Recognition of Hg²⁺ and Cr³⁺ in Physiological Conditions by a Rhodamine Derivative and Its Application as a Reagent for Cell-Imaging Studies

Abstract

A new rhodamine-based receptor, derivatized with an additional fluorophore (quinoline), was synthesized for selective recognition of Hg²⁺ and Cr³⁺ in an acetonitrile/HEPES buffer medium of pH 7.3. This reagent could be used as a dual probe and allowed detection of these two ions by monitoring changes in absorption and the fluorescence spectral pattern. In both instances, the extent of the changes was significant enough to allow visual detection. More importantly, the receptor molecule could be used as an imaging reagent for detection of Hg²⁺ and Cr³⁺ uptake in live human cancer cells (MCF7) using laser confocal microscopic studies. Unlike Hg­(ClO₄)₂ or Hg­(NO₃)₂ salts, HgCl₂ or HgI₂ failed to induce any visually detectable change in color or fluorescence upon interaction with <b>L</b>_<b>1</b> under identical experimental conditions. Presumably, the higher covalent nature of Hg^II in HgCl₂ or HgI₂ accounts for its lower acidity and its inability to open up the spirolactam ring of the reagent <b>L</b>_<b>1</b>. The issue has been addressed on the basis of the single-crystal X-ray structures of <b>L</b>_<b>1</b>·HgX₂ (X^– = Cl^– or I^–) and results from other spectral studies