Development of a zinc ion-selective luminescent lanthanide chemosensor for biological applications

Detection of chelatable zinc (Zn²⁺) in biological studies has attracted much attention recently, because chelatable Zn²⁺ plays important roles in many biological systems. Lanthanide complexes (Eu ³⁺, Tb³⁺, etc.) have excellent spectroscopic properties for biological applications, such as long luminescence lifetimes of the order of milliseconds, a large Stoke's shift of >200 nm, and high water solubility. Herein, we present the design and synthesis of a novel lanthanide sensor molecule, [Eu-7], for detecting Zn²⁺. This europium (Eu³⁺) complex employs a quinolyl ligand as both a chromophore and an acceptor for Zn²⁺. Upon addition of Zn²⁺ to a solution of [Eu-7], the luminescence of Eu³⁺ is strongly enhanced, with high selectivity for Zn²⁺ over other biologically relevant metal cations. One of the important advantages of [Eu-7] is that this complex can be excited with longer excitation wavelengths (around 340 nm) as compared with previously reported Zn²⁺-sensitive luminescent lamthanide sensors, whose excitation wavelength is at too high an energy level for biological applications. The usefulness of [Eu-7] for monitoring Zn²⁺ changes in living HeLa cells was confirmed. This novel Zn²⁺-selective luminescent lanthanide chemosensor [Eu-7] should be an excellent lead compound for the development of a range of novel luminescent lanthanide chemosensors for biological applications.