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A chemically modified glassy carbon (GC) electrode was developed as an amperometric sensor for detection of biological thiols. The electrode was modified by inclusion of co-enzyme pyrroloquinoline quinone (PQQ) and a co-catalyst of oxidized single wall carbon nanotubes (Ox-SWNT) into a gold polypyrrole (Au-PPy) nanocomposite matrix. The electrode (PQQ/Ox-SWNT/ Au-PPy/GC) was characterized using scanning electron microscopy and cyclic voltammetry. Optimal conditions for the PQQ/Ox-SWNT/Au-PPy/GC electrode were determined and then utilized for the amperometric detection of L-cysteine, N-acetyl-L-cysteine, L-penicillamine and D, L-glutathione. The electrochemical response for each thiol in pH 3.2 citrate phosphate buffer at +450 mV (vs. Ag/AgCl) was found to be linear with limit of detections (LOD, S/N=3) ranging from 0.50 mM for L-penicillamine to 1.55 mM for D, L-glutathione with sensitivities of 30.2 nA/mM and 3.6 nA/mM respectively. The electrode design is simple and easy to construct using a minimum amount of co-enzyme and co-catalyst, resulting in detection methods with very good stability and improved sensitivity for thiol detection. [ABSTRACT FROM AUTHOR]

Abstract A nanocomposite of graphene oxide (GO), yttrium oxide (Y 2 O 3) nanoparticles, and Nafion was prepared and used to modify a glassy carbon electrode (GCE) by a simple drop casting technique. The modified electrode sensor was then used for the electrochemical detection of l -3,4-dihydroxyphenylalanine (l -DOPA). Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were used for the characterization of the modified electrode. Square wave voltammetry (SWV) and cyclic voltammetry (CV) were also employed for electrochemical detection of l -DOPA and characterization of the modified electrode. The effect of SWV parameters, ratio of GO:Y 2 O 3 in the composite, and the electrolyte solution pH on the oxidation peak currents were examined to optimize experimental conditions. The results of the optimization steps revealed the best experimental parameters based on the current response for oxidation of l -DOPA were 55 mV for pulse amplitude, 10 mV for step potential, 30 Hz for frequency, 2:1 for the GO:Y 2 O 3 ratio, and 3.5 for pH. Under these conditions, the GCE modified with GO-Y 2 O 3 showed a linear current response for oxidation of l -DOPA in the range of 0.5–350 μM with a limit of detection (LOD) of 0.05 μM, a limit of quantification (LOQ) of 0.17 μM, and a heterogeneous rate constant (k°) of 1.07 × 10−2 cm s−1. Increased sensitivity compared to a GO modified electrode was clearly observed. The electrode also exhibited selectivity towards l -DOPA in the presence of common interferences such as ascorbic and uric acid, and excellent stability and reproducibility over time. Finally, the modified electrode was utilized for the detection of l -DOPA in biological samples. [ABSTRACT FROM AUTHOR]