Selectivity of Gold Nanoparticle Adsorption in a Weak Polyelectrolyte Brush Controlled Through Grafting Density Changes

Understanding how the grafting density of weak polyelectrolyte brushes influences nanoparticle adsorption is key to optimizing their performance in selective separation and sensing technologies. Poly(2-vinyl pyridine) (P2VP) brushes provide a tunable environment in which surface compatibility and electrostatic interactions can be precisely controlled through grafting density and pH. Here, we investigated how variations in the grafting density of amine-end-functionalized P2VP brushes (P2VP-NH₂, M_n = 40.6 kg/mol) affect the adsorption kinetics and selectivity during immersion in mixed diameter (13- and 23-nm) solutions of spherical citrate-coated gold nanoparticles (AuNPs) at pH 4.0 and 6.2. AFM and ellipsometry confirmed the dry and in situ brush morphologies of P2VP-NH₂ brushes, which exhibited grafting densities ranging from 0.10 - 0.17 chains/nm². The brushes were immersed for seven days in mixed-diameter AuNP solutions at pH 4.0 and 6.2, followed by imaging with helium ion microscopy to quantify adsorption. At pH 4.0, 23 nm AuNPs exhibited optimal adsorption at intermediate grafting densities, while at pH 6.2, adsorption of 13 nm AuNPs (relative to 23 nm AuNPs) increased with grafting density. Across both conditions, higher grafting density generally enhanced nanoparticle uptake. These results demonstrate how grafting density and solution environment cooperatively dictate size-selective adsorption in weak polyelectrolyte brushes, providing design principles for responsive filtration and sensing interfaces.