Catalytic Motors

Research on nanoscale robots is opening a completely new trend in the way we look at the world, and has derived interest from various quarters of science and technology.  If the robots at nanoscale do their job as desired, their potential and utility are unbound. One of the major hurdles towards such a design is the interaction of the robots with its local environment. It is well know that when things shrink to the nanoscale, the Brownian motion plays a vital role in the movement of such nanoscale object, and an ‘extra’ amount of force has to be spent by the object to overcome the ‘random walk’. At these scales, Brownian motion makes it all but impossible to keep a steady direction of motion while immersed in a fluid. In fact, all molecular-scale motors in nature—including muscle proteins and the enzymes that produce ATP—are either constrained to run along a track or embedded in a membrane due to this. But this does not discourage us from building motors at nanoscale. In fact, there are now many smart methods to overcome the above disadvantage.

One such method is the Catalytic motor. The idea behind this is very simple: create a chemical reaction at the surface of an object which is coated with a catalyst. When the reaction takes place, the product will escape the surface, which further propels the object in the opposite direction. Let’s see an example of this.

In 2001, Rustem Ismagilov (now at Chicago) and George Whitesides, both then at Harvard University found that centimeter-scale “boats” with catalytic platinum strips on their stern would spontaneously move on the surface of a tank of water and hydrogen peroxide ( H₂O₂). The platinum promotes the breakup of H₂O₂ into oxygen and water, and bubbles of oxygen formed that seemed to push the boats ahead by recoil, the way the exhaust coming out the back of a rocket gives it forward thrust.

Wonderful !.....isn’t it….now the question is how do you bring this down to nanoscale. Ayusman Sen and coworkers at Penn State University came up with a smart idea: Their miniaturized version of the Harvard engine was a gold-platinum rod about as long as a bacterial cell (two microns) and half as wide (350 nanometers). The rods were mixed into the solution, rather than floating on the surface. Like the ATP-powered molecular motors inside the cell, these tiny catalytic cylinders were essentially immersed in their own fuel. And they did indeed move autonomously, at speeds of tens of microns per second ! But the reason why they moved was different from the Harvard engine. The way these nanorods actually work is that they apply a continuous force to prevail over the drag with no need for gliding. At the platinum end, each H₂O₂ molecule is broken down into an oxygen molecule, two electrons and two protons. At the gold end, electrons and protons combine with each H₂O₂ molecule to produce two water molecules. These reactions generate an excess of protons at one end of the rod and a dearth of protons at the other end; consequently, the protons must move from platinum to gold along the surface of the rod. Like all positive ions in water, protons attract the negatively charged regions of water molecules and thus drag water molecules along as they move, propelling the rod in the opposite direction as dictated by Newton’s law of motion that every action has an equal and opposite reaction. And thus a catalytic motor works…..still there are plenty of things to venture, and am sure more progress will be made in this field in coming years.

Every time I come across these nano-excitements in science, a quote by Feynman always resonates in my mind “There is plenty of room at the bottom”, and what a prophetic statement it has turned out to be !