Voltammetric studies of indigo adsorbed on pre-treated carbon paste electrodes

The adsorptive behaviour of indigo on the surface of pre-treated carbon pas te electrodes (CPE) has been studied in various aqueous media at different pH, by cyclic (CV) and alternating current voltammetry (ACV). Cyclic voltam mograms of this molecule are characterised by two main electrodic processes in the potential range from -0.7 V to +0.9 V (vs. Ag/AgCl/sat. KCl), which are reversible in the pH range of 3-11, these being irreversible when both are recorded at pH 1. The first one at pH 1 becomes purely reversible when the applied potential range is from - 0.2 V to +0.2 V and constitutes the best analytical signal. Due to these electrodic features a significant incr ease in sensitivity was achieved when the signal was recorded by alternatin g current voltammetry. Indigo was quantified by CV and ACV, in 0.1 M HClO4 pH 1 and 0.1 M Tris-HCl pH 7.2 solutions. Limits of detection in the sub-na nomolar range were achieved for a 5-min accumulation time by ACV. These dat a provides useful information about the suitability of this electrodic proc ess as a detection scheme in the development of alkaline phosphatase (AP) b ased voltammetric affinity devices, in which indigo is generated by the enz ymatic hydrolysis of the 3-indoxyl phosphate substrate. (C) 2000 Elsevier S cience S.A. All rights reserved.