Unveiling the highly selective time-dependent colorimetric response of an 8-hydroxyquinoline- and indolinium-conjugated donor–π–acceptor (D–π–A) hybrid for the detection of copper(ii) ions

Abstract

A novel indolinium–8-hydroxyquinoline-based donor–π–acceptor colorimetric chemosensor, IQS, was synthesized and its time-dependent colorimetric response for the selective detection of Cu²⁺ ions was systematically investigated. The probe exhibited a distinct, time-dependent color change with copper(II) ions in a DMSO–H₂O (8 : 2, v/v) medium, enabling visual detection. UV-visible absorption studies revealed pronounced bathochromic shifts and the formation of new absorption bands upon Cu²⁺ coordination. Furthermore, Job's plot analysis confirmed a 2 : 1 (IQS : Cu²⁺) binding stoichiometry. The probe displayed excellent selectivity with negligible interference from other metal ions. The limit of detection for Cu²⁺ was 3.98 × 10⁻⁵ M within 5 min and it reduced to 1.31 × 10⁻⁵ M after 24 h. The binding constant values were 2.40 × 10² M⁻² (within 5 minutes) and 2.73 × 10² M⁻² (after 24 h), indicating strong affinity toward Cu²⁺ ions. Density functional theory (DFT) and TD-DFT analyses demonstrated a decrease of the HOMO–LUMO energy gap and enhanced charge-transfer character upon complexation. These results confirm that IQS is a robust colorimetric chemosensor, facilitating the selective and time-dependent detection of Cu²⁺.