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Mimicking Atomic Bonding

Materials Science: Sticky patches bestow valency on colloidal particles

by Bethany Halford
November 1, 2012 | A version of this story appeared in Volume 90, Issue 45

Credit: Nature
These scanning electron micrographs show colloidal particles of various valencies with “sticky” patches of differing sizes. Scale bars are 500 nm.
SEMs of colloidal particles with various valencies and different sized-sticky patches.
Credit: Nature
These scanning electron micrographs show colloidal particles of various valencies with “sticky” patches of differing sizes. Scale bars are 500 nm.

Thanks to a new assembly strategy, colloidal particles can now be coaxed into sophisticated structures in much the same way that atoms combine to form molecules (Nature, DOI: 10.1038/nature11564). The development could lead to novel functional materials such as a diamond lattice made from colloidal-sized particles with specific optical properties.

Whether in organic molecules or atomic lattices, atoms’ predictable ways of arranging themselves via valency give chemists the ability to create sophisticated structures. Colloid scientists, on the other hand, aren’t so lucky. Colloidal particles are uniformly sticky across their spherical surfaces and they assemble in nonspecific ways, making it difficult to design and assemble three-dimensional structures from the particles.

Now, researchers led by New York University’s David J. Pine have come up with a way to create valency in colloidal particles so that they assemble into predictable structures just as atoms combine to form molecules. They take cross-linked amidinated polystyrene microspheres, 540 nm or 850 nm in diameter, and allow them to assemble into clusters with shapes such as dumbbells, triangles, and tetrahedra.

They then add styrene and polymerize it to grow the cluster. This creates a particle that possesses a new central core with protrusions from the original cluster.

The protrusions can be selectively functionalized with single strands of DNA so that they stick to complementary strands of the oligonucleotide that are attached to other particles. With this strategy, the team created colloidal molecules with linear, triangular, and tetrahedral symmetry.

“It has long been the goal of many colloid scientists to synthesize ‘artificial atoms,’ ” write Northwestern University materials scientists Matthew R. Jones and Chad A. Mirkin in a commentary that accompanies the paper. “This work is a major advance on earlier attempts to generate directional interactions between particles, and greatly increases the sophistication of structures that can be built ‘bottom up’ from smaller components.”



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