Rama K. Layek

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.

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.

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.

Fluorescence Resonance Energy Transfer from Sulfonated Graphene to Riboflavin: A Simple Way to Detect Vitamin B2

We have prepared sulfonated graphene (SG) by diazonium coupling technique and it has been characterized by UV-vis absorption spectroscopy, Raman spectroscopy, electron microscopy, energy-dispersive spectroscopy (EDS), EDS elemental mapping, X-ray photoelectron spectroscopy (XPS), and FTIR spectroscopy. The photoluminescence (PL) property of SG at different pH (pH 4, 7, and 9.2) has been investigated and SG shows highest PL-intensity and quantum yield at pH 4 compared to those at higher pH and that of GO at pH 4. Due to the strong overlap between the emission spectrum of SG and absorption spectrum of riboflavin (RF, vitamin B2) at pH 4, it has been tactfully used as donor for the fluorescence resonance energy transfer (FRET) process. However, graphene oxide (GO) does not exhibit any FRET with RF at an identical condition due to its much lower quantum yield. We have demonstrated a selective detection of vitamin B2 in presence of nucleic acid (DNA, RNA), protein (BSA), amino acid (Lysine) and other water-soluble vitamins (Becosules, Zevit capsules) based on the spontaneous FRET from PL-active SG (donor) to RF (acceptor). The calibration curve indicates excellent affirmation to detect vitamin B2 using FRET and it is superior to the ordinary fluorescence method of detecting RF in presence of different biomolecules.

Polythiophene-g-poly(dimethylaminoethyl methacrylate) doped methyl cellulose hydrogel behaving like a polymeric AND logic gate

The variation of photoluminescence (PL) property of polythiophene-g-poly(dimethylaminoethyl methacrylate) (PT-g-PDMA, PD) with temperature and pH is used to develop a fully polymeric fluorescent AND logic gate type material using methyl cellulose (MC) hydrogel. The PL intensity gradually increases with increasing temperature of the PD doped aqueous MC solution and with increasing pH of the medium. In contrast, the PL-intensity of the PD solution decreases with increase in temperature for all of the pH values studied here due to collapsing of PDMA chains on the PT core, signifying that the PL intensity increases in the MC gel after compensating for the above decrease. The truth table suggests that it acts as an AND fluorescent molecular logic gate type system with fluorescence as output and temperature and pH as inputs. The maximum sensitivity of this logic gate is at higher pH (pH 9.2) than at neutral or acidic pH (pH 4) and at 45 °C. The reason is discussed from the viewpoint of the change in polarity at the microenvironment of the polythiophene chain in PD in the MC gel due to the change in temperature and pH.

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.

Nano-structured poly(3-hexyl thiophene) grafted on poly(vinylidene fluoride) via poly(glycidyl methacrylate)

Poly(vinylidene fluoride)-g-poly(glycidyl methacrylate)-g-poly(3-hexyl thiophene) (PGHT) co-polymer was synthesized using atom transfer radical polymerization (ATRP) of glycidyl methacrylate (GMA) on a poly(vinylidene fluoride) (PVDF) backbone in ethylene carbonate (EC), followed by the oxidative polymerization of 3-hexyl thiophene (3-HT) from the anchored thiophene unit in nitromethane. The poly(vinylidene fluoride)-g-poly(glycidyl methacrylate (PG) and PGHT graft co-polymers are characterized by 1H NMR, FTIR and GPC analysis. The PG graft co-polymer exhibits an open spherulitic morphology which further worsens with increasing polymerization time. In PGHT, P3HT exhibits nanosphere morphology of diameter 2.9–5.5 nm that decreases with increased PG polymerization time. The lamellar structure of PVDF deteriorates with the progress of PG polymerization, however, upon further grafting with P3HT the lamellar structure of PVDF reappears. In the PG co-polymers PVDF exists in the α-polymorph but in PGHT, it transforms into the piezoelectric β-polymorph. Both the PG and PGHT graft co-polymers exhibit high thermal stability. The PVDF melting point in the PG co-polymers has decreased by 12–19°. However, in PGHT the PVDF melting point remains the same and the P3HT melting point increases. In PGHT, the π–π* transition peak shows a small red shift emitting at 14–18 nm lower wavelength than that of pristine P3HT. The above spectral shift is attributed to the self organized structure of grafted P3HT chains in PGHT forming a nanosphere morphology. The dc conductivity of PGHT is lower than that of P3HT.