Mitochondria are central to the intrinsic apoptotic pathway of programmed cell death. Commitment to cell death occurs upon mitochondrial outer membrane permeabilization, and dysregulation of this vital apoptotic control point is implicated in various pathologies. Provided herein are novel sensors which can detect, in real time and with great sensitivity, the release of protons from intact mitochondria caused by mitochondrial outer membrane permeabilization. Embodiments include graphene based ion-sensitive field effect transistors.