Fabry–Pérot probing of humidity-responsive swelling kinetics in confined poly(vinyl alcohol)/poly(acrylamide) thin films

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Understanding water sorption and swelling in substrate-confined polymer films is essential for developing responsive thin-film devices, yet their kinetic characterization remains challenging. Here, poly(vinyl alcohol)/polyacrylamide (PVA/PAAm) blend films were incorporated as humidity-responsive spacers in Al/(PVA/PAAm)/Ag nanocube Fabry–Pérot cavities, enabling optical readout of humidity-induced changes in film optical thickness through cavity resonance shifts. Under identical spin-coating conditions, increasing the PAAm content produced dry spacer thicknesses ranging from approximately 82–258 nm and systematically shifted the initial cavity resonance. Upon increasing the relative humidity from 15% to 85%, all films exhibited reversible redshifts, with a maximum wavelength modulation of approximately 85 nm. The response was relatively weak at low and intermediate humidity but became more pronounced above approximately 60–70% RH, suggesting enhanced water uptake and swelling at high humidity. Humidity-step measurements revealed an unusual decrease in response time across the composition series, from approximately 2.8–0.1 s, despite the increase in initial film thickness. This trend is qualitatively consistent with a less-swellable interfacial bound layer whose relative contribution is larger in thinner films. The cavities maintained stable wavelength switching over 30 humidity cycles. Incorporation of glutathione-derived carbon dots further enabled cavity-modulated fluorescence, with enhancement factors of approximately 1.9–3.4 and humidity-dependent dual optical outputs. These results demonstrate a simple Fabry–Pérot-based optical transduction strategy for rapid humidity sensing while providing a convenient platform for probing swelling kinetics in confined hydrophilic polymer films.