nanoparticles (BiNPs) in basic aqueous media.

Nanostructures of bismuth, the heaviest element among the ‘safe ones’ earning the status of a ‘green element’,¹ are particularly interesting due to their large magnetoresistance and excellent thermoelectric properties.^2–4 Bismuth nanoparticles (BiNPs) are the most extensively used nanostructures of bismuth. For example, BiNPs are utilized as contrast agents for computed tomography, photoacoustic imaging and infrared thermal imaging,^5,6 catalysts for the reduction of nitroaromatic compounds,^7,8 and removal of NO from air.⁹ Furthermore, BiNPs can act as intermediates for the synthesis of other nanostructures of bismuth, such as thermoelectric Bi₂Te₃ (ref. 10) and seeds for the solution–liquid–solid growth of nanowires.^11,12 Techniques for the synthesis of pure BiNPs could be categorized into (i) thermal decomposition,¹² (ii) mechanochemical processing,¹³ (iii) photochemical reduction,¹⁴ and (iv) solution-phase chemical reduction methods.^6,15–19 The thermal decomposition method requires harsh preparation conditions, expensive organometallic precursors, high temperature, and long reaction time, while producing high-quality monodispersed BiNPs. The mechanochemical processing technique is advantageous in terms of using inexpensive and nontoxic bulk bismuth pellets as precursors. However, the energy consumption and costly instrumentations are the limitations. The photochemical reactions typically require long time for sufficient conversion to bismuth nanoparticles, while they can also produce highly monodispersed BiNPs. The solution-phase chemical reduction methods are most popular due to the facile procedures and accessible reagents. However, stabilizers and toxic reducing agents often used are the limitations. To resolve the above-mentioned limitations, simple and green alternative methods are highly demanded.The use of abundant plant sources, which has been applied for synthesis of various metal nanoparticles such as Ag and Au, is a promising solution.^20,21 Plant sources contain a wide variety of biomolecules potentially serving as reducing and capping agents. Edible plant sources are obviously safest. To the best of our knowledge, there is no report on the synthesis of crystalline BiNPs using plant sources, while P. Poltronieri et al. reported synthesis of amorphous BiNPs using hydroalcoholic extract of Moringa oleifera.²² We focused on lemon, a very common fruit containing abundant antioxidants such as polyphenols, limonoids, citric acid, ascorbic acid, and vitamins potentially reduce ions with high oxidation states. Some of the phytochemicals, namely carbohydrates and proteins bearing ionic moieties, can be capping agents. Accordingly, lemon juice was successfully applied for the formation and in situ stabilization of silver and gold nanoparticles in aqueous media.^23,24 We presumed that a similar mechanism also works for BiNPs.In this communication, we introduce a greener strategy for the synthesis of crystalline BiNPs using lemon juice as a reducing and capping agent. For example, the synthesis was carried out using Bi(NO₃)₃·5H₂O (0.25 mmol) and freshly prepared lemon juice (25 mL) at 80 °C for 2 h under an aerobic basic condition. The X-ray diffraction (XRD) pattern (Fig. 1) of the obtained product was indexed to the pure rhombohedral phase of elemental bismuth (JCPDS no. 44-1246), indicating that the obtained product is BiNPs without detectable oxide phases. The average crystallite size was calculated to be 20 nm applying the Scherrer''s equation on the peaks of the (012), (104) and (110) plane. This result indicates that the phytochemicals present in lemon juice have ability to reduce bismuth salts to form BiNPs. The plausible phytochemicals for reduction are ascorbic acid, citric acid, and sugars. The reduction of Bi³⁺ with glucose⁸ and ascorbic acid¹⁷ was reported, and we accordingly performed the control experiment using possible reducing agents contained in lemon juice, namely ascorbic acid, glucose, and starch without any stabilizing agents. All of them could form elemental bismuth in highly aggregated forms as confirmed by the XRD patterns (ESI, Fig. S9^†) and SEM images (ESI, Fig. S10^†).Open in a separate windowFig. 1XRD pattern of obtained BiNPs synthesized using lemon juice with that of authentic Bi (JCPDS no. 44-1246).Electron microscopy was employed to confirm the size and the morphology of synthesized BiNPs. The scanning electron micrography (SEM) image (Fig. 2a) shows spherical objects having the size in the range of 50 to 100 nm agglomerated and covered with amorphous substances presumably originating from lemon juice. This agglomeration occurred during drying as confirmed by the stable water dispersibility of BiNPs with an average hydrodynamic diameter (D_h) of 255 nm investigated by dynamic light scattering (DLS) (ESI, Fig. S1a^†). The high colloidal dispersibility of the BiNPs and the coating layer observed in the SEM image suggest that phytochemicals present in lemon juice act also as capping agents. The larger D_h measured by DLS than the size observed by SEM can be attributed to the surrounding hydration layer and swelled phytochemicals attached to the surface of the BiNPs. We then performed transmission electron microscopy (TEM) analysis to get the actual size of the Bi cores. The TEM images of the BiNPs (Fig. 2b and c) indicated the presence of heavy elements in the matrix of light elements. The particles are spherical, and the diameter ranges from 8 to 30 nm. The high-resolution TEM image (Fig. 2d) shows the lattice fringes of 0.298 nm, 0.253 nm, and 0.224 nm of the typical crystallite agreeing well with the distance of the (012), (104) and (110) plane, respectively, of the rhombohedral Bi(0).Open in a separate windowFig. 2(a) SEM and (b–d) TEM images of BiNPs synthesized using lemon juice.The EDX spectrum (ESI, Fig. S2^†) reveals that obtained BiNPs comprise bismuth (Bi), carbon (C), and oxygen (O). The strongest signal of bismuth testifies the successful synthesis of BiNPs, whereas the C and O signals demonstrate the presence of the capping layer on the BiNPs.The FT-IR spectrum of the synthesized BiNPs was analyzed to presume the possible components on the BiNPs surface with the comparison with the solid content of lemon juice (Fig. 3). The FT-IR spectrum of the BiNPs shows the following major absorption bands. The broad absorption band at 3050–3500 cm⁻¹ is assignable to the O–H stretching of alcohol moieties. The absorption band around 2840–2940 cm⁻¹ is assignable to the C–H stretching of alkane, and the absence of the sharp absorption around 2950–3050 cm⁻¹ suggests the absence of trace contents of unsaturated C–H groups. The absence of the adsorptions originating from COO–H around 2400–3100 cm⁻¹ and C O around 1700–1730 cm⁻¹ indicates that citric acid^25,26 and other carboxylic acids contained sufficiently in lemon juice are not the major components on the BiNPs, while the presence of the strong absorption around 1600 cm⁻¹ is assignable to carboxylate moieties. The absorption bands around 930–1130 cm⁻¹ assignable to C–C and C–O stretching and around 1200–1420 cm⁻¹ assignable to O–C–H, C–C–H and C–O–H bending suggest the presence of sugars on BiNPs.^25,26 The content of the organic moieties was estimated approximately 14% by thermogravimetric analysis from weight loss that occurred at the temperature range from 130 to 450 °C (ESI, Fig. S3^†), at which the negligible C and O signals were observed in the EDX spectrum of the residue after TGA (ESI, Fig. S2^†).Open in a separate windowFig. 3FT-IR spectra of solid content of lemon juice and obtained BiNPs synthesized using lemon juice.The ¹H and ¹³C NMR and FTIR spectroscopic analysis (ESI, Fig. S4–S8^†) of the ethanol and chloroform extracts of the BiNPs suggests that the major capping phytochemicals are polysaccharides and fatty acid derivatives. Other minor possible components may include amino acids, terpenes and phenolic compounds contained in lemon juice.The successful formation of BiNPs made us enthusiastic to investigate their catalytic performance in the reduction of nitroaromatic compounds, problematic pollutants arising from explosives, analgesic, and antipyretic drugs and dyes.²⁷ Herein, the catalytic performance of our BiNPs was investigated by selecting the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by NaBH₄ at room temperature as a model reaction.^7,8,28 The progress of the reduction was monitored by UV-vis absorption spectroscopy. The colour of the 4-NP aqueous solution changed from light yellow to deep yellow upon the addition of NaBH₄ to produce 4-nitrophenolate ion and an intense absorption peak at 403 nm appeared instead of the original absorption peak of 4-NP at 316 nm in the UV-vis spectrum.⁷ In the absence of BiNPs, the colour of the solution and the intensity of the peak were retained even after 12 h. In the presence of BiNPs, the deep yellow mixture became almost colourless within 180 min (Fig. 4a). The intensity of the absorption peak at 403 nm decreased overtime, and simultaneously a new absorption peak appeared and grew at 299 nm, indicating the formation of 4-AP (Fig. 4a).^8,27,29 The reaction rate almost follows pseudo-first-order kinetics agreeing with the Langmuir–Hinshelwood mechanism; in which two reactants react after adsorption on the solid surface, and then the product desorbs. The rate constant (k) was determined from the plot of ln(A/A₀) vs. time (Fig. 4b) according to the previous reports.^7,8,28,29 The k value of 0.0134 min⁻¹ (activity factor = 0.1 s⁻¹ g⁻¹) is lower than previously reported PVP-coated bismuth nanodots (6.033 s⁻¹ g⁻¹)⁸ and starch coated BiNPs (0.02751 s⁻¹).⁷ The lower catalytic rate of our BiNPs can be attributed to the electrostatic repulsion of 4-nitrophenolate ions with negatively charged BiNPs (zeta potential value = −31.7 mV) in the reaction mixture. The initial rate is slower than the rate after 50 min. Possible factors are (i) the inductive period for the surface activation of the nanoparticles at the initial stage as reported for bismuth nanodots by Liang et al.⁸ and (ii) partial and gradual release of phytochemicals under the basic conditions. After the catalytic reaction, the D_h value measured by DLS (195 nm) became smaller than the original one maintaining the good water dispersibility, suggesting unravelling of primary particles fused by the capping layer (ESI, Fig. S1b^†). In addition, the weight loss in the TGA curve became lower (ESI, Fig. S3^†), and the SEM image implies the partial loss of the coating (ESI, Fig. S11^†). This decrease of the organic moieties is attributable to the partial removal of coating substances such as fatty acids during the catalytic reaction under the basic and reductive conditions, supported by the disappearance of the absorption band around 1600 cm⁻¹ assignable to carboxylate moieties in the FTIR spectrum of BiNPs after the catalytic reaction (ESI, Fig. S12^†). Carboxylic acids and their esters are reported to be reduced to alcohol by NaBH₄ in the presence of electrophiles.^30–33 The initial presence of the amphiphilic and anionic layer plausibly delays the catalytic reaction.Open in a separate windowFig. 4(a) Optical images and absorption spectra of catalytic reduction of 4-NP ([4-NP] = 15 ppm) by NaBH₄ (4.28 × 10⁻⁴ M) in presence of BiNPs (142 mg L⁻¹); (b) pseudo-first order kinetic plot of catalytic reduction.In conclusion, we have demonstrated a green, cost-effective, and successful approach for synthesis of crystalline BiNPs using lemon juice as a reducing as well as capping agent. The obtained BiNPs effectively catalysed the reduction of 4-NP to 4-AP by NaBH₄. The importance of this method lies in the simple synthetic procedure, uses of safe and low-cost lemon, and good dispersibility over conventional chemical approaches.     

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

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

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

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

Solvent-assisted linker exchange as a tool for the design of mixed-linker MIL-140D structured MOFs for highly selective detection of gaseous H2S

A MIL-140D-sdc framework has been used as a highly stable backbone for the introduction of 4,4′-azobenzene dicarboxylic acid (H₂abdc) via solvent-assisted ligand exchange (SALE). The implemented azo groups can serve as coordination sites for copper ions. These can exchange ligands with different gases, but show a high selectivity against H₂S, which makes this material promising for potential sensor applications.

Solvent-assisted linker exchange was used as tool to modify a MIL-140D-sdc (sdc = 4,4′-stilbenedicarboxylate) MOF with azostilbene dicarboxylic acid. The azo groups can act as coordination sites for copper ions and allow the use of this material as sensor for gaseous H₂S.

Hydrogen sulfide (H₂S) is a toxic gas with harmful effects on human health and is responsible for different diseases, like liver cirrhosis.¹ At concentrations above just 50 ppm, irritation of the respiratory tract can occur.² For the detection of H₂S, chemical sensors containing semiconducting metal oxides are widely used. Although these exhibit high sensitivities, they also have some disadvantages due to their high working temperatures and a lack of selectivity.³Especially in the areas of selectivity and cross-sensitivity, metal–organic frameworks (MOFs) can overcome the limits that plague many other sensors. Due to their modular design built up by metal nodes and organic linker molecules, they can be tailored to the desired application.⁴ Functionalities attached to the linkers of MOFs are often required in order to be able to use these materials for sensor applications.⁵Many MOFs are described for the detection of H₂S. Apart from one example of a capacitive sensor,⁶ the detection is almost always based on a fluorescence turn-on probe. Here functional groups of the MOF (–N₃ or –NO₂) are reduced by H₂S to amino groups resulting in turning on the fluorescence of the linker. However, in these investigations the MOFs are dispersed in buffered aqueous or ethanolic solutions.⁷ In addition, no gaseous H₂S is used, but sulfides-containing sodium salts such as Na₂S, that dissociate in solution to HS⁻, which is required for the reaction.⁸ To the best of our knowledge, only one example is described in literature where a MOF is used for a luminescent-based detection of gaseous H₂S.⁹Azo components have the special ability to coordinate metal ions. This has already been investigated in detail with individual azo compounds and metal salts of palladium¹⁰ or copper.^11,12 Until now, a transfer of this reaction behaviour into a MOF is rare and not yet described for copper species but for palladium species.¹³ Nevertheless, these copper-azo complexes can be used for the colourimetric detection of both aqueous HS⁻ and gaseous H₂S.¹⁴ The sensor principle is based on a displacement reaction. The coordinated copper ions react with gaseous H₂S to CuS as already shown with CuO-loaded metal oxide semiconductors.¹⁵ Considering copper-based MOFs such as the HKUST-1, they show a lack of stability under normal conditions, making them unsuitable for use in sensor technology. Nevertheless, a reaction between H₂S and the metal centres is still possible, however, this leads to the formation of CuS and the complete breakdown of the framework.¹⁶With the UiO family, a isoreticular series of Zr-based MOFs was reported.¹⁷ Particularly, the UiO-66 and its derivatives exhibit an exceptional thermal and chemical stability. By the use of the same precursors but higher synthesis temperatures, Guillerm et al. presented highly stable Zr-MOFs, the so-called MIL-140 series.¹⁸ In MIL-140 frameworks, there are one-dimensional chains of zirconium oxide (c.n. 7) as inorganic building units (IBUs) orientated parallel to the c-axis of the structures. Each chain is connected via linker molecules to six other chains resulting in a one-dimensional pore system (see ESI Fig. S1^†). Especially those MOFs with longer linker molecules exhibit an improved stability in comparison to their UiO analogues. This can be explained with the different structures of these compounds.^18,19By using post-synthetic modifications (PSM) further functionalized MOF materials can be obtained.²⁰ In case of exchanging an unfunctionalized linker molecule with a functionalized, the solvent-assisted ligand exchange (SALE) is an attractive possibility to adjust the properties of an already synthesized MOF and enhance the tunability.²¹We recently reported a low-temperature synthesis of MIL-140 frameworks and a new MIL-140-structured MOF, based on 4,4′-stilbenedicarboxylic acid (H₂sdc), the MIL-140D-sdc (see Table S1 and ESI Fig. S1^†).²² This MOF shows a high thermal and chemical stability which can be compared to the former reported MIL-140 MOFs.¹⁸ A synthesis for an azo-based MIL-140D-H with H₂abdc as linker is also described in literature (see ESI Table S1^†).²³ However, the resulting framework is instable under ambient conditions and a transition to a unknown nearly nonporous phase occurs after only a few days.²³ These are unfavourable properties for a stable sensor material.In this work, we demonstrate the efficiency of the SALE to produce MOFs that combine high stability and high functionality. Therefore, we use the MIL-140D-sdc framework as stable backbone. For the preparation of the mixed-linker MOF (MIL-140D-sdc/abdc), we substitute sdc²⁻ with abdc²⁻via SALE by immersing the MIL-140D-sdc in a DMF solution of H₂abdc and storing it at 120 °C for 24 h (up to 1 eq. H₂abdc with respect to the Zr cations in the framework, see ESI Table S2^†).For verifying the successful SALE, the exchanged MOF samples were disassembled and analysed with ¹H-NMR spectroscopy (ESI Fig. S2 and S3^†). The evaluation has shown that the level of conversion rises with an increasing amount of provided linkers (H₂abdc). The maximum exchange rate is slightly below 50%.Powder X-ray diffraction (PXRD) is used to check the stability of the compounds after each reaction step. After exchanging the MIL-140D-sdc with abdc²⁻ at 120 °C, the solids exhibit the same diffraction pattern, but the colour has changed from white to reddish (Fig. 1a).Open in a separate windowFig. 1Presentation of the conducted modification, incorporation and gas exposure with MIL-140D-sdc structured MOFs. (a) Powder XRD pattern of the samples prepared with a SALE starting from MIL-140D-sdc and the resulting colour change with different amounts of H₂abdc. (b) Powder XRD pattern after the incorporation of increasing amounts of Cu²⁺ into the MIL-140D-sdc/abdc framework and the observed colour change and (c) powder XRD pattern and pictures of the same materials after the exposure to 100 ppm H₂S.The advantage of MIL-140 structured MOFs results from the stacking of the linkers and the resulting short distance between the azo groups. This arrangement should be promising for the coordination of copper ions, since in literature the molecules either have additional functional groups for the coordination or the metal ions are located between two azo groups.²⁴ Subsequently, copper ions are incorporated into the framework. For this purpose, the linker-exchanged MOF is dispersed in DMF and stirred for 1 hour at room temperature while adding copper chloride. The amount of copper chloride used is identical to the previously used amount of added linker H₂abdc (see ESI Table S3^†). After the copper ions were integrated into the framework, the colour of the MOF changed from red to green (Fig. 1b). Although the samples still show the same diffraction pattern, a closer look reveals differences. Thus, the 200-reflection (at 5° 2Θ) shifts slightly to smaller angles and in the range of 10° 2Θ the intensity of two reflections increase (ESI Fig. S4^†). Neither the shifted reflection nor the colour change can be observed during the treatment of UiO-abdc and MIL-140D-H with copper chloride. Additionally, pure MIL-140D-H appears not to be stable during this reaction (ESI Fig. S5 and S6^†).Nitrogen physisorption measurements verified that there is no significant quantity of linker molecules in the tunnel-like pores after the SALE (ESI Fig. S12 and Table S5^†). In order to demonstrate that the integrated copper ions do not inhibit the accessibility of the pores, N₂ physisorption measurements have been repeated (ESI Fig. S13^†). Indeed, the experimental BET areas barely change and are only about 100 m² g⁻¹ lower (ESI Table S6^†). It can therefore be assumed that the copper ions are coordinated at the azo groups of the linkers and thus hardly contribute to a loss of surface area.For further simplification, at this point the focus is on the samples with the highest exchanged amount of linker and highest stored quantity of Cu²⁺ (1 eq. each). For a more detailed evaluation of the pore size and thus the location of the incorporated copper ions, argon physisorption measurements were carried out. Compared to the originally used sample MIL-140D-sdc, the BET areas are only slightly reduced and also the calculated pore size distributions for all samples show a good correlation (Fig. S15, S16 and Table S8^†). Both measurements indicate a coordination of the copper ions at the azo groups of the linkers. Furthermore, this result contradicts an excessive adsorption of the copper ions in the tunnel-like pores.The amount of copper coordinated in the MOF is estimated by EDX spectroscopy. For this purpose, the MOF was offered two different quantities of copper salt (0.5 and 1.0 eq.) for complexation. In comparison to the SALE sample, the EDX measurements show the same copper content of 0.4 copper atoms per sum formula, regardless of whether 0.5 or 1.0 eq. copper salt was added (see ESI Fig. S17–S19 and Table S9^†). The similar value is due to the fact that only half of the linkers are exchanged during the SALE and again suggests the coordination of the copper ions at the azo groups.The thermogravimetric data are a further indication for the successful storage of Cu²⁺ in the framework. The sample obtained after the SALE shows a similar thermal decomposition as the starting material MIL-140D-sdc. Both materials have a thermal stability of about 350 °C, but the residual mass is lower than calculated in case of the SALE sample (ESI Fig. S21 and Table S10^†). After incorporation, the residual mass increases by approximately 4.5m%, which is equivalent to a copper amount of approximately 0.45 per sum formula and in good agreement with the EDX spectroscopy. The reflections of the powder diffraction pattern of the residue can be accurately assigned to ZrO₂ and CuO (see ESI Fig. S7^†). In addition, the percentage of guests in the framework is increasing drastically. This could be an indication that the copper ions act as Lewis acidic sites where solvent molecules preferentially coordinate. In order to estimate the Lewis acidity of a MOF, the shift of the acetone absorption band can be considered.²⁵ In the presence of strongly Lewis-acidic MOFs, the stretching vibration of the carbonyl group shifts to smaller wave numbers. In this case, the acetone vibration is located at 1690 cm⁻¹ and thus 25 cm⁻¹ lower than for uncoordinated acetone (see ESI Fig. S10^†).In the last step, the incorporated Cu²⁺ ions will be used for the detection of H₂S. With this approach it is possible to transfer the properties of the molecular complexes¹¹ into a solid state material, as we have already shown with a calixarene-based MOF for highly selective NO₂ detection.²⁶ It is generally assumed that the implicit chemical reaction is the formation of CuS.¹⁵ Upon this reaction, a fast and impressive colour change occurs from green to black which can be simply observed with the naked eye. Taking the diffraction patterns into account, it is noticeable that the above-mentioned shift of the 200-reflection is reversed upon H₂S exposure, indicating a change in the coordination of the copper ions (see Fig. 1c and ESI Fig. S4^†). After the exposure to H₂S, the BET area of all samples is drastically reduced (see ESI Fig. S14 and Table S7^†) which can be explained by pore blocking effects due to CuS formation.The formation of CuS can additionally be verified with Raman spectroscopy. Here, a weak CuS band can be found at 471 cm⁻¹ (see ESI Fig. S11^†). Again, this is an indication for the formation of CuS located inside the pore channels or on the surface of the MOFs as pore blocker. This is also an explanation for the decrease of the BET surfaces from the physisorption measurements. After the MOF has been exposed to H₂S, the acetone vibration disappears completely from the IR spectrum. This observation supports the thesis that the coordination of the copper ions at the azo groups and thus the Lewis acidic effect is no longer present after this step.Additionally, the experimental residue of the MOF is similar to the Cu²⁺ incorporated sample (see ESI Fig. S21 and Table S10^†) and is composed of ZrO₂ and CuO (see ESI Fig. S8^†). Moreover, the EDX measurement show a sulphur content in this sample which is comparable to the amount of copper and obviously CuS is formed (see ESI Fig. S20 and Table S9^†).The chemical reaction of the sensor response is the formation of CuS and the associated colour change from green to black. For the application as H₂S sensor material, the spectroscopic characteristics were determined. As already can be seen in the photographs in Fig. 1, the colouration becomes more pronounced with increasing amount of provided linker during the SALE and thus also of the amount of incorporated Cu²⁺. This observation is supported by the UV/Vis measurements. Here, the absorption also intensifies with increasing amounts of H₂abdc after the SALE in the range between 430 and 500 nm. The pure MIL-140D-sdc shows no absorption in this region at all (see ESI Fig. S22a^†). The integration of azo groups is essential for the incorporation of copper ions into the MOF and thus for the detection of H₂S. When a pure MIL-140D-sdc is immersed in a copper chloride solution it can be shown that no copper is coordinated by the MOFs by comparison of the UV/Vis spectra. The absorption does not change because no copper ions can be complexed by the sdc²⁻ linker of pure MIL-140D-sdc. When the copper treated MIL-140D-sdc was exposed for 30 minutes to 100 ppm H₂S no measurable difference in the spectra occurs (see Fig. S24^†). As a result, it can be concluded that no copper ions were deposited in the framework at all.After the incorporation of copper ions in the MIL-140D-sdc/abdc, a second absorption maximum at 730 nm is visible (see ESI Fig. S22b^†). As before, the maximum absorption increases with the amount of copper salt used. After exposure to H₂S, all samples immediately change their colour, whereby the highest absorption can be observed in the sample with the highest copper and abdc²⁻ content. At this point, the entire absorption in the visible range of the light spectrum increases (see ESI Fig. S22c and S23^†).During a 30 minute exposure to 100 ppm H₂S a cycled UV/Vis measurement was conducted to observe directly the change in the absorbance. Within less than 1 minute, an increase of the absorption between 550 and 600 nm is visible. After 30 minutes the saturation is reached and absorbance is approximately 450 percent higher compared to the starting material after the Cu²⁺ incorporation (see Fig. 2). For a more precise analysis, the sample was exposed to 100 ppm H₂S for 30 minutes and the absorbance at 580 nm was recorded every five seconds. Here the MOF exhibits almost no absorption after the incorporation of Cu²⁺. After the exposure to H₂S, the absorbance increases immediately and is again reaching a saturation point with an absorbance twice as high (see Fig. 3).Open in a separate windowFig. 2UV/Vis spectra of the MIL-140D-sdc/abdc after the SALE with H₂abdc (red), after the incorporation of Cu²⁺ (green) and after the exposure to 100 ppm H₂S with different time steps ranging from 1 to 30 minutes.Open in a separate windowFig. 3UV/Vis measurement of the Cu²⁺ incorporated sample (1 eq.) at 580 nm. Before the gas exposure (100 ppm H₂S) a baseline was measured under room conditions.The next step was to investigate the selectivity. For this purpose, the MOF was exposed to various gases such as CO₂ and CO (100 ppm), NO₂ (10 ppm) or stored in an NH₃ and diethyl ether (DEE) atmosphere for 30 minutes. In terms of stability, a partly decomposition occurs only in presence of CO due to its interaction with the azo group (see ESI Fig. S9^†).²⁷ Furthermore, the exposure to these gases results in a colour change for each sample, which can be explained by a ligand exchange at the coordinated copper ions (ESI Fig. S25^†).²⁸ Each sample was again investigated with UV/Vis spectroscopy and the main differences in the spectra are in the range of 400 to 550 and 650 to 800 nm (see ESI Fig. S26^†).The other examined gases cause only minor changes in the range of 580 nm. Only in the case of H₂S a significant change in the spectrum at this wavelength was observed, which should enable quantitative detection of this gas. The absorbances of the different samples at 580 nm are compared and set in relation to the original absorbance of the copper stored sample. The result is shown in Fig. 4, which demonstrates the high selectivity of this material for H₂S measured under these conditions.Open in a separate windowFig. 4Comparison of the differences in the absorbance of the sensor material in the presence of different gases (exposure time: 30 minutes). The measured values are all given in relation to the starting material whose absorbance was defined as 100%.In summary, it was demonstrated that through the post-synthetic step of the SALE a mixed-linker MOF has been obtained, the composition of which can be controlled by the amount of H₂abdc used in the SALE process. Furthermore, the new MIL-140D-sdc/abdc combines the stability of the MIL-140D-sdc framework and the functionality of the introduced azo groups. This again emphasizes the unique nature of the SALE for the production of differently functionalized frameworks and increases the tunability of manufactured materials. For the first time it was possible to use these linkers in a porous solid as coordination sites for copper ions. The porosity of the MOF is barely affected, making the coordinated metal ions accessible to guests. The guests can be different gases that coordinate under a ligand exchange with the copper ions. Nevertheless, this material has a high selectivity towards H₂S, which is expressed by the colour change from green to black, most likely due to the formation of CuS.The MIL-140D-sdc/abdc seems to be an interesting starting material for the coordination of various metals, due to the close arrangement of the linkers. Further, more elaborated, functional groups could help to optimize the sensing process or enhance the reversibility of the sensing reaction. This property can be used for further post-synthetic modifications of the linker and might also result in other attractive sensor materials.     

Removal of per- and polyfluoroalkyl substances (PFAS) from water by ceric(iv) ammonium nitrate

Ceric(iv) ammonium nitrate (CAN) in aqueous medium acts as an excellent precipitating agent for perfluorooctanesulfonic acid (PFOS). The Ce(iv) center plays a crucial role. Interestingly, Ce(iii) chloride showed much less effectiveness under similar conditions. The efficacy of CAN was reduced upon changing the substrate to perfluorooctanoic acid (PFOA).

Removing per- and polyfluoroalkyl substances (PFASs) from water by ceric ammonium nitrate (CAN).

Per- and polyfluoroalkyl substances (PFAS) are toxic and xenobiotic compounds that have been widely used in fire extinguishers, carpet guards, paper, and non-stick cookware.¹ However, these groups of organofluorines are highly resistant to degradation and are not easily separable by the usual water purification techniques.^2–4 The environmental and health hazards caused by PFAS contamination in drinking water and ground water have stimulated research for innovative strategies to remove PFASs.⁵ An additional risk while developing these removal techniques is the toxicity of PFASs even at very low concentrations.^6,7 Easily available chemicals that can efficiently degrade/precipitate out low concentrations of PFAS from water can provide a futuristic design to develop valuable water purification materials.The large C–F bond dissociation energy (∼485 kJ mol⁻¹) makes it resistant to oxidation and reduction, and together with simultaneous hydro- and oleophobicity of PFAS, make the overall degradation/separation process challenging.^8,9 Methods to degrade/remove PFAS from water include in situ generated radicals, heat treatment, photocatalysis, or strong reducing or oxidizing agents.^10–12 There are also important reports on PFAS remediation techniques using carbonaceous nanomaterials that are high surface area sorbents or In₂O₃ type, which possess oxygen vacancies in the monocrystalline structure.^13–15There are also some reports on the removal of PFASs directly by powdered-activated carbon (PAC) and granular-activated carbon (GAC) adsorption processes.^16,17 The conventional coagulation processes (e.g., by iron chloride, alum etc.) are usually inefficient at low concentrations of PFAS. They are slow, produce a lot of sludge, and are usually expensive.¹⁸ A natural coagulant Moringa oleifera seeds has been reported to be a better alternative and produces less sludge.¹⁹ Nevertheless, the effectiveness at low concentration of PFAS remains as a challenge.Herein, we report a new approach for precipitation of the commercially available PFOS (as potassium salt) from water (<20 μM) using ceric(iv) ammonium nitrate (CAN). This study was initiated from the perspective of generating in situ high valent/mixed valent oxide species/nanoparticles from FeCl₂ and CoCl₂ that can oxidatively degrade PFAS. In preliminary studies, CAN was used as an oxidant and resulted in significant reduction in the concentration of PFAS of the supernatant aqueous solution whenever CAN was used. Thereafter a range of concentrations of CAN (from 0.094 mM to 0.75 mM) were tested in consecutive experiments keeping PFOS concentration unchanged at 15 μM (Fig. S3^†). After the treatment with 0.38 mM concentration CAN, the supernatant shows ∼80% removal of PFOS (Fig. 1, column 1 and 2). Very slow formation of an off-white precipitate was seen from the solution after seven days of undisturbed standing.Open in a separate windowFig. 1(1) The LC-MS detected concentrations of 15 μM of PFOS in water before treatment, (2) after treatment with 0.38 mM CAN ([(NH₄)₂Ce^IV(NO₃)₆]), (3) 0.38 mM Ce^IV(SO₄)₂, (4) 0.38 mM CAN + 0.38 mM FeCl₂, (5) 0.38 mM CAN + 0.38 mM CoCl₂, (6) 0.38 mM FeCl₂, (7) 0.38 mM CoCl₂, (8) 0.38 mM CeCl₃, and (9) 0.38 mM AN (NH₄NO₃) (all experiments were repeated in triplicate and the dilution details are provided in the ESI^†).Two 1st row transition metal salts [CoCl₂ and FeCl₂] and also a combination of these salts with CAN [CAN : CoCl₂ (1 : 1 equivalent); CAN: FeCl₂ (1 : 1 equivalent)] were tested under identical conditions. In all these cases, very little (∼15–20%) diminution of PFOS concentration (in LC-MS) was observed in the absence of CAN (Fig. 1, column 4–7). Since CAN itself was very reactive and either pure Fe and Co salts or a 1 : 1 mixture appeared to be much less effective, higher ratios of CAN : CoCl₂ or CAN : FeCl₂ were not tested further.^§¶ The lower efficacy of these 1 : 1 combinations can be explained by possible partial use of CAN to oxidize Fe(ii) and Co(ii).²⁰To check whether Ce^IV is playing a crucial role, another Ce^IV containing commercially available and strong oxidant, Ce^IV(SO₄)₂ (0.38 mM) was tested and showed ∼60% disappearance of PFOS under identical conditions (Fig. 1, column 1 and 3). To find if this is a general trend for any oxidation state of cerium, the impact of CeⁱⁱⁱCl₃ (commercially available) was also investigated. It clearly showed significantly lower efficiency in removing PFOS (at best ∼33%) (Fig. 1 and column 8) from the aqueous medium. No precipitate was found even after two weeks of undisturbed standing. Observing the noteworthy reactivity difference between CAN and CeCl₃, another set of experiments was performed using 0.38 mM of ammonium nitrate (AN) (Fig. 1, column 9) and 15 μM of PFOS. Almost no change (<5%) in PFOS concentration indicates strongly towards the significance of the Ce(iv) center.In order to obtain clarity on the possible species that are precipitating out from the CAN–PFOS interaction, higher concentrations were used. LC-MS analysis indicated that CAN is more effective at higher concentration and complete removal of PFOS (1.5 mM) is possible at around 18.75 mM concentration of CAN (by LC-MS) (Fig. 2) within 3 minutes of continuous stirring after mixing. An off-white precipitate formed over couple of days after mixing CAN and PFOS.Open in a separate windowFig. 2The LC-MS detected 1.5 mM of PFOS in water before ([PFOS : Ce(iv)] = 1 : 0) and after the treatment (3 minute) with 2.34 mM, 4.69 mM, 9.38 mM, 18.75 mM and 37.5 mM of CAN.The solutions before and after the treatment with CAN and the precipitate formed during the experiment were also investigated by 400 MHz ¹H and 376 MHz ¹⁹F NMRs (Fig. 3, S6–S9).^† These NMR studies (Fig. 3) of the supernatant liquid showed complete removal (in the NMR detection limit) of PFOS from water. The slight change in the chemical shift values and the splitting pattern of the NMR signals indicate CAN-PFOS interaction or possible complex formation.Open in a separate windowFig. 3(a) The colour and texture of CAN that was used for this study, (b) pictorial representation of CAN''s chemical structure, (c) and (d) are the 376 MHz ¹⁹F NMR of aqueous solutions (1.5 mM) of PFOS in DMSO-d₆, before and after the treatment with 18.75 mM CAN [Ce(iv)] respectively showing complete disappearance of PFOS from water, and (e) 376 MHz ¹⁹F NMR of the precipitate in DMSO-d_6. Inset shows magnified picture of c, d and e from −129 ppm to −114 ppm, indicating complexation or strong interaction between CAN [Ce(iv)] and PFOS.To generalize our findings, we also tested another commercially available and highly regulated PFAS, perfluorooctanoic acid (PFOA, Fig. 6) as a substrate.²¹ At 15 μM starting concentration, at best 65% of the PFOA could be removed with 0.75 mM of CAN (Fig. S4^†). At 1.5 mM PFOA, 78% removal was possible, with 18.75 mM CAN. However, complete removal of PFOA (1.5 mM) was detected (by LC-MS) at a much higher CAN concentration (0.15 M), after 3 minutes of continuous stirring (Fig. 4).Open in a separate windowFig. 4The LC-MS detected concentrations of 1.5 mM of PFOA in water before ([PFOA : Ce(iv)] = 1 : 0) and after the treatment (3 minute) with 18.75 mM, 37.5 mM, 75 mM and 150 mM of CAN.Open in a separate windowFig. 6Chemical structures of the two commercially available PFASs used in this study.In this case, an off-white precipitate was observed after a week of undisturbed standing. Similar NMR investigations were performed by 400 MHz ¹H and 376 MHz ¹⁹F NMR (Fig. 5, and S10–S13^†) spectroscopy. The complete disappearance of the fluoride signal from water (in the NMR detection limit) and the change in the chemical shift values together with the splitting pattern of the precipitate also suggest complexation between Ce and PFOA.²² It is interesting to see more prominent changes in the chemical shift values in the case of the Ce–PFOA complex than that of the Ce–PFOS complex. In the case of Ce–PFOA complex, initial deprotonation of PFOA is expected prior to complexation/interaction with Ce(iv).Open in a separate windowFig. 5(a) and (b) are the 376 MHz ¹⁹F NMR of an aqueous solution (1.5 mM) of PFOA in DMSO-d₆, before and after the treatment with 0.15 M CAN [Ce(iv)] respectively showing complete disappearance of PFOA from water. (c) Represents the 376 MHz ¹⁹F NMR of the precipitate in DMSO-d_6. Inset shows magnified picture of a, b and c from −128 ppm to −113 ppm, indicating complexation between Ce(iv) and PFOA.Very similar precipitation was observed with Ce^IV(SO₄)₂ at the higher concentration study. PFOS (1.5 mM) precipitated out much faster (within a few hours) and complete removal (in the NMR detection limit) (Fig. S16^†) was possible at 18.75 mM of Ce^IV(SO₄)₂. In comparison, faint white precipitation from PFOA (1.5 mM) and 18.75 mM of Ce^IV(SO₄)₂ was observed over a period of two days. NMR of the supernatant aqueous solution shows incomplete removal of PFOA under identical condition (Fig. S17^†).It is clear that PFOS reacts with CAN and Ce^IV(SO₄)₂ more efficiently (Fig. 2 and ​and4)4) and precipitates out faster than PFOA. This difference in reactivity can be due to the better donor ability (nucleophilicity) of the sulfonate group in PFOS than the carboxylic acid group in PFOA. Due to the higher acidity of the sulfonic acid, the conjugate base sulfonate (nucleophile) remains as the major species in solution in the presence of CAN in aqueous medium.²³ In the case of PFOA, to be equally effective, it needs higher concentration of CAN. Moreover, being shorter than PFOS, PFOA has been reported to show a higher tendency to stay in the aqueous phase, and therefore is difficult to precipitate out.²⁴ With AN (NH₄NO₃), no such precipitation was observed, and NMR results also confirm no detectable interaction between AN and PFOS (Fig. S14 and S15^†).All these observations suggest a possible complexation between Ce(iv) of CAN and these PFASs, followed by precipitation where PFOS and PFOA act as a nucleophile (ligand). No distinct new proton signal in 400 MHz ¹H NMR spectra of the aqueous solution and the precipitate after the CAN treatment, confirms the absence of any newly formed hydrocarbons.HRMS analysis of the diluted (1 : 20 DMSO : water) solutions of the precipitates (from PFOA + CAN and PFOS + CAN) were quite complicated (Fig. S18 and S19^†). It is reported that on the way to complexation with Ce(iv)/Ce(iii), PFAS can trigger radical induced pathways to undergo C–C and C–F bond cleavages.^25,26 In this regard, the formation of the polymeric compounds [e.g. (Ce)_m(PFOA)_n(NO₃)_x(NH₄)_y(H₂O)_z, where m, n, x, y and z can be any arbitrary numbers from 0, 1, 2 to n] cannot be completely ruled out. We were unable to identify any new species with reasonable precision from the HRMS experiments. The precise identification of these species needs further detailed investigation with various other spectroscopic techniques.In conclusion, CAN effectively precipitates out both PFOS and PFOA even at low concentration (<20 μM) and clearly reacts more efficiently with the former. A clear trend of CAN > Ce(SO₄)₂ > CeCl₃ > CoCl₂ ≥ FeCl₂ > NH₄NO₃ was observed for precipitating out PFOS and PFOA from the aqueous medium. These PFASs are reported among the most difficult ones to degrade.²⁷ The Ce^IV center plays an important part in this, as reflected by the comparative studies using CAN, Ce(SO₄)₂, CeCl₃ and AN. The oxidation state of the cerium center, as well as the size of the PFASs play crucial roles in the successful removal/precipitation from the aqueous medium. Thus, a similar simple strategy can potentially be used in future for the design of highly effective filter beds (e.g., using a combination of GAC/PAC and CAN) for purifying PFAS contaminated water at larger scale.