In the last decade, we have seen the size of electronic devices shrink as transistors have replaced vacuum tubes and chips have replaced transistors. “But there is a limit to how small we can make an electronic device using ‘top-down’ design — such as starting with a piece of silicon and converting it to a chip,” says Harry Gibson, Virginia Tech chemistry professor. “For the trend to miniaturize to continue, a ‘bottom-up’ approach will be necessary.”

As in nature, engineers and scientists are assembling molecules to perform specific jobs, such as to conduct electricity; block or "grab" other molecules to filter salt from sea water or radioactive materials from ground water; or join together to create new materials.

"We are trying to learn from nature," says Gibson. "In biological systems, 'self-assembly' is key.”

Work on "self assembly" of non-biolgical molecules began in 1967 when Dupont chemist Carl Pederson invented molecules called crown ethers that interact with metal ions and can be used as catalysts. It was the birth of a field known as supramolecular chemistry — or chemistry based on controling the way molecules join so they will combine in new and useful ways.

In 1985, Pederson, Donald Cram, and Jean-Marie Lehn received a Nobel Prize for their work in the field. Building on the idea of supramolecular interactions, Cram developed "container molecules" that could trap small molecules. Lehn explored many kinds of molecules and applications that could arise from "control over the intramolecular bond."

Now, supramolecular chemistry is at the edge of two fields:

— the study of fundamental biological processes, to design better drugs and implants, for instance; and

— the development of new materials and devices, such as molecular wires and switches.

All biological systems, such as DNA and the transfer of materials through cell walls, operate by self-assembly. Attraction and repulsion that result in spontaneous and specific interactions and aggregation of molecules is the basis of supramolecular interaction. That is why development of purification molecules to desalinate sea water depends upon what we can learn about manipulating molecular recognition and interactions.

Gibson and his students are working to understand how to control self-assembly to create polymeric materials. New materials with specific engineered attributes could be used to build everything from light-weight, heat-resistant engines and softer, moisture-tolerant dental implants to solvents, filters, and materials for which uses have not yet been invented.

Gibson is using internal molecular attraction properties to build bonds. “Imagine cooked spaghetti as long molecules bonded together. We are trying to make attractant forces strong enough to, for example, make macaroni self-assemble into spaghetti. We are asking, ‘What controls it? How big can we make a supramolecule?’”

Gibson and colleagues have developed a new material that is insoluble. The process joins polyamide molecules, such as make up nylon and protein. A very strong bond occurs between the molecules and the resulting material “turns out to be insoluble in anything except something that destroys it,” Gibson says.

He illustrates this by tying plastic rings together with lengths of string, representing ring and linear molecules. The structure is further modified by linear molecules threading through the rings and then attaching themselves to other rings so they can't be unthreaded- at least not easily.

"What we think is happening is a tangle of molecules," says Gibson. "It is a physical network rather than a chemical bond."

The process changes the shape of the polymer molecule and has a significant impact on its viscosity. When treated with some solvents, the material absorbs the solvent and becomes gel like. With other solvents, the material is simply impervious.

Gibson's ability to control molecular recognition has resulted in the creation of supramolecules called polyrotaxanes - linear molecules threaded through cyclic or ring-shaped molecules so that the rings can slide up and down the string.

When applied to polymers, "These novel combinations may provide a unique and powerful means of improving properties such as blending and adhesion, solution viscosity, and strength and toughness of plastics," says Gibson.

Adding ring-shaped molecules changes the way large molecules pack, he explains.

When the string and the ring are made of different kinds of molecules, the potentials multiply. It is possible to change the thermal properties of a polymer by using different ring molecules. It has been demonstrated that the softening or melting point can be lowered, which implies that it can also be increased "once we make rings with a higher melting point," says Gibson.

Crystallization has also been observed, which means that the macrocycles, or rings, are lending this property to polymers, which don't ordinarily crystallize. This phenomena of polyrotaxanes could be used to strengthen polymers or to make them elastomeric (rubbery).

"It's hard to predict what we'll see in 50 years,"Gibson concludes. "Look at the computing power sitting on people's desks that wasn't there two years ago. Without the computer, we couldn't collect the data and do the analysis and predictions on molecules before we make them. Twenty years ago, it would take a year. Now we can do it in half a day."

— Written by Susan Trulove