Diameter Au Nanoparticles Research Articles

Ion beam irradiation-induced tuning of SPR of Au nanoparticles in fullerene C70 matrix: dependence of energy loss

We investigated the effect of energy loss of ions on the ion irradiation-induced tuning of surface plasmon resonance (SPR) wavelength of Au nanoparticles (NPs) in fullerene C70 matrix. The transformation of fullerene C70 into amorphous carbon (a-C) under ion irradiation was used to tune the SPR wavelength of Au–C70 nanocomposite thin films. It is found that the range of tuning of SPR wavelength increases with increase in electronic energy loss of the incoming beam. The growth of Au NPs with increasing fluence was observed in all the cases and total growth is proportional to the electronic energy loss. The average diameter of Au NPs in pristine film is ~4.8 nm and a maximum growth of ~3 nm was observed at a fluence of 3 × 1013 ions/cm2, when the film was irradiated with 120 MeV Ag ions. It was also observed that nuclear energy loss via collision cascades has lower efficiency for SPR tuning in comparison with the electronic excitations.

A simultaneous increase in the thermal and electrical transport in carbon nanotube yarns induced by inter-tube metallic welding

Vertically aligned arrays of multiwall carbon nanotubes (MWCNT) were decorated with gold (Au) nanoparticles of different diameter and areal densities and spun into yarns. The melting point of Au nanoparticles determined by differential scanning calorimetry was approximately 260°C, well below the oxidation temperature of carbon. A continuous yarn was formed while pulling out a bundle of CNTs from the metalized CNT array. Relatively low temperature (300°C) thermal processing of the metalized yarn resulted in a 30% improvement in thermal conductivity, 40% increase in electrical conductivity and a 4× increase in elastic modulus. Cross-sections of the yarn were examined with transmission electron microscopy to characterize the physical nature of the metal–nanotube interface. The deposition procedure described to decorate the nanotube yarns is easily scalable to larger CNT arrays or other configurations for commercial applications, such as medical implants, lightweight conductors, smart uniforms for the soldiers, and conformal electronics in aerospace industry.

Fabrication of ordered Au nanoparticles array and its optical absorption properties

Three kinds of polystyrene spheres with different sizes were fabricated on glasses by drifting method of nanosphere lithography. Au thin films with various thicknesses were prepared by magnetron sputtering under controlled deposition time. Triangle-shaped Au nanoparticles array in three different heights after removing the PS template were observed using scanning electron microscopy. The absorption characteristics of all samples were measured by ultraviolet-visible spectrophotometer. Surface plasmon resonance peak was seen to shift to the longer wavelength with increasing diameter of Au nanoparticles. And the surface plasmon resonance peak was shifted to the shorter wavelength with increasing height of Au nanoparticles. Based on Mie theory, we simulated the extinction properties, which vary with the diameter of Au nanoparticles, using Matlab software. The conclusion is in accordance with our experimental results.

The self-assembling of DNA-templated Au nanoparticles into nanowires and their enhanced SERS and catalytic applications

A simple photochemical technique is described for the synthesis of organized assemblies of Au nanoparticles (NPs) on a DNA template in a shorter time scale. The smaller Au nanoparticles (NPs) are initially formed in the DNA solution, self-assembled together, and generate the nanowire structures. The diameter and the length of the nanowires can be tuned by controlling the different reaction parameters. The average diameters of the individual Au NPs could be varied in the range of 10–50 nm, whereas their lengths could be extended to the level of several micrometres. The mechanism for the formation of the self-assembled structure of Au NPs on DNA is elaborated upon. The DNA–Au nanowire shows a highly stable and reproducible signal in surface enhanced Raman scattering (SERS) studies. The SERS activity was examined with Rhodamine 6 G (R6G) as a model dye molecule and the observed enhancement factor (EF) was ∼106. The catalytic activity was tested for the reduction of 4-nitroaniline (4-NA) with sodium borohydride in aqueous solution. The synthesized Au nanowires were found to be stable for more than six months under ambient conditions at room temperature without any change of the optical properties. The superior SERS and catalytic activities of the material might be useful in future for different applications in organic catalysis reactions and in a variety of SERS based detections of bio-molecules, as sensors etc.

Controlling Nanoparticle Dynamics in Conical Nanopores

Conical nanopores are a powerful tool for characterizing nanoscale particles and even small molecules. Although the technique provides a wealth of information, such pores are limited in their ability to investigate particle dynamics due to high particle velocities through a very short sensing zone. In this report, we demonstrate the use of applied pressure to balance electrokinetic forces acting on 8 nm diameter Au nanoparticles as they translocate through a ∼10 nm diameter orifice. This force balance provides a means to vary nanoparticle velocity by 3 orders of magnitude, allowing for their detection and characterization on time scales as long as 100 ms. We studied nanoparticles having different zeta potentials by varying salt concentration, applied pressure, and voltage to reveal the point at which forces are balanced and the particle velocities approach zero. Variation of the voltage around this force balance point provides a means to precisely control the magnitude and direction of the particle translocation velocity. Nanoparticle velocities computed from finite-element simulations as a function of applied pressure and voltage yield predictions in semiquantitative agreement with the experimental results. Optimizing the conditions of these techniques will allow the characterization of particles and their dynamics down to the smallest end of the nanoscale range.

Nanoporous Ge electrode as a template for nano-sized ( < 5 nm) Au aggregates

In this paper we present the extremely peculiar electrical properties of nanoporous Ge. A full and accurate electrical characterization showed an unexpected and extremely high concentration of positive carriers. Electrochemical analyses showed that nanoporous Ge has improved charge transfer properties with respect to bulk Ge. The electrode behavior, together with the large surface-to-volume ratio, make nanoporous Ge an efficient nanostructured template for the realization of other porous materials by electrodeposition. The pores were efficiently decorated by Au nanoparticles of diameter as low as 1–5 nm, prepared by electrochemical deposition. These new results demonstrate the potential and efficient use of nanoporous Ge as a nanostructured template for nano-sized Au aggregates, opening the way for the realization of innovative sensor devices.

Cytotoxic Effects of Gold Nanoparticles: A Multiparametric Study

The in vitro labeling of therapeutic cells with nanoparticles (NPs) is becoming more and more common, but concerns about the possible effects of the NPs on the cultured cells are also increasing. In the present work, we evaluate the effects of poly(methacrylic acid)-coated 4 nm diameter Au NPs on a variety of sensitive and therapeutically interesting cell types (C17.2 neural progenitor cells, human umbilical vein endothelial cells, and PC12 rat pheochromocytoma cells) using a multiparametric approach. Using various NP concentrations and incubation times, we performed a stepwise analysis of the NP effects on cell viability, reactive oxygen species, cell morphology, cytoskeleton architecture, and cell functionality. The data show that higher NP concentrations (200 nM) reduce cell viability mostly through induction of reactive oxygen species, which was significantly induced at concentrations of 50 nM Au NPs or higher. At these concentrations, both actin and tubulin cytoskeleton were deformed and resulted in reduced cell proliferation and cellular differentiation. In terms of cell functionality, the NPs significantly impeded neurite outgrowth of PC12 cells up to 20 nM concentrations. At 10 nM, no significant effects on any cellular parameter could be observed. These data highlight the importance of using multiple assays to cover the broad spectrum of cell-NP interactions and to determine safe NP concentrations and put forward the described protocol as a possible template for future cell-NP interaction studies under comparable and standardized conditions.

Oxidation of Highly Unstable &lt;4 nm Diameter Gold Nanoparticles 850 mV Negative of the Bulk Oxidation Potential

Here we describe the oxidation of <4 nm diameter Au nanoparticles (NPs) attached to indium tin oxide-coated glass electrodes in Br(-) and Cl(-) solution. Borohydride reduction of AuCl(4)(-) in the presence of hexanethiol or trisodium citrate (15 min) led to Au NPs <4 nm in diameter. After electrochemical and ozone removal of the hexanthiolate ligands from the thiol-coated Au NPs, Au oxidation peaks appeared in the range 0-400 mV vs Ag/AgCl (1 M KCl), which is 850-450 mV negative of the bulk Au oxidation peak near 850 mV. The oxidation potential of citrate-coated Au NPs is in the 300-500 mV range and those of 4 and 12 nm diameter Au NPs in the 660-780 mV range. The large negative shift in potential agrees with theory for NPs in the 1-2 nm diameter range. The oxidation potential of Au in Cl(-) solution is positive of that in Br(-) solution, but the difference decreases dramatically as the NP size decreases, showing less dependence on the halide for smaller NPs.

Photonic–Plasmonic Mode Coupling in On-Chip Integrated Optoplasmonic Molecules

We investigate photonic-plasmonic mode coupling in a new class of optoplasmonic materials that comprise dielectric microspheres and noble metal nanostructures in a morphologically well-defined on-chip platform. Discrete networks of optoplasmonic elements, referred to as optoplasmonic molecules, were generated through a combination of top-down fabrication and template-guided self-assembly. This approach facilitated a precise and controllable vertical and horizontal positioning of the plasmonic elements relative to the whispering gallery mode (WGM) microspheres. The plasmonic nanostructures were positioned in or close to the equatorial plane of the dielectric microspheres where the fields associated with the plasmonic modes can synergistically interact with the evanescent fields of the WGMs. We characterized the far-field scattering spectra of discrete optoplasmonic molecules that comprised two coupled 2.048 μm diameter polystyrene microspheres each encircled by four 148 nm diameter Au nanoparticles (NPs), through far-field scattering spectroscopy. We observed a broadening of the TE and TM modes in the scattering spectra of the optoplasmonic dimers indicative of an efficient photonic-plasmonic mode coupling between the coupled photonic modes of the WGM resonators and the localized surface plasmon modes of the NPs. Our experimental findings are supported by generalized multiple particle Mie theory simulations, which provide additional information about the spatial distributions of the near fields associated with the photonic-plasmonic hybrid modes in the investigated optoplasmonic molecules. The simulations reveal partial localization of the spectrally sharp hybrid modes outside of the WGM microspheres on the Au NPs where the local E-field intensity is enhanced by approximately 2 orders of magnitude over that of an individual Au NP.

Investigation of methanol electrooxidation on Au/C catalyst in alkaline medium

Gold nanoparticles supported on activated carbon (Au/C) are prepared by rapid reduction with KBH 4, after AuCl 4 − was partially deposited on the surface of the activated carbon by the reaction of AuCl 4 − and ammonia. Through the characterization of the transmission electron microscope and X-ray powder diffraction, the mean diameter of the Au nanoparticles (AuNPs) decreases with the increase of the Au loading. The energy dispersive X-ray spectroscopy analysis is carried out for measuring the Au loadings of the Au/C catalysts. The results exhibit the Au/C catalyst with 20 wt% Au has the highest loading efficiency (94.5%). The origin of the catalytic activity of Au/C catalysts for the methanol electrooxidation (MEO) is investigated by the cyclic voltammetry, which indicates that the current densities normalized by the actual Au loading for the MEO increase with a decrease in the mean diameter of AuNPs by a factor of 2.42–3.17. Based on this result, the active sites (corners, edges and step sites) for the MEO are proposed.

Illuminating Epidermal Growth Factor Receptor Densities on Filopodia through Plasmon Coupling

Filopodia have been hypothesized to act as remote sensors of the cell environment, but many details of the sensor function remain unclear. We investigated the distribution of the epidermal growth factor (EGF) receptor (EGFR) density on filopodia and on the dorsal cell membrane of A431 human epidermoid carcinoma cells using a nanoplasmonic enabled imaging tool. We targeted cell surface EGFR with 40 nm diameter Au nanoparticles (NPs) using a high affinity multivalent labeling strategy and determined relative NP binding affinities spatially resolved through plasmon coupling. Distance-dependent near-field interactions between the labels generated a NP density (ρ)-dependent spectral response that facilitated a spatial mapping of the EGFR density distribution on subcellular length scales in an optical microscope in solution. The measured ρ values were significantly higher on filopodia than on the cellular surface, which is indicative of an enrichment of EGFR on filopodia. A detailed characterization of the spatial distribution of the NP immunolabels through scanning electron microscopy (SEM) confirmed the findings of the all-optical plasmon coupling studies and provided additional structural details. The NPs exhibited a preferential association with the sides of the filopodia. We calibrated the ρ-dependent spectral response of the Au immunolabels through correlation of optical spectroscopy and SEM. The experimental dependence of the measured plasmon resonance wavelength (λ(res)) of the interacting immunolabels on ρ was well described by the fit λ(res) = 595.0 nm - 46.36 nm exp(-ρ/51.48) for ρ ≤ 476 NPs/μm(2). The performed correlated spectroscopic/SEM studies pave the way toward quantitative immunolabeling studies of EGFR and other important cell surface receptors in an optical microscope.

A hybrid substrate for surface‐enhanced Raman scattering spectroscopy: coupling metal nanoparticles to strong localised fields on a micro‐structured surface

AbstractElectromagnetic coupling between localised plasmons on metal nanoparticles and the strong localised fields on a micro‐structured surface is demonstrated as a means to increase the enhancement factor in surface‐enhanced Raman scattering (SERS) spectroscopy. Au nanoparticles of diameter 20 nm were deposited on a micro‐structured Au surface consisting of a periodic array of square‐based pyramidal pits (Klarite). The spectra of 4‐aminothiophenol (4‐ATP) were compared before and after deposition of Au nanoparticles on the micro‐structured surface. The addition of Au nanoparticles is shown to provide significantly higher signal intensities, with improvements of the order of ∼103 per molecule compared with spectra obtained from the micro‐structured substrate alone. This hybrid approach offers promise for combining nanoparticles with micro‐ and nano‐structured surfaces in order to design SERS substrates with higher sensitivities. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Real-time manipulation of gold nanoparticles inside a scanning electron microscope

The forces needed to overcome static friction and move 150 nm diameter Au nanoparticles on an oxidized Si substrate were measured in Normal and Shear oscillation modes inside a scanning electron microscope (SEM) in real time. The experimental setup consisted of a quartz tuning fork (QTF) mounted onto a high-precision 3D nanomanipulator used with a glued silicon or tungsten tip as a force sensor. Static friction was found to range from tens of nN to several hundred nN. Large variations in static friction values were related to differences in particle shape. Kinetic friction tended to be close to the detection limit and in most cases did not exceed several nN. The influence of thermal treatment in reducing the static friction of nanoparticles was considered.

Generation of reactive oxygen species induced by gold nanoparticles under x-ray and UV Irradiations

The radiosensitizing effect of 5--250 nm diameter Au nanoparticles (AuNPs) in water was investigated under irradiations of diagnostic x-ray and UV light. Enhanced generations of hydroxyl radical ( OH) and superoxide anion (O 2 −) were confirmed from their dependencies on the absorbed energy, ethanol concentration and AuNPs' concentration. Two kinds of fluorescent probes revealed that the reactive oxygen species (ROS) generation rate under x-ray irradiation was enhanced by factors of 1.46 for OH and 7.68 for O 2 −. Photo- and Auger electron charge transfer is possibly relevant to generation of O 2 − near the particle surface, whereas fluorescent x-rays are involved in generation of OH in the secondary water radiolysis. Smaller diameter AuNPs with larger surface area showed a greater yield of ROS. An inverse proportion of ROS generation to the AuNPs' diameter suggests a catalytic function of AuNPs' surface for enhanced ROS generation. From the Clinical Editor This paper investigates the effects of UV and X-ray irradiation on reactive oxygen species induction of gold nanoparticles, concluding that smaller diameter AuNPs with larger surface area lead to a greater yield of ROS probably due to catalytic effects. The paper may be important for the development of novel non-toxic radiation sensitizers.

Gold volatile compound generation: optimization, efficiency and characterization of the generated form

The generation of an analytically useful volatile form of Au has been studied. The flow injection generation was performed in a dedicated generator consisting of a special mixing apparatus and gas–liquid separator design in the presence of surfactants (Triton X-100, Antifoam B) and diethyldithiocarbamate. The on-line atomization in the quartz tube multiatomizer for atomic absorption (AAS) detection has been employed as the convenient atomization/detection means. The optimization of generation and atomization conditions resulted in an analytical procedure yielding the detection limit of 17 ng ml−1 and a very good long range reproducibility of the analytical signal. A 198,199Au radioactive indicator of high specific activity together with AAS measurements was used to track quantitatively the transfer of analyte in the course of generation and transport to the atomizer and to determine the generation efficiency of 11.9 ± 0.1% at the Ar carrier flow rate optimized for the multiatomizer of 240 ml min−1. The efficiency was twice as high at the Ar carrier flow rate of 600 ml min−1. In situ trapping in GF for AAS was explored as an alternative to the on-line atomization. The detection limit of 3.0 ng ml−1 was achieved even though the Ar flow rate optimum for trapping (115 ml min−1) was too low for efficient generation: the overall efficiency of generation and trapping was 1.11 ± 0.03%. Transmission electron microscopy measurements proved the presence of Au nanoparticles of diameter of approximately 10 nm and smaller transported from the generator by the flow of carrier Ar.

Facile green synthesis of gold nanoparticles with gum arabic as a stabilizing agent and reducing agent

A facile, completely green, and cheap route for the synthesis of Au nanoparticles at 25–75° has been developed by using only hydrogen tetrachloroaurate as the precursor and gum arabic (GA) simultaneously as a reducing agent and a stabilizing agent. No extra reagents are needed. From the analyses of UV/VIS absorption spectra, TEM, HRTEM, SAED, and XRD patterns, the formation of Au nanoparticles with a fcc structure was recognized. The synthesis reaction was usually finished in 2–4 h. Increasing the reaction temperature increased the formation rate but had no significant effect on the optical property and size of Au nanoparticles. With increasing Au(III) ion concentration or GA concentration, the mean diameter of Au nanoparticles slightly increased. Also, the particle size distribution became broader at higher Au(III) ion concentration or lower GA concentration due to the insufficient protection. Although raising the GA concentration was helpful to reduce Au(III) ions completely and stabilize the Au nanoparticles, too high GA concentration was not suitable for the stabilization of Au nanoparticles because the increased intermolecular force of GA might hinder the dispersion of Au nanoparticles. Furthermore, the resultant Au nanoparticles were found to remain highly stable in the NaCl solution.

Enzyme:nanoparticle bioconjugates with two sequential enzymes: stoichiometry and activity of malate dehydrogenase and citrate synthase on Au nanoparticles.

We report the synthesis and characterization of bioconjugates in which the enzymes malate dehydrogenase (MDH) and/or citrate synthase (CS) were adsorbed to 30 nm diameter Au nanoparticles. Enzyme:Au stoichiometry and kinetic parameters (specific activity, kcat, KM, and activity per particle) were determined for MDH:Au, CS:Au, and three types of dual-activity MDH/CS:Au bioconjugates. For single-activity bioconjugates (MDH:Au and CS:Au), the number of enzyme molecules adsorbed per particle was dependent upon the enzyme concentration in solution, with multilayers forming at high enzyme:Au solution ratios. The specific activity of adsorbed enzyme increased with increasing number adsorbed per particle for CS:Au, but was less sensitive to stoichiometry for MDH:Au. Dual activity bioconjugates were prepared in three ways: (1) by adsorption of MDH followed by CS, (2) by adsorption of CS followed by MDH, and (3) by coadsorption of both enzymes from the same solution. The resulting bioconjugates differed substantially in the number of enzyme molecules adsorbed per particle, the specific activity of the adsorbed enzymes, and also the enzymatic activity per particle. Bioconjugates formed by adding CS to the Au nanoparticles before MDH was added exhibited higher specific activities for both enzymes than those formed by adding the enzymes in the reverse order. These bioconjugates also had 3-fold higher per-particle sequential activity for conversion of malate to citrate, despite substantially fewer copies of both enzymes present.

Observation of Discrete Au Nanoparticle Collisions by Electrocatalytic Amplification Using Pt Ultramicroelectrode Surface Modification

After growing a thin layer of oxide (PtOx) by anodization of a Pt electrode, it changed from catalytically active for electrochemical NaBH4 oxidation into an inactive electrode. When held at a potential where the oxide film was maintained, collisions of individual 14 nm diameter Au nanoparticles (NPs) that catalyzed NaBH4 oxidation were successfully observed as discrete current pulses (spikes or blips) for each NP interaction with the modified Pt electrode via amplification from NaBH4 oxidation. The current response is affected by NP concentration and the applied potential.

Effects of gold nanoparticles on electro-optical properties of a polymer-dispersed liquid crystal

We present evidence for relatively large changes in the electro-optical properties of a polymer-dispersed liquid crystal (PDLC) due to inclusion of small concentrations of 14 nm diameter Au-nanoparticles (NPs). We observe that NPs lower switch-on electric field, increase optical transmission at certain voltages, and influence the frequency response of the PDLC.

Enhancement of GaN Nanowires Synthesis with a Controlled Diameter by Using Au Nanoparticles on Plasma-Activated Si Substrate

This study demonstrates an easy and fast method of controlling the average diameter and decreasing the standard deviation (SD) of Au nanoparticles (NPs) and GaN nanowires (NWs). Plasma, immersion, and heat-treatments were conducted before the growth of GaN NWs. After the plasma treatment of the Si(111) substrate, immersion treatment was used to change the diameter of Au NPs adsorbed on the substrate surface. Results indicated that the Au NP diameter can be controlled using the immersion time. The Au NPs had an SD of 8 nm, which is much lower than that of a high temperature annealed thin film (20 nm). The experimental results indicated that the GaN NW diameter depended on the Au NP diameter. The GaN NW diameter was in the range of 27–44 nm. The GaN NWs had an SD of 10 nm, whereas GaN NWs grown from a high temperature annealed thin film had an SD of 35 nm. X-ray photoelectron spectroscopy and NMR analysis spectra showed the Si–C bond and carbon–carbon double bond on Au NPs, respectively. The X-ray diffraction pattern confirmed the hexagonal structure of GaN NWs. The blueshift of photoluminescence emission was verified when the NW diameter decreased from 89 to 31 nm.