Tunable Interference Colors in Nanofibril–Crystal Composite Films via Integrated Salt-Assisted Assembly

Cellulose nanofibrils (CNFs) are strong, flexible biopolymers, whereas producing large-area (centimeter-scale) optical films with tunable, vivid interference colors with them remains difficult. The amorphous regions within the fibrils reduce birefringence and prevent uniform color formation. Here, we develop an integrated salt-assisted assembly (iSAA) strategy that combines metal-ion crosslinking with organic-inorganic coassembly to fabricate nanofibril-crystal composite films with tunable, vibrant colors. By adjusting the concentration of aluminum salt during assembly, we precisely control optical phase retardation from about 400 nm to over 2,500 nm, covering the full range of the interference colors. The resulting films appear naturally transparent while displaying intense colors across the visible spectrum when viewed under polarized light. We further demonstrate the universality of the iSAA strategy by fabricating films from different biopolymer fibrils (e.g., chemically modified CNFs and protein amyloid fibrils) on hydrophilic and hydrophobic substrates. Additionally, coupling the transmitted colors of these films and their intrinsic optical haze produces a novel spectrally selective polychromatic lighting effect. These results establish the iSAA strategy as a general platform for engineering nanofibril-based birefringent materials with programmable polarization optics, expanding their potential in advanced optical and energy-efficient applications.