Duke University and the University of Massachusetts have created a unique set of conditions in which tiny particles within a solution will consistently assemble themselves into these and other complex shapes.
By manipulating the magnetization of a liquid solution, the researchers have for the first time coaxed magnetic and non-magnetic materials to form intricate nano-structures. The resulting structures can be "fixed," meaning they can be permanently linked together. This raises the possibility of using these structures as basic building blocks for such diverse applications as advanced optics, cloaking devices, data storage and bioengineering.
Changing the levels of magnetization of the fluid controls how the particles are attracted to or repelled by each other. By appropriately tuning these interactions, the magnetic and non-magnetic particles form around each other much like a snowflake forms around a microscopic dust particle.
"We have demonstrated that subtle changes in the magnetization of a fluid can create an environment where a mixture of different particles will self-assemble into complex superstructures," said Randall Erb, fourth-year graduate student. He performed these experiments in conjunction with another graduate student Hui Son, in the laboratory of Benjamin Yellen, assistant professor of mechanical engineering and materials science and lead member of the research team.
The results of the Duke experiments appear in Feb. 19 issue of the journal Nature.
The nano-structures are formed inside a liquid known as a ferrofluid, which is a solution consisting of suspensions of nanoparticles composed of iron-containing compounds. One of the unique properties of these fluids is that they become highly magnetized in the presence of external magnetic fields. The unique ferrofluids used in these experiments were developed with colleagues Bappaditya Samanta and Vincent Rotello at the University of Massachusetts.
"The key to the assembly of these nano-structures is to fine-tune the interactions between positively and negatively magnetized particles," Erb said. "This is achieved through varying the concentration of ferrofluid particles in the solution. The Saturn and flower shapes are just the first published examples of a range of potential structures that can be formed using this technique."
According to Yellen, researchers have long been able to create tiny structures made up of a single particle type, but the demonstration of sophisticated structures assembling in solutions containing multiple types of particles has never before been achieved. The complexity of these nano-structures determines how they can ultimately be used.
"It appears that a rich variety of different particle structures are possible by changing the size, type and or degree of magnetism of the particles," Yellen said.
Yellen foresees the use of these nano-structures in advanced optical devices, such as sensors, where different nano-structures could be designed to possess custom-made optical properties. Yellen also envisions that rings composed of metal particles could be used for antenna designs, and perhaps as one of the key components in the construction of materials that display artificial "optical magnetism" and negative magnetic permeability.
In the Duke experiments, the nano-structures were created by applying a uniform magnetic field to a liquid containing various types of magnetic and non-magnetic colloidal particles contained between transparent glass slides to enable real-time microscopic observations of the assembly process. Because of the unique nature of this "bulk" assembly technique, Yellen believes that the process could easily be scaled up to create large quantities of custom-designed nano-structures in high-volume reaction vessels. However, the trick is to also be able to glue the structures together, because they will fall apart when the external field is turned off, he said.
"The magnetic forces assembling these particles are reversible," Yellen said. "We were able to lock these nano-structures in their intended shapes both by using chemical glues and by simple heating."
The Duke team plans to test different combinations of particles and ferrofluids developed by the University of Massachusetts team to create new types of nano-structures. They also want to try to make even smaller nano-structures to find the limitations of the assembly process, and study the interesting optical properties which are expected from these structures.
The Ethics of Nanotechnology
What kind of world do we wish to inhabit and leave for following generations? Our planet is in trouble if current trends continue into the future: environmental degradation, extinction of species, rampant diseases, chronic warfare, poverty, starvation and social injustice.
Are suffering and despair humanity's fate? Not necessarily. We have within our grasp the technology to help bring about great progress in elevating humanity. Or we can use our evolving knowledge for destructive ends. We are already immersed in fiery debates on genetic engineering, cloning, nuclear physics and the science of warfare. Nanotechnology, with its staggering implications, will create a whole new set of ethical quandaries. A strong set of operating principles is needed -- standards by which we can guide ourselves to a healthier destiny.
The following are some ethical guidelines gleaned from both Foresight and our own philosophy and experience in this field:
* Nanotechnology's highest and best use should be to create a world of abundance where no one is lacking for their basic needs. Those needs include adequate food, safe water, a clean environment, housing, medical care, education, public safety, fair labor, unrestricted travel, artistic expression and freedom from fear and oppression.
* High priority must be given to the efficient and economical global distribution of the products and services created by nanotechnology. We recognize the need for reasonable return on investment, but we must also recognize that our planet is small and we all depend upon each other for safety, stability, even survival.
* Military research and applications of nanotechnology must be limited to defense and security systems, and not for political purposes or aggression. And any government-funded research that generates useful non-military technological advances must be made available to the public.
* Scientists developing and experimenting with nanotechnology must have a solid grounding in ecology and public safety, or have someone on their team who does. Scientists and their organizations must also be held accountable for the willful, fraudulent or irresponsible misuse of the science.
* All published research and discussion of nanotechnology should be accurate as possible, adhere to the scientific method, and give due credit to sources. Labeling of products should be clear and accurate, and promotion of services, including consulting, should disclose any conflicts of interest.
* Published debates over nanotechnology, including chat room discussions, should focus on advancing the merits of the arguments rather than personal attacks, such as questioning the motives of opponents.
* Business models in the field should incorporate long-term, sustainable practices, such as the efficient use of resources, recycling of toxic materials, adequate compensation for workers and other fair labor practices.
* Industry leaders should be collaborative and self-regulating, but also support public education in the sciences and reasonable legislation to deal with legal and social issues associated with nanotechnology.
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