Potentiometric sensors require the implementation of conducting wires for signal transduction, but this is impractical for the readout of individual nanoparticles. It is here demonstrated for the first time that the potentiometric response of ion-selective nanospheres can be observed with voltage-sensitive dyes, thereby converting nanoscale electrochemical signals into an optical readout. No reference electrode is needed since the readout is by fluorescence. The results strongly support the potentiometric origin for the fluorescence response. The ion-selective nanospheres exhibit excellent selectivity and respond to ion concentration changes independent of sample pH, providing a new platform of potentiometric nanosensors with optical readout compatible with optical imaging equipment.