Nanocellulose-based cobalt(II) coordinated malonic acid hybrid aerogels exhibiting reversible thermochromism and moisture sensor properties

The emergence of sustainable polymers and technologies has led to the development of innovative materials with minimal carbon emissions which find extensive applications in wearable electronics, biomedical sensors, and Internet of Things (IoT)-based monitoring systems. Nanocellulose which can be generated from abundant biomass materials has been widely recognized as a sustainable alternative for a diverse range of applications due to its remarkable properties and eco-friendly nature. By making use of the unique and easily accessible coordination transformation property of Co(II) ions and associated visible light absorption changes, we report a novel Co(II) cation-incorporated nanocellulose/malonic acid hybrid aerogel material that exhibits reversible thermochromism induced by thermal stimulus in the presence of atmospheric moisture. This effect is accentuated by the highly porous nature of the nanocellulose aerogel material we have developed. Besides the reversible thermochromic property which Co(II) ions exhibit, the metal ions act as very efficient reinforcing units contributing significantly to the structural stability and rigidity of the hierarchical aerogels by coordinative cross-linking through carboxylate moieties present in the TEMPO-oxidized cellulose nanofibers (TCNF) and additionally adding malonic acid to provide sufficient COO− for cross-linking. Thorough characterization and detailed investigation of as-prepared hybrid aerogels was conducted to evaluate their overall properties including reversible thermochromism and moisture sensor behaviour. Further, an Android mobile-based application was developed to demonstrate the real-world application of the aerogels for atmospheric humidity sensing.