Colorimetric detection of Hg2+ and CH3Hg+ by a novel spirooxazine derivative as a highly sensitive and selective probe

Abstract

6'-(indolin-1-yl)-1-(4-methoxybenzyl)-3,3-dimethylspiro[indoline-2,3′-naphtho[2,1-b][1,4]oxazine] (SPO), a novel colorimetric probe in a spirooxazine class, was successfully synthesized. Its molecular structure was confirmed by the single crystal and spectroscopic analyses. Upon UV activation, SPO in a 1,4-dioxane solution possessed superior selectivity and sensitivity for colorimetric detection of CH3Hg+ over other cations, including Hg2+. The results from the absorption study showed that the mixtures of SPO and Hg2+ or CH3Hg+ exhibited a new absorption band at 590 nm. This indicates the structural isomerization of SPO into an open-form merocyanine which acts as a ligand to form an SPO-Hg2+ or SPO-CH3Hg+ complex. The stoichiometric ratio of SPO to each mercuric cation in the complexes was determined using Job's method and the results suggested that the complexes were formed at a 1:1 mol ratio. The detection limit of Hg2+ and CH3Hg+ in 1,4-dioxane was investigated and the results showed the values at nM level (152 nM for Hg2+ and 150 nM for CH3Hg+). Moreover, the test strips coated with SPO were also demonstrated to provide a practically highly sensitive detection of CH3Hg+ in mixed 1,4-dioxane:water solutions.