o-Pah (20 μM) changes color upon the addition of 100 μM various metal ions, with a further addition of 20 μM Cu 2+ in a DMSO/HEPES buffer solution [1]. Procedures for metal ion sensing: stock solutions of metal ions (100 mM and 10 mM) were prepared in deionized water [1]. A stock solution of o-Pah (0.5 mM) is prepared in DMSO and then diluted to 5 μM with HEPES buffer solution (50 mM KNO3, 50 mM HEPES, pH 7.2). During titration experiments, 2 mL solutions of o-Pah (5 μM, 0.5 μM) are placed in a 1 cm quartz optical cell, and Cu 2+, Hg 2+ stock solutions are added gradually with a micropipette. In selectivity experiments, test samples are prepared by adding appropriate amounts of the metal ion stock solution into the 2 mL solution of o-Pah (5 μM). During fluorescence measurements, the excitation wavelength is 483 nm, and emission spectra are collected between 495-600 nm.