Ultrathin Gold Nanowires Can Be Obtained by Reducing Polymeric Strands of Oleylamine−AuCl Complexes Formed via Aurophilic Interaction

Tailor-Made Gold Nanomaterials for Applications in Soft Bioelectronics and Optoelectronics

In modern nanoscience and nanotechnology, gold nanomaterials are indispensable building blocks that have demonstrated a plethora of applications in catalysis, biology, bioelectronics, and optoelectronics. Gold nanomaterials possess many appealing material properties, such as facile control over their size/shape and surface functionality, intrinsic chemical inertness yet with high biocompatibility, adjustable localized surface plasmon resonances, tunable conductivity, wide electrochemical window, etc. Such material attributes have been recently utilized for designing and fabricating soft bioelectronics and optoelectronics. This motivates to give a comprehensive overview of this burgeoning field. The discussion of representative tailor-made gold nanomaterials, including gold nanocrystals, ultrathin gold nanowires, vertically aligned gold nanowires, hard template-assisted gold nanowires/gold nanotubes, bimetallic/trimetallic gold nanowires, gold nanomeshes, and gold nanosheets, is begun. This is followed by the description of various fabrication methodologies for state-of-the-art applications such as strain sensors, pressure sensors, electrochemical sensors, electrophysiological devices, energy-storage devices, energy-harvesting devices, optoelectronics, and others. Finally, the remaining challenges and opportunities are discussed.

Sub-millisecond microfluidic mixers coupled to time-resolved in situ photonics to study ultra-fast reaction kinetics: the case of ultra-small gold nanoparticle synthesis

We report a continuous microreactor platform achieving sub-millisecond homogeneous reagent mixing (∼300 μs) for a time-resolved study on the synthesis of ultra-small gold nanoparticles (NPs). The microreactor (coupled with small angle X-ray scattering, UV-vis, and X-ray absorption spectroscopy for in situ and in operando characterizations), operates within mixing time frames below system characteristic times, providing a unique opportunity to deepen the comprehension of reaction and phase transition pathways with unprecedented details. The microreactor channel length can be approximated to a given reaction time when operated in continuous mode and steady state. As a result, the system can be statically investigated, eliminating technique-dependent probing time constraints and local inhomogeneities caused by mixing issues. We have studied Au(0) NP formation kinetics from Au(III) precursors complexed with oleylamine in organic media, using triisopropylsilane as a reducing agent. The existence of Au(III)/Au(I) prenucleation clusters and the formation of a transient Au(I) lamellar phase under certain conditions, before the onset of Au(0) formation, have been observed. Taking advantage of the high frequency time-resolved information, we propose and model two different reaction pathways associated with the presence or absence of the Au(I) lamellar phase. In both cases, non-classical pathways leading to the formation of NPs are discussed.

Water-phase synthesis of Au and Au-Ag nanowires and their SERS activity

Metal nanowires (NWs) with a diameter of a few nanometers have attracted considerable attention as a promising one-dimensional nanomaterial due to their inherent flexibility and conductive properties and their weak plasmon absorption in the visible region. In a previous paper, we reported the synthesis of ultrathin 1.8 nm-diameter Au NWs using toluene-solubilized aqueous solutions of a long-chain amidoamine derivative (C18AA). This study investigates the effect of different organic solvents solubilized in C18AA aqueous solutions on the morphology of the Au products and demonstrates that solubilizing methylcyclohexane yields thick 2.7 nm-diameter Au NWs and 3.3 nm-diameter Au-Ag alloy NWs. Further, the surface-enhanced Raman scattering sensitivity of ultrathin Au NWs, thick Au NWs, and thick Au-Ag alloy NWs were assessed using 4-mercaptopyridine and found that their enhancement factors are 10⁴-10⁵ and the order is Au-Ag NWs > thick Au NWs > ultrathin Au NWs.

Flexible and transparent electrodes imprinted from Au nanowires: stability and ageing

We study the stability of flexible transparent electrodes (FTEs) that were self-assembled from ultra-thin gold nanowires (AuNW) by direct nanoimprinting of inks with different particle concentrations (1 to 10 mg mL^-1). The resulting lines were less than 3 μm wide and contained bundles of AuNW with oleylamine (OAm) ligand shells. Small-angle X-ray scattering confirmed a concentration-independent bundle structure. Plasma sintering converted the wire assemblies into lines with a thin metal shell that contributes most to electrical conductivity and covers a hybrid core. We studied the relative change in sheet resistance and the morphology of the FTEs with time. The sheet resistance increased at all concentrations, but at different rates. The metal shell aged by de-wetting and pore formation. The hybrid core de-mixed and densified, which led to a partial collapse of the shell. Residual organics migrated through the shell via its pores. Lines formed at low concentration (c _Au = 2 to 3 mg mL^-1) contained less residual organics and aged slower than those formed at high c _Au ≥ 5 mg mL^-1. We passivated the conductive shell with thin, adsorbed layers of PEDOT:PSS and found that it decelerated degradation by slowing surface diffusion and hindering further rupture of the shell. Thick capping layers prevented degradation entirely and stopped pore formation.

Oleic acid/oleylamine ligand pair: a versatile combination in the synthesis of colloidal nanoparticles

A variety of colloidal chemical approaches has been developed in the last few decades for the controlled synthesis of nanostructured materials in either water or organic solvents. Besides the precursors, the solvents, reducing agents, and the choice of surfactants are crucial for tuning the composition, morphology and other properties of the resulting nanoparticles. The ligands employed include thiols, amines, carboxylic acids, phosphines and phosphine oxides. Generally, adding a single ligand to the reaction mixture is not always adequate to yield the desired features. In this review, we discuss in detail the role of the oleic acid/oleylamine ligand pair in the chemical synthesis of nanoparticles. The combined use of these ligands belonging to two different categories of molecules aims to control the size and shape of nanoparticles and prevent their aggregation, not only during their synthesis but also after their dispersion in a carrier solvent. We show how the different binding strengths of these two molecules and their distinct binding modes on specific facets affect the reaction kinetics toward the production of nanostructures with tailored characteristics. Additional functions, such as the reducing function, are also noted, especially for oleylamine. Sometimes, the carboxylic acid will react with the alkylamine to form an acid-base complex, which may serve as a binary capping agent and reductant; however, its reducing capacity may range from lower to much lower than that of oleylamine. The types of nanoparticles synthesized in the simultaneous presence of oleic acid and oleylamine and discussed herein include metal oxides, metal chalcogenides, metals, bimetallic structures, perovskites, upconversion particles and rare earth-based materials. Diverse morphologies, ranging from spherical nanoparticles to anisotropic, core-shell and hetero-structured configurations are presented. Finally, the relation between tuning the resulting surface and volume nanoparticle properties and the relevant applications is highlighted.

Flexible and transparent electrodes imprinted from metal nanostructures: morphology and opto-electronic performance

We directed the self-assembly of nanoscale colloids via direct nanoimprint lithography to create flexible transparent electrodes (FTEs) with metal line widths below 3 μm in a roll-to-roll-compatible process. Gold nanowires and nanospheres with oleylamine shells were imprinted with soft silicone stamps, arranged into grids of parallel lines, and converted into metal lines in a plasma process. We studied the hierarchical structure and opto-electronic performance of the resulting grids as a function of particle geometry and concentration. The performance in terms of optical transmittance was dominated by the line width. Analysis of cross-sections indicated that plasma sintering only partially removed the insulating ligands and formed lines with thin conductive shells and a non-conductive core. We provide evidence that the self-assembly of high-aspect nanowires can compensate for defects of the stamp and substrate irregularities during imprinting, while spheres cannot. The wire-based electrodes thus outperformed the sphere-based electrodes at ratios of optical transmittance to sheet resistance of up to ≈ 0.9% Ω_sq ^-1, while spheres only reached ≈ 0.55% Ω_sq ^-1.

Facile and Scalable Synthesis of Whiskered Gold Nanosheets for Stretchable, Conductive, and Biocompatible Nanocomposites

Noble metal nanomaterials have been studied as conductive fillers for stretchable, conductive, and biocompatible nanocomposites. However, their performance as conductive filler materials is far from ideal because of their high percolation threshold and low intrinsic conductivity. Moreover, the difficulty in large-scale production is another critical hurdle in their practical applications. Here we report a method for the facile and scalable synthesis of whiskered gold nanosheets (W-AuNSs) for stretchable, conductive, and biocompatible nanocomposites and their application to stretchable bioelectrodes. W-AuNSs show a lower percolation threshold (1.56 vol %) than those of gold nanoparticles (5.02 vol %) and gold nanosheets (2.74 vol %), which enables the fabrication of W-AuNS-based stretchable nanocomposites with superior conductivity and high stretchability. Addition of platinum-coated W-AuNSs (W-AuNSs@Pt) to the prepared nanocomposite significantly reduces the impedance and improved charge storage capacity. Such enhanced performance of the stretchable nanocomposite enables us to fabricate stretchable bioelectrodes whose performance is demonstrated through animal experiments including electrophysiological recording and electrical stimulation in vivo.

Highly sensitive, room temperature operated gold nanowire-based humidity sensor: adoptable for breath sensing

A novel, highly sensitive gold nanowire (AuNW) resistive sensor is reported here for humidity sensing in the relative humidity range of 11% to 92% RH as well as for breath sensing. Both humidity and breath sensors are widely needed. Despite a lot of research on humidity and breath sensors, there is a need for simple, inexpensive, reliable, sensitive and selective sensors, which will operate at room temperature. Here we have synthesized gold nanowires by a simple, wet chemical route. The nanowires synthesized by us are 4–7 nm in diameter and a few micrometers long. The nanowires are amine functionalized. The sensor was prepared by drop casting gold nanowires on an alumina substrate to form a AuNW layer with different thicknesses (10, 20, 30 μm). The AuNW sensor is highly selective towards humidity and shows minimum cross sensitivity towards other gases and organic vapors. At an optimum thickness of 20 μm, the humidity sensing performance of the AuNW sensor over 11% to 92% RH was found to be superior to that of 10 and 30 μm thick layers. The response time of the sensor is found to be 0.2 s and the recovery time is 0.3 s. The response of the AuNW sensor was 3.3 MΩ/% RH. Further, the AuNW sensor was tested for sensing human breathing patterns.

A novel, highly sensitive gold nanowire (AuNW) resistive sensor is reported here for humidity sensing in the relative humidity range of 11% to 92% RH as well as for breath sensing.

Cow-to-cow variation in nanocrystal synthesis: learning from technical-grade oleylamine

Many technical-grade reagents, including oleylamine, are broadly used as ligands in nanocrystal synthesis, allowing for cost-effective, and more environmentally friendly, preparation of materials in useful quantities. Impurities can represent 30% or more of these reagent blends, and have frequently emerged as substantial drivers of nanocrystal morphology, assembly, or other physical properties, making it important to understand their composition. Some functional alkyl reagents are derived from natural sources (e.g. often beef tallow, in the case of oleylamine), introducing alkyl chain structures very different than those that might be expected as side products of synthesis from pure feedstocks. Additionally, impurities can exhibit variations based on biological factors (e.g. species, diet, season). In biology, blends of alkyl chains allow for surprisingly sophisticated function of amphiphiles in the cell membrane, pointing to the possibility of similar control in synthetic materials if reagent composition were either better controlled or better understood. Here, we provide brief context on the breadth of roles technical-grade impurities have played in nanocrystal materials, followed by a perspective on oleylamine impurities, their physical properties, and their potential contributions to nanomaterial function.

Recent Progress in the Transfer of Graphene Films and Nanostructures

The one-atom-thick graphene has excellent electronic, optical, thermal, and mechanical properties. Currently, chemical vapor deposition (CVD) graphene has received a great deal of attention because it provides access to large-area and uniform films with high-quality. This allows the fabrication of graphene based-electronics, sensors, photonics, and optoelectronics for practical applications. Zero bandgap, however, limits the application of a graphene film as electronic transistor. The most commonly used bottom-up approaches have achieved efficient tuning of the electronic bandgap by customizing well-defined graphene nanostructures. The postgrowth transfer of graphene films/nanostructures to a certain substrate is crucial in utilizing graphene in applicable devices. In this review, the basic growth mechanism of CVD graphene is first introduced. Then, recent advances in various transfer methods of as-grown graphene to target substrates are presented. The fabrication and transfer methods of graphene nanostructures are also provided, and then the transfer-related applications are summarized. At last, the challenging issues and the potential transfer-free approaches are discussed.

Lipids: An Atomic Toolkit for the Endless Frontier

The evolution of lipids in nanoscience exemplifies the powerful coupling of advances in science and technology. Here, we describe two waves of discovery and innovation in lipid materials: one historical and one still building. The first wave leveraged the relatively simple capability for lipids to orient at interfaces, building layers of functional groups. This simple form of building with atoms yielded a stunning range of technologies: lubricant additives that dramatically extended machine lifetimes, molecules that enabled selective ore extraction in mining, and soaps that improved human health. It also set the stage for many areas of modern nanoscience. The second wave of lipid materials, still growing, uses the more complex toolkits lipids offer for building with atoms, including controlling atomic environment to control function (e.g., pK_a tuning) and the generation of more arbitrary two-dimensional and three-dimensional structures, including lipid nanoparticles for COVID-19 mRNA vaccines.

Nanowelding in Whole-Lifetime Bottom-Up Manufacturing: From Assembly to Service

The continuous miniaturization of microelectronics is pushing the transformation of nanomanufacturing modes from top-down to bottom-up. Bottom-up manufacturing is essentially the way of assembling nanostructures from atoms, clusters, quantum dots, etc. The assembly process relies on nanowelding which also existed in the synthesis process of nanostructures, construction and repair of nanonetworks, interconnects, integrated circuits, and nanodevices. First, many kinds of novel nanomaterials and nanostructures from 0D to 1D, and even 2D are synthesized by nanowelding. Second, the connection of nanostructures and interfaces between metal/semiconductor-metal/semiconductor is realized through low-temperature heat-assisted nanowelding, mechanical-assisted nanowelding, or cold welding. Finally, 2D and 3D interconnects, flexible transparent electrodes, integrated circuits, and nanodevices are constructed, functioned, or self-healed by nanowelding. All of the three nanomanufacturing stages follow the rule of "oriented attachment" mechanisms. Thus, the whole-lifetime bottom-up manufacturing process from the synthesis and connection of nanostructures to the construction and service of nanodevices can be organically integrated by nanowelding. The authors hope this review can bring some new perspective in future semiconductor industrialization development in the expansion of multi-material systems, technology pathway for the refined design, controlled synthesis and in situ characterization of complex nanostructures, and the strategies to develop and repair novel nanodevices in service.

Preparing Alumina-Supported Gold Nanowires for Alcohol Oxidation

The development of shape-controlled noble metal nanocrystals such as nanowires (NWs) is progressing steadily owing to their potentially novel catalytic properties and the ease with which they can be prepared by reducing the metal ions in a particular solution as capping agents. Recently, many reports have been presented on the preparation of shape-controlled Au nanocrystals, such as nanostars and nanoflowers, by a one-pot method using 2-[4-(2-hydroxyethyl)-1-piperazinyl] ethanesulfonic acid (HEPES) as capping and reducing agents. The catalytic activity is depressed due to the adsorption of the capping agent onto a Au surface. Since HEPES has low binding affinities on the Au surface, shape-controlled nanocrystals obtained using HEPES are effective for application as nanocatalysts because HEPES was easily removed from the Au surface. In this study, we report the preparation of AuNWs, with an average diameter of 7.7 nm and lengths of a few hundred nanometers, in an aqueous solution containing HEPES and sodium borohydride. A γ-Al2O3-supported AuNW (AuNW/γ-Al2O3) catalyst was obtained using catalytic supporters and a water extraction method that removed HEPES from the Au surface without morphological changes. AuNW/γ-Al2O3 was then utilized to catalyze the oxidation of 1-phenylethyl alcohol to acetophenone. The formation rate of acetophenone over AuNW/γ-Al2O3 was 3.2 times that over γ-Al2O3-supported spherical Au nanoparticles (AuNP/γ-Al2O3) with almost the same diameter.

Switchable Binding Energy of Ionic Compounds and Application in Customizable Ligand Exchange for Colloid Nanocrystals

The ability to engineer the surface ligands or adsorbed molecules on colloid nanocrystals (NCs) is important for various applications, as the physical and chemical properties are strongly affected by the surface chemistry. Here, we develop a facile and generalized ionic compound-mediated ligand-exchange strategy based on density functional theory calculations, in which the ionic compounds possess switchable bonding energy when they transfer between the ionized state and the non-ionized state, hence catalyzing the ligand-exchange process. By using an organic acid as the intermediate ligand, ligands such as oleylamine, butylamine, polyvinylpyrrolidone, and poly(vinyl alcohol) can be freely exchanged on the surface of Au NCs. Benefiting from this unique ligand-exchange strategy, the ligands with strong bonding energy can be replaced by weak ones, which is hard to realize in traditional ligand-exchange processes. The ionic compound-mediated ligand exchange is further utilized to improve the catalytic properties of Au NCs, facilitate the loading of nanoparticles on substrates, and tailor the growth of colloid NCs. These results indicate that the mechanism of switchable bonding energy can be significantly expanded to manipulate the surface property and functionalization of NCs that have applications in a wide range of chemical and biomedical fields.

Single-parameter-tuned synthesis for shape-controlled gold nanocrystals stimulated by iron carbonyl

Shape-controlled synthesis is essential for functional nanomaterials, allowing deeper insights intothe relationship between the structures and the catalytic properties. Synthesis of nanocrystals with particular morphologies are usually studied independently among various synthetic methods, those underline that different surface capping ligands or shape-directing agents bring about disparate shapes. However, a single quantitative parameter method is still lacking to realize precise control of well-defined morphology nanocrystals, especially anisotropic structures, which is essential to understanding the growth process of nanocrystals. Herein, we proposed a single-parameter-tuned synthesis strategy for preparation of shape-controlled gold nanocrystals by regulating the amount of iron carbonyl, by which we produced highly monodisperse Au nanocrystals with various shapes in organic phase including nanoplates (diameter of 16.02 ± 1.13 nm and thickness of 5.35 ± 0.58 nm), nanorods (length of 37.53 ± 3.73 nm and width of 5.26 ± 0.37 nm) and nanospheres (diameter of 8.26 ± 0.38 nm). The single-parameter-tuned method reveals the dual roles of iron carbonyl for controlling the shapes of gold nanocrystals including reductant and oxidative etchant and empowers versatility in synthetic methodology for other noble metals. Moreover, catalytic activity shifting in shapes of nanocrystals was revealed based on the reduction of 4-nitrophenol, showing that the as-synthesized Au nanoplates displayed the enhanced catalytic performance with the lowest activation energy. Our work provides a brand-new pathway for shape-controlled synthesis of noble-metal nanocrystals and has a strong practical value in application fields.

Camouflaged Gold Nanodendrites Enable Synergistic Photodynamic Therapy and NIR Biowindow II Photothermal Therapy and Multimodal Imaging

Gold nanodendrite (AuND)-based nanotheranostic agents with versatile capabilities were fabricated by optimizing the geometrical configurations (dendrite length and density) of AuND to achieve localized surface plasmon resonance (LSPR) in near-infrared biowindow II (NIR-II), and then subsequently functionalizing with a mitochondria-targeting compound (triphenylphosphonium, TPP), loading with an NIR-photosensitizer (indocyanine green, ICG) and coating with the macrophage cell membrane (MCM) to trap ICG within AuND and selectively interact with MDA-MB-231 cells. The novel AuND-TPP-ICG@MCM system enabled the integration of multimodal fluorescence/photoacoustic/surface-enhanced Raman imaging with synergistic therapies of NIR-II photothermal therapy and NIR-I photodynamic therapy for cancer treatment. Enhanced hyperthermia and elevated production of reactive oxygen species within the tumors via MCM coating and mitochondria targeting afforded a synergistic efficacy for tumor eradication with limited side effects. The demonstrated biocompatibility, multi-imaging capability, and high therapeutic efficiency under NIR laser irradiation indicate the potentials of this multifunctional nanotheranostic platform for clinical utility in cancer therapy.

Au-Polyoxometalates A-B-A-B Type Copolymer-Analogue Sub-1 nm Nanowires

Here it is shown that polyoxometalate (POM) clusters (H₃ PW₁₂ O₄₀ ·xH₂ O, PW₁₂ ) can be introduced to interact with Au nanoclusters to form the "A-B-A-B" type building block ("A" represents Au nanoclusters and "B" stands for PW₁₂ clusters), which continue to grow into copolymer-analogue Au-PW₁₂ sub-1 nm nanowires. Due to the synergetic effect of Au nanoclusters and POMs, the obtained Au-PW₁₂ sub-1 nm nanowires efficiently perform catalytic activity in the photo-electrochemical converting CO₂ into CO. Under light, the catalyst maintains remarkable faradic efficiency (FE) of ≈99% from -0.7 to -0.9V (RHE), which is better than that in dark (FE of 66.4-90.64% from -0.7 to -0.9 V (RHE)). Density functional theory calculations and cryo-electron microscope images support the "A-B-A-B" type of structure and mechanistic studies also reveal the higher reactivity toward COOH* formation and CO adsorption on the catalyst, which lead to the superior catalytic activity in CO₂ reduction reaction.

Gold Nanowires from Nanorods

Gold nanowires (AuNWs) possess strong potential application in micro- and nanoelectronics as well as in plasmonic waveguides because of their low electrical resistance. However, the synthesis of pure solvent-dispersible AuNWs with full control over their length still remains a challenge. All the previously reported methods produce AuNWs with other impurities such as smaller nanorods, platelets, and spherical particles and are limited to a certain length (typically below 10 μm). This article describes a one-step synthesis of extremely long AuNWs (up to 25 μm) with great control over their dimensions by using pentahedrally twinned gold nanorods (AuNRs) as seed particles. To induce the AuNW growth, the reduction of Au(I) to Au(0) was carried out on the surface of AuNRs at a very low pH by introducing HCl into the growth solution. The slow conversion of Au(I) to Au(0) due to the increase in reduction potential at lower pH promoted the preferential deposition of metallic gold on the more reactive tips of AuNRs compared to their sides, resulting in the formation of AuNWs. In analogy to the "living" polymerization reaction, the length of the AuNWs was proportional to the amount of Au(I) added to the growth solution; thus, the desired length of AuNWs was achieved by controlling the supply of Au(I) ions in the reaction mixture. The AuNWs longer than 6 μm were found to be responsive to microwave radiation. When an aqueous solution of AuNWs was exposed to microwaves, the formation of sharp kinks was observed in several locations of AuNWs without their disintegration into smaller pieces.

Molecular gold strings: aurophilicity, luminescence and structure-property correlations

This review covers the compound class of one-dimensional gold strings. These compounds feature a formally infinite repetition of gold complexes as monomers/repeating units that are held together by aurophilic interactions, i.e. direct gold-gold contacts. Their molecular structures are primarily determined in the solid state using single crystal X-ray diffraction. The chemical composition of the employed gold complexes is diverse and furthermore plays a key role in terms of structure characteristics and the resulting properties. One of the most common features of gold strings is their photoluminescence upon UV excitation. The emission energy is often dependent on the distance of adjacent gold ions and the electronic structure of the whole string. In terms of gold strings, these parameters can be fine-tuned by external stimuli such as solvent, pH value, pressure or mechanical stress. This leads to direct structure-property correlations, not only with regard to the photophysical properties, but also electric conductivity for potential application in nanoelectronics. Concerning these correlations, gold strings, consisting of self-assembled individual complexes as building blocks, are the ideal compound class to look at, as perturbations by an inhomogeneity in the ligand sphere (such as the end of a molecule) can be neglected. Therefore, the aim of this review is to shed light on the past achievements and current developments in this area.

Cold welding assisted self-healing of fractured ultrathin Au nanowires

In nano-electronic field, cold welding is a simple novel method to join ultrathin noble metal nanowires (NWs) without introducing extra energy and defects. In previous works, it always occurred between ultrathin noble metal NWs, tensile fracture parts of a single NW, or a NW formation from nanoparticles. However, some external force is still needed to drive the materials as close to each other as possible before the process. Here, we proposed a new method to achieve cold welding without introducing artificial loadings. The bending fractured ultrathin gold (Au) NW can be self-healed assisted by cold welding during the removal of the tungsten (W) tip by in situ transmission electron microscope (TEM). A new interface with lattice mismatch formed in the welding zone after multiple periodic cycles, leaving an angle between the two rebonded fracture parts. Furthermore, the cold welding assisted self-healing of the bending fractured ultrathin Au NW and atom evolutions were also confirmed by molecular dynamics (MD) simulations. The successful implementation of cold welding makes the self-healing come true when the ultrathin Au NW fractures under the unexpected vibrations.

Key factors affecting Rayleigh instability of ultrathin 4H hexagonal gold nanoribbons

Rayleigh instability was originally used to describe the phenomenon of a cylindrical fluid jet that transforms into a chain of droplets. Very recently, it has been extended to metallic nanostructures like gold (Au) and silver (Ag) nanowires (NWs), as well as mixed alloy NWs by some thermodynamic processes. To date, the key factors affecting the Rayleigh instability have not been well studied. To clarify this, we systematically investigate the features of Rayleigh instability in ultrathin 4H hexagonal Au nanoribbons (NRBs) under electron beam (E-beam) irradiation. We prove that by decreasing the initial widths of 4H Au NRBs and the E-beam current density, as well as the irradiation time and intensity per unit area, the Rayleigh instability can be effectively restrained. Our work thus sheds light on how to effectively reduce or even eliminate the Rayleigh instability of one dimensional nanomaterials.

Gold nanowire growth through stacking fault mechanism by oleylamine-mediated synthesis

Tadpole-like gold nanowires were obtained by the oleylamine-mediated synthesis presenting an unusual mixture of fcc and hcp phases. Structural analyses were performed to understand their structure and growth using aberration-corrected high-resolution scanning transmission electron microscopy and electron diffraction at the tail region of tadpoles showing that the anisotropic shape occurred due to stacking fault defects. Stacking faults and twin defects are responsible for the hcp phase inferring a defect dependent growth. The stacking fault model used in X-ray diffraction (XRD) refinement resulted in 60% of hcp stacking sequences. Temperature-dependent XRD analyses showed that the faults become unstable around 120 °C, and it is completely converted to the fcc phase at 230 °C. We attribute the nanowire formation to a stacking fault mechanism of growth that begins in the later stage of nanoparticle growth. The UV-Vis spectrum presented two localized surface plasmon resonance bands at 500 nm and from 800 nm extending to near-infrared, associated with transverse and longitudinal modes, respectively. A surprising ferromagnetic behavior is also observed with a blocking temperature near 300 K.

Polymer Lamellae as Reaction Intermediates in the Formation of Copper Nanospheres as Evidenced by In Situ X-ray Studies

The classical nucleation theory (CNT) is the most common theoretical framework used to explain particle formation. However, nucleation is a complex process with reaction pathways which are often not covered by the CNT. Herein, we study the formation mechanism of copper nanospheres using in situ X-ray absorption and scattering measurements. We reveal that their nucleation involves coordination polymer lamellae as pre-nucleation structures occupying a local minimum in the reaction energy landscape. Having learned this, we achieved a superior monodispersity for Cu nanospheres of different sizes. This report exemplifies the importance of developing a more realistic picture of the mechanism involved in the formation of inorganic nanoparticles to develop a rational approach to their synthesis.

Excited-State Turn-On of Aurophilicity and Tunability of Relativistic Effects in a Series of Digold Triazolates Synthesized via iClick

iClick reactions between Au(I) acetylides PPh₃Au-C≡CR, where R = nitrophenyl (PhNO₂), phenyl (Ph), thiophene (Th), bithiophene (biTh), and dimethyl aniline (PhNMe₂), and Au(I)-azide PPh₃AuN₃ provide digold complexes of the general formula R-1,5-bis-triphenylphosphinegold(I) 1,2,3-triazolate (Au₂-R). Within the digold triazolate complexes the Au(I) atoms are held in close proximity but beyond the distance typically observed for aurophilic bonding. Though no bond exists in the ground state, time-dependent density functional theory interrogation of the complexes reveals excited states with significant aurophilic bonding. The series of complexes allows for tuning of the excited-state "turn-on" of aurophilicity, where ligand to metal charge transfer (LMCT) induces the aurophilic bonding. Complexes containing ligand-localized excited states, however, do not exhibit aurophilicity in the excited state. As a control experiment, a monogold complex was synthesized. The computed excited state of the monogold species exhibited LMCT to the gold ion as in the dinuclear cases, but without a partnering gold ion only a distinct N-Au-P bending occurs, revealing a potential mechanism for the excited-state turn-on of aurophilic bonding. Analysis of the steady-state electronic spectra indicates that LMCT states are achievable for compounds with sufficiently strong electron-donating ligands, and in digold complexes this is associated with enhanced fluorescence, suggestive of an aurophilic interaction.

Facile morphology control of gold(0) structures from aurophilic assemblies

Different gold microstructures have been synthesized by using supramolecular gold(i) organometallic compounds as templates and Ag nanoparticles as reducing agents. The use of fibers resulting from supramolecular assemblies of neutral gold(i) compounds gives rise to the formation of microrods. The use of supramolecular assemblies from ionic molecules results in spherical or square-based prism gold microstructures, depending on the shape of the supramolecular gold(i) precursor assembly. In addition to temperature and reaction time, solvents exert a strong influence on the formation and morphology of gold structures, as borne out by the example that well-defined star-like morphologies have been obtained in chloroform.

Multiscale Soft-Hard Interface Design for Flexible Hybrid Electronics

Emerging next-generation soft electronics will require versatile properties functioning under mechanical compliance, which will involve the use of different types of materials. As a result, control over material interfaces (particularly soft/hard interfaces) has become crucial and is now attracting intensive worldwide research efforts. A series of material and structural interface designs has been devised to improve interfacial adhesion, preventing failure of electromechanical properties under mechanical deformation. Herein, different soft/hard interface design strategies at multiple length scales in the context of flexible hybrid electronics are reviewed. The crucial role of soft ligands and/or polymers in controlling the morphologies of active nanomaterials and stabilizing them is discussed, with a focus on understanding the soft/hard interface at the atomic/molecular scale. Larger nanoscopic and microscopic levels are also discussed, to scrutinize viable intrinsic and extrinsic interfacial designs with the purpose of promoting adhesion, stretchability, and durability. Furthermore, the macroscopic device/human interface as it relates to real-world applications is analyzed. Finally, a perspective on the current challenges and future opportunities in the development of truly seamlessly integrated soft wearable electronic systems is presented.

Fabrication of single-crystalline gold nanowires on cellulose nanofibers

Cellulose nanofibers (CNF) are promising nanomaterials for functional inks and printed sensors, although the potential applications are currently limited by the available functionalization methods. This work outlines a convenient method to grow a novel and highly conductive network of single-crystalline gold nanowires (AuNW) on CNF for use in conductive inks and printed sensors. The CNF are able to reduce Au (III) precursors to Au (0) monomers and generate nucleation sites for the subsequent monomer-by-monomer growth of Au nanocrystals; sodium citrate is used to control the reduction kinetics and the crystal growth. The growth of these AuNW/CNF materials is a three-step process of redox reaction, isotropic nucleation and anisotropic crystallization: the morphology and crystal structure of Au nanocrystals on CNF can be controlled by adjusting the reaction temperature and concentrations of citrate and CNF. The AuNW/CNF materials obtained have been formulated into highly conductive and atmospherically stable inks for use in either directly writing or screen printing. We have demonstrated AuNW/CNF-printed sensors with highly controllable electrical conductivity as well as excellent stability against rinsing and immersion by water and ethanol.

An Application Exploiting Aurophilic Bonding and iClick to Produce White Light Emitting Materials

The paper focuses on exploiting aurophilic bonding to produce white light emitting materials. Inorganic Click (iClick) is employed to link two or four Au(I) metal ions through a triazolate bridge. Depending on the choice of phosphine ligand (PEt₃ or PPh₃), dinuclear Au₂-FO or tetranuclear Au₄-FO complexes can be controllably synthesized (FO = 2-(9,9-dioctylfluoreneyl-)). The iClick products Au₂-FO and Au₄-FO are characterized by combustion analysis and multinuclear NMR, TOCSY 1D, ¹H-¹³C gHMBC, and ¹H-¹³C gHSQC. In addition, the photophysical properties of Au₂-FO and Au₄-FO were examined in THF solution. Transient absorption spectroscopy was employed to elucidate the excited state features of the gold compounds. Solution processed OLEDs were fabricated and characterized, which gave white light electroluminescence with CIE coordinates (0.34, 0.36), as seen referenced to CIE standard illuminant D65 (0.31, 0.32). TDDFT computational analysis of Au₂-FO and Au₄-FO reveals the origin of light emission. In the case of Au₄-FO, direct excitation leads to increased aurophilic bonding in the excited state, and as a result the emission profile is broadened to cover a larger region of the visible spectrum, thus giving white light emission. Designing molecules that can access or increase aurophilic bonding in the excited state provides another tool for fine-tuning the emission profiles of gold complexes.

The synthesis of high-aspect-ratio Au microwires with a biomolecule for electrochemical sensing

Gold (Au) crystalline microwires with an unprecedented diameter of >500 nm and an aspect ratio >400 were synthesized using l-tyrosine as a reducing and capping agent. The Au microwires possessed high conductivity and electrocatalytic activities towards glucose and Hg(ii). Their large diameters and aspect ratios also offered maneuverability, and it was easy to produce Au microelectrodes.

Seeded growth of gold–silver ultrathin wire–dot hybrid nanostructures

Gold–silver hybrid nanostructures in the form of “ultrathin wire–dots” are prepared in high purity via seeded growth.

Entropy Can Bundle Nanowires in Good Solvents

Surfaces with surface-bound ligand molecules generally attract each other when immersed in poor solvents but repel each other in good solvents. While this common wisdom holds, for example, for oleylamine-ligated ultrathin nanowires in the poor solvent ethanol, the same nanowires were recently observed experimentally to bundle even when immersed in the good solvent n-hexane. To elucidate the respective binding mechanisms, we simulate both systems using molecular dynamics. In the case of ethanol, the solvent is completely depleted at the interface between two ligand shells so that their binding occurs, as expected, via direct interactions between ligands. In the case of n-hexane, ligands attached to different nanowires do not touch. The binding occurs because solvent molecules penetrating the shells preferentially orient their backbone normal to the wire, whereby they lose entropy. This entropy does not have to be summoned a second time when the molecules penetrate another nanowire. For the mechanism to be effective, the ligand density appears to best be intermediate, that is, small enough to allow solvent molecules to penetrate, but not so small that ligands do not possess a clear preferred orientation at the interface to the solvent. At the same time, solvent molecules may be neither too large nor too small for similar reasons. Experiments complementing the simulations confirm the predicted trends.

Solvent-triggered reversible interconversion of all-nitrogen-donor-protected silver nanoclusters and their responsive optical properties

Surface organic ligands are critical in determining the formation and properties of atomically precise metal nanoclusters. In contrast to the conventionally used thiolate, phosphine and alkynyl ligands, the amine ligand dipyridylamine is applied here as a protecting agent in the synthesis of atomically precise metal nanoclusters. We report two homoleptic amido-protected Ag nanoclusters as examples of all-nitrogen-donor-protected metal nanoclusters: [Ag₂₁(dpa)₁₂]SbF₆ (Ag₂₁) and [Ag₂₂(dpa)₁₂](SbF₆)₂ (Ag₂₂) (dpa = dipyridylamido). Single crystal X-ray structural analysis reveals that both clusters consist of a centered-icosahedron Ag₁₃ core wrapped by 12 dpa ligands. The flexible arrangement of the N donors in dpa facilitates the solvent-triggered reversible interconversion between Ag₂₁ and Ag₂₂ due to their very different solubility. The successful use of dpa in the synthesis of well-defined silver nanoclusters may motivate more studies on metal nanoclusters protected by amido type ligands.

Noble metal nanoclusters are commonly protected by thiolate, phosphine, or alkynyl ligands. Here, the authors synthesize two homoleptic amido-protected silver clusters, whose structures interconvert easily with changes of solvent due to the coordination flexibility and diverse binding modes of the nitrogen-donor ligands.

Crystallization of Gd2O3 nanoparticles: evolution of the microstructure via electron-beam manipulation

NaGdF4 is one of the most commonly employed phosphor host matrices for lanthanide doping and is one of the most efficient infrared-to-visible up-conversion fluorescent host materials. Although the structure, morphology and luminescence properties of NaREF4 have been sufficiently investigated, there are very few reported instances of introducing localized order/crystallinity by electron-beam (e-beam) irradiation. In this work, we studied the phase transformation of Gd2O3 from an amorphous to crystalline form via manipulation by e-beam irradiation. The amorphous Gd2O3 occurs as an impurity in the cubic-NaGdF4 nanoparticles (NPs). The structural evolutions, including the transformation from amorphous to crystalline, the recrystallization process and the formation of the graphene@NP core-shell structure, are discussed in detail. We also propose an evolution scheme, in which the e-beam manipulation of the organic-containing NPs induces a subtle structural transformation, depending in principle on the microenvironment of the NPs.

Sequential Ligand Exchange of Coordination Polymers Hybridized with In Situ Grown and Aligned Au Nanowires for Rapid and Selective Gas Sensing

Combining polymeric materials and conductive one-dimensional metal nanostructures is able to achieve enhanced chemical and electrical properties, but the control over their morphology and spatial arrangement remains a big challenge. Herein, by replacing benzenedicarboxylate (BDC) in ZnBDC nanoplates with oleylamine (OAM) in the presence of HAuCl₄, Zn-OAM nanobelts with a highly ordered laminar structure were obtained, on which ultrathin Au nanowires (Au NWs) were deposited and aligned along the long axes of the nanobelts. The resulting Zn-OAM/Au NW hybrid further underwent an OAM-to-2-methylimidazole ligand exchange, resulting in the formation of porous nanobelts composed of ZIF-8 nanocrystals interwound with aligned Au NWs. Due to the synergistic effect between the polymeric and metallic structures, the Zn-OAM/Au NW hybrid nanobelts and ZIF-8/Au NW porous nanobelts demonstrated fast and selective gas sensing at ambient conditions, in sharp contrast to the nonresponsive Au NWs or Zn-based polymers alone.

Effect of the Air/Water Interfacial Properties of Amine Derivatives on the in Situ Fabrication of Microsized Gold Sheets

Development of new methods for producing large-area nanocrystals with specific shapes is crucial for advancements in various fields. In this study, submillimeter-sized sheet-structured gold crystals with nanoscale thicknesses were fabricated by chemical reduction of HAuCl₄ in the presence of long-chain amidoamine-derived surfactants (C nAOH; n = 12, 14, 16, or 18) in aqueous solutions. Using the C18AOH system at 30 °C, large-area sheet-structured crystals with widths of ∼100 μm and thicknesses of 30 nm were effectively obtained at the air/water interface. The crystal size depended on the temperature and the alkyl-chain length of the surfactant. An investigation of the relationship between the crystal growth and the interfacial properties of C nAOH revealed that large-area crystals were obtained when densely packed molecular layers of long-chain C nAOH were formed at the air/water interface. The interfacial molecular layer of C18AOH showed most effective soft-templating effect and contributed in promoting the growth of sheet-structured gold crystals.

Tunable synthesis of ultrathin AuAg nanowires and their catalytic applications

Metallic nanowires (NWs) are very interesting and important nanomaterials with unique properties and a number of potential applications. Herein we report a tunable synthesis of water soluble ultrathin AuAg NWs. By using TEM and UV-vis spectroscopy, we demonstrate that these NWs can be produced by a new two-step process, which involves the formation of NW templates during the aging period and the subsequent formation of thicker NWs by a solvent driven fusion and wetting process. Our control studies further show that silver concentration plays a key role in the formation of these nanowires. We also demonstrate that these nanowires can effectively catalyse the reduction of 4-nitrophenol to the corresponding 4-aminophenol. Interestingly, the larger diameter ultrathin nanowires (av. 8 nm) exhibit a greater catalytic performance than the thinner nanowires (av. 3 nm). We believe that these studies are important for further development of one dimensional metal based nanomaterials, which may find a range of potential research and technological applications.

Morphology-Controlled Versatile One-Pot Synthesis of Hydrophobic Gold Nanodots, Nanobars, Nanorods, and Nanowires and Their Applications in Surface-Enhanced Raman Spectroscopy

Many previously reported syntheses of gold nanoparticles required lengthy reaction times, complicated operations, high temperatures, or multi-step manipulations. In this work, a morphology-controlled versatile one-pot synthesis of hydrophobic gold nanodots, nanobars, nanorods, and nanowires has been developed. A series of gold nanomaterials ranging from round nanodots, short nanobars, and long nanorods to ultrathin and ultralong nanowires (diameter <2 nm, length >2 μm) have been readily prepared by simply adjusting the feeding ratio of chloroauric acid to oleylamine, oleic acid, and triphenylsilane. The silk-like ultralong and ultrathin nanowires were found to have a single crystalline structure and may have significant potential applications in microelectronics and biosensors. Large sizes of gold spherical nanoparticles were obtained from gold nanodots via a seed-mediated growth approach. These nanoparticles and ultralong nanowires showed excellent surface-enhanced Raman scattering (SERS) activity in organic solvents and, therefore, were employed as efficient organic-soluble SERS substrates for the detection of hydrophobic food toxicants, such as 3,4-benzopyrene, and carcinogens, such as benzidine.

Au-Ag Nanoflower Catalysts with Clean Surfaces for Alcohol Oxidation

Shape-controlled metal nanocrystals, such as nanowires and nanoflowers, are attractive owing to their potentially novel catalytic properties and bimetallic nanocrystals composed of two distinct metals are expected to act as highly active catalysts. However, their catalytic activities are limited because of the capping agents adsorbed on the metal surfaces, which are necessary for the preparation and dispersion of these nanocrystals in solvents. Therefore, the preparation of bimetallic shape-controlled noble metal nanocrystals with clean surfaces, devoid of almost all capping agents, are expected to have high catalytic activity. Herein, we report the preparation of bimetallic Au-Ag nanoflowers using melamine as the capping agent. The bimetallic Au-Ag nanoflowers with a clean surface were subsequently obtained by a support and extraction method. The bimetallic nanoflowers with a clean surface were then used for the aerobic oxidation of 1-phenylethyl alcohol and they exhibited high rates for the formation of acetophenone compared to Au nanoflowers and spherical nanoparticles with almost the same size and Au/Ag ratio. We also show that Au-Ag nanoflowers containing only 1 % Ag (Au₉₉ -Ag₁ NFs) exhibit the highest rate of acetophenone formation among Au-Ag nanoflowers with different Au/Ag ratios owing to an increase in the electron density of the Au atoms that act as active sites for the oxidation of 1-phenylethyl alcohol.

Photocatalytic synthesis of gold nanoparticles using TiO2 nanorods: a mechanistic investigation

Herein, we investigate the mechanism of photocatalytic synthesis of noble metal nanoparticles by reductive photoelectrons generated from semiconductors.