Redox competition and generation-collection modes based scanning electrochemical microscopy for the evaluation of immobilised glucose oxidase-catalysed reactions

Abstract

Redox competition (RC-SECM) and generation-collection (GC-SECM) modes of scanning electrochemical microscopy were applied for the evaluation of a glucose oxidase (GOx)-modified non-conducting poly(methyl methacrylate) surface. The current vs. distance curves in RC-SECM mode were registered at −600 mV vs. Ag/AgCl to determinate local O₂ concentration, taking into account that the O₂ is consumed in the GOx-catalysed enzymatic reaction. This measurement was performed in phosphate–acetate buffer at pH 6.6 with 0–30 mmol L⁻¹ of glucose using a platinum ultramicroelectrode (UME) as the moving working electrode in a three-electrode electrochemical cell. The UME current, which is related to oxygen reduction rate, decreased when glucose was added to the solution. Another part of the investigation was performed in GC-SECM mode at +600 mV vs. Ag/AgCl in order to measure local H₂O₂ concentration, which is formed during the GOx-catalysed enzymatic reaction. The same SECM mode was used for imaging the GOx-catalysed reaction without any redox mediator. The imaging distance was chosen based on both the RC-SECM and GC-SECM experimental results. The RC-SECM and GC-SECM modes are described, and the processes that occurred on the UME- and GOx-modified surfaces are revealed.