On-off-on fluorescent nanosensing: Materials, detection strategies and recent food applications

Background: The detection of food quality and safety is important in the whole food supply chain. In recent years, fluorescent detection technology has been extensively applied in food analysis. In particular, nanomaterial-based "on-off-on" fluorescent sensors have received much attention because of their high sensitivity, wide detection range, strong anti-interference ability, and flexible design. Nevertheless, there is no review focusing on the "onoff-on" fluorescent sensors for food analysis.Scope and approach: In the current review, several common nanomaterials are introduced, including quantum dots (QDs), carbon dots (CDs), graphene quantum dots (GQDs), and metal nanoclusters (MNCs). Main detection strategies of "on-off-on" fluorescent sensors are elaborated, involving redox reaction, competitive reaction, DNA hybridization reaction and enzymatic reaction. In addition, recent applications of "on-off-on" fluorescent sensors for trace level detection of nutrients, food additives, drug residues, pesticide residues, foodborne pathogenic bacterias, mycotoxins, and heavy metal ions are also summarized. Finally, advantages, challenges and future prospects for developing the "on-off-on" fluorescent sensors are presented.Key findings and conclusions: The "on-off-on" fluorescent sensors exhibit a wide detection range, strong antiinterference ability, and excellent linear responses to various analytes. However, current detection applications are still at the laboratory stage, commercial and industrial applications are rarely reported. Future research should focus on developing high-throughput detection techniques, and portable or visual fluorescent sensors, including kits and filter papers based sensors. Apps on smartphones should also be developed in future work.