Ultrasensitive laser-induced fluorescence detection in hydrodynamically focused flows

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.