Abstract: The invention relates to a biosensor comprising living cells that express a chemosensor, or receptor, on their surface. When grown on a microarray comprising electrodes, the cells can be induced, by binding of a ligand to the receptor, to secrete a molecule. This secretion event is detected with millisecond temporal resolution via electrochemical oxidation of the secreted molecule on the electrode which is voltage-clamped slightly above its redox potential. The current so generated is indicative of the amount of the ligand bound to the receptor.