title: High-efficiency molecular counting in solution:  single-molecule detection in electrodynamically focused microdroplet streams.
abstract: We report fluorescence detection of individual rhodamine 6G molecules using a linear quadrupole to focus streams of microdroplets through the waist of a counterpropagating cw Ar(+) laser. Since the terminal velocity scales as the square of the droplet diameter, the droplet-laser interaction time was "tunable" between 5 and 200 ms by using water samples spiked with a small, variable (2-5% v/v) amount of glycerol. Fluorescence bursts from droplets containing single molecules were clearly distinguished from the blanks in real time with an average signal-to-noise ratio of about 10, limited primarily by photobleaching and droplet size fluctuations (<1%). The volume throughput rates associated with this approach (∼10 pL/s) are roughly 10(3) higher than those associated with particle levitation techniques, with minimal sacrifice in sensitivity. Total molecular detection efficiencies of about 80% (at >99% confidence) were obtained for 100 and 15 fM rhodamine 6G solutions, in good agreement with detailed theoretical calculations and statistical limitations.
authors.affiliation: Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Mail Stop 6142, P.O. Box 2008, Oak Ridge, Tennessee 37831.
authors.forename: N
authors.initials: N
authors.lastname: Lermer
authors.forename: M D
authors.initials: MD
authors.lastname: Barnes
authors.forename: C Y
authors.initials: CY
authors.lastname: Kung
authors.forename: W B
authors.initials: WB
authors.lastname: Whitten
authors.forename: J M
authors.initials: JM
authors.lastname: Ramsey