Nanofluidics with controllable gating properties have promising and exciting uses in various applications requiring regulated fluid flow and particle selectivity, such as drug delivery, filtration, and molecular sorting. Stimuli–responsive polymer brushes are ideal candidates for such applications. The variation of their conformations in response to thermal, chemical, or optical changes in the surrounding environment makes them well suited for designing smart, functional surfaces for valve-like nanofluidic channels. Here, we show that the selective functionalization of the side walls of nanofluidic channels with stimuli-responsive polymers yields the conformations and structural properties necessary for controlled gating. Specifically, we explored the grafting of poly(dimethylaminoethyl methacrylate) from the walls of submicron channels in a silicon grating. To probe changes in the polymer brush structure in response to variations in the solution pH and ionic strength, we utilized high-resolution specular and off-specular neutron reflectometry capabilities. Simultaneous analysis of the specular and off-specular signals enabled a 3D reconstruction of the sample profile and yielded a detailed description of the brush thickness and conformations under the different solvent conditions. Our analysis is based on a dynamical theory model, which provides an exact solution for scattering from periodic structures. Using an advanced computational optimization protocol, the brush structure was obtained from simultaneous fits of the dynamical theory model to both specular and off specular signals. Our results indicate that the polymer chains are in a collapsed state under basic pH (open gate) and expand upon decreasing solvent pH (equivalent to a partially closed gate). In all cases, we observed a dense polymer brush region near the channel wall and a less dense region toward the middle of the channels.

Characterization of polymer-grafted channels for nanofluidic gating applications

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.