Dual-Mode Luminescent Nanopaper Based on Ultrathin g–C3N4 Nanosheets Grafted with Rare-Earth Upconversion Nanoparticles

Ultrathin graphite-like carbon nitride (g–C3N4) nanosheets have attracted considerable attention due to the enhanced intrinsic photoabsorption and photoresponse with respect to bulk g–C3N4. For the first time, a dual-mode of down- and upconversion luminescent g–C3N4 nanopaper with high optical transparency and mechanical robustness was successfully fabricated through a simple thermal evaporation process using chitosan as a green cross-linking agent. The dual-mode of down- and upconversion fluorescence emission originated from the amino terminated ultrathin g–C3N4 nanosheets functionalized with carboxylic acid modified multicolored rare-earth upconversion nanoparticles (cit–UCNPs) via EDC/NHS coupling chemistry. The homogeneously distributed cit–UCNPs@g–C3N4 nanoconjugates with excellent hydrophilicity displayed good film-forming ability and structural integrity; thus, the photoluminescence of each ingredient was substantially maintained. Results indicated that the freestanding chitosan cross-linked cit–UCNPs@g–C3N4 luminescent nanopaper possessed high transmittance, excellent mechanical properties, and remarkable dual-mode emission. The smart design of high performance luminescent nanopaper based on ultrathin g–C3N4 nanosheets grafted with multicolored UCNPs offers a potential strategy to immobilize other multifunctional luminescent materials for easily recognizable and hardly replicable anticounterfeiting fields.