Ahmad Dehestani

Probing Variable Amine/Amide Ligation in NiIIN2S2 Complexes Using Sulfur K-Edge and Nickel L-Edge X-ray Absorption Spectroscopies: Implications for the Active Site of Nickel Superoxide Dismutase

Nickel superoxide dismutase (NiSOD) is a recently discovered metalloenzyme that catalyzes the disproportionation of O2(*-) into O2 and H2O2. In its reduced state, the mononuclear Ni(II) ion is ligated by two cis-cysteinate sulfurs, an amine nitrogen (from the protein N-terminus), and an amide nitrogen (from the peptide backbone). Unlike many small molecule and metallopeptide-based NiN2S2 complexes, S-based oxygenation is not observed in NiSOD. Herein we explore the spectroscopic properties of a series of three Ni(II)N2S2 complexes (bisamine-ligated (bmmp-dmed)Ni(II), amine/amide-ligated (Ni(II)(BEAAM))(-), and bisamide-ligated (Ni(II)(emi))(2-)) with varying amine/amide ligation to determine the origin of the dioxygen stability of NiSOD. Ni L-edge X-ray absorption spectroscopy (XAS) demonstrates that there is a progression in ligand-field strength with (bmmp-dmed)Ni(II) having the weakest ligand field and (Ni(II)(emi))(2-)) having the strongest ligand field. Furthermore, these Ni L-edge XAS studies also show that all three complexes are highly covalent with (Ni(II)(BEEAM))(-) having the highest degree of metal-ligand covalency of the three compounds studied. S K-edge XAS also shows a high degree of Ni-S covalency in all three complexes. The electronic structures of the three complexes were probed using both hybrid-DFT and multiconfigurational SORCI calculations. These calculations demonstrate that the nucleophilic Ni(3d)/S(pi)* HOMO of these NiN2S2 complexes progressively decreases in energy as the amide-nitrogens are replaced with amine nitrogens. This decrease in energy of the HOMO deactivates the Ni-center toward O2 reactivity. Thus, the Ni-S bond is protected from S-based oxygenation explaining the enhanced stability of the NiSOD active-site toward oxygenation by dioxygen.

Ultrathin Epitaxial Cu@Au Core–Shell Nanowires for Stable Transparent Conductors

Copper nanowire networks are considered a promising alternative to indium tin oxide as transparent conductors. The fast degradation of copper in ambient conditions, however, largely overshadows their practical applications. Here, we develop the synthesis of ultrathin Cu@Au core-shell nanowires using trioctylphosphine as a strong binding ligand to prevent galvanic replacement reactions. The epitaxial overgrowth of a gold shell with a few atomic layers on the surface of copper nanowires can greatly enhance their resistance to heat (80 °C), humidity (80%) and air for at least 700 h, while their optical and electrical performance remained similar to the original high-performance copper (e.g., sheet resistance 35 Ω sq-1 at transmittance of ∼89% with a haze factor <3%). The precise engineering of core-shell nanostructures demonstrated in this study offers huge potential to further explore the applications of copper nanowires in flexible and stretchable electronic and optoelectronic devices.

Benzoin Radicals as Reducing Agent for Synthesizing Ultrathin Copper Nanowires

In this work, we report a new, general synthetic approach that uses heat driven benzoin radicals to grow ultrathin copper nanowires with tunable diameters. This is the first time carbon organic radicals have been used as a reducing agent in metal nanowire synthesis. In-situ temperature dependent electron paramagnetic resonance (EPR) spectroscopic studies show that the active reducing agent is the free radicals produced by benzoins under elevated temperature. Furthermore, the reducing power of benzoin can be readily tuned by symmetrically decorating functional groups on the two benzene rings. When the aromatic rings are modified with electron donating (withdrawing) groups, the reducing power is promoted (suppressed). The controllable reactivity gives the carbon organic radical great potential as a versatile reducing agent that can be generalized in other metallic nanowire syntheses.

Synthesis of Silver Nanowires with Reduced Diameters Using Benzoin-Derived Radicals to Make Transparent Conductors with High Transparency and Low Haze

Reducing the diameter of silver nanowires has been proven to be an effective way to improve their optoelectronic performance by lessening light attenuation. The state-of-the-art silver nanowires are typically around 20 nm in diameter. Herein we report a modified polyol synthesis of silver nanowires with average diameters as thin as 13 nm and aspect ratios up to 3000. The success of this synthesis is based on the employment of benzoin-derived radicals in the polyol approach and does not require high-pressure conditions. The strong reducing power of radicals allows the reduction of silver precursors to occur at relatively low temperatures, wherein the lateral growth of silver nanowires is restrained because of efficient surface passivation. The optoelectronic performance of as-prepared 13 nm silver nanowires presents a sheet resistance of 28 Ω sq-1 at a transmittance of 95% with a haze factor of ∼1.2%, comparable to that of commercial indium tin oxide (ITO).