Zinc selective chemosensor based on pyridyl-amide fluorescence

Chemosensors are developed to image zinc ions. Fluorescence enhancement due to Zn²⁺ binding is an excellent way to detect its presence. A chemosensor for Zn²⁺ based on dipicolylamine (DPA) groups connected by a pyridyl amide backbone has been synthesized. Addition of 2-chloroacetyl chloride to 2,6-diaminopyridine affords 2,6-bis(chloroethylamido)pyridine, which is converted to the sensor BADPA-P by 2,2′-dipicolylamine displacement of chlorine. This compound along with two others, the mono-DPA, ADPA-P and the benzyl in place of pyridyl, BADPA-B, present three potential Zn²⁺ sensors. It was found that BADPA-P in the presence of Zn²⁺ shows a large increase in fluorescence, whether in polar organic or aqueous environments. Its fluorescence in the presence of Cd²⁺, unlike with Zn²⁺, is not enhanced when excited at longer wavelengths. Proton NMR measurements, indicate two Zn²⁺ ions bind to BADPA-P. Also, Zn²⁺ enhances fluorescence even when other metal ions are present.