PHOTON BURST DETECTION OF SINGLE NEAR-INFRARED FLUORESCENT MOLECULES

The ability to observe the burst of photons from single fluorescent molecules in solution with high efficiency (>90%) is a technically challenging task and has important applications in many areas of analytical chemistry. The ability to observe the photon burst from visible fluorescent dye molecules using pulse-laser excitation and time-gated detection in order to reduce the scattered photon contribution from the background has recently been demonstrated. The detection efficiency of these molecules was limited due to the excitation of fluorescent impurities resulting in small amplitude bursts observed in the solvent blank requiring the need for a high discriminator threshold to reduce the number of errors from false positives. We wish to report the first observation of the photon bursts arising from single near-infrared (near-IR) dye molecules transiting a focused Gaussian laser beam in solution with a high detection efficiency. Near-infrared excitation and detection were used to reduce the fluorescent impurity contribution to the background, which time-gating and spectral filtering cannot overcome in most cases. The single molecule detection apparatus uses a mode-locked Tl:sapphire laser as the excitation source and electronics configured in conventional time-correlated single photon counting configuration so that a time gate could be used to help minimize the scattered photon contribution to the background. The photodetector was a solid-state single photon avalanche diode (SPAD), which possesses a large quantum efficiency in the near-IR and a timing response suitable for time-gated detection of dyes with subnanosecond fluorescence lifetimes. The single molecule detection efficiency for the near-infrared dye IR-132 was found to be approximately 97%, a significant improvement when compared to the detection efficiency for the visible dye R6G even though fewer photons were detected per near-IR dye molecule. The average number of photons detected per molecule for the near-IR dye was determined to be approximately 18.