Biological photocathodes.

Biological surfaces emit electrons when subjected to UV light. This emission is increased greatly after exposure to cesium vapor. Increases from 2 to 3 orders of magnitude are observed, depending on the biochemicals present. Heme and chlorophyll exhibit unusually high photoemission currents, which are increased further after cesiation. Photoemission from proteins and lipids is much less but also is increased by exposure to cesium. The formation of photocathodes with cesium greatly increases the practical magnifications attainable in photoelectron microscopy of organic and biological specimens. Photoelectron micrographs taken at magnifications greater than or equal to X 100,000 of chlorophyll-rich thylakoid membranes and of colloidal gold-labeled cytoskeleton preparations of cultured epithelial cells demonstrate the improvement in magnification. The selectivity and stability of the photocathodes suggest the possibility of detecting chromophore binding proteins in membranes and the design of photoelectron labels for tagging specific sites on biological surfaces.