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

Abstract

We report development of a novel instrument to measure tracer diffusion in water under nano-scale 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 to 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 nm precision. Fluctuations in fluorescence intensity due to diffusion of a dye molecule in water confined between 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 measurement of diffusion laws under confinement. We found that the diffusion of a tracer molecule is slowed down by more than ten times for the probe-substrate separations of 5 nm and below.

Translational Diffusion of a Fluorescent Tracer Molecule in Nanoconfined Water

“…It is clear from our findings that the diffusion appears slowed down due to adsorption–desorption. Then, the apparent diffusion coefficient calculated from the slower residential time scale due to adsorption will be a few orders less even for our data . It is possible that the slow-down reported by Zhong et al and Kievsky et al is due to adsorption–desorption interactions.…”

“…All experiments are performed with a home-built instrument . A schematic of the instrument is shown in Figure .…”

“…The quantification of the diffusion coefficient may have systematic errors, and hence, its determination may not have significant confidence. We can also estimate diffusion coefficients from the experimental data using a Monte Carlo based fitting procedure …”

“…All experiments are performed with a home-built instrument. 37 A schematic of the instrument is shown in Figure 1. In order to create a small controlled gap in which water is nanoconfined, a sharp tip that is pulled out of single-mode optical fiber is used (Thorlabs 460HP, 125 μm diameter with cladding).…”

“…We can also estimate diffusion coefficients from the experimental data using a Monte Carlo based fitting procedure. 37 The schematic of the simulation box is given in As the values of concentration and fluorescence properties used in simulation and alignments in experiments can have slight variations from ideal expectations, the intercept of the simulated autocorretation may not match with the experimental autocorrelation. Thus, before fitting the simulated autocorrelation with the experimental one, the intercept is fitted.…”