Molecular Imprinting-Based Ratiometric Fluorescence Nanosensor and Kit for Rapid and Visual Detection of Folic Acid

Molecular imprinting-based ratiometric fluorescence (MI-RFL) nanosensors receive increasing concerns in various fields such as food security, owing to high selectivity and sensitivity, nondestruction, convenience, visualization, etc. Herein, a ternary-emission MI-RFL nanosensor was facilely constructed by postimprinting mixing (PIM) red-emission molecularly imprinted polymers (r-MIPs) and green MIPs (g-MIPs) for rapid and visual detection of folic acid (FA) in food samples. The two fluorescent MIPs were prepared using a sol–gel surface imprinting one-pot method, with FA as template molecules, SiO2 nanoparticles as supporting materials, 3-mercaptopropionic acid-modified red CdTe quantum dots (CdTe QDs), and glutathione-modified green CdTe QDs as fluorescence sources, respectively. Under the optimal conditions, a good linearity ranging from 0.05 to 50 ppm was attained accompanied by rich color evolution, and limit of detection was down to 0.005 ppm. Then, FA tablets and milk powder were tested, and almost consistent FA contents with that of actual results were found, and recoveries were 89.39–103.43% with relative standard deviations less than 3.37%. Furthermore, the nanosensor-based kit was fabricated to analyze FA tablets by observing color change in 9 min, indicating kit's high accuracy and practicality. This study can provide a universal point-of-care testing method for on-site monitoring of targets in complicated matrices.