Structured color humidity indicator from reversible pitch tuning in self-assembled nanocrystalline cellulose films

Iridescence is an example of structured color that is widespread in the biosphere, exhibited by multilayer inorganic thin films for optical filters, photonic crystals and other materials in which the periodic patterning of matter interacts with an electromagnetic field. Nanocrystalline cellulose (NCC) can be cast in the form of thick iridescent films whose color originates in the multi-domain chiral nematic texture created by self-assembly of the rigid rod crystallites. Scanning electron microscopy confirms the periodic layer structure that arises from the helical twist axis of the chiral nematic mesophase film. In effect, the film comprises multi-domain Bragg reflectors. On exposure to liquid water, and high relative humidity (RH), a reversible shift in the film iridescence from dry state blue-green to wet state red-orange is observed. This color change, which requires no pigment, is quantified by reflectance spectroscopy. The color transition is attributed to sorption of water that causes the pitch of the Bragg reflector to enlarge, and this leads to a red shift in the iridescence. The subsequent expansion of the film thickness was observed using polarized optical microscopy. The effect resembles molecular dopant and electric field induced pitch tuning along the helicoid axis in one-dimensional photonic crystal-like chiral nematic molecular systems. The color shift for a 40 μm thick NCC film is slow, occurring on timescale of 1-3 min. Thinner films change color in less than 2 s.