Electroanalytical exploitation of quinone-thiol interactions: application to the selective determination of cysteine.

Square wave voltammetry was applied to the detection of cysteine through the use of an indirect assay that exploits the reaction of the thiol with a quinone indicator. Voltammetric discrimination between unreacted quinone and the corresponding quinone-cysteine adduct is possible with clear resolution of the latter peak providing a linear response from 5 to 47 microM. The selectivity of the approach was assessed with no interference from cystine, lysine, paracetamol or 4-aminophenol. The response recorded in the presence of a massive excess of ascorbic acid was also investigated and the integrity of the approach confirmed. The effects of other sulfhydryl thiols, homocysteine and glutathione, were also assessed and found to present no appreciable change in the voltammetric profile. The practical utility of the approach was investigated through examining the response to cysteine in urine.

Evaluation of phenolic assays for the detection of nitrite.

A series of generic nitrite assay systems based on the single step nitrosation of phenol derivatives are presented. The chemical reactivity offered by the C-nitroso compounds provides an opportunity to pursue a number of analytical strategies of which three spectroscopic (UV/Vis) and two electrochemical options (linear sweep voltammetry/differential pulse voltammetry) were evaluated. The capacity for multiple detection options from a single analyte species without significant sample manipulation is a major advantage with each assay system providing sub ppm detection limits with linear ranges up to milli-molar concentrations of nitrite. The influence of common interferents such as nitrate, ascorbate and paracetamol was investigated. The applicability of the assay procedures to the analysis of authentic biological samples (saliva and urine samples) was assessed with the analytical accuracy independently corroborated with a standard Griess protocol. In addition, a brief comparison with alternative nitrite detection strategies is also presented.

Amperometric enzyme-amplified immunoassays.

Enzyme-amplified immunoassays have been adapted for electrochemical measurement, using an NAD+/NADH redox cycle coupled to an electrode via the active site of diaphorase. Two amperometric methods are described, the first employs an organic conducting salt electrode, NMP+/TCNQ-; the second a platinum wire with ferricyanide as electron transfer mediator. In an immunoenzymometric assay for human prostatic acid phosphatase the sensitivities of the electrochemical methods were comparable to that achieved with the existing optical technique, but the dynamic range of the electrochemical assays was increased by at least two orders of magnitude. It is proposed that electrochemical enzyme-amplified immunoassays may eventually replace their optical counterparts.