A fiber-optic-based instrument was developed to make simultaneous measurements of inter- and intracellular pH and membrane potential of vesicles containing bacteriorhodopsin in the presence of high-intensity actinic light. Fluorescent probes were incorporated into the membrane or intracellular space, and were excited at their characteristic wavelength. Most frequently, pyrinine and oxanol V or VI correlated independently with pH and ion-selective electrodes.
