A Quinoxaline-Naphthaldehyde Conjugate for Colorimetric Determination of Copper Ion

Abstract

This work facilitates detection of bivalent copper ion by a simple Schiff base probe QNH based on a quinoxaline-naphthaldehyde framework. The detailed study in absorption spectroscopy and theoretical aspects and crystal study of the probe and probe-copper complex has been discussed. The detection limit of the probe in the presence of Cu²⁺ is 0.45 µM in HEPES-buffer/acetonitrile (3/7, v/v) medium for absorption study. The reversibility of the probe-copper complex has been investigated by EDTA. The selective visual detection of copper has been established also in gel form.

Keywords: Cu2+ colorimetric detection; ab initio calculations; absorption study; quinoxaline−naphthaldehyde conjugate.

Figure 1

Visible colour change of QNH after addition of cations in HEPES−buffer/acetonitrile (3/7, v/v) medium. The concentration of the metals in all vials is 10⁻⁴ M.

Figure 2

UV–Visible titration of QNH (10^–5 M) in the presence of various concentrations of Cu²⁺. (Upward arrow indicates gradual increase in absorption at 552 nm with increasing concentration of externally added Cu²⁺). (Inset: absorbance of QNH at 552 nm as a function of [Cu²⁺]).

Figure 3

The left panel shows the computed absorption spectra of QNH and QNH in the presence of Cu²⁺, plotted with blue and black lines, respectively. The right panel shows computed frontier molecular orbitals (HOMO and LUMO) for QNH and QNH−Cu²⁺ model systems. Values in parenthesis are the theoretical absorption wavelengths in nm. Surfaces are generated with an isovalue of 0.02.

Figure 4

Absorption spectrum of QNH–Cu²⁺ complex in the presence of different amino acids in (3:7 v/v) HEPES buffer: acetonitrile medium.

Scheme 1

Schematic representation of QNH–Cu²⁺ complex in the presence of histidine.

Figure 5

Proposed sensing mechanism of Cu²⁺ by QNH. The displayed chemical structures shown as ball and sticks correspond to the DFT-optimized models. Colour code: C = grey; H = white; N = blue; O = red; Cu = brown.