A Highly Sensitive and Selective Fluorescein-Based Cu Probe and Its Bioimaging in Cell.

Copper is a vital trace metal in human body, which plays the significant roles in amounts of physiological and pathological processes. The application of copper-selective probe has attracted great interests from environmental tests to life process research, yet a few of sensitive Cu tests based on on-site analysis have been reported. In this paper, a novel fluorescein-based fluorescent probe N4 was designed, synthesized, and characterized, which exhibited high selectivity and sensitivity to Cu comparing with other metal ions in ethanol-water (1/1, v/v) solution. The probe N4 bonded with Cu to facilitate the ring-opening, and an obvious new band at 525 nm in the fluorescence spectroscopy appeared, which could be used for naked-eye detection of Cu within a broad pH range of 6-9. Meanwhile, a good linearity between the fluorescence intensity and the concentrations of Cu ranged 0.1-1.5 eq. was observed, and the limit of detection of N4 to Cu was calculated to be as low as 1.20 μm. In addition, the interaction mode between N4 and Cu was found to be 1:1 by the Job's plot and mass experiment. Biological experiments showed that the probe N4 exhibited low biological toxicity and could be applied for Cu imaging in living cells. The significant color shift associated with the production of the N4-Cu complex at low micromolar concentrations under UV light endows N4 with a promising probe for field testing of trace Cu ions.