Two-channel sequential single-molecule measurement.

Experimental results are given which demonstrate the sequential detection of single molecules with a measurement efficiency of near unity. IR140 dye molecules are detected in sequential probe volumes within a flowing stream through a 9 μm i.d. capillary. The measurement of single molecules was confirmed by means of autocorrelation, photobleaching, visual observation, and cross-correlation analysis. The number of single molecules photobleached prior to being measured in the second probe region was in excellent agreement with the bulk studies described by a photodestruction curve. A cross-correlation peak with a temporal delay corresponding to the interprobe volume transit time and a width in agreement with parabolic flow give a clear indication of sequential detection. The near unity measurement efficiencies for both channels indicates the great potential for rapid quantitative analysis of dilute solutions.