MATHEMATICAL-MODELING OF AN AMPEROMETRIC ENZYME ELECTRODE BASED ON A POROUS MATRIX OF STOBER GLASS-BEADS

The mathematical model of a glucose sensor based on the amperometric d etection of hydrogen peroxide using immobilized glucose oxidase (GOD) has been described. In this sensor GOD is immobilized on Stober glass beads that are attached to a platinum electrode. The influence of the bead radius (r(b), ranging 20, 45, 70, 100 and 200 nm) on the performa nce of the sensor has been analyzed. The total enzyme concentration de fined per unit interfacial area increases directly with the bead radiu s and the effective diffusivity of the substrate in the enzyme layer d ecreases with increasing bead radius. The model describes approximate analytical solutions for the behavior of the system, which is assumed to follow the Michaelis-Menten scheme of reaction. Two distributions o f the enzyme in the bead layer have been taken into consideration in t he discussion. Numerical solutions have also been presented to give a complete picture of the behavior of the system. Comparison of numerica l solutions and approximate analytical solutions suggests that the mod el is consistent in the regions of approximations. The model predicts different behavior of the system on either side of the critical radius (approximately 26 nm). The process is essentially diffusion controlle d for the sensors with beads of radius smaller than the critical radiu s and the current response of the sensors in this case increases with the increase in bead radius. The current response of the sensors with beads of radius greater than the critical radius decreases with the in crease in bead radius. The regime of operation (kinetic control or dif fusion control) for this case depends on the value of the Thiele modul us. (C) 1996 Elsevier Science Limited