Selective Detection of Chromate and Permanganate Ions Using Gallic Acid Capped CaF₂:Tb³⁺ Nanocrystals

In this study, we have developed ligand-sensitized Ln³⁺-doped nanocrystals (NCs) for the selective sensing of Cr₂O₇ ^2- and MnO₄ ^- ions in nanomolar concentrations. This is accomplished with the gallic acid capped-CaF₂:Tb³⁺ NCs. These NCs display bright green emission through an efficient energy transfer from surface functionalized gallic acid molecules to Tb³⁺ ions upon UV light excitation. The luminescence from Tb³⁺ ions are selectively quenched by the addition of Cr₂O₇ ^2- and MnO₄ ^- anions. The reduction in the luminescence intensity is found to be quite selective, as the addition of other strong oxidizing species (I^-, F^-, Br^-, Cl^-, PO₃ ^2-, SO₄ ^2-, VO₃ ^-, WO₄ ^2-, IO₃ ^-, ClO₄ ^-,) had minimal impact on the luminescence intensity of Tb³⁺ ions. The calculated limit of detection from the experimental results (for the 3σ/slope criterion) is 77 nM and 55 nM for K₂Cr₂O₇ and KMnO_4, respectively. The findings show that tuning the resonance energy transfer (RET) between analytes and Tb³⁺ inside the NCs serves as a tool for the detection of dichromate and permanganate ions selectively.