Turn-on fluorescence ferrous ions detection based on MnO₂ nanosheets modified upconverion nanoparticles

A turn on upconversion fluorescence probe based on the combination of ~32 nm NaYF₄: Yb/Tm nanoparticles and MnO₂ nanosheets has been established for rapid, sensitive detection of Fe²⁺ ions levels in aqueous solutions and serum. X-ray diffraction (XRD), transmission electron microscopy (TEM), absorption and emission spectra have been used to characterize the crystal structure, morphology and optical properties of the samples. MnO₂ nanosheets on the surface of UCNPs act as a fluorescence quencher, resulting in the quenching of the blue fluorescence (with excitation/emission maximum of 980/476 nm) via fluorescence resonance energy transfer from upconversion nanoparticles to MnO₂ nanosheets. With the adding of Fe²⁺, upconversion fluorescence of the nanocomposites recovers due to the reduction of MnO₂ to Mn²⁺. Because of the low background of the probe offered by upconversion fluorescence, this probe can be used for detecting Fe²⁺ in aqueous solutions in the range of 0.1-22 μM with detection limit of 0.113 μM. The developed method has also been applied to detect 10 μM Fe²⁺ ions in serum with recoveries ranging from 97.6 to 105.3% for the five serum samples. Significantly, the probe shows fast response and stable signal, which is beneficial for long-time dynamic sensing. Thus, the proposed strategy holds great potential for disease diagnosis and treatment.