NANOMATERIALS
C R E D I T: S C I . A DV. ( M IC ROC U BES ) ; N AT UR E ( S U P E R LAT T I C E) ; PRO C. N AT L . ACA D. S CI . U SA (3 - D M O DE L )
▸ Nanocrystals surprise with superlattice While using small-angle X-ray scattering to study palladium nanocrystal formation, SLAC National Accelerator Laboratory postdoc Liheng Wu and Stanford University graduate student Joshua J. Willis texted their adviser, Stanford’s Matteo Cargnello: “Something weird is happening.” The nanocrystals had suddenly and unexpectedly formed a three-dimensional superlattice—a material akin to a typical inorganic crystal but with nanocrystals serving as the “atoms.” Such superlattices have potential applications in magnetics, electronics, and catalysis, and they are typically prepared slowly through solvent evaporation at low temperatures. Micrograph of In the research led a superlattice by Cargnello and composed SLAC’s Christoof palladium nanocrystals (small pher J. Tassone, Wu and Willis saw spheres). superlattices form in just seconds at 230 °C (Nature 2017, DOI: 10.1038/nature23308). They were able to tune the superlattice structure by changing the synthesis conditions and the structure of a carboxylic acid surfactant. The approach also worked to create superlattices of lead telluride and iron nanocrystals. The researchers suggest that the nanocrystals grow individually until reaching a critical diameter—about 5 nm for palladium, 11 nm for iron. At that size, attractive forces between the nanocrystals cause them to come together, but the surfactant prevents aggregation, resulting in a superlattice.—JYLLIAN KEMSLEY
MATERIALS
Magnetic microbot colloids Hours of practice enable marching band members to smoothly step through one orderly formation after another. Some colloidal particles can also do that fancy footwork, but they don’t need to practice. A team of researchers including Koohee Han and Orlin D. Velev of North Carolina State University made polymer cubes with 10-µm-long edges and selectively coated one face of each cube with a 100-nm-thick film of cobalt, which can be magnetized. Then they formed aqueous suspensions of the microcubes and showed that by controlling the way magnetic fields were applied to the suspensions, including switching the fields on and off and superimposing fields from multiple electromagnets, the cubes could be made to spontaneously and reversibly assemble in a variety of shapes and patterns (Sci. Adv. 2017, DOI: 10.1126/sciadv.1701108). In
Switching a magnetic field on and off causes magnetic microcubes to reorient controllably. some cases, the cubes reversibly switched between a linear chain and ringlike configuration. In others, the cubes underwent complex folding, unfolding, and rotational motions. In yet another display of control, the team used a pattern of cubes to capture and transport a live cell and then release it. The researchers propose that this strategy may one day be used to develop microbots, artificial muscles, and other biomimetic devices.—MITCH JACOBY
the complex and their two-dimensional arrangement, the 3-D organization of the complex is less well understood. Now, a team led by R. Scott Hawley, Brian D. Slaughter, and Jay R. Unruh of Stowers Institute for Medical Research has combined two microscopy techniques to get a better 3-D picture of the fruit fly version
MICROSCOPY
▸ Revealing 3-D organization of meiosis complex A multiprotein structure called the synaptonemal complex helps chromosomes segregate properly during the cell division process known as meiosis. Although much is already known about the proteins in
A proposed 3-D model shows how multiple proteins (pink, brown, blue, and green) in the fruit fly synaptonemal complex are arranged relative to one another. The gray sheets represent proteins that weren’t included in this study. The gray loops represent the DNA strands.
of the complex (Proc. Natl. Acad. Sci. USA 2017, DOI: 10.1073/pnas.1705623114). The researchers modified a technique called expansion microscopy to make it compatible with superresolution microscopy. In expansion microscopy, a gel matrix with an embedded sample is uniformly inflated, which separates the components in the sample while keeping them correctly oriented relative to one another. To use the method with superresolution microscopy, the researchers modified the method so they could thinly slice the samples after embedding them in the matrix but before expanding the matrix. They expanded the samples to about four times their normal size. Then they acquired images using a superresolution microscopy method called structured illumination microscopy. The images revealed that the complex has two layers that mirror each other. The researchers propose that the layers possibly connect a sister chromatid (one of two identical copies of a replicated chromosome) from one homologous chromosome to a sister chromatid of the other homolog.—CELIA ARNAUD AUGUST 7, 2017 | CEN.ACS.ORG | C&EN
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