pH-Mediated Size-Selective Adsorption of Gold Nanoparticles to Polymer Brushes

The design of smart functional surfaces that interact with nanoparticles is essential to precisely control adsorption, desorption, and diffusion dynamics during the nanoparticle transport in a variety of nanoscale applications. Here, we demonstrate that a weak polyelectrolyte brush can be used as a functional surface to selectively adsorb nanoparticles according to their size by adjusting the pH of the buffer solution. We first developed an effective brush preparation method using a symmetric polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer. We characterized the uniform P2VP brush layer using X-ray reflectivity and atomic force microscopy. The buffer pH effectively adjusts the interactions between the citrate-coated AuNPs surfaces and P2VP brushes. Low pH (~4.0) yields highly stretched brush chain conformations with sufficiently attractive interaction pairs, while high pH (~6.5) produces slightly stretched brushes with limited attractive interaction pairs. Quartz crystal microbalance with dissipation monitored the adsorption thermodynamics as a function of AuNP diameter (11 and 21 nm) and buffer pH. We find that high pH provides limited penetration depth for nanoparticles and promotes size selectivity for 11-nm AuNP adsorption. This selectivity for the smaller AuNPs was also observed in mixed AuNP suspensions, thus demonstrating the potential application of P2VP brushes for size separation of nanoparticles.