To detect Cd ion in aqueous solution we used AuNP colloidal. Inspections by UV-Vis spectrum reveal shifting in absorption peak in the UV range toward visible region, these shifting and their intensities increased with Cd ions concentration. I didn't find an explanation for this phenomena.

We used in our work 2 ratios for Au colloidal to aqueous solution. For 0.5/0.5, in the Vis region, the plasmon resonance peak intensities decreased with Cd increased, while for 0.7/0.3 plasmon resonance peak intensities increased with Cd increased. These different results need some explanation,since all the references talking about decreasing intensities with Cd ions.