Fig. 2. (a) The UV–vis spectra of ETC-Au-Fe3O4NPs solution with various concentrations of Cd2+(from 0 to 190 M). Inset: the photographic images of ETC-Au-Fe3O4NPssolution before (purple) and after (blue) the addition of Cd2+(190 M). (b) The changes of the ratio (R: A565/A900) as a function of the concentration of Cd2+. Inset: The linearcorrelation between R and the concentration of Cd2+.