There is increasing interest in understanding the properties of solutions confined within nanotubes and synthetic or biological nanopores. How the ionic content of a nanopore-confined solution differs from that of a contacting bulk salt solution is of particular importance, for example, to water desalinization, industrial electrolysis, and all living systems. This paper explores ionic content, ionic interactions, and ion-transport properties of solutions confined within the 10 nm diameter pores of a synthetic polymer membrane. The membrane has a fixed negative pore-wall and surface charge due to ionizable carbonate groups. As a result, under some conditions, the nanopore-confined solution contains only cations and no anions or salt present in a contacting solution, ideal cation permselectivity. This anion- and salt-rejecting ability varies greatly with the cation of the salt, a result that is in contradiction to the prevailing model for permselectivity in nanopores. The extant model fails because it does not account for specific chemical interactions between the cation and the carbonate groups. The nature of these ion-selective interactions is discussed here.

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.

MnO2 has been proposed as an electrode material in electrochemical energy storage devices. However, poor cycle life, especially in aqueous electrolytes, remains a detriment to commercialization. Prior studies have suggested a number of explanations for this capacity loss; however, experiments aimed at elucidating the details of the degradation process (es) are sparse. We describe here a microtube-membrane construct that allows for electrodeposition of monodisperse MnO2 microparticles distributed across the membrane surface, and for subsequent electrochemical cycling of these MnO2 particles. This allowed for a detailed analysis of the effect of cycling on the MnO2, by simply imaging the membrane surface before and after cycling. When an aqueous electrolyte was used, gross changes in particle shape, size and morphology were observed over the course of 500 cycles. Partial dissolution occurred as well. No such changes were observed when the MnO2 particles were cycled (up to 500 times) in a propylene carbonate electrolyte solution.

Ion permselectivity is an important ionic transport property exhibited in nanopores and nanotubes. Permselectivity develops due to the strong interactions between ions in solution and the nanopore/nanotube surfaces. Here, we study the ion permselectivity of gold-plated nanotube membranes. Gold-plated membranes were obtained by using an electroless gold plating method to deposit gold nanotubes into the pores and on the faces of track-etched polycarbonate membranes. These membranes have the potential to display cation permselectivity due to the excess surface charge density present on the membrane from chemisorbed Cl- anions. The cation permselectivity of membranes with 9.9 + 0.6 nm diameter nanotubes was studied potentiometrically by using a concentration cell. A new Debye-sphere based model was presented here to describe the extent of permselectivity. However, some ion specific effects with the gold-plated membrane are observed that cannot be interpreted by Debye theory. These ion-specific surface interactions lead to a reduction in the negative surface charge, causing no permselectivity for some cations as demonstrated by their lower than ideal cation transference numbers. Quantification of the surface charge density by XPS indicated the gold-plated membranes are highly charged due to Cl- chemisorption with a surface charge density of ~ 5.3 uC per cm2 corresponding to 3.3 x 1013 Cl--ions per cm2.

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.