Rapid and efficient detection of single chromophore molecules in aqueous solution

The first experiments on the detection of single fluorescent molecules in a flowing stream of an aqueous solution with high total efficiency are reported. A capillary injection system for sample delivery causes all the dye molecules to pass in a diffusion-broadened stream within a fast-moving sheath flow, through the center of the tightly focused laser excitation beam. Single-molecule detection with a transit time of ~1 ms is accomplished with a high-quantum-efficiency single-photon avalanche diode and a low dead-time time-gating circuit for discrimination of Raman-scattered light from the solvent.