A Switchable Fluorescent Sensor Based on NH₂-MIL-53(Al) for Detection of Ascorbic Acid in Fresh Fruits

The detection of ascorbic acid (AA) in foods is important for ensuring nutritional quality and monitoring the stability and shelf life of food products. In the present work, the amino-based metal-organic framework NH₂-MIL-53(Al) was used for highly sensitive and selective detection of AA. The NH₂-MIL-53(Al) nanocrystals were synthesized using a facile solvothermal method and characterized using different techniques. It showed a strong blue fluorescence emission at 440 nm at the excitation wavelength of 330 nm. It was found that Fe³⁺ had an exceptional fluorescence quenching effect on NH₂-MIL-53(Al), resulting from the formation of a non-fluorescent complex between NH₂-MIL-53(Al) and Fe³⁺. Upon the addition of AA, the quenched fluorescence of NH₂-MIL-53(Al)/Fe³⁺ was successfully restored due to the reduction of Fe³⁺ to Fe²⁺ by AA. Therefore, this switchable ("on-off-on") sensor was applied for the detection of AA in a linear range of 0-75 µM with a detection limit (LOD) of 280 nM. The sensor showed excellent specificity towards AA against several other substances including metal ions and amino acids. Furthermore, the developed sensor was successfully validated for AA detection in fresh fruits with good recoveries (96-105%). Hence, NH₂-MIL-53(Al) proves to be highly useful in determining AA content during food processing in a rapid, sensitive, and traceable manner.