Detection and characterization of single molecules in aqueous solution

Using a confocal microscope with a single-photon avalanche photodiode as detector, we studied photon bursts of single Rhodamine 6G (R6G) and Rhodamin B-zwitterion (RE) molecules in aqueous solution by excitation of the lowest excited singlet state S-1 with a frequency-doubled titanium:sapphire laser. Multichannel scaler traces, the fluorescence autocorrelation function and fluorescence decay times determined by time-correlated single-photon counting have been measured simultaneously. The time-resolved fluorescence signals were analyzed with a maximum likelihood estimator. Fluorescence lifetime patterns in steps of 100 ps were generated by convolution with the excitation pulse. The lifetime of the S, state was derived from the Kullback-Leibler minimum discrimination information. We are able to demonstrate for the first time identification of two different single dye molecules via their characteristic fluorescence lifetimes of 1.79 +/- 0.33 ns (RE) and 3.79 +/- 0.38 ns (R6G) in aqueous solution.