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 excitatio n of the lowest excited singlet state S-1 with a frequency-doubled tit anium:sapphire laser. Multichannel scaler traces, the fluorescence aut ocorrelation function and fluorescence decay times determined by time- correlated single-photon counting have been measured simultaneously. T he time-resolved fluorescence signals were analyzed with a maximum lik elihood estimator. Fluorescence lifetime patterns in steps of 100 ps w ere generated by convolution with the excitation pulse. The lifetime o f the S, state was derived from the Kullback-Leibler minimum discrimin ation information. We are able to demonstrate for the first time ident ification of two different single dye molecules via their characterist ic fluorescence lifetimes of 1.79 +/- 0.33 ns (RE) and 3.79 +/- 0.38 n s (R6G) in aqueous solution.