Ion-Track-Etched Membranes as Nanoionic Platforms: Fabrication, Nanoscale Ion Transport, and Device Applications─A Review

Ion-track-etched membranes (ITEMs) have emerged as a distinct class of nanostructured materials and represent a versatile platform that offers precise control over pore shape, size, and areal density. These membranes offer considerable mechanical and chemical stability, making them valuable for a variety of applications, including nanoionic rectifiers, sensors, nanoiontronics devices, and energy harvesting systems. The chemical etching process of ITEMs introduces carboxylic acid groups on the surface, which serve as the sites for functionalization. Functionalization is crucial for using the ITEMs in sensor applications. ITEM-based sensors provide considerable sensitivity and selectivity. Moreover, the sensors have a low limit of detection compared to other traditional sensors. ITEMs also exhibit memristive behavior (hysteresis loop) and can perform logic gate functions. These attributes are crucial for nanoionic fluidic-based neuromorphic computing. Moreover, ITEMs can serve as efficient platforms for osmotic energy harvesting (OEH) as well as membrane-assisted cooling and dehumidification applications. This review article articulates the fundamentals of ITEMs, fabrication strategies, the origins of surface-charge-dominated ion-transport mechanisms under nanoconfinement, and chemical functionalization approaches. In addition to that, concepts of ionic rectification, hysteresis, and nonlinear functionalities related to the memristor and neuromorphic computing are also discussed. Moreover, special attention was paid to OEH systems, and power densities obtained with different membranes were also discussed and compared. The review article also provides insights into next-generation nanoionic systems by combining the concepts of nanofluidics and electrochemistry. In addition, a comprehensive analysis of the current challenges and future opportunities in developing multifunctional, scalable, and sustainable ITEM technologies is also presented.