Molecular sensors based on the photoelectric effect of bacteriorhodopsin: Origin of differential responsivity

Bacteriorhodopsin is a popular advanced material. When oriented in a membrane or a thin film deposited on a transparent electrode, it responds to sudden changes of the illumination level with spikes of fast photoelectric signal, of which the amplitude and polarity reflect the extent and sense of changes (a phenomenon known as differential responsivity). The present article explain how light-induced rapid charge separation and recombination in the membrane leads to differential responsivity. This article also evaluates several alternative proposed mechanisms appearing in the literature.