Fiber-optic two-photon fluorescence correlation spectroscopy for remote cell flow velocity measurements

We report the use of a sensitive double-clad fiber (DCF) probe for remote cell flow velocity measurement by the means of two-photon excited fluorescence correlation spectroscopy (FCS). The ability to measure the flow velocities of labeled cells in whole blood has been demonstrated. A flow velocity of as high as 40 cm/s has been measured. The theoretical lower limit is the self-diffusion of the cell, which is almost zero compared to typical body fluid flow. Owing to the unique feature of two-photon excitation, we can monitor multiple fluorescent markers simultaneously. Therefore, using high brightness nanoparticles to generate the reference signal, we can calibrate the real time flow velocity or even calculate the average size of the cells under measurement. The ability to conduct in vivo flow velocity measurement with single cell resolution using the fiber probe would provide a unique way for disease diagnosis or surveillance after treatment.