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.