Investigation of concentration profiles inside operating biocatalytic sensors with scanning electrochemical microscopy (SECM)

Scanning electrochemical microscopy (SECM) with amperometric or potentiometric measuring tips was used to investigate biocatalytic reactions inside the enzyme layer of a biosensor during its operation. The well known glucose oxidase catalyzed oxidation of glucose has been selected for the studies. Local, instantaneous concentration of dissolved oxygen and hydrogen peroxide was studied observing the amperometric current while miniaturized potentiometric tip served for local pH measurements. Liquid enzyme layer immobilized with Cellophane membrane or cross linked polyacrilamide gel membrane containing entrapped enzyme served for biocatalytic media in the SECM imaging. Local maximum of H(2)O(2) and minimum of O(2) profiles were found at approximately 200 microm far from the substrate/enzyme layer boundary. From the experimental findings guidelines to design well functioning biocatalytic sensors could be concluded. The concentration profiles obtained with SECM techniques were compared with the results of simple model calculations carried out with the method of finite changes. Most of earlier made SECM studies dealing with enzyme reactions imaged the electrolyte being in contact with the immobilized enzyme. The data in our investigation, however, were collected inside the working catalytic layer.