A Highly Sensitive and Selective Fluorescein-Based Cu(2+) 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(2+) 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(2+) comparing with other metal ions in ethanol–water (1/1, v/v) solution. The probe N4 bonded with Cu(2+) 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(2+) within a broad pH range of 6–9. Meanwhile, a good linearity between the fluorescence intensity and the concentrations of Cu(2+) ranged 0.1–1.5 eq. was observed, and the limit of detection of N4 to Cu(2+) was calculated to be as low as 1.20 μm. In addition, the interaction mode between N4 and Cu(2+) 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(2+) imaging in living cells. The significant color shift associated with the production of the N4-Cu(2+) complex at low micromolar concentrations under UV light endows N4 with a promising probe for field testing of trace Cu(2+) ions.