Determination of 2, 6-dipicolinic acid as an Anthrax biomarker based on the enhancement of copper nanocluster fluorescence by reversible aggregation-induced emission

The weak fluorescence efficiency of copper nanoclusters (Cu NCs) limits their wide applications in biosensing and bioimaging areas, while the aggregation-induced emission (AIE) effect is anticipated to increase their luminescence intensity. Herein, the weak red emission of Cu NCs is increased considerably by the addition of lanthanide Tb 3+ , ascribed to the AIE effect. Monitoring of spores contamination can be carried out by determining the level of 2, 6-dipicolinic acid (DPA), which is a marker of spores. Due to the stronger synergy between DPA and Tb 3+ for its clamped configuration of adjacent pyridine nitrogen group with the carboxylic acid group, the addition of DPA leads Tb 3+ to be taken away from Cu NCs through a stronger coordination effect, causing Cu NCs to return to the dispersed state and weakened fluorescence. Based on this, an "off–on-off" fluorescent probe for DPA sensing was built, in which Tb 3+ was used as a bridge to achieve AIE enhanced fluorescence effect on Cu NCs as well as a specific recognizer of DPA. The detection range for DPA was 0.1–60 μM and the detection limit was 0.06 μM, which was much lower than the infectious dose of anthrax spores. Since DPA is a unique biomarker for bacterial spores, the method was applied to the detection of actual bacterial spores and satisfactory results were obtained with a detection limit of 4.9*10 3  CFU mL −1 . Graphical Abstract