Single molecule detection in solution

This work reports the first experiments demonstrating the high overall detec-tion efficiency of single chromophore molecules in solution. The measurements are accomplished using Iciser-induced fluorescence with time-gated photon counting to isolate solvent molecule signals from promptly scattered light. Compared to prior experiments, the system sensitivity is improved by over an order of magnitude with more efficient light collection and a custom-modified solid-state single photon de-tector, which is found to have > 50% quantum efficiency and subnanosecond time jitter. A custom time-gating circuit is implemented to avoid electronic pile-up and loss of signal due to the high count rate (> 3 x 10⁵ s^-1). The increased photon detection rate permits single molecules to be detected using flow velocities that are about 40 times faster than in the prior experiments. A novel sample delivery system causes almost all molecules to pass in a narrow stream through the center of the laser beam, whereas in prior experiments only a very small fraction of the molecules would pass through the beam and be detected. In the new system, sample molecules are injected into a high velocity sheath flow through a fine capillary placed directly above the laser beam to reduce diffusional spreading. For sulforhodamine 101 molecules in water, an overall molecule detection efficiency of about 80 % and a quantitative signal indicated by a well defined peak in the histogram of burst amplitudes are obtained. These results agree well with calculations and simulations performed under conditions used in the experiments.