A fluorescent probe for sequential sensing of MnO4− and Cr2O72− ions in aqueous medium based on a UCNS/TMB nanosystem

Abstract

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.