Cellulose Nanocrystal-Based Colored Thin Films for Colorimetric Detection of Aldehyde Gases

We demonstrate a controllable and reliable process for manifesting color patterns on solid substrates using cellulose nanocrystals (CNCs) without the use of any other chemical pigments. The color can be controlled by adjusting the assembly conditions of the CNC solution during a dip-and-pull process while aiding the close packing of CNCs on a solid surface with the help of ionic-liquid (1-butyl-3-methylimidazolium) molecules that screen the repelling electrostatic charges between CNCs. By controlling the pulling speed from 3 to 9 μm/min during the dip-and-pull process, we were able to control the film thickness from 100 to 300 nm, resulting in films with different colors in the visible range. The optical properties were in good agreement with the finite-difference time-domain simulation results. By functionalizing these films with amine groups, we developed colorimetric sensors that can change in color when exposed to aldehyde gases such as formaldehyde or propanal. A principal component analysis showed that we can differentiate between different aldehyde gases and other interfering molecules. We expect that our approach will enable inexpensive and rapid volatile organic compound detection with on-site monitoring capabilities.