Dual-modulation of lanthanide valence and MOF phase transition: advancing ratiometric thermometry in Eu-doped ZIF-L films for high-sensitivity thermal sensing

Abstract

Ratiometric optical thermometry has emerged as a vital platform for non-contact temperature monitoring in industrial and scientific applications, demanding sensors with high sensitivity, non-invasiveness, and spatial resolution. Although zeolitic imidazolate framework-8 (ZIF-8) possesses attractive thermal stability and structural tunability, its limited luminescence intensity hinders practical thermometric applications. Meanwhile, ZIF-L, a two-dimensional (2D) polymorph of ZIF-8 with identical ligands, remains unexplored for temperature sensing. Herein, we propose a dual-modulation strategy synergizing lanthanide valence engineering with MOF phase control to develop a multi-emission center Eu³⁺/Eu²⁺ co-doped ZIF-L film (Eu/ZIF-L). By introducing Eu³⁺ into ZIF-8 precursors, the partial reduction of Eu³⁺ to Eu²⁺ under thermal treatment triggers a structural evolution from 3D ZIF-8 to 2D ZIF-L. The resultant Eu/ZIF-L exhibits dual emission peaks at 450 nm (Eu²⁺) and 495 nm (ligand-centered), with their intensity ratio (I₄₉₅/I₄₅₀) demonstrating remarkable linear temperature dependence over 296–373 K and achieving a relative sensitivity of 1.95% K⁻¹. This work pioneers the integration of valence-state manipulation and MOF phase transition for thermometric optimization, providing a novel paradigm to design advanced ratiometric MOF thermometers for industrial thermal management.