Synthesis of Europium based Fluorescent Films for High Sensitivity Detection of Iron Ions via Layered Precursor Anion Exchange

In this study, based on the anion exchange strategy, layered europium hydroxide (LEuH) film was used as a precursor to prepare luminescence 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 change of the interlayer and host layer during the process were clarified, and the transformation mechanism of LEuH to the ' dissolution-recrystallization ' of tungsten-molybdate film was revealed. It can be seen from the anion source concentration and reaction time that the exchange tendency of WO42- is higher than that of MoO42-. NaEu(WO4)2 and NaEu(MoO4)2 films have stable tetragonal crystal structure and efficient luminescence properties (the fluorescence intensity is 78 times and 109 times that of LEuH, respectively), showing higher comprehensive potential than LEuH in Fe3+ fluorescence detection. Their excellent luminescent properties, Eu3+ characteristic red light emission (~617 nm) and Fe3+ specific fluorescence quenching make them directly used as fluorescent probes to achieve in-situ rapid detection of Fe3+.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 (KSV : 2.825 × 104 L/mol, LOD: 4.9 μmol/L), which is significantly better than the NaEu(MoO4)2 film (KSV : 8.84 × 103 L/mol, LOD: 7.3 μmol/L).