Fluorescent color analysis of ascorbic acid by ratiometric fluorescent paper utilizing hybrid carbon dots-silica coated quantum dots

Monitoring the concentration of ascorbic acid (AA) in biological samples is essential to food quality and healthcare. Various fluorescence methods widely reported for the detection of AA are usually based on turn-on or turn-off principle. However, few ratiometric fluorescent sensors were reported for AA determination. Additionally, the application of ratiometric fluorescence probe is usually limited by the narrow range of color changes and insensitivity color change of the AA concentration. Herein, a novel ratiometric fluorescent test paper to achieve the continuous color changes from red to blue was developed for the fluorometric qualitative and quantitative determination of AA. The nanoprobe consists of blue-emitting carbon dots (C-dots) and red-emitting silica coated quantum dots (QDs@SiO2) with certain ratiometric fluorescence intensity of blue to red. The fluorescence of blue-emitting CDs (peaking at 450 nm) is quenched by Fe3+, then the blue fluorescence is restored consecutively with the gradual increasing of AA concentration due to the reduction from Fe3+ to Fe2+ by using AA. While the fluorescence of red-emitting QD@SiO2 (peaking at 630 nm) works as a stable internal standard. Therefore, a distinguish color transition from red to blue can be observed by naked eyes intuitively with the increasing of AA concentration. By calculating the ratiometric fluorescent intensity at 450 nm and 630 nm or observing the color of the mixture solution by naked eyes, the concentration of AA can be detected easily and accurately. Under optimized conditions, the fluorescent intensity ratio (I450/I630) shows linear relationship with the concentration of AA from 0 to 70 μM with the detection limit as low as 3.17 μM. Finally, a ratiometric fluorescent sensor paper was further developed for AA detection with satisfactory results in fruit juice. The method proposed here gives the wide applications of fluorescent test paper in food and biological assays. © 2020 Elsevier Ltd