Development of a Fluorescein-Based Probe with an "Off-On" Mechanism for Selective Detection of Copper (II) Ions and Its Application in Imaging of Living Cells.

Copper is a common metallic element that plays an extremely essential role in the physiological activities of living organisms. The slightest change in copper levels in the human body can trigger various diseases. Therefore, it is important to accurately and efficiently monitor copper ion levels in the human body. Recent studies have shown that fluorescent probes have obvious advantages in bioimaging and Cu detection. Therefore, a novel Cu probe (N2) was designed and synthesized from fluorescein, hydrazine hydrate and 5-p-nitrophenylfurfural that is sensitive to and can detect Cu within 100 s. The response mechanism of the N2 probe to Cu was studied by several methods such as Job's plots and MS analysis, which showed that the Cu and the N2 probe were coordinated in a complexation ratio of 1:1. In addition, compared with other cations investigated in this study, the N2 probe showed excellent selectivity and sensitivity to Cu, exhibiting distinct fluorescence absorption at 525 nm. Furthermore, in the equivalent range of 0.1-1.5, there is a good linear relationship between Cu concentration and fluorescence intensity, and the detection limit is 0.10 μM. It is worth mentioning that the reversible reaction between the N2 probe and Cu, as well as the good biocompatibility shown by the probe in bioimaging, make it a promising candidate for Cu biosensor applications.