Issue Date: January 22, 2007
Nanoparticles Act Like Atoms
Using a little topology and a few thiol ligands, materials scientists have managed to corral a gold nanoparticle's thousands of atoms and make them behave like one divalent atom (Science 2007, 315, 358).
Transformed from a multivalent mass to tidy two-handled building blocks, the nanoparticles can then be hooked together into a tiny string of golden beads. "It's the nanoscale equivalent of a polymer," says Francesco Stellacci, the Massachusetts Institute of Technology professor who spearheaded the research.
To establish an orderly divalent bonding motif in the unruly nanoparticles, Stellacci's group took advantage of what topologists call the "hairy-ball theorem." Basically, the theorem states that if you've got a sphere covered in hair, it's impossible to comb those hairs so they all lie flat. No matter what you do, two hairs at opposite points on the sphere will stand straight up.
Applying the theorem to a gold nanoparticle "sphere" coated with a mixture of two types of thiol ligand "hairs," the researchers reasoned they could swap the thiols at those polar positions with thiol chains bearing a terminal carboxylic acid. "We knew that the exchange reaction would take place orders of magnitude faster there than at any other point on the sphere," Stellacci explains.
With an acid-functionalized chain at each of its poles, the particle selectively bonds with other molecules as though it were a divalent atom. Once they'd done the thiol swap, Stellacci's group simply condensed the divalent particles with 1,6-diaminohexane, creating polymer-like strands up to 20 nanoparticles long.
"Introducing valency into nanostructures through face- or edge-selective functionalization" is a major fundamental advance, comments Northwestern University's Chad A. Mirkin, an expert in controlling nanoscale architecture. The work will help researchers "realize more sophisticated and highly functional architectures," Mirkin adds. "It is analogous to transitioning from atoms to molecules."
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