A new method for measurement and quantification of tracer diffusion in nanoconfined liquids

We report development of a novel instrument to measure tracer diffusion in water under nanoscale confinement. A direct optical access to the confinement region, where water is confined between a tapered fiber and a flat substrate, is made possible by coating the probe with metal and opening a small aperture (0.1 μm-1 μm) at its end. A well-controlled cut using an ion beam ensures desired lateral confinement area as well as adequate illumination of the confinement gap. The probe is mounted on a tuning-fork based force sensor to control the separation between the probe and the substrate with nanometer precision. Fluctuations in fluorescence intensity due to diffusion of a dye molecule in water confined between the probe and the sample are recorded using a confocal arrangement with a single photon precision. A Monte Carlo method is developed to determine the diffusion coefficient from the measured autocorrelation of intensity fluctuations which accommodates the specific geometry of confinement and the illumination profile. The instrument allows for measurement of diffusion laws under confinement. We found that the diffusion of a tracer molecule is slowed down by more than 10 times for the probe-substrate separations of 5 nm and below.