Arun K. Nandi

Highly Fluorescent Graphene Oxide-Poly(vinyl alcohol) Hybrid: An Effective Material for Specific Au3+ Ion Sensors

We have developed a new highly fluorescent graphene oxide (GO)/poly(vinyl alcohol) (PVA) hybrid (GO-PVA) in an acidic medium (pH 4). Fourier transform infrared (FTIR) spectra indicate the formation of hydrogen bonds between the hydroxy group of PVA and the hydroxy groups of GO. The hybrid is highly fluorescent, because of passivation by hydrogen bonding, as evident from Raman spectra. The quantum yields of GO-PVA hybrids are higher than that of GO. The fluorescent microscopic images of the hybrids exhibit a fibrillar morphology, and all of them emit highly intense green light. Field-emission scanning electron microscopy (FESEM) micrographs also show a fibrillar morphology, which is produced due to the supramolecular organization of GO-PVA complex. The highly fluorescent GO-PVA1 hybrid has been used as a fascinating tool for selective sensing of Au(3+) ions in aqueous media with a detectable limit of ~275 ppb. The sensitivity of the Au(3+) ion (300 μM) in the presence of 600 μM concentrations of each ion (Cu(2+), Ag(+), Mg(2+), Ca(2+), Zn(2+), K(+), Pb(2+), Co(2+), Ni(2+), Pd(2+), Fe(2+), Fe(3+), and Cr(3+)), taken together, is unique, exhibiting a quenching efficiency of 76%. The quenching efficiency in the presence of a biologically analogous mixture (d-glucose, d-lysine, BSA, Na(+), K(+), Ca(2+), Mg(2+), Zn(2+)) (600 μM each) is 73%, which suggests that the GO-PVA1 hybrid is an efficient sensor of Au(3+) ions. The average lifetime of GO at pH 4 increases in the GO-PVA1 hybrid, indicating the formation of a more stable excited state but the increase in lifetime value after addition of Au(3+) salt solution to the hybrid solution indicates dynamic quenching. The selectivity of sensing of Au(3+) is attributed to its reduction potential being higher than that of other metal ions and XPS data of GO-PVA1 hybrid with 300 μM Au(3+) substantiate the reduction of Au(3+) to Au(0), because of the transfer of excitons from the hybrid facilitating the selective photoluminescence (PL) quenching.

Supramolecular assembly of melamine and its derivatives: nanostructures to functional materials

The last twenty years have witnessed increasing research activity in the area of supramolecular chemistry of 1 : 1 co-assembly of melamine (M)–cyanuric acid (CA), since the historic discovery of the M·CA aggregate in crystal form and its structural analysis by Wang and his coworkers in 1990. Its useful chemical structure and fascinating H-bonding interaction sites distinguish M and its analogous derivatives as scaffolding components in the field of supramolecular chemistry to develop desired nano-to-micro scaled architecture. To date, M-based supramolecular assemblies are known in diverse forms which include fascinating nano- to micromorphological structures, molecular guest boxes, small molecular gels, membrane, sensor and liquid crystal development, polymeric scaffolds etc. In this review, we have covered the development of M and its derivatives, encompassing both nano/micro-ordered structures and advanced functional materials.

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-g-poly[(diethylene glycol methyl ether methacrylate)-co-poly(N,N-dimethylaminoethyl methacrylate)] [PT-g-P(MeO2MA-co-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 (ID/IG) is noticed during GQD and PG formation. In contrast to GO, GQDs do not exhibit any absorption peak for its smaller-sized sp(2) 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 up-converted 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 (Jsc) of 4.36 mA/cm(2), an open-circuit voltage (Voc) 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 Voc and Jsc increase with increasing GQD concentration, showing a maximum PCE of 2.09%. The PG composite exhibits better cell viability than the components.

Organogels from Different Self-Assembling New Dendritic Peptides: Morphology, Reheology, and Structural Investigations

Three new peptide based dendrimers with different generations were synthesized, purified, and characterized. Each of these dendrimers form efficient organogels under suitable conditions and these gels were characterized by field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), atomic force microscopy (AFM), Fourier transformed infrared (FT-IR) spectroscopy, differential scanning calorimetry (DSC) and rheology. It has been observed that gel forming propensity increases from first to second generation dendrimer and it decreases from second to third generation. The hydrogen bonding interaction is the main driving force for the formation of aggregated structure that leads to the formation of a fibrillar network, responsible for gelation. The morphology is network type consisting of taped or twisted fibrils spanning throughout the space. DSC measurements show the thermoreversible first-order phase transition. Rheological studies indicate that flow behavior and segmental motion of these gels are different for different peptidic gels, obtained from various generations of dendritic peptides.

Time sensitive, temperature and pH responsive photoluminescence behaviour of a melamine containing bicomponent hydrogel

Melamine (M) produces supramolecular complexes MQ31, MQ11 and MQ13 with 6,7-dimethoxy-2,4[1H,3H]-quinazolinedione (Q) (numbers indicate respective molar ratios of components) and they produce thermoreversible hydrogels [≥0.1% (w/v)] at 30 °C. Optical microscopy shows fibrillar network morphology and on irradiation with 300 nm light, the fibrils emit blue light. DSC results indicate a thermoreversible first order phase transition and the storage modulus (G′) is invariant with frequency supporting the formation of thermoreversible hydrogel. The magnitude of G′ follows the order MQ11 > MQ13 > MQ31 and a probable cause is attributed to number of H-bonds, which follows the same order in the nanofibers. FTIR-spectra suggest H-bonding interaction between components and WAXS results indicate a different crystalline structure of the complexes. The UV-vis spectra of the MQ systems shows red shift of both π–π* and n–π* bands and a maximum red shift is observed for the MQ13 system. The PL intensity decreases with increase in M concentration and lifetime data indicates PL quenching due to the formation of less fluorescent complexes. The PL property of the gel depends on aging time. A gradual development of a new emission peak at the expanse of an initial peak suggests the formation of an initial metastable state, which transforms into the stable state after 24 h of aging. The gel is stable in the pH range 6–9, above or below which the gel breaks down showing a significant PL-quenching and a blue shift of the emission peak. With increase of temperature, the emission peak intensity increases at first up to 45 °C, followed by an abrupt decrease at ≥55 °C. Possible reasons are attributed to the gradual disassembly of the complexes converting thinner fibers and finally melting.

Physical properties of poly(vinylidene fluoride) composites with polymer functionalized multiwalled carbon nanotubes using nitrene chemistry

Poly(methyl methacrylate) (PMMA) functionalized multi-walled carbon nanotubes (MWNT) (f-MWNT) are prepared using nitrene chemistry and atom transfer radical polymerization. The >CO groups in the f-MWNT interact with the >CF2 groups of poly(vinylidene fluoride) (PVDF) to achieve a compatible blend. An increase of the glass transition temperature (Tg), relaxation temperatures of the crystal-amorphous interface and crystalline phases are observed in the composites over pristine PVDF. The thermal stability of the composites increases with increasing f-MWNT concentration. The storage modulus increases significantly with increasing f-MWNT concentration and the highest increase of 120% over PVDF is observed for 1 wt% f-MWNT at 50 °C. An increase in tensile strength with a decrease of the strain at break, and increase of Youngs modulus and toughness indicate the formation of a very hard and ductile composite material. The electrical conductivity is high (1 × 10−4 S cm−1, for 5% f-MWNT) and shows a very low percolation threshold (0.36% w/w at 30 °C). Analysis of the conductivity data yields the magnitude of percolation exponent 2.1, suggesting three-dimensional percolation is a suitable model for the conduction of the composites. The temperature variation of the conductivity suggests that the conduction might occur through a temperature fluctuation induced tunnelling mechanism. The I–V characteristic curves indicate the semiconducting nature of the composites.

Enhanced fluorescent intensity of graphene oxide–methyl cellulose hybrid in acidic medium: Sensing of nitro-aromatics

Graphene oxide (GO) in acidic media (pH = 4) emits blue light but in neutral and alkaline media (pH = 7 and 9.2) the emission is negligible. On addition of 0.85, 1.7 and 3.4% (w/v) methyl cellulose (MC) to GO solution (0.005% w/v) the emission intensity increases dramatically at every pH but with an increase in pH the PL (photoluminescence) intensity decreases for every composition of the hybrid solution. The average lifetime of GO at pH = 4 increases on addition of MC. Fluorescent microscopic images of GO–MC hybrids for different MC content indicate that the morphology of the hybrids at pH 4 is ribbon type but at pH 7 and 9.2 no characteristic morphology is produced. The decrease of glass transition temperature by 9 °C of the GMC0.85 system (produced from drying GO-MC hybrid solution containing 0.85% MC solution) from that of pure MC suggests the presence of supramolecular interaction in the system. There is a drastic decrease in PL intensity on addition of nitroaromatics to the GMC0.85 system and it is very large (91%) for the addition of picric acid. Thus, the hybrid system acts as a good sensor for the detection of nitro aromatics by instantaneous photoluminescence quenching with a detectable limit of 2 ppm.

Bicomponent hydrogels of lumichrome and melamine: photoluminescence property and its dependency on pH and temperature.

Lumichrome (L) and melamine (M) produce thermoreversible hydrogels in LM31 and LM11 compositions, but LM13 composition does not produce hydrogel (the numbers indicate the respective molar ratio of the components). The formation of thermoreversible gels is confirmed from morphology, DSC, and rheological experiments where LM13 system does not meet the required characteristics of thermoreversible gels. FTIR spectra suggest that H-bonding between L and M produces the supramolecular complex, and (1)H NMR spectra suggest that pi-stacking of the complex produce fibrillar network structure entrapping a large amount of water producing the hydrogels. The nonplanar structure of LM13 complex probably causes difficulty in pi-stacking, prohibiting the gel formation. The UV-vis spectra show a blue shift of the pi-pi* transition band (354 nm) indicating H-aggregate formation but the pi-pi* band coupled with n-pi* transition (386 nm) shows a constant red shift by 7 nm, indicating independency of pi-stacking on the n-pi* transition in the different LM systems. The PL intensities of LM11 and LM31 gels become more quenched than the intensity of pure L due to formation of nonfluorescent complex (static quenching) in the gels. In the LM13 sol the degree of quenching is less than that of the gels because of absence of energy transfer through the junction points of gels. The increased lifetime values of LM gels compared to that of pure L is also indicative of H-aggregate formation. The PL intensity increases linearly with increase of temperature due to thinning of the fibers decreasing the exciton energy transfer. The emission peak shows a red shift with rise in temperature, indicating H- to J-aggregate transformation, and at the melting temperature it shows a sharp decrease. With both increase and decrease of pH from the neutral pH 7, the gels exhibit higher PL intensity because of sol formation.

Self-sustaining, fluorescent and semi-conducting co-assembled organogel of Fmoc protected phenylalanine with aromatic amines

N-Flourenylmethoxycarbonyl (Fmoc) phenylalanine (F) produces co-assembled organogel with 2-aminoanthracene (AA) and 2-aminonaphthalene (NA) at a 1 : 1 molar ratio of the components. The deep green co-assembled F-AA gel is rigid and can be cut into different shapes. At lower concentration, 0.2% (w/v), it shows a mixture of fibre and flake morphology, while at 1.5% (w/v) concentration only flake morphology is observed but the F-NA co-assembled gel produces tape morphology. The powder diffraction data of F-AA co-assembled gel indicate π–π stacking and lamellar packing which is supported by DFT calculation. The melting point of F gel is 15 °C higher over F-AA gel but the gel strength and stiffness of the F-AA co-assembled gel is 94 and 2.5 times higher than that of F gel. The F gel shows a smooth gel breaking point at 4 Pa but the F-AA co-assembled gel shows only a slippage at 160 Pa due to its high stiffness. The UV-vis spectra suggest the formation of H-aggregates and a charge transfer complex in the F-AA gel. The emission peak of AA shows a red shift in the F-AA co-assembled gel where both fluorescence intensity and peak position decrease with an increase in temperature. The F-AA xerogel shows semiconducting behaviour with a dc conductivity value 2.3 × 10−8 S cm−1 and the I–V characteristic curves indicate a semiconducting nature with a signature of negative differential resistance.

Improved mechanical and photophysical properties of chitosan incorporated folic acid gel possessing the characteristics of dye and metal ion absorption

The folic acid (F) gel, prepared in a 1:1 dimethyl sulfoxide (DMSO)–water mixture (v/v), at 0.2% (w/v) concentration has a nano-fibrous network morphology. FTIR spectra indicate the presence of intermolecular H-bonding interactions in the gel. The folic acid–chitosan (C) hybrid (FC) gel (prepared by mixing 0.1 ml of 1% (w/v) C solution with 2 ml 0.2% (w/v) F solution) appears as a yellow transparent semi-solid mass consisting of a nano-fibrous network with a higher density of branches. The fibrillar diameter of the FC hybrid gel (10.5 nm) is much lower than that of F gel (17.3 nm). The FTIR band at 3390 cm−1 of F gel shows a shift to 3435 cm−1 in the FC hybrid gel indicting a hydrogen bonding interaction between C and F. The fluorescence intensity of the FC hybrid gel is enhanced by 2.75 times than that of F gel. The shear viscosity of FC hybrid gel is 3 orders higher than that of the F gel and both gels exhibit shear thickening at a low shear rate (<10−3 s−1) but above that, shear-thinning occurs. In the creep phase, the FC hybrid gel exhibits strain recovery but the F gel does not. The complex modulus (G*) of both systems initially exhibit a sharp increase, followed by a slow increase with time (t) and dG*/dt varies with C concentration in the FC hybrid gel showing a maximum. The WAXS pattern of the FC hybrid xerogel does not exhibit any crystalline peak, suggesting that the additive (C) inhibits the crystallization of F. The dye absorption by the hybrid gel material is maximum for Eosin Yellow, (83.5%), however, Methyl Orange and Methylene Blue are absorbed to a lesser extent (∼40%). The FC hybrid gel absorbs 55% of Cu2+, 67.2% of Cr3+ and 49% of Co2+ from their respective solutions.