Fluorescence Ratio Nanoprobe Consisting of a Carbon Nanodots-Quantum Dots Composite for Visual Detection of Folic Acid in Dry Milk Powders

Folic acid (FA) is a vitamin essential for human physiology; thus, rapid, accurate, and sensitive methods for its detection and quantification in different biological samples are necessary. A fluorescent ratiometric nanoprobe (the complex composed of quantum dots and carbon dots) based on carboxyl-modified quantum dots (QDs) and amino-modified graphene-based carbon dots (CDs) has been developed for the visual determination of folic acid (FA). The nanoprobe is the composition in which QDs were covalently bonded with CDs. Red and blue fluorescence of nanoprobe can be selectively quenched and enhanced by FA, respectively. Thus, the nanoprobe is capable to generate clearly visible fluorescence color change from pink to blue upon exposure to UV light. Depending on the color change and its intensity, FA can be identified, and their concentrations can be determined. The sensor prepared based on the nanoprobe demonstrated good selectivity and sensitivity toward FA with exceptionally low detection limit equal to 0.039 μM. We also prepared a paper-based sensor to demonstrate convenient and simple approach to visually detect FA. Thus, the nanoprobe demonstrated feasibility of rapid real-time quantitative FA detection; considering its simple, effective, and practicality, expectedly it can be used for practical application.