Detection and identification of single molecules in solution

We have extended our recent experiments in the detection of single fluorescent molecules in solution to the exploration of spectroscopy at the single-molecule level. As a first step we have developed a technique that can efficiently distinguish between two species of dye molecules on the basis of differences in their emission spectra. We have also demonstrated that another spectroscopic property, fluorescence lifetime, can be accurately determined at the single-molecule level. Spectroscopic properties can be used to identify fluorescent molecules and to reveal static or slowly varying aspects of the microenvironment of each molecule, thereby yielding information unavailable from bulk studies.