Tracking Nanoparticle Growth

A new technique enables scientists to follow nanoparticle growth with a “label”—a metallic seed that is distinguishable from the rest of the nanoparticle via electron microscopy.

Using the method, Chad A. Mirkin of Northwestern University and coworkers have mapped how metallic nanoparticles change shape as they grow (Science, DOI: 10.1126/science.1225653). Knowing how nanoparticles evolve into certain shapes could help scientists better control the materials’ electronic and catalytic properties, the team says.

“We have done for nanoparticle mechanistic work what the isotopic-labeling folks and fluorescent-labeling folks have done for synthetic chemistry and biology, respectively,” Mirkin says.

Oftentimes, he adds, “nanoparticles are made in a black-box type of operation.” A graduate student might be an expert at making a particular particle shape under a certain set of conditions, he explains, but typically little is known about why the shape forms. Such syntheses can be quite useful, Mirkin adds, but it is difficult to adapt nanoparticle recipes to grow other desired shapes without better understanding how they form.

His group’s new study addresses this knowledge gap. Starting with gold seed crystals, the Northwestern team grew an assortment of differently shaped nanoparticles—octahedra, tetrahedra, and icosahedra, to name a few—by shining 550-nm light on the seeds in the presence of silver nitrate. The light helps catalyze deposition of silver onto the seeds’ surfaces.

Because Au and Ag scatter electrons differently, the researchers could distinguish the two elements by electron microscopy: The seeds were plainly visible inside the final Ag-coated particles, indicating where nanoparticle growth was initiated.

The team then set out to learn how complex nano­particle shapes evolve from specifically shaped Au seeds. In one experiment, they grew octahedral nanoparticles from cubic Au seeds. In another, they showed that octahedral seeds led to tetrahedral particles. On the basis of these and other investigations, the team proposed an overall growth pathway for the nanoparticles starting from a single Au seed and ending with an icosahedron—the most complex shape synthesized—via a number of intermediate shapes.

“What is really fascinating about this paper is that the team can image the embedded seed particle once the overall final nanoparticle is made,” says Catherine J. Murphy, a chemist at the University of Illinois, Urbana-Champaign. “Thus, more direct information about how the seed dictates the final ‘outer’ nanoparticle crystal shape is obtained.”

“The labeling technique,” says Younan Xia, a chemist at Georgia Tech, “could become a very powerful tool for studying the nucleation and growth mechanisms of nanocrystals.”

Mirkin says the team is now trying to generalize its technique by exploring materials other than the Ag-Au system for tracking nanoparticle growth.