Iridescent Chiral Nematic Cellulose Nanocrystal/Polyvinylpyrrolidone Nanocomposite Films for Distinguishing Similar Organic Solvents

Fast and easily distinguishable color change is the simplest sensor signal recognized by naked eyes. Rapid color change has been done via filling or removing liquid inside channels constructed by multilayered reflectors in biological species for camouflage or signaling. By mimicking it, a test paper for fast detection of similar organic solvents or water-content in ethanol has been designed based on nanocomposite films composed of cellulose nanocrystals (CNC) and polyvinylpyrrolidone (PVP) with chiral nematic structure. The chiral nematic architecture and structural color of CNC can be kept in CNC/PVP nanocomposites in a wide range of PVP content, up to 70 wt %. Moreover, we have observed that the nanocomposite containing higher weight percentage of PVP showed more distinguishable color difference while dipping in similar solvents. Owing to the wide solubility of PVP in organic solvents and the magnifying effect via increasing content of PVP, CNC/PVP nanocomposite films can work as discrimination sensors by presenting apparent color change while being dipping in several groups of similar organic solvents, such as homologues (methanol/ethanol), skeletal isomers (1-propanol/2-propanol), halogenated hydrocarbons (choloroform/dichloromethane), and even ethanol containing a small amount of water. The fast color change of CNC/PVP has been induced by the sensitive response of its structural color to the volume fraction change of CNC in the swollen state.