We first report on the development of new microscope means that reduce background contributions in fluorescence fluctuation methods: i) excitation shutter, ii) electronic switches, and iii) early and late time-gating. The elements allow for measuring molecules at low analyte concentrations. We first found conditions of early and late time-gating with time-correlated single-photon counting that made the fluorescence signal as bright as possible compared with the fluctuations in the background count rate in a diffraction-limited optical set-up. We measured about a 140-fold increase in the amplitude of autocorrelated fluorescence fluctuations at the lowest analyte concentration of about 15 pM, which gave a signal-to-background advantage of more than two-orders of magnitude. The results of this original article pave the way for single-molecule detection in solution and in live cells without immobilization or hydrodynamic/electrokinetic focusing at longer observation times than are currently available.
Keywords: Single-molecule fluorescence fluctuation spectroscopy and imaging, solution, fluorescence correlation, FCS, scattered background, autofluorescence, pulsed excitation, modulated excitation, detector gating, time-correlated single-photon counting