New Family of Self-Assembled Nanolattices Created
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Hong and colleagues synthesized the polymer by atom transfer radical polymerization. They used a modified pyrene as the initiator, which fixes the fluorophore onto one end of the growing chain, and chose sulfadimethoxine as their monomer for its physiological-range dissociation constant. After polymerization, coumarin 343 was added to the free end of the chain through an ester linkage. At pH 7.5 and above, the primary emission is from the pyrene, as would be expected from the excitation wavelength of 334 nm; at pH 7.0 and below, the researchers observed a broad peak at 491 nm, corresponding to coumarin 343 emission as well. Since coumarin 343 is not excited by 334-nm light, the researchers ascribe this peak to a fluorescence resonance energy transfer from the pyrene to the coumarin due to physical proximity. This occurs as the pH is lowered below the dissociation constant of the polymer chain and the chain transitions from a coil to a globule state, pulling the two fluorophores close together. According to the researchers, this process and the appearance of the signal peak from the coumarin are reversible, suggesting that this sensor could be used to monitor multiple pH changes over time. KRISTA L. NIECE
As reported in the January 5 issue of Nature (DOI: 10.1038/nature04414; p. 55), O’Brien and postdoctoral research scientist Elena Shevchenko of Columbia; researchers Dmitri Talapin and Christopher Murray at the IBM Watson Research Center in Yorktown Heights, New York; and Nicholas Kotov of the University of Michigan, Ann Arbor, have formed more than 15 different binary nanoparticle superlattice (BNSL) structures using combinations of semiconducting, metallic, and
magnetic nanoparticles (see Figure 1). They demonstrated that by tuning the charge state of the nanoparticles, they were able to direct the self-assembly process. “Combining native solutions of 6.2-nm PbSe and 3.0-nm Pd nanoparticles [with a particle concentration ratio of ~1:5] results in the formation of several BNSL structures with MgZn 2 and cuboctahedral AB13 lattices dominating,” the researchers reported. “However, the same nanoparticles assemble into orthorhombic AB-
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New Family of Self-Assembled Nanolattices Created Inspired by the way most solids form in nature, with free-floating molecules spontaneously assembling themselves into a rigid, highly uniform array, researchers from Columbia University and IBM have learned how to create a new family of intricate structures out of artificial nanoscale crystals. Because the nanocrystals can be chosen for their precise magnetic and electronic properties, these new structures could have broad application for magnetic storage and nanoscale electronics. “You can think of nanocrystals as building blocks like the toy Lego,” said the group’s spokesperson, Stephen O’Brien of Columbia, “in which a larger structure can be assembled by locking in the pieces according to their shape and the way they prefer to join to each other. Except all of this is on an incredibly small lengt
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