Voltage-Triggered Structural Switching of Polyelectrolyte-Modified Nanochannels

Synthetic solid-state nanochannels modified with polyelectrolyte brushes are an important class of stimuli-responsive nanofluidic devices. This work theoretically addresses the design of a voltage-triggered nanomechanical gate using the collapse transition of a hydrophobic polyelectrolyte brush within a long nanochannel. In poor solvent conditions, a polyelectrolyte brush grafted to the inner surface of a nanochannel can either collapse to its walls or stretch toward its axis in order to form a central dense plug. An applied transmembrane potential favors polyelectrolyte chain conformations that are tilted in the direction of the electric field, and therefore, the transmembrane potential can trigger a transition from the collapsed-to-the-center state to the collapsed-to-the-wall state. This work studied this transition as a function of the length of the polyelectrolyte chains, the hydrophobicity of the polymer backbone, and the pH and ionic strength of the solution. The optimal conditions to achieve a sharp voltage-triggered transition between the collapsed-to-the-wall and the collapsed-to-the-center structures were identified. This work also explored the effect of the voltage-triggered collapse transition on the transport of probe particles of different sizes. It is shown that there is a balance between the permeability of the channel and the selectivity of the two different collapse states for the particle. In the particular system explored in this work, this balance makes the structural transition mostly effective to gate the transport of species with radii in the ∼1 nm range.