Molecular Devices Based on Photoelectric Effects: Principles and Applications

Felix T. Hong

Department of Physiology, Wayne State University School of Medicine, Detroit, Michigan 48201, USA

e-mail: fhong@med.wayne.edu
 

The application of an external electric field Prototype molecular devices are often configured as layered molecular thin films on a transparent metal electrode or as bilayers exposed to an aqueous environment. This configuration mimics the visual and the photosynthetic membranes natural versions of nanobionic devices. The fundamental processes involved are light-induced vectorial charge movements (photovoltaic effect) and light-induced photoconductivity, which are macroscopically observable electric signals. We have developed a systematic methodology of analyzing photoelectric and photoconductive effects on the basis of theoretical and experimental analysis of thin films or bilayers that contain bacteriorhodopsin, a retinal protein pigment from Halobacterium salinarium. The approach is equally applicable to other types of films made from other kinds of photopigments. Thus, the approach, which combines electrochemical and electrophysiological techniques, is relevant to the construction of molecular devices based on either light-induced electron transfers or proton transfers.

Two types of photovoltaic effect are considered. The transient photosignal, generated by stimulation with a brief light pulse, is capacitative in nature (AC photoelectric effect). The steady state photosignal, generated by illumination with a continuous light, is resistive (conductive) in nature (DC photoelectric).

Since the AC photoelectric signal is highly sensitive to the chemical compositions of the adjacent aqueous solution, the device can be constructed in two configurations: a) a photon sensor modulated by the chemical environment, or b) a chemical sensor gated by light pulses (e.g., as ISFET devices). Devices developed in the laboratories of other investigators will be used to illustrate the underlying principle.

The DC photoelectric effect of bacteriorhodopsin is relevant for artificial solar energy conversion. However, bacteriorhodopsin is not a photodiode. Rather, it relies on a photon-gated proton-conduction channel to minimize charge recombination in the dark.

Literature

F. T. Hong, Biomolecular electronics. In: Handbook of Chemical and Biological Sensors, R. F. Taylor and J. S. Schultz (eds.), Institute of Physics Publishing Ltd, Bristol, 257-286, (1996).

F. T. Hong, Interfacial photochemistry of retinal proteins, Prog. Surf. Sci. 62 (1-6), 1-237 (1999).