Theory Predicts that Cycloaddition Functionalizations May be Used to Manipulate CNT Conductance
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10/31/2006
12:33 PM
Page 847
RESEARCH/RESEARCHERS
Yang examined receptor endocytosis, the process by which cells absorb materials—such as a drug attached to folic acid—that have been captured by receptors on the cell surface. The compound is then broken down and processed, releasing the drug. One of the key mechanisms of this breakdown is disulfide reduction, which involves the breaking of chemical bonds. It was thought that disulfide reduction relied on the movement of the material along microtubules (hollow tubelike structures) and fusion with special digestive-enzyme-containing compartments within the cell called lysosomes. However, the research showed that disulfide reduction occurred even when such components were removed from the process. By inactivating different cellular components, Yang discovered which components are essential to the disulfide reduction process. “It was surprising to learn that many other components of the cell, aside from those previously assumed to be responsible, were capable of releasing the drug from folic acid,” Yang said. “This significantly increases the opportunity for the drug to be released. For instance, we used to believe it had to get to a specific location to be released, and now we know it can happen almost anywhere during endocytosis.”
Theory Predicts that Cycloaddition Functionalizations May be Used to Manipulate CNT Conductance Nicola Marzari, an associate professor at the Massachusetts Institute of Technology (MIT), and Young-Su Lee, an MIT graduate student in materials science and engineering, have used density functional theory to determine that cycloaddition functionalizations can be used to manipulate carbon nanotube (CNT) conductances. The researchers report their findings in the September 15 issue of Physical Review Letters (#116801; DOI: 10.1103/PhysRevLett. 97.116801). With an internal bonding structure rivaling that of diamond, CNTs are extraordinarily strong and can be highly efficient electrical conductors. However, one problem in working with them is that there is no reliable way to arrange CNTs into a circuit, partly because growing them can result in a randomly oriented structure. Researchers have attached to the sidewalls of the CNTs chemical molecules that work as “handles” that allow the nanotubes to be assembled and manipulated. However, these molecular bonds also change the CNTs’ structure and destroy their conductivity. Now, Marzari and Lee have identified a class of molecules— carbenes and nitrenes—that preserve the metallic properties of CNTs and their near-perfect ability to conduct electricity with little resistance (see Figure 1).
Figure 1. Certain molecules can attach themselves to metallic carbon nanotubes without interfering with the nanotubes’ exceptional ability to conduct electricity. At left, the high conductance state has two molecular orbitals, shown in green. Some molecules let the nanotube switch between (left) highly conductive and (right) poorly conductive (with one red molecular orbital), creating the potential for new appli
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