Photo-Patternable Polymer Nanofilims for Humidity and Temperature Sensing and Recording

Relative humidity (RH) and temperature-responsive nanofilms have been developed as colorimetric sensors based on light interference, with the sensors providing rapid and easy detection, high accuracy, good mobility, wearability, and power-free sensing ability. However, current studies on RH and temperature-sensitive colorimetric films focus only on the effect of either temperature or RH on the sensing ability. With an inherent relationship between temperature and RH, this constraint leads to significant limitations for the use of thin film sensors in practical applications. Furthermore, the inherent reversible color-changing behavior of these films does not allow them to record temperature and RH changes in the past time. Herein, we developed a photo-patternable colorimetric polyelectrolyte multilayered (PEM) nanofilm using chitosan (CHI) and carboxymethyl cellulose azide (CMC-N₃). This nanofilm displayed uniform colors covering the entire visible spectra, while in the low temperature (2-10°C) and high RH (~80%) regimes, the film demonstrated vivid color changes with 1°C resolution. Notably, we address the current limitations by proposing a novel 3-dimensional (3D) non-linear regression model to describe the correlation between the normalized thickness (color), temperature, and RH; with the thickness prediction showing high agreement with experimental results. Moreover, by taking advantage of different swelling behaviors of CHI/CMC-N₃ films before and after UV treatment, a variety of colors and crosslinked patterns are hidden/displayed at selected RHs and temperatures. This enables a tunable multimodal display of pattern-encoded information. Furthermore, we developed a smart device based on this colorimetric polymer nanofilm for effectively record an irreversible change of temperatures. This device has shown quick color response in temperature ranges from 23°C to -30°C in fewer than 50 seconds. Remarkably, when the device experiences temperature disruption above a certain threshold time (t_th), it shows irreversible color-changing behavior in response to the temperature change from subzero (-30°C and -15°C) to room temperature or above. It was demonstrated that t_th, from minutes to days, of the TTI device can be precisely tuned by adjusting moisture absorber type and weight, interior RH, and storage temperature.