A microfluidic sensor device includes a substrate having patterned thereon at least one Ag/AgCl electrode (working electrode) and an inner chamber overlying the at least one Ag/AgCl electrode. The device includes an ion selective permeable membrane permeable to TPP+ disposed on one side of the first chamber and a sensing chamber overlying the ion selective permeable membrane. A separate reference electrode is inserted into the sensing chamber. The working electrode and reference electrode are coupled to a voltmeter to measure voltage. This voltage can then be translated into a TPP+ concentration which is used to determine the mitochondrial membrane potential (ΔΨm).