Carbon Nanotubes Used as Nanoscale Mass Conveyors
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RESEARCH/RESEARCHERS
Magnetic Nanoparticles Assembled by Micromanipulation of Magnetotactic Bacteria A variety of methods have been developed to pattern nanocrystals and assemble them into desired nanostructures. Examples include the formation of singleelectron devices by trapping semiconducting nanoparticles between electrodes and the use of biomineralization by genetically engineered viruses to construct ordered nanostructures. Magnetic nanoparticles can function as singledomain magnets and have applications in spintronics, magnetic memory, and drug delivery. Previously, magnetic nanowires were self-assembled by taking advantage of their large aspect ratios. In addition, magnetic properties have been used to assemble magnetic microstructures. H. Lee, R.M. Westervelt, and coworkers from Harvard University have used microelectromagnets and microfluidics to control the motion of magnetotactic bacteria, which grow intracellular magnetic nanoparticles through a biomineralization process, and have thereby assembled magnetic nanoparticles into ordered structures. As they reported recently in Nano Letters (Web release date, April 8), Westervelt and co-workers selected the magnetotactic bacteria Magnetospirillum magnetotacticum, which grow intracellular, spherical, magnetite (Fe3O4) nanoparticles contained within a phospholipid membrane. The 50 nm nanoparticles, which have a narrow size distribution and act as single-domain permanent magnets, are typically assembled into single or multiple chains that are anchored within the cell. Using either optical or electron-beam lithography, the researchers fabricated two types of microelectromagnets on a Si/SiO2 substrate: a ring trap, which consists of a wire following a circular path and an insulating layer; and a matrix, which consists of two perpendicular arrays of straight conducting wires that are separated and capped with insulating layers. The researchers used soft lithography to fabricate the microfluidic chamber from poly(dimethylsiloxane) with dimensions chosen to minimize viscous drag on the bacteria. After treating both the electromagnets and the chamber with O 2 plasma to render the surfaces hydrophilic, the researchers conformally sealed the chamber on top of the microelectromagnets. Currents were chosen such that the magnitude of the magnetic field was about 0.1 T, which enabled stable trapping of the bacterium, each of which have a magnetic moment of about 10–15 A m2, MRS BULLETIN/JUNE 2004
Carbon Nanotubes Nanotubes Used Used as as Nanoscale Nanoscale Mass Mass Conveyors Conveyors Carbon The very very concept concept of of nanotechnology nanotechnology necessitates necessitates nanofabrication, nanofabrication, including including the the The manipulation and and transport transport of of atoms atoms and and molecules. molecules. Scanning Scanning probe probe techniques techniques manipulation have already already been been used used to to manipulate manipulate atoms atoms and and molecules molecules on on surfaces, surfaces, but but they they have cannot effici
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