Development of Photopatternable Colorimetric Polymer Nanofilms for Temperature and Humidity Sensing

Advanced sensors play a crucial role across various fields, including environmental monitoring, national security, agriculture, food industry, and healthcare. In particular, relative humidity (RH) and temperature sensors have found widespread applications across multiple industries, contributing to improved efficiency and safety. For example, to help protect the integrity of bioreagents such as COVID-19 vaccines, a reliable temperature monitoring, and recording device is needed to prevent them wasted due to loss of potency from out-of-range temperature storage in manufacturing or transportation. Current electronic-based temperature monitoring and logging devices are costly, not directly monitoring the temperature of the bioreagents, and facing cyber insecurity with data transmission. A non-electronic-based temperature monitoring and recording sensor based on the inherent properties of smart materials can be a potential solution to overcome the limitations of current devices. Polymeric 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, recording the changes in subzero temperatures and humidity over certain periods are essential for pharmaceutical, food-beverage, and cold storage industries, as product quality can be hampered during storage and transportation due to temperature disruptions. Traditional electronic subzero time-temperature indicators (TTIs) are widely used; however, they can be energy-inefficient, fragile, and non-recyclable. Though the responsive colorimetric nanofilms do not have these limitations, the inherent reversible color-changing behavior of these films does not allow them to record temperature changes in the past. In this dissertation, we developed a photopatternable colorimetric polyelectrolyte multilayered (PEM) nanofilm using chitosan (CHI) and carboxymethyl cellulose azide (CMC-N3). Remarkably, the 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 examining the correlation between structural color (thickness), temperature, and RH. We propose a novel 3-dimensional (3D) non-linear regression model to describe the correlation between the normalized thickness, temperature, and RH; with the thickness prediction showing high agreement with experimental results. Moreover, due to the photoreactive nature of carboxymethyl cellulose-azido derivative (CMC-N3), the CHI/CMC-N3 films can covalently be crosslinked upon UV irradiation. 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 TTI device based on this colorimetric polymer nanofilm to effectively record an irreversible change of temperatures. The irreversible temperature monitoring (iRTM) device has been integrated using: 1) a polymeric nanofilm that displays full-spectrum colors when environmental temperature or RH changes, and 2) a drying module containing a precise amount of moisture absorber, in a transparent sealed container. 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 (tth), 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 tth, from minutes to days, of the TTI device can be precisely tuned by adjusting moisture absorber type and weight, interior RH, and storage temperature. Several field tests have demonstrated good versatility and applicability of the device.