Faceted Gold Research Articles

Site-Selective Deposition of Silica Nanoframes and Nanocages onto Faceted Gold Nanostructures Using a Primer-free Tetraethyl Orthosilicate Synthesis.

The Stöber method for forming spherical silica colloids is well-established as one of the pillars of colloidal synthesis. In a modified form, it has been extensively used to deposit both porous and protective shells over metal nanomaterials. Current best-practice techniques require that the vitreophobic surface of metal nanoparticles be primed with a surface ligand to promote silica deposition. Although such techniques have proved highly successful in forming core-shell configurations, the site-selective deposition of silica onto preselected areas of faceted metal nanostructures has proved far more challenging. Herein, a primer-free TEOS-based synthesis is demonstrated that is capable of forming architecturally complex nanoframes and nanocages on the pristine surfaces of faceted gold nanostructures. The devised synthesis overcomes vitreophobicity using elevated TEOS concentrations that trigger silica nucleation along the low-coordination sites where gold facets meet. Continued deposition sees the emergence of a well-connected frame followed by the lateral infilling of the openings formed over gold facets. With growth readily terminated at any point in this sequence, the synthesis distinguishes itself in being able to achieve patterned and tunable silica depositions expressing interfaces that are uncorrupted by primers. The so-formed structures are demonstrated as template materials capable of asserting high-level control over synthesis and assembly processes by using the deposited silica as a mask that deactivates selected areas against these processes while allowing them to proceed elsewhere. The work, hence, extends the capabilities and versatility of TEOS-based syntheses and provides pathways for forming multicomponent nanostructures and nanoassemblies with structurally engineered properties.

Process Optimization for the Bioinspired Synthesis of Gold Nanoparticles Using Cordyceps militaris, Its Characterization, and Assessment of Enhanced Therapeutic Efficacy.

The promising therapeutic implications of nanoparticles have spurred their development for biomedical applications. An eco-friendly methodology synthesizes gold nanoparticles using Cordyceps militaris, an edible mushroom (Cord-Au-NPs), using a quality-by-design approach (central composite design). UV-visible spectroscopy analysis revealed an absorption peak at 540-550 nm, thus confirming the synthesis of gold nanoparticles. Cord-Au-NPs have a crystalline structure, as evidenced by the diffraction peaks. The zeta potential value of -19.42 mV signifies the stability of Cord-Au-NPs. XRD study shows gold facets and EDX analysis revealed a strong peak of spherical nanoparticles in the gold region with a mean particle size of 7.18 nm and a polydispersity index of 0.096. The obtained peaks are closely associated with phenolic groups, lipids, and proteins, as examined by FTIR, suggesting that they function as the reducing agent. Cord-Au-NPs exhibited dose-dependent antioxidant, antidiabetic, and antibacterial activity. The method is eco-friendly, nontoxic, less time-consuming, and does not use synthetic materials, leading to higher capabilities in biomedical applications.

Optical suppression of energy barriers in single molecule-metal binding.

Transient bonds between molecules and metal surfaces underpin catalysis, bio/molecular sensing, molecular electronics, and electrochemistry. Techniques aiming to characterize these bonds often yield conflicting conclusions, while single-molecule probes are scarce. A promising prospect confines light inside metal nanogaps to elicit in operando vibrational signatures through surface-enhanced Raman scattering. Here, we show through analysis of more than a million spectra that light irradiation of only a few microwatts on molecules at gold facets is sufficient to overcome the metallic bonds between individual gold atoms and pull them out to form coordination complexes. Depending on the molecule, these light-extracted adatoms persist for minutes under ambient conditions. Tracking their power-dependent formation and decay suggests that tightly trapped light transiently reduces energy barriers at the metal surface. This opens intriguing prospects for photocatalysis and controllable low-energy quantum devices such as single-atom optical switches.

Self-assembled monolayers of O-(2-Mercaptoethyl)-O′-methyl-hexa(ethylene glycol) (EG7-SAM) on gold electrodes. Effects of the nature of solution/electrolyte on formation and electron transfer blocking characteristics

The self-assembly of O-(2-Mercaptoethyl)-O’-methyl-hexa(ethylene glycol) (EG7) on gold substrates produces monolayers whose structure should depend primarily on the solvents used for their formation. Although this should be also the case for any self-assembled monolayer (SAM) system, the presence of oxygen atoms in the EG7 chains brings about specific interactions with the solvent molecules other than just the van der Waals interactions taking place with alkanethiols. In this work we investigate the effects of the nature of the solutions for EG7-SAM formation in the reductive desorption processes using gold substrates either polycrystalline or single crystal electrodes. The patterns obtained in polycrystalline substrates are compared to the peaks observed at gold single crystal electrodes and it has been found that the main peaks correspond to the molecules desorbed from the different gold facets contained in the polycrystalline substrate. These single peaks are in fact composed of at least two contributions that can be explained as the presence of domains where the EG7 molecules are organized with different orientations, giving place to intermolecular interactions of different magnitude. The blocking properties of these films are strongly determined by the nature of the electrolyte used for the analysis and not by the solvent used in their formation. Although no experimental evidence of the specific interaction and/or retention inside the film of Na+ or phosphate or both ions is obtained, a specific effect that accounts for an electron transfer rate constant of an order of magnitude lower than the obtained with other electrolytes is observed. These results can contribute to increase the understanding of the relationship between the EG7-SAMs structure and functionalities.

Search for safe havens and resilience to global financial volatility: Response of GCC equity indexes to GFC and Covid-19

The bear Gulf Cooperation Council (GCC) sectoral equity markets in the midst of the COVID-19 pandemic presented us a test case to revisit the previously proclaimed refuge facets of gold and other precious commodities, as well as to find the resilience of these markets to global financial volatility. We further make a comparison of our findings with those obtained for the 2008 Global Financial Crisis (GFC) period. New evidence from our study based on bivariate Dynamic Conditional Correlation Generalized Autoregressive Conditional Heteroskedasticity model confirms the potential of gold to serve as a weak safe haven amid both financial crises albeit with higher effectiveness during the early phase of the COVID-19 pandemic. As a whole, silver and crude oil couldn't offer sanctuary for GCC's sectoral equity markets. Further results show that unless hedged, returns of GCC equity markets are vulnerable to volatility and risk in the global financial markets.

Tuning the morphology of bimetallic gold-platinum nanorods in a microflow synthesis

An automated microfluidic system with computer-controlled syringe pumps was applied for screening a three-dimensional concentration space for the formation of binary gold-platinum metal nanorods. Leveraging the micro segmented flow technique, precise residence and reactant addition timings as well as concentration spaces were addressed. The density and thickness of quasi-isotropic platinum shells on gold nanorod cores were tuned from isolated spots to a dense arrangement of high-aspect-ratio columns. The changing optical properties of the particles in the platinum deposition were used for monitoring the reaction progress and the products by the means of a fiber based micro flow-through spectrophotometer allowing to optimize process times. From our data, we propose an electrochemical model, postulating a diode-like effect and limitations for the formation of Pt nuclei on the gold surface and the formation of nano local elements. This point of view is supported by the observed decoration effects of gold facets and to the formation of columnar structures of the platinum shell.

Optical response of noble metal nanostructures: quantum surface effects in crystallographic facets

Noble metal nanostructures are ubiquitous elements in nano-optics, supporting plasmon modes that can focus light down to length scales commensurate with nonlocal effects arising from quantum confinement and spatial dispersion in the underlying electron gas. Quantum and nonlocal effects can be more prominent in crystalline noble metals, due to their lower intrinsic loss (when compared with their polycrystalline counterparts), and because particular crystal facets give rise to distinct electronic surface states whose signatures can be imprinted in the optical response of a structure. Here, we employ an atomistic method to describe nonclassical effects impacting the optical response of crystalline noble metal surfaces and demonstrate that these effects can be well captured using a set of surface-response functions known as Feibelman d -parameters determined from such quantum-mechanical models. In particular, we characterize the d -parameters associated with the (111) and (100) crystal facets of gold, silver, and copper, emphasizing the importance of quantum surface effects associated with electron wave function spill-out/spill-in and with the surface-projected band gap emerging from the atomic-layer corrugation. Furthermore, we show that the extracted d -parameters can be straightforwardly applied to describe the optical response of various nanoscale metal morphologies of interest, including metallic ultrathin films, graphene–metal heterostructures hosting ultraconfined acoustic graphene plasmons, and crystallographic faceted metallic nanoparticles supporting localized surface plasmons. We envision that the d -parameters presented here, along with the prescription to extract and apply them, could help circumvent computationally expensive first-principles atomistic calculations to describe quantum nonlocal effects in the optical response of mesoscopic crystalline metal surfaces, which are becoming widely available with increasing control over morphology down to atomic length scales for state-of-the-art experiments in nano-optics.

One‐Step Preparation of Biocompatible Gold Nanoplates with Controlled Thickness and Adjustable Optical Properties for Plasmon‐Based Applications

AbstractThe ability to synthesize plasmonic nanomaterials with well‐defined structures and tailorable size is crucial for exploring their potential applications. Gold nanoplates (AuNPLs) exhibit appealing structural and optical properties, yet their applications are limited by difficulties in thickness control. Other challenges include a narrow range of tunability in size and surface plasmon resonance, combined with a synthesis conventionally involving cytotoxic cetyltrimethylammonium (CTA) halide surfactant. Here, a one‐step, high‐yield synthesis of single‐crystalline AuNPLs is developed, based on the combined use of two structure‐directing agents, methyl orange and FeBr3, which undergo preferential adsorption onto different crystalline facets of gold. The obtained AuNPLs feature high shape homogeneity that enables mesoscopic self‐assembly, broad‐range tunability of dimensions (controlled thickness from ≈7 to ≈20 nm, accompanied by modulation of the edge length from ≈150 nm to ≈2 µm) and plasmonic properties. These merits, coupled with a preparation free of CTA‐halide surfactants, have facilitated the exploration of various uses, especially in bio‐related areas. For example, they are demonstrated as biocompatible photothermal agents for cell ablation in NIR I and NIR II windows. This work paves the way to the innovative fabrication of anisotropic plasmonic nanomaterials with desired attributes for wide‐ranging practical applications.

Design of Magnetic-Plasmonic Nanoparticle Assemblies via Interface Engineering of Plasmonic Shells for Targeted Cancer Cell Imaging and Separation.

Magnetic-plasmonic nanoparticles have received considerable attention for widespread applications. These nanoparticles (NPs) exhibiting surface-enhanced Raman scattering (SERS) activities are developed due to their potential in bio-sensing applicable in non-destructive and sensitive analysis with target-specific separation. However, it is challenging to synthesize these NPs that simultaneously exhibit low remanence, maximized magnetic content, plasmonic coverage with abundant hotspots, and structural uniformity. Here, a method that involves the conjugation of a magnetic template with gold seeds via chemical binding and seed-mediated growth is proposed, with the objective of obtaining plasmonic nanostructures with abundant hotspots on a magnetic template. To obtain a clean surface for directly functionalizing ligands and enhancing the Raman intensity, an additional growth step of gold (Au) and/or silver (Ag) atoms is proposed after modifying the Raman molecules on the as-prepared magnetic-plasmonic nanoparticles. Importantly, one-sided silver growth occurred in an environment where gold facets are blocked by Raman molecules; otherwise, the gold growth is layer-by-layer. Moreover, simultaneous reduction by gold and silver ions allowed for the formation of a uniform bimetallic layer. The enhancement factor of the nanoparticles with a bimetallic layer is approximately 107 . The SERS probes functionalized cyclic peptides are employed for targeted cancer-cell imaging and separation.

Shapes of epitaxial gold nanocrystals on SrTiO3 substrates.

Morphological control of gold nanocrystals is important as their catalytic and optical properties are highly shape dependent. In this paper we report the shapes of gold nanocrystals which deviate from the equilibrium Wulff shape due to the influence of the SrTiO3 single crystal substrates. The gold crystals are characterized by scanning tunneling microscopy (STM) and scanning electron microscopy (SEM). The nanocrystals have an equilibrium shape of a truncated octahedron with {111} and {001} facets. On all three substrate surfaces, i.e., SrTiO3(001)-(2 × 1), SrTiO3(001)-c(4 × 2), and SrTiO3(111)-(4 × 4) + (6 × 6), the height-to-width ratio of the gold crystals is not a constant as would be expected for equilibrium crystals, but instead it increases with crystal height. We propose that as the crystals increase in size, their aspect ratio heightens to relax the interfacial strain. The ratio between the {111} and {001} surface areas of our gold crystals is found to differ on the three substrates, which we speculate is due to the selective adsorption of surfactants on the {111} and {001} gold facets resulting from the different substrate surfaces. Reentrant facets of gold crystals that should be present according to their Wulff shape are not observed because these concave sites typically grow out due to kinetic considerations. This study demonstrates the significant effect of the crystal facet termination and surface reconstruction of an oxide substrate on the shape of supported gold nanocrystals.

Structural transitions and bilayer formation of CTAB aggregates

Controlled CTAB self-assembly is an essential prerequisite for the formation of gold nanorods with tailored shape and monodispersity. In this paper, we exploit the use of salt concentration and co-surfactant decanol for the preparation of CTAB aggregates with different morphologies. To this end we use a model of CTAB recently developed by ourselves, and perform electronic structure calculations in order to improve current point charge parametrization of decanol. Using molecular dynamics simulations with the new models, we find a sequence of structural transitions of CTAB aggregates induced by salt concentration and added cosurfactant. In pure solutions, CTAB aggregates form spherical micelles with a compact ionic shell and a diffuse double layer that can be qualitatively described with Poisson–Boltzmann theory. Addition of decanol as a cosurfactant induces a sequence of dramatic structural transitions at low surfactant concentration and allows the stabilization of compact ordered bilayers in a well defined range of intermediate decanol/CTAB ratios. At low and high decanol/CTAB ratios spherical micelles are transformed into wormlike cylindrical micelles. At intermediate decanol/CTAB ratios, fully formed bilayers are observed, with surfactants exhibiting a compact structure with strong positional and orientational order. We discuss how the controlled self-assembly of compact CTAB bilayers at low global CTAB concentration can pave the way for the selective passivation of gold facets and the controlled formation of monodisperse gold nanorods.

Ab Initio atomistic thermodynamics modeling of adsorption of oxygen on gold and gold-silver surfaces

A theoretical study of oxygen adsorption on gold and gold-silver surfaces by means of density functional theory (DFT) calculations with an atomistic thermodynamic model is performed. The (111) and (211) facets of gold and gold-silver alloy surfaces are considered, and their stabilization is discussed upon adsorption of oxygen depending on O and Ag coverage. The details of how the DFT-based atomistic thermodynamic model can apply to the transition metal surface are also presented in this work.

Correlating Selective Electrocatalysis of Dopamine and Ascorbic Acid Electrooxidation at Nanoporous Gold Surfaces with Structural-Defects

Interlinking kinetics of electrode processes of electroactive compounds with structural properties of the electrode materials like nanoporous gold (NPG) can be a smart strategy to design an electrocatalyst for specific applications. Accordingly, NPG materials electrochemically synthesized and characterized to confirm their structure and morphology were employed as platform to investigate the electrode reactions of ascorbic acid (AA) and dopamine (DA). A systematic study revealed that DA heterogeneous electron transfer (HET) rate is slower and independent of the electrode materials, whereas AA oxidation process becomes faster as a function of the NPG thickness. Such observations were correlated with changes in the mass transport and the specific surface interactions of AA and DA with changing film thickness of NPG. DA adsorbs strongly and the electrochemical process is essentially influenced by the larger surface area of the NPG film. On the other hand, AA interacts preferentially with the exposed high energy and more reactive (200), (220) and (311) crystalline facets of gold, which increase in thicker NPG films as demonstrated by High Resolution Transmission Electron Microscopy and X-Ray Diffraction, enhancing the HET rate.

Modeling of a C-end rule peptide adsorbed onto gold nanoparticles.

The RPAR peptide, a prototype C-end Rule (CendR) sequence that binds to neuropilin-1 (NRP-1), has potential therapeutic uses as internalization trigger in anticancer nanodevices. Recently, the functionalization of gold nanoparticles with CendR peptides has been proved to be a successful strategy to target the NRP-1 receptor in prostate cancer cells. In this work, we investigate the influence of two gold surface facets, (100) and (111), on the conformational preferences of RPAR using molecular dynamics simulations. Both clustering and conformational analyses revealed that the peptide backbone becomes very rigid upon adsorption onto gold, which is a very fast and favored process, the only flexibility being attributed to the side chains of the two Arg residues. Thus, the different components of RPAR tend to adopt an elongated shape, which is characterized by the pseudo-extended conformation of both the backbone and the Arg side chains. This conformation is very different from the already known bioactive conformation, indicating that RPAR is drastically affected by the substrate. Interestingly, the preferred conformations of the peptide adsorbed onto gold facets are not stabilized by salt bridges and/or specific intramolecular hydrogen bonds, which represent an important difference with respect to the conformations found in other environments (e.g. the peptide in solution and interacting with NRP-1 receptor). However, the conformational changes induced by the substrate are not detrimental for the use of gold nanoparticles as appropriate vehicles for the transport and targeted delivery of the RPAR. Thus, once their high affinity for the NRP-1 receptor induces the targeted delivery of the elongated peptide molecules from the gold nanoparticles, the lack of intramolecular interactions facilitates their evolution towards the bioactive conformation, increasing the therapeutic efficacy of the peptide.

Increase in the β-Sheet Character of an Amyloidogenic Peptide upon Adsorption onto Gold and Silver Surfaces.

Fibrillation of amyloid beta (Aβ) peptide is the hallmark of Alzheimer's disease. Given that interactions at the bio-nano interface affect the fibrillation tendency of this peptide, an understanding of the interactions at Aβ peptide-inorganic surfaces on the microscopic level can help to determine the possible neurotoxicity of nanoparticles. Here, the interactions between a fibril-forming peptide, Aβ25-35 , and (111) and (100) facets of gold and silver surfaces have been studied by conducting atomistic molecular dynamics simulations. The obtained results indicate that the adsorption onto gold and silver surfaces force the peptide into the β-sheet-rich conformations, which is prone to aggregation, suggesting a new mechanism for the acceleration of fibril formation upon interaction with nanoparticles. To quantify the β-sheet content for a single peptide, a new metrics based on the Ramachandran probability distribution is introduced.

Simulation of CTAB bilayer adsorbed on Au (100), Au (110), and Au (111) surfaces: structure stability and dynamic properties

Anisotropic growth of gold nanorods is often attributed to the adsorption of surfactant cetyltrimethyl ammonium bromide (CTAB) bilayer on gold facets. In this work, we performed DFT (density function theory) calculations to investigate possible binding sites of Br ions on Au (100), Au (110), and Au (111) facets. The CTAB bilayers on these facets were then constructed, and the structures of these interdigitated bilayers were studied by using molecular dynamics simulation. In addition, the dynamic properties of CTAB bilayer were investigated, and the results indicate that the outer layer exhibits noticeable lateral diffusion and undergoes fluctuation along surface normal. Both lateral diffusion and surface normal fluctuation of CTAB bilayer on Au (111) facet are more profound. The stability of CTAB bilayers on gold surfaces was estimated in terms of the required energy for dissociation of a CTA+ ion from the outer layer. The stability of CTAB bilayer on Au (111) is considerably weaker than on Au (100) and Au (110). Higher volatility and weaker stability may be due to lower packing density of CTAB bilayer on Au (111) facet, and both of them induce the growth tendency of gold nanoparticles along the (111) direction.

Prominent electrocatalytic methanol oxidation from cauli-flower shape gold with high-index facets

Nano-sized gold has been recognized as a potential catalyst in methanol fuel cells for both low cost and no formation of poisoning species compared with Pt. Previous techniques to prepare highly effective pure gold catalysts for methanol fuel cells are dealloying, which needs stern conditions. In this paper, cauli-flower shape gold nanoparticles with exposed high-index facets were obtained by a mild hydrothermal method. During the reaction process, sodium dodecyl sulfonate selectively protected high-index facets of gold, and H2O2 kept etching low-index ones and growing new ones at the same time. A possible growth mechanism for high-index facets was proposed. The as-prepared gold nanoparticles were employed as electrocatalysts in the methanol oxidation. The results show enhanced anode current of 1249.8 μA cm−2 at 0.18 V in solutions with 0.5 M KOH and 1.0 M CH3OH, which is 10.68-fold larger than the previous reported value based on nanoporous gold catalysts. The preparation method of cauli-flower shape gold was easy to be operated and controlled, which is also universal in the synthesis of other materials with high-index facets.

The gold/ampicillin interface at the atomic scale.

In the fight against antibiotic resistance, gold nanoparticles (AuNP) with antibiotics grafted on their surfaces have been found to be potent agents. Ampicillin-conjugated AuNPs have been thus reported to overcome highly ampicillin-resistant bacteria. However, the structure at the atomic scale of these hybrid systems remains misunderstood. In this paper, the structure of the interface between an ampicillin molecule AMP and three flat gold facets Au(111), Au(110) and Au(100) has been investigated with numerical simulations (dispersion-corrected DFT). Adsorption energies, bond distances and electron densities indicate that the adsorption of AMP on these facets goes through multiple partially covalent bonding. The stability of the AuNP/AMP nanoconjugates is explained by large adsorption energies and their potential antibacterial activity is discussed on the basis of the constrained spatial orientation of the grafted antibiotic.

Facet Recognition and Molecular Ordering of Ionic Liquids on Metal Surfaces

Ionic liquids are widely used as solvents and reaction media due to low volatility, stability up to high temperature, and large dipole moment. Emergent applications also aim at the anisotropic growth of metal nanostructures in ionic liquids through facet-selective interactions although the governing mechanisms remain poorly understood. We employed a combination of quantum mechanical and classical simulations to analyze the structure and energetics of the self-assembly of ionic liquids on metal surfaces from single ion pairs to multilayers, using the example of 1-ethyl-3-methylimidazolium ethyl sulfate ([EMIM][ES]) on the crystallographic {111}, {100}, and {110} facets of gold. Adsorption is controlled by the interplay of soft epitaxy, ionic interactions, induced charges, and steric effects related to the geometry of the cation and anion. These factors lead to characteristic molecular patterns on individual surfaces. Binding energies are similar irrespective of surface coverage and only slightly increase f...

Crystal Face Dependent Chemical Effects in Surface-Enhanced Raman Scattering at Atomically Defined Gold Facets

Among electromagnetic and chemical (CM) contributions to surface-enhanced Raman scattering (SERS), the former is becoming controllable according to the recent progress in nanofabrication of plasmonic metal structures. However, it is still difficult to control the latter effect. Here, the degree of each contribution to SERS signals is examined on well-defined single crystalline facets of gold by using optical field localization within sphere-plane type plasmonic cavities. Crystal face dependent SERS studies of aminobenzenthiol adsorbates clearly show the distinction between CM enhancements on different surfaces, suggesting that the CM-activity of "SERS-hotspots" is closely related to interfacial dipoles formed at metal-molecular junctions.