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Single-Molecule Detection in the Near-IR Using Continuous-Wave Diode Laser Excitation with an Avalanche Photon Detector

Volume 52, Number 1 (Jan. 1998) Page 1-6

Soper, Steven A.; Legendre, Benjamin L.

While single-molecule detection in flowing sample streams has been reported by a number of groups, the instrumentation can be somewhat prohibitive for many applications due to the complexity and extensive expertise required to operate such a device. In this paper we report on the construction of a single-molecule detection device that is rugged, compact, inexpensive, and easily operated by individuals not well trained in optics and laser operations. The singlemolecule detection apparatus consists of a semiconductor diode laser operating in a continuous-wave (CW) mode and a single photon avalanche diode transducer for converting the detected photons into transistor-transistor logic (TTL) pulses for displaying the data. In addition, the sampling volume is produced by a single-component lens, to create a volume on the order of 1 pL, allowing the sampling of microliter volumes of material on reasonable time scales. The device is targeted for operation in the near-IR region (700-1000 nm), where matrix interferences are minimal. Our data will demonstrate the detection of single molecules for the near-IR dyes IR132 and IR-125, in methanol solvents in flowing sample streams at sampling rates of 100-250 samples/s. Detection efficiencies for the investigated near-IR dyes were found to be 98% for IR-132 and 50% for IR-125. Previous attempts in our laboratory to detect single molecules of IR-125 using time-gated detection were unsuccessful because of the short upper-state lifetime of this fluorophore ( tau f = 472 ps).