Two-Photon Fluorescence Correlation Spectroscopy of Gold Nanoparticles under Stationary and Flow Conditions

In this work we report two-photon fluorescence correlation spectroscopy experiments run on gold nanoparticles in aqueous solution. We compare the photophysical properties of nanoparticles obtained by the Turkevich method and by laser ablation as a function of increasing laser power. Fluorescence correlation spectroscopy curves for all nanoparticles show the rotational diffusion contribution at short lag times, even if the residual rotation contrast coefficient at high excitation laser power is lower for laser ablation nanoparticles with respect to Turkevich syntheses. Moreover, in contrast to Turkevich nanoparticles, laser ablation ones do not show an increase of the number of bright tracers in the excitation volume under higher excitation power. Nanoparticles from Turkevich synthesis are also tested as possible flow tracers inside a very simple microfluidic device featuring a single straight channel. Our findings confirm that gold nanoparticles can indeed be used as extremely sensitive probes to observe local flow inside micrometric size channels, for example, such as mammalian capillaries, since it can distinguish subtle changes in flow speed, as the ones imparted by a syringe pump.