A dual turn-on/turn-off fluorescence sensing system based on N-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 N-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 (L2)) 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 L2 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. © 2011 American Chemical Society.