Metal triggered fluorescence sensing of citrate using a synthetic receptor

A metal containing fluorescent chemosensor was designed, synthesized, and studied for the quantification of citrate in common beverages. The sensor consists of Cu(II) bound by a 1,10-phenanthroline ligand which is attached to a bis(aminoimidazolium) receptor (5). Receptor 5 was designed such that binding of the metal creates an additional binding site for citrate. This additional binding interaction was found to increase the metal and citrate binding constants in a cooperative manner, yielding a minimum 2.0 fold increase in the citrate binding constant and a minimum 2.0 fold increase in the Cu(II) binding constant. Further, 5 was designed so that binding of Cu(II) quenches a photo-excited state of the 1,10-phenanthroline fluorophore. Thus, addition of citrate to 5-Cu(II) resulted in an increase of the fluorescence of the system. The nature of the fluorescence modulation upon citrate binding was probed using a model compound (6-Cu(II)). The data support an increase of electron density on the metal due to the donating ability of a carboxylate anion of citrate. In a sensing assay, the receptor is effective for measuring citrate concentrations in the micromolar range in highly competitive media. We believe this is the first demonstration of anion sensing in which the fluorescence emission is modulated due to a perturbation in the metal quenching effect upon analyte binding.