Determination of ascorbic acid and ascorbate oxidase based on quaternary CuInZnS QDs/thiochrome ratiometric...

In this work, a rapid and sensitive ratiometric fluorescence sensing method for the detection of ascorbic acid (AA) and ascorbate oxidase (AA-Ox) was developed. MPA capped quaternary CuInZnS QDs with fluorescence emission wavelength of 580 nm was synthesized by hydrothermal synthesis. Thiamine (VB1) can be oxidized by KMnO₄ to yield thiochrome (oxVB1) with a fluorescence emission peak at 459 nm. So positively charged thiamine and negatively charged MPA modified CuInZnS QDs can form a ratiometric fluorescence system via electrostatic interaction. The presence of AA will consume KMnO₄, which can inhibit the oxidization of thiamine, accompanied with a decrease of the fluorescence intensity at 459 nm, while the fluorescence of CuInZnS QDs at 580 nm remains the same when AA is present, so the fluorescence intensity ratio (I₄₅₉/I₅₈₀) decreased. When AA was oxidized by AA-Ox, KMnO₄ could not be reduced by AA, which caused the fluorescence intensity of thiochrome at 459 nm restored, and the fluorescence intensity ratio (I₄₅₉/I₅₈₀) increased again. Therefore, AA and AA-Ox can be monitored by detecting the fluorescence intensities ratio at 459 and 580 nm. Moreover, the developed detection system for AA and AA-Ox displayed a good linear relationship from 0.05 to 0.25 μmol L^-1 and 0.1-5 mU·mL^-1, and the detection limit are 0.011 μmol L^-1 and 0.078 mU·mL^-1, respectively. The proposed method was applied to the determination of AA-Ox in human serum samples with satisfactory results.