Over the past decade, research groups around the globe have created a variety of tiny particles that move on their own, powering themselves forward. Eventually, researchers hope to use such particles to deliver drugs inside the body and whisk up chemical spills. Now, two teams of researchers have given these microparticles a couple of new skills. One enables them to swim upstream, mimicking the way certain bacterial pathogens find their targets; the other churns out hydrogen gas for fuel cells at an unprecedented rate.
Self-powered microparticles—only a millionth of a meter or so across—use different chemicals in solution as fuel, breaking them down to propel themselves forward. In 2004, for example, researchers designed nanosized particles that, when placed in solution containing hydrogen peroxide, generate hydroxide ions that create a small electric field around the particles; because the particles themselves are charged, they respond to that field and are pulled through the solution.
Now, researchers have taken that work a step further. A team led by researchers at New York University reports today in Science Advances that it has made hydrogen peroxide–propelled microparticles that, under the right conditions, swim upstream against fluid flowing through a small capillary. Parasitic bacteria use much the same method to push against the fluid pressure in the urinary tract to colonize the bladder.
In this case, the synthetic particles are micrometer-sized white plastic spheres that contain a small round patch of hematite, or iron oxide. In solution, the hematite splits oxygen away from hydrogen. This reaction sets up chemical and local electric gradients, causing the polymer beads to surf forward on those flows in the direction of their hematite protrusions. But it does so only in the presence of blue light. Without the light, the particles just float about. When the light is switched on, however, the hematite absorbs enough extra energy to carry out its chemical reaction and move. The researchers also found that when they push their solution through a pipette, the particles swim against the current.
“This is very exciting,” says Tom Mallouk, a chemist and micromotor expert at Pennsylvania State University, University Park, who was not involved with the work. He notes that researchers have produced many different types of micromotors that move through chemical gradients—a process known as chemotaxis—but that this is the first one that responds to fluid flow, or rheotaxis. “It’s a new method of control that is broadly applicable to micromotors,” Mallouk says. He adds that it might be possible to harness the effect to improve drug delivery inside tumors, which typically exert pressure against compounds trying to enter.
In the second study, a team led by nanotechnologist Joseph Wang of the University of California, San Diego, used microparticle motors to explore a very different application. In this case, their goal was to supply hydrogen to a fuel cell that is used to convert chemical energy to electricity that can be harnessed to power a car or home. In these cells, hydrogen gas (H2) is broken down at one electrode into positively charged hydrogen ions and electrons. The stream of electrons produces electricity, which is sent through a wire for powering devices. The current then returns to a second electrode, where the electrons meet up with the positively charged hydrogen ions and oxygen from the air to generate water.
But although hydrogen fuel cells are highly efficient at converting the gases to electricity, the gases themselves take up a large volume. That makes it difficult to store enough of them near the fuel cell to provide enough electricity to power a car over a long distance, for example.
One option is to use catalysts to react with energy-dense liquid fuels such as sodium borohydride to generate large amounts of hydrogen that can then be fed into a fuel cell. Such catalysts have been made using tiny microparticles. But there’s a catch: The catalytic particles produce hydrogen by forming bubbles where they come in contact with the sodium borohydride solution. Those bubbles, it turns out, tend to cling to the particles and prevent the catalyst from continuing its work, dramatically slowing the reaction.
So for their study, Wang and his colleagues formed two-part “Janus” particles, named after the two-faced Roman god. One half of the particle was made of titanium, which doesn’t react with sodium borohydride. The other half was platinum, which reacts vigorously, ripping off H2 molecules. Because the particles eject H2 only from the platinum side, they shoot through the solution and away from the H2 bubbles as they form. As a result, the platinum surface is continually exposed to sodium borohydride and able to convert it to H2. In a recent article, published online before print in Angewandte Chemie International Edition, Wang and his colleagues reported that their particles increased the hydrogen generation some 20-fold over previous efforts.
Mallouk, who wasn’t involved with this research either, says the work marks a clever proof of concept for increasing the rate of H2 production and could give hydrogen fuel cell cars a big boost. More broadly, he says, it shows how making catalysts mobile might speed up many different types of reactions. “That could be a powerful and useful concept.”