Synthesis and alignment of silver nanorods and nanowires and the formation of Pt, Pd, and core/shell structures by galvanic exchange directly on surfaces

Here we describe the synthesis of Ag nanorods (NRs) (aspect ratio <20) and nanowires (NWs) (aspect ratio > or =20) directly on surfaces by seed-mediated growth. The procedure involves attaching gold seed nanoparticles (Au NPs) to 3-mercaptopropyltrimethoxysilane (MPTMS)-functionalized silicon or glass surfaces and growing them into NRs/NWs by placing the substrates into a solution containing cetyltrimethylammonium bromide (CTAB), silver nitrate, and ascorbic acid with the pH ranging from 7 to 12. Under our conditions, Ag NRs/NWs grow optimally at pH 10.6 with a 3% yield, where spherical, triangular, and hexagonal nanostructures represent the other byproducts. The length of Ag NRs/NWs ranges from 50 nm to more than 10 microm, the aspect ratio (AR) ranges from 1.4 to >300, and the average diameter is approximately 35 nm. Approximately 40% of the 1D structures are NRs, and 60% are NWs as defined by their ARs. We also report the alignment of Ag NRs/NWs directly on surfaces by growing the structures on amine-functionalized Si(100) surfaces after an amidation reaction with acetic acid and a method to improve the percentage of Ag NRs/NWs on the surface by removing structures of other shapes with adhesive tape. Surface-grown Ag NRs/NWs also react with salts of palladium, platinum, and gold via galvanic exchange reactions to form high-surface-area 1D structures of the corresponding metal. The combination of the seed-mediated growth of Ag on Au NRs followed by the galvanic exchange of Ag with Pd leads to interesting core/shell NRs grown directly on surfaces. We used scanning electron microscopy, UV-vis spectroscopy, and X-ray photoelectron spectroscopy to characterize the surface-grown nanostructures.     

Selective shortening of single-crystalline gold nanorods by mild oxidation

Gold nanorods (NRs) have received much attention due to their size-dependent surface plasmon-related optical properties. A seed-mediated approach has recently been developed for the synthesis of Au NRs with varying length-to-diameter aspect ratios. With the introduction of silver ions in the growth solution, Au NRs of narrow size distributions can be produced in high yields. Herein we describe an approach for the continuous and selective shortening of Au NRs synthesized by the silver ion-assisted seed-mediated method through oxidation with environmentally benign oxygen at slightly elevated temperatures. UV-visible extinction measurements indicate that the longitudinal surface plasmon band of Au NRs decreases in intensity and blue-shifts as a function of the oxidation time. Transmission electron microscopy (TEM) imaging shows that the length of Au NRs decreases with oxidation and their diameter stays almost constant, which suggests that oxidation starts at the ends of Au NRs. The size distributions of shortened Au NRs are similar to those of starting NRs. Further oxidation transforms Au NRs into nanospheres, which become smaller in diameter and finally completely disappear. It has been found that the oxidation rate of Au NRs can be controlled by temperature and acid concentration. Furthermore, high-resolution TEM studies reveal that Au NRs synthesized by the silver ion-assisted seed-mediated method are single crystalline and they stay single crystalline during oxidation. It is expected that Au NRs of any aspect ratio with narrow size distributions within the limit of that possessed by starting NRs can be produced by this mild oxidation approach.     

Directly monitoring the growth of gold nanoparticle seeds into gold nanorods

This paper describes the use of atomic force microscopy to directly image surface-attached 3-5 nm diameter gold nanoparticle seeds before and after seed-mediated growth into gold nanorods (Au NRs) and other shapes (spheres, triangles, and hexagons). Results show that Au NRs form from seeds growing in either one or two directions. A direct correlation exists between seed diameter and NR diameter; small diameter seeds form small diameter NRs. However, correlation between seed diameter and nanostructure shape or NR length is less evident. We describe our results in terms of growth mechanisms proposed in the literature and discuss possible reasons for the large size dispersity observed for surface-grown Au NRs. A better understanding of Au NR and other metal and semiconductor one-dimensional (1D) growth processes is necessary to improve synthesis, tailor their properties, and utilize 1D nanostructures for useful technological applications.     

Well-controlled synthesis of Au@Pt nanostructures by gold-nanorod-seeded growth

Pt nanodots were formed on Au nanorods (NRs) by using a simple seed-mediated growth. Their density and distribution on the Au NR can be finely tuned by varying the reaction parameters. At lower Pt/Au ratios, the Pt nanodots mainly appear at endcaps and side edges of the Au rod. At higher Pt/Au ratios, they distribute homogeneously over the whole Au rod. The obtained Pt nanostructure is a single crystal owing to the epitaxial growth of Pt on the Au rod. Due to the unique surface plasmon resonance (SPR) features of the Au NRs, the Au core/Pt shell (Au@Pt) nanostructures also exhibit well-defined and red-shifted longitudinal SPR bands in the visible and near-infrared region. The position and intensity can be regulated by the thickness and amount of the Pt shell. At a thinner Pt thickness, the Au@Pt NRs show higher dielectric sensitivity than the corresponding Au NRs. It thus opens up the potential of Pt nanostructures for SPR-based sensing.     

Defining rules for the shape evolution of gold nanoparticles

The roles of silver ions and halides (chloride, bromide, and iodide) in the seed-mediated synthesis of gold nanostructures have been investigated, and their influence on the growth of 10 classes of nanoparticles that differ in shape has been determined. We systematically studied the effects that each chemical component has on the particle shape, on the rate of particle formation, and on the chemical composition of the particle surface. We demonstrate that halides can be used to (1) adjust the reduction potential of the gold ion species in solution and (2) passivate the gold nanoparticle surface, both of which control the reaction kinetics and thus enable the selective synthesis of a series of different particle shapes. We also show that silver ions can be used as an underpotential deposition agent to access a different set of particle shapes by controlling growth of the resulting gold nanoparticles through surface passivation (more so than kinetic effects). Importantly, we show that the density of silver coverage can be controlled by the amount and type of halide present in solution. This behavior arises from the decreasing stability of the underpotentially deposited silver layer in the presence of larger halides due to the relative strengths of the Ag(+)/Ag(0)-halide and Au(+)/Au(0)-halide interactions, as well as the passivation effects of the halides on the gold particle surface. We summarize this work by proposing a set of design considerations for controlling the growth and final shape of gold nanoparticles prepared by seed-mediated syntheses through the judicious use of halides and silver ions.     

金纳米棒的表面改性及与H_2O_2的作用

通过调控过氧化氢与金纳米棒相互作用时溶液的H~+和Br~-浓度,考察了过氧化氢刻蚀金纳米棒的条件.通过静电相互作用将聚苯乙烯磺酸钠修饰到带正电的金纳米棒表面,并探讨了表面配体变化对过氧化氢与金纳米棒相互作用的影响,比较了聚苯乙烯磺酸钠浓度改变对过氧化氢刻蚀金纳米棒所引起的等离子体吸收峰的变化.结果表明,过氧化氢与金纳米棒作用过程中,H~+浓度增加可以加快刻蚀反应速率,Br~-起到稳定金离子的作用.采用聚苯乙烯磺酸钠修饰抑制了过氧化氢对金纳米棒的刻蚀,当聚苯乙烯磺酸钠与金纳米棒表面的CTAB完全作用后,复合材料电位接近零,金纳米棒的稳定性降低,继续增加聚苯乙烯磺酸钠的量至电位为负,复合材料稳定性增加.     

Synthesis of gold nanorod/single-wall carbon nanotube heterojunctions directly on surfaces

This contribution describes the synthesis of gold nanorod (Au NR)/single-wall carbon nanotube (SWCNT) heterojunctions assembled directly on Si/SiOx substrates. SWCNTs are attached to amine-functionalized Si/SiOx substrates, and Au monolayer-protected clusters (MPCs) are adsorbed to the surface of SWCNTs through hydrophobic interactions. Seed-mediated reduction of HAuCl4 with ascorbic acid in the presence of cetyltrimethylammonium bromide (CTAB) onto the Au MPCs leads to the growth of larger Au nanostructures directly on the SWCNTs. Au NRs account for 19% of the nanostructures, some of which are attached directly to the sidewall and some at the ends of the SWCNTs. Raman spectroscopic measurements of SWCNTs before and after growth of the Au nanostructures reveal that the presence of Au leads to an approximately 50-fold enhancement of the Raman scattering signal. Combining 1D nanostructures of different materials (Au and carbon in this example) is of fundamental interest and may find use in nanoelectronics, chemical sensing, electrochemical, and spectroscopy applications.     

A facile method for gold decoration of Te@CdTe nanorods in aqueous solution

Colloidal synthesis of metal-semiconductor hybrid nanostructures is mainly achieved in organic solution. In some applications of hybrid nanoparticles relevant in aqueous media, phase transfer of hydrophobic metal-semiconductor hybrid nanostructures is essential. In this work, we present a simple method for direct synthesis of water-soluble gold (Au) decorated Te@CdTe hybrid nanorods (NRs) at room temperature by using aqueous Te@CdTe NRs as templates, which were preformed by using CdTe nanocrystals (NCs) as precursor in the presence of hydrazine hydrate (N(2)H(4)). Our results showed that NRs were decorated with Au islands both on tips and along the surface of the NRs. The size and density of Au islands can be controlled by varying the amount of Au precursor (mixture of HAuCl(4) and thioglycolic acid (TGA)) and TGA/HAuCl(4) ratio. A possible growth mechanism for the Au decoration of Te@CdTe NRs is concluded as three steps: (1) the formation of AuTe(1.7) via the substitution reaction of Cd(2+) by Au(3+), (2) adsorption of Au-TGA complex onto the preformed AuTe(1.7) anchors and following reduction by CdTe and N(2)H(4), leading to the formation of small Au NCs, (3) Au NCs grow to bigger ones, followed by reduction of more Au precursor by N(2)H(4).     

Sensitive electrochemiluminescence resonance energy transfer(ECL-RET) between Ru(bpy)_3~(2+) and Au nanorod for hydrogen peroxide detection

Here,we developed a novel electrochemiluminescence resonance energy transfer(ECL-RET) approach between Ru(bpy)_3~(2+) and Au nanorods(NRs) for sensitive determination of H_2O_2.Au NRs were synthesized through silver ion-assisted seed-mediated method which exhibited an obvious absorption peak at about 627 nm.They were modified at glassy carbon electrode(GCE) surface which showed a significant ECL quenching efficiency about 56.5%due to the ECL-RET process.This Au NRs modified electrode was then utilized to measure the concentration of H_2O_2 on the basis of the significant quenching effect of H_2O_2 on Ru(bpy)_3~(2+) ECL.Results demonstrated that the decrement of ECL intensity at Au NRs modified electrode had ~ 6.6-fold enhancement as compared with that at bare electrode.     

Synthesis of Uniform Gold Nanorods with Large Width to Realize Ultralow SERS Detection

In this work, we successfully demonstrate high-yield synthesis of high-quality gold nanorods (Au NRs) with width ranging from 6.5 nm to 175 nm by introducing heptanol molecules as secondary templating agents during cetyltrimethylammonium bromide-templated, seeded growth method. The results show that an appropriate concentration of heptanol molecules not only alter the micellization behavior of CTAB in water, but also help silver ions impact the symmetry-breaking efficiency of additional Au−NP seeds in addition to enhancing the utilization of gold precursors. Moreover, the generality and versatility of the present strategy for synthesis of Au NRs with flexible controlled dimensions are further demonstrated by successful synthesis of Au NRs with the assistance of other fatty alcohols with properly long alkyl chains. Furthermore, when arrays of vertically aligned Au NRs with large width (AVA−Au_120×90 NRs) are used as SERS substrates, they can achieve the ultralow limit of detection for crystal violet (10⁻¹⁶ M) with good reliability and reproducibility, and the rapid detection and identification of residual harmful substances.     

Growth of anisotropic gold nanostructures on conducting glass surfaces

In this paper, we describe a method for the growth of gold nanowires and nanoplates starting from a bilayer array of gold seeds, anchored on electrically conducting indium tin oxide (ITO) substrates. This is based on a seed-mediated growth approach, where the nanoparticles attached on the substrate through molecular linkages are converted to nanowires and nanoplates at certain cetyltrimethylammonium bromide (CTAB) concentration. Our modified approach can be used to make nanowires of several tens of micrometers length at a lower CTAB concentration of 0.1 M. The length of the nanowires can be varied by adjusting the time of the reaction. As the concentration of CTAB was increased to 0.25 M, the nanoparticles got converted to nanoplates. These Au nanoplates are (111) oriented and are aligned parallel to the substrate.     

Seed free high yield gold nanorods synthesis from single precursor gold(I) dithiophosphate complex

In this work, we demonstrate a simple, one pot and seed free synthetic route for the formation of gold nanorods (Au NRs) via thermal decomposition of gold(I) dithiophosphate {[Au₂{S₂P(OⁱPr)₂}₂]_n,} 1 complex as a molecular precursor in presence of 4′‐amino‐biphenyl‐4‐carboxylic molecule. Here [Au₂{S₂P(OⁱPr)₂}₂]_n, complex functioned as gold (Au) source and 4′‐amino‐biphenyl‐4‐carboxylic molecule stabilized gold (Au) nanorods (NRs) through the unidirectional coating of Au surface during its growth in the reaction medium.     

Shape tunable synthesis of anisotropic gold nanostructures through binary surfactant mixtures

In recent years, much effort has been made to produce gold (Au) nanorods of different sizes through the use of binary surfactant mixtures via a seed-mediated growth approach. However, how the ratio of two different surfactants influences the shape of the resulting Au nanoparticles remains to be elucidated. Here, we report the shape-controlled synthesis of Au nanoparticles using a binary surfactant mixture of CTAB (cetyltrimethylammonium bromide) and DDAB (didodecyldimethylammonium bromide) via a silver-assisted seed-mediated growth approach. Decreasing the CTAB/DDAB ratio results in a shape transition from Au nanorods to elongated tetrahexahedra and finally to Au bipyramids. The results showed significant improvement in the yield of Au bipyramidal type nanoparticles in different sizes (nm to μm) by using binary surfactant mixtures without any need for shape selection procedure. By varying the pH and concentration of ascorbic acid, we can control the shape and size of Au nanoparticles (i.e., truncated bipyramids, dogbones, and nanodumbbells) at fixed CTAB/DDAB ratios. A preliminary growth mechanism has been proposed based on the change in the mixed micelle soft-template induced by the increasing concentration of DDAB and reaction parameters (i.e., pH, concentration of ascorbic acid). These results constitute the advances in the understanding for synthesizing anisotropic Au nanoparticles of tunable optical properties via engineering the design of a soft-template. These anisotropic Au nanoparticles, especially, bipyramids of different morphologies and sizes are potential candidates for the enhancement of the optical response and developing label-free biosensing devices.     

An investigation of gold adsorption from a binary mixture with selective mesoporous silica adsorbents

Gold-selective adsorbents were prepared from mesoporous MCM-41 silica by grafting organic amine groups (i.e., RNH2, R2NH, and R3N; R=propyl). NH2-MCM-41, NRH-MCM-41, and NR2-MCM-41 displayed strong affinity for gold and at 1 mmol/g loading adsorbed 0.40, 0.33, and 0.20 mmol/g of gold. Copper and nickel were not adsorbed on these adsorbents. Grafting surface chemical moieties introduces heterogeneity on an otherwise uniform MCM-41 pore surface and metal adsorption is best described by the Freundlich adsorption model. A series of binary adsorption equilibrium studies with NH2-MCM-41 containing 2.2 mmol RNH2/g shows that NH2-MCM-41 adsorbs only gold from solutions containing copper and nickel with an adsorption capacity of 0.6 mol of Au/mol of RNH2 (1.1 mmol of Au/g of NH2-MCM-41). Copper and nickel were not adsorbed by NH2-MCM-41 regardless of the solution concentration, composition, and pH (i.e., 2 to 4) in the presence of gold. The LeVan and Vermeulen adsorption model based on a single component Freundlich isotherm and corrected for the anion effect accurately predicted the binary adsorptions. The adsorbed gold was completely recovered by a simple acid wash and the recovered gold solution is 99% pure. The regenerated NH2-MCM-41 remained 100% selective for gold removal and exhibited the same adsorption capacity even after several uses.     

Mechanism of ZnO nanotube growth by hydrothermal methods on ZnO film-coated Si substrates

ZnO nanorods (NRs) and nanotubes (NTs) have been synthesized by a hydrothermal method on Si substrates that had been precoated (by pulsed laser deposition (PLD)) with a thin ZnO film. High-resolution transmission electron microscopy and selected area electron diffraction analysis confirm that the NTs are ZnO single crystals and that their growth direction is along [0001] (the c-axis). Scanning electron microscopy points to the early-time formation of two classes of NRs on the PLD ZnO coating, one of which is longer and displays higher length/diameter aspect ratios than the other. The morphologies of NRs belonging to the first of these classes were seen to evolve with time, progressively tapering, and producing volcano-like surface structures that develop into NTs. In contrast, NRs belonging to the other (shorter) class retain their hexagonal cross-section and have flat tops. To explain these emergent structures and, in particular, the selective growth of ZnO NTs, we have undertaken a systematic investigation of the effects of different substrates (e.g., borosilicate glass, Pt-coated glass, and both bare and PLD ZnO-coated Si wafers) and of the reactive solution on the growth properties of ZnO NRs, NTs, and the ZnO nanopowders that precipitate from the reactive mixture. The experimental findings suggest the following ZnO NT growth mechanism. The PLD ZnO film consists of many nanocrystallites, with a preferred c-axis alignment. These serve to nucleate the hydrothermal growth of (c-axis aligned) NRs. The NRs are deduced to be Zn-polar, but can be either Zn-atom or O-atom terminated. It is proposed that the different surface terminations influence (by electrostatic interactions) the cation (Zn(2+) and ZnOH(+)) to anion (OH(-)) concentration ratio in the double layer at the growing polar surface. Zn-atom termination causes a reduction in the local Zn(2+)/OH(-) (and ZnOH(+)/OH(-)) ratios (i.e., the extent of solution supersaturation) relative to those in the bulk solution, thereby encouraging tapered NR growth and, as the zinc concentration falls further, the emergence of volcano-like structures on the polar surface, which seed the subsequent growth of ZnO NTs.     

pH Dependence of the size and crystallographic orientation of the gold nanoparticles prepared by seed-mediated growth

The effect of the pH of the growth solution on the size and crystallographic orientation of gold nanoparticles (GNPs) was studied during the course of the preparation of surface-confined spherical GNPs following a two-step protocol (electrochemical and chemical). GNPs were first electrodeposited onto a clean glassy carbon (GC) electrode and these GNPs were used as seeds. Seed-mediated growth of the electrodeposited GNPs was performed in a solution of H[AuCl(4)] at various pHs (5.0 to 0.5) using NH(2)OH as a reducing agent. The thus-prepared GNPs were characterized by electrochemical, UV-visible absorption spectral, SEM, and TEM studies. The nucleation (i.e., formation of the new seeds) was found to dominate over growth (i.e., enlargement of the seed particles) process at higher pH during NH(2)OH seeding, whereas only growth was recognized at low pH (0.5). Nonspherical byproducts were noticed when the seed-mediated growth was performed at higher pHs, but at pH 0.5 only spherical GNPs were observed. The present method provides a way out for the preparation of GNPs with homogeneous shape resolving the problem of simultaneous formation of nonspherical byproducts during the seed-mediated growth as well as for the preparation of GNPs with a Au(111) facet ratio as high as 97%. On the basis of the obtained results, the mechanism of the growth process at low pH is also discussed. Interestingly, an enhanced electrochemical response was obtained for the oxidation of H(2)O(2) using the GNPs prepared at pH 0.5.     

A novel electrochemical sensor based on gold nanorods and Nafion-modified GCE for the electrocatalytic oxidation of nitrite

The high-quality CTAB-stabilized gold nanorods (Au NRs) were prepared by the way of seed-mediated protocol. The microstructure and composition of the Au NRs were identified by transmission electron microscopy, field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy and UV–visible spectroscopy. Further, a novel non-enzymatic electrochemical sensor of nitrite based on Au NRs–Nafion-modified glassy carbon electrode (GCE) was successfully developed. Under the optimum experimental conditions, the electrochemical behaviors of nitrite on the Au NRs–Nafion-modified GCE were systematically studied by electrochemical impedance spectroscopy, cyclic voltammetry and chronoamperometry. The electrochemical investigations indicated that the Au NRs–Nafion-modified GCE had a wide linear range of 3.0 × 10^?6–6.0 × 10^?3 mol L^?1, an acceptable sensitivity of 130.9 ± 0.05 μA mM^?1 cm^?2, a fast response time of 3 s and a low detection limit of 0.64 ± 0.02 μmol L^?1 at the signal-to-noise ratio of 3 (S/N = 3). Additionally, the electrochemical sensor also showed good stability and favorable anti-interference capability for the detection of nitrite.     

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.     

Medium effects for very fast electron transfer reactions at electrodes: the [Ru(NH3)6]3+/2+ system in water

The reduction kinetics of [Ru(NH(3))(6)](3+) was studied at Au(111) and Au(100) single-crystal ultramicroelectrodes in dilute perchloric acid electrolytes. Both heterogeneous rate constants and experimental transfer coefficients varied with the crystallographic orientation of the gold surface. The value of the heterogeneous rate constant at Au(111) was significantly larger than that at Au(100). The experimental transfer coefficients also increased but in the opposite order. Standard rate constants at both electrodes increased with an increase in electrolyte concentration. Using double-layer data obtained in 0.01 M HClO(4), it is shown that the true transfer coefficient for this reaction is 0.5 within experimental error. The effective charge on the reactant which has a nominal charge of +3 is close to +1. The latter result reflects the distribution of charge within the polyatomic reactant.     

Characterization of gold nanorods in vivo by integrated analytical techniques: their uptake, retention, and chemical forms

Integrated analytical techniques were used to study the tissue distribution and structural information of gold nanorods (Au NRs) in Sprague-Dawley rats through tail intravenous injection. Before in vivo experiments were conducted, careful characterization of Au NRs was performed. The zeta potential proved that adsorption of bovine serum albumin on Au NRs turned the surface charges from positive to negative as in an in vitro simulation. The biodistribution of Au NRs was investigated quantitatively by inductively coupled plasma mass spectrometry at different time points after injection. As target tissues, both liver and spleen were chosen to further demonstrate the intracellular localization of Au NRs by the combination of transmission electron microscopy and energy-dispersive X-ray spectroscopy. Moreover, synchrotron-radiation-based X-ray absorption spectroscopy was employed and it was observed that long-term retention of Au NRs in liver and spleen did not induce obvious changes in the oxidation states of gold. Therefore, the present systematic method can provide important information about the fates of Au NRs in vivo and can also be extended to study the biological effects of other metallic nanomaterials in the future.