Each issue of Gold Bulletin contains key highlights from the research and patent literature. Authors who publish high quality work in other journals are invited to send a copy of their publication to the editor for inclusion in the next issue.

Catalysis

Solvent-free oxidation of primary carbon-hydrogen bonds in toluene using Au-Pd alloy nanoparticles

The chemoselective oxidation of unfunctionalised hydrocarbons to functionalised hydrocarbons (rather than the chemoselective but trivial combustion to CO2) has long been considered the holy grail of oxidation catalysis. Clearly, the readily available molecular oxygen would be the preferred oxidant, avoiding the generation of waste and co-products which can occur with other oxidants. The commercial significance of such reactions would be immense. Gold catalysts are very good oxidation catalysts, as demonstrated by the early work of Haruta, Rossi and Hutchings. Thus scientists have explored gold catalysts for the oxidation of hydrocarbons. Explorative studies in homogeneous catalysis by Periana showed that gold complexes oxidize methane to methanol in concentrated sulfuric acid with selenic acid as the oxidizing reagent at 180°C with a maximum turnover number (TON) of only 30 (see Angew. Chem. Int. Ed. 2004, 43, 4626). Different heterogeneous systems gave slightly better results, but a large step forward has now been achieved by Kesavan et al. (Science, 2011, 331, 6014, 195). They oxidize toluene to benzyl benzoate in good chemoselectivity under solvent-free conditions with an oxygen pressure of 10 bar/0.1 MPa. A TON of 3,150 for Au/Pd nanoparticles on carbon could be reached, and good activity was observed in the range of 80–160°C. An uncatalysed background reaction only became significant at 190°C. This paper from the Hutchings lab stresses once more that gold catalysis is an increasingly important tool in the field of green chemistry (Professor Stephen Hashmi, University of Heidelberg).

Fully reversible metal deactivation effects in gold/ceria–zirconia catalysts: role of the redox state of the upport

The reversible modification of the properties of supported Au nanoparticles caused by alternating oxidizing and reducing pretreatment of Au/CeO2–ZrO2 catalysts have been revealed in a new article by Cíes et al. (Angewandte Chemie International Edition, 2010, volume 49, Issue 50, 9744–9748). The team used a methodology that combines FTIR spectroscopy, studies on the volumetric adsorption of CO and ultimate oxygen storage capacity, determination of metal dispersion by electron microscopy, and X-ray photoelectron spectroscopy.

Titania-supported gold nanoparticles as photocatalyst

This perspective by Primo et al. (Phys. Chem. Chem. Phys., 2011, 13, 886–910) is focused on the photocatalytic activity of gold nanoparticles supported on titania (Au/TiO2). Titania is the most widely used photocatalyst, but its limited activity under visible light irradiation has motivated the quest for modified titania materials absorbing visible light. Data supporting and confirming the photoactivity of gold particles are briefly presented to justify the possibility of gold photosensitization of titania by electron injection into the conduction band. After describing the most common procedures used to prepare Au/TiO2, the central part of this article is focused on the photocatalytic activity reported for Au/TiO2 for hydrogen generation, dye decoloration, phenol decomposition and carboxylic acid degradation. One important point has been to distinguish those reports using visible light from those other in which direct titania excitation by UV light has been used. The authors argue that Au/TiO2 photocatalysts can find real applications in the near future for both environmental remediation and in the field of hydrogen generation.

Efficient silver-free gold (I)-catalyzed hydration of alkynes at low catalyst loading

The use of [(IPr)AuOH] as versatile, air- and moisture-stable pre-catalyst permits the in situ generation of the cationic gold (I) species [(IPr)Au]X after reaction with a Brønsted acid, as described in the paper by Nun et al. (Journal of Organometallic Chemistry, 2011, volume 696, Issue 1, 7–11). This catalytic system offers as a main advantage the lack of use of a silver salt activator or co-catalyst which is often air-, light- and moisture-sensitive. A general gold (I)-catalyzed procedure using this in situ activation at very low catalyst loading is described for the hydration of a broad range of internal and terminal alkynes.

A bioinspired approach for controlling accessibility in calix[4]arene-bound metal cluster catalysts

De Silva et al. (Nature Chemistry, 2010, 2, 1062–1068) have used calix[4]arene macrocycles bearing phosphines as crude mimics of the rigid backbones of proteins, demonstrating the synthesis of gold clusters and the control of their accessibility through an interplay between the sizes of the calixarene ligands and metal cores. In enzymes, the electronic and steric environments of active centers, and therefore their activity in biological processe, are controlled by the surrounding amino acids. In a similar way, organic ligands have been used for the “passivation” of metal clusters, that is, inhibition of their aggregation and control of their environment. However, the ability of enzymes to maintain large degrees of accessibility has remained difficult to mimic in synthetic systems in which little room, if any, is typically left to bind to other species. The results presented showed or 0.9-nm cores, 25% of all the gold atoms within the cluster bind to the chemisorption probe 2-naphthalenethiol. This accessibility dramatically decreases with 1.1 and 4 nm gold cores.

Gold catalyzes the Sonogashira coupling reaction without the requirement of palladium impurities

In this research, through a combination of kinetic and theoretical studies, Corma et al. (Chem. Commun., 2011, 47, 1446–1448) have concluded that gold is intrinsically active to perform the Sonogashira coupling reaction between phenylacetylene and iodobenzene. The presence of Pd impurities is not mandatory for catalyst activity.

Plasmonic enhancement of photocatalytic decomposition of methyl orange under visible light

By integrating strongly plasmonic Au nanoparticles with strongly catalytic TiO2, Hou et al. (Journal of Catalysis, 2011, volume 277, Issue 2, 149–153) have observed enhanced photocatalytic decomposition of methyl orange. Irradiating Au nanoparticles at their plasmon resonance frequency creates intense electric fields, which can be used to increase electron–hole pair generation rate in semiconductors. As a result, the photocatalytic activity of large bandgap semiconductors, like TiO2, can be extended into the visible region of the electromagnetic spectrum. In this research, the authors report a ninefold improvement in the photocatalytic decomposition rate of methyl orange driven by a photocatalyst consisting of strongly plasmonic Au nanoparticles deposited on top of strongly catalytic TiO2. Finite-difference time-domain simulations indicate that the improvement in photocatalytic activity in the visible range can be attributed to the electric field enhancement near the metal nanoparticles. The intense local fields produced by the surface plasmons couple light efficiently to the surface of the TiO2. This enhancement mechanism is particularly effective because of TiO2’s short exciton diffusion length, which would otherwise limit its photocatalytic efficiency. Electromagnetic simulations of this process suggest that enhancement factors many times larger than this are possible if this mechanism can be optimized.

The effect of gold on platinum oxidation in homogeneous Au–Pt electrocatalysts

Oxidation of Au–Pt thin films has been carried out by Wolter et al. (Applied Surface Science, 2010, volume 257, Issue 5, 1431–1436) in ambient air at room temperature and characterized by X-ray photoelectron spectroscopy. The homogeneous films were prepared by RF co-sputtering with concentrations varying from Au9Pt91 to Au89Pt11 and compared to pure Pt and Au thin films. Spectral deconvolution of the Au 4f and Pt 4f core levels revealed linear peak shifts for both the Au–Au and Pt–Pt bonding components as a function of alloy mixture and metallic component peak asymmetry which remained constant for all alloy stoichiometries. The predominant oxidation products were PtO and PtO2 and were characterized by stable core level binding energies for all films. A gradual decline in the Pt–O x products and corresponding levels of elemental oxygen was observed with increasing Au content but was similar in proportion to the metallic Pt components. Based on these results, variations in Pt oxide phases and/or concentration do not appear to contribute to enhanced electrocatalytic activity for oxygen reduction observed for the intermediate alloy stoichiometries

Electronics

A micromechanism study of thermosonic gold wire bonding on aluminum pad

Wire bonding remains the most important of electronic packaging technologies. A micromechanism of thermosonic gold wire bonding was elaborated by examining its interfacial characteristics as a result of the bonding process, including the fragmentation of the native aluminum oxide layer on Al pads, and formation of initial intermetallic compounds (IMCs). Xu et al. (J. Appl. Phys. 2010, 108, 113517) have found that the presence of an approximately 5 nm thick native oxide layer on original Al pads has a significant effect on the bonding, and the nucleation of IMCs during the bonding process must overcome this relatively inert thin film. Bonding strength was fundamentally determined by the degree of fragmentation of the oxide films, through which the formation of IMCs can be initiated due to the direct contact of the metal surfaces to be bonded. The authors showed that the extent of fracture the oxide layer was influenced by the level of ultrasonic power, as at its high level alumina fragmentation becomes pervasive resulting in contiguous alloy interfaces and robust bonds. The IMCs formed at the interfaces were identified as Al4Al and AuAl2 with a thickness of 150–300 nm. The formation mechanism of such IMCs was explained by the effective heat of formation theory.

An effective lift-off method for patterning high-density gold interconnects on an elastomeric substrate

Guo and De Weerth (Small, 2010, volume 6, Issue 24, 2847–2852) describe high-resolution, high-density gold interconnects are effectively patterned on an elastomeric substrate. A 3-cm cable of ten gold wires with 10 μm width and 20 μm pitch is achieved, successfully demonstrating density increases of more than one order of magnitude from previously established work. The authors contend that many applications in the fields of stretchable electronics and conformable neural interfaces will benefit from these fabrication developments.

Nanosized-induced low-temperature alloying in binary and ternary noble alloy systems for micro-interconnect applications

Kao et al. (Acta Materialia, 2011, volume 59, Issue 3, 1184–1190) have demonstrated the concept of low-temperature alloying due to the nanosize effect. The interactions between noble metallic nanoparticle deposits (NPD), including Au and bimetallic Ag3Au, and bulk substrates (Ag, Cu, and Ni) upon heating were investigated systematically. According to the experimental results, in the very early stage of heating at temperatures higher than the melting points of nanoparticles, which are substantially lower than those in the bulk state, the supercooled liquid reacted with the substrate and gave rise to binary or ternary alloying. The reaction products under such non-equilibrium conditions reveal a variety of metallurgical features depending on the nature of the NPD/substrate systems.

Interfacial reaction between Au–Sn solder and Au/Ni-metallized Kovar

Gold-tin (Au–Sn) solder and Kovar alloy have been extensively used in mechanical engineering, the atomic energy industry, aerospace facilities and electronic devices. Solder bonds strongly to the metallized substrate by forming IMCs at the interface. The IMC layer may adversely affect the reliability of the joints due to excessive growth and thermal fatigue during storage and service. Therefore, knowledge of the interfacial reactions between the Au–Sn solder and Au/Ni-metallized Kovar in microelectronic and optoelectronic packaging is essential. In this study Yoon et al. (Journal of Materials Science: Materials in Electronics, 2011, volume 22, number 1, 84–90), describe the microstructural evolution and interfacial reactions between the Au–Sn solder and Au/Ni-plated Kovar substrate during aging at 180 and 250°C for up to 1,000 h. The authors showed that the microstructure of the Au–Sn/Ni/Kovar joint was stable during aging at 180°C. The solid-state interfacial reaction was much faster at 250°C than at 180°C. The joints aged at 250°C fractured along the interface, thereby demonstrating brittle failure potentially because of the brittle IMC layer at the interface. The complete consumption of the thin Ni layer significantly weakened the joint interface during aging at 250°C and clearly demonstrated the need for a thicker Ni layer in order to ensure the high temperature reliability of the Au–Sn/Ni/Kovar joint above 250°C.

Method of bonding to gold surface and resultant combinations

This new US Patent Application 20100310888 relates to a method of bonding a gold surface to a second surface which comprises heating a hybrid organic-inorganic melting gel >50°C, applying the melting gel to either the gold surface or the second surface. The melting gel is heated to above 130°C until the melting gel has cured sufficiently to bond the surfaces together. The invention also relates to a combination of a gold surface and a second surface that is bonded together with a hybrid organic-inorganic melting gel. In another aspect of the invention, the hybrid organic–inorganic melting gel is heated to a workable viscosity and cast into a film, sheet, block, or lens. The cast gel is cured or partially cured and then applied between the gold surface and the second surface. Additional uncured melting gel may be applied. The constructed joint is heated to above 130°C until the melting gel has cured sufficient to bond the surfaces together.

Medical and dental

Small molecule-capped gold nanoparticles as potent antibacterial agents that target gram-negative bacteria

This report by Zhao et al. (J. Am. Chem. Soc., 2010, 132 (35), 12349–12356) illustrates a new strategy in designing antibacterial agents—a series of commercially available compounds, amino-substituted pyrimidines (themselves completely inactive as antibiotics), when presented on gold nanoparticles (NPs), show antibacterial activities against multidrug-resistant clinical isolates, without external sources of energy such as infrared. These pyrimidine-capped gold NPs exert their antibiotic actions via sequestration of magnesium or calcium ions to disrupt the bacterial cell membrane, resulting in leakage of cytoplasmic contents including nucleic acids from compromised cell membranes, and via interaction with DNA and inhibition of protein synthesis by internalized NPs. These amino-substituted pyrimidine-capped gold NPs induce bacterial resistance more slowly compared with small-molecule antibiotics and appear harmless to human cells. The authors believe the NPs may therefore be useful for clinical applications.

Fluorescence-surface enhanced Raman scattering co-functionalized gold nanorods as near-infrared probes for purely optical in vivo imaging

Gold nanorods (GNRs) have been widely used for bio-imaging. However, their use in assisted optical in vivo deep tissue imaging is severely restricted due to signal attenuation, low contrast and low real-timing. To overcome these problems, Qian et al. (Biomaterials, 2011, volume 32, issue 6, 1601–1610) have functionalized GNRs with both near-infrared fluorescence and surface enhanced Raman scattering and utilized these co-functionalized GNRs for purely optical in vivo imaging of live mice. The authors proposed technology has the combined advantages of high real-timing, high imaging contrast, and deep detection ability. The distribution and excretion of intravenously injected GNRs in deep tissues of live mice were observed in vivo for the first time through purely optical imaging. We also demonstrated successfully in vivo biomedical applications of the co-functionalized GNRs to sentinel lymph node mapping and tumor targeting of mice. The authors believe their work has applicability for disease diagnosis and clinical therapies.

Engineering nanocomposite materials for cancer therapy

Cancer accounted for 13% of all deaths worldwide in 2005. Although early detection is critical for the successful treatment of many cancers, there is sensitivity limitations associated with current detection methodologies. Furthermore, many traditional anticancer drug treatments exhibit limited efficacy and cause high morbidity. The unique physical properties of nanoscale materials can be utilized to produce novel and effective sensors for cancer diagnosis, agents for tumor imaging, and therapeutics for cancer treatment. Functionalizing inorganic nanoparticles with biocompatible polymers and natural or rationally designed biomolecules offers a possible route towards engineering responsive and multifunctional composite systems. Although only a few of these innovations have reached human clinical trial to date, nanocomposite materials based on functionalized metal and semiconductor nanoparticles promise to transform the way cancer is diagnosed and treated. This review presented by Minelli et al. (Small, 2011, volume 6, issue 21, 2336–2357) summarizes the current state-of-the-art in the development of inorganic nanocomposites for cancer-related applications including gold-related science.

Inhibition of influenza virus infection by multivalent sialic acid-functionalized gold nanoparticles

Papp et al. (Small, 2010, volume 6, issue 24, 2900–2906) describe an efficient synthesis of sialic acid-terminated glycerol dendron to chemically functionalize 2 and 14 nm gold nanoparticles (AuNPs). These nanoparticles are highly stable and show high activity towards the inhibition of influenza virus infection. As the binding of the viral fusion protein hemagglutinin to the host cell surface is mediated by sialic acid receptors, a multivalent interaction with sialic acid-functionalized AuNPs is expected to competitively inhibit viral infection. Electron microscopy techniques and biochemical analysis show a high binding affinity of the 14 nm AuNPs to hemagglutinin on the virus surface and, less efficiently, to isolated hemagglutinin. The functionalized AuNPs are nontoxic to the cells under the conditions studied. The authors claim that this approach allows a new type of molecular-imaging activity correlation and is of particular relevance for further application in alternative antiviral therapy.

Nanotechnology

Synthesis of Au(core)/Ag(shell) nanoparticles and their conversion to AuAg alloy nanoparticles

Shore et al. (Small, 2011, volume 7, issue 2, 230–234) at North Carolina State University show how a technique that is commonly used for making single-metal nanoparticles can be extended to create nanoparticles consisting of two metals—and that have tunable properties. The researchers created core/shell nanoparticles with a gold core and silver shell, as well as alloy nanoparticles, which mix the gold and silver. The researchers also characterized the optical properties of these nanoparticles. Tuning the optical properties of nanoparticles is of interest for applications such as security technology, and for use in making chemical reactions more efficient—which has multiple industrial and environmental applications.

Enhancement of luminescent quenching-based oxygen sensing by gold nanoparticles: comparison between luminophore/matrix/nanoparticle thin films on glass- and gold-coated substrates

For weak luminescence, quenching of insensitive luminophores by proximity to a gold film improves signal to noise by suppression of background luminescence of Ru(4,7-diphenyl-1,10-anthroline)3Cl2 according to recent research. Initially Roche et al. (J. Nanophoton., 2010, Vol. 4, 043521) expected that the effects of gold film quenching- and nanoparticle-enhanced luminescence could be combined to give a summative improvement, but the increase caused by the nanoparticles generates a larger signal-to-noise ratio and greater sensitivity of those luminophores to the dynamic quenching by gaseous oxygen. Impressive detection limits were achieved on gold-coated glass and plain glass, where detection limit was 0.05% and 0.004% and sensitivity 0.02 and 0.05%, respectively.

Binary self-assembly of gold nanowires with nanospheres and nanorods

Sánchez-Iglesias et al. (Angewandte Chemie International Edition, 2010, volume 49, issue 51, 9985–9989) have used gold nanowires as templates to direct the one-dimensional assembly of nanospheres and nanorods under suitable evaporation conditions, and using oleylamine as a capping agent. The overall optical response of the film can be tuned by varying the distances between the nanoparticles, according to the publication.

Assembly of strands of multiwall carbon nanotubes and gold nanoparticles using alkanedithiols

The possibility of using a new method to chemically bond multiwall carbon nanotubes (MWCNTs) to lower the contact resistance of thin CNT films is presented. MWCNT and AuNP were assembled into strands by cross-linking with alkanedithiols as described by Park et al. (Carbon, 2011, volume 49, issue 2, 487–494). Long MWCNT strands were first shortened to ~0.25 μm by chemical oxidation followed by ball milling, and then thiolated by reaction with cysteamine. The thiol groups on the surfaces of the MWCNT strands combined with Au nanoparticles to produce MWCNT–AuNP strands. A simple mixing of these strands with alkanedithiols resulted in an assembly of strands linked by the alkanedithiols which adsorbed onto the surfaces of the AuNPs attached to the MWCNT–AuNP strands. Short MWCNT–AuNP strands connected to one another in a parallel arrangement, whereas long strands assembled in a crossing arrangement.

Preparation and high-resolution microscopy of gold cluster-labeled nucleic acid conjugates and nanodevices

Nanogold and undecagold are covalently linked gold cluster labels which enable the identification and localization of biological components with molecular precision and resolution. They can be prepared with different reactivities which means they can be conjugated to a wide variety of molecules, including nucleic acids, at specific, unique sites. The location of these sites can be synthetically programmed in order to preserve the binding affinity of the conjugate and impart novel characteristics and useful functionality. Methods for the conjugation of undecagold and nanogold to DNA and RNA are discussed by Powell and Hainfeld (Micron, 2011, volume 42, issue 2, 163–174), and applications of labeled conjugates to the high-resolution microscopic identification of binding sites and characterization of biological macromolecular assemblies are described. As well as providing insights into their molecular structure and function, high-resolution microscopic methods also show how nanogold and undecagold conjugates can be synthetically assembled, or self-assemble, into supramolecular materials to which the gold cluster labels impart useful functionality.