pH and anion sensitive silver(I) coordinated melamine hydrogel with dye absorbing properties: metastability at low melamine concentration

Ag(I)-melamine coordination polymer produces a metastable hydrogel at 1:1 molar ratio but 1:2 molar ratio produces a stable gel which has the helical fibrillar morphology, the ability of selective dye absorption and is highly sensitive to pH and addition of anions

Bi-component hydrogel of perylene-3, 4, 9, 10-tetracarboxylic potassium salt and L-tyrosine

Potassium perylene-3,4,9,10-tetracarboxylate (P) and L-tyrosine (T) produces supramolecular PT complexes viz. PT11, PT12, PT13 and PT14 where the numbers indicate respective molar ratios. PT11 does not produce a hydrogel but the other three produce hydrogels with a minimum gelation concentration of 1.30% (w/v). All PT gels exhibit a fibrillar morphology and the gel melting temperature follows the order PT14 > PT13 > PT12. The frequency independent G' (G' > G'') for PT12, PT13 and PT14 systems characterize them as a gel and the critical yield stress values are 6.0, 3.18 and 0.76 Pa for PT14, PT13 and PT12, respectively. The presence of H-bonding and electrostatic interactions in PT complexes is evident from the FTIR spectra. NMR spectra suggest p-p stacking in both gels and in the PT11 complex, however, there is less p-stacking in the latter. The red shift of the absorption peaks of PT gels compared to that of P is attributed to the J-aggregate formation. The emission peaks also show red shifts and the emission intensity increases significantly. The average life time value decreases in the gels. With increasing temperature, PL-intensity gradually decreases due to the de-stacking of the J-aggregates but the emission peak position shows a step like increase due to batch-wise slipping of PT motifs from J-aggregates

Selective colorimetric sensing of mercury(II) using turn off-turn on mechanism from riboflavin stabilized silver nanoparticles in aqueous medium

A simple Hg<SUP>2+</SUP> sensor has been developed using the vitamin B2 (riboflavin) stabilized Ag nanoparticle via a "turn off"-"turn on" mechanism; both the colour and photoluminescence properties of the riboflavin solution are used as sensitizing tools showing a sensitivity up to 5 nM Hg<SUP>2+</SUP> concentration

Variation of physical and mechanical properties in the bicomponent hydrogels of melamine with positional isomers of hydroxybenzoic acid

The positional isomers of hydroxybenzoic acid have a significant influence on the physical and mechanical properties of their bicomponent hydrogels with melamine (M) produced at 1:1 molar composition. In the bicomponent hydrogels of salicylic acid (s), meta-hydroxy benzoic acid (m) and para-hdroxy benzoic acid (p), a gradation of morphology of the xerogels from tape to fibre occurs. WAXS study suggests different structures of the gels. FTIR spectra indicate that H-bonded complexes are produced in the gels. The aromatic protons in the gels become upfielded suggesting π-π stacking that follows the order Mp > Ms ≈ Mm. The UV-vis spectrum indicates better H-type aggregate formation in Mp than those in Ms and Mm systems. The photoluminescence (PL) intensity of the gels has increased by 200, 25 and 350 times those of pure acids for Ms, Mm and Mp gels, respectively. With the increase of temperature the PL intensity decreases and at 30 °C the highest PL intensity is observed at pH 6.8 but at pH 4 and 9.2 PL the intensity is drastically reduced due to breaking of the gel network. The melting point values suggest that the thermal stability of the gels follow the order Mp > Ms > Mm and both storage modulus (G') and critical strain also follow the same order. In G' vs. temperature plots, the Mp system exhibits a linear variation whereas the Ms and Mm systems exhibit peaks at 82 and 63 °C, respectively. This is attributed to the disaggregation of the gel macromolecule at high temperature forming a microgel and shear assisted aggregates

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

Mesoporous carbon cubes derived from fullerene crystals as a high rate performance electrode material for supercapacitors

We report the thermal conversion of three-dimensional (3D) mesoporous crystalline fullerene C 70 cubes (MCFC) into mesoporous carbon with retention of the original cubic morphology. This newly synthesized cube morphology mesoporous carbon shows excellent electrochemical capacitive properties. Cyclic voltammetry and chronopotentiometry (charge-discharge) measurements reveal that this nanostructured carbon material exhibits very high specific capacitance ca. 286 F g -1 at a scan rate of 5 mV s -1 and 205 F g -1 at a current density of 1 A g -1 . Furthermore, it exhibits high rate performance and excellent cyclic stability, no loss of specific capacitance is observed after 10,000 cycles. We believe that this novel mesoporous cubic carbon can be used as an excellent electrode material for advanced electrochemical supercapacitors

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

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