Fluorimetric Analysis on Individual Nanoliter Sample Droplets

Abstract

Fluorescence from rhodamine 6G molecules in 225-μm-diameter (6-nL) droplets of ethanol is measured as they fall through a 514.5-nm Ar-ion laser beam, which is operated at a power density of 250 W/cm2. The fluorescence, measured at 555 nm, is imaged onto the entrance slit of a monochromator and measured with a photomultiplier tube. The droplets are produced by repeatedly plunging a 190-μm glass stylus into and out of a sample solution. The frequency of droplet production (150 Hz) corresponds to a flow rate of about 50 μL/min. The present detection limit of 3.7 fg, or 7.8 amol of rhodamine 6G, is limited by stray light and dark-current fluctuations.