Humidity Induced Swelling of Sustainable Chitosan Nanocomposite Thin Films for Optical Sensors

Humidity sensors play an essential role in monitoring product quality and lifetime in the pharmaceutical and manufacturing industries. Recently thermoplastic polymers and inorganic semiconductors-based humidity sensors are raising environmental concerns. Biodegradable and renewably sourced chitosan-based nanocomposite films could provide a more sustainable alternative. The protonation of NH₂ groups of chitosan increases the affinity of these films towards moisture. Here, thin chitosan films of 120-350nm thickness were coated on Si substrate. Rapid swelling of the thin films in humid environment was spontaneously identified with visible changes in color explained by thin-film interference phenomenon. At 95% relative humidity, film thicknesses increased 50% compared to dry state, confirmed by in-situ ellipsometry and interferometer. The response to humidity change was ultrafast (~5s) and the absorption-desorption process exceeded 50 cycles with confidence. The moisture absorption kinetics followed Fickian to non-Fickian type diffusion pathway. By blending 0.5-2wt% polar nanofillers like graphene oxide (GO), the moisture absorption kinetics followed similar pathway with rapid response while maintaining mechanical stability owing to the interaction of oxygen-rich groups of GO and NH₂ groups of chitosan. This highly sensitive humidity dependent colorimetric property of chitosan nanocomposite films enables its potential as a biopolymer-based optical sensor.