Translational Diffusion of a Tracer Molecule in Nanoconfined Water

Abstract

Water in nanoscale slits and pores has many unexpected features like fast permeation in carbon nanotubes, permeation in helium-tight membranes, suppression of freezing, etc. The knowledge of nanoconfined water finds applications in water filtration, nano-fluidics, and studies of protein folding. There exist many studies which looked at the dynamics of confined water. Most of the spectroscopic measurements find the self-diffusion of water in nano-gaps similar to the bulk water. In contrast, the rheological measurements find the viscosity of surface-bound water to be many orders larger than its bulk values. It is not apparent how this large viscosity will manifest in the solute/tracer dynamics. Measuring tracer dynamics in the context of confined water have tremendous potential to bring clarity about the nature of nanoconfined water. However, such studies are lacking in the literature. This thesis tries to fill this gap. A new instrument is made to probe tracer diffusion in nanoconfined water. This instrument integrates ideas of Atomic Force Microscope(AFM), Near-field Scanning Optical Microscope(NSOM), and Fluorescence Correlation Spectroscopy(FCS). We could create a nanoscale pore between the tip and glass coverslip with AFM-like actuation technology. With NSOM-like tips, we could locally illuminate a small volume in the nanoconfined water beneath the tip. We could measure the diffusion time scales of the fluorescent molecule crossing an observation volume beneath the tip by autocorrelation of fluorescent intensities from a tracer dye confined between the tip-substrate gap. The necessary instrument automation and data acquisition protocol are developed in-house. Furthermore, we developed a Monte-Carlo method to analyze the data from the instrument to obtain the diffusion coefficient of tracer dye under nanoconfinement. We studied the diffusion of different fluorophores in nanoconfined water. Most of the studies are done for the fluorophore called Coumarin 343. Two-time scales are observed for the tracer molecule to cross the observational volume. From control experiments, we found the slow time scale is due to adsorption of fluorophore while the other time scale is due to diffusion. We found that even for confinements as low as 2nm, the translational diffusion of fluorophores is similar to the bulk diffusion. Measurements with another fluorescent molecule: Alexa Fluor 568, confirmed these conclusions. At the same time, we couldn't analyze the data from the molecule, Rhodamine 6G, as it is adsorbing strongly to the confining surfaces. This thesis work establishes a new instrument with a new analysis procedure. We used the instrument to study the translational diffusion of fluorescent tracer molecules in single nanopore-confined water.