Excitation-emission orthogonalized luminescent upconversion nanoparticles (OUCNPs), which can respond to changes in external stimuli accordingly, show great promise in many intelligent applications. However, the construction of such materials mostly relies on the selective absorption of Nd³⁺ and Yb³⁺ at different wavelengths and the long-range energy migration between the layers, resulting in complex structures and limited orthogonal luminescence intensity. Herein, we developed a relatively simple structure of OUCNPs (β-NaErF₄@NaLuF₄@NaYF₄:20%Yb, 2%Er@NaLuF₄), where the fluorescence emission switches from red to green when the excitation wavelength is shifted from 808 to 980 nm. This structure exhibits high-quality, independent, and non-interfering orthogonal luminescence properties without Nd³⁺ sensitization and long-range energy migration. As a proof of concept, we demonstrate the application of the designed OUCNPs in anti-counterfeiting. We also prepared OUCNPs@PEI (PEI = polyethylenimine) self-referencing fluorescent probes to enable quantitative analysis of trinitrotoluene (TNT) in solution with a detection limit of 3.04 μM. The probes can be made into test strips for portable on-site visual detection of TNT, and can also be used to image latent fingerprints and detect explosive residues in fingerprints simultaneously. The concept proposed in this work can be extended to the visual detection of a larger range of organic and biological molecules, and is highly promising for practical applications.

LiErF₄ was commonly used as a dipolar-coupled antiferromagnet, and was rarely considered as a luminescent material. Herein, we achieved the strong red upconversion emission of LiErF₄ simply by an inert shell coating, i.e., LiErF₄@LiYF₄. Owing to the unique and intrinsic ladder-like energy levels of Er³⁺ ions, this LiErF₄ core-shell nanostructures present red emission (~ 650 nm) under multi-band excitation in the near-infrared (NIR) region (~ 808, ~ 980, and ~ 1,530 nm). A brighter and monochromic red emission can be further obtained via doping 0.5% Tm³⁺ into the LiErF₄ core, i.e., LiErF₄: 0.5% Tm³⁺@LiYF₄. The enriched Er³⁺ ions and strong monochromic red emission natures make LiErF₄: 0.5% Tm³⁺@LiYF₄ nanocrystals very sensitive for trace water probing in organic solvents with detection limit of 30 ppm in acetonitrile, 50 ppm in dimethyl sulfoxide (DMSO), and 58 ppm in N, N-dimethyl- formamide (DMF) under excitation of 808 nm. Due to their superior chemical and physical stability, these nanoprobes exhibit excellent antijamming ability and recyclability, offering them suitable for real-time and long-term water monitoring.