Unlocking Molecular Self-Assembly Secrets

Drs. Jan Genzer and Orlando Rojas try not to delve too deeply into their fiber and materials science research. What happens on the surface is much more interesting, they say. Genzer, a chemical and biomolecular engineering professor in the College of Engineering, and Rojas, an associate professor in the Department of Wood and Paper Science in the College of Natural Resources, are studying the surface properties of natural and synthetic molecules to gain a better understanding of nanoparticles and create nanoscale biosensors.

Several years ago, while devising ways to make paper stronger and improve its optical and wetting properties, Rojas began studying intermolecular forces and surface interactions. He quickly became interested in the nanoscale potential of cellulose fibers and natural polymers, which often have a chain-like organization that can be spliced or assembled into nanostructures. Now, he has a handful of research projects going, from coating cellulose nanofibers with catalytic particles to create biosensors to using lignin from woody plants to make carbon nanofibers. “The forest products sector is a mature industry,” he says, “but we have an opportunity in nanotechnology as scientists look to exploit biological systems like trees and plants.”

In the biosensor project, Rojas and Genzer are working to get polymer molecules and reactive agents to self-assemble—build naturally from a cellulose base—making the resulting nanoscale system sensitive to changes in such conditions as temperature, pH, or electrolyte concentration. A change would trigger the polymer to collapse or expand, altering the surface density and thickness to produce a visible signal.

Molecular self-assembly has been used to produce sensors and test beds where physical and chemical properties gradually change across the surface of glass or silicon substrates.

Genzer’s research team has previously used molecular self-assembly to produce sensors and test beds where physical and chemical properties gradually change across the surface of glass or silicon substrates. A gradient is produced by adjusting the time allowed for molecules to evaporate from a source and deposit across the substrate. By controlling the density and length of polymers that assemble on a glass base, for example, the team developed a way to test hundreds of density/length combinations at once—similar to a biotech microarray—to find the right polymer coating to attract or repel proteins in applications ranging from ship hulls to contact lenses.

Working with the National Institute of Standards and Technology, Genzer’s team recently became the first to uncover a basic principle of self-assembly. In a confined system, molecules don’t order themselves in a classic diffusion manner but grow in a wave-like fashion that spreads from a source point. “We don’t yet fully understand all molecular aspects of the process,” he says. “But understanding the nature of this dynamic phenomenon could have remarkable implications for studying other propagating systems, like the spread of diseases or urban growth.”




Drs. Jan Genzer, left, and Kirill Efimenko have found that molecules in a confined area arrange themselves in a wave-like pattern from a source point.

This diagram shows polymer molecules assembling on a silica base as they become more tightly packed.

Dr. Orlando Rojas examines a gold-coated quartz crystal that is covered with a cellulose film to act as a biosensor.