A fluorescent probe for sequential sensing of MnO₄^- and Cr₂O₇^2- ions in aqueous medium based on a UCNS/TMB nanosystem

This paper reports the successful design of an UC fluorescence nanosystem for distinguishable and sequential detection of MnO4- and Cr2O72- based on the inner filter effect (IFE) between Gd2O3:Yb,Er upconversion nanospheres (UCNS) and 3,3',5,5'-tetramethylbenzidine (TMB). The detection of MnO4- was realized by a stoichiometric oxidation reaction between TMB and Mn(vii), which resulted in quenching of the upconversion luminescence (UCL) by the blue product through IFE. The assay of Cr2O72-, in contrast, was based on the formation of complexes and the consumption of the oxidation product of the previous reaction. The sensing of MnO4- did not interfere with that of Cr2O72- when Pb2+ was added as a masking agent. This provides a new strategy for sensitive and selective detection of MnO4- and Cr2O72-. The detection of MnO4- showed not only a low LOD of 0.243 μM but also high selectivity over other possible interfering ions. The sensitivity assay of MnO4- can be performed in the linear range of 1.85 to 69 μM. An excellent linear relationship was also found in the sensing of Cr2O72-, with a linear range of 18.3 to 250 μM and a LOD of 2.95 μM. The selectivity towards Cr2O72- was also proved to be good. Based on these properties, the UCNS-TMB nanoplatform is a potential chemosensor for both MnO4- and Cr2O72- ions in practical assays for waste water.