Color-tunable luminescent Ln³⁺ composite as self-referencing and ratiometric sensor for low-level water detection in ethanol

Graphical abstract: A new color-tunable luminescent Ln³⁺ (Ln = Eu, Tb or Eu and Tb in different molar ratio) composite was prepared through a simple method, which can be applied as self-referencing and ratiometric sensor for detecting low-level water content (0.01-0.5 vol%) in ethanol. - Highlights: • A novel Ln³⁺ (Ln = Eu, Tb or Eu and Tb in different molar ratio) complex with tunable luminescence from red to green was prepared through a simple procedure. • The luminescence color of the Eu/Tb co-doped complex can be tuned by water through the luminescence quenching mechanism as well as the variation of energy transfer efficiency from Tb³⁺ to Eu³⁺. • The complex with Eu³⁺:Tb³⁺ = 1:1 can be applied as self-referencing and ratiometric luminescent sensor for detecting low-content water (0.01–0.5 vol%) in ethanol. - Abstract: Developing simple, fast and sensitive detection of low-level water in organic solvents is in high demand for academic and industrial applications. In this paper, a novel color-tunable lanthanide composite has been prepared by simply mixing 2,2':6',2"-terpyridine-4'-carboxylic acid (Hctpy) and Ln³⁺ with a molar ratio of 3:1 in absolute ethanol at room temperature, in which the Ln³⁺ ions are mainly coordinated with carboxyl moieties rather than with terpyridine moieties of Hctpy. In addition, the emission colors of the composite can also be tuned by water, ascribed to the luminescence quenching mechanism and the variation of Tb-to-Eu energy transfer efficiency. Based on these properties, a self-referencing and ratiometric luminescent water detector with the Eu/Tb molar ratio of 1:1 has been developed, showing proportional relationship between the intensity ratio of ⁵D₀ → ⁷F₂ transition of Eu³⁺ to ⁵D₄ → ⁷F₅ transition of Tb³⁺ (I_Eu/I_Tb) and the water concentration (0.01–0.5 vol%) in ethanol.