Ultrasensitive laser-induced fluorescence detection in hydrodynamically focused flows

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Abstract
The detection sensitivity for fluorescent species in hydrodynamically focused flows has been significantly enhanced. Improved flow parameters, efficient detection optics, and photon counting are the keys to this enhancement. A detection limit of 800 Rhodamine 6G molecules flowing through our 0.6-pL probe volume during a 1-sec counting interval has been achieved. At the detection limit, the probability of a Rhodamine 6G molecule’s being in the probe volume is 0.06. Our current apparatus is capable of detecting a single species containing the equivalent of eight Rhodamine 6G fluorophores during the passage of this species through the focused laser beam. A number of instrumental improvements are being implemented that will further enhance the detection sensitivity projected to the limit of a single molecule.