Flexible, Bio-Compatible Nanofluidic Ion Conductor

Ion transport in nanochannels has unique behaviors such as charge selectivity and high ionic conductivity. Due to their simple structure, the photolithographic nanochannels have been widely used to understand the fundamentals of nanofluidic ion transport. However, for practical applications, especially biorelated applications, a scalable, flexible, mechanically stable, and biocompatible nanofluidic device engineered with charge selectivity and high ionic conductivity is more desirable. Herein, we report a scalable biomass material, namely, bacterial cellulose (BC), with excellent mechanical strength, flexibility, and biocompatibility for nanofluidic ion transport. The BC film is a 3-dimensional (3D) interconnected network of 10–30 nm thick cellulose nanofibers, containing 1–2 nm nanochannels with large negatively charged surface groups. After a facile TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) oxidation treatment, the zeta potential of the cellulose nanofibers significantly improves from −13 mV to −45 mV, and the ionic conductivity increases 40 times from 2.5 × 10–5 S/cm to 1.0 × 10–3 S/cm at low salt concentrations. As a proof of concept, we successfully demonstrate an ultrasensitive humidity sensor with the BC nanofluidic film for wearable health monitor applications.