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

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.