Differential detection of Zn2+ and Cd2+ ions by BODIPY-based fluorescent sensors

Two monostyryl BODIPY derivatives that contain one or two bis(hydroxyamido)amino group(s) as the metal chelator have been prepared. The effects of various metal ions on their electronic absorption and fluorescence properties have been studied in detail in MeCN or in phosphate buffered saline (PBS). The results show that the derivative with two hydroxyamide chains can selectively detect Zn(2+) ions in MeCN. The compound and ions bind in a 1:1 stoichiometry with an association constant of 2.2(±0.1)×10(4)  M(-1). The intensity of the fluorescence emission increases remarkably and is substantially blue-shifted from 624 to 572 nm, owing to the inhibition of intramolecular charge transfer, thus allowing its use as a ratiometric fluorescent sensor for Zn(2+) ions. The derivative with four hydroxyamide chains behaves differently: It responds selectively toward Cd(2+) ions in phosphate buffered saline. The compound and ions bind in a 1:2 stoichiometry, with first and second association constants of 4.4(±0.9)×10(4)  M(-1) and 1.3(±0.1)×10(4)  M(-1), respectively. Upon the addition of 80 equivalents of Cd(2+) ions, the fluorescence quantum yield increases 15-fold. Both of these compounds exhibit differential sensing of Zn(2+) and Cd(2+) ions and the associated color changes can be easily seen by the naked eye.