Colorimetric Xylenol Orange: A Long-Buried Aggregation-Induced Emission Dye and Restricted Rotation for Dual-Mode Sensing of pH and Metal Ions

Abstract

As the third largest class of dyes in the world, triphenylmethane dyes are widely applied in colorimetric sensing. However, triphenylmethane dyes are commonly nonfluorescent, which limits their sensing applications. It is worthwhile to study the fluorescence off/on control of triphenylmethane dyes and promote the applications of triphenylmethane dyes in sensing technology. In this work, the fluorescence off/on control was investigated by employing a triphenylmethane dye xylenol orange (XO), which is a colorimetric indicator for pH and metal ions. It was discovered that XO exhibited aggregation-induced emission (AIE), and thus, its fluorescence off/on was controlled by intramolecular rotation. This discovery broadens the optical properties of XO and transforms XO from a colorimetric dye to a colorimetric/fluorescent dual-mode AIE dye. It was further verified that the AIE-based fluorescence off/on control improved the sensing performance of XO. A bovine serum albumin-based rotation suppression method was applied to enhance the fluorescence emission of XO for colorimetric/fluorescent dual-mode indication of pH and metal ions. Compared with colorimetric sensing, colorimetric/fluorescent dual-mode sensing exhibits higher accuracy, ascribed to the self-validation effect. This work uncovers AIE-based fluorescence off/on control of triphenylmethane dyes and breathes new life into the sensing applications of triphenylmethane dyes.