Filling the Void

The glowing hollow spheres above--as well as cylinders, solid rings and flat disks--are too small to be seen with the naked eye but they are the giants of the molecular world. And incredibly, they don't require laborious synthesis to combine their millions of atoms: they literally build themselves. By modifying the starting molecules, scientists can make these structures grow into their intended shapes, much in the same way as proteins and cells are genetically encoded to grow and arrange themselves into functioning entities.

Created by researchers from the University of Rochester, these structures are the largest self-assembling molecules yet--1,000 times larger, in fact, than any previously made. Larger than bacteria and most human cells--and clearly visible under an optical microscope--they fill a critical void between very tiny, molecular nanostructures, such as buckyballs (soccer ball-shaped cages of carbon atoms), and the microscale world of industrial colloids and encapsulants.

"In self-assembly, bigger, complex objects that function are the dream," says Samson Jenekhe, a professor of chemical engineering, chemistry and materials science and lead author of a paper in the March 20 issue of Science. "What we have done is to expand the size of the microscopic world, bridging from small molecules to the size of materials used in industry."

Jenekhe, who also described his work on March 20 at a meeting of the American Physical Society in Los Angeles, foresees myriad practical applications for the molecular structures, from delivering drugs to formulating cosmetics, adhesives, paints, imaging media and pesticides. Encapsulating such products in tiny containers now involves coating droplets of the substance with a rigid covering. Also, because some of the objects are composed mainly of a hollow cavity surrounded by a fluorescent shell, they may even be useful in making microscopic lasers.

The largest of the structures created by Jenekhe and his colleague, X. Linda Chen, are just 50 microns long. But each of these microspheres can hold millions of nanostructures, such as buckyballs. Some studies have shown that buckyballs can shield living cells from damage and might be used in treating such diseases as diabetes. Shuttling billions of them into the body in a self-assembled shell might prove a convenient means of delivery.

The Rochester engineers made their molecular behemoths by using larger molecules as a starting point. Instead of single atoms or the building blocks of cells used by previous researchers, they selected the precursors of large polymers used in making commercial plastics. "Most researchers have used small molecules, believing it would be impossible to tame polymers into self-assembling," Jenekhe notes. "A few have tried to self-assemble polymers, but without much success."

Jenekhe and his colleagues chose poly(phenylquinoline)-block polystyrene, a polymer used in paints, adhesives and foam polystyrene cups. This chain consists of a rigid half and a flexible half--and like oil and water, the two ends behave very differently under some conditions. By manipulating the behavior of these ends, the team forces the polymer chains to assemble into specified shapes and sizes. Once the polymer is prepared, it takes the molecules just minutes to organize themselves into discrete microscopic objects.

Key to the success of Jenekhe and Chen was their ability to incorporate hydrogen bonds into polymer structures. The hydrogen bonds give them the same source of stability that helps DNA and self-assembled proteins in nature arrange themselves into functioning objects. Jenekhe says the University of Rochester has filed a patent on the technique.