Pressure-Dependent Diffusion of PEG-Functionalized Gold Nanoparticles Probed by X-ray Photon Correlation Spectroscopy.

Dynamic properties of nanoparticles under high pressure are poorly understood due to experimental challenges. Here, we use X-ray photon correlation spectroscopy (XPCS) to investigate the diffusion dynamics of polyethylene glycol (PEG)-functionalized gold nanoparticles dispersed in water at pressures up to 6 kbar. Complementary small-angle X-ray scattering reveals no measurable pressure-induced changes in the gold core size, confirming structural stability across the studied pressure range. XPCS measurements show a systematic slowing down of nanoparticle dynamics with increasing pressure, with relaxation rates retaining the expected q2-dependence characteristic of Brownian motion. The extracted diffusion coefficients decrease continuously with pressure and align with predictions based on the pressure-dependent viscosity of water and the Stokes-Einstein relation, assuming a constant hydrodynamic particle size. Within experimental uncertainty, no additional contribution from pressure-induced ligand-shell modifications is observed in these dilute dispersions.