Tiny movers and shakers
doi:10.1038/nindia.2009.118 Published online 30 April 2009
It is eminently possible to create very small metallic 'vehicles' that run on fuel amassed from their surroundings and deliver things from one place to another. The beauty of these tiny machines is that billions of them can fit into something like the size of a pinhead and power themselves, says Ayusman Sen, a chemical scientist who has spent most of his life in the laboratory looking at them.
Sen, an alumnus of the University of Calcutta and Indian Institute of Technology, Kanpur, now heads the chemistry department at Penn State University. Along with his co-worker Thomas Mallouk, director of the Centre for Nanoscale Science, he has been trying to make these tiny, metallic objects move on their own power.
Nanostructures, he says, are brilliant creations but even more wonderful is to make them move. "These structures have to use what's around them. In nature, it's done with catalytic reactions using substances from the surrounding environment," Sen explains.
Nanoscale moving systems are currently the subject of intense interest, he says, due to their potential applications in nanomachinery, nanoscale assembly, robotics, tribology, fluidics, and chemical/biochemical sensing. Using such machines, one can deliver drugs to tissues, perform difficult surgeries and even communicate the outside world from inside the body. They mimic biological motors by using catalytic reactions to create forces based on chemical gradients.
Sen's work is driven by catalysis, the chemical phenomenon whereby a substance accelerates a chemical reaction but emerges unchanged at the end of the process. He and his team of students and colleagues focus their efforts on redox (reduction-oxidation) chemical reactions, where electrons and protons are broken away from their parent atoms and pumped back and forth between substances, liberating energy.
Writing in the upcoming issue of Scientific American1, Sen and Mallouk describe how to make such nanotech motors. "For a micron-size body in water, swimming is a bit like wading through honey. A nanomotor has no memory of anything that pushed on it—no inertia—and inertial propulsion schemes (such as drifting after the recoil from bubbles) are hopeless. The way our nanorods actually work is that they apply a continuous force to prevail over the drag with no need for gliding," they explain the unique mechanism.
The group is currently working on the mechanism of how to offload the cargo that these nanomotors can carry on them. They are looking at photosensitive linker molecules that will break and drop the cargo when exposed to light.
Earlier, Sen and his team had reported the movement of gold-platinum bimetallic nanorod motors towards hydrogen peroxide fuel when they are placed in a fuel gradient — the first time such behavior was seen outside of the biological world2.