Nanoparticles Seen as Artificial Atoms

In the growth of crystals, do nanoparticles act as "artificial atoms" forming molecular-type building blocks that can assemble into complex structures? This is the contention of a major but controversial theory to explain nanocrystal growth. A study by researchers at the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) may resolve the controversy and point the way to energy devices of the future.

These are sequential color TEM images showing the growth of Pt3Fe nanorods over time, displayed as minutes:seconds. At the far right, twisty nanoparticle chains straighten and stretch into nanorods [Credit:  Haimei Zheng]

Led by Haimei Zheng, a staff scientist in Berkeley Lab's Materials Sciences Division, the researchers used a combination of transmission electron microscopy and advanced liquid cell handling techniques to carry out real-time observations of the growth of nanorods from nanoparticles of platinum and iron. Their observations support the theory of nanoparticles acting like artificial atoms during crystal growth.

"We observed that as nanoparticles become attached they initially form winding polycrystalline chains," Zheng says. "These chains eventually align and attach end-to-end to form nanowires that straighten and stretch into single crystal nanorods with length-to-thickness ratios up to 40:1. This nanocrystal growth process, whereby nanoparticle chains as well as nanoparticles serve as the fundamental building blocks for nanorods, is both smart and efficient."

Zheng is the corresponding author of a paper describing this research in the journal Science. The paper is titled "Real-Time Imaging of Pt3Fe Nanorod Growth in Solution." Co-authors are Hong-Gang Liao, Likun Cui and Stephen Whitelam.

If the near limitless potential of nanotechnology is to even be approached, scientists will need a much better understanding of how nano-sized particles can assemble into hierarchical structures of ever-increasing organization and complexity. Such understanding comes from tracking nanoparticle growth trajectories and determining the forces that guide these trajectories.

Through the use of transmission electron microscopy and liquid observation cells, scientists at Berkeley Lab and elsewhere have made significant progress in observing nanoparticle growth trajectories, including the oriented attachment of nanoparticles -- the chemical phenomenon that starts the growth of nanocrystals in solution. However, these observations have typically been limited to the first few minutes of crystal growth. In their study, Zheng and her colleagues were able to extend the time of observation from minutes to hours.

"The key to studying the growth of colloidal nanocrystals with different shapes and architectures is to maintain the liquid in the viewing window long enough to allow complete reactions," Zheng says. "We dissolved molecular precursors of platinum and iron in an organic solvent and used capillary pressure to draw the growth solution into a silicon-nitride liquid cell that we sealed with epoxy. The sealing of the cell was especially important as it helped keep the liquid from turning viscous over time. Previously, we'd often see the liquids become viscous and this would prevent the nanoparticle interactions that drive crystal growth from taking place."

Zheng and her colleagues chose to study the growth of platinum iron nanorods because of the electrocatalytic material's promising potential for use in next generation energy conversion and storage devices. They were able to observe these nanoparticles assemble into nanorod crystals using powerful transmission electron microscopes at Berkeley Lab's National Center for Electron Microscopy, including TEAM 0.5 (Transmission Electron Aberration-corrected Microscope), which can produce images with half‑angstrom resolution -- less than the diameter of a single hydrogen atom.

"From what we observed only single nanoparticles exist at the beginning of crystal growth, but, as growth proceeds, small chains of nanoparticles become dominant until, ultimately, only long chains of nanoparticles can be seen," Zheng says. "Our observations provide a link between the world of single molecules and hierarchical nanostructures, paving the way for the rational design of nanomaterials with controlled properties."

Source: DOE/Lawrence Berkeley National Laboratory [May 24, 2012]

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Scientists generate electricity from viruses

Imagine charging your phone as you walk, thanks to a paper-thin generator embedded in the sole of your shoe. This futuristic scenario is now a little closer to reality. Scientists from the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a way to generate power using harmless viruses that convert mechanical energy into electricity.

The scientists tested their approach by creating a generator that produces enough current to operate a small liquid-crystal display. It works by tapping a finger on a postage stamp-sized electrode coated with specially engineered viruses. The viruses convert the force of the tap into an electric charge.

Their generator is the first to produce electricity by harnessing the piezoelectric properties of a biological material. Piezoelectricity is the accumulation of a charge in a solid in response to mechanical stress.

The milestone could lead to tiny devices that harvest electrical energy from the vibrations of everyday tasks such as shutting a door or climbing stairs.

It also points to a simpler way to make microelectronic devices. That's because the viruses arrange themselves into an orderly film that enables the generator to work. Self-assembly is a much sought after goal in the finicky world of nanotechnology.

The scientists describe their work in a May 13 advance online publication of the journal Nature Nanotechnology.

"More research is needed, but our work is a promising first step toward the development of personal power generators, actuators for use in nano-devices, and other devices based on viral electronics," says Seung-Wuk Lee, a faculty scientist in Berkeley Lab's Physical Biosciences Division and a UC Berkeley associate professor of bioengineering.

He conducted the research with a team that includes Ramamoorthy Ramesh, a scientist in Berkeley Lab's Materials Sciences Division and a professor of materials sciences, engineering, and physics at UC Berkeley; and Byung Yang Lee of Berkeley Lab's Physical Biosciences Division.

The piezoelectric effect was discovered in 1880 and has since been found in crystals, ceramics, bone, proteins, and DNA. It's also been put to use. Electric cigarette lighters and scanning probe microscopes couldn't work without it, to name a few applications.

But the materials used to make piezoelectric devices are toxic and very difficult to work with, which limits the widespread use of the technology.

Lee and colleagues wondered if a virus studied in labs worldwide offered a better way. The M13 bacteriophage only attacks bacteria and is benign to people. Being a virus, it replicates itself by the millions within hours, so there's always a steady supply. It's easy to genetically engineer. And large numbers of the rod-shaped viruses naturally orient themselves into well-ordered films, much the way that chopsticks align themselves in a box.

These are the traits that scientists look for in a nano building block. But the Berkeley Lab researchers first had to determine if the M13 virus is piezoelectric. Lee turned to Ramesh, an expert in studying the electrical properties of thin films at the nanoscale. They applied an electrical field to a film of M13 viruses and watched what happened using a special microscope. Helical proteins that coat the viruses twisted and turned in response—a sure sign of the piezoelectric effect at work.

Next, the scientists increased the virus's piezoelectric strength. They used genetic engineering to add four negatively charged amino acid residues to one end of the helical proteins that coat the virus. These residues increase the charge difference between the proteins' positive and negative ends, which boosts the voltage of the virus.

The scientists further enhanced the system by stacking films composed of single layers of the virus on top of each other. They found that a stack about 20 layers thick exhibited the strongest piezoelectric effect.

The only thing remaining to do was a demonstration test, so the scientists fabricated a virus-based piezoelectric energy generator. They created the conditions for genetically engineered viruses to spontaneously organize into a multilayered film that measures about one square centimeter. This film was then sandwiched between two gold-plated electrodes, which were connected by wires to a liquid-crystal display.

When pressure is applied to the generator, it produces up to six nanoamperes of current and 400 millivolts of potential. That's enough current to flash the number "1" on the display, and about a quarter the voltage of a triple A battery.

"We're now working on ways to improve on this proof-of-principle demonstration," says Lee. "Because the tools of biotechnology enable large-scale production of genetically modified viruses, piezoelectric materials based on viruses could offer a simple route to novel microelectronics in the future."

Source: DOE/Lawrence Berkeley National Laboratory [May 13, 2012]

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'Backpacking' bacteria help ferry nano-medicines inside humans

To the ranks of horses, donkeys, camels and other animals that have served humanity as pack animals or beasts of burden, scientists are now enlisting bacteria to ferry nano-medicine cargos throughout the human body. They reported on progress in developing these "backpacking" bacteria -- so small that a million would fit on the head of a pin -- in San Diego on March 29 at the 243rd National Meeting & Exposition of the American Chemical Society (ACS). 

Bacterial cells could deliver diagnostics, therapeutics or sensors to where they are needed most in the body ]Credit:Sean Parsons, ACS]

"Cargo-carrying bacteria may be an answer to a major roadblock in using nano-medicine to prevent, diagnose and treat disease," David H. Gracias, Ph.D., leader of the research team said. Gracias explained that nanotechnology is the engineering of ultra-small machines and other devices. These devices generally lack practical self-sustaining motors to move particles of medication, sensors and other material to diseased parts of the body. So why not attach such cargo to bacteria, which have self-propulsion systems, and have them hike around the human body? 

"Currently, it is hard to engineer microparticles or nanoparticles capable of self-propelled motion in well-defined trajectories under biologically relevant conditions," Gracias said. He is with Johns Hopkins University in Baltimore, Maryland. "Bacteria can do this easily, and we have established that bacteria can carry cargo." 

In addition, bacteria can respond to specific biochemical signals in ways that make it possible to steer them to desired parts of the body. Once there, bacteria can settle down, deposit their cargo and grow naturally. Bacteria already live all over the body, particularly in the large intestine, with bacterial cells outnumbering human cells 10-to-1. Despite their popular reputation as disease-causers, there are bacteria in the human body, especially in the intestinal tract, that are not harmful, and the backpackers fall into that category. 

Gracias' bacteria don't really carry little nylon or canvas backpacks. Their "backpacks" are micro- or nano-sized molecules or devices that have useful optical, electrical, magnetic, electrical or medicinal properties. The cargos that the team tested also varied in size, shape and material. So far, the team has loaded beads, nanowires and lithographically fabricated nanostructures onto bacteria. 

Other scientists are seeking to enlist bacteria in transporting nano-cargo. They already have established, for instance, that large numbers of bacteria -- so-called "bacterial carpets" -- can move tiny objects. Gracias' research focuses on attaching one piece of cargo to an individual bacterium, rather than many bacteria to much larger cargo. The bacteria, termed "biohybrid devices," can still move freely, even with the cargo stuck to them. 

"This is very early-stage exploratory research to try and enable new functionalities for medicine at the micro- and nanoscale by leveraging traits from bacteria," explained Gracias. "Our next steps would be to test the feasibility of the backpacking bacteria for diagnosing and treating disease in laboratory experiments. If that proves possible, we would move on to tests in laboratory mice. This could take a few years to complete." 

Source: American Chemical Society [March 29, 2012]

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