Chemists at Rice University have found a way to assemble a bigger nanostructure from smaller particles. And the instructions are remarkably simple: just add water.

The process takes advantage of the hydrophobic properties of oils and lipids — or the simple fact that oil and water don't mix. 

The research, to appear in the Nov. 29 issue of the Journal of the American Chemical Society, will help chemists create useful materials including potent cancer drugs and efficient catalysts for the chemical industry.

Researchers took a particle of gold about two nanometres thick and attached it to an amphiphile — a molecule that is hydrophobic at one end, but bonds with water at the other end.

When two amphiphiles meet in water, the hydrophobic ends join together to get as far away from the water as possible, just as drops of oil collect together in a sink full of water.

When thousands of amphiphiles are put in water, the hydrophobic ends of the amphiphiles all try to come together, forming a membrane of amphiphiles similar to the protective layer of lipids that protects living cells. This membrane is called a micelle.

As more amphiphiles connect together, they form more complex shapes like spheres and cyclinders, with the water-bonding ends on both the outside and inside of the structures and the hydrophobic ends all connected together into one solid shape. And since the gold is part of the hydrophobic part of the molecule, it too forms a solid structure.

The shapes created depend on the solvents used, size of the particles and the lengths of the amphiphiles. Structures as large as 1,000 nanometers could be created using the process, researchers said.

"When the micelle forms, the process drives the packing of all the junction points, which connect the hydrophobic and the hydrophilic part [the end that bonds with water] of an amphiphile, into a high-density array," said Eugene Zubarev, the Norman Hackerman-Welch Young Investigator and assistant professor of chemistry at the Texas university.

"We can, in effect, use micellization as a means to assemble billions and billions of individual nanoparticles into well-defined one-dimensional superstructures that are soluble in water," he said.

Gold has practical applications in cancer treatment because it is biologically inert and yet can absorb infrared light and convert it to heat. Targeted tiny gold nanostructures could then kill cancer cells without harming surrounding tissue. A report on these gold nano-bullets was published by Rice University scientists in 2003.