Quinoline derivatives as fluorescent probes for zinc determination in living cells

Fluorescence sensors are one of rapid technics for metal cation determination in cells, tissues and biological fluids. 8-Hydroxyquinoline (8HQ) derivatives have possess favorable photophysical and coordination properties for developing fluorescent metal sensors. 8HQ derivatives 2-(4-methoxystyryl)quinolin-8-ol (STQ-Me), 2-(4-ethoxyphenylethenyl)quinolin-8-ol (STQ-Et) and 2-(4-(trifluoromethyl)styryl)quinolin-8-ol (STQ-CF3) were investigated. Absorbance and fluorescence spectra of 50 µM 8HQ derivatives in saline were evaluated in the presence of Mg2+, Ca2+, and Zn2+. The investigations were carried out on epidermal carcinoma of the mouth KB cell line. Among the studied compounds, STQ-Me demonstrates sufficient stability in saline and appears to be able to bind Zn2+ with presumably some selectivity in the presence of Ca2+ and Mg2+. The fluorescence intensity of STQ-Me solution increased with rising Zn2+ concentration in the presence of Ca2+ and Mg2+. Compounds STQ-Et and STQ-CF3 demonstrated either weak divalent cation selectivity, poor stability and a pronounced tendency to aggregate in saline. In live KB cells incubated with ZnO NPs, STQ-Me produced a zinc-dependent fluorescence pattern similar to the reference probe TSQ. However, STQ-Me was likely unable to chelate protein-bound zinc, as evidenced by the dynamic fluorescence parameters of KB cells. Overall, STQ-Me demonstrates potential as a fluorescent probe for monitoring labile zinc in living cells, but further molecular optimization is required to enhance fluorescence output and improve binding to protein-associated zinc pools.