Scientists at North Carolina State University have developed a new technique for the manufacture of nanoparticles that consist of two metals with tunable properties. The approach, which has been extended from the method of making single-metal nanoparticles, has the ability to tune the optical properties of nanoparticles and could have potential applications in security technology, as well as in creating more efficient chemical reactions.
For the study, published in the journal Small [Shore et al. Small (2010) doi: 10.1002/smll.201001138], the researchers developed core/shell nanoparticles that had a gold core and silver shell, as well as alloy nanoparticles which mix the gold and silver, and characterize the optical properties of these nanoparticles.
Dr. Joe Tracy, co-author of the paper, commented “Silver and gold have unique optical properties arising from their specific interactions with the electric field of light. By manipulating the ratio of the metals, and whether the nanoparticles have core/shell or alloy structures, we can alter their optical properties with control.”
The team was able to synthesize the nanoparticles through digestive ripening, an approach already used to develop gold nanoparticles, and seen as a straightforward way of synthesizing different kinds of core/shell and alloy nanoparticles. Digestive ripening depends on ligands: small organic molecules with parts that bond directly onto metals, and which are normally linked to the metal cores of the nanoparticles and prevent the nanoparticles from clumping together. This happens when the ligands are able to transport metal atoms from the core of one nanoparticle to another, resulting in a more homogenous size distribution among the nanoparticles.
Once digestive ripening had made a solution of similarly sized gold nanoparticles, silver acetate was introduced, with the ligands moving Ag atoms to the surfaces of the gold nanoparticles, so that the nanoparticles had gold cores and silver shells. They then transferred the nanoparticles into a solution that contained a different ligand, which was heated to 250 oC, resulting in the metals diffusing into each other, creating nanoparticles made of a gold–silver alloy.
Tracy said “This study, along with related work by others, shows that digestive ripening is a viable method for creating multi-component metal nanoparticles. We used gold and silver, but the same principles would likely apply to other metals.”

The research has helped to confirm the viability of digestive ripening as an approach for synthesizing multicomponent nanoparticles with stoichiometric and nanostructural control, and the possibilities of the AuAg system. In terms of applications, it may lead to improved catalysis and taggants for the security industry. With the use of AuAg core/shell and alloy nanoparticles with the same elemental composition that have different optical properties it will be more difficult to counterfeit inks based on such nanoparticles without first having identified the nanostructure.

Laurie Donaldson