Easy-to-synthesize N-alkylaminopyrazole ligands are used in a proof-of-concept fluorescent sensor that can operate as a simple 'paper test' for rapid and highly sensitive mercury detection in water. A significant enhancement of fluorescence emission in aqueous media for the Hg2+ detection at concentrations lower than 0.1 ppb is achieved. A linear range from 10 to 100 ppb [Hg2+] was obtained in the paper-based system. The respective Turn-ON and Turn-OFF fluorescence mechanisms in the paper test and water solution (pH 1) systems are also shown. © The Royal Society of Chemistry 2012.