This paper presents a mathematical model of a biosensor based on a chemically modified electrode. The model is solved numerically and results of the simulations are validated with analytical solutions for a specific set of parameter values. The mathematical model of the biosensor comprises three layers: an enzyme layer, a dialysis membrane and an outer diffusion layer. The dialysis membrane is merged with the diffusion layer in the mathematical model by introducing an effective diffusion coefficient. The main purpose of this work was to determine at which parameter values the three-layer model can be replaced with the two-layer model with desired accuracy. Additionally, a possibility to use the maximal gradient of the biosensor current density instead of the steady-state current density is investigated. Numerical experiments showed that using the maximal gradient of the output current density can be very attractive to improve biosensor sensitivity.