by Don Wood
Chemist Albert Stiegman wanted to make a pretty glass that would change color when exposed to light.
He failed, and he’s delighted he did.
He got his pretty glass, sure enough, but its indifference to light ultimately led to a discovery that may help scientists better understand the curious world of nanomagnetism.
The buzz of science and engineering these days, nanoscienceand its applied side, nanotechnologyis revolutionizing how researchers in chemistry, physics and biology approach their work. The field is focused entirely on the atomic world, where size is indeed everything.
Everything super-small, that is. The prefix “nano,” from the Greek, means “billionth.” Things within the “nano-world” are literally at least a billionth of a meter in diameter or overall lengthand often far smaller.
Stiegman’s newest “pretty glass” (he’s created several varieties) is a study in nanoscience with a surprising—if not yet fully understood—twist. So far, the substance is little more than a scientific curiosity, a happy accident—and that may be all it ever is. Any talk of the discovery opening a door to a snappy new high-tech product is wishful thinking, says Stiegman, an inorganic chemist at FSU.
But the fact that this novel material exists at all intrigues more than a few—last summer the discovery drew the attention of the Chemical & Engineering News, a leading professional journal, which reported on Stiegman’s find.
For years, Stiegman has experimented with making special, highly colorful glasses in his lab using what is called the “sol-gel” process. Basically, the technique is fairly simple: two or more compounds are mixed together, and a resulting liquid slowly gels to a Jello-like consistency that eventually hardens into a transparent solid.
The process produces glass that resembles ordinary glass—but with a key difference. Stiegman’s glass is shot through with tiny tunnels and caverns not much wider than a molecule. These nano-sized pores can allow small molecules of various compounds to penetrate the glass and react with any number of chemicals that the glass’ gel can be doped with. The resulting chemical reaction can do some neat thingsfrom changing the glass’ color to making it fluoresce all its own.
A few years back, Stiegman figured out how to make his glass using a compound made out of vanadium, a silvery-white, soft metal often used to make high-strength steel alloys.
Using vanadium oxide, he soon found that he could turn the glass into various colors by exposing it to different chemicals. For example, in the presence of hydrogen sulfide (the “rotten egg” odor of freshman chemistry courses) his glass would turn amber. Formaldehyde would turn it green. His glass thus had potential for being a rather dramatic detector for these and other pollutants.
This time around, Stiegman wanted to put a compound in the glass so that it would change color when exposed to light, a property called photochromism. The phenomenon already is evident in the marketplace, a common example being sunglasses that automatically darken when you step outside. Engineers also are interested in such materials as a way to store optical data in devices such as CDs, DVDs and computer memories.
For this experiment, instead of vanadium Stiegman chose a dye from a venerable class of compounds known as Prussian blues (see box, left). These brilliant blue dyes are made up of molecules built from two metal ions tied together by a molecule of cyanide. In this case, Stiegman used a dye whose metal ions were cobalt and iron.
Stiegman assigned graduate student Joshua Moore to work out a way to put the Prussian blue into the sol-gel glass.
Moore soon realized his prof had thrown him a curveball. What he faced was no simple problem—Prussian blues are typically made by mixing together two reagents containing the right concentrations of metallic ions. The positive and negative ions immediately join together to form the desired molecule, which promptly “precipitates out”as the chemists sayor clumps together and collects at the bottom of the container.
So, if you mix the sol-gel components and the Prussian blue components together in the same vessel, the Prussian blue will be sitting benignly on the bottom while the sol-gel is still slowly hardening above—without any dye in it.
Moore eventually solved the problem by diluting the amount of dye in his glass recipe. This allowed more control over the precipitation problem. Finally, he tinkered with this and other parameters until he hit what Stiegman called the “sweet spot”–just the right formula so that the Prussian blue particles were trapped by the hardening gel before they had a chance to fall out of solution. Delighted, Stiegman dubbed the process “arrested precipitation.” The result was a beautiful purple glass with some surprising properties.
With help from FSU physicist Eric Lochner, a member of the university’s Center for Materials Research and Technology, Stiegman and Moore began testing the material’s optical properties. Alas, the hoped-for color change didn’t appear. As far as making a photochromic material was concerned, the experiment was a bust.
One day, while Moore and Lochner were discussing these depressing results, Lochner suggested testing the material for magnetic properties. He knew that Prussian blue compounds were magnetic and thought something interesting might turn up.
When Moore took the idea to Stiegman, his prof was skeptical. Stiegman thought there would be far too few particles too widely scattered in the glass to show any interesting magnetic behavior. Still, Moore wanted to pursue the idea and Stiegman didn’t want to squelch his enthusiasm. So, collaborating with Lochner, they tested their glass for magnetic properties.
What they found was astonishing. The Prussian blue particles trapped in the glass proved to be superparamagnetic–a phenomenon of considerable interest to scientists and engineers working in the field of nanotechnology.
Superparamagnetism is a daunting word, but the concept isn’t too hard to understand—especially if you skip over the quantum mechanics. The phenomenon occurs when magnetic particles are made small enough—on the order of ten billionths of a meter. At that size, each particle acts like a tiny magnet, but thermal agitation–the random jiggling of molecules due to heat–prevents adjacent particles from lining up with each other. Instead, the particles are all randomly oriented and behave like lonely, magnetic islands with no common interaction. As a consequence, the material as a whole doesn’t act like a magnet at all.
But applying an external magnetic field can force the particles to align so that the material temporarily becomes magnetic. As soon as you switch off the magnetic field, thermal agitation scrambles the particles again and you’re back where you started with a nonmagnetic material. The phenomenon is so efficient that researchers get excited over the prospects of using superparamagnetic devices as tiny magnetic switches to control a variety of electronic gadgets.
The possibilities have helped generate excitement about Stiegman’s discovery. (Lochner mentioned one novel idea–trapping both a superparamagnetic particle and a bit of a drug in a membrane, then using an external magnet to direct the drug to where you want it to go in the body.)
Another interesting discovery about the Prussian dye-impregnated glass is that the amount of magnetism can be tuned using a combination of ammonia and light. Moore and Stiegman found that by shining light on the glass they could increase its magnetic strength, and doping it with ammonia changed its reaction to the light. The upshot was a clever means of tuning the glass’ magnetic properties. This property suggests the possibility of someday writing to a magnetic memory using a laser, then reading the data back with a magnet, said Stiegman.
But he’s quick to put the finding into perspective.
“This is a scientific discovery, and I’m not sure it’s an industrial discovery,” he said. “The novelty is in how it was made and the kind of particles that make it superparamagnetic.”
The novelty of the method is the ease with which the nano-sized particles are created, he said. Making these things is usually a difficult process involving many steps. In this case, the Prussian blue nanoparticles appear spontaneously as a result of the arrested precipitation in the sol-gel.
Still, in the field of materials science, the finding is a first. In the C&EN article last August, Geoffrey F. Strouse, an inorganic chemist at U. California, Santa Barbara, lauded the discovery for its potential place in technology, noting the importance of having a magnetic substance that can be molded.
“It’s truly a neat application with real potential in the design and development of magnetic materials,” said Strouse, who is scheduled to join FSU’s chemistry department later this year. “This could give rise to some fascinating magnetic structures that can literally be cast into a device, rather than having to be formed (via conventional) methods.”
Stiegman’s lab, meanwhile, is extending the sol-gel process to create other kinds of entrapped nanoparticles (he’s already succeeded with a different Prussian blue) and using heat, light and chemical reactions to transform the particles into other substances of interest.
As for the pretty glass that changes color when exposed to light–for now, that project sits on the shelf.
“My original idea didn’t amount to anything,” Stiegman said. “We set out to do one thing and wound up doing something else.
“But the something else was even better than the thing we were trying to do, so I’m not complaining. I believe in serendipity.”