Multimode Anticounterfeiting Labels Based on a Flexible and Water-Resistant NaGdF₄Yb³⁺,Er³⁺@Carbon Dots Chiral Fluorescent Cellulose Film

Advanced fluorescent anticounterfeiting technology has attracted a great deal of attention and inspired researchers to develop reliable fluorescent materials with multimodal luminescence. Herein, hydrophobic NaGdF4:Yb3+,Er3+ nanoparticles with dual-mode luminescence were prepared through a one-step solvothermal method using oleic acid as a chelating agent and octadecene as a solvent. The as-synthesized sample was found to provide both green upconversion luminescence and blue downconversion luminescence at excitation wavelengths of 365 and 980 nm. The structure and composition analysis showed that the as-obtained samples were spherical NaGdF4:Yb3+,Er3+@carbon dots (CDs) with a 14.5 ± 0.2 nm particle size and good dispersity in nonpolar solvents. Subsequently, NaGdF4:Yb3+,Er3+@CDs were coated on a cholesteric nanocellulose film with structural color to prepare multimode circularly polarized fluorescent materials. The prepared composite film is flexible and water-resistant, meaning that it is very suitable for anticounterfeiting labels, not only providing a simple and rapid synthesis method with which to prepare multimode fluorescent materials but also presenting a novel anticounterfeiting technology.