Glucose oxidase entrapped in polypyrrole on high-surface-area Pt electrodes: a model platform for sensitive electroenzymatic biosensors

Enzyme entrapped in an electrosynthesized polymer film on a high-surface-area electrode is shown to be an attractive platform for the important class of amperometric biosensors based on oxidases and electrooxidation of the H2O2 generated by enzyme-catalyzed oxidation of analyte. Two Pt electrode surface morphologies, Pt black (Pt-BLK) and Pt nanowire brush (Pt-NW) were electrodeposited from chloroplatinic acid. Glucose oxidase (GOx) served as the model enzyme, which was entrapped in an electropolymerized polypyrrole (PPY) film on the high-surface-area Pt electrodes. The glucose sensitivity of the GOx electrodes can be controlled simply through Pt deposition conditions and varies roughly linearly with electrode microscopic roughness factor. A sensitivity of 103 μA mM−1 cm−2 was achieved with the Pt-BLK/PPY-immobilized GOx system at an underlying electrode surface roughness of 318, which is 150-fold higher than the sensitivity of a bare evaporated Pt/PPY-immobilized GOx electrode with a surface roughness of ∼2-3. The apparent Michaelis constant for PPY-immobilized GOx (∼30 mM) is similar to that for free enzyme (25 mM) suggesting that the overall electrode kinetics are not limited by glucose mass transfer. Scanning electron micrographs of enzyme electrode surfaces and a fluorescence assay to determine immobilized GOx concentration revealed that the PPY-GOx layer is deposited as a thin film that follows the contours of the electrode surface and that up to 3-fold greater enzyme is entrapped on the roughest electrodes examined. The orders of magnitude greater sensitivity of enzyme electrodes based on high-surface-area Pt appears likely to be due to increased H2O2 electrooxidation efficiency.