Doctoral Work, Part 1: Bi-metallic Nano-Swimmers

Let's say you were bored one day, and you had a bottle of hydrogen peroxide and a piece of platinum wire lying around. Let's say you wanted to find out what would happen if you dropped the platinum into the peroxide (you are really bored). What would you see? As soon as you drop the wire into the solution, a chemical reaction will immediately begin to occur that will cause tiny bubbles to form all over the metal surface (see note 1 below). Although bubbles would form, the wire would not move around in the container at all. Why would it, right? It would simply settle to the bottom. Similarly, if you dropped a piece of gold into the peroxide, it would not move.

But what if instead you dropped a microscopic metal cylinder made of half platinum and half gold into the peroxide? What would happen then? It would not be unreasonable to assume that the cylinder would act similarly to the individual platinum and gold pieces and simply settle to the bottom of the container. However, the actual result is quite different.

You see, it is possible to make these mini-cylinders, and they are known as 'bimetallic nanorods.'  When immersed in hydrogen peroxide, bimetallic nanorods move entirely on their own at 10-100 body lengths per second. No outside driving force is required. The rods always move along their axis, and always with the platinum end forward (which is a clue as to how they move).  For comparison, the Space Shuttle moved at a bit over 100 body lengths per second while achieving orbit.

Diagram of a typical nanorod.  The black arrow indicates the direction of motion.

Even though the rods move fairly fast relative to their size, it should be emphasized that "their size" is really quite diminutive. Bimetallic nanorods are typically about 2 micrometers (or "microns") in length and about 300 nanometers in diameter (see note 2). To get an idea of how small that is, consider that an average human hair has a diameter of about 100 microns. You would need to lay 50 nanorods end-to-end to equal the diameter of an average human hair.

If that doesn't convince you that these things are tiny, take a look at the picture below. That container has about 100 billion (1 followed by 11 zeros) nanorods in it (see note 3). And they're not even taking up the entire container - most of them are gathered in that dark smudge at the bottom.

~100,000,000,000 nanorods.  Seriously.

At this point, two good questions have probably occurred to you.

1. Why should we care that these little cylinders can swim in peroxide? What can we do with them that would be useful or helpful?
2. How in the world do they manage to move on their own? Why do they only move in hydrogen peroxide?

To answer the first question, bimetallic nanorods have a wide variety of potential applications in nanotechnology. One especially promising application is the enhancement of drug delivery technology. Traditional drug delivery methods include oral ingestion or injection. When you take a drug using one of these methods, the medication goes to a lot of places in your body besides where it is needed. Thus, in most cases, only a small portion of the medication reaches the intended "target." Targeted drug delivery aims to deliver medication only where it is needed in the body. This would make it possible to deliver a more concentrated dose of medication in the intended location while reducing the relative concentration of the medication elsewhere in the body, improving the efficacy of the drug while reducing its side effects.

So, how do bimetallic nanorods come in? It has been experimentally shown that nanorods are capable of attaching themselves to tiny spheres (1-micron diameter) made out of polystyrene (the same material that foam cups are made of) and tugging them around (see video below). One day we may be able to replace the sphere with a drug, and have the nanorods bind to the drug, seek out a certain site (e.g. a tumor) in the body and deliver the drug there (see note 4). One of the reasons that hasn't happened yet is that nobody has figured out how to make nanorods move in fuels other than peroxide. And the main reason that hasn't happened yet is that nobody knows exactly why they move in hydrogen peroxide.

Nanorods can be steered using external magnetic fields and can also pick up and release cargo at pre-determined locations.  Here, the cargo is a spherical polystyrene particle with a magnetic coating.  When it gets close enough to the cargo, the rod snaps on in much the same way that magnets "snap" onto your refrigerator door.

Which brings us to the second question: how do bimetallic nanorods move in hydrogen peroxide?Until my work, a complete physical theory had not been formulated, but clearly it has something to do with the peroxide itself. Somehow, the nanorods are using it as a fuel - they convert the chemical energy stored in the H2O2 molecules into mechanical energy (motion) (see note 5). We know this because they barely move at all in pure water, and because their swimming speed increases with the concentration of hydrogen peroxide; that is, the more peroxide per unit volume you have, the faster the nanorods go. So the peroxide is definitely the energy source for the autonomous motion. Of course, humans can also move autonomously in water (i.e., swim), and we also do it by converting chemical energy (stored in the body) into motion (using our muscles). But nanorods don't have muscles - they're just microscopic pieces of metal! We know they get their energy from the peroxide, but how do they convert it into the mechanical energy that propels them forward?

That question is the subject of my thesis research. There are several prevailing theories behind the nanorods' motion, although it has not been proven which one, if any, is the correct one. We are currently formulating our own theory, which I am using a computer model to simulate. The goal is to successfully recreate the motion of a nanorod on a computer. We are also conducting our own experiments with nanorods. If the results of my simulations agree well with the experimental results found by our group and by other groups, that will constitute strong support for my model. We will be that much closer to being able to use nanorods in the human body and elsewhere in nanotechnology.

Notes

1. What is happening here is that the platinum is acting as a catalyst to initiate the chemical reaction. Remember that the chemical formula for hydrogen peroxide is H2O2. Roughly speaking, the platinum gives the peroxide molecules a little "nudge" that makes them "want" to break down into water and oxygen gas. The oxygen gas shows up as the bubbles you see.

2. Just as there are 1000 millimeters in 1 meter, there are 1000 micrometers in 1 millimeter and 1000 nanometers in 1 micrometer. The general rule is that if one feature on an object is less than 1 micron in length (in this case, the diameter), the nano- prefix is used, hence the name nanorods.

3. You might ask how we know there are that many nanorods in there. (We didn't count them!) The answer has to do with the method used to make them. Nanorods are actually grown by depositing metal into tiny pores in a membrane made of aluminum oxide. The inner diameter of the pores is about the same as the diameter of the nanorods. We know how many pores there are in a given area of the membrane, so for a membrane of any given size we can have a fairly accurate estimate of how many nanorods we made.

4. Targeted drug delivery was recently realized in a proof-of-concept experiment: some colleagues of ours at UC-San Diego used a different type of self-propelling rod (made of zinc) and administered orally to some rather unsuspecting mice.  The nifty thing about these swimmers is that they use stomach acid as a fuel, and deliver cargo payloads into the stomach lining of some unsuspecting mice.  For the technically inclined, the journal article is here, and you can read a non-technical press release here.  This work is a major step forward toward the goal of using self-propelling particles to deliver drugs in nanomedicine.

5. By the way, there are naturally occurring nanoscale motors that are capable of doing a similar thing. For example, several different types of biomolecules are capable of harvesting energy from energy-rich molecules such as adenosine triphosphate (ATP) to initiate autonomous motion.