Colorimetric indicators for specific recognition of Cu2+ and Hg2+ in physiological media: Effect of variations of signaling unit on optical response

Abstract

Easy to synthesize probes are designed using bispicolyl moiety as the receptor unit with two different signaling moieties, anthraquinone and bisindolyl. Both the compounds show ‘naked-eye’ sensing of Cu2+ and Hg2+ in ∼100% aqueous medium. Not only selectivity, the compounds also show high sensitivity towards the target metal ions, as evident by their sufficiently low detection limits. The present system can detect Cu2+ as low as 0.06 ppm and Hg2+ up to 0.03 ppm from visible color change. The sensors are found equally effective in the detection of metal ions even in the protein-bound state. The impact of the signaling unit on the sensing properties of probes having same binding units has been observed and investigated. Mechanistic investigations revealed that variations in signaling unit influence the pKa of the bispicolyl nitrogens, which subsequently controls the stoichiometry of the probe-metal ion complexes as well as their luminescence property. Additionally, high specificity and good accuracy with recovery values ranging from 98.0 to 104.0% were obtained during Cu2+/Hg2+ estimation in spiked water samples. Low cost, reusable paper strips were developed for on-site detection of the metal ions which does not require the assistance of an optical instrument.