The glucose sensitivity and oxygen dependence of a variety of implantable biosensors based on glucose oxidase (GOx), incorporating an electrosynthesized poly-o-phenylenediamine (PPD) permselective barrier on 125-mu m diameter Pt disks (Pt-D) and cylinders (Pt-C, 1-mm length), were measured and compared. Full glucose calibrations and experimental monitoring of solution oxygen concentration allowed us to determine apparent Michaelis-Menten parameters for glucose and oxygen. In the linear region of glucose response, the most sensitive biosensor design studied was Pt-D/PPD/GOx (enzyme deposited over polymer) that was similar to 20 times more sensitive than the more widely used Pt-C/GOx/PPD (enzyme immobilized before polymer deposition) configuration. The oxygen dependence, quantified as K-M(O-2), of both active and less active designs was surprisingly similar, a finding that could be rationalized in terms of an increase in K-M(G) with increased enzyme loading. The Pt-D/PPD/GOx design will now enable us to explore glucose concentration dynamics in smaller and layered brain regions with good sensitivity and minimal interference from fluctuations in tissue pO(2). (c) 2005 Elsevier B.V. All rights reserved.