Effect of detergents on the fluorescence from CdS-Q clusters prepared using variable excess Cd2+ concentrations

The effect of surfactants Triton X-100 (TX-100, neutral), cetyltrimethylammonium chloride (CTAC, cationic) and sodium dodecyl sulfate (SDS, anionic) on the intensity and lifetime of several fluorescence bands of Q-CdS clusters, which were prepared with variable excess Cd2+ concentration, was studied. The fluorescence intensity of the several clusters of Q-CdS is strongly increased by adding a little excess of Cd2+. When more Cd2+ is added, a moderate increment of fluorescence intensity is observed and then, at higher excess Cd2+ concentrations, the fluorescence intensity remains nearly constant. The effect of the detergents on the fluorescence was found to depend on the excess of Cd2+ added to the cluster. The quenching of the fluorescence from Q-CdS by CTAC and SDS render linear Stern-Volmer plots. The corresponding values of the Stern-Volmer constants (K-sv) are lowered when the excess cadmium concentration is increased. For TX-100 the Stern-Volmer plots were non-linear for all the clusters with all the excess cadmium concentrations used. Two different zones, corresponding to different responses of the system in two concentration ranges are observed: the first one at TX-100 concentrations lower (zone I) and the second one at TX-100 concentrations higher (zone II), than 2 x 10(-4) M, which is equal to the concentration of the polyphosphate used to stabilize the colloid. According to the experimental results, we believe that TX-100 quenching, for all the CdS clusters' fluorescence bands in the range of final cadmium concentration used for the experiments, is a polyphosphate-assisted process in zone I, while in zone II this assistence is not evident. The fluorescence decay curves were also determined in all the cases. They were not altered, for any of the clusters, by the addition of TX-100 and SDS, pointing to static quenching processes in both cases. On the other side, the fluorescence lifetimes were shortened by the addition of CTAC, which is indicative that, at least in part, the quenching process is dynamic for this cationic detergent. (C) 2000 Elsevier Science S.A. All rights reserved.