3,5-Dihydroxy Benzoic Acid-Capped CaF₂:Tb³⁺ Nanocrystals as Luminescent Probes for the WO₄ ^2- Ion in Aqueous Solution

We report a facile and effective luminescence method for the determination of the WO₄ ^2- ion in aqueous medium at initial pH = 6.3. This is achieved using 3,5-dihydroxybenzoic acid-capped CaF₂:Tb³⁺ (5%) nanocrystals (NCs) as a luminescent probe. This is accomplished based on the energy transfer luminescence from the WO₄ ^2- ion to the Tb³⁺ ion in small-size CaF₂:Tb³⁺ NCs. Hydroxyl groups on the surface ligand helps in binding the tungstate ion to the surface of the NCs. With the gradual addition of the WO₄ ^2- ion, the intensity of the Tb³⁺ excitation and emission spectra significantly increased. The linear range of the detection was from 1 to 10 μM for the WO₄ ^2- ion (R ² = 0.99). The calculated detection limit was 0.4 μM (by applying the 3σ/K criterion).