Metal Particles That Power Themselves to Move Like Living Cells

Platinum/gold nanorods immersed in distilled water migrate en masse from lower right to upper left toward a source of hydrogen peroxide that has been introduced into this region of the distilled water. As the hydrogen peroxide seeps into the distilled water it creates a chemical gradient along which the nanorods swim. The metallic 1,500 x 400 nanometer rods propel themselves toward the highest concentration of hydrogen peroxide by catalyzing the oxidation of hydrogen peroxide to oxygen at their platinum heads and to water at their gold tails.

Ayusman Sen makes tiny, non-living objects do something they have never been able to do on their own -- he makes them move. He also makes them chase food, engulf prey, tug cargo, gather like beach-goers on a steamy summer day, and disperse as though a shark just paid a visit to the swimming area. Upon first glance, the micro-movies created by Sen and his group appear to belong in the Department of Biology, rather than in the Department of Chemistry where Sen serves as head at Penn State. But chemistry is where they belong, because these nanoscale objects have never been alive. Nonetheless, these tiny pieces of metal, which are measured in billionths of meters, ambulate their way around a microscope slide as though performing an impromptu water ballet when stimulated by the effects of their self-produced catalysis.

Sen has applied catalysis, his central research interest, to the difficult problem of making nanoscale objects move purposefully from one place to another. "People make wonderful nanostructures, but they don't move around," he said. "Practically, if you want to move a nano-object from one point to another, you need a nano-motor and you have to power it. But these objects are so small you can't hook them up with a piece of wire. Neither can they carry around their own fuel, so they have to use what's around them. In nature it's done with catalytic reactions using substances from the surrounding environment. For instance, with sperm cells or flagellar bacteria, a catalyst converts ATP (adenosine triphosphate) to ADP (adensosine diphosphate) producing mechanical motion in the process. The fact that our metallic particles move around at this size regime is a first example of an inorganic object behaving like a living system."

Three fundamental scientific phenomena underlie Sen's technology: 1) catalysis, which is the chemical phenomenon whereby a substance accelerates a chemical reaction but ends up in its original chemical condition at the end of the process; 2) redox reactions in which electrons and protons are broken away from their parent atoms and are pumped back and forth between substances, which results in the liberation of energy and consequent changes in electrical charge; and 3) electrophoresis, which is the tendency of small particles suspended in solution to move toward electric fields of unlike charge and away from those of like charge.

In simple stepwise terms: catalysis triggers a redox reaction between particle and solution, which results in a difference in charge in different areas of the solution, which in turn causes the object to undergo electrophoresis and to move preferentially toward or away from concentrations of charge. While the directed movement of clusters or ensembles of microscopic and nanoscopic objects by means of electricity and magnetism is not new, self-powered autonomous motion of individual objects never has been demonstrated before.

UNCOMMON BEHAVIOR WITH COMMON MATERIALS

Ayusman Sen

The fluid that Sen and his associates use most often is a dilute solution of the medicine-cabinet standard, hydrogen peroxide (H2O2) which, in addition to serving as a convenient swimming hole for tiny metal objects, also serves as their energy supply.

The metallic objects most often are silver, gold, or platinum particles and structures measuring between one-thousand and two-thousand nanometers, or billionths, of a meter. Although smaller particles are expected to move in exactly the same way, seeing them do so is constrained by the limits of optical microscopy, which cannot resolve objects smaller than half the wavelength of visible light -- around 250 nanometers.

The fluid environment in which Sen's particles move about is produced as a consequence of the parallel oxidation and reduction of hydrogen peroxide, which yields a flow of protons and electrons, and the formation of oxygen and water. Together the reactions are known as redox reactions -- shorthand for reduction, where atoms gain electrons, and oxidation, where they lose them. Redox reactions frequently produce electrically unbalanced atoms or molecules called ions, which come in two flavors: atoms and molecules called anions, which have more electrons than protons and a negative electric charge; and those called cations, which have fewer electrons than protons and a positive charge. Unlike evenly balanced (ground state) atoms and molecules, which are electrically neutral, ions of either species have an affinity for other atoms, molecules, particles, and solutions with opposite charges and an antipathy for those with like charges.

By definition, compounds made up of ions are salts, so in laboratory jargon the process of producing ions by means of redox reactions is sometimes referred to as pumping salts. During the salt-pumping process, as ions disperse, they create an electrical gradient in the fluid surrounding the metal particles that generated them. The difference in electrical charge results in the preferential movement of the particles toward or away from concentrations of charged areas of the fluid.

Although hydrogen peroxide along with gold and platinum are convenient materials for exploiting the transport characteristics of ionic systems, many combinations of metal catalysts and fluid energy sources are possible.

TAXIS! TAXIS!

The scientific term for motion caused by a chemical gradient is chemotaxis. Similarly, motion caused by magnetism is known as magnetotaxis. And that caused by light is phototaxis. The phenomena were thought to be limited to living things until Sen and his colleagues proved otherwise. "Bacteria will find out where the food is and they will swim toward it, or find out where the poison is and swim away," he explained. "Previously it was thought that in order to do that bacteria needed memory to know where the food is and where it is not. As a result of our experiments with non-living objects, we now think that it may be a purely physical phenomenon."

The group's achievements are embodied in a nanoscale catalytic motor with no moving parts. The cylindrical 1,500-x-400-nanometer (nm) nanorods, as they are called, are made of alternating segments of platinum and gold. The nanorods are fabricated by electrodepositing alternating layers of metals that exhibit different catalytic properties in an appropriately sized mesh and are extracted en masse.

"The platinum end catalyzes the oxidation of the hydrogen peroxide, converting hydrogen peroxide to positively charged protons and negatively charged electrons along with oxygen," Sen explained. "At the same time, the gold end consumes protons and electrons to reduce hydrogen peroxide to water. So there's a net electron flow from the platinum end to the gold end. Since electrons can't move on their own, protons have to move at the same time. And since protons have a slight attraction to water they tend to drag water molecules resulting in a fluid flow from the platinum end to the gold end. If the nanorod is free to move, it moves in the direction opposite the fluid flow. But if it is stationary, it pumps fluid around itself. So the protons act as paddles, either moving the nanorod or pumping the fluid around it."

GETTING NANO TO WALK A STRAIGHT LINE

Nonetheless, getting the nanorods to move from one point to another in a straight line is fraught with obstacles for several reasons. First, unless the two discrete parts of the redox reaction happen in different locations along the nanorod, there will be no difference in charge along the nanorod, which disallows electrophoresis. Sen and his colleagues have answered this problem by designing the nanorods asymmetrically, ensuring a difference in the electrical gradient between the platinum and gold ends.

Secondly, due to the very smallness of nanoscale objects, they are not very much affected by inertia, which is the law of physics that keeps moving things in motion and non-moving things in place. This is because the solid/liquid friction between objects and the fluids in which they are suspended is governed by the ratio of the mass of the object to the viscosity of the fluid. Since nanoscale objects have almost no mass due to their infinitesimally small size, the fluids in which they float become effectively more viscous than they would be at bulk scale. Sen gave a practical illustration: "For a human being to feel what a bacteria feels, he or she should try swimming in molasses." Despite the drag, the large difference in ion concentration between nanorod and fluid causes sufficient attraction between the electron-rich nanorod and its self-catalyzed, proton-rich fluid for the nanorod to move.

The nanorods' insignificant mass makes them especially susceptible to disturbances by thermal turbulence and random molecular collisions that interfere with their straight-line movement. As a consequence, even though their end-to-end asymmetric design confers directional stability, much like that of the head and tail of a fish or the bow and stern of a ship, without some type of guidance mechanism the nanorods amble about in aimless circles, spirals, pirouettes, and geometrically dissolute patterns. In answer to this problem, the Penn State team has developed three different guidance systems based on three scientific phenomena: magnetism, chemistry, and light.

NANO MAGNETICS

The magnetic guidance system that the group developed several years ago to steer nanorods incorporates two thin layers of magnetized nickel interposed between the platinum head and two gold tail sections of the nanorod. The magnetized nickel layers respond to an external magnet by coaxing the nanorods to swim in a direction that is perpendicular to its field lines. Because the nickel layers are thinner than the nanorod's diameter, their magnetic poles run across the nanorod rather than along its axis, thereby facilitating linear guidance. Although the magnetic field offers directional guidance, it does not act as a propulsive force since it effectively pulls the nanorods from side to side, rather than from back to front.

Coincidentally, the magnetic system mimics a biological system found in bacteria that live deep in mud. As their name suggests, magnetotactic bacteria are guided by magnetic forces, specifically, those of Earth's geomagnetic poles. They live in aquatic muck at the interface of the oxygen-containing and non-oxygen-containing layers. These animals have evolved the ability to biomineralize iron (magnetite and greigite) from water and to sequester it in vesicles within their single-celled bodies. The iron-containing cell vesicles act like compass needles responsive to Earth's magnetic field, guiding the animals to swim in a direction that is parallel to Earth's geomagnetic field lines, which reduces the chance of their wandering away from their food supply by ensuring that they swim only toward or away from Earth's magnetic north and south poles, depending on which hemisphere they live in. Coincidentally, Sen's nanorods are about the same size and possess the same magnetic characteristics as these single-celled animals.

NANO-CHEMISTRY

The group's chemical guidance system is demonstrated by one of the Sen group's micro-movies, in which a piece of gel soaked in hydrogen peroxide is placed in a drop of distilled water containing one-micron (1,000-nanometer) platinum/gold nanorods. The nanorods move along a chemical gradient, created as the hydrogen peroxide disperses into the water, toward the highest concentration of dissolved hydrogen peroxide. To the untrained eye, the moving particles look more like migrating biological organisms than like little pieces of metal.

NANO IN THE SPOTLIGHT

With yet another form of phoresis, called phototaxis, movement is initiated when light-sensitive materials become excited to a higher energy level, thereby triggering catalysis. In a demonstration video, silver-chloride particles suspended in a solution of distilled water remain stationary until an ultraviolet light is shone upon them, whereupon they gather at the center of the ultraviolet light where they continue to aggregate, even when the light is turned off.

PUTTING NANO TO WORK

While getting metal particles to move toward or away from specific targets is an admirable achievement, as a practical matter it won't be very valuable unless and until the objects can do something once they get there. That something might be delivering materials for self-assembling superstructures, transporting drugs inside the human body, functioning as roving sensors, or serving as fluid pumps. In order to prove the feasibility of such functions, Sen and his group currently are at work on attaching microscale cargo to their nanorods and hauling them to specific locations. At this point, cargo can be attached to nanorods in one of two ways: either electrostatically, by depositing an additional segment of charged material to the tail of the nanorod and charging the cargo oppositely, or chemically, with a synthesized sulfur-based tether.

Summarizing his group's progress thus far, Sen reports, "We know how to load up cargo. We know how to move it. Once we learn how to unload cargo, we will have the equivalent of nano dump trucks that can deliver cargo to a designated site or lay down patterns to create superstructures.

"With drug delivery, at present a drug is injected or absorbed and wanders around the bloodstream until it gets where it needs to go by passive diffusion. If you couple our technology with chemotaxis, a nano-motor would actively seek a specific target and swim to the chosen site."

Citing another set of practical applications for autonomous nanomotors, Sen points to the need for active mobile sensors. "All sensors now sit in one place and wait for whatever they're designed to sense to come to them. Our sensors would actually hunt down their target because they would move along a gradient trail."

In another scenario of the technology's promise, Sen talks about nanoscale particles as though they were hunter and quarry. "We also have predator/prey situations in which a prey particle catalyzes the formation of food or energy source for a predator particle," he said. "In this scenario, electrophoresis naturally causes the predator to pick up the gradient and follow the prey."

SHARK! SHARK!

An example of such activity is an enlightening experimental accident in which Michael Ibele, one of Sen's doctoral students, discovered a dust particle swimming in distilled water on a microscope slide amidst a large crowd of silica tracer particles. While viewing the slide, Ibele found the silica crowd steadfastly moving en masse out of the dust particle's way whenever it approached. The aggregate movement looks uncannily like the swimming area at a July seashore into which a solitary shark has intruded. While he is attempting to duplicate the event, Ibele speculates that the dust particle was pumping some species of salt into the fluid, thereby creating a chemical gradient repulsive to the beach-going particles. The dust particle has been affectionately dubbed "nanoshark boat."

While many of the prospective uses for Sen's autonomous nanomotors exist outside the bounds of macroscale mechanics, the group's gold nano-gears are powered catalytically by gradient-generating platinum-tipped gear teeth. The gears spin freely in fluid and look just like the macroscale gears we use every day in everything from automotive transmissions to cuckoo clocks. The group currently is investigating ways to mount the gears on axles in order to enable them to do useful work.

"We can do a lot of things that formerly were thought to be biological in origin," Sen said. "The public is often worried that scientists are producing things that will do evil things. The good news is that we can make thousands of these things in an afternoon, but our things can't reproduce."

Contact:

Ayusman Sen, (814) 863-2460, asen@psu.edu

For more information about Dr. Sen's research, visit: http://research.chem.psu.edu/axsgroup/

About the writer:

Tom Imerito is president of Science Communications, a Pittsburgh technology communications consultancy (www.science-communications.com). He can be reached at 412-833-4932 or at thomas@science-communications.