Science Concentrates ENERGY STORAGE
▸ Cryo-electron microscopy images battery dendrites
Standard microscopy methods corrode lithium dendrites and cause substantial electron-beam damage (left). In contrast, cryo-TEM preserves dendrites’ native structures and composition (right). loading a specimen into a microscope for TEM analysis expose the sample to air. In the case of room-temperature lithium dendrites, that process causes corrosion, which irreversibly changes their structure and composition. To preserve the dendrites, in an effort to understand and ultimately prevent their growth, the Stanford team assembled coin-type lithium-ion batteries in an inert atmosphere, applied electrical current to cause dendrite growth, and then used customized techniques and equipment to do TEM analysis on pristine, cryogenically cooled samples. The study shows that dendrites grow along select lattice directions as faceted, single-crystalline nanowires and that the nanostructure details depend on
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C&EN | CEN.ACS.ORG | NOVEMBER 6, 2017
Untreated patterns printed with a MOF-based ink are invisible under ambient and UV light (left) but shine brightly under UV light after exposure to a halide salt.
NANOMATERIALS
Making invisible inks more invisible Invisible inks based on “smart” fluorescent materials have been shining brightly (if only you could see them) in the data-encryption/decryption arena lately. Researchers have debuted various organic dyes, nanocrystals, and other materials to help keep confidential information secure and to develop anticounterfeiting strategies. But some of the materials are costly or difficult to prepare, and many of these inks remain somewhat visible when illuminated with ambient or ultraviolet light. Liang Li and coworkers at Shanghai Jiao Tong University may have come up with a way to get around those problems. The team prepared a colorless solution of an inexpensive lead-based metal-organic framework (MOF) compound and used it in an ink-jet printer to create completely invisible patterns on paper. Then they exposed the paper to a methylammonium bromide decryption solution, thereby converting the starting material to luminescent, MOF-encased methylammonium lead tribromide nanocrystals, which shine brightly under UV light, revealing the pattern (Nat. Commun. 2017, DOI: 10.1038/ s41467-017-01248-2). They rendered the pattern invisible again by briefly treating the paper with a polar solvent, which destroys the structure of the MOF-encased crystals and quenches their luminescence. Now, the team is working to develop tin-based or other lead-free starting materials and procedures for using the ink repeatedly.—MITCH JACOBY
the type of liquid electrolyte used in the battery.—MITCH JACOBY
BIOLOGICAL CHEMISTRY
▸ How TB sugars interact with mammalian receptors Researchers have synthesized 60 glycans representative of all the classes of carbohydrates found on the tuberculosis bacterium and screened them against some human receptor proteins. By probing the way these glycans and proteins interact, the study provides insights into molecular mechanisms that enable TB bacteria to invade
human cells and survive there during infections. Glycans found on surfaces of mycobacteria, the group to which the TB bacterium belongs, are known to cause host immune responses, but it has been difficult to profile the host proteins, called lectins, that recognize these sugars. Hypothetically, the glycans could be made enzymatically or isolated from bacteria. But most enzymes that make them are not known, and isolated glycans are hard to purify and characterize and lack linkers needed to immobilize them on arrays. Todd L. Lowary of the University of Alberta and coworkers therefore chemically synthesized the 60 representative glycans with linkers, attached them to bovine serum albumin, and printed them on glass slides. The team, also including the groups of Maureen E. Taylor and Kurt Drickamer at Imperial College London, then screened
C R E D I T: NAT. CO M MU N. ( I N K ) ; S CI ENC E ( MI C ROG RA P H )
An analytical technique famous for imaging biological structures in exquisite detail can do a heck of a job when adapted to energy storage materials, according to a study. A research team led by Stanford University materials scientists Yuzhang Li, Yanbin Li, and Yi Cui applied cryo-transmission electron microscopy (cryo-TEM), the subject of the 2017 Nobel Prize in Chemistry, to study formation of needlelike lithium dendrites inside lithium-ion batteries (Science 2017, DOI: 10.1126/science.aam6014). Deposition of lithium during charging cycles can lead to dendrites that grow large enough to pierce a battery’s insulating separator and make contact with both of its electrodes. That process short-circuits the battery, causing it to fail and occasionally burst into flames. Standard methods for
Carbohydrate recognition domain
* Label Glycans
2-D MATERIALS
How to make ultrathin materials with liquid metals
Bovine serum albumin Glass slide
Representative glycans from several families (shown here in red and green) found on the TB bacterium’s surface are immobilized on an array via links to bovine serum albumin and screened against fluorescently labeled lectin fragments. the resulting array with a panel of fluorescently labeled carbohydrate-recognition fragments from one cow lectin and several human lectins (ACS Chem. Biol. 2017, DOI: 10.1021/acschembio.7b00797). Among other results, screening revealed binding of the mannose receptor and the protein DC-SIGN to two families of TB glycans, interactions that promote entry of the mycobacteria into immune-system macrophages.—STU BORMAN
C R E D I T: A DA PT ED FRO M ACS C H E M . B I O L . (G L AS S SL I DE )
INORGANIC CHEMISTRY
▸ First triazenyl radicals stabilized and characterized
Over the past decade, researchers have succeeded in making radical compounds of main group elements by stabilizing them with N-heterocyclic carbenes. Nitrogen-containing triazenyl radicals are attractive for their potential reactivity and physical properties. Free triazenyl radicals have previously been detected by electron paramagnetic resonance (EPR) spectroscopy and as ligands in transition-metal complexes, but they’ve been challenging to stabilize and isolate. ReR R searchers led by Eunsung Lee of the Institute N N N for Basic Science and N N Pohang University of N N Science & Technology R R have now synthesized Triazenyl radical and characterized a pair of triazenyl radicals stabilized by N-heterocyclic carbenes (J. R= or Am. Chem. Soc. 2017,
Just as slugs leave behind trails of goo, liquid metals can leave behind atomically thin films. Researchers at the Royal Melbourne Institute of Technology have introduced a new method that uses oozy liquid metals to create solid two-dimensional metal oxides (Science 2017, DOI: 10.1126/ science.aao4249). This approach could create 2-D films from a variety of metals, potentially broadening the range of ultrathin materials with attractive optical and electronic properties, the researchers say. Jonathan N. Coleman, a 2-D materials guru at Trinity College Dublin who was not involved in the project, calls the work “a real breakthrough.” Watch our video describing the technique at cenm.ag/liquidmetals.
DOI: 10.1021/jacs.7b08753). The researchers reacted chloroimidazolium chlorides with azides to form triazenyl cations, which they then reduced to the corresponding triazenyl radicals via one-electron reduction with potassium metal in toluene. The researchers characterized the radicals by single-crystal X-ray diffraction, EPR spectroscopy, and UV-vis absorption spectroscopy. Density functional theory calculations suggest that the unpaired electron in both radicals is strongly delocalized over the entire conjugated π system, which may be how the N-heterocyclic carbenes stabilize the triazenyl radical. The researchers demonstrated a potential application by using one of the triazenyl radicals as a cathode material in a lithium-ion battery.—CELIA ARNAUD
MOLECULAR ELECTRONICS
▸ Antiaromaticity flips single-molecule switch Ultrasmall electronic devices require ultrasmall components. That’s why scientists have been working for the past few decades on electronic circuitry in which function is controlled by just a single molecule. Those molecules nearly always sport conjugated aromatic backbones because aromaticity in those molecules usually goes hand-in-hand with enhanced conductivity. But a team led by Latha Venkataraman and Luis M. Campos of Columbia University and Jeffrey B. Neaton of Lawrence Berkeley National Laboratory has shown that aromatic isn’t the
Pinned between gold electrodes (yellow), TBTP serves as a single-molecule switch as its core undergoes reversible oxidation between a nonconducting neutral state (red) and a conducting antiaromatic dication (green). S
S
TBTP
S
S –2 e– +2 e–
only way to go: The team has demonstrated a highly Off conducting, reversible, single-molecule switch based on antiaromaticity (Sci. Adv. 2017, DOI: 10.1126/sciadv. aao2615). In classical organic chemistry, Hückel’s rule states that conjugated cyclic molecules with 4n+2 π electrons exhibit enhanced stability because they are aromatic. Related molecules with 4n π electrons are comparatively unstable and described as antiaromatic, a concept pioneered by Columbia’s Ronald Breslow, who died Oct. 25. Intrigued by the possibility that a so-called unstable antiaromatic molecule might function as a switch, the team used a scanning tunneling microscope to pin a nonconducting thiophenylidene derivative (TBTP, shown) between gold electrodes. They showed that a quick electrooxidation step reversibly removes two electrons, converting the neutral molecule to a highly conducting antiaromatic dication that works stably and reproducibly as a switch.—MITCH JACOBY NOVEMBER 6, 2017 | CEN.ACS.ORG | C&EN
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