Nanomolar detection of p-nitrophenolvia in situ generation of p-aminophenol at nanostructured microelectrodes

Abstract

A new method and a new buffer medium, expected to be practical for miniaturized electrochemical immunoassays, were developed for rapid detection of nanomolar levels of p-nitrophenol (p-NP). The method exploits rapid reduction of p-NP to p-aminophenol (p-AP) by fast scan cyclic voltammetry at 500 V s⁻¹ at nanostructured carbon fiber microdisk electrodes (∼7 μm dia.) fabricated from polyacrylonitrile (PAN) fibers. Large surfaces of the nanostructured microdisks facilitate the rapid reduction of p-NP to p-aminophenol (p-AP), as confirmed by the overlap with the analytical signals of the standards, which is then rapidly detected by fast scan cyclic voltammetry. A new 2-amino-2-(hydroxymethyl)propane-1,3-diol (Tris)–HAc buffer medium was developed in order to allow adaptation of this detection strategy to alkaline phosphatase (ALP)-based immunoassays. Tris–HAc is a stable medium, while the traditional Tris–HCl buffer medium produces large residual faradaic currents attributed to chloride oxidation. Addition of sodium acetate to a Tris–HAc buffer medium allows sensitivity enhancement by a factor of 2 to 0.85 nA μM⁻¹, similar to the best sensitivity reported at the nanostructured PAN carbon fiber microdisk sensors.