Ultrasensitive and high-throughput fluorescence analysis of droplet contents with orthogonal line confocal excitation.

This paper describes a simple modification to traditional confocal fluorescence detection that greatly improves signal-to-noise (s/n) for the high-speed analysis of droplet streams. Rather than using the conventional epi geometry, illumination of the droplet was in the form of a line that is orthogonal to both the direction of flow and the light-collection objective. In contrast to the epi geometry where we observed high levels of scattering background from the droplets, we detected more than 10-fold less background (depending on the laser power used) when orthogonal-line-confocal illumination was used. We characterized this improvement using a standard microfluidic platform over a range of analyte concentrations and observed an improvement in limits of detection of greater than 10. Using this method, we were able to analyze picomolar concentrations of analytes contained within picoliter-volume droplets at a rate of greater than 350 droplets per second.