Luminescence lifetimes in biological systems

Abstract

Photo-luminescence (fluorescence, phosphorescence and delayed fluorescence) can be used as a probe of the molecular environment (polarity, micro-viscosity and mobility), and has very widespread uses in biological systems. Modern laser-based techniques for the measurement of fluorescence decay times in the nanosecond to picosecond time-domain, time-gated fluorescence spectra, and fluorescence anisotropy, are outlined, with recent results on several biological systems with both intrinsic and extrinsic probes used as examples. Anisotropy measurements on fluorescent probes in lipid bilayers and membranes are described and application of this technique to the study of the enzyme subtilisin in natural and modified form is discussed in detail. Particular emphasis is placed on studies of surfaces by using the relatively new technique of time-resolved evanescent wave-induced fluorescence spectroscopy, the example here being the study of surface-adsorbed bovine serum albumin, with use of rhodamine dyes as probes. Fluorescence imaging is widely used in medical and biological applications, and several recent developments in three-dimensional confocal techniques are described. The potential of time-resolved fluorescence imaging is explored, and some results are presented on the fluorescence decay of dye in living cells and on the time-gated imaging of these dyes in tissue.