Single-molecule analysis of ultradilute solutions with guided streams of 1-mu m water droplets

We describe instrumentation for real-time detection of single-molecule fluo rescence in guided streams of 1-mu m (nominal) water droplets. In this tech nique, target molecules were confined to droplets whose volumes were compar able with illumination volumes in diffraction-limited fluorescence microsco py and guided to the waist of a cw probe laser with an electrostatic potent ial. Concentration detection limits for Rhodamine 6G in water were determin ed to be similar to 1 fM, roughly 3 orders of magnitude lower than correspo nding limits determined recently with diffraction-limited microscopy techni ques for a chemical separation of similar dyes. In addition to its utility as a vehicle for probing single molecules, instrumentation for producing an d focusing stable streams of 1-2-mu m-diameter droplets may have other impo rtant analytical applications as well. (C) 1999 Optical Society of America.