A simple ninhydrin–hydrazone probe for highly selective and sensitive colorimetric detection of Cu2+ ions

Abstract

Copper ions (Cu²⁺) are essential in numerous biological and environmental processes, yet their excess can lead to severe toxicity. Hence, developing selective and sensitive colorimetric sensors for Cu²⁺ detection remains a critical research pursuit. In this study, a newly synthesized Nin–PyHz-based chemosensor, 2-(2-(pyridin-2-yl)hydrazono)-1H-indene-1,3(2 H)-dione (L), was explored for its selective Cu²⁺ recognition ability. The sensor exhibited a distinct optical response exclusively toward Cu²⁺ ions, while other metal ions and anions caused negligible interference. The free ligand showed an absorption band at 382 nm, which shifted to 434 nm upon Cu²⁺ binding, accompanied by a perceptible color change, indicating complex formation. This bathochromic shift arises from modulation of the intramolecular charge transfer (ICT) process upon coordination with Cu²⁺ ions. The binding constant, derived from the Benesi–Hildebrand relation, was found to be 1.83 × 10⁵ M⁻¹, and the limit of detection (LOD) was determined as 0.69 µM. Job’s plot confirmed a 1:1 stoichiometric interaction between L and Cu²⁺ ions. The sensor exhibited optimal performance in the pH range of 6.0–8.0, highlighting its excellent selectivity and applicability for Cu²⁺ detection in real or complex samples.