Size-Dependent Electrostatic\nAdsorption of Polymer-Grafted\nGold Nanoparticles on Polyelectrolyte Brushes

Designing a functional surface that selectively adsorbs\nnanoparticles\nbased on their size and shape is essential for developing an advanced\nadsorption-based, postsynthesis nanoparticle separation device. We\ndemonstrate selective adsorption of larger nanoparticles from solution\nonto a polyelectrolyte brush by tuning the salt concentration. Specifically,\na positively charged polyelectrolyte brush is created by converting\npyridine groups of poly(2-vinylpyridine) to <i>n</i>-methylpyridinium\ngroups using methyl iodide. The adsorption kinetics and thermodynamics\nof poly(ethylene glycol)-grafted, negatively charged gold nanoparticles\n(diameters of 12 and 20 nm) were monitored as a function of salt concentration.\nIn a salt-free solution, the polyelectrolyte brush adsorbs gold nanoparticles\nof both sizes. As the salinity increases, the areal number density\nof adsorbed nanoparticles monotonically decreases and becomes negligible\nat high salinity. Interestingly, there is an intermediate range of\nsalt concentrations (i.e., 15–20 mM of NaCl) where the decrease\nin nanoparticle adsorption is more pronounced for smaller particles,\nleading to size-selective adsorption of the larger nanoparticles.\nAs a further demonstration of selectivity, the polyelectrolyte brush\nis immersed in a binary mixture of 12 and 20 nm nanoparticles and\nfound to selectively capture larger particles with ∼90% selectivity.\nIn addition, the size distribution of as-synthesized gold nanoparticles,\nwith an average diameter of 12 nm, was reduced by selectively removing\nlarger particles by exposing the solution to polyelectrolyte brush\nsurfaces. This study demonstrates the potential of a polyelectrolyte\nbrush separation device to remove larger nanoparticles by controlling\nelectrostatic interactions between polymer brushes and particles.