Squaraine-based near-infrared colorimetric chemosensor for highly sensitive and selective detection of Cu2+ ions

A near-infrared responsive squaraine-based colorimetric probe, designated as SQ-68, has been designed, synthesized, and thoroughly characterized for the selective and sensitive detection of Cu2+ ions. The probe exhibits remarkable specificity for Cu2+ over a broad spectrum of competing metal ions in acetonitrile solution, as confirmed by the absorption spectral investigations. Upon interaction with Cu2+, SQ-68 undergoes significant absorption quenching at 672 nm, accompanied by a visually detectable colorimetric shift from cyan to colourless, enabling rapid and intuitive detection. Job's plot analysis revealed that the binding stoichiometry of the SQ-68-Cu2+ complex follows a 1:1 ratio. The binding constant (Ka) was determined to be 9.82 × 104 M−1 based on the Benesi-Hilderbrand method. The probe exhibited remarkable sensitivity, achieving a detection limit of 77 nM and a limit of quantification of 258 nM, confirming its efficiency for ultra-trace Cu2+ detection. Additionally, the binding interaction between SQ-68 and Cu2+ is chemically reversible, as evidenced by the restoration of the probe's absorption spectrum upon the introduction of ethylenediaminetetraacetic acid. Given its excellent performance, SQ-68 presents a robust and cost-effective platform for real-time Cu2+ monitoring in environmental and biological systems, making it a highly promising tool for water quality assessment and heavy metal ion sensing.

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