Body-centered-cubic" unit cells of the 3-D nanoparticle crystals. One type of nanoparticle occupies each corner of the cube and a second type of nanoparticle is located centrally inside. These unit cells, measuring tens of nanometers, form a repeating lattice that extends more than a micron (1,000 nanometers) in three dimensions. (Credit: Image courtesy of DOE/Brookhaven National Laboratory)
'Holy Grail' Of Nanoscience: DNA Technique Yields 3-D Crystalline Organization Of Nanoparticles
In an achievement some see as the "holy grail" of nanoscience, researchers at the U.S. Department of Energy's Brookhaven National Laboratory have for the first time used DNA to guide the creation of three-dimensional, ordered, crystalline structures of nanoparticles (particles with dimensions measured in billionths of a meter). The ability to engineer such 3-D structures is essential to producing functional materials that take advantage of the unique properties that may exist at the nanoscale - for example, enhanced magnetism, improved catalytic activity, or new optical properties.
"From previous research, we know that highly selective DNA binding can be used to program nanoparticle interactions," said Oleg Gang, a scientist at Brookhaven's Center for Functional Nanomaterials (CFN), who led the interdisciplinary research team, which includes Dmytro Nykypanchuk and Mathew Maye of the CFN, and Daniel van der Lelie of the Biology Department. "But while theory has intriguingly predicted that DNA can guide nanoparticles to form ordered, 3-D phases, no one has accomplished this experimentally, until now."
As with the group's previous work, the new assembly method relies on the attractive forces between complementary strands of DNA - the molecule made of pairing bases known by the letters A, T, G, and C that carries the genetic code of living things. First, the scientists attach to nanoparticles hair-like extensions of DNA with specific "recognition sequences" of complementary bases. Then they mix the DNA-covered particles in solution. When the recognition sequences find one another in solution, they bind together to link the nanoparticles.
This first binding is necessary, but not sufficient, to produce the organized structures the scientists are seeking. To achieve ordered crystals, the scientists alter the properties of DNA and borrow some techniques known for traditional crystals.
Importantly, they heat the samples of DNA-linked particles and then cool them back to room temperature. "This 'thermal processing' is somewhat similar to annealing used in forming more common crystals made from atoms," explained Nykypanchuk. "It allows the nanoparticles to unbind, reshuffle, and find more stable binding arrangements."
The team also experimented with different degrees of DNA flexibility, recognition sequences, and DNA designs in order to find a "sweet spot" of interactions where a stable, crystalline form would appear.
Results from a variety of analysis techniques, including small angle x-ray scattering at the National Synchrotron Light Source and dynamic light scattering and different types of optical spectroscopies and electron microscopy at the CFN, were combined to reveal the detail of the ordered structures and the underlying processes for their formation. These results indicate that the scientists have indeed found that sweet spot to create 3-D nanoparticle assemblies with long-range crystalline order using DNA.
The crystals are remarkably open, with the nanoparticles themselves occupying only 5 percent of the crystal lattice volume, and DNA occupying another 5 percent. "This open structure leaves a lot of room for future modifications, including the incorporation of different nano-objects or biomolecules, which will lead to enhanced nanoscale properties and new classes of applications," said Maye. For example, pairing gold nanoparticles with other metals often improves catalytic activity. Additionally, the DNA linking molecules can be used as a kind of chemical scaffold for adding small molecules, polymers, or proteins.
Furthermore, once the crystal structure is set, it remains stable through repeated heating and cooling cycles, a feature important to many potential applications.
The crystals are also extraordinarily sensitive to thermal expansion - 100 times more sensitive than ordinary materials, probably due to the heat sensitivity of DNA. This significant thermal expansion could be a plus in controlling optical and magnetic properties, for example, which are strongly affected by changes in the distance between particles. The ability to effect large changes in these properties underlies many potential applications such as energy conversion and storage, as well as sensor technology.
The Brookhaven team worked with gold nanoparticles as a model, but they say the method can be applied to other nanoparticles as well. And they fully expect the technique could yield a wide array of crystalline phases with different types of 3-D lattices that could be tailored to particular functions.
"This work is the first step to demonstrate that it is possible to obtain ordered structures. But it opens so many avenues for researchers, and this is why it is so exciting," Gang says.
The research will be published in the January 31, 2008, issue of the journal Nature.
This research was funded by the Division of Materials Science and Engineering in the Office of Basic Energy Sciences within the U.S. Department of Energy's Office of Science.
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Nanotechnology: holy grail or grey goo?
As governments at the 6th WTO Ministerial in Hong Kong bristle with the thorny politics of trade, the report that ETC Group releases today, Oligopoly, Inc. 2005, serves as a reminder that what looks like buying and selling between countries is most often the redistribution of capital among subsidiaries of the same parent multinational corporation. Researcher Mark Baxendale looks behind the hype and the scare stories at 'the next big thing' in science, nanotachnology
Capitalism, forever in search of updated means of production, has seized upon nanotechnology as a panacea for its present ills and invested huge amounts in research programmes. Nanotechnology is the control of the properties of matter by defining shape and size at the nanometre scale—billionths of a metre. Nanoscience is the study of physical phenomena at atomic and molecular scales.
The possibilities offered by nanotechnology and nanoscience have been hyped to attract investors to such an extent that it is almost guaranteed to be a disappointment. The short term benefits of nanotechnology will be very mundane or frivolous. The biggest private sector investors in nanoparticle research are cosmetics companies.
Nanotechnology has also generated serious concerns among anti-capitalist activists, echoing the debates around genetically modified foods. Some of this concern draws on Eric Drexler's 1986 book, Engines of Creation, in which he predicted self-reproducing nanoscale machines.
The fear generated by this vision, popularised by Michael Crichton's novel Prey, is that a self-reproducing molecular machine could be designed to consume life and reduce us all to "grey goo".
In 2000, one informed commentator, Bill Joy, said that research into nanotechnology should stop immediately, as developments in the wrong hands could end life as we know it.
There is debate about whether the grey goo theory is a real possiblity. The nanoparticles being researched and used today are not self-reproducing and several hundred years of scientific endeavour have given us little insight into how to achieve self-reproduction.
Self-reproduction is a feature of biology, for example ribosome synthesises protein molecules according to a specification embedded in an organism's DNA. But nature has had a "research and development" time of several billion years, and the prospect of us out-designing nature is remote.
However, there are pressing concerns about the health implications of nanoparticles in the body. Nanoparticles can pass through biological cell walls so the interaction with our bodies is at a much deeper level than for larger molecules such as asbestos that get trapped in the lungs.
Lobby groups have raised this issue—Greenpeace have called for 10 percent of funding to be dedicated to health studies. Such studies have commenced but there has been no sign of any research funding from New Labour yet.
We should insist on the highest safety standards for those working with free nanoparticles, but we should also do so for by far the greatest producer of carbon nanoparticles, namely the car engine. We should also insist on the highest standards of toxicology for those cosmetics companies already using nanoparticles.
Nanotechnology does promise to bring real benefits—especially in healthcare and the search for renewable energy sources. At the tiny scales nanoscience deals with, the properties of matter differ significantly from those of our familiar everyday world, opening up new possibilities for science and technology.
For example, the gold and silver used in jewellery is inert—it is stable and unreactive. But gold nanoparticles can speed up certain chemical reactions and silver nanoparticles kill bacteria.
Embedding nanoparticles in another material can also drastically alter its properties. For example, rubber can be strengthened by mixing in carbon nanoparticles and dispersed gold nanoparticles give glass a deep red colour.
These changes to the properties of rubber and glass have been known about for some time. What's new is that through nanoscience we are beginning to understand why these changes take place.
New developments, particularly in microscopy, microelectronics and molecular biology, have provided tools for us to explore nature on the nanoscale.
The manufacture of components in microelectronics now takes places on such a small scale that the "top-down" processes (analogous to carving a statue out of rock) are converging with the "bottom-up" processes (analogous to building a house from individual bricks).
Scientists are now exploring the possibility of self-assembled electronic components using technologies borrowed from molecular biology.
The convergence of different fields of science as old boundaries break down at the nanoscale is an important aspect of nanoscience, and one of the joys of working in this field. Another example of this is the quantum dot—a device developed for telecommunications now used in the body to selectively kill off cancerous cells.
If we are to maximise the benefits of nanotechnology, we should not leave control of this field to the "band of warring brothers", as Karl Marx called the capitalist classes of the world.
Social movements, including some of those involved in this work, can be a powerful force arguing that nanotechnology should be used to meet social needs.
