Simple and sensitive electrochemical detection of aminopeptidase N using monolayer-modified Au interdigitated array electrodes with moderate electrocatalytic activities

Abstract

Electrochemical–electrochemical (EE) redox cycling using interdigitated array (IDA) electrodes is a simple and efficient approach for signal amplification. However, IDA electrodes with high or low electrocatalytic activities are inappropriate for obtaining high signal-to-background ratios in serum containing various interfering redox-active species. Accordingly, herein, Au IDA electrodes are modified with a monolayer of 6-mercapto-1-hexanol (MCH) to achieve moderate electrocatalytic activities, and the modified electrodes are applied to the simple and sensitive electrochemical detection of aminopeptidase N (APN) in human serum. APN liberates redox-active species (namely, 4-amino-1-naphthol, AN) via proteolytic cleavage of AN-conjugated substrates, and the released AN participates in the EE redox cycling. Among facilely synthesized five possible AN-conjugated substrates, AN-conjugated alanine is most rapidly cleaved by APN. MCH modification significantly decreases capacitive currents and the occurrences of unwanted faradaic reactions. The detection limit for APN in a 10-fold diluted human serum in the case of EE redox cycling without a washing step (3.8 ng/mL) is 40 times lower than that obtained without EE redox cycling (150 ng/mL). The proposed detection method exhibits high selectivity toward APN regardless of the presence of other proteases. This simple and selective method is practically promising for APN analysis in human serum.