How low does the oxygen concentration go within a sandwich-type amperometric biosensor? Part 2: Theory for PPO biosensors

A mathematical model to describe the amperometric response of a sandwich-type biosensor containing polyphenol oxidase as the recognition catalytic element is proposed. The model was solved numerically and the resulting nonlinear solution was used to simulate chronoamperometric curves as well as to estimate the concentration profiles of reagents and products of the enzymatic reaction within the sensor. Fromm's systematic method was applied to get a kinetic expression for the analysis of polyphenol oxidase enzymes. The simulated data are compared with curves corresponding to a biosensor prepared with an oxidase enzyme that reacts according to a ping-pong mechanism. Although in both cases oxygen is used as mediator of the enzymatic reaction, the electrochemical step can generate it back from the H2O2 released by enzymes such as glucose oxidase but it cannot do the same in the case of working with polyphenol oxidase. Most of the calculated profiles and related data are presented using dimensioned variables so that they can be directly compared with experimental results. Relevant parameters such as limit current, response-time, and sensitivity are analyzed as function of the thickness of membranes, concentration of enzyme and concentration of substrate.