Design variations of a polymer-enzyme composite biosensor for glucose: Enhanced analyte sensitivity without increased oxygen dependence

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-μm diameter Pt disks (PtD) and cylinders (PtC, 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 PtD/PPD/GOx (enzyme deposited over polymer) that was ∼20 times more sensitive than the more widely used PtC/GOx/PPD (enzyme immobilized before polymer deposition) configuration. The oxygen dependence, quantified as KM(O2), of both active and less active designs was surprisingly similar, a finding that could be rationalized in terms of an increase in KM(G) with increased enzyme loading. The PtD/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 pO2.