Atanu Kuila

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.

Optoelectronic and photovoltaic properties of graphene quantum dot–polyaniline nanostructures

In aqueous dispersions of graphene quantum dots (GQDs) produced by a sono-Fenton method, aniline is in situ polymerized to produce different polyaniline (PANI)–GQD hybrids (PAGD) without using external dopant. FTIR studies indicate that the carboxylic acid groups of the GQDs dope PANI well. The UV-Vis spectra exhibit a π to polaron band transition of the PAGD hybrids and show a gradual red shift with increasing intensity for increasing amounts of GQDs due to the gradual uncoiling and increase of polarons in the doped PANI chains. The fluorescence intensity of the GQDs is drastically quenched in the PAGD hybrids suggesting effective charge transfer between the GQDs and PANI chains. The X-ray diffraction study suggests the presence of a lamellar structure with a lamellar thickness of 13.57 A. The morphologies of the PAGD hybrids studied using field emission scanning electron microscopy exhibit a change from flakes to rods with increasing GQD concentration, which has been attributed to the change from a flat to cylindrical lamella formation. The thermogravimetric analysis result indicates that, in comparison to HCl-doped PANI, the PAGD hybrids exhibit better thermal stability. In the PAGD composites the dc conductivity increases by three orders compared to that of the GQDs due to polaron formation in the PANI chains. The current–voltage (I–V) characteristics of the PAGD composites indicate semiconducting behaviour and on irradiation with light an almost reversible photoresponse occurs. Dye-sensitized solar cells (DSSCs) fabricated with the PAGD hybrids and N719 dye indicate a highest power conversion efficiency (PCE) of 3.12%. Impedance data of the PAGD hybrids exhibit semicircular Cole–Cole plots indicating the presence of a resistance (R)–capacitance (C) circuit where the capacitance is in parallel to the bulk resistance which increases with increasing GQD concentration. The Debye plot and the dielectric permittivity values also support the variation of the photovoltaic properties of the PAGD hybrids. The impedance spectra of the DSSCs indicate the presence of three semicircles exhibiting a complex equivalent circuit composed of three R–C circuits, and analysis of the data yields the lifetime values of photo-injected electrons supporting the PCE variation of the PAGD hybrids.

Amphiphilic poly(N-vinyl pyrrolidone) grafted graphene by reversible addition and fragmentation polymerization and the reinforcement of poly(vinyl acetate) films

The reversible addition and fragmentation (RAFT) polymerization of vinyl pyrrolidone (VP) from graphene oxide (GO) is used to produce GO-g-PVP (GP) and the grafting is confirmed from Fourier transformed infrared (FTIR) and nuclear magnetic resonance spectra. The average thickness of GP (8.2 nm) obtained from atomic force microscopy is higher than that of GO (1.2 nm), indicating the wrapping of grafted PVP on the GO sheets. Transmission electron microscopy of GP exhibits swollen domains (white spots) characterizing the grafted PVP chains from the GO surface. The dispersibility of the GP sheets becomes greatly improved over that of GO and they are dispersible in the solvents of Hansen solubility parameter (δp + δH) range 6.3–58. Three nanocomposites GP1, GP3 and GP5, produced by mixing with 1, 3 and 5 (w/w)% GP with poly(vinyl acetate) (PVAc), produce a stable dispersion in dimethyl formamide, although mixtures of GO and PVAc do not. The field emission scanning electron microscopy of the GP5 sample indicates a good homogeneous dispersion of GP sheets within the PVAc matrix, although both GO and PVP are individually immiscible with PVAc. The FTIR data indicates a specific interaction between GP and PVAc. The glass transition temperature (Tg) of the pure PVAc increases in the GP composites, but in the GO composite it remains unchanged. In the GPP5 hybrid containing the GO, PVP and PVAc mixture produced at the same composition as in GP5, an increase of Tg is seen to a lesser degree than that of GP, indicating that GO acts as a compatibilizer of a PVP and PVAc immiscible blend. The mechanical properties of PVAc exhibit a strong reinforcement and the Youngs modulus & tensile strength data show a 190% and 169% increase over PVAc in the GP5 sample due to the homogenous dispersion and unidirectional (parallel) orientation of GP sheets in the composite film.

On the pH sensitive optoelectronic properties of amphiphilic reduced graphene oxide via grafting of poly(dimethylaminoethyl methacrylate): a signature of p- and n-type doping

Poly(N,N′-dimethylaminoethyl methacraylate) (PDMAEMA) functionalized reduced graphene oxide (rGO) is synthesized by atom transfer radical polymerization, followed by attachment to rGO via diazonium coupling. The rGO-PDMAEMA (RGP) is characterized by 1H NMR, UV-Vis, FTIR and Raman spectroscopy. TEM and AFM studies demonstrate the formation of molecular brushes of PDMAEMA chains over rGO surface, and the TGA thermograms indicate 55 wt% grafting of PDMAEMA. RGP is dispersible in the widest spectrum of solvents from CCl4 to water [solubility parameter (δp + δh), 0.6 to 58]. RGP exhibits strongly pH dependent fluorescence properties: at pH 4, it exhibits two emission peaks, but at pH 7 and pH 9.2, a single and broad emission peak is observed. Two emission peaks at pH 4 are attributed to radiative decay of excitons to two kinds of holes in the rGO originating from illumination and p-type doping, which is also characterized by a blue shift of the Raman D band. Moreover, n-type doping of RGP at pH 9.2 is also evident due to a similar Raman shift. The dc-conductivity of RGP at pH 4 is 2 orders higher than that of pH 9.2 and the I–V characteristic curve at pH 7 exhibits a bimodal NDR property with a rectification ratio of 5.5. The bimodal NDR property is explained with a model using density of states and polaronic band.

Temperature triggered antifouling properties of poly(vinylidene fluoride) graft copolymers with tunable hydrophilicity

A water soluble polymer poly(diethylene glycol methyl ether methacrylate) (PMeO2MA) is grafted on a poly(vinylidene fluoride) (PVDF) backbone via coupled atom transfer radical coupling (ATRC) followed by atom transfer radical polymerization (ATRP). The PVDF-g-PMeO2MA copolymers are designated as PD-24, PD-16, etc. depending on the polymerization time and are characterized using 1H NMR spectroscopy, FTIR spectroscopy and gel permeation chromatography. AFM images indicate a change in morphology from spherulitic PVDF to the self-organized nano-sphere morphology with hairy PMeO2MA chains at the surface corona. The TGA data of PD graft copolymers indicate a two-stage degradation whose temperatures are higher compared to those of the components. The glass transition temperatures of the PD graft copolymers are higher than that of PMeO2MA, and both melting point & crystallinity decrease progressively with an increase of graft conversion. Dynamic light scattering (DLS) data indicate that PD graft copolymers possess a lower critical solution temperature (LCST) at ∼30 °C which can be tuned by changing the composition of the graft copolymer. The antifouling properties of the PD-24 film, produced specifically by water treatment at 15 °C (PD-24-15) and 37 °C (PD-24-37), are tested with bovine serum albumin (BSA) at below and above the LCST and a lower protein adsorption is noticed at 37 °C indicating a temperature triggered antifouling property of the PD graft co-polymers. The surface hydrophilicity of the graft copolymer, measured from the contact angle measurement, is higher in the PD graft co-polymer than that of PVDF and the contact angle decreases more significantly for the PD-24-15 film than that for the PD-24-37 film with time. The filtration of BSA solution using these two films monitored through fluorescence intensity indicates ∼60% protein absorption during filtration through the PD-24-15 film but PD-24-37 does not exhibit any change of fluorescence intensity, indicating superior antifouling properties.

Phase Behavior of Poly(vinylidene fluoride)-graft-poly(diethylene glycol methyl ether methacrylate) in Alcohol-Water System: Coexistence of LCST and UCST.

A thermoresponsive polymer poly(diethylene glycol methyl ether methacrylate) (PMeO2MA) is grafted from poly(vinylidene fluoride) (PVDF) backbone by using a combined ATRC and ATRP technique with a high conversion (69%) of the monomer to produce the graft copolymer (PD). It is highly soluble polymer and its solution property is studied by varying polarity in pure solvents (water, methanol, isopropanol) and also in mixed solvents (water-methanol and water-isopropanol) by measuring the hydrodynamic size (Z-average) of the particles by dynamic light scattering (DLS). The variation of Z-average size with temperature of the PD solution (0.2%, w/v) indicates a lower critical solution temperature (LCST)-type phase transition (T(PL)) in aqueous medium, an upper critical solution temperature (UCST)-type phase transition (T(PU)) in isopropanol medium, and no such phase transition for methanol solution. In the mixed solvent (water + isopropanol) at 0-20% (v/v) isopropanol the TPL increases, whereas the T(PU) decreases at 92-100% with isopropanol content. For the mixture 20-90% isopropanol, PD particles having larger sizes (400-750 nm) exhibit neither any break in Z-average size-temperature plot nor any cloudiness, indicating their dispersed swelled state in the medium. In the methanol + water mixture with methanol content of 0-30%, T(PL) increases, and at 40-60% both UCST- and LCST-type phase separations occur simultaneously, but at 70-90% methanol the swelled state of the particles (size 250-375 nm) is noticed. For 50 vol % methanol by varying polymer concentration (0.07-0.2% w/v) we have drawn a quasibinary phase diagram that indicates an approximate inverted hourglass phase diagram where a swelled state exists between two single phase boundary produced from LCST- and UCST-type phase transitions. An attempt is made to understand the phase separation process by temperature-dependent (1)H NMR spectroscopy along with transmission electron microscopy.

Deciphering the Effect of Polymer-Assisted Doping on the Optoelectronic Properties of Block Copolymer-Anchored Graphene Oxide

Doping facilitates the tuning of band gap, providing an opportunity to tailor the optoelectronic properties of graphene in a simple way, and polymer-assisted doping is a new route to combine the optoelectronic properties of graphene with the properties of a polymer. In this endeavor, a linear diblock copolymer, polycaprolactone-block-poly(dimethyl aminoethyl methacrylate) (PCL13-b-PDMAEMA117) (GPCLD) is grafted from the graphene oxide (GO) surface via consecutive ring opening and atom transfer radical polymerization. GPCLD is characterized using proton nuclear magnetic resonance (1H NMR), Fourier transform infrared spectroscopy, atomic force microscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, and Raman spectroscopy. The phase transition behavior of the GPCLD solution with varying temperature and pH is monitored using fluorescence spectroscopy and dynamic light scattering. Temperature-dependent 1H NMR spectra at pH 9.2 indicate the influence of temperature on the interaction between GPCLD and solvent (water) molecules causing the phase separation. Fluorescence spectra at pH 4 and 9.2 give the evidence of localized p- and n-type doping of graphene assisted by the pendent PDMAEMA chains. In the impedance spectra of GPCLD films, the Nyquist plots vary with pH; at pH 4, they exhibit a semicircle at higher frequencies and a spike at lower frequencies; at pH 7.0, the spike is replaced by an arc; and at pH 9.2, the semicircle at higher frequencies vanishes and only a spike is noticed, all of these suggesting different types of doping of graphene at different pH values. The dc-conductivity also varies with pH and temperature because of the different types of doping. The current (I)-voltage (V) property of GPCLD at different pH values is very unique: at pH 9.2, an interesting feature of negative differential resistance (NDR) is observed; at pH 7, the rectification property is observed; and at pH 4, again the NDR property is observed. The temperature-dependent I-V property at pH 7 and 9.2 clearly indicates a signature of doping, dedoping, and redoping because of the change in the interaction of GO with the grafted polymer arising from coiling and decoiling of polymer chains.

Supramolecular grafting of doped polyaniline leads to an unprecedented solubility enhancement, radical cation stabilization, and morphology transformation

We have demonstrated an entirely supramolecular approach for polymer grafting where polyaniline (PANI) is grafted with polyethylene glycol (PEG) coupled with β-cyclodextrin forming a pseudorotaxane with the aniline moiety. The resultant supramolecularly grafted PANI (βCD-PEG-PANI) in the doped state shows an extremely high solubility towards aqueous as well as organic solvents. Furthermore, the grafted PANI exhibits a higher degree of doping and a highly efficient radical cation stabilization, compared to a control PANI system synthesized under identical conditions. The redox switching behavior of PANI is also fully retained in βCD-PEG-PANI. An unusual disk-like morphology has been observed in βCD-PEG-PANI irrespective of the nature of the solvent. The pseudo-micellar assembly formation of βCD-PEG-PANI has been attributed to the high solubility, efficient radical cation stabilization, and unconventional morphological behavior of the grafted PANI. The present system represents a powerful integration of three key elements: first, fascinating electronic and physicochemical properties of parent PANI; second, extremely high aqueous solubility; and third, biocompatibility transcribed from PEG and βCD. While the first two characteristics are highly beneficial for technology generation, a combination of all three can open up a new era for biocommunication systems. The non-specific mode of interactions between the monomer (aniline in this case) and βCD-PEG implies that such a supramolecular strategy can be employed for a wide range of polymers including other conducting polymers.