PH and potential-controlled multi-modal mass transport in block copolymer nanochannel membranes

Abstract

Inspired by the hydrophobic gating for achieving fast and selective ion/molecular transport in cell membranes, wetting/dewetting transition in solid-state nanopores controlled by external stimuli such as voltage, pH, electrostatics, and light have attracted increasing attention. For an accurate and better understanding, a single nanopore or low-density array of nanopores was preferred to investigate the wetting and dewetting transitions owing to their well-defined chemical functions and physical structures. However, high-density nanochannel membranes capable of processing high-throughput and multi-modal mass transport are more beneficial with the aim of practical use. In this regard, pH- and potential-responsive nanochannel membranes consisting of a polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer (BCP) are prepared to demonstrate a multi-modal transport system with high-throughput capability. At pH < pKa(P4VP) (pKa ∼ 4.8), the cylindrical P4VP nanodomains are hydrophilic and positively charged, acting as an anion-exchange membrane. In contrast, at pH > pKa(P4VP), the P4VP domains switch to be charge-neutral and hydrophobic, naturally blocking the mass transport through the nanochannels. Applying a sufficiently positive potential to a BCP membrane-coated electrode may induce oxidative wetting in the hydrophobic nanochannels to facilitate mass transport across the membrane with no charge-selectivity. Releasing the bias makes the hydrophobic nanochannel membranes retrieve the original dewetted state, blocking the transport again. In addition, direct observation of the wetting-dewetting transition dynamics in the hydrophobic nanochannels is investigated by monitoring potential-correlated electrochemiluminescence (ECL) signals arising from Ru(bpy)32+ and co-reactant tripropylamine (TPA) under potential modulations. ECL signals tend to decrease with increasing membrane thickness ranging from 0 nm to 820 nm because it requires higher potentials to induce wetting in the nanochannels due to elongated hydrophobic nanochannels. The multi-modal transport system developed in the present work will be useful for applications such as water treatment, biosensors, and smart valve systems like controlled drug release/delivery.