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 Cr2O72- 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 emissions from Tb3+ ions are selectively quenched by the addition of Cr2O72- 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-, PO32-, SO42-, VO3-, WO42-, IO3-, ClO4-,) had minimal impact on the luminescence intensity of Tb3+ ions. The calculated limit of detection from the experimental results (for the 3/slope criterion) is 77 nM and 55 nM for K2Cr2O7 and KMnO4, 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.

4-Mercaptobenzoic acid capped terbium(III)-doped CaF2 nanocrystals: a fluorescent probe for nitroaromatic pollutants.

The authors report on an energy transfer based fluorometric approach for the detection of nitroaromatic pollutants. This is achieved using 4-mercaptobenzoic acid (4-MBA)-capped CaF2:Tb3+ nanocrystals that were synthesized by a microwave procedure. 4-MBA acts as both a capping agent and a sensitizer for the Tb3+ ions in CaF2 host matrix. This approach is different from the earlier studies where Ce3+ is generally used as the sensitizer for the Ln3+ ions. The use of capping ligand as sensitizer has the feature that binding of nitroaromatics directly to the sensitizer can alter the energy transfer efficiency between the sensitizer and the Tb3+ ions. The fluorescent nanocrystal probe doped with 2% of Tb3+ displays green emission with a peak at 542 nm if photoexcited at 311 nm. The emission is quenched if the nanocrystals are exposed to nitroaromatic compounds such as 4-nitrophenol, 2,4-dinitrophenol, 2,4,6-trinitrophenol (picric acid), 4-nitrotoluene, 2,4-dinitrotoluene and 2,4,6-trinitrotoluene. These analytes also cause a (longwave/shortwave) shift in the excitation maxima which helps in identifying the individual nitroaromatic compound using single nanoprobe. The respective detection limits (by applying the 3σ/K criterion) are 0.86 µM, 0.83 µM, 0.78 µM, 0.36 µM, 1.5 µM, and 1.96 µM. Graphical abstract Schematic illustration of the use of 4-mercaptobenzoic acid (MBA)-capped CaF2:Tb3+ nanocrystals as a fluorescent nanoprobe for the detection of nitroaromatic analytes. The Tb3+ ions show strong green fluorescence via 4-MBA-induced ligand sensitization. The specific π interaction between 4-MBA capped CaF2 nanocrystals and nitroaromatics leads to reduction in the fluorescence intensity by inhibiting the energy transfer from 4-MBA to Tb3+ ion in CaF2 nanocrystals.