Redox-Responsive Copper(I) Metallogel: A Metal–Organic Hybrid Sorbent for Reductive Removal of Chromium(VI) from Aqueous Solution

Herein, we report a new strategy to remove toxic Cr­(VI) ion from aqueous solution using metal–organic hybrid gel as sorbent. The gel could be easily synthesized from the commercially available organic ligand 2-mercaptobenzimidazole (2-MBIm) and copper­(II) chloride in alcoholic medium. The synthesis involves one-electron reduction of Cu­(II) to Cu­(I) by 2-MBIm, and then gel formation is triggered through Cu­(I)–ligand coordination and extensive hydrogen-bonding interactions involving the “–NH” protons (of 2-MBIm ligand), solvent molecules, and chloride ions. The gel shows entangled network morphology. Different microanalytical techniques (FTIR, powder XRD, FESEM, TEM, rheology etc.) have been employed for complete characterizations of the gel sample. Both Cu­(I) (<i>in situ</i> formed) and Cl^– ions trigger the gel formation as demonstrated from systematic chemical analyses. The gel also exhibits its stimuli-responsive behavior toward different interfering chemical parameters (pH, selective metal ions and anions, selective complexing agents, etc.). Finally the gel shows its redox-responsive nature owing to the distinguished presence of Cu­(I) metal centers throughout its structural backbone. And this indeed helps in the effective removal of Cr­(VI) ions from aqueous solution. Reduction of Cr­(VI) to Cr­(III) ions and its subsequent sorption take place in the gel matrix. The reductive removal of Cr­(VI) has been quantitatively interpreted through a set of different kinetic measurements/models, and the removal capacity of the gel matrix has been observed to be ∼331 mg g^–1 at pH ∼ 2.7, which is admirably higher than the commonly used adsorbents. However, the capacity decreases with the increase in pH of the solution. The overall removal mechanism has been clearly demonstrated. Again, the gel could also be recycled. Thus, the low-cost and large-scale fabrication of the redox-active metallogel makes it an efficient matrix for the toxic ion removal and hence indicates the high promise of this new generation hybrid material for environmental pollution abatement

Co-assembled White-Light-Emitting Hydrogel of Melamine

A coassembled light-harvesting hydrogel of melamine (M), 6,7-dimethoxy-2,4­[1H, 3H]-quinazolinedione (Q) with riboflavin (R), is used to produce a white-light-emitting hydrogel (W-gel) by mixing with the dye rhodamine B (RhB) in a requisite proportion. Addition of R to the Q solution causes both static and dynamic quenching to the emission of Q as evident from the Stern–Volmer plot and the emission of R shows a gradual increase in intensity. On addition of RhB to an aqueous solution of R, fluorescence resonance energy transfer (FRET) occurs, showing an emission peak at 581 nm. In a solution of constant molar ratio of Q and R, addition of RhB causes a quenching of emission of R with no effect on the emission of Q, indicating that the energy transfer takes place only between R and RhB. In the MQR coassembled hydrogel containing RhB, the gel melting temperature is lower than those of MQ and MQR gel, but the storage modulus remains almost unaffected. The oscillatory stress experiment indicates a gradual decrease of critical stress values for breaking of MQ, MQR, and W-gels attributed to the coassembly. In contrast to the solution of Q and R, energy transfer occurs on addition of RhB to the MQ gel. By varying the RhB and R concentration in the 1:1 MQ gel white light emission is observed for the W-gel composition having molar ratio of M:Q:R:RhB = 100:100:0.5:0.02 with the Commission Internationale de L’eclairage (CIE) coordinates of 0.31 and 0.36 for the excitation at 360 nm. However, in the sol state, the CIE coordinates of the hybrid differ significantly from those of the white light

Improved Mechanical and Electronic Properties of Co-assembled Folic Acid Gel with Aniline and Polyaniline

Co-assembled folic acid (<b>F</b>) gel with aniline (<b>ANI</b>) (<b>ANI</b>:<b>F</b> = 1:2, w/w) is produced at 2% (w/v) concentration in water/DMSO (1:1, v/v) mixture. The gel is rigid and on polymerization of the gel pieces in aqueous ammonium persulfate solution co-assembled folic acid - polyaniline (<b>F-PANI</b>) gel is formed. Both the co-assembled <b>F-ANI</b> and <b>F-PANI</b> gels have fibrillar network morphology, the fiber diameter and its degree of branching increase significantly from those of <b>F</b> gel. WAXS pattern indicates co-assembled structure with the <b>F</b> fiber at the core and <b>ANI/PANI</b> at its outer surface and the co-assembly is occurring in both <b>F-ANI</b> and <b>F-PANI</b> systems through noncovalent interaction of H-bonding and π stacking processes between the components. FTIR and UV–vis spectra characterize the doped PANI formation and the MALDI mass spectrometry indicates the degree of polymerization of polyaniline in the range 24-653. The rheological experiments support the signature of gel formation in the co-assembled state and the storage (<i>G</i>′) and loss (<i>G</i>″) modulii increase in the order <b>F</b> gel< <b>F-ANI</b> gel < <b>F-PANI</b> gel, showing the highest increase in <i>G</i>′ ≈ 1100% for the <b>F-PANI</b> gel. The stress at break, elasticity, and stiffness also increase in the same order. The dc-conductivity of <b>F-ANI</b> and <b>F-PANI</b> xerogels is 2 and 7 orders higher than that of <b>F</b> xerogel. Besides, the current (<i>I</i>)–voltage (<i>V</i>) curves indicate that the <b>F</b>-xerogel is insulator, but <b>F-ANI</b> xerogel is semiconductor showing both electronic memory and rectification; on the other hand, the <b>F-PANI</b> xerogel exhibits a negative differential resistance (NDR) property with a NDR ratio of 3.0

Co-Assembled Conductive Hydrogel of <i>N</i>‑Fluorenylmethoxycarbonyl Phenylalanine with Polyaniline

A metastable coassembled hydrogel of <i>N</i>-Fluorenylmethoxycarbonyl (Fmoc) phenylalanine (FP) with aniline (FP–ANI), upon polymerization, produces a stable green-colored coassembled FP–polyaniline (FP–PANI) hydrogel. The coassembly is produced by supramolecular interactions between FP and ANI/PANI. WAXS spectra suggest that structures of FP powder, FP–ANI, and FP–PANI xerogels are different from each other. The FP–ANI gel exhibits a mixture of doughnut and fiber morphology, but the FP–PANI gel exhibits a nanotubular morphology. UV–vis spectroscopy suggests that the doped state of PANI and the fluorescence property of FP completely vanish in the FP–PANI gel. The storage and loss modulii (<i>G</i>′ and <i>G</i>″) of the FP–PANI gel are higher than those of the FP–ANI gel. The FP–ANI gel breaks at a lower oscillator stress (57 Pa) than the FP–PANI gel (93 Pa), which exhibits a good strain recovery demonstrating excellent viscoelastic properties. The FP–PANI gel also exhibits a dc conductivity (1.2 × 10^–2 S·cm^–1) that is seven orders higher than that of the FP–ANI gel because of the doped nature of PANI. The current–voltage (<i>I–V</i>) characteristic curve of FP–PANI xerogel resembles the behavior of a semiconductor–metal junction, and upon white light irradiation, it exhibits a reversible on–off cycle with a constant photocurrent value of 0.1 mA. The Nyquist plot obtained from impedance measurements of the FP–PANI xerogel is different from that obtained for the FP–ANI xerogel, and it exhibits almost a semicircle, indicating the existence of both resistive and capacitive features connected in parallel mode

Effect of Pretreatment Conditions on the Precise Nanoporosity of Graphene Oxide

Nanoscale pores in graphene oxide (GO) control various important functions. The nanoporosity of GO is sensitive to low-temperature heating. Therefore, it is important to carefully process GO and GO-based materials to achieve superior functions. Optimum pretreatment conditions, such as the pre-evacuation temperature and time, are important during gas adsorption in GO to obtain accurate pore structure information. This study demonstrated that the pre-evacuation temperature and time for gas adsorption in GO must be approximately 333–353 K and 4 h, respectively, to avoid the irreversible alteration of nanoporosity. In situ temperature-dependent Fourier-transform infrared spectra and thermogravimetric analysis–mass spectrometry suggested significant structural changes in GO above the pre-evacuation temperature (353 K) through the desorption of “physically adsorbed water” and decomposition of unstable surface functional groups. The nanoporosity of GO significantly changed above the aforementioned pre-evacuation temperature and time. Thus, standard pretreatment is indispensable for understanding the intrinsic interface properties of GO

Redox-Switchable Copper(I) Metallogel: A Metal–Organic Material for Selective and Naked-Eye Sensing of Picric Acid

Thiourea (TU), a commercially available laboratory chemical, has been discovered to introduce metallogelation when reacted with copper­(II) chloride in aqueous medium. The chemistry involves the reduction of Cu­(II) to Cu­(I) with concomitant oxidation of thiourea to dithiobisformamidinium dichloride. The gel formation is triggered through metal–ligand complexation, i.e., Cu­(I)-TU coordination and extensive hydrogen bonding interactions involving thiourea, the disulfide product, water, and chloride ions. Entangled network morphology of the gel selectively develops in water, maybe for its superior hydrogen-bonding ability, as accounted from Kamlet–Taft solvent parameters. Complete and systematic chemical analyses demonstrate the importance of both Cu­(I) and chloride ions as the key ingredients in the metal–organic coordination gel framework. The gel is highly fluorescent. Again, exclusive presence of Cu­(I) metal centers in the gel structure makes the gel redox-responsive and therefore it shows reversible gel–sol phase transition. However, the reversibility does not cause any morphological change in the gel phase. The gel practically exhibits its multiresponsive nature and therefore the influences of different probable interfering parameters (pH, selective metal ions and anions, selective complexing agents, etc.) have been studied mechanistically and the results might be promising for different applications. Finally, the gel material shows a highly selective visual response to a commonly used nitroexplosive, picric acid among a set of 19 congeners and the preferred selectivity has been mechanistically interpreted with density functional theory-based calculations

Integration of Poly(ethylene glycol) in <i>N</i>‑Fluorenylmethoxycarbonyl‑l‑tryptophan Hydrogel Influencing Mechanical, Thixotropic, and Release Properties

Polyethylene glycol (<b>PEG</b>) is incorporated to improve the mechanical properties of <i>N</i>-fluorenylmethoxycarbonyl-l-tryptophan (<b>FT</b>) hydrogel producing the hybrid (<b>FTP</b>) gels designated as <b>FTP1</b>, <b>FTP2.5</b>, etc. having <b>PEG</b> concentrations of 0.05 and 0.125% (w/v), respectively. Both the <b>FT</b> and <b>FTP1</b> gels exhibit fibrillar network morphology; the fibers of the <b>FTP1</b> gel are thinner than those of the <b>FT</b> gel. <b>FTP</b> gels exhibit a magnificent improvement in mechanical properties, and the storage and complex moduli increase with a maximum of ∼2800% for the <b>FTP2.5</b> gel. Creep recovery experiment exhibits a maximum strain recovery of 90% for the <b>FTP1</b> gel. The thixotropic property is observed for both <b>FT</b> and <b>FTP</b> gels and the rate of recovery increases with increase of <b>PEG</b> concentration; the latter acts as a molecular adhesive to the gel fibers bringing back the network structure easily. Gelation of <b>FT</b> causes a 5-fold increase of fluorescence intensity due to molecular aggregation, and with increase of <b>FT</b> concentration the ratio of fluorescence intensities at 470 and 395 nm increases. Exploiting the thixotropic property of <b>FT</b> and <b>FTP</b> hybrid gels, doxorubicin (DOX) is successfully encapsulated, and tunable release of DOX using appropriate amount of <b>PEG</b> in the gel matrix under physiological conditions is observed

Nanoengineering of a Supramolecular Gel by Copolymer Incorporation: Enhancement of Gelation Rate, Mechanical Property, Fluorescence, and Conductivity

In the quest to engineer the nanofibrillar morphology of folic acid (F) gel, poly­(4-vinylpyridine-<i>co</i>-styrene) (PVPS) is judiciously integrated as a polymeric additive because of its potential to form H-bonding and π-stacking with F. The hybrid gels are designated as F-PVPS<i>x</i> gels, where <i>x</i> denotes the amount of PVPS (mg) added in 2 mL of F gel (0.3%, w/v). The assistance of PVPS in the gelation of F is manifested from the drop in critical gelation concentration and increased fiber diameter and branching of F-PVPS<i>x</i> gels compared to that of F gel. PVPS induces a magnificent improvement of mechanical properties: a 500 times increase of storage modulus and ∼62 times increase of yield stress in the F-PVPS5 gel compared to the F gel. The complex modulus also increases with increasing PVPS concentration with a maximum in F-PVPS5 gel. Creep recovery experiments suggest PVPS induced elasticity in the otherwise viscous F gel. The fluorescence intensity of F-PVPS<i>x</i> gels at first increases with increasing PVPS concentration showing maxima at F-PVPS5 gel and then slowly decreases. Gelation is monitored by time-dependent fluorescence spectroscopy, and it is observed that F and F-PVPS<i>x</i> gels exhibit perfectly opposite trend; the former shows a sigmoidal decrease in fluorescence intensity during gelation, but the latter shows a sigmoidal increase. The gelation rate constants calculated from Avrami treatment on the time-dependent fluorescence data manifest that PVPS effectively enhances the gelation rate showing a maximum for F-PVPS5 gel. The hybrid gel exhibit 5 orders increase of dc conductivity than that of F-gel showing semiconducting nature in the current–voltage plot. The Nyquist plot in impedance spectra of F-PVPS5 xerogel exhibit a depressed semicircle with a spike at lower frequency region, and the equivalent circuit represents a complex combination of resistance–capacitance circuits attributed to the hybrid morphology of the gel fibers

A Comparative Account of the Kinetics of Light-Induced <i>E</i>–<i>Z</i> Isomerization of an Anthracene-Based Organogelator in Sol, Gel, Xerogel, and Powder States: Fiber to Crystal Transformation

The organogel of (<i>E</i>)-<i>N</i>′-(anthracene-10-ylmethylene)-3,4,5-tris­(dodecyloxy)­benzohydrazide (<b>I</b>) in methyl cyclohexane having a fibrillar network structure exhibits excellent fluorescence, which decreases sharply with time upon photoirradiation at λ = 365 nm. It has been attributed to the transformation of the <i>E</i> isomer of <b>I</b> to the <i>Z</i> isomer, and the kinetics of <i>E</i>–<i>Z</i> isomerization are compared for the sol, gel, xerogel, and powder states. The rate constants at different temperatures are measured from Avrami plots and its increase with an increase in temperature, indicating temperature acts as a promoter for photoirradiated <i>E</i>–<i>Z</i> isomeization along the imine (CN) bond. In the powder form, the rate constant values are the lowest compared to those of other states for all temperatures and the xerogels exhibit the highest rate of <i>E</i>–<i>Z</i> isomerization. The rate constants of sol and gel states mostly lie between the two. The wide-angle X-ray scattering pattern changes after ultraviolet (UV) irradiation with the generation of new sharp peaks whose intensities increase with an increase in irradiation time. A polarized optical microscopic study indicates formation of small crystalline dots on the fibers in the gels, dendritic morphology on the xerogel fibers, and large needlelike morphology at the surface boundary of the solid. The dried <b>I</b> gel exhibits a melting peak at 96.7 °C, but upon irradiation, two peaks are observed at 98.5 and 152.7 °C; the latter has been attributed to the melting of crystals of <i>Z</i> isomers. Similar higher melting peaks are observed both for the xerogel and for powders after UV irradiation; the powders exhibit the highest meting peak at 159.4 °C. Possible reasons for the variation of rate constant values in the four different states and the difference in morphology and melting points of crystals of <i>Z</i> isomers of <b>I</b> are discussed

Graphene Quantum Dots from a Facile Sono-Fenton Reaction and Its Hybrid with a Polythiophene Graft Copolymer toward Photovoltaic Application

A new and facile approach for synthesizing graphene quantum dots (GQDs) using sono-Fenton reaction in an aqueous dispersion of graphene oxide (GO) is reported. The transmission electron microscopy (TEM) micrographs of GQDs indicate its average diameter as ∼5.6 ± 1.4 nm having a lattice parameter of 0.24 nm. GQDs are used to fabricate composites (PG) with a water-soluble polymer, polythiophene-<i>g</i>-poly­[(diethylene glycol methyl ether methacrylate)-<i>co</i>-poly­(<i>N</i>,<i>N</i>-dimethylaminoethyl methacrylate)] [PT-<i>g</i>-P­(MeO₂MA-<i>co</i>-DMAEMA), P]. TEM micrographs indicate that both P and PG possess distinct core–shell morphology and the average particle size of P (0.16 ± 0.08 μm) increases in PG (0.95 ± 0.45 μm). Fourier transform infrared and X-ray photoelectron spectrometry spectra suggest an interaction between −OH and −COOH groups of GQDs and −NMe₂ groups of P. A decrease of the intensity ratio of Raman D and G bands (<i>I</i>_D/<i>I</i>_G) is noticed during GQD and PG formation. In contrast to GO, GQDs do not exhibit any absorption peak for its smaller-sized sp² domain, and in PG, the π–π* absorption of polythiophene (430 nm) of P disappears. The photoluminescence (PL) peak of GQD shifts from 450 to 580 nm upon a change in excitation from 270 to 540 nm. PL emission of PG at 537 nm is quenched, and it shifts toward lower wavelength (∼430 nm) with increasing aging time for energy transfer from P to GQDs followed by <i>up-converted</i> emission of GQDs. Both P and PG exhibit semiconducting behavior, and PG produces an almost reproducible photocurrent. Dye-sensitized solar cells (DSSCs) fabricated with an indium–titanium oxide/PG/graphite device using the N719 dye exhibit a short-circuit current (<i>J</i>_sc) of 4.36 mA/cm², an open-circuit voltage (<i>V</i>_oc) of 0.78 V, a fill factor of 0.52, and a power conversion efficiency (PCE, η) of 1.76%. Extending the use of GQDs to fabricate DSSCs with polypyrrole, both <i>V</i>_oc and <i>J</i>_sc increase with increasing GQD concentration, showing a maximum PCE of 2.09%. The PG composite exhibits better cell viability than the components