A Highly Selective and Efficient Copper(II) – “Turn-On” Fluorescence Imaging Probe for l-Cysteine

A simple copper(II) complex of the anthracenyl-appended terpyridine ligand 4′-(anthracen-9-yl)-2,2′:6′,2′′-terpyridine (L), [Cu(L)Cl₂] (1, ESI-MS, m/z = 507.08), is reported as a highly selective “turn-on” optical imaging probe for l-cysteine. Probe 1 shows a Cu^II/Cu^Iredox potential [E_1/2= –0.194 V versus normal hydrogen electrode (NHE)] within a biologically viable range. The copper center in 1 adopts a square-pyramidal geometry (τ = 0.0851), and the Cu–N_pybond (1.970 Å) of the middle pyridine (py) ring is shorter than the other two Cu–N_pybonds (2.069 and 2.040 Å). The Cu–Cl bonds (2.444 and 2.027 Å) are labile enough to be replaced by solvent molecules. The square-based geometry is further supported by the A_∥value of 156.8 × 10^–4cm^–1determined by electron paramagnetic resonance (EPR) spectroscopy at 70 K. The d–d transition of 1 in acetonitrile/4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (acetonitrile/HEPES) buffer at pH 7.34 appears at λ =760 nm (ε = 198 m^–1cm^–1), and the ligand-based transitions are observed at λ = 327–384 nm. The ligand shows strong fluorescence at λ = 430 nm with a quantum yield of 11 %, but its emission intensity is completely quenched on coordination with Cu²⁺ions, as in 1. Probe 1 detects l-cysteine selectively through turn-on fluorescence intensity at λ_em= 431 nm with a limit of detection of 1.9 × 10^–8m at pH 7.34. Interestingly, probe 1 is able to visualize exogenously added l-cysteine in Henrietta Lack cervical cancer (HeLa) cells, human liver hepatocellular carcinoma (HepG2) cells, and human embryonic kidney 293 (HEK293) cells under identical conditions at pH 7.4 in confocal microscopy imaging.