Size-Dependent Electrostatic Adsorption of Polymer-Grafted Gold Nanoparticles on Polyelectrolyte Brushes

In this study, we demonstrate that by tuning the salt concentration a polyelectrolyte brush selectively adsorbs larger nanoparticles. Specifically, we successfully created a positively charged polyelectrolyte brush by quaternizing pyridine groups of poly(2-vinylpyridine) brushes using methyl iodide. Using X-ray photoelectron spectroscopy and X-ray reflectivity we characterize the charge fraction and out-of-plane structure of the polyelectrolyte brush. A quartz crystal microbalance with dissipation monitored the adsorption kinetics and thermodynamics of polyethylene glycol-grafted, negatively-charged gold nanoparticles (diameters of 12 and 20 nm) as a function of salt concentration. In a salt-free solution, the polyelectrolyte brush adsorbs gold nanoparticles at both nanoparticle sizes. As the salinity increases, the number of adsorbed nanoparticles monotonically decreases and eventually becomes negligible at high salinity. Interestingly, there is a range of salt concentrations where the decrease in nanoparticle adsorption is more pronounced for smaller particles, leading to size-selective adsorption of the 20-nm nanoparticles. As further proof of selectivity, when the polyelectrolyte brush is immersed in a mixture of large and small nanoparticles, the brush selectively captures the larger particles. In addition, we demonstrate that the size distribution of synthesized gold nanoparticles can be reduced by selectively removing larger particles.