Study of Bi-directional detection for ascorbic acid and sodium nitrite based on Eu-containing luminescent polyoxometalate.

Eu-containing polyoxometalate KEu(SiMoWO)·28HO (Eu-SiMoW) owns the stimu-chromic and photoluminescence properties. An ingenious test of ascorbic acid (AA) and sodium nitrite (NaNO) was carried out based on the dual properties of Eu-SiMoW in solutions. First, the redox reaction of Eu-SiMoW and AA generated the blue reduced Eu-SiMoW, accompanied by fluorescence quenching; then the redox reaction of the reduced Eu-SiMoW and NaNO made Eu-SiMoW back to its original pale yellow state with red luminescence. Accordingly, the content of AA and NaNO could be measured by the reversible change of color and luminescence of Eu-SiMoW. This bi-directional detection method is first discovered and proven to be a simple and effective method for the detection of AA and NaNO. The proposed method exhibited a linear response range (LRR) from 0.1 to 0.9 mmol L with a limit of detection (LOD) of 0.53 µmol L in UV-vis spectra and 4.67 µmol L in luminescence spectra for AA as well as a LRR from 0.05 to 0.4 mmol L with a LOD of 1.16 µmol L in UV-vis spectra and 5.39 µmol L in luminescence spectra for NaNO. Moreover, the fluorescence switching of Eu-SiMoW could be realized by reacting with reductant and oxidant through the redox reaction. The detection mechanism is considered as a fluorescence resonance energy transfer process between discolor component SiMoW and luminescence component Eu in Eu-SiMoW.