Au Core Research Articles

Asymmetric Core–Shell Gold Nanoparticles and Controllable Assemblies for SERS Ratiometric Detection of MicroRNA

AbstractJanus nanogap gold nanoparticles (JAuNNPs) with varying proportions of Au shell coverage of (ca. 100/75/50/25 %) are presented. The internal nanogap between the partial Au shell and core caused asymmetric optical behavior; tunability depends on the degree of Au shell coverage and structural asymmetry. The shell‐to‐shell or core‐to‐core JAuNNDs(50 %) were self‐assembled from amphiphilic JAuNNPs(50 %) by tuning the hydrophilic and hydrophobic polymer brushes on the Au core or shell. The positions of electromagnetic field enhancement of JAuNNDs varied with geometrical configurations because of hybridized plasmonic coupling effects. Furthermore, DNA linkers were utilized to form JAuNND12(50 %). By combining with Raman molecules, ratiometric SERS signals could be generated, enabling JAuNND12(50 %) to image the distribution of miR‐21 in living cells and tumors. Asymmetric JAuNNPs allowed facile conjugation of various linkage molecules to fabricate dimeric nanostructures.

Formation and Application of Core-Shell of FePt-Au Magnetic-Plasmonic Nanoparticles.

Monodispersed FePt core and FePt–Au core–shell nanoparticles (NPs) have been chemically synthesized in liquid solution and with controllable surface-functional properties. The NP size was increased from 2.5 nm for FePt to 6.5 nm for FePt–Au, which could be tuned by the initial concentration of gold acetate coated onto FePt seeding NPs via a seed-mediated formation of self-assembled core–shell nanostructures. The analyses of the interplanar spacing obtained from the high-resolution transmission electron microscopy (HRTEM), selective electron diffraction pattern (SAED), and x-ray diffraction (XRD) confirmed that both FePt core and Au shell belong to the face-centered cubic (fcc) structure. FePt–Au NPs have a surface plasmon resonance (SPR) peak at 528 nm in the visible optical band region, indicating the red shift compared with the typical theoretical value of 520 nm of pure Au NPs. The surface modification and ligand exchange of FePt–Au was using mercaptoacetic acid (thiol) as a phase transfer reagent that turned the NPs hydrophilic due to the functional carboxyl group bond on the surface of presented multifunctional magnetic–plasmonic NPs. The water-dispersible FePt-based NPs conjugated with biomolecules could reach the different biocompatibility requirements and also provide enough heating response that acted as a potential agent for magnetic fluid hyperthermia in biomedical engineering research fields.

Formation of Co–Au Core–Shell Nanoparticles with Thin Gold Shells and Soft Magnetic ε-Cobalt Cores Ruled by Thermodynamics and Kinetics

Bimetallic core–shell nanoparticles (CSNPs), where a ferromagnetic core (e.g., Co) is surrounded by a noble-metal thin plasmonic shell (e.g., Au), are highly interesting for applications in biomedi...

Defective-tin-oxide wrapped gold nanoparticles with strong sunlight harvesting and efficient charge separation for photocatalysis

Strong sunlight harvesting and efficient charge separation are of great significances for solar energy utilization of metal oxides, but it is a challenge to realize these two aspects simultaneously. Here, we strategize oxygen-vacancy-rich (or defective)-SnO2 wrapped gold nanoparticles ([email protected]2 NPs) for this. The designed NPs are fabricated just by adding precursors into boiling pre-prepared Au colloidal solution and then re-dispersing the reacted NPs in water, based on surface alloying and selective oxidation. Typically, the fabricated [email protected]2 NPs (with 12 nm thick shell) exhibit strong absorption in full solar spectrum (200–2000 nm) with an energy absorption efficiency as high as 84%. Meanwhile, the photocurrent response of the typical NPs is 2.3 and 13.6 times as high as that of [email protected]2 NPs (with poor oxygen vacancies) and commercial SnO2 NPs, respectively, showing efficient charge separation. Such excellent performance is attributed to the synergistic effect of surface plasmon resonance (SPR) of the Au core and the band gap narrowing induced by the abundant oxygen vacancies. Further, the excellent photocatalytic performance of the [email protected]2 NP catalyst has been demonstrated via exemplary photocatalytic removal of gaseous ammonia. The catalytic rate is about 4.2 and 25 times as high as that of the [email protected]2 NP and the commercial SnO2 NP catalysts, respectively. It is expected that such excellent photocatalytic performance will also be obtained on other defective oxides wrapped metal structures. This work provides a preparation method for [email protected]2 NPs, which can be used for efficient solar energy utilization.

Study on surface enhanced Raman scattering of Au and Au@Al2O3 spherical dimers based on 3D finite element method

In this paper, the surface enhanced Raman scattering (SERS) characteristics of Au and Au@Al2O3 nanoparticle dimers were calculated and analyzed by using finite element method (3D-FEM). Firstly, the electric field enhancement factors of Au nanoparticles at the dimer gap were optimized from three aspects: the incident angle of the incident light, the radius of nanoparticle and the distance of the dimer. Then, aluminum oxide is wrapped on the Au dimer. What is different from the previous simulation is that Al2O3 shell and Au core are regarded as a whole and the total radius of Au@Al2O3 dimer is controlled to remain unchanged. By comparing the distance of Au nucleus between Au and Au@Al2O3 dimer, it is found that the electric field enhancement factor of Au@Al2O3 dimer is much greater than that of Au dimer with the increase of Al2O3 thickness. The peak of electric field of Au@Al2O3 dimer moves towards the middle of the resonance peak of the two materials, and it is more concentrated than that of the Au dimer. The maximum electric field enhancement factor 583 is reached at the shell thickness of 1 nm. Our results provide a theoretical reference for the design of SERS substrate and the extension of the research scope.

Control over the Morphology and Plasmonic Properties of Rod-like Au-Pd Bimetallic Nanostructures

Bimetallic nanostructures are interesting materials for many applications ranging from plasmonics to catalysis. However, synthesizing bimetallic nanorods with good control over composition, morphology and maintaining an anisotropic shape is challenging. Here, we have shown a reproducible approach to the synthesize the bimetallic core-shell nanorods within a stabilizing mesoporous silica shell and with an Au core and a Pd metal shell with excellent control over their properties. Specifically, we demonstrate that the thickness and morphology of the Pd metal shell (rough/smooth) can be precisely tuned and studied the plasmonic properties of the resulting bimetallic nanostructures in detail. Our approach can be naturally extended to different or more than two metals leading to rod-shaped metal particles with highly tunable materials properties that are of interest in their use in optical and catalytic applications.

One-pot synthesis of Au@mSiO2 yolk-shell nanoparticles with enhanced catalytic and surface-enhanced Raman scattering (SERS) properties

The yolk-shell nanoparticles (YSNs) are promising functional nanomaterials for a variety of applications such as bio-medicine, catalysis, sensor, lithium battery and detection due to their unique structures and excellent properties. In the past few decades, the synthesis of YSNs generally requires multiple steps to complete, and the process is relatively tedious and complicated. Here, Au@mSiO2 YSNs with good dispersibility were successfully synthesized by a facile one-pot process. The Au core with average diameter of 22 nm can move freely in the cavity and the silica shell with a thickness of 15 nm can effectively prevent Au particle aggregation. The method is a one-pot one-step synthesis of Au@mSiO2 core-shell nanoparticles (CSNs) combined with an organosilicon-assisted etching process. Thanks to the unique properties of Au@mSiO2 YSNs, such as low density and large surface area, we have conducted a series of studies on its properties. The redox reaction of p-nitrophenol (4-NP) was monitored by UV–vis absorption spectrum to investigate the catalytic performance of Au@mSiO2 YSNs. The surface-enhanced Raman scattering (SERS) effect of Au@mSiO2 YSNS was studied using p-aminophenol (PATP) as probe molecule. The results show that Au@mSiO2 YSNs have higher catalytic activity and stronger SERS effect than that of Au@mSiO2 CSNs. The Au@mSiO2 YSNs will have great application prospects in catalysis, biosensing and detection, etc.

Gold(I) N-heterocyclic carbene precursors for focused electron beam-induced deposition.

Seven gold(I) N-heterocyclic carbene (NHC) complexes were synthesized, characterized, and identified as suitable precursors for focused electron beam-induced deposition (FEBID). Several variations on the core Au(NHC)X moiety were introduced, that is, variations of the NHC ring (imidazole or triazole), of the alkyl N-substituents (Me, Et, or iPr), and of the ancillary ligand X (Cl, Br, I, or CF3). The seven complexes were tested as FEBID precursors in an on-substrate custom setup. The effect of the substitutions on deposit composition and growth rate indicates that the most suitable organic ligand for the gold precursor is triazole-based, with the best deposit composition of 15 atom % gold, while the most suitable anionic ligand is the trifluoromethyl group, leading to a growth rate of 1 × 10−2 nm3/e−.

Nonlinear optical behavior of Au@Ag core- shell nanostructures

Core-shell structure has been getting tremendous research interest due to their tunable multifunctional properties, induced as a result of the co-occurrence of more than one nanostructured phases. Here we report detailed nonlinear optical studies of Au@Ag core-shell nanostructures under ns pulse excitation. The size and shell thickness of the core-shells were obtained from TEM images. A significant blue shift from 519 nm to 411 nm in surface plasmon maxima is observed on forming Ag (shell) layer on Au (core) nanosphere and a slight red shift (411 nm to 425 nm) was observed as the shell thickness increases from 7.5 to 22.5 nm. All the samples exhibited reverse saturable absorption behavior and the extent of optical limiting activity enhanced on increasing the shell thickness. Nonlinear refraction properties of the samples were also analyzed and all the samples were found to be exhibiting negative nonlinearity. Improved optical limiting activity on increasing shell thickness of the studied system could be attributed to enhanced local field effect and nonlinear scattering effect, which is confirmed from the finite difference time domain simulation analysis. The good nonlinear optical properties of Au@Ag core-shell structures ensure its potential for future nonlinear optical applications.

Structural Control over Bimetallic Core-Shell Nanorods for Surface-Enhanced Raman Spectroscopy.

Bimetallic nanorods are important colloidal nanoparticles for optical applications, sensing, and light-enhanced catalysis due to their versatile plasmonic properties. However, tuning the plasmonic resonances is challenging as it requires a simultaneous control over the particle shape, shell thickness, and morphology. Here, we show that we have full control over these parameters by performing metal overgrowth on gold nanorods within a mesoporous silica shell, resulting in Au–Ag, Au–Pd, and Au–Pt core–shell nanorods with precisely tunable plasmonic properties. The metal shell thickness was regulated via the precursor concentration and reaction time in the metal overgrowth. Control over the shell morphology was achieved via a thermal annealing, enabling a transition from rough nonepitaxial to smooth epitaxial Pd shells while retaining the anisotropic rod shape. The core–shell synthesis was successfully scaled up from micro- to milligrams, by controlling the kinetics of the metal overgrowth via the pH. By carefully tuning the structure, we optimized the plasmonic properties of the bimetallic core–shell nanorods for surface-enhanced Raman spectroscopy. The Raman signal was the most strongly enhanced by the Au core–Ag shell nanorods, which we explain using finite-difference time-domain calculations.

Significantly Enhanced Overall Water Splitting Performance by Partial Oxidation of Ir through Au Modification in Core–Shell Alloy Structure

Developing efficient bifunctional electrocatalysts for overall water splitting in acidic conditions is the essential step for proton exchange membrane water electrolyzers (PEMWEs). We first report the synthesis of core-shell structure nanoparticles (NPs) with an Au core and an AuIr2 alloy shell (Au@AuIr2). Au@AuIr2 displayed 4.6 (5.6) times higher intrinsic (mass) activity toward the oxygen evolution reaction (OER) than a commercial Ir catalyst. Furthermore, it showed hydrogen evolution reaction (HER) catalytic properties comparable to those of commercial Pt/C. Significantly, when Au@AuIr2 was used as both the anode and cathode catalyst, the overall water splitting cell achieved 10 mA/cm2 with a low cell voltage of 1.55 V and maintained this activity for more than 40 h, which greatly outperformed the commercial couples (Ir/C||Pt/C, 1.63 V, activity decreased within minutes) and is among the most efficient bifunctional catalysts reported. Theoretical calculations coupled with X-ray-based structural analyses suggest that partially oxidized surfaces originating from the electronic interaction between Au and Ir provide a balance for different intermediates binding and realize significantly enhanced OER performance.

Combining the core-shell construction with an alloying effect for high efficiency ethanol electrooxidation

Summary Combining core-shell construction with an alloying effect is effective for optimizing the electronic and lattice strain effects for achieving highly catalytic efficiency for a given electrochemical reaction. Here, we report the use of core-shell Au@FePd nanoparticles with an Au core and a sub-nano FePd alloy shell to demonstrate this concept. The Au core modifies the electronic configuration of Pd atoms, while the Fe component in thin shells reduces the lattice expansion of Pd induced by the Au core, both of which endow the Pd shell with an appropriate d-band center, favorable for the electrooxidation of ethanol through the C1 pathway. In particular, the as-prepared core-shell Au@FePd nanoparticles at an optimized Fe:Pd ratio exhibit a mass activity of 13.3 A mg−1 and a specific activity of 20.2 mA cm−2 for the ethanol electrooxidation in an alkaline medium, which outperform those of a majority of the recently reported Pd-based electrocatalysts.

Plasmon-pyroelectric nanostructures used to produce a temperature-mediated reactive oxygen species for hypoxic tumor therapy

Photodynamic therapy (PDT) has attracted considerable attention for tumor treatment, however, its efficacy is dramatically limited by the low reactive oxygen species (ROS) generation efficiency at hypoxic tumor microenvironment. Herein, new plasmon-pyroelectric gold (Au)@barium titanate (BTO) core-shell (CS) nanostructures (NSs) were designed to produce a temperature-mediated oxygen (O2)-independent PDT for hypoxic tumor therapy. Upon 808 nm laser irradiation, the plasmonic heating of Au cores was excited and transferred a heat to pyroelectric BTO shell, where the temperature elevation could decrease spontaneous polarization of BTO, leading to abundant hole released from the BTO surface and promoting to hydroxyl radical formation through an O2-independent approach. Simultaneously, [email protected] CSNSs still remain excellent photothermal performance originating from Au cores. Actually, 808 nm laser irradiated and polyethylene glycol modified [email protected] CSNSs could exhibit promising phototherapeutic effects on 4T1 cells at hypoxic conditions and 4T1 tumor-bearing mice, showing great potential for hypoxic tumor therapy.

Sinapic Acid Loaded Secondary Growth of SiO2–Au Core Shell Nanostructure as an Effective Antiarthritic Agent to Treat Collagen-Induced Arthritis

Systemic autoimmune disease of rheumatoid arthritis (RA) is a severe case of joint deformity. Serious off-targeted drugs are not capable of curing RA and lead to increased risk of infection. Nanoparticles-based arthritis therapy is a promising approach to promote effective treatment strategy in inflammatory tissues. In particular, an inorganic-metal hybrid form of core–shell nanostructure has a significant role in the drug application due to its unique properties arising from either core or shell materials. Presently, the Au–SiO2 core–shell nanomaterials with an average diameter of 78 nm were successfully synthesized. The nanomaterials revealed a very high sinapic acid (SA) loading to eliminate the inflammatory response in RA by avoiding the side effects of conventional drugs. Various characterization techniques revealed that the as-fabricated Au–SiO2 has a higher-contrast core and aggregates of SiO2 on their surfaces/of the shell. Besides, Au–SiO2 core–shell exhibits the large extended pore structure to store a large quantity of SA drugs. Then, the mice were treated with different doses of SA loaded Au–SiO2 core–shell to assess the various biochemical parameters, including body weight, arthritic score analysis, hind paw volume, organ indices measurement, histological analysis pro-inflammatory cytokines, and anti-serum antibody analysis. The obtained results have proven that SiO2–Au develop a core–shell skin structure and each with its unique SA loading might act as a potential anti-inflammatory agent, which can be effectively used to treat collagen-induced arthritis compared to Au–SiO2 and SA. Therefore, SA loaded Au–SiO2 core–shell nanomaterials might employ as an interesting candidate in future arthritis applications.

Concentration dependent thermo-optical properties and nonlinear optical switching behavior of bimetallic Au-Ag nanoparticles synthesized by femtosecond laser ablation

Concentration, morphology and composition-dependent thermal diffusivity changes occurring in bimetallic Au-Ag nanoparticles synthesized by femtosecond laser ablation were investigated using a dual- beam mode matched thermal lens technique. The synthesis utilized the variation in ablation time of silver plate kept immersed in already synthesized gold nanoparticle seeds. Thermal diffusivity values were found to increase both with an increase in concentration of the samples as well as an increase in Ag/Au weight percentage ratio. Au-Ag core–shell and nanoalloy structure show much greater variation in the thermal diffusivity compared to the other samples. A possible explanation could be the modified surface state density of the bimetallic nanoparticles. A single beam z scan technique using an Nd:YAG laser operating at a wavelength of 532 nm was employed for the nonlinear optical characterization of the samples. At a fixed laser input fluence, a switching behavior from saturable absorption (SA) to reverse saturable absorption (RSA) was observed as the ablation time of the silver plate in gold nanoparticle solution increased. The effect of laser input fluence on nonlinear optical properties of core–shell and nanoalloy structures was also studied and the optical limiting threshold of nanoalloy was calculated. The results revealed an increase in the nonlinear absorption coefficient with an increase in input fluence. A linear variation of the limiting threshold was also observed with an increase in input fluence. Thermal diffusivity studies find applications in photo thermal therapies and optical limiting properties find application in optoelectronic switches.

Redox Cycling within Nanoparticle-Nucleated Protein Superstructures: Electron Transfer between Nanoparticulate Gold, Molecular Reductant, and Cytochrome c.

We previously described how thousands of the heme protein cytochrome c (cyt.c) self-organize into multilayered, roughly spherical superstructures as initiated by nucleation around one colloidal gold or silver nanoparticle. Within these superstructures, the protein is stabilized to unfolding in buffered media and survives superstructure encapsulation within silica gels and processing to form bioaerogels. We now report that Au∼cyt.c superstructures in buffered media are not simply static groupings of proteins, but that the Au core and protein corona exhibit dynamic electron-transfer reactions within the superstructure as verified by UV-visible and resonance Raman spectroscopy. Within the superstructure, hundreds to thousands of ferricytochrome c (FeIII-cyt.c) are reduced to ferrocytochrome c (FeII-cyt.c) following first-order kinetics with an average apparent forward rate constant of 1.9 ±0.4 × 10-5 s-1. The reducing power in the microheterogeneous medium is derived from two multielectron reductants: tannic acid used to stabilize the commercial gold sol and the Au nanoparticle at the center of the protein superstructure. Fluorescence monitoring of guanidinium chloride-induced unfolding reveals that superstructure-associated cyt.c is stabilized to unfolding before and after chemical reduction of FeIII-cyt.c to form FeII-cyt.c, indicating that the superstructures remain intact during microheterogeneous redox reactions. Smaller nucleating Au nanoparticles or lower ionic strength in the buffered medium yields a greater extent of cyt.c reduction. Partial oxidation of the cyt.c-associated nanoparticulate Au is verified by X-ray photoelectron spectroscopy. The Au nanoparticle at the heart of the superstructure functions as a direct electron donor to the heme with oxidized Au atoms being recycled back to Au(0) as long as residual tannic acid, derived from the Au sol mother liquor, is present in the aqueous microheterogeneous medium.

Core size optimized silver coated gold nanoparticles for rapid screening of tricyclazole and thiram residues in pear extracts using SERS

Silver-coated gold nanoparticles (Au@AgNPs) were optimized with different gold (Au) core sizes and modified surface with mercaptooctane (MCO) to determine fungicide residues in pear fruit extracts. The developed MCO/Au@AgNPs with a gold core size of 28 nm and silver shell thickness of 6 nm exhibited high sensitivity and significant enhancement of Raman scattering in detecting tricyclazole (TCZ) and thiram fungicides in standard and pear fruit samples. The detection limits for TCZ and thiram in the pear fruit extracts were 0.005 and 0.003 ppm with coefficients of determination of (R2) of 0.9984 and 0.9971, percent recovery ranging from 82.7 to 107.4% and 80.7 to 109.8%, and relative standard deviation (RSD) of 1.5 to 7.9% and 3.7 to 9.5%, respectively. Results showed that this easily prepared method could be employed as SERS active platforms for the rapid assessment of trace contaminants of agrochemicals in agriculture production.

Origins of Visible and Near-Infrared Emissions in [Au25(SR)18]- Nanoclusters.

The origin of photoluminescence in [Au25(SR)18]- nanoclusters remains elusive, and there is not a universal model that can well explain the experimental result. Here, we design Au25 nanoclusters protected by four different types of ligands for investigation of the photoluminescence mechanism by looking into the visible to near-infrared emissions. On the basis of time-resolved emission and nanosecond transient absorption spectroscopy analyses, we propose a model that can well explain the emission bands of Au25 nanoclusters. The visible and near-infrared emissions have different lifetimes and are found to arise from the core-shell charge transfer state and the Au13 core state, respectively. The obtained insight will help to understand how the excited state deactivates and to further engineer the photoluminescence of metal nanoclusters.

Highly Stable Ag–Au Core–Shell Nanowire Network for ITO‐Free Flexible Organic Electrochromic Device

AbstractSolid and flexible electrochromic (EC) devices require a delicate design of every component to meet the stringent requirements for transparency, flexibility, and deformation stability. However, the electrode technology in flexible EC devices stagnates, wherein brittle indium tin oxide (ITO) is the primary material. Meanwhile, the inflexibility of metal oxide usually used in an active layer and the leakage issue of liquid electrolyte further negatively affect EC device performance and lifetime. Herein, a novel and fully ITO‐free flexible organic EC device is developed by using Ag–Au core–shell nanowire (Ag–Au NW) networks, EC polymer and LiBF4/propylene carbonate/poly(methyl methacrylate) as electrodes, active layer, and solid electrolyte, respectively. The Ag–Au NW electrode integrated with a conjugated EC polymer together display excellent stability in harsh environments due to the tight encapsulation by the Au shell, and high area capacitance of 3.0 mF cm−2 and specific capacitance of 23.2 F g−1 at current density of 0.5 mA cm−2. The device shows high EC performance with reversible transmittance modulation in the visible region (40.2% at 550 nm) and near‐infrared region (−68.2% at 1600 nm). Moreover, the device presents excellent flexibility (>1000 bending cycles at the bending radius of 5 mm) and fast switching time (5.9 s).

Dielectric Effects in FeO x -Coated Au Nanoparticles Boost the Magnetoplasmonic Response: Implications for Active Plasmonic Devices.

Plasmon resonance modulation with an external magnetic field (magnetoplasmonics) represents a promising route for the improvement of the sensitivity of plasmon-based refractometric sensing. To this purpose, an accurate material choice is needed to realize hybrid nanostructures with an improved magnetoplasmonic response. In this work, we prepared core@shell nanostructures made of an 8 nm Au core surrounded by an ultrathin iron oxide shell (≤1 nm). The presence of the iron oxide shell was found to significantly enhance the magneto-optical response of the noble metal in the localized surface plasmon region, compared with uncoated Au nanoparticles. With the support of an analytical model, we ascribed the origin of the enhancement to the shell-induced increase in the dielectric permittivity around the Au core. The experiment points out the importance of the spectral position of the plasmonic resonance in determining the magnitude of the magnetoplasmonic response. Moreover, the analytical model proposed here represents a powerful predictive tool for the quantification of the magnetoplasmonic effect based on resonance position engineering, which has significant implications for the design of active magnetoplasmonic devices.