RESUMO
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.
RESUMO
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.