Assembly Of Au Research Articles

Heating process and damage threshold analysis of Au film coated on Cu substrate for femtosecond laser

The heating processes of a two-layer film assembly of Au padded with Cu irradiated by femtosecond laser pulse are studied using a two-temperature model. It is found that the chosen substantially influence the energy transport, and consequently the temperature variation, and thermal equilibrium time. At the same laser fluence, the different thickness of gold film leads to a change of gold surface temperature. By choosing the thickness of the gold layer in the two-layer film assemblies, the damage threshold of the gold film can be maximized. The results can be used to optimize the damage threshold of gold coating optical components.

Plasmon coupling of R6G-linked gold nanoparticle assemblies for surface-enhanced Raman spectroscopy

This paper presents the study of surface-enhanced Raman scattering (SERS) for Rhodamine 6G (R6G)-linked gold (Au) assemblies. The nano-assemblies fabricated with Au nanoparticles, having diameters ( D) of ∼5–40 nm, possessed interparticle gaps of ∼0.03–0.2 D, respectively. The R6G-linked Au assemblies displayed weak spectral shifts of localized surface plasmon resonance (LSPR) mode at shorter wavelength region and large fractional red-shifts Δ λ ∼ 0.07–0.47 λ o at longer wavelength region. The resonant spectral coupling of the nano-assemblies LSPR mode at the longer wavelength region and the optical excitation at ∼785 nm, in addition to the highly localized hot spots, could support the large SERS enhancement of R6G-linked Au assemblies.

CdSe quantum dot-sensitized Au/TiO2 hybrid mesoporous films and their enhanced photoelectrochemical performance

Novel CdSe quantum dot (QD)-sensitized Au/TiO2 hybrid mesoporous films have been designed, fabricated, and evaluated for photoelectrochemical (PEC) applications. The Au/TiO2 hybrid structures were made by assembly of Au and TiO2 nanoparticles (NPs). A chemical bath deposition method was applied to deposit CdSe QDs on TiO2 NP films with and without Au NPs embedded. We observed significant enhancements in photocurrent for the film with Au NPs, in the entire spectral region we studied (350–600 nm). Incident-photon-to-current efficiency (IPCE) data revealed an average enhancement of 50%, and the enhancement was more significant at short wavelength. This substantially improved PEC performance is tentatively attributed to the increased light absorption of CdSe QDs due to light scattering by Au NPs. Interestingly, without QD sensitization, the Au NPs quenched the photocurrent of TiO2 films, due to the dominance of electron trapping over light scattering by Au NPs. The results suggest that metal NPs are potentially useful for improving the photoresponse in PEC cells and possibly in other devices such as solar cells based on QD-sensitized metal oxide nanostructured films. This work demonstrates that metal NPs can serve as light scattering centers, besides functioning as photo-sensitizers and electron traps. The function of metal NPs in a particular nanocomposite film is strongly dependent on their structure and morphology.

Electrochemical detection of oligonucleotide by attaching redox probes onto its backbone

An approach was demonstrated to detect oligonucleotide by attaching redox probes onto its backbone. First, peptide nucleic acid (PNA) with a neutral backbone was immobilized onto a gold (Au) electrode surface as a capture. Second, when the PNA capture hybridized with a target oligonucleotide (a short DNA), an assembly of Au–PNA–DNA formed and phosphate groups were thus brought into the assembly from the DNA's backbone. The linker ion of Zr 4+ exhibits a strong coordination interaction with the phosphate group and the carboxylic group. The hybridized target DNA provides the phosphate group while a derivatized redox probe of ferrocene (Fc) carboxyl acid offers the carboxylic group. Therefore, the redox probe can be attached to the phosphate group by the linker to form an assembly of Au–PNA–DNA–Zr–Fc. Its redox process was studied and the detection conditions of oligonucleotide were optimized. A limit of detection of 1.0 × 10 −12 M or ∼2 attomol was reached.

Amino acid-assisted one-pot assembly of Au, Pt nanoparticles onto one-dimensional ZnO microrods

In this contribution, a facile one-pot strategy is developed for the assembly of noble metal (Au and Pt) nanoparticles onto one-dimensional ZnO microrods using a green non-toxic reagent amino acid, lysine with two amino functional groups as the capping agent. Noble metal nanoparticles with a small size capped by lysine are formed and simultaneously anchored onto the surface of ZnO. No pre-functionalization of the ZnO support is needed, hence simplifying the synthesis and reducing the fabrication cost. Inspired by the catalytic properties of Au nanoparticles in both catalyst and sensor materials, we have examined the gas sensing performances towards ethanol detection. Obtained results demonstrate that after decoration by Au nanoparticles, the sensor shows significantly enhanced sensing performances in terms of high sensitivity, fast response and recovery, excellent reproducibility and good selectivity. The improved sensor properties are probably ascribed to the catalytic Au promoters and the Schottky barriers at the metal and semiconductor interface.

In Situ Generation of Two-Dimensional Au-Pt Core-Shell Nanoparticle Assemblies.

Two-dimensional assemblies of Au–Pt bimetallic nanoparticles are generated in situ on polyethyleneimmine (PEI) silane functionalized silicon and indium tin oxide (ITO) coated glass surfaces. Atomic force microscopy (AFM), UV–Visible spectroscopy, and electrochemical measurements reveal the formation of core–shell structure with Au as core and Pt as shell. The core–shell structure is further supported by comparing with the corresponding data of Au nanoparticle assemblies. Static contact angle measurements with water show an increase in hydrophilic character due to bimetallic nanoparticle generation on different surfaces. It is further observed that these Au–Pt core–shell bimetallic nanoparticle assemblies are catalytically active towards methanol electro-oxidation, which is the key reaction for direct methanol fuel cells (DMFCs).

Controlled Assembly of Au, Ag, and Pt Nanoparticles with Chitosan

A simple method for the controlled assembly of metal nanoparticles into one-dimensional chains has been developed. The chain-formation process could be accounted for by the electrostatic interaction of the negatively charged carboxylic groups on the citrate ions surrounding metal nanoparticles and the positively charged chitosan polymer. Three representative types of nanoparticles comprising Au, Ag, and Pt, respectively, have been used to illustrate the generic applicability of the proposed methodology. Facile control of the chain length of the nanoparticle assembly could be achieved by adjusting the concentrations of negatively or positively charged species. We anticipate the ready adaptability of the methodology to nanoparticles of more complex compositions.

Silver Fused Conducting Fiber Formation of Au–Ag Core-Shell Nanoparticles Mediated by Ascorbic Acid

In this paper, we report the spontaneous formation of fibrous structures consisting of assemblies of Au–Ag core-shell nanoparticles (NPs) from a solution consisting of Au–Ag core-shell NPs and l-ascorbic acid (AA). AA acted both as the reducing agent for the generation of NPs and also as the mediator for the formation of fibers. The process of fiber formation involved three steps—reduction of HAuCl4 to Au NPs by AA, subsequent formation of Au–Ag core-shell NPs after addition of AgNO3, and spontaneous formation of fibers from the mixtures in water. It took typically about 30 days to form complete fibers that are of lengths of several hundred micrometers to millimeters, although nanofibers started forming from the first day of solution preparation. The width of each of these fibers was typically about 1–4 µm with length of each segment of fiber bundle, on the order of 40 µm. Formation of fibers was also observed in absence of AgNO3. These fibers consisted of Au NPs and polymer of AA degradation products and were not electrically conducting. Also, low concentrations of AgNO3 produced fibers with low electrical conductivity. However, it was observed that increase in the amount of AgNO3 leads to the formation of fibers that were electrically conducting with conductivity values in the range of metallic conductivity. Spectroscopic and electron microscopic investigations were carried out to establish the formation of fibers. The details of fiber formation mechanism under different conditions and electrical conductivities of the fibers are discussed in the article.

Directed assembly of Au and Fe nanoparticles on a TiOx/Pt(111) ultrathin template: the role of oxygen affinity

The essential role of O affinity in the directed assembly of size-selected Au and Fe nanoparticles (NPs) on a TiO(x)/Pt(111) ultrathin oxide phase, an effective template for size selected metal NP growth, is revealed through scanning tunneling microscopy and density-functional calculations. A weakly interacting element (Au) diffuses rapidly and gets trapped in the vacancy defects (picoholes) located inside parallel rows (troughs, spaced 1.44 nm apart) peculiar to the film structure, producing size-selected NPs arranged in regular linear arrays aligned along the troughs. In contrast, an element with greater O affinity (Fe) experiences higher diffusion barriers, and the growth is dominated by kinetic effects, with a less effective preferential nucleation and the appearance of irregular NP morphologies.

Formation of Gold and Silver Nanoparticle Arrays and Thin Shells on Mesostructured Silica Nanofibers

AbstractMesostructured silica nanofibers synthesized in high yields with cetyltrimethylammonium bromide as the structure‐directing agent in HBr solutions are used as templates for the assembly of Au and Ag nanoparticles and the formation of thin Au shells along the fiber axis. Presynthesized spherical Au and Ag nanoparticles are adsorbed in varying amounts onto the silica nanofibers through bifunctional linking molecules. Nonspherical Au nanoparticles with sharp tips are synthesized on the nanofibers through a seed‐mediated growth approach. The number density of nonspherical Au nanoparticles is controlled by varying the amount of seeded nanofibers relative to the amount of supplied Au precursor. This seed‐mediated growth is further used to form continuous Au shells around the silica nanofibers. Both the Au‐ and Ag‐nanoparticle/silica‐nanofiber hybrid nanostructures and silica/Au core/shell fibers exhibit extinction spectra that are distinct from the spectra of Au and Ag nanoparticles in solution, indicating the presence of new surface plasmon resonance modes in the silica/Au core/shell fibers and surface plasmon coupling between closely spaced metal nanoparticles assembled on silica nanofibers. Spherical Au‐ and Ag‐nanoparticle/silica‐nanofiber hybrid nanostructures are further used as substrates for surface‐enhanced Raman spectroscopy, and the enhancement factors of the Raman signals obtained on the Ag‐nanoparticle/silica‐nanofiber hybrid nanostructures are 2 × 105 for 4‐mercaptobenzoic acid and 4‐mercaptophenol and 7 × 107 for rhodamine B isothiocyanate. These hybrid nanostructures are therefore potentially useful for ultrasensitive chemical and biological sensing by using molecular vibrational signatures.

Synthesis and assembly of Au–Pt bimetallic nanoparticles

Au–Pt bimetallic nanoparticles (NPs) were synthesized by reducing the mixture of HAuCl4 and K2PtCl6 with ethanol in the presence of cinnamic acid (C6H5CHCHCO2H, CA) through a thermal process. It was found that the isolated NPs could gradually self-assemble into chain-like structures, ultimately to 3-dimensional network nanostructures by adjusting the molar ratio of CA to K2PtCl6. Energy-dispersive Spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction was used to confirm the formation of Au–Pt bimetallic nanostructures. It was worthwhile noting that the bimetallic NPs with the novel structures prepared by our method exhibited an attractive catalytic activity for the hydrogen evolution reaction in an acidic solution.

For a capability to protect: Mass killing, the African Union and NATO

Click to increase image sizeClick to decrease image size Acknowledgement The author wishes to acknowledge and thank Refugees International for giving him the support and encouragement to develop this idea. Notes 1. The United States plan is to contribute about $600 million over five years and has challenged its G-8 partners to expand their programmes likewise. The US plan would at least triple the current annual contributions to the African Contingency Operations Training Assistance programme for peacekeeping training. 2. Terms like ‘peace enforcement’ and ‘peacemaking’ are sometimes used to describe international operations when conditions are not entirely permissive. But such terms may not adequately describe situations when forceful military action is needed to stop mass killing or defeat forces doing the killing whether or not there is a peace to be enforced or made. For the purpose of this essay, we will rely on more descriptive, if less diplomatic, terms like ‘forcible humanitarian intervention’. 3. The origins, evolution and limitations of traditional peacekeeping are well explained in John Hillen's Blue Helmets (London: Brassey's, 1998). 4. Public statements of UN Secretary General Kofi Annan have done much to establish these principles. See, for example, ‘Two Concepts of Sovereignty’, The Economist, 18 September 1999. Also, see the Report of the International Commission on Intervention and State Sovereignty, ‘The Responsibility to Protect’, December 2001, http://www.dfait-maeci.gc.ca/iciss-ciise/report-en.asp. 5. The concept of sovereignty qualified by an obligation to treat people humanely could also be extended to encompass the idea of regime change. 6. Globalisation, the spread of information technology, and the ubiquity and accessibility of news media are all contributing to a growing public sense of and concern for ‘the human condition’, which will in turn strengthen norms that limit the violence of sovereigns against people and demand action to uphold those norms. It seems inevitable that this trend will continue. 7. See Clifford H. Bernath and David C. Gompert, ‘Power to Protect’: Using New Military Capabilities to Stop Mass Killings (Washington DC: Refugees International, 2003). 8. Hillen's Blue Helmets provides a good account of these limitations and the corresponding reliance on sub-contracting of military powers to conduct intervention and enforcement. 9. The advantage of striking killing forces – which typically want to kill but not to fight – was demonstrated by the British in Sierra Leone, who showed not the slightest hesitation in punishing, frightening and thus scattering the Revolutionary United Front. 10. This was the case in Rwanda, as noted, but also in the eastern Democratic Republic of the Congo, where Rwandan forces have largely had their way – not always with desirable results. 11. See David C. Gompert, Courtney Richardson, Richard L. Kugler and Clifford H. Bernath, Learning from Darfur: Building a Net-capable African Force to Stop Mass Killing (Washington DC: Center for Technology and Security Policy, National Defense University, 2005). 12. Among the obstacles has been the opposition of the permanent members of the UN Security Council who are concerned about the erosion of their decision authority if the world body has a force ready to act. 13. This idea is suggested by Peter Gantz of Refugees International. Whether such arrangements would require additional legal obligations, like those of a treaty-based alliance, or could instead rely on political and practical understandings, needs more study. 14. Among the Western countries with the kinds of capabilities that an AU humanitarian intervention could fall back on are the UK, France, Italy, Germany, the Netherlands, Canada and Sweden. 15. Other examples include the British aircraft carrier off Sierra Leone and US amphibious forces off Liberia – in both cases providing escalation dominance and sending a clear message to the killers. 16. It is important to note that the Constitutive Act (or charter) of the African Union explicitly provides for forcible humanitarian intervention without the unanimity of the AU Assembly or the consent of a recalcitrant sovereign, if needed to stop crimes against humanity. Additional informationNotes on contributorsDavid C. GompertDavid C. Gompert is an advisor to Refugees International and a Senior Fellow at the RAND Corporation. He is former Senior Advisor for National Security and Defense, Iraq, and has served in senior positions in several US administrations, in private industry and at RAND.

Assembly of Bimetallic Gold−Silver Nanoparticles via Selective Interparticle Dicarboxylate−Silver Linkages

This article describes findings of an investigation of the thin-film assembly of binary metal nanoparticles via interparticle linkages of dicarboxylic acid mediators at selective metal sites. Decanethiol- (DT-) capped gold−silver (Au−Ag) alloy nanoparticles of ∼3-nm core sizes were chosen as a model system because of the unique carboxylate−Ag+ binding chemistry and the well-defined thiolate-capping capability at Au. The dicarboxylate-mediated thin-film assembly of Au−Ag nanoparticles was characterized using UV−vis, TEM, XPS, FTIR, and NMR techniques. The results from controlling the oxidative or reductive conditions reveal that the oxidation of Ag to Ag+ on the alloy nanocrystal surface played an important role in the selective ligand exchange and interparticle linkage by dicarboxylic acids (e.g., hexadecanedioic acid, HDA). The formation of the thin-film assembly is dependent on both the alloy composition of the Au−Ag nanoparticles and the chain length of the dicarboxylic acids. The degree for an HDA−DT/Au−Ag exchanging and linking system is determined to yield a value of 16% of DTs per Au−Ag particle being displaced by HDAs. In comparison to similar thin-film assemblies of Au−Ag and Au nanoparticles by other mediators, the results support the conclusion that the HDA-mediated assembly of Au−Ag nanoparticles occurs via −CO2-−Ag+ linkages at silver sites forming a mediator (HDA)−template (DT) nanostructure. The nanostructured thin-film assemblies exhibit interesting optical and electronic properties, which have important implications for the exploration of binary nanoparticle-structured materials for sensing and catalytic applications where fine-tuning of either the binary nanocrystal cores or the bifunctional interparticle structures is desired.

Stabilization of Pt nanoparticles by single stranded DNA and the binary assembly of Au and Pt nanoparticles without hybridization

The non-specific interaction between single stranded DNA (ssDNA) and 12 nm Pt nanoparticles is investigated in this work. The data show a strong and non-specific interaction between the two which can be exploited for the stabilization of Pt nanoparticles in aqueous solutions. Based on the experimental findings, a non-hybridization based protocol to assemble 17 nm Au and Pt nanoparticles (12 nm cubic and 3.6 nm spherical) by single-stranded DNA was developed. Transmission electron microscopy (TEM) and UV–visible spectroscopy confirmed that Au and Pt nanoparticles could be assembled by the non-specific interaction in an orderly manner. The experimental results also caution against the potential pitfalls in using DNA melting point analysis to infer metal nanoparticle assembly by DNA hybridization.

Surface‐Enhanced Raman Scattering of p‐Aminothiophenol on a Au(core)/Cu(shell) Nanoparticle Assembly

Surface-enhanced Raman scattering (SERS) of p-aminothiophenol (PATP) molecules adsorbed onto assemblies of Au(core)/Cu(shell) nanoparticles is reported. We compare it with the SERS spectrum of PATP adsorbed onto gold nanoparticles: both the absolute and relative scattered intensities of various bands in the two spectra are very different. The difference in relative intensity can be ascribed to chemical effects; the chemical enhancement ratio of the two substrates is approximately 3-5. A theoretical analysis based on a charge-transfer model is carried out, which yields a consistent result and shows that the difference in chemical enhancement is mainly due to the state densities and Fermi levels of the substrates. The difference in absolute intensity originates from electromagnetic (EM) enhancement. EM enhancement of Au(core)/Cu(shell) nanoparticles is unlike that of single-component gold or copper SERS-active substrates. The core/shell particle size for optimal enhancement is about 20 nm in the case of a 632.8 nm incident laser (the size ratio of the core and shell layers is about 0.6).

Kinetically Controlled Pt Deposition onto Self-Assembled Au Colloids: Preparation of Au (Core)−Pt (Shell) Nanoparticle Assemblies

Assemblies of Au (core)−Pt (shell) nanoparticles were prepared by chemical deposition of Pt metal onto colloidal Au particles that self-assembled onto substrates, since PtCl62- may be reduced to Pt(0) by the mild reducing agent NH2OH in the presence of gold nanoparticles. It is shown that the Pt deposition is isotropic and uniform on each particle via the combination of scanning electron microscopy and atomic force microscopy analysis. And the linear dependence of the particle size on the deposition time suggests that the deposition procedure is kinetically controlled. What is more, the kinetics of this Pt deposition onto Au nanoparticles is analyzed, which further reveals that the growth rate of the core−shell nanoparticles depends on the concentration of PtCl62-, the molar mass and density of Pt metal, and a constant related to the surface reaction.

The Status of the findings of the African Commission: from moral persuasion to legal obligation

Within the African regional human rights system, the African Commission on Human and Peoples' Rights is mandated to promote and protect human and peoples' rights. As part of its protective mandate, the Commission, in terms of an individual complaints procedure, can find that state parties are in violation of the provisions of the African Charter and has recommended remedies in cases where violations have been found. However, concerns have been raised about the non-compliance of state parties with the findings and recommendations issued by the African Commission. Although many possible reasons for this phenomenon could be explored, this contribution investigates one of the factors that may play a role, namely the argument that the findings of the Commission are not legally binding, and that states are consequently not obliged to abide by them. Arguments pertaining to the Commission's lack of competence to decide individual communications and its lack of competence to issue remedies, usually forwarded by those holding the view that the Commission's findings are not legally binding, are addressed here in a bid to clarify the status of the Commission's findings. A conclusion is reached that the findings of the Commission may be viewed as legally binding, firstly on a textual basis (the provisions of the African Charter) and secondly on account of the subsequent “ratification” (or “adoption”) of the findings by the AU Assembly.

PH Dependence of Interparticle Coupling for Gold Nanoparticle Assemblies Formation: Electrostatic Attraction and Hydrogen Bonding

Abstract With the addition of ω-sulfanylalkanoic or ω-alkyldisulfanylalkanoic acids to sodium-contained aqueous Au colloidal solutions, close-packed self-assembly occurred UV–vis, TEM, solid-state 13C NMR, and FT-IR spectra were employed to study the resulting aggregation arrays. The resulting two-dimensional Au assemblies exhibited interparticle coupling via both hydrogen bonding and electrostatic interactions at pH 6. Either acidic (pH 3) or base (pH 11) generated significant three-dimensional close-packed aggregates. It is suggested that electrostatic interactions might lead to the aggregates during the dry-up process in the preparation of TEM samples of the Au nanoparticles capped with COO− terminals at pH 11. For the acidic solutions (pH 3), coagulation was governed by hydrogen-bonding forces.

A novel hydrogen peroxide sensor based on horseradish peroxidase immobilized on colloidal Au modified ITO electrode

A novel method to fabricate a hydrogen peroxide sensor was developed by immobilizing horseradish peroxidase (HRP) on colloidal Au modified ITO conductive glass support. The cleaned glass support was modified with (3-aminopropyl)trimethoxysilane (APTMS) first to yield an interface for the assembly of colloidal Au. Then 15 nm colloidal Au particles were chemisorbed onto the amine groups of the APTMS. Finally, HRP was adsorbed onto the surface of the colloidal Au. The immobilized HRP displayed excellent electrocatalytical response to the reduction of hydrogen peroxide. The performance and factors influencing the resulted biosensor were studied in detail. The resulted biosensor exhibited fast amperometric response (within 5 s) to H 2O 2. The detection limit of the biosensor was 8.0 μmol l −1, and linear range was from 20.0 μmol l −1 to 8.0 mmol l −1. Furthermore, the resulted biosensor exhibited high sensitivity, good reproducibility, and long-term stability.

Micro-patterning of 3-aminopropyltrimethoxy- silane self-assembled monolayers with colloidal gold

This paper provides a new method to the micro-patterning of 3-aminopropyl-trimethoxysilane (ATS) self-assembled monolayers by colloidal Au. Photolithographing of ATS self-assembled monolayers on SiO 2-coated Si substrates and subsequent assembly of colloidal Au to the amino terminal groups in the unirradiated regions lead to the formation of Au microstructures. As revealed by scanning electron microscopy, the colloidal Au closely replicates the mask patterns on the unirradiated regions. The present approach may be applicable to fabrication of microelectronic circuits and microbiosensors.