Aminoquinoxaline-Based Dual Colorimetric and Fluorescent Sensors for pH Measurement in Aqueous Media

Abstract

This research is focused on the development of pH indicators based on the quinoxaline signaling group for acidic aqueous solutions (pH 1–5). A push–pull quinoxaline QC1 in which two electron-donating (3-aminopropyl)amino substituents are attached to positions 6 and 7 of the electron-deficient quinoxaline moiety was prepared using the palladium-catalyzed C–N cross-coupling reaction. The 3-aminopropyl residues are mostly protonated in aqueous solutions below pH 8, thus serving as hydrophilizing substituents that render quinoxaline derivative QC1 water-soluble in this range of acidities and useful for measurements in the pH range of 1–5. This chromophore is a dual optical chemosensor that exhibits shifts of both absorption and emission bands in response to external stimuli. The presence of naturally relevant metal cations (13 ions) does not interfere with spectrophotometric and fluorescence measurements of the optical response of aminoquinoxaline in the visible region. Moreover, these spectral changes are easily observed by the naked eye, allowing for rapid semi-quantitative analyses under “in-field” conditions.