Résumé
Nanocrystals are frequently synthesized in high-boiling organic solvents using strongly bound organic ligands to stabilize them. Sizing methods for nanoscale particles and biomolecules based on the Stokes–Einstein-Sutherland equation assume nonslip boundary conditions at the surface, which has proved to be difficult to confirm. Here we combine analytical Taylor Dispersion Analysis (TDA) and molecular dynamics simulations to address this issue. Our results show that the nonslip boundary condition holds down to a core diameter of at least 3 nm in organic solvents but that solvent penetration into the ligand shell becomes increasingly important. Taylor Dispersion Analysis provides precise and accurate measurements of the diffusion coefficient for particles ranging in radius from 1.4 up to 4.5 nm. We show that the oleic acid ligand shell has a constant effective thickness over this size range, which enables extraction of the inorganic core radius. This conclusion is supported by molecular simulations. The results further demonstrate that the Stokes–Einstein-Sutherland equation holds for spheres down to 1.4 nm in radius in Newtonian fluids. The molecular simulations predict that for even smaller sizes, the shell thickness will become dependent on core size. For particle sizing, TDA has advantages over dynamic light scattering for strongly absorbing particles or for ultrasmall nanoparticles (typically below 5 nm) which do not scatter light strongly.