Our 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). Our 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, bearing an eerie resemblance under the microscope to live swimming bacteria [see video at www.SciAm.com/nanomotor]. As often happens in science, however, the hypothesis that led to the experiment was wrong. We had imagined our nanorods spewing tiny bubbles off their back and being pushed along by recoil. But what they actually do is more interesting, because it reminds nanotechnologists that we must think very differently about motion on small length scales.
At the macroscale, the notion of recoil makes good sense. When someone swims or rows a boat, their arms, legs or oars push water backward, and the recoil force pushes the body or boat forward. In this way, a swimmer or boat can glide forward even after one stops pushing. How far an object glides is determined by the viscous force, or drag, and by the inertia, a body’s resistance to changes in its velocity. The drag is proportional to the object’s width, whereas the inertia is proportional to the object’s mass, which in turn is proportional to the width to the third power. For smaller objects, inertia scales down much faster than drag, becoming negligible, so that drag wins out. On the micron scale, any gliding ends in about one microsecond, and the glide distance is less than one 100th of a nanometer. Hence, for a micronsize 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 conafter 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. 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.
Once this principle was established (with the help of our students and our Penn State collaborators Vincent H. Crespi, Darrell Velegol and Jeffrey Catchmark), several other catalytic nanomotor designs followed. And Adam Heller’s research group at the University of Texas at Austin and Joseph Wang’s group at Arizona State University showed that mixtures of different fuels— glucose and oxygen or H2O2 and hydrazine— could make motors run faster than they do with a single fuel.
Whereas freely suspended metal nanorods move with respect to the bulk solution, an immobilized metal structure in the presence of H2O2 will induce fluid flows at the interface between the structure and the fluid, thereby potentially powering the motion of something else immersed in the fluid. We have demonstrated this fluid-pumping effect on a gold surface patterned with silver.
Source of Information : Scientific American(2009-05)