Synthetic membranes containing asymmetrically shaped pores have been shown to rectify the ionic current flowing through the membrane. Ion-current rectification means that such membranes produce nonlinear current–voltage curves analogous to those observed with solid-state diode rectifiers. In order to observe this ion-current rectification phenomenon, the asymmetrically shaped pores must have pore-wall surface charge. Pore-wall surface charge also allows for electroosmotic flow (EOF) to occur through the membrane. We have shown that, because ion-current is rectified, EOF is likewise rectified in such membranes. This means that flow through the membrane depends on the polarity of the voltage applied across the membrane, one polarity producing a higher, and the opposite producing a lower, flow rate. As is reviewed here, these ion-current and EOF rectification phenomena are being used to develop new sensing technologies. Results obtained from an ion-current-based sensor for hydrophobic cations are reviewed. In addition, ion-current and EOF rectification can be combined to make a new type of device—a chemoresponsive nanofluidic pump. This is a pump that either turns flow on or turns flow off, when a specific chemical species is detected. Results from a prototype Pb2+ chemoresponsive pump are also reviewed here.

Ion-exchange membranes have been used in commercial water desalination and wastewater treatment centers for decades. The key feature of these membranes is their permselectivity. The primary mechanism that governs permselectivity is highly debated, especially as these pores reach molecular dimensions. In this work, a 30 nm commercially available polycarbonate membrane is gold-plated using an electroless template synthesis method. By varying gold-plating time, one can create pores with diameters as small as 1 nm. These membranes are exposed to various chloride salt solutions, which forms a layer of adsorbed chloride along the faces of the membrane and pore walls. This fixed negative charge allows for the rejection of anions and the transport of cations across the membrane. Using a varying concentration cell, the selectivity of these membranes can be investigated potentiometrically by the transference numbers. These membranes express ideal cation permselectivity so long as the thickness of the electrical double layer is larger than the radius of the nanotubes. Individual cation influence on membrane permselectivity is investigated using gold plated pores with diameters smaller than 10 nm.

Quaternary ammonium salts (QAS) are cationic surfactants widely used in domestic and industrial products like detergents, disinfectants, personal care products, and more. Due to their substantial use, QAS are accidentally or intentionally released into the environment and have been detected in surface waters, wastewaters, and soil sediments. The concentration of QAS in groundwaters has greatly increased, and this poses serious health threats. Therefore, various methods to purify wastewaters and remove the containment QAS have been proposed. Membrane-based technologies have been used extensively, and wastewater treatment facilities often use polymeric membrane-based systems for purification. Quaternary ammonium salts are known to accumulate on the polymeric membrane surfaces and within the membrane pores. This leads to membrane fouling, which is a major obstacle for efficient operation of these membrane systems. Direct evidence is presented here for the adsorption of aqueous QAS adsorbed on a polycarbonate membrane studied. Potentiometric experiments, in accordance with surface contact angle and infrared spectroscopy measurements, illustrate the extent of adsorption on the membrane surface and within the membrane nanopores. Using a homologous quaternary ammonium series, we focus on how the hydrophobicity of the adsorbing cationic surfactant directly affects membrane separation performance, specifically cation transport and membrane permselectivity. It was found that the strength of the interaction between the QAS and the polycarbonate membrane was directly proportional to QAS hydrophobicity (i.e. carbon number). Excessive adsorption to the membrane surface and pore walls deteriorated membrane separation performance and required extra rinsing using an aqueous KCl solution to remove the QAS foulant.