Time resolved fluorescence microscopy has been used to measure the radiative rate of molecules embedded in single spherical microcavities. The value of the measured rate is found to be a function of the diameter of the selected microcavity. The values of radiative rate enhancement (RRE) have been obtained for various cavity sizes and different concentrations. It is shown here for the first time that the concentration dependence of RRE can be used for sensitive detection of changes in the concentration of molecules in microscopic space. (c) 2005 Elsevier B.V. All rights reserved.