Taylor dispersion measurements of monolayer protected clusters: A physicochemical determination of nanoparticle size

We describe the use of the Taylor dispersion method to determine the diffusion coefficients of monolayer protected clusters' (MPCs). The MPCs have nanometer Au cores (similar to 1-5 nm diameter) coated with a passivating, self-assembled monolayer of thiolate ligands (similar to 40-600). The results are found to agree with electrochemically measured diffusion coefficients, validating both approaches for measuring nanoparticle diffusivities. Taylor diffusion coefficients are used to calculate MPC hydrodynamic diameters under the sticking and the slip boundary conditions of the Stokes-Einstein equation, which are compared to the overall MPC diameter-estimated as the core diameter (measured by TEM) plus the length (2L) of two fully extended ligands. Better agreement,between calculated and experimental hydrodynamic diameters of alkanethiolate-coated MPCs in organic solvents are obtained using slip boundary conditions, implying free draining of solvent through the outer portion of the extended chains. The hard-sphere (sticking) version of the Stokes-Einstein relation predicts unrealistically small hydrodynamic diameters. Similar but less clear-cut results are obtained for tiopronin-coated MPC ions in aqueous media.