Ultra-Sensitive water detection based on NaErF4@NaYF4 high-level-doping upconversion nanoparticles

[Display omitted] •An ultra-sensitive water detection probe (i.e., NaErF4@NaYF4) was developed.•The limit of water detection (LOD) in DMF solvent can go down to 5 ppm.•The NaYF4 shell thickness play a two-sides role in the sensing performance. Water-sensitive detection plays a vital role in our society and life, such as the chemical industry, environmental monitoring, and food inspection. In this work water sensitivity as high as 5 ppm (0.0005 vol%) is realized based on a novel upconversion nanophosphor. Different from the popularly employed low-doping strategies, an efficient high-level-doping nanostructure, i.e., NaErF4@NaYF4, is tailored for the desired upconversion emission, resulting in more than one order of magnitude enhancement of the sensitivity compared to the record of the low-doped counterparts. The superiority of this structure is determined to come from the robust interactions between the Er3+ ions and the water molecules adsorbed on the particle surface. This design offers a new strategy of nanophosphor structures for ultrasensitive water sensing and sheds light on further optimization of nanophosphor based sensors.