Single-Molecule Detection Limits in Levitated Microdroplets

Laser-excited fluorescence from electrodynamically levitated microdroplets Is used to detect small numbers of rhodamlne-6G molecules. The small sample volume, typically a few plcollters reduces the background due to solvent and Impurity Raman and fluorescence emission. With 514.5-nm excitation from an argon Ion laser, as few as 12 molecules have been detected In glycerol-water droplets. Our present detection limit, due to variations In the Impurity concentration In the blanks, corresponds to a slgnal-to-nolse ratio of 3 for a single molecule of rhodamlne-6G contained In a 1-pL volume (dro plet diameter of ≃12 µm).