Naked Eye Detection of Cr3+ and Co2+ Ions by Gold Nanoparticle Modified with Azomethine

This paper reports the synthesis of azomethine-modified gold nanoparticles with azomethine (azomethine-AuNPs) in aqueous media, which were characterized by FT-IR spectroscopy, ultraviolet–visible spectroscopy (UV-Vis), dynamic light scattering (DLS), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). The azomethine-AuNPs were employed as colorimetric for Cr³⁺ and Co²⁺ ions at pH 6.2–7.5 and 8.1–9.1, at room temperature in aqueous solution. In the presence of Cr³⁺ and Co²⁺, the azomethine-AuNPs induce aggregation of the nanoparticles. Upon aggregation, the surface plasmon absorption band red-shifts so that the nanoparticle solution appears a blue color. The sensitivity of azomethine-AuNPs towards other metal ions, Mg²⁺, Mn²⁺, Cr⁶⁺, Na⁺, Ni²⁺, Ag⁺, Al³⁺, Ca²⁺, Cd²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Hg²⁺, Cd²⁺, K⁺, Co³⁺, Ni²⁺, Pb²⁺, and Zn²⁺ are negligible. This highly selective sensor allows a direct quantitative assay of Co²⁺ and Cr³⁺ with colorimetric detection limits of 83.22 and 108 nM, respectively.