Ascorbic acid-triggered electrochemical-chemical-chemical redox cycling for design of enzyme-amplified electrochemical biosensors on self-assembled monolayer-covered gold electrodes

L-Ascorbic acid 2-phosphate (MP) is an optimal substrate for alkaline phosphatase (ALP) in electrochemical bioassays because of its low cost, good water solubility, less electrode passivation and high signal-to-background ratio. However, developing of electrochemical sensors with AAP as the enzyme substrate on self-assembled monolayer (SAM)-covered electrode is limited because the insulating SAM hinders the electron transfer between the electrode and ascorbic acid (AA, the enzymatic product of AAP). In this work, we first reported a strategy for developing AAP-based electrochemical biosensors on SAM-covered gold electrode. The method is based on AA-triggered "outer-sphere to inner-sphere" electrochemical chemical chemical (ECC) redox cycling with ferrocenecarboxylic acid (FcA) as the redox mediator. Specifically, AA produced from AAP facilitated the regeneration of FcA from its electrochemical-oxidation product (referred to as FcA(+) in the text), leading to an increase in the anodic current of FcA. Electrochemically inert tris(2-carboxyethyl)phosphine (TCEP) was used as a chemical reducing reagent to regenerate AA from its oxidation product, thus amplifying the electrochemical signal. The applications and performances of the proposed method were demonstrated in the competitive assays of beta-amyloid (A beta) peptides. The theoretical simplicity and high sensitivity indicated that our work would be valuable for developing simple and sensitive electrochemical biosensors. (C) 2014 Elsevier B.V. All rights reserved.