Metal triggered fluorescence sensing of citrate using a synthetic receptor

A metal containing fluorescent chemosensor was designed, synthesized, and s tudied for the quantification of citrate in common beverages. The sensor co nsists 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 bi nding of the metal creates an additional binding site for citrate. This add itional binding interaction was found to increase the metal and citrate bin ding constants in a cooperative manner, yielding a minimum 2.0 fold increas e 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) qu enches 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 bind ing was probed using a model compound (6-Cu(II)). The data support an incre ase of electron density on the metal due to the donating ability of a carbo xylate anion of citrate. In a sensing assay, the receptor is effective for measuring citrate concentrations in the micromolar range in highly competit ive media. We believe this is the first demonstration of anion sensing in w hich the fluorescence emission is modulated due to a perturbation in the me tal quenching effect upon analyte binding.