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

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 KMnO 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 KMnO, 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 (I/I) decreased. When AA was oxidized by AA-Ox, KMnO could not be reduced by AA, which caused the fluorescence intensity of thiochrome at 459 nm restored, and the fluorescence intensity ratio (I/I) 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 and 0.1-5 mU·mL, and the detection limit are 0.011 μmol L and 0.078 mU·mL, respectively. The proposed method was applied to the determination of AA-Ox in human serum samples with satisfactory results.