Green synthesis of a thermo/photochromic doped cellulose polymer: a biocompatible film for potential application in cold chain visual tracking†

To mitigate food losses and ensure a robust cold chain in transportation, sensors play a pivotal role in swiftly and visibly monitoring storage conditions. The most commonly used indicators for reporting temperature violations are based on devices capable of signaling when a threshold temperature has been reached or exceeded or on disposable colorimetric sensors. A potential alternative, which uses reusable colorimetric sensors, may come from utilizing systems capable of displaying reversible color changes upon temperature variations; in this regard, molecules exhibiting thermo- and photochromic properties such as N-salicylideneaniline derivatives (anils) have emerged as promising candidates due to the simplicity of their synthesis and their ability to respond to temperature and light stimuli. In this study we have synthesized a family of anils through mechanochemistry, focusing on H/F substituents on the bromoaniline residue. The compounds were fully characterized using XRD and thermal techniques, and their thermo- and photochromic properties were explored via infrared spectroscopy. Among the series, the most suitable compound, i.e., a photochromic one showing a neat color change (from white to red/orange) quickly naked eye-detectable and whose back reaction is slow or virtually negligible at low temperatures, was identified and incorporated into a carboxymethyl cellulose (CMC) biopolymer matrix to produce a composite film, which was further characterized via XRD, thermal analyses and mechanical tests. The selected compound maintained its photochromic behavior upon embedding, and UV irradiation induced a color change in the film from colorless to red, while reversibility was evaluated at different temperatures (−19 °C, 4 °C and RT) using UV-vis spectroscopy. The composite film maintained a deep red color at −19 °C and 4 °C for seven weeks, while rapidly reversing to white/yellowish at room temperature, making it a suitable candidate for the development of sensors for cold chain transport and scenarios requiring rapid visual inspection of storage conditions.