Optimized colorimetric detection of cobalt ions (Co²⁺) using alliin-Ag-Au nanoparticles

Cobalt is a vital trace element that is necessary for biological functions but poses toxicity risks at higher levels, necessitating its accurate detection in environmental and occupational contexts. Traditional cobalt detection methods, including spectroscopy and chromatography, often lack affordability and environmental friendliness. This study explores the development of a highly sensitive, selective, and eco-friendly nanosensor for detecting cobalt ions (Co²⁺) using bimetallic silver-gold nanoparticles (Ag-Au NPs) synthesized with alliin extracted from garlic. The properties of the synthesized Ag-Au NPs were investigated using a comprehensive suite of techniques including UV-visible spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, electron microscopy and computational quantum mechanical modelling (DFT). The nanoparticles exhibited a distinct surface plasmon resonance peak at 533 nm, confirming their formation. Sensitivity studies revealed a detection limit of 0.09 nM for Co²⁺ ions, outperforming many established methods. Selectivity tests demonstrated a significant absorbance shift and colour change in the presence of Co²⁺ ions, differentiating it from other metals like Mn²⁺, Cr³⁺, Al³⁺, Ni²⁺, Pd²⁺, Cd²⁺, Ba²⁺, and Zn²⁺. The reaction kinetics indicated pseudo-first-order behaviour, emphasizing the specificity of the interaction between Co²⁺ and Ag-Au NPs. Molecular orbital analysis and Gibbs free energy calculations affirmed the stability of the Co-containing complexes. This novel colorimetric sensor, leveraging garlic-derived biomaterials, provides a sustainable, efficient and environment friendly means for detecting cobalt ions, with potential use in environmental assessment and healthcare applications.