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Noteworthy Chemistry

October 14, 2013


Platinum(II) complexes improve on the platins. Platinum-based anticancer drugs such as cisplatin, carboplatin, and oxaliplatin are commonly used in chemotherapy. Despite their extensive use, the platin family is plagued by severe side effects and increasing cross-resistance. To improve upon the platins, Y. Li and colleagues at the Kunming Institute of Precious Metals (Yunnan, China), the Chinese Academy of Sciences (Yunnan), and the University of the Chinese Academy of Sciences (Beijing) developed two Pt(II) complexes that contain mixed ammonia–amine ligands that show enhanced activity toward tumor cells.

Dichloroacetate esters have been used for many years to induce apoptosis in the mitochondria of cancer cells. The authors added dichloroacetate groups to dicarboxylate ligands to exploit the synergistic effect between dichloroacetate and platin-like Pt(II) complexes.

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They prepared complexes 1 and 2 from the commercially available starting material K[Pt(NH3)Cl3] in four steps. Both complexes exist as pairs of cis and trans isomers. When they are hydrolyzed, the complexes release platinum pharmacophores and dichloroacetate.

The results of in vitro cytotoxicity tests show that complexes 1 and 2 are at least as efficient for inducing apoptosis in cancer cells as cisplatin, carboplatin, and similar complexes that do not contain dichloroacetate groups. Both complexes, however, are much less cytotoxic to normal cells; and their performance is not compromised by cisplatin-resistant SK-VO-3 cancer cells. (Sci. Rep. 2013, 3, No. 2462; Xin Su)

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Make biodegradable block polyester nanofibers in water. A. Greiner and coauthors at Phillips University Marburg and the University of Bayreuth (both in Germany) prepared surfactant-free, nonwoven electrospun mats using water as the solvent. With their method, they copolymerized hexamethylene adipate (HA) or ring-opened ε-caprolactone (CL) with methoxypoly(ethylene glycol) (MPEG) to form PHA-b-MPEG and PCL-b-MPEG, respectively.

The authors tuned the micelle-forming, semicrystalline block copolymers by varying the hydrophobic PHA and PCL block sizes at a constant MPEG molecular weight (5 kDa). The key to their success is purification that removes any unreacted MPEG. As measured by dynamic light scattering, the particle size of the dispersions increases with longer hydrophobic (PHA or PCL) bond length.

To make them suitable for electrospinning, the dispersions were dialyzed to increase the solids content to balance stability and prevent gelation. Electrospinning was accomplished by cospinning PCL-b-MPEG and PHA-b-MPEG with high–molecular weight poly(ethylene oxide) (PEO; 300 or 900 kDa). The amount of PEO added depended upon the template PEO size and potential interactions with the MPEG block.

After PEO was removed by soaking in water at 20 °C, smooth, intact fibers, predominantly ≈275 to 750 nm long, were obtained. Fibers produced by electrospinning copolymers that contained shorter hydrophobic PHA or PCL blocks exhibited significant swelling and fiber collapse.

The authors’ work demonstrates a sustainable process for manufacturing biodegradable polyester copolymers. (Macromolecules 2013, 46, 7034–7042; LaShanda Korley)

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Synthesize solid-state light emitters inexpensively. Conjugated molecules are often prepared by Suzuki or Sonogashira coupling reactions. These reactions are useful, but the moisture- and air-sensitive palladium catalysts and the boronic acid starting materials are expensive. Using common starting materials, F. Turksoy, C. Tanyeli, and coauthors at the Middle East Technical University (Ankara, Turkey) and the Scientific and Technological Research Council of Turkey (Gebze Kocaeli) synthesized a series of conjugated luminogens (13) under mild reaction conditions.

Inexpensively synthesized solid-state light emitters

The reactions proceeded well under environmentally friendly conditions. Luminogens 13 do not fluoresce in solution, but they emit efficiently at 495–503 nm in the solid state. They are thermally stable and lose little weight when heated to 516 ºC. Electroluminescence devices made from these luminogens perform well, with maximum brightness values as high as 18,000 cd/m2 and external quantum efficiencies up to 3.2%. (J. Mater. Chem. C 2013, 1, 7081-7091; Ben Zhong Tang)

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Use liquid metals for 3-D printing. The 3-D printing technique has become tremendously popular, especially since the beginning of this decade. Molten polymers that cool rapidly and solidify are the dominant raw materials for most 3-D printers. When 3-D conductive microstructures are needed, however, polymers often do not qualify, even in the presence of conductive additives.

Liquid metals, however, are difficult to adapt to certain patterns because of restraints set by Plateau–Rayleigh instability. M. D. Dickey and co-workers at North Carolina State University (Raleigh) developed a way to pattern free-standing 3-D microstructures with liquid metal by using a direct-write method.

The authors used a low-viscosity liquid eutectic alloy that consists of 75 wt% gallium and 25 wt% indium and has a melting point of 15.7 ºC. (They state that other gallium alloys work similarly.) In the direct-write method, the liquid metal is extruded from a syringe needle 30–200 μm diam onto substrates that are controlled by a motorized translational stage. The resulting freestanding wires can be as tall as ≈1 cm; their diameters are determined by the size of the syringe nozzle.

During wire formation, a thin layer (≈1 nm) of gallium oxide forms on the surface of the wires. The oxide skin yields when the wires are under tension so that they can elongate. The presence of the oxide is critical for the wires to retain their shapes because it prevents the droplets from coalescing. The pressure applied to the liquid metal cannot be too low or too high; otherwise, the wire may collapse or grow with radial bulges.

The authors made other freestanding microstructures, such as metal filaments and stacked droplets, in a similar manner. They produced an array of in-plane metal lines by injecting the liquid metal into poly(dimethylsiloxane)-based microfluidic molds and then removing molds by chemical etching. (Adv. Mater. 2013, 25, 5081–5085; Xin Su)

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Layered perovskites might form room-temperature spintronic devices, according to a theoretical study. Q.-F. Liang, L.-H. Wu, and X. Hu* at the National Institute for Materials Science (Tsukuba, Japan) and Shaoxing University (China) propose fine-tuning the degrees of freedom of electron spin, sublattices, and valleys (conduction band minima) in perovskite G–type antiferromagnetic insulators. These materials would be grown along the [111] direction and achieve a new topological insulator state. (In G-type ordering, two sublattices have spins that are antiparallel to each other.)

Bulk topological insulators are insulating, but their quantum-edge states can transport current with no dissipation. These states are realized only at low temperatures, at which the edge-state spin-up and spin-down electrons are mixed. This study indicates that an antiferromagnetic insulating state with a reversible spin-polarized quantum edge current can be achieved at room temperature or higher by tuning a combination of the electric potential, antiferromagnetic field, and spin–orbit coupling.

The researchers were inspired by the quantum spin Hall effect seen in graphene and other materials with honeycomb-like crystal lattices. They extended the concept to perovskite insulators ABO3 in which magnetic B atoms exhibit G-type antiferromagnetic ordering and are arranged in buckled honeycomb lattices along the [111] direction. The figure shows a six-layer ABB′X perovskite with one layer of B (blue) replaced by B′ (gray).

Replacing one layer of magnetic B atoms with a layer of nonmagnetic B′ atoms and applying a uniform electric field induce a staggered electric potential for the two sublattices. The G-type antiferromagnetic ordering of the B atoms on either side of the B′ layer provides an antiferromagnetic exchange field.

The authors performed first-principles calculations on a model solid that consisted of six layers of LaCrO3 with one layer of La2Ag2O6 or La2Au2O6 sandwiched into the center. They estimated a spin–orbit coupling of several tens of meV, which makes this new topological state available at room temperature. Recent developments in laser molecular beam epitaxy allow the growth of perovskite structures along the [111] direction with atomic precision, so that verifying the calculations in the lab should be feasible. (New J. Phys. 2013, 15, No. 063031; Nancy McGuire)

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A six-layer ABB'X perovskite

Tune hybrid coatings for surface-mediated drug delivery. M. Zhu, B. Städler, and coauthors at Donghua University (Shanghai), Aarhus University (Denmark), CSIRO Materials Science and Engineering (Clayton, Australia), and Monash University (Clayton) investigated the co-assembly of highly branched, thermoresponsive poly(N-isopropylacrylamide) (pNiPAAm) polymers. They synthesized the polymers via reversible addition−fragmentation chain transfer (RAFT) and assembled them with polydopamine (PDA) into films for biomedical coatings.

The lower critical solution temperature (LCST) of pNiPAAm ranged from 30 to 40 °C. High degrees of assembly and deposition onto a silica substrate occurred near the LCST. Surface roughness increased when pNiPAAm-PDA was deposited and could be tuned by varying the assembly temperature. Films deposited at 39 ºC exhibited similar levels of protein adsorption regardless of pNiPAAm content, whereas at 24 ºC deposition temperature the extent of protein adsorption decreased with increasing pNiPAAm content.

The authors evaluated the pNiPAAm-PDA films as capping layers for entrapped liposomes. The success of minimally deposed films on layer-by-layer constructs containing the liposomes depended on the substrate used. The authors discuss the effects of this and other variables such as capping layer permeability and underlying polymer layers on the preparation of surface coatings for drug delivery. (J. Phys. Chem. B 2013, 117, 10504–10512; LaShanda Korley)

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