Molecular insights into structure and solvation of temperature-responsive PNIPAM-grafted gold nanoparticle.

Abstract

Efficient design of thermoresponsive polymer-grafted metal nanoparticles necessitates a microscopic understanding of polymer conformational reorganization, interfacial interactions and the role of hydration in response to a change in temperature. This study focuses on investigating the structure and solvation of poly(N-isopropylacrylamide (PNIPAM)-grafted gold nanoparticles (AuNPs) with varying temperature, using molecular dynamics simulations. Our findings demonstrate that the temperature-dependent reduction in the effective size of the nanoparticles can be achieved when the polymer chains either collapse into "mushroom-like" conformations or when they are "bent" on the gold surface. These conformational preferences arise due to an interplay of polymer (de)hydration and polymer-Au interactions. The polymer collapse is accompanied by a decline in PNIPAM-water hydrogen bonds, increased buried surface area per chain and strengthening of PNIPAM-AuNP interaction energy, leading to thermally driven rearrangement of PNIPAM close to the AuNP surface. The mean residence times indicate that the weakly bound water molecules near the amide groups of PNIPAM lead to weakly activated desorption upon increasing the temperature. However, the strongly bound water molecules near the AuNP surface require more energy for desorption. The hydration dynamics transitions from a regime dominated by flexible, water-accessible short chains to the one characterized by compact, partially dehydrated long-chains with larger AuNP surface coverage. Overall, the results underscore the coupled effects of structural dehydration, energetic stabilization of polymer-surface contact, and accelerated interfacial water dynamics in determining the thermoresponsive behaviour of nanoparticles. The findings offer molecular design principles for designing thermoresponsive nanoparticles with tunable hydration and adsorption characteristics.