An anthraimidazoledione-based charge transfer probe for dual mode sensing of calcium ions: role of the counter ion in signal improvement

Abstract

An anthraimidazoledione-based amphiphilic probe has been designed for dual-mode sensing of Ca²⁺ ions at physiological pH in a buffered medium. The compound showed deep pink color in the native state due to intramolecular charge transfer from the imidazole to the anthraquinone moiety. The addition of Ca²⁺ ions resulted changes in solution color to orange with a concomitant appearance of blue-colored fluorescence. The mechanistic investigations indicate that the Ca²⁺ ion binds to the APTRA moiety on the donor site, which leads to a blue-shift in the absorption maxima. Most importantly, because of the naked-eye response, this method does not need sophisticated visualizing instruments for the analysis. Interestingly, the counter ion showed a significant impact on the extent of Ca²⁺ sensing. The fluorescence response was large when Cs⁺ (loose ion pair) was considered as the counter ion instead of K⁺ (tight ion pair). Furthermore, the present system can achieve the detection of Ca²⁺ ions in real-life water samples and also in the presence of serum albumin protein. The high recovery values along with small standard deviations indicate the suitability of the present method in analyzing real-life samples. Finally, dye-coated paper strips were developed for rapid, on-location detection of Ca²⁺ ions.