Characterizing the Brownian Diffusion of Nanocolloids and Molecular Solutions: Diffusion-Ordered NMR Spectroscopy vs Dynamic Light Scattering.

Hydrodynamic size is a characteristic dimension that reflects the Brownian diffusion of objects, such as proteins, macromolecules, and various colloids when dissolved/dispersed in fluid phases. This property is crucial when investigating the utility of colloidal nanocrystals and polymeric materials in biology. Dynamic light scattering (DLS) has been widely used to measure the diffusion coefficient and hydrodynamic size of such systems. Comparatively, diffusion-ordered NMR spectroscopy (DOSY-NMR) is a relatively new analytical method that has provided researchers with an alternative experimental approach to access such information. Here, we apply DLS and DOSY-NMR simultaneously to characterize the diffusion coefficient and hydrodynamic size of several sets of nanocolloids, including dispersions of gold nanoparticles and luminescent quantum dots that are surface-capped with either hydrophobic or hydrophilic coatings, as well as a monomer and a low-molecular-weight polymer. We compare, side by side, the findings acquired from each measurement, which has allowed us to identify the benefits and constraints of each technique. Our results show that the two approaches provide comparable data when larger size nanocolloids are probed. However, we find that DOSY is substantially more effective in characterizing nanocolloids that are fluorescent and/or have very small dimensions, as well as molecular-scale organic ligands, where DLS reaches its limit. Additionally, we find that, compared to DLS, DOSY tends to require higher solute concentrations and longer collection time to generate data with high signal-to-noise ratios.