Thiol-capped Gold Nanoparticles Research Articles

Chemical synthesis and structural studies of thiol-capped gold nanoparticles

We report the chemical synthesis and structural studies of thiol-capped Au nanoparticles (NPs) using extended X-ray absorption fine structures (EXAFS) and high-resolution transmission electron microscopy (HRTEM). Synthesis of Au NPs was conducted in one case in a toluene/water two-phase system using alkanethiols with varied hydrocarbon chain length (C6, C12, and C18), resulting in NPs of sizes ranging from 1.6 nm to 5.4 nm. Au L3-edge EXAFS reveals a systematical trend of the local structure of Au in the NPs when the Au/S ratio and chain-length of thiols are varied. In another synthesis, thiol-capped Au NPs were prepared on the surface of silicon nanowires, which act as both substrates and reducing agents. HRTEM reveals that not only spherical particles but also very small quasi-1D nanostructures of Au were formed. The formation and structure of these Au NPs was discussed in terms of ligand and template effect associated with the silicon nanowire substrates.Key words: thiol-capped Au nanoparticles, EXAFS, silicon nanowires, electroless deposition, quasi-1D Au nanostructures.

Influence of the Capping Molecule on the Magnetic Behavior of Thiol-Capped Gold Nanoparticles

Gold nanoparticles with an average particle size below 3 nm have been synthesized and stabilized with different thiol-derivatized molecules for studying the influence of the capping molecule on the magnetic behavior. Thiolated-alkane chains with different lengths as well as a thiol-containing biomolecule (tiopronin) have been selected as protecting shells for the synthesized NPs. Magnetic characterization indicates that the appearance of a ferromagnetic-like behavior is related not only with the formation of Au-S bonds linking the protective molecules to the nanoparticle surface but also with the formation of the nanoparticle itself as well as with the geometry of the capping molecule. The later seems to determine whether the protective monolayer shell is ordered or not. The simultaneous presence of Au-Au and Au-S bonds together with a reduced particle diameter, and the formation of an ordered monolayer protective shell, have been proved to be key parameters for the ferromagnetic-like behavior exhibited by thiol-functionalized gold NPs.

Surface plasmon resonance and magnetism of thiol-capped gold nanoparticles

Surface plasmon resonance measurements and magnetic characterization studies have beencarried out for two types of thiol-capped gold nanoparticles (NPs) with similar diametersbetween 2.0 and 2.5 nm and different organic molecules linked to the sulfur atom:dodecanethiol and tiopronin. In addition, Au NPs capped with tetraoctyl ammoniumbromide have also been included in the investigation since such capping molecules weaklyinteract with the gold surface atoms and, therefore, this system can be used as amodel for naked gold NPs; such particles presented a bimodal size distributionwith diameters around 1.5 and 5 nm. The plasmon resonance is non-existent fortiopronin-capped NPs, whereas a trace of such a feature is observed for NPs covered withdodecanethiol molecules and a bulk-like feature is measured for NPs capped withtetralkyl ammonium salts. These differences would indicate that the modification ofthe surface electronic structure of the Au NPs depends on the geometry andself-assembling capabilities of the capping molecules and on the electric charge transferredbetween Au and S atoms. Regarding the magnetization, dodecanethiol-cappedNPs have a ferromagnetic-like behaviour, while the NPs capped with tioproninexhibit a paramagnetic behaviour and tetralkyl ammonium-protected NPs arediamagnetic across the studied temperature range; straight chains with a well-definedsymmetry axis can induce orbital momentum on surface electrons close to the bindingatoms. The orbital momentum not only contributes to the magnetization butalso to the local anisotropy, giving rise to permanent magnetism. Due to thedomain structure of the adsorbed molecules, orbital momentum is not induced fortiopronin-capped NPs and the charge transfer only induces a paramagnetic spincomponent.

Chemically Induced Permanent Magnetism in Au, Ag, and Cu Nanoparticles: Localization of the Magnetism by Element Selective Techniques

We report a direct observation of the intrinsic magnetization behavior of Au in thiol-capped gold nanoparticles with permanent magnetism at room temperature. Two element specific techniques have been used for this purpose: X-ray magnetic circular dichroism on the L edges of the Au and 197Au Mössbauer spectroscopy. Besides, we show that silver and copper nanoparticles synthesized by the same chemical procedure also present room-temperature permanent magnetism. The observed permanent magnetism at room temperature in Ag and Cu dodecanethiol-capped nanoparticles proves that the physical mechanisms associated to this magnetization process can be extended to more elements, opening the way to new and still not-discovered applications and to new possibilities to research basic questions of magnetism.

Thiol-Capped Gold Nanoparticles on Graphite: Spontaneous Adsorption and Electrochemically Induced Release

Gold nanoparticle-(AuNP)-modified carbon graphite surfaces have been prepared by simple immersion of highly oriented pyrolytic graphite (HOPG) in hexane solutions containing 3 nm diameter butanethiol- or nonanethiol-capped nanoparticles. The AuNP adsorb on the HOPG surface free of unbounded thiols, remaining unchanged with time. The amount of adsorbed thiol-protected AuNP depends on concentration and time. The reductive desorption of thiols from the AuNP produces an efficient release of more than 90% of the AuNP from the carbon surface to the aqueous solution. The remaining thiol-free Au nanoparticles do not sinter on the HOPG, forming stable and electrochemically active islands. These results could open interesting possibilities for easy transfer of thiol-capped metallic NP from one environment to another, for controlled release of biomolecules from metallic NP, and for the preparation of catalytic or decontamination systems on large area C surfaces.

Evolution of the microstructure, chemical composition and magnetic behaviour during the synthesis of alkanethiol-capped gold nanoparticles

In the present paper, we show an exhaustive microstructural characterization of thiol-capped gold nanoparticles (NPs) with two different average particle sizes. These samples are compared with the polymer-like Au(I) phase formed as a precursor during the synthesis of the thiol-capped gold NPs. The set of analysed samples shows different microstructures at the nanoscale with different proportions of Au atoms bonded either to S or to Au atoms. It has been experimentally shown that the presence of a ferromagnetic-like behaviour is associated to the formation of NPs with simultaneous presence of Au–Au and Au–S bonds. In order to explain such magnetic behaviour a possible model is proposed based on the spin–orbit coupling so that localized charges and/or spins (Au–S bonds) can trap conduction electrons (Au–Au bonds) in orbits.

Structural and magnetic characterization of oleic acid and oleylamine-capped gold nanoparticles

In this work the study of oleic acid and oleylamine-capped gold nanoparticles is presented. The structural characterization of the sample shows 6.7 nm gold nanoparticles with a narrow size distribution. The experimental optical absorption spectrum has a maximum at 2.35 eV. The calculated optical absorption spectrum is shifted and narrower than the experimental one, indicating that the oleic acid and oleylamine do not merely passivate the metallic nanoparticles but modify its electronic structure. These gold nanoparticles show in addition a kind of magnetic order similar to other organic passivated gold nanoparticles as thiol-capped gold nanoparticles. Although the magnetic interactions seem to be weaker than in thiol-capped ones, the magnetic behavior looks similar to that, i.e., an invariant temperature dependence of the magnetization from 5 to 300 K and a noticeable coercive field. We analyze the influence of the organic layer bonding the nanoparticles on the magnetic behavior.

Preparation of Thiol-Capped Gold Nanoparticles by Chemical Reduction of Soluble Au(I)−Thiolates

A novel preparation method of thiol-stabilized gold nanoparticles is described. Soluble Au(I)−thiolate complexes were prepared, isolated, and reduced with Superhydride (lithium triethyl borohydride) in suitable inert solvents. The dimensions of the resulting gold nanoparticles were compared to those prepared using conventional methods (i.e., that use a mixture of Au(III) salts and thiols as the initial reagents). The sizes (and size dispersity) of the thiol-capped gold nanoparticles synthesized here are comparable to those prepared by the conventional methods used.

Fabrication, morphology, structure, and photoluminescence of ZnS and CdS nanoribbons

One-dimensional semiconductor nanoribbons of hexagonal wurtzite sulfides (ZnS and CdS) have been prepared in bulk quantity by a thermal evaporation technique using thiol-capped gold nanoparticles as catalysts. Compared to their starting materials, ZnS and CdS powders, the band-gap photoluminescence excited by ultraviolet light from ZnS and CdS nanoribbons at room temperature was significantly enhanced. X-ray-excited optical luminescence at the S K edge confirms the near-band-gap and the defect origin of the luminescence.

Fabrication of gold nanocrystal-coated polypyrrole nanotubules

A method of fabricating gold nanocrystals coated polypyrrole (PPy) tubules was developed by coating carboxylic acid–thiol-capped or citrate-reduced gold nanoparticles on polypyrrole tubule templates. Electrostatic interaction between gold nanoparticles and PPy tubules was used as a driving force for this assembly process. The gold nanoparticle coated PPy tubule can be applied as signal-enhancing cuvettes for Raman spectroscopic analysis of trace molecules. The Raman scattering of trace Rhodamine B on the Au nanoparticle-coated tubule was enhanced by a factor of 104–105.

Quantitative analysis of gold nanoparticles from synchrotron data by means of least-squares techniques

Powder samples of thiol-capped gold nanoparticles in the size range of 2-4 nm were quantitatively characterized by means of synchrotron X-ray diffraction data, with respect to their structure, size and strain distributions. A novel Rietveld-like approach was applied, refining domain size distribution, strain-size dependence and structure type concentrations. Three structure types (cuboctahedron, icosahedron, decahedron) were considered in this analysis and a detail study of the strain content was performed by comparing different models. The results showed a strong influence of the strain model and a careful analysis is presented. Final domain size and strain distributions agree well with the existence of both single-domain and imperfectly formed or multi-domain nanoparticles, but the final strain profiles seem to be mostly related to the different degree of structural perfection at different sizes as a result of the synthesis process. The present work represents an important step towards the development of robust methods to determine strain profiles in nanosystems, aiming to fulfill the description of these important but complex systems.

Size-Dependent Solubility of Thiol-Derivatized Gold Nanoparticles in Supercritical Ethane

It is shown that alkane thiol-capped gold nanoparticles can be dissolved in supercritical ethane. Importantly, the solubility was found to be dependent on the core diameter thus allowing separation by size. Nanoparticles with a core diameter less than 1.7 nm were soluble in the supercritical fluid.

Structure Population in Thiol-Passivated Gold Nanoparticles

Chemically synthesized nanoparticles can be regarded as building blocks for more complex systems. Hence, the characterization of their attributes is a key step toward controlling and tuning the properties of new nanocomposite materials. We present a structural characterization of thiol-capped gold nanoparticles as a function of size (2−4 nm) by X-ray diffraction (XRD). In addition to the fcc bulk structure, nanometric gold may present decahedral or icosahedral structures depending on particle size. To evaluate the structure evolution, we have applied Debye function analysis to the XRD profiles. The analysis reveals a high proportion of defective and complex structures in all samples, which may have become trapped by the growth conditions. Nevertheless, the fcc structure is more prominent with increasing diameter, indicating that thermodynamics also plays an important role in the average structure of this system. The results underline the importance of performing careful structural characterization of chem...

Inter-atomic distance contraction in thiol-passivated gold nanoparticles

Size-dependent inter-atomic distance contraction was investigated by Extended X-ray Absorption Fine Structure in thiol-capped gold nanoparticles. A slight nearest-neighbor distance reduction was observed as a function of particle diameter (2–4 nm range) but, for all samples, it was less than 1%. This value contrasts with the larger effect expected and found in other systems, especially for the smallest particles (2 nm) where it is twice as small. Our analysis also points out a short metal–ligand bond, suggesting a rather strong surface interaction. We interpreted these results as a passivant effect where the metal–ligand interaction partially compensates the expected lattice contraction.