Nanoscience imitating nature

t’s tough to build things that are 100,000 times smaller than the diameter of a human hair. Biology has had a few billion years to perfect the craft of building from the bottom up. That’s why UC Berkeley nanoscientist Matthew Francis collaborates closely with Mother Nature. Francis and his research group use organic chemistry to assemble nanoscale devices with unprecedented capabilities that could revolutionize cancer treatment or lead to the development of highly efficient solar cells.

     “Our goal is to address a big challenge in nanoscience, which is how to position objects with exquisite resolution so that the exciting components people are developing can be combined into functional devices,” Francis says.

	DELIVERY (left) Empty MS2 capsids like these could be used to house an anticancer agent in a drug delivery system.
TUBES (right) The tube-like capsid of the tobacco mosaic virus provides a promising template for the construction of a bio-artificial light harvesting system.

     In recent years, he explains, materials scientists have developed a wide array of impressive building blocks for nanoscale systems, from computer components fashioned from single molecules to promising drug delivery systems. The problem, though, is constructing useful devices from these materials. Some of the new nanoparticles are too small for current lithographic techniques like those used to fabricate integrated circuits. Others are a bit oversized for precise “bottom up” positioning using organic chemistry.

     “Biology has an enormous number of proteins that self-assemble into structures with feature sizes that are at exactly the right length scales,” Francis says. “So we can use proteins as positioning scaffolds to place these interesting components into functional arrays.”

     For example, the researchers have transformed the shell of a bacteriophage called MS2 into a capsule that could deliver anti-cancer drugs only to tumor cells. First, the group developed a method to remove the viral genome so that an anti-cancer drug can be stored inside. Once the drug was in place, the viral shell could then be coated with a polymer that protects the cargo from triggering unwanted side effects or degrading prematurely as it travels through the body to the tumor site. Finally, the linkage attaching the drug might be functionalized to release the anticancer agent only when the nanocapsule reaches the tumor.

     The first step though, Francis says, is developing the tools of organic chemistry.

     “Our specialty is designing reactions that can link the nanoparticles and organic molecules to the proteins to make these new hybrid structures.”

     One of his favorite examples of nature’s own nanoengineering, he says, is the photosynthetic light harvesting system that enables plants and certain bacteria to convert sunlight into organic energy. In plants, an array of molecules called chromophores form a “collector antenna” that gathers the photons from the sunlight and transfers the energy to a central engine that does the energy conversion.

     Research groups have already synthesized artificial chromophores with tunable optical characteristics. The goal is to devise solar cells that convert sunlight into electricity far more efficiently than today’s technology. Artificial or not, though, these kinds of chromophore-based light harvesting systems function properly only if each molecule is approximately three to five nanometers away from its neighbor. How can scientists possibly achieve such a precise pattern? One answer, of course, is to go back to nature.

     In recent experiments, Francis and his research group have functionalized the tobacco mosaic virus, harmless to humans, so that its shell can be used as a template for the self-assembly of a wide-variety of nanomaterials. Indeed, Francis says, the rod- and disc-like shapes of the obtained assemblies are particularly well suited as a chassis for a light harvesting system inspired by nature.

     “For me, the real excitement of nanoscience is that it allows you to build devices with functions that simply don’t exist now and can only occur at the nanoscale,” he adds. go to next article
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This article originally appeared in the March issue of ScienceMatters@Berkeley, a monthly online publication produced by the Colleges of Letters & Science and Chemistry. The magazine can be read in its entirety at http://sciencematters.berkeley.edu