Excitation orthogonalized upconversion nanoprobe for instant visual detection of trinitrotoluene

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.