Methyl Red: A Fluorescent Sensor for Hg²⁺ over Na⁺, K⁺ , Ca²⁺, Mg²⁺, Zn²⁺, and Cd²⁺

Methyl red (MR), a very well known acid base indicator (pK_a = 5.2), shows fluorescence emission in the range 320 nm to 480 nm, with an emission maximum at 375 nm on excitation by photons at 310 nm. A fluorescent intensity titration of MR by Hg²⁺ ion, a well known toxic heavy metal ion, shows quenching of the fluorescent intensity of MR. The quenching continues till the concentration ratio between MR and Hg²⁺ ion becomes 1:1. Similar titration with a number of other metal ions, namely Na⁺, K⁺, Ca²⁺, Mg²⁺, Zn²⁺, and Cd²⁺, has very small effect on the fluorescent intensity of MR. Thus MR acts as a selective fluorescent sensor for Hg2+ ion by selective fluorescence quenching. The efficiency of fluorescence quenching value is found to be 8.0. The binding ratio and binding constant (β) between MR and Hg²⁺ ion are calculated from the fluorescence and UV/visible spectral data, and are found to be 1:1 and log β =4.68. The Hg:MR complex was found to be stable in the pH range 5.5 to 8.2.