Medium Dependent Dual Turn-On/Turn-Off Fluorescence System for Heavy Metal Ions Sensing

Abstract

A dual turn-on/turn-off fluorescence sensing system based on <i>N</i>-alkylaminopyrazole ligands for heavy metal ions, where the response can be tuned upon medium change, is developed. The synthesis and characterization of Zn^II, Cd^II and Hg^II complexes with two <i>N</i>-alkylaminopyrazole ligands, used as metal receptors, are first presented. The acidity and complexation constants for a selected ligand (1-[2-(octylamino)ethyl]-3,5-diphenylpyrazole ligand (<b>L2</b>)) with Zn^II, Cd^II, and Hg^II are also determined. The fluorescent behavior of these complexes can be tuned by the different medium used (e.g., MeOH or HCl) giving rise to two different sensing mechanisms. The <b>L2</b> ligand can be applied as a global heavy metal warning chemosensor (for Pb^II, Zn^II, Cd^II, or Hg^II ions) for water samples achieving detection limits lower than the maximum concentration recommended by the environmental agencies (detection limit lower than 0.3 ng/mL for any of the mentioned metal ions). The utility of the developed sensing system for Hg^II detection in seawater without any previous sample pretreatment with interest for future in-field sensing kit like applications is also discussed