Visual Color Response in CDs@YF ₃ : Eu ³⁺ Composite Nanophosphor

The development of high‐performance composite materials, particularly functionalized carbon dots (CDs), is of great significance for advancing applications in visual sensing and anti‐counterfeiting. Herein, a dual‐functional fluorescent nanocomposite CDs@YF 3 : Eu 3+ for visual Cu 2+ detection and anti‐counterfeiting, is designed by coupling blue‐emitting CDs with red‐emitting YF 3 : Eu 3+ nanoparticles, exhibiting exhibits ratiometric fluorescent characteristics under ultraviolet excitation, with an average primary particle size of ∼46 nm, which is significantly smaller than pure YF 3 : Eu 3+ . Based on this color‐switching behavior, a multi‐level security feature utilizing the two complementary colors is developed, which greatly enhances the security of information encryption by requiring a specific decryption key. Crucially, the system enables qualitative detection of Cu 2+ in aqueous solutions through fluorescence quenching, which is unaffected by the pH of the solution. Furthermore, anti‐counterfeiting labels with a unique, flexible, and waterproof microstructure are produced by electrospinning technology through the random embedding of composite materials within nanofibers, with an average fiber diameter of ∼545 nm. This work has pioneered a novel sensing and security platform based on advanced optical sensing capabilities, which supports both precise target analysis and tamper‐proof security validation with visual color response.