Assembly of AIEgen‐Based Fluorescent Metal–Organic Framework Nanosheets and Seaweed Cellulose Nanofibrils for Humidity Sensing and UV‐Shielding

Abstract Integrating synthetic low‐dimensional nanomaterials such as metal–organic framework (MOF) nanosheets with a sustainable biopolymer is a promising strategy to endow composites with attractive structural and functional properties for expanded applications. Herein, aggregation‐induced‐emission luminogen (AIEgen)‐based MOF bulk crystals are successfully exfoliated into ultrathin 2D nanosheets. Seaweed cellulose nanofibrils (CNFs) are assembled with low amounts (0.3 to 4.0 wt%) of the 2D nanosheets to generate luminescent composites. The 2D nanosheets are adsorbed onto the CNFs in dilute water suspensions owing to the flexibility of the MOF nanosheets and the high aspect ratio of the CNFs. Transparent films are prepared by solution casting from a water suspension of the CNF‐MOF assembly. The fluorescence emission of the composite films is enhanced because of the favored affinity between MOF nanosheets and CNFs. Remarkably, these films demonstrate excellent UV‐shielding capacity and high optical transmittance at the visible wavelength range. The composite films also show reversible changes in fluorescence emission intensity in response to ambient humidity. The tensile strength and modulus of the composite films are also enhanced owing to the increased adhesion between CNFs through the adsorbed MOF nanosheets. This work provides a novel pathway to fabricate luminescent CNFs‐based composites with tunable optical properties for functional materials.