Single-molecule detection with axial flow into a micrometer-sized capillary.

We characterize a new geometry for single-molecule detection with flow for use with a submilliliter drop of sample on an inverted confocal microscope. The solution is sucked into a glass capillary positioned above the ellipsoidal confocal volume so that molecules traverse the longest axis of the ellipsoid for greatest photon yield. Decreased spacing between the capillary tip and laser focus gives increased flow speed, as measured by fluorescence correlation spectroscopy, but also increased background from capillary autofluorescence. Flow can alleviate localized triplet and photobleaching effects and speed single-molecule sampling rates for fluorescence fluctuation spectroscopy determinations of slowly diffusing biomolecules in pharmaceutical drug discovery research.