Adsorption and Denaturation of Structured Polymeric Nanoparticles at an Interface.

Nanoparticles (NPs) have been widely applied in fields as diverse as energy conversion, photovoltaics, environment remediation, and human health. However, the adsorption and trapping of NPs interfaces is still poorly understood, and few studies have characterized the kinetics quantitatively. In many applications, such as drug delivery, understanding NP interactions at an interface is essential to determine and control adsorption onto targeted areas. Therapeutic NPs are especially interesting because their structures involve somewhat hydrophilic surface coronas, to prevent protein adsorption, and much more hydrophobic core phases. We initiated this study after observing aggregates of nanoparticles in dispersions where there had been exposure of the dispersion to air interfaces. Here, we investigate the evolution of NP attachment and structural evolution at the air-liquid interface over time scales from 100 ms to 10s of seconds. We document three distinct stages in NP adsorption. In addition to an early stage of free diffusion and a later one with steric adsorption barriers, we find a hitherto unrealized region where the interfacial energy changes due to surface "denaturation" or restructuring of the NPs at the interface. We adopt a quantitative model to calculate the diffusion coefficient, adsorption rate and barrier, and extent of NP hydrophobic core exposure at different stages. Our results deepen the fundamental understanding of the adsorption of structured NPs at an interface.