Synthesis of europium-based fluorescent films for high-sensitivity detection of iron ions <i>via</i> layered precursor anion exchange

In this study, based on the anion exchange strategy, a layered europium hydroxide (LEuH) film was used as a precursor to prepare luminescent films of sodium europium tungstate (NaEu(WO₄)₂) and sodium europium molybdate (NaEu(MoO₄)₂). By changing the concentration of anion sources (Na₂WO₄ and Na₂MoO₄), the structural evolution, morphological transformation and chemical composition changes in the interlayer and host layer during the process were elucidated, and the transformation mechanism of LEuH towards the 'dissolution-recrystallization' of the tungsten-molybdate film was revealed. It can be seen from the anion source concentration and reaction time that the exchange tendency of WO₄^2- is higher than that of MoO₄^2-. NaEu(WO₄)₂ and NaEu(MoO₄)₂ films have a stable tetragonal crystal structure and efficient luminescence properties (the luminescence intensity is 78 times and 109 times higher than that of LEuH, respectively), showing higher comprehensive potential than LEuH in Fe³⁺ luminescence detection. NaEu(WO₄)₂/NaEu(MoO₄)₂ films have inherent advantages as fluorescent probes due to their excellent luminescent properties, including Eu³⁺ characteristic red light emission with a peak at ∼617 nm. When applied to Fe³⁺ detection, Fe³⁺ ions compete with Eu³⁺ emission centers for energy absorption. This specific quenching effect enables the film to achieve rapid <i>in-situ</i> detection of Fe³⁺ through changes in luminescence intensity. Among them, the matrix CT band (∼250-300 nm) of NaEu(WO₄)₂ is highly overlapped with the strong absorption region (∼250-300 nm) of Fe³⁺, while the CT band (∼300-350 nm) of NaEu(MoO₄)₂ only overlaps with the weak absorption band (∼200-350 nm) of Fe³⁺. Therefore, the NaEu(WO₄)₂ film probe exhibits higher sensitivity and lower detection limit (<i>K</i>_SV: 2.825 × 10⁴ L mol^-1, LOD: 4.9 μmol L^-1), which is significantly better than the NaEu(MoO₄)₂ film (<i>K</i>_SV: 8.84 × 10³ L mol^-1, LOD: 7.3 μmol L^-1).