Aggregation and aging of nanoparticle–protein complexes at interfaces studied by evanescent-light scattering microscopy

Plasma protein-induced aggregation of nanoparticles (NPs) is a crucial issue in many applications, such as drug delivery. Although great efforts have been made to investigate the protein adsorption kinetics or protein-induced NPs coalescence in bulk solutions, limited evidence has been uncovered for interfacial circumstances. Diet, disease, medicine, or senility could thoroughly change interfacial physicochemical properties of the inner lining of blood vessels. Implants including stents and artificial heart valves also have varied and evolutionary interfaces. Hence, there is an urgent need to understand the mechanism behind the non-specific protein adsorption and NP-protein aggregation in such interfacial cases. Here, we use evanescent light scattering to observe polystyrene NPs‒fibrinogen aggregation at substrates with varying surface properties. A density-fluctuation correlation function is utilized to reveal the relaxation dynamics of the aggregates. Both time-resolved and spatial-correlated evidence shows that the aging process of such soft materials is out-of-equilibrium, where the dynamics faster and slower than exponential can coexist in one single relaxation process. Besides, corona formation, inner stress, and interconnection together determine the microstructure, local adhesion, and structural relaxation of the aggregates, which can further correspond to the protein-to-NP ratio as well as the surface chemistry of NPs and substrates.