Construction of self-sensitized LiErF₄: 0.5% Tm³⁺@LiYF₄ upconversion nanoprobe for trace water sensing

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.