Synthesis, Structure and Growth Mechanism of Size and Shape Tunable Au/Ag Bimetallic Nanoparticles

By simply adding ascorbic acid in advance of AgNO₃, the size and shape controllable Au/Ag bimetallic nanoparticles (NP) were prepared in the traditional Au growth solution free of seed at room temperature. The size distribution of NP is well uniform with ca. 10%–15% standard deviation in diameter. By changing CTAB concentration, the size and shape of NPs are tunable. After researching the surface‐enhanced Raman spectroscopy (SERS) behavior of the prepared NPs, an enhancement factor varied from 4.3×10⁴ to 1.1×10⁵ was obtained for the NP centered at ca. (64±8) nm. Electrochemical cyclic voltammetric results revealed that the so formed nanoparticles were Au riched Au/Ag bimetallic NP, and this formation might be due to the disproportionation reaction of Au⁺ prompted by Ag⁺ and the under potential deposition process of Ag⁺ on Au.     

Combining optical lithography with rapid microwave heating for the selective growth of Au/Ag bimetallic core/shell structures on patterned silicon wafers

We demonstrate a novel approach for the production of patterned films of nanometer-sized Au/Ag bimetallic core/shell nanoparticles (NPs) on silicon wafers. In this approach, we first self-assembled monodisperse Au NPs, through specific Au...NH(2) interactions, onto a silicon substrate whose surface had been modified with a pattern of 3-aminopropyltrimethoxysilane (APTMS) groups to form a sandwich structure having the form Au NPs/APTMS/SiO(2). These Au NPs then served as seeds for growing the Au/Ag bimetallic core/shell NPs: we reduced silver ions to Ag metal on the surface of Au seeds under rapid microwave heating in the presence of sodium citrate. Energy-dispersive X-ray analysis confirmed that the Au/Ag bimetallic core/shell NPs grew selectively on the regions of the surface of the silicon wafer that had been patterned with the Au seeds. Scanning electron microscopy images revealed that we could synthesize well-scattered, high-density (>82%) thin films of Au/Ag bimetallic core/shell NPs through the use of this novel strategy. The patterned structures that can be formed are simple to produce, easily controllable, and highly reproducible; we believe that this approach will be useful for further studies of nanodevices and their properties.     

Ascorbic acid as a mediator and template for assembling metallic nanoparticles

In this paper we report the results on the use of L-ascorbic acid (AA) in assembling metal nanoparticles (NPs) into three-dimensional fibrous structures. The degradation product of AA leads to the formation of fibrous structures, which has been used as a template for deposition of metal NPs such as Au, Pt, and Ag. We also report that AA can be used as the reducing agent in generating Au NPs. The spontaneous fiber formation and formation of Au NPs by AA have been coupled to generate fibers made up of composite of Au NPs and the polymer from the degradation products of AA. These fibers appear in the form of a fiber bundle with branched structures having overall dimensions on the order of several millimeters. They have typical widths of 1-4 microm with length of each segment of fiber bundle on the order of 40 microm. The composite fiber bundle has been found to be electrically conducting with surface resistivity on the order of 2.16x10(3) Omegacm. UV-vis spectroscopy, X-ray diffraction, transmission and scanning electron microscopic measurements were used to establish the formation of fibrous structures in the medium.     

Controlled gold nanoparticle diffusion in nanotubes: Platfom of partial functionalization and gold capping

Multifunctionality of nanotubes (NTs) is essential in biomedical and biotechnological applications, such as drug/gene delivery, bioseparation, and single-molecule detection. Each functionality should be located at optimal positions, depending on their roles such as targeting, tracking, and transporting. This enables avoidance of possible malfunctions or interference caused by having randomly distributed multiple groups (e.g., hydrophobic and hydrophilic) in the same space. In the aspect of multifunctionality, however, a general selective partial functionalization method of NT inner surfaces still remains a challenge. For this reason, we investigated a selective partial functionalization method of NTs using controlled gold nanoparticle (Au NP) diffusion in nanotubes and the preparation method of Au-capped silica nanotubes. Silica nanotubes (SNTs) were prepared using template sol-gel synthesis, and the inside of SNT was selectively modified with (3-trimethoxysilylpropyl) diethylenetriamine (DETA-silane). Au NPs of 2-nm size were then incubated with SNTs with DETA layer inside. Spontaneous diffusion of negatively charged Au NPs from bulk into the positively charged nanochannels of SNTs led trapped Au NPs onto the inner surface of SNTs. The degree of functionalization was controlled by the channel diameter, Au NP concentration, and solvent type. These SNTs partially modified with Au NPs were then used for localized selective chemical functionalization of SNTs. This was accomplished by the reaction between thionylated Au NPs trapped on the inside of SNTs and Alexa555-maleimide. Au-capped SNTs were prepared from SNTs with Au NPs inside by seed-mediated gold growth.     

Organosulfur-functionalized Au, Pd, and Au-Pd nanoparticles on 1D silicon nanowire substrates: preparation and XAFS studies

A hybrid preparative method was developed to prepare organosulfur-functionalized Au nanoparticles (NPs) on silicon nanowires (SiNWs) by reacting HAuCl(4) with SiNW in the presence of thiol. A number of organosulfur molecules-dodecanethiol, hexanethiol, 1,6-hexanedithiol, and tiopronin-were used to functionalize the Au surface. Size-selected NPs ranging from 1.6 to 7.5 nm were obtained by varying the S/Au ratio and the concentration of HAuCl(4). This method was further extended to the preparation Pd and Pd-Au bimetallic NPs on SiNWs. The morphology of the metal nanostructures was examined by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The local structure and bonding of the SiNW-supported metal nanostructures were studied using X-ray absorption fine structures (XAFS) [including both X-ray near-edge structures (XANES) and extended X-ray absorption fine structures (EXAFS)] at the Au L(3)-, Pd K-, S K-, and Si K-edges. It was also found that the annealing of the thiol-capped Au NPs up to 500 degrees C transforms the surface of the thiol-capped NPs to gold sulfide, as identified using Au L(3)- and S K-edge XANES. We also illustrate that this preparative approach can be used to form size-controllable Au NPs on carbon nanotubes.     

In Situ Single‐Nanoparticle Spectroscopy Study of Bimetallic Nanostructure Formation

Bimetallic nanostructures (NSs), with utility in catalysis, are typically prepared using galvanic exchange (GE), but the final catalyst morphology is dictated by the dynamics of the process. In situ single nanoparticle (NP) optical scattering spectroscopy, coupled with ex situ electron microscopy, is used to capture the dynamic structural evolution of a bimetallic NS formed in a GE reaction between Ag and [PtCl₆]^2?. We identify an early stage involving anisotropic oxidation of Ag to AgCl concomitant with reductive deposition of small Pt clusters on the NS surface. At later stages of GE, unreacted Ag inclusions phase segregate from the overcoated AgCl as a result of lattice strain between Ag and AgCl. The nature of the structural evolution elucidates why multi‐domain Ag/AgCl/Pt NSs result from the GE process. The complex structural dynamics, determined from single‐NP trajectories, would be masked in ensemble studies due to heterogeneity in the response of different NPs.     

Fabrication of Au‐Nanoparticle/TiO2‐Nanotubes Electrodes Using Electrochemical Methods and Their Application for Electrocatalytic Oxidation of Hydroquinone

Gold nanoparticle (Au‐NPs)‐Titanium oxide nanotube (TiO₂‐NTs) electrodes are prepared by using galvanic deposition of gold nanoparticles on TiO₂‐NTs electrodes as support. Scanning electron microscopy and energy‐dispersive X‐ray spectroscopy results indicate that nanotubular TiO₂ layers consist of individual tubes of about 60–90 nm diameters and gold nanoparticles are well‐dispersed on the surface of TiO₂‐NTs support. The electrooxidation of hydroquinone of Au‐NPs/TiO₂‐NTs electrodes is investigated by different electrochemical methods. Au‐NPs/TiO₂‐NTs electrode can be used repeatedly and exhibits stable electrocatalytic activity for the hydroquinone oxidation. Also, determination of hydroquinone in skin cream using this electrode was evaluated. Results were found to be satisfactory and no matrix effects are observed during the determination of hydroquinone content of the “skin cream” samples.     

Preparation of Ag Nanowire @ Au Nanoparticle Hybrid Nanowires and their Influence on the Fluorescence Properties of P3HT

In this study, the galvanic displacement reaction between silver and AuCl₄⁻ was carried out to synthesize a series of silver nanowire (Ag NW) @ gold nanoparticle (Au NP) hybrid nanowires. The influence of Ag NW @ Au NP hybrid nanowires on the fluorescence properties of the poly (3‐hexylthiophene) (P3HT) was investigated. The particle sizes of Au NPs on the hybrid nanowires could be adjusted by varying the reaction time and the concentration of the HAuCl₄ solution. Furthermore, steady‐state fluorescence measurements showed that the fluorescence intensity of the P3HT films was higher on various Ag NW @ Au NP hybrid nanowires compared to that on a bare silicon substrate. This was due to the increase in the intensity of electromagnetic field by the localized surface plasmon resonances of Au NPs and surface plasmon polaritons of Ag NWs from the hybrid nanowires. The results were further confirmed by the Raman spectra of the P3HT films on different substrates.     

Open tubular capillary electrochromatography using capillaries coated with films of alkanethiol-self-assembled gold nanoparticle layers

We have used alkanethiol self-assembly and dithiol layer-by-layer (LBL) self-assembly processes to prepare an Au nanoparticle (NP)-coated open tubular capillary electrochromatography (OTCEC) column for the separation of three neutral steroid drugs (testosterone, progesterone, and testosterone propionate). The CEC column was fabricated through LBL self-assembly of Au NPs on a 3-aminopropyltrimethoxysilane (APTMS)-modified fused-silica capillary and subsequent surface functionalization of the Au NPs through self-assembly of alkanethiols. We investigated the electrochromatographic properties of the resulting Au NP-coated CEC column using a "reversed phase" test mixture of three steroid drugs. We found that the key factors affecting the separation performance were the number of Au NP layers, the length of the carbon-atom chain of the alkanethiol self-assembled on the Au NPs, the percentage of organic modifier, and the pH of the running electrolyte. This study reveals that the self-assembly of alkanethiols and dithiols onto Au NPs provides stationary phases for CEC separation that are easy to prepare and whose retention behavior is highly controllable and reproducible. We believe that our findings will contribute to further studies of the application of nanotechnology to separation science.     

The role of physical and chemical properties of Pd nanostructured materials immobilized on inorganic carriers on ion formation in atmospheric pressure laser desorption/ionization mass spectrometry

Fundamental parameters influencing the ion‐producing efficiency of palladium nanostructures (nanoparticles [Pd‐NP], nanoflowers, nanofilms) during laser irradiation were studied in this paper. The nanostructures were immobilized on the surface of different solid inorganic carrier materials (porous and mono‐crystalline silicon, anodic porous aluminum oxide, glass and polished steel) by using classical galvanic deposition, electroless local deposition and sputtering. It was the goal of this study to investigate the influence of both the nanoparticular layer as well as the carrier material on ion production for selected analyte molecules. Our experiments demonstrated that the dimensions of the synthesized nanostructures, the thickness of the active layers, surface disorders, thermal conductivity and physically or chemically adsorbed water influenced signal intensities of analyte ions during surface‐assisted laser desorption/ionization (SALDI) while no effects such as plasmon resonance, photoelectric effect or catalytic activity were expected to occur. Excellent LDI abilities were seen for Pd‐NPs immobilized on steel, while Pd nanoflowers on porous silicon exhibited several disadvantages; viz, strong memory effects, dependency of the analytical signal on amount of physically and chemically adsorbed water inside porous carrier, reduced SALDI activity from unstable connections between Pd and semiconductor material, decrease of the melting point of pure silicon after Pd immobilization and resulting strong laser ablation of metal/semiconductor complex, as well as significantly changed surface morphology after laser irradiation. The analytical performance of Pd‐NP/steel was further improved by applying a hydrophobic coating to the steel surface before galvanic deposition. This procedure increased the distance between Pd‐NPs, thus reducing thermal stress upon LDI; it simultaneously decreased spot sizes of deposited sample solutions. Copyright © 2014 John Wiley & Sons, Ltd.     

Electro-oxidation of hydrazine at gold nanoparticle functionalised single walled carbon nanotube network ultramicroelectrodes

Networks of pristine single walled carbon nanotubes (SWNTs) grown by catalysed chemical vapour deposition (cCVD) on an insulating surface and arranged in an ultramicroelectrode (UME) format are insensitive to the electro-oxidation of hydrazine (HZ) in aqueous solution, indicating a negligible metallic nanoparticle content. Sensitisation of the network towards HZ oxidation is promoted by the deliberate and controlled electrodeposition of "naked" gold (Au) nanoparticles (NPs). By controlling the deposition conditions (potential, time) it is possible to control the size and spacing of the Au NPs on the underlying SWNT network. Two different cases are considered: Au NPs at a number density of 250 ± 13 NPs μm(-2) and height 24 nm ± 5 (effective surface coverage, θ = 92%) and (ii) Au NPs of number density ~ 22 ± 3 NPs μm(-2) and height 43 nm ± 8 nm (θ = 35%). For both morphologies the HZ oxidation half-wave potential (E(1/2)) is shifted significantly negative by ca. 200 mV, compared to a gold disc UME of the same geometric area, indicating significantly more facile electron transfer kinetics. E(1/2) for HZ oxidation for the higher density Au NP-SWNT structure is shifted slightly more negative (by ~25 mV) than E(1/2) for the lower density Au NP electrode. This is attributed to the lower flux of HZ at NPs in the higher number density arrangement (smaller kinetic demand). Importantly, using this approach, the calculated HZ oxidation current density sensitivities for the Au NP-SWNT electrodes reported here are higher than for many other metal NP functionalised carbon nanotube electrodes.     

Stereoselective and Chiroselective Surface Plasmon Resonance (SPR) Analysis of Amino Acids by Molecularly Imprinted Au‐Nanoparticle Composites

Au nanoparticles (NPs) functionalized with thioaniline and cysteine are used to assemble bis‐aniline‐bridged Au‐NP composites on Au surfaces using an electropolymerization process. During the polymerization of the functionalized Au NPs in the presence of different amino acids, for example, L ‐glutamic acid, L ‐aspartic acid, L ‐histidine, and L ‐phenylalanine, zwitterionic interactions between the amino acids and the cysteine units linked to the particles lead to the formation of molecularly imprinted sites in the electropolymerized Au‐NP composites. Following the elimination of the template amino acid molecules, the electropolymerized matrices reveal selective recognition and binding capabilities toward the imprinted amino acid. Furthermore, by imprinting of L ‐glutamic or D ‐glutamic acids, chiroselective imprinted sites are generated in the Au‐NP composites. The binding of amino acids to the imprinted recognition sites was followed by surface plasmon resonance spectroscopy. The refractive index changes occurring upon the binding of the amino acids to the imprinted sites are amplified by the coupling between the localized plasmon associated with the Au NPs and the surface plasmon wave.     

Various Au nanoparticle organizations fabricated through SiO2 monomer induced self-assembly

A novel method has been developed to fabricate the assembly of Au colloidal nanoparticles (NPs) using SiO(2) monomers. The key strategy was the use of a controlled sol-gel procedure including hydrolysis, deposition, and condensation of tetraethyl orthosilicate (TEOS). Namely, the assembly of Au NPs was created by the anisotropic deposition of SiO(2) monomers and subsequent permanent fixing by the growth of a SiO(2) shell. Various assemblies of Au NPs such as dimer, trimer, and pearl-chain morphology were fabricated by systematically changing the concentration and injection speed of TEOS. A longitudinal plasmon resonance band was observed as a result of the assembly of Au NPs and can be tuned from visible to near-infrared by altering the length of pearl-chain morphology. In addition, single Au NP was homogeneously coated with a SiO(2) shell by means of controlling the deposition rate of SiO(2) monomers during a Sto?ber synthesis without the use of a silane coupling agent or bulk polymer as the surface primer to render the Au surface vitreophilic. The Au NPs (mean size 11.4 nm in diameter) were thus encapsulated into SiO(2) beads with a wide range of sizes (from 20 to 50 nm in diameter). These pure SiO(2)-coated Au beads with tunable shell thickness should be crucial for biosensors, particularly as Raman-tag particles.     

AuAg bimetallic nanoparticles film fabricated based on H2O2-mediated silver reduction and its application

A method to fabricate AuAg bimetallic nanoparticles film by H₂O₂-mediated reduction of silver was reported. Gold nanoparticles (Au NPs) were first adsorbed onto the surface of a self-assembled 2-aminoethanethiol monolayer-modified gold film or 3-aminopropyltriethoxysilane (APTES) monolayer-modified quartz slide. Upon further treatment of this modified film with the solution containing silver nitrate (AgNO₃) and H₂O₂, silver was deposited on the surface of Au NPs. The size of the AuAg bimetallic particles could be readily tuned by manipulating the concentration of H₂O₂. Surface plasmon resonance (SPR) was used to investigate the process, the deposition of silver on Au NPs modified gold film resulted in an obvious decrease of depth in the SPR reflectance intensity and minimum angle curves (SPR R-θ curves), which may be utilized for the quantitative SPR detection of the analyte, H₂O₂. Combination of the biocatalytic reaction that could yield H₂O₂ by using the enzyme, glucose oxidase, with the deposition of silver may enable the design of a glucose biosensor by SPR technique. Furthermore, we evaluated the AuAg bimetallic nanoparticles film for their ability to be an effective substrate for surface-enhanced Raman scattering (SERS).     

Synthesis and optical properties of nanorattles and multiple-walled nanoshells/nanotubes made of metal alloys

The galvanic replacement reaction between silver and chloroauric acid has been exploited as a powerful means for preparing metal nanostructures with hollow interiors. Here, the utility of this approach is further extended to produce complex core/shell nanostructures made of metals by combining the replacement reaction with electroless deposition of silver. We have fabricated nanorattles consisting of Au/Ag alloy cores and Au/Ag alloy shells by starting with Au/Ag alloy colloids as the initial template. We have also prepared multiple-walled nanoshells/nanotubes (or nanoscale Matrioshka) with a variety of shapes, compositions, and structures by controlling the morphology of the template and the precursor salt used in each step of the replacement reaction. There are a number of interesting optical features associated with these new core/shell metal nanostructures. For example, nanorattles made of Au/Ag alloys displayed two well-separated extinction peaks, a feature similar to that of gold or silver nanorods. The peak at approximately 510 nm could be attributed to the Au/Ag alloy cores, while the other peak was associated with the Au/Ag alloy shells and could be continuously tuned in the spectral range from red to near-infrared.     

A one-step etching method to produce gold nanoparticle coated silicon microwells and microchannels

Gold (Au) nanoparticles (NPs) have large surface areas and novel optical properties and can be readily functionalized using thiol-based chemistry; hence, they are useful in bioanalytical chemistry. Here, we describe a one-step, plasma-etching process that results in the spontaneous formation of Au NP coated recessed microstructures in silicon (Si). Mechanistically, the plasma etch rate of Si was enhanced in the vicinity of 10-100 nm thick Au patterns resulting in the formation of microwells or microchannels uniformly coated with 20-30 nm sized Au NPs. The methodology provides versatility in the types of microstructures that can be formed by varying the shape and dimensions of the Au patterns and the etch time. We also describe selective binding of antibodies to Au NP coated Si microwells using thiol-based surface modification.     

Versatile scheme for the step-by-step assembly of nanoparticle multilayers

A versatile scheme for the preparation of nanoparticle (NP) multilayers is presented. The method is based on the step-by-step assembly of NPs and bishydroxamate disulfide ligand molecules by means of metal-organic coordination using easily synthesized tetraoctylammonium bromide (TOAB)-stabilized gold NPs. The assembly of NP multilayers was carried out via a Zr(IV)-coordinated sandwich arrangement of the hydroxamate ligands on Au and glass surfaces. The latter were precoated with electrolessly deposited Au clusters to enable binding of the first NP layer. The new method avoids the need to perform elaborate colloid reactions to prepare the NP building blocks. Au NP monolayer and multilayer films prepared in this manner were characterized by UV-vis spectroscopy, atomic force microscopy (AFM), and cross-sectional transmission electron microscopy (TEM), showing a regular growth of NP layers. The use of coordination chemistry as the binding motif between repeat layers allows for the convenient assembly of hybrid nanostructures comprising molecular and NP components. This was demonstrated by the construction of Au NP multilayers with controlled spacing from the surface or between two NP layers. Drying the samples during or after the construction process induces NP aggregation and changes in the film morphology and optical properties.     

A distinct platinum growth mode on shaped gold nanocrystals

An ultralow amount of platinum can be deposited on the gold surface using copper underpotential deposition and galvanic exchange. The platinum tended to deposit as layers on the octahedral gold nanocrystals with an Au(111) surface, while it aggregated and formed small particles on the cubic gold nanocrystals with an Au(100) surface.     

Coordination-based gold nanoparticle layers

Gold nanoparticle (NP) mono- and multilayers were constructed on gold surfaces using coordination chemistry. Hydrophilic Au NPs (6.4 nm average core diameter), capped with a monolayer of 6-mercaptohexanol, were modified by partial substitution of bishydroxamic acid disulfide ligand molecules into their capping layer. A monolayer of the ligand-modified Au NPs was assembled via coordination with Zr4+ ions onto a semitransparent Au substrate (15 nm Au, evaporated on silanized glass and annealed) precoated with a self-assembled monolayer of the bishydroxamate disulfide ligand. Layer-by-layer construction of NP multilayers was achieved by alternate binding of Zr4+ ions and ligand-modified NPs onto the first NP layer. Characterization by atomic force microscopy (AFM), ellipsometry, wettability, transmission UV-vis spectroscopy, and cross-sectional transmission electron microscopy showed regular growth of NP layers, with a similar NP density in successive layers and gradually increased roughness. The use of coordination chemistry enables convenient step-by-step assembly of different ligand-possessing components to obtain elaborate structures. This is demonstrated by introducing nanometer-scale vertical spacing between a NP layer and the gold surface, using a coordination-based organic multilayer. Electrical characterization of the NP films was carried out using conductive AFM, emphasizing the barrier properties of the organic spacer multilayer. The results exhibit the potential of coordination self-assembly in achieving highly controlled composite nanostructures comprising molecules, NPs, and other ligand-derivatized components.