By GABRIEL POPKIN
In 1959, the famous physicist Richard Feynman gave a talk titled "There's Plenty of Room at the Bottom." Feynman's message to his peers was that the nano-world — the creation and control of matter at the scale of billionths of a meter — had barely been explored.
More than 50 years later, nanoscience has exploded. Universities around the world have launched nanoscience centers and initiatives; the United States funds a National Nanotechnology Initiative to the tune of more than $1 billion a year. More than 1,600 nanotechnology-based consumer products have hit the market, from high-tech coatings and sunscreens to catalysts and artificial tissue components.
And yet, a talk with Feynman's title would still be apt. In fields from electronics to data storage to medicine, many scientists feel that nanotechnology's potential is only starting to be realized.
"It's a vast field, and there's still new basic research coming out," said Vivien Zapf, a physicist at the National High Magnetic Field Laboratory. "Especially the ability to sense and manipulate individual atoms is a whole other field."
Scientists haven't always had the tools to study small-scale matter with the required precision. Ironically, studying the world's smallest constituents often requires some of the largest and most sophisticated hardware. (Just think of the 27-kilometer, $10 -billion Large Hadron Collider needed to discover the subatomic Higgs boson.)
The same is true for nanoscience. Some of the field's most cutting-edge work is now being done in high magnetic field labs around the world, where clever uses of the world's strongest magnetic fields are teasing out microscopic matter's secrets.
Nanocages may hold key to advances in health, energy
In 1985, chemist Harry Kroto and colleagues at Rice University in Houston zapped a pile of graphite with a laser and discovered a strange new molecule. It consisted of 60 carbon atoms arranged in a pattern similar to that of a soccer ball.
They named their new material buckminsterfullerene, after the geodesic domes of visionary architect Buckminster Fuller; others gave it the catchier term "buckyballs."
The discovery inspired an avalanche of research into the molecule's unique mechanical and electrical properties, and netted its discoverers the 1996 Nobel Prize in Chemistry.
Today, Paul Dunk, a chemist at the National MagLab in Tallahassee, Florida, who worked with Kroto until his death in 2016, is helping to make these properties useful. He's not studying C60, but other strange and wondrous forms that appear when he zaps graphite, metals and other elements with a laser.
Hints of the field's potential appeared as early as 1999, when scientists unexpectedly found carbon molecules made up of 80 atoms, rather than 60. Like C60, C80 was stable, robust and symmetrical. But these larger buckyballs held a surprise. Inside each molecule's walls were three atoms of a metal called scandium and a nitrogen atom. C80 could, as it turned out, encapsulate other atoms.
Left: Dunk's research has illuminated how carbon nanocages form when graphite, a metal and nitrogen are vaporized with a laser. (Credit: Paul Dunk); Right: Paul Dunk. (Credit: Stephen Bilenky)
The discovery of these "nanocages" opened up the possibility of using buckyballs to transport tiny cargoes. Gadolinium, for example, is an excellent contrast agent for the magnetic resonance imaging (MRI) technology often used in medicine, but it is also toxic. Gadolinium in a nanocage, however, could safely travel through the body to a site where it’s needed.
But there is a holdup: No one understands the violent, chaotic reactions that bring these nanocages into being, much less how to manufacture them consistently and cheaply.
Dunk and collaborators in Texas and Spain are trying to change that. They vaporize concoctions of graphite mixed with different metals and other elements, and study the resulting nanocages in an ion cyclotron resonance mass spectrometer, a device built around a powerful magnet that weighs molecules. In this machine, each molecule circulates at a slightly different frequency that depends on its mass and electric charge. Dunk and his colleagues have used the device to study virtually all the elements in the periodic table to show which metallic atoms can take up residence inside fullerenes.
"The variety is staggering," he said.
The difference between the frequencies in mass spectrometry becomes more pronounced as the magnetic field strength increases, so the National MagLab's world-record instruments help Dunk separate and identify the vast diversity of molecules he produces in the lab.
"With other mass spectrometry methods that don't use high magnetic fields, you can't get that ultra-high resolution, so most of these nanocages have been basically invisible and undetectable," Dunk said. "That high magnetic field allows us to probe many self-assembly processes for the first time, and to develop new nanocages. It fills a really huge gap in nanoscience."
Beyond creating new metallofullerenes, Dunk and his colleagues test theories of how these compounds form by looking for hypothesized intermediate molecules between the original reactants and end products. They have shown that, unlike what many scientists believed, the cages do not shrink from or break off of larger globs of carbon, but rather nucleate around the metal, carbon atom by carbon atom. A paper they have written describing the process is slated for publication later this year.
With this improved understanding, the researchers hope to pave the way toward more controllable and efficient methods for manufacturing cluster nanocages for technologies ranging from new light-based electronics (or photovoltaics) to molecular electronics.
"There are a lot of new structures that have unique properties that have not been found,” Dunk said. “We hope to accelerate that process."
Added Dunk, "If (cluster nanocages) can be formed in sufficient quantity at the right price, the applications that tackle human health and energy concerns could become an immediate reality."
Drug delivery is rocket science
Think of it as door-to-door drug delivery — on the cellular level.
Peter Christianen, a physicist at the High Magnetic Field Laboratory in Nijmegen, the Netherlands, envisions a day when tiny rockets, careening through a patient's veins, will dispatch medicine directly to the site where it's needed.
Technically, the "rockets" are stomatocytes, artificial cells with mouth-like openings. Similar to a cell or vesicle in your body, stomatocytes have membranes consisting of long carbon chains, each with a water-seeking and a water-repelling end. These chains, pressed against each other, form a water-tight boundary that divides inside from out.
Recent research in this area by Christianen and his collaborators is a good example of how serendipity can lead scientific discovery into unexpected, but very promising, directions.
A few years ago, some of Christianen's colleagues placed platinum and hydrogen peroxide into a tiny stomatocyte. They found that the chemical reaction that ensued — the liquid peroxide "fuel" split into water and oxygen gas that blasted out of the cell — made the vesicle into a tiny rocket.
Christianen and his team suspected that they could use magnetic fields to reshape and steer the "nanorockets." But when placed in a strong, 20-tesla magnet, the stomatocytes didn’t deform much.
Top Left: The vesicle begins, without any magnetic field, closed. Then, after a magnetic field is applied, the mouth opens. Finally, after the cargo is loaded and the field is switched off, the filled capsule closes. When a magnetic field is reapplied (not shown), the vesicle will open and release its cargo. (Credit: Peter Albers); Bottom Left: Electron microscope images of vesicles. In the images on the left, they are outside a magnetic field and have only a small opening. In the images on the right, taken in a 20-tesla magnetic field, the vesicles are deformed, resulting in a large opening. (Credit: Radboud University); Right: Peter Christianen. (Credit: Radboud University)
However, the magnetic field did cause one crucial change: The vesicles' mouths opened. Immediately the team had a new idea: Stomatocytes could be loaded with drugs, shuttled to specified sites in the body, and zapped with a magnetic field to force them to disgorge the drugs. The scientists dubbed them magneto-valves.
"We thought, 'OK, we can use it as a capture-and-release device for drug delivery,'" Christianen said. "It's totally reproducible and reversible."
Developing nanorockets turned out to be just a first step. The next was learning how to mold them, a process that also involved some science serendipity.
Christianen learned that osmotic pressure (determined by the concentrations of organic solvents inside and outside the vesicles) influences a vesicle’s shape. So he and his colleagues tried a new experiment. First, they added water to a mixture of long carbon chains that had been dissolved in an organic solvent; the chains formed spherical vesicles. Then the researchers changed the osmotic pressure by diluting the solvent outside the vesicles; they responded by buckling into various shapes, each suggesting different applications.
Disc-like structures may have different flow properties or be better for cell-adhesion, said Christianen, while spherical vesicles could be used as nanoreactors for chemical reactions, and stomatocytes as drug delivery vehicles.
"So different shapes give different functionalities," he said.
Then Christianen's team turned again to magnetic fields — this time to determine exactly what shapes they were creating. They employed a technique called magnetic birefringence, which uses polarized laser light to image how objects are oriented in a magnetic field. This allowed them to precisely map how vesicles’ shapes change as osmotic pressure changes.
This measurement and control of stomatocyte nanorockets brings Christianen's team a step closer to one of medicine’s holy grails: precision drug delivery. The technique is especially promising because it would require nothing fancier than a run-of-the-mill MRI machine.
With more hard work — and perhaps a dash more serendipity — that vision seems bound to come to pass.
Magnetism for miniaturization
Miniaturization isn't just happening in medicine. In physics, researchers are probing different kinds of molecules for properties that will lead to the next generation of electronics, even quantum computers.
In the hot field of spintronics, for example, scientists are looking for ways to use the "spin state" of electrons (or whether an electron is oriented up or down) as the ones and zeros of binary code.
At the National MagLab's Pulsed Field Facility, physicist Vivien Zapf is leading a team down a different path to nano-level electronics: multiferroics.
In multiferroic materials, one behavior can be used to control another, akin to the way electricity can generate magnetism and vice versa. But in multiferroics, that coupling is between magnetism and something called ferroelectricity. And just as magnetism and electricity help make the modern world go round, so too, some physicists believe, will the pairing of magnetism and ferroelectricity help the world of the future go round.
In ferroelectric molecules, the distribution of electric charge is uneven — one side is more positive, the other more negative. If you apply an electric field to it, you can get that polarity to flip. So ferroelectricity is a way of moving electric charges around, and in that sense is like electricity. However, ferroelectricity is a lot greener.
Top: Zapf’s research was conducted in this world-record 100-tesla pulsed field magnet. (Credit: Dave Barfield); Bottom: Vivien Zapf. (Credit: Richard Sandberg)
"Unlike spintronics, you're using voltages instead of electric current, so you reduce the power consumption," Zapf said.
That means ferroelectricity, when paired with magnetism in multiferroics, could pave the way to smaller, more powerful electronic devices that are far more energy-efficient than could be achieved with spintronics.
To that end, Zapf and her collaborators, including National MagLab physicist Shalinee Chikara, try to create multiferroic molecules. They believe they have found a promising candidate: a molecular magnet featuring a manganese atom surrounded by rings containing carbon and nitrogen — or MnIII(pyrol)3(tren), if you want to get technical.
By putting the molecule in very high magnetic fields up to 65 teslas, the scientists cause the electron spins to transition to a different state.
It's a new way to pursue multiferroics.
"Instead of thinking about which way the spins are pointing, you actually change their size," Zapf said. "Which means you're changing the configuration of electrons within one atom so that, overall, the atom ends up with a different degree of magnetism than it had before." That change in magnetism triggers the ferroelectricity, making the material multiferroic.
There's something else special about Zapf's molecule: While the majority of work in multiferroics has focused on inorganic materials (they don't contain carbon), Zapf's group is one of a few that studies materials with organic molecules, which do contain carbon. Their success opens up hundreds of thousands of similar organic materials for previously unexplored multiferroic effects, Zapf said.
"That has us pretty excited," Zapf said. "These hybrid inorganic-organic materials are a new route to designing magnetism."
Zapf may never have pursued this promising line of research if she hadn't decided to venture outside her discipline and attend a chemistry conference a few years ago. There, she learned about spin state transitions — an area most physicists don't know much about.
"By letting physics and chemistry intersect, we get access to this new way of approaching multiferroics," Zapf said.
With her chemist colleagues from around the world, Zapf is cracking the door on a host of possible applications. Multiferroics could lead to new, highly sensitive magnetic sensors and new designs for small-scale, high-frequency devices such as antennas, power transformers or MRI magnets.
But figuring that out is someone else’s job, she said.
"There are people who specialize in applications. I'm more into, 'Let's design crazy new ways to make ferroelectrics talk to ferromagnets,'" she said. "I'm one of the people who feeds them the new ideas and new approaches."
Zapf's colleagues on her latest paper include National MagLab physicists Shalinee Chikara and John Singleton, chemists Nathan Smythe and Brian Scott of Los Alamos National Laboratory, theorists Shizeng Lin and Cristian Batista (University of Tennessee), and a student project at Harvey Mudd College with Jim Eckert and Elizabeth Krenkel.