We have characterized a commercial confocal scanning head for the detection of single molecule fluorescence by two-photon excitation. We have verified that the distribution of the fluorescence emitted by dyes and labeled proteins on glass substrates is discrete with quanta proportional to a common reference signal. We describe and test a simple and quantitative tool to discriminate between single molecules and molecular aggregates on single snapshots based on the analysis of the intensity distribution. We have verified the square dependence of the fluorescence intensity vs. the excitation power, suggesting that no appreciable saturation and fast photo-damage of the chromophores takes place at the excitation power employed here.
Copyright 2001 Wiley-Liss, Inc.