Zn2+ fluorescent chemosensors and the influence of their spacer length on tuning Zn2+ selectivity

Abstract

Zn²⁺ fluorescent chemosensors based on the chelation-enhanced fluorescence (CHEF) mechanism were prepared and fine-tuning of their Zn²⁺ selectivity was achieved by control of the spacer length. The fluorescence emission response to metal ions shows a fluorescence decrease for Co²⁺, Cu²⁺ and Ni²⁺ and an increase for Ca²⁺, Cd²⁺ and Zn²⁺. Maximum Zn²⁺ selectivity over Ca²⁺ and Cd²⁺ was achieved with 3. The basic binding properties (stoichiometry, apparent dissociation constant, and pH dependence) were measured by fluorescence spectroscopy. Analysis of the Job plot for the 3–Zn²⁺ complex indicates the formation of a 1 ∶1 complex. The apparent dissociation constant K_d was determined as 0.98 (±0.43) nM for 3–Zn²⁺ at pH 7.5 [50 mM HEPES buffer I = 0.1 (NaNO₃)]. The plateaus in the fluorescence intensity of 2 and the 2–Zn²⁺ complex in regions with a pH exceeding 7 imply that the fluorescence measurements should be little affected by physiological pH changes.