Chitosan Nanocomposite Thin Films as Sustainable Biopolymer Based Colorimetric Humidity Sensors

Humidity sensors play an essential role in monitoring product quality and lifetime in the manufacturing and pharmaceutical industries for vaccine and drug viability. Recently thermoplastic polymers and inorganic semiconductors-based humidity sensors are raising environmental concerns owing to non-degradability despite their performances. Biodegradable and renewably sourced chitosan-based nanocomposite films could provide a more sustainable alternative. Chitosan has randomly distributed β-linked D-glucosamine and N-acetyl-D-glucosamine and 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 <i>thin-film interference</i> phenomenon. Over the full relative humidity range of 95%, film thicknesses increase 50% compared to dry state, confirmed by <i>in-situ</i> ellipsometry and interferometric techniques. The response to humidity change was ultrafast (~5s) and the absorption-desorption of moisture exceeded 50 cycles with confidence. The moisture absorption kinetics followed non- Fickian type diffusion pathway. By blending polar nanofillers like graphene oxide (GO) in a small amount (0.5-2wt%), the swelling behavior of these blend films followed similar trend of rapid moisture absorption while maintaining film stability because of the interaction of oxygen-rich groups of GO and NH₂ groups of chitosan. The nanocomposite thin films had reliable thermal properties and stable tensile strength. This highly sensitive humidity dependent colorimetric property of chitosan nanocomposite films enables its potential as a biodegradable sensor for monitoring quality assurance systems benefiting several industries.<br/>Funding: AFOSR W911NF2010281