Ashok K. Sundramoorthy

Highly Selective Mercury Detection at Partially Oxidized Graphene/Poly(3,4-Ethylenedioxythiophene):Poly(Styrenesulfonate) Nanocomposite Film-Modified Electrode

Partially oxidized graphene flakes (po-Gr) were obtained from graphite electrode by an electrochemical exfoliation method. As-produced po-Gr flakes were dispersed in water with the assistance of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS). The po-Gr flakes and the po-Gr/PEDOT:PSS nanocomposite (po-Gr/PEDOT:PSS) were characterized by Raman spectroscopy, Fourier transform-infrared spectroscopy (FT-IR), UV-Vis spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). In addition, we demonstrated the potential use of po-Gr/PEDOT:PSS electrode in electrochemical detection of mercury ions (Hg2+) in water samples. The presence of po-Gr sheets in PEDOT:PSS film greatly enhanced the electrochemical response for Hg2+. Cyclic voltammetry measurements showed a well-defined Hg2+ redox peaks with a cathodic peak at 0.23 V, and an anodic peak at 0.42 V. Using differential pulse stripping voltammetry, detection of Hg2+ was achieved in the range of 0.2 to 14 µM (R2 = 0.991), with a limit of detection (LOD) of 0.19 µM for Hg2+. The electrode performed satisfactorily for sensitive and selective detection of Hg2+ in real samples, and the po-Gr/PEDOT:PSS film remains stable on the electrode surface for repeated use. Therefore, our method is potentially suitable for routine Hg2+ sensing in environmental water samples.

Low-temperature solution process for preparing flexible transparent carbon nanotube film for use in flexible supercapacitors

Single-walled carbon nanotubes (SWNTs) possess high conductivity, mechanical strength, transparency, and flexibility, and are thus suitable for use in flexible electronics, transparent electrodes, and energy-storage and energy-harvesting applications. However, to exploit these properties, SWNTs must be de-bundled in a surfactant solution to permit processing and use. We report a new method to prepare a SWNT-based transparent conducting film (TCF) using the diazo dye 3,3′-([1,1′-biphenyl]-4,4′-diyl)bis(4-amino naphthalene-1-sulfonic acid), commonly known as Congo red (CR), as a dispersant. Uniform 20-nm-thick TCFs were prepared on rigid glass and flexible polyethylene terephthalate (PET) substrates. The CR-SWNT dispersion and the CR-SWNT TCFs were characterized via UV-Vis-NIR, Raman spectroscopy, FT-IR spectroscopy, transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM) and dynamic light scattering (DLS) measurements. The sheet resistivity of the CRSWNT TCF was ~34 ± 6.6 Ω/□ with a transmittance of 81% at 550 nm, comparable to that of indium tin oxide-based films. Unlike SWNT dispersions prepared in common surfactants, such as sodium dodecyl sulfate (SDS), sodium cholate (SC), and Triton X-100, the CR-SWNT dispersion was amenable to forming TCF by drop coating. The CR-SWNT TCF was also very stable, maintaining a very low sheet resistivity even after 1,000 consecutive bending cycles of 8 mm bending radius. Further, manganese dioxide (MnO2) was electrochemically deposited on the CR-SWNT-PET film (MnO2-CR-SWNT-PET). The as-prepared MnO2-CR-SWNT-PET electrode exhibited high specific capacitance and bendability, demonstrating promise as a candidate electrode material for flexible supercapacitors.

Highly selective colorimetric and electrochemical sensing of iron (III) using Nile red functionalized graphene film.

We report a highly selective method for identification and detection of iron (III) (ferric iron, Fe3+) using Nile red (NR) as a complexing agent. Fe3+ preferentially binds with NR in dimethylformamide (DMF)/water (1:1) solution over other cations such as Fe2+, Cu2+, Pb2+, Hg2+, Mn2+, Ni2+, Zn2+, Co2+ and Cd2+ at room temperature. In the presence of Fe3+, the color of NR solution changes from purple to dark brown, which is detectable with bare eyes. Using UV-vis spectroscopy, we could measure the amount of Fe3+ in the sample solution by monitoring changes in absorption from 540 to 580nm; the linear range and the limit of detection are 30-1000µM and 24.9µM, respectively. Taking advantage of the NR selectivity, we treated partially oxidized graphene sheets (po-Gr) with NR to obtain po-Gr-NR dispersion by ultrasonication. The NR-treated po-Gr flakes (po-Gr-NR) were characterized by UV-vis, FT-IR, and Raman spectroscopies and FE-SEM, which indicated attachment of NR on po-Gr sheets. The po-Gr-NR hybrid film deposited glassy carbon electrode (po-Gr-NR/GCE) served as the Fe3+ sensor. Differential pulse voltammetry was used to investigate the detection of Fe3+ in 0.05M HCl+0.05M KCl solution. The linear range and the limit of detection of Fe3+ were from 37.5nM to 21.53µM and 18.7nM, respectively. Furthermore, this sensor was successfully used to measure Fe3+ content in red wine samples.

Azo dye functionalized graphene nanoplatelets for selective detection of bisphenol A and hydrogen peroxide

Graphene nanoplatelets (GNP) have emerged as promising electrode material for electrochemical sensing applications because of their high conductivity, large surface-to-volume ratio, biocompatibility, and low cost. However, GNP are not soluble in water. We dispersed GNP in water with the assistance of a tri-azo dye, Direct Blue 71 (DB). Thin films prepared with GNP/DB dispersions were characterized by scanning electron microscopy (SEM), Raman spectroscopy, and UV-vis spectroscopy, which indicated the binding of DB with GNP films. A GNP/DB film coated glassy carbon electrode (GCE) exhibited high electrocatalytic activity to oxidation of bisphenol A (BPA) and reduction of hydrogen peroxide (H2O2). The GNP/DB film greatly enhanced the BPA oxidation peak current at +524 mV, and the H2O2 reduction peak current at −400 mV vs. Ag/AgCl in pH 7.0 phosphate-buffered saline (PBS) solution. The oxidation peak current was proportional to the BPA concentration from 10 nM to 100 nM and 100 nM to 25 μM, with a limit of detection of 1.23 nM. The GNP/DB-modified GCE also showed a remarkable decrease of over-potential for the reduction of H2O2 with a fast amperometric response of less than 2 s, a good linear range of 10 μM to 1.9 mM, and a high sensitivity of 57.6 μA mM−1. The fabricated sensor shows good reproducibility and stability with limited interference. Furthermore, the sensor was successfully applied to determine BPA in spiked commercial milk and juice samples.

Simultaneous reduction and covalent grafting of polythiophene on graphene oxide sheets for excellent capacitance retention

Herein, we report room temperature reduction and covalent grafting of graphene oxide sheets by thiophene derivatives to produce pseudocapacitive electrodes, which are capable of delivering high capacitance (230 F g−1 at 1 mV s−1) and, most important, 100% cycling retention after 5000 cycles. Raman and FTIR spectroscopies confirmed the strong interaction of PTh with rGO in the PTh-g-rGO hybrids.