Digestive ripening, a technique used to make single-metal nanoparticles, also can be extended to create nanoparticles that consist of two metals and that also have tunable properties, according to a study from researchers at North Carolina State University.

The method relies on the use of ligands, usually anchored to the metal cores of the nanoparticles to prevent the nanoparticles from clumping together. This allows them to be suspended in solution. When the ligands can transport metal atoms from the core of one nanoparticle to another, the size distribution among the nanoparticles is more homogeneous.

The group used digestive ripening to create a solution of gold nanoparticles similar in size. When they introduced silver acetate into the solution, the ligands transported silver atoms to the surfaces of the gold nanoparticles, resulting in nanoparticles with gold cores and silver shells.

Putting the nanoparticles into a second solution with a different ligand and heating that solution to 250 °C caused the metals to diffuse into each other, creating gold-silver alloy nanoparticles.

The researchers also tried skipping the shell-creation step, introducing silver acetate into the second solution and again raising the temperature to 250 °C. The shortcut method created gold-silver alloy nanoparticles with simpler control over the alloy’s gold-to-silver ratio.

The results likely would apply to other metals, according to the researchers, whose paper was published online Dec. 13, 2010, by the journal Small.