Recently, I prepared Ag, Cu, Fe, and Cu/Fe NPs using plant extract for reduction and a water-soluble film forming polymer for capping. Although I noticed the change in color of the salt during addition to the color corresponding to each NP, I did not find the peak related to the nanoparticle for any of them. Even AgNPs, which is easily noticed with high absorbance even at little concentration.
First figure from left to right: Cu/Fe, Fe, Cu ,Ag, and Blank