Direct electrochemical oxidation of disulfides at anodically pretreated boron-doped diamond electrodes

Anodically oxidized diamond electrodes have been used to oxidize disulfides, thiols, and methionine in aqueous acidic media and tested for amperometric detection of these compounds after chromatographic separation. Cyclic voltammetric signals for 1 mM glutathione disulfide (GSSG) were observed at 1.39 and 1.84 V vs SCE, the values being less positive than those of its as-deposited counterpart as well as glassy carbon electrode. The voltammetric and chronocoulometric results have indicated the high stability of the electrode with negligible adsorption. A positive shift in the peak potential with increasing pH indicated the attractive electrostatic interaction between the anodically oxidized diamond surface and the positively charged GSSG in acidic media that promoted its analytical performance. The results of the electrolysis experiments of disulfides and thiols showed that the oxidation reaction mechanism of glutathione (GSH) and GSSG involves oxygen transfer. Following separation by liquid chromatography (LC), the determination of both GSH and GSSG in rat whole blood was achieved at a constant potential (1.50 V vs Ag/AgCl), and the limits of detection for GSH and GSSG were found to be 1.4 nM (0.028 pmol) and 1.9 nM (0.037 pmol) with a linear calibration range up to 0.25 mM. These detection limits were much lower than those reported for the amperometry using Bi-PbO2 electrodes and LC-mass spectrometry, and the LC method using diamond electrodes were comparable with enzymatic assay in real sample analysis. The high response stability and reproducibility together with the possibility of regeneration of the electrode surface by on-line anodic treatment at 3 V for 30 min further support the applicability of anodically pretreated diamond for amperometric detection of disulfides.