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

Eu-containing polyoxometalate K₁₃Eu(SiMoW₁₀O₃₉)₂·28H₂O (Eu-SiMoW) owns the stimu-chromic and photoluminescence properties. An ingenious test of ascorbic acid (AA) and sodium nitrite (NaNO₂) 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 NaNO₂ made Eu-SiMoW back to its original pale yellow state with red luminescence. Accordingly, the content of AA and NaNO₂ 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 NaNO₂. The proposed method exhibited a linear response range (LRR) from 0.1 to 0.9 mmol L^-1 with a limit of detection (LOD) of 0.53 µmol L^-1 in UV-vis spectra and 4.67 µmol L^-1 in luminescence spectra for AA as well as a LRR from 0.05 to 0.4 mmol L^-1 with a LOD of 1.16 µmol L^-1 in UV-vis spectra and 5.39 µmol L^-1 in luminescence spectra for NaNO₂. 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.