Naphthyridine-based symmetrical and unsymmetrical pyridinium amides in sensing of biotin salt

Two naphthyridine-based receptors have been designed and synthesised for biotin salt. The compounds serve as good hosts for the detection of biotin carboxylate rather than biotin ester. The correct dispositions of the binding groups under an isophthaloyl spacer enable the receptors to bind both the cyclic urea and the carboxylate ends simultaneously with moderate binding constant values. The receptors are effective for the binding of tetrabutylammonium salt of biotin with a concomitant increase in the fluorescence of naphthyridine and show appreciable binding of biotin salt in CH3CN containing 1.2% DMSO. The binding was monitored in CH3CN containing 1.2% DMSO and DMSO using 1H NMR, UV–vis and fluorescence spectroscopic methods.