This work presents a biosensor based on living cells immobilised on a light-addressable potentiometric sensor (LAPS). The set-up is sensitive to the concentrations of nutrients metabolised by bacteria, which results in an extracellular acidification. This is exemplarily demonstrated with glucose as nutrient and Escherichia coli as microorganism. The function of this biosensor is comparable to an enzyme-based field-effect sensor when using diffusion limitation. With the help of addressability of the LAPS an on-chip differential set-up was developed. Thus, external influences such as sensor drift, temperature and external pH changes can be minimised.