Scalable, cationic chitosan nanofluidic cables with aligned nanochannels

Due to the unique behavior of ion transport within confined nanochannels, nanofluidics has gained ever-increasing popularity in power generation, desalination, sensor and DNA sequencing. Both the anionic and cationic nanofluidic devices are of importance in their practical applications as the movement of the ions can be effectively regulated by the surface charge of the nanochannels. However, the emerging technology primarily focuses on anionic nanofluidic devices, while the straightforward, scalable and economical fabrication of cationic nanofluidic devices remains challenging. Herein, a bottom-up approach to aligned cationic nanofluidic films/cables, which fully harnesses the inherent positive charges on chitosan chains, is proposed. The cationic nanofluidic films are prepared directly through the ionic crosslinks between positively charged chitosan chains and negatively charged sodium dodecyl sulfate micelles via interfacial-diffusion-induced gelation, which feature favorable optical transparency, flexibility, thermal stability, tensile strength, anti-swelling ability, long-term stability and environmental tolerance. Furthermore, the resulting films can be stretched into aligned films and further twisted into ionic cables with a tensile strength of 131 MPa. Thanks to the well-aligned nanofluidic channels and the inherently positively-charged surface, the ionic cables exhibit a surface-charge-governed ion transport phenomenon as well as anion selectivity. The ionic conductivity of the ionic cable reaches 2.4 × 10−4 S·cm−1 at low salt concentrations (4.1 × 10−4 S·cm−1 for aligned films). This work not only provides a straightforward, scalable, cost-effective strategy for cationic nanofluidic devices through leveraging the inherent properties of natural polymers, but also offers promising opportunities for the underutilized biomass to be transformed into value-added products.