Stimuli-responsive adsorption of poly(acrylic acid) onto silver nanoparticles: Role of polymer chain length and degree of ionization

The preparation of stimuli-responsive metal nanoparticles is a key challenge in developing smart multifunctional nanoscale devices. The efficient strategy relies on coating of nanoparticles with polymers or surfactants, which are capable of undergoing physicochemical changes in response to external stimuli. Herein, we report a computational strategy towards the development of an atomic-scale molecular dynamics (MD) model for pH-responsive metal nanoparticles comprising an inorganic silver core and a polymeric shell consisting of single-chain poly(acrylic acid) (PAA) of varying sizes. PAA is a weak polyelectrolyte, which contains ionizable carboxylic side-chain groups, capable of donating or accepting protons upon changes in pH, and responds to environmental changes by altering conformations of its macromolecule. We investigated the adsorption behavior of PAA onto silver nanoparticles (AgNPs) upon varying (i) a polymer chain length from 220 up to 1540 units, (ii) the degree of ionization a, ranging from 0 to 1. The latter is determined by the content of neutral and ionized carboxylic groups, mimicking of pH changes, ranging between the two extreme cases: fully neutral (alpha = 0) and fully ionized (alpha = 1) states, respectively. Our MD simulations show that, being fully neutral, PM collapses and adsorbs onto AgNP similar to traditional non-ionizable polymers. However, we found out that the interfacial behavior of PM could be tuned by changing of the degree of ionization of the carboxylic groups. We observed that electrostatic-driven variations in a shape and chain packing density of PM were able to trigger the degree of nanoparticle-polymer interactions. Our results show that PM-grafted AgNPs can provide a simple model for designing of novel pH-responsive composite nanomaterials. (C) 2018 Elsevier B.V. All rights reserved.