They're tiny, invisible to the naked eye. Yet they tower over their counterparts in the molecular world.
The three-dimensional objects created by two University of Rochester engineers are the largest synthetic structures ever made by a technique known as self-assembly, where molecules organize themselves into larger structures. What's more, they glow, or fluoresce, and they are among the most well defined, discrete structures scientists have ever created through self-assembly. Engineers report this work in the March 20 issue of Science.
The colorful hollow spheres and cylinders, solid rings, and flat disks are 1,000 times bigger than the previous largest synthetic self-assembled structures, say the investigators. Each is made up of millions of molecules that organized themselves together.
"In the world of self-assembly, these structures are giant," says graduate student X. Linda Chen. "We've created microstructures through self-assembly; others have only been able to create nanostructures."
The largest of the team's structures are 50 micrometers long, larger than most of the cells in the human body and bigger than the largest bacteria known. Even so, the objects are tiny by everyday standards -- none is larger than the width of a human hair. The harmless synthetic objects bear an uncanny resemblance to bacteria that cause illnesses like tuberculosis, blood poisoning, and strep infections.
"In self-assembly, bigger, complex objects that function are the dream," says Samson Jenekhe, Chen's adviser and a professor of chemical engineering, chemistry, and materials science. Just a few years ago, chemists believed the self-assembly of such large objects would remain elusive until the early decades of the 21st century.
Chen and Jenekhe were able to create larger objects because they started out with a larger building block: a carefully designed polymer, the type of macromolecule that forms plastics. Once the polymer is prepared, it takes the molecules just minutes to organize themselves into discrete microscopic objects.
"Most researchers have used small molecules, believing it would be impossible to tame polymers into self-assembling," says Jenekhe, whose work is supported by the National Science Foundation and the Office of Naval Research. "A few have tried to self-assemble polymers, but without much success. Our design guarantees arrangement into well defined, stable shapes."
Their polymer, poly(phenylquinoline)-block-polystyrene, is a type of block co-polymer -- a material used in paint to keep it from sticking together, and in materials like adhesives and Styrofoam cups. The polymer chain features both a rigid half and a flexible half. Like oil and water, the two ends of the chain behave very differently under some conditions. It's by understanding precisely how such hybrid chains interact and then manipulating their behavior that the team forces the polymers to assemble into specified shapes and sizes. Key to the team's success was its ability to incorporate hydrogen bonds into polymer structures, giving them the same source of stability that helps DNA and self-assembled proteins in nature arrange themselves into functioning objects.
"We first realized last summer that this polymer had special potential for self-assembly, and began working night and day to synthesize and study it," says Chen. "It has not been easy to put molecules together to create such beautiful shapes."
One possible application is drug delivery. In one experiment the team used its self-assembled hollow spheres as tiny containers to carry buckyballs, or fullerenes, stuffing each sphere with billions of the soccer-ball-shaped carbon molecules. Some studies have shown that buckyballs can shield certain cells from many different types of damage, but their usefulness is limited because they are not very soluble. Shuttling billions of them via a self-assembled shell might prove a convenient means of delivery.
Jenekhe predicts that the new self-assembly technique may be useful in a wide range of other fields as well: cosmetics, adhesives, pesticides, biomaterials, sensors, pollution control, composites, coatings and paints, photographic and imaging media, catalysis, microfabrication, microencapsulation, and microelectronics. Since some of the objects are composed mainly of a hollow cavity surrounded by a fluorescing shell, they may even be useful in making microscopic lasers.
The research is part of the growing field of self-assembly, where scientists design molecules that can assemble themselves into much larger, functioning objects. In essence, they are drawing inspiration from nature, where proteins and cells are genetically encoded to grow and precisely arrange themselves into functioning entities.
"A human being is the ultimate in self-assembly: an egg and sperm join, and the embryo grows and develops into a human being," Jenekhe says. "Through our research not only do we take advantage of some of the self-assembly techniques nature uses, but we hope to shed light on the biological process as well."