Diffusion-Controlled Fluorescence Quenching in Micelles

Abstract

The overall decay of fluorescence from an ensemble of small micelles containing fluorescent probes and quenchers is adequately described by a wellknown model due to Infelta et al.(Infelta PP, Grätzel M, Thomas JK (1988) J Phys Chem 78:190). It is determined mainly by the statistics of distribution of the quenchers: in micelles containing quenchers the decay is rapid, whereas the decay from probes excited in micelles without quenchers is slow and equal to that of the unquenched excited state, if no migration occurs.

If the micelles are allowed to grow two important changes occur: the micelle size distribtuion usually becomes polydisperse, and the inherent decay in a micelle with quenchers becomes strongly nonexponential (for growth in one or two dimensions). The Infelta model is not valid under these conditions. For a slight uni-dimensional growth the polydispersity is most important for the characteristics of the observed decay; from measurements with varying quencher concentrations the width of the micelle size distribution can be estimated in addition to the average micele size. In very long micelles quenching will be observed only if the average distance from the excited species to a quencher is much shorter than the length of the micelle. There will be many quenchers in each micelle so that the statistics of distribution is unimportant, and the excited probe and quencher will not explore the full length of the micelle before deactivation; the quenching is then controlled by one-dimensional diffusion in an effectively infinite cylindrical micelle.