Microbead-based electrochemical immunoassay with interdigitated array electrodes

Abstract

The objective of this study was to develop a sensitive and miniaturized immunoassay by coupling a microbead-based immunoassay with an interdigitated array (IDA) electrode. An IDA electrode amplifies the signal by recycling an electrochemically redox-reversible molecule. The microfabricated platinum electrodes had 25 pairs of electrodes with 1.6-μm gaps and 2.4-μm widths. An enzyme-labeled sandwich immunoassay on paramagnetic microbeads with mouse IgG as the analyte and β-galactosidase as the enzyme label was used as the model system. β-Galactosidase converted p-aminophenyl β- d-galactopyranoside to p-aminophenol (PAP). This enzyme reaction was measured continuously by positioning the microbeads near the electrode surface with a magnet. Electrochemical recycling occurred with PAP oxidation to p-quinone imine (PQI) at +290 mV followed by PQI reduction to PAP at −300 mV vs Ag/AgCl. Dual-electrode detection amplified the signal fourfold compared to single-electrode detection, and the recycling efficiency reached 87%. A calibration curve of PAP concentration vs anodic current was linear between 10 −4 and 10 −6 M. A signal from 1000 beads in a 20-μL drop was detectable and the immunoassay was complete within 10 min with a detection limit of 3.5 × 10 −15 mol mouse IgG.