Colloid stability of thymine-functionalized gold nanoparticles

Gold nanoparticles surface-coated with thyminethiol derivatives containing long hydrocarbon chains have been prepared. The diameter of the particles is 2.2 and 7.0 nm, respectively, with a relatively narrow size distribution. Thyminethiol derivatives are attached to the gold particle surfaces with thymine moieties as the end groups. The colloid stability of the gold nanoparticles as a function of the type and concentration of monovalent salt, pH, and particle size was investigated in alkaline, aqueous solutions. The gold particles are stable in concentrated NaCl and KCl solutions, but are unstable in concentrated LiCl and CsCl solutions. The larger gold particles are more sensitive to salt concentration and aggregate at lower salt concentrations. The reversible aggregation and dispersion of the gold particles can be controlled by changing the solution pH. The larger gold particles can be dispersed at higher pH and aggregate faster than the smaller particles, due to stronger van der Waals forces between the larger particles. Hydration forces play an important role in stabilizing the particles under conditions where electrostatic forces are negligible. The coagulation of the gold nanoparticles is attributed to van der Waals attraction and reduced hydration repulsion in the presence of LiCl and CsCl.     

Fivefold symmetry in superlattices of monolayer-protected gold nanoparticles

Three-dimensional (3D) superlattices of gold nanoparticles were prepared at an air/solution interface. The surface of the gold nanoparticles used is protected by N-acetylglutathione (NAG). Morphological studies revealed that the superlattices formed fivefold symmetric structures such as pentagonal rod, decahedron, and icosahedron, which were probably developed by multiple twinning. Moreover, high-resolution surface images of the superlattices in fivefold symmetry showed excellent ordered arrangements of nanoparticles with both close-packed and non-close-packed structures.     

Selective surface patterning for the coadsorption of self-assembled gold and semiconductor nanoparticles

The predetermined patterned adsorption of two types of nanoparticles on the same substrate may be of considerable importance in various applications, among others, to enhance the absorption of semiconductor nanoparticles by the plasmonic effect of metal NPs. We describe here a simple method for self-assembling 2D lateral patterns in which both gold and semiconductor nanoparticles are adsorbed, each in a predesigned area. Our method is based on a one-step lithographic process and the adsorption of two distinct self-assembled monolayers that can selectively bind only one type of nanoparticle.     

Expeditious synthesis of water-soluble, monolayer-protected gold nanoparticles of controlled size and monolayer composition

A protocol is reported for the preparation of water-soluble, thiol-protected Au nanoparticles (Au-MPC) where dioctylamine is used as a stabilizing agent when the gold cluster is formed using the two-phase Brust and Schiffrin procedure. The amount of amine controls the size of the nanoparticles in the 1.9-8.9 nm diameter range. The final stabilization of the gold clusters by addition of functionalized thiols is performed under very mild conditions compatible with most biomolecules. The procedure is suitable for a wide variety of functional groups present in the thiol and allows one to use thiol mixtures with a precise control of their composition in the monolayer. As a proof of principle, examples of nanoparticles protected with thiols comprising functional groups ranging from polyethers, saccharides, polyamines and ammonium ions are reported.     

Organic mediator-induced structural transformation in superlattices of monolayer-protected gold nanoparticles

N-acetylglutathione (NAG)-protected gold nanoparticles self-assemble into three-dimensional (3D) face-centered cubic (fcc)-type superlattices at an air/water interface under highly acidic conditions. To prepare the well-defined superlattices, 1month's incubation is at least necessary since the size growth of the as-prepared nanoparticles is essential. Addition of 4-pyridinecarboxyic acid (PyC), a bifunctional hydrogen-bonding mediator, promotes the formation of the superlattices, which are created for about 2weeks' storage. Interestingly, PyC-induced nanoparticle superlattices are in a body-centered tetragonal (bct) structure. The fcc-to-bct phase transformation would be due to stronger interaction between NAG and PyC than that between NAG molecules on the gold nanoparticle surfaces.     

Quenching of molecular fluorescence on the surface of monolayer-protected gold nanoparticles investigated using place exchange equilibria

The insertion of fluorescently labeled thiols into the protecting self-assembled monolayer on the surface of gold nanoparticles through place exchange reactions and the effects of this insertion on the photophysical properties of the fluorophores are investigated. Analysis of solution-phase fluorescence data using a dynamic equilibrium model yields the equilibrium constant for the place exchange equilibrium, as well as the relative fluorescence brightness of the fluorophores on the particle surface. In all cases we find a significant quenching of the fluorescence, and potential reasons for this quenching are discussed. In the case of these relatively small particles (4.5 nm diameter), the quenching appears to be mainly related to enhanced nonradiative deactivation pathways. The place exchange equilibrium constant reveals a reduced affinity of the fluorescently labeled thiols for insertion into the nonfluorescent alkylthiol monolayer (K(eq) approximately 0.2) compared to unlabeled alkylthiols.     

Laser desorption/ionization mass spectrometry analysis of monolayer-protected gold nanoparticles

Monolayer-protected gold nanoparticles (AuNPs) feature unique surface properties that enable numerous applications. Thus, there is a need for simple, rapid, and accurate methods to confirm the surface structures of these materials. Here, we describe how laser desorption/ionization mass spectrometry (LDI-MS) can be used to characterize AuNPs with neutral, positively, and negatively charged surface functional groups. LDI readily desorbs and ionizes the gold-bound ligands to produce both free thiols and disulfide ions in pure and complex samples. We also find that LDI-MS can provide a semi-quantitative measure of the ligand composition of mixed-monolayer AuNPs by monitoring mixed disulfide ions that are formed. Overall, the LDI-MS approach requires very little sample, provides an accurate measure of the surface ligands, and can be used to monitor AuNPs in complex mixtures.      

Layer-by-layer self-assembled multilayers of redox polyelectrolytes and gold nanoparticles

Construction and characterization of structural and charge transport properties of electrostatically LbL self-assembled multilayers of gold nanoparticles and a viologen-based redox-active polyelectrolyte is reported.     

Surface effects of monolayer-protected gold nanoparticles on the redox reactions between ferricyanide and thiosulfate

The intense research interest in nanosized particle materials is mainly fueled by their unique physical, chemical, electronic, and magnetic properties[1]. Among these, they have shown great application po-tentialities in the catalytic regulation of elec-t…     

Quantised charging of monolayer-protected nanoparticles

Metal nanoparticles coated with an organic monolayer, so-called monolayer protected clusters (MPCs), can show quantised charging at room temperature due to their sub-attofarad capacitance arising from the core size and the nature of the protecting monolayer. In this tutorial review, we examine the factors affecting the energetics of MPC charging. In the first section, the underlying physics of quantised charging is outlined and we give an overview of the various methods that can be used to measure single electron transfer to nanoparticles. In the subsequent sections, we discuss how electrochemical measurements can be used to give information on the quantised charging of freely diffusing and films of immobilised MPCs. The predictions of models used to determine MPC capacitance are compared with experimental data from the literature.     

Formation process and solvent-dependent structure of a polyproline self-assembled monolayer on a gold surface

The formation process and structure of a self-assembled monolayer (SAM) of lipoic-acid-terminated polyproline on a gold surface in aqueous solution were investigated by several techniques. The amount of polyproline molecules on the gold surface was determined from the area of the reductive desorption peak, and orientation and thickness of the polyproline SAM were determined in situ by attenuated total reflection infrared (ATR-IR) spectroscopy and ellipsometry. The kinetics of the polyproline SAM formation process were discussed on the basis of these results. The in situ IR study confirmed that the conformation of the polyproline SAM was changed by changing the solvent from water to methanol and methanol to water, as is the case for polyproline dissolved in solution.     

Characterization of the surface structure of gold nanoparticles and nanorods using structure sensitive reactions

The surface structure of gold nanorods has been determined by studying the behavior of electrochemical reactions sensitive to the structure and compared to that obtained by other structure characterization techniques. Lead underpotential deposition (UPD) reveals that the surface of the nanorods is composed by (111) and (110) domains, while (100) domains are practically absent from the surface. In the case of the oxygen reduction reaction, the formation of hydrogen peroxide as a final product of the reaction in the whole potential range also indicates that (100) domains are absent on the surface of the nanoparticles, corroborating the previous result. These results are compared with other surface structure information provided by other techniques.     

Humidity effect on the monolayer-protected gold nanoparticles coated chemiresistor sensor for VOCs analysis

Two gold-thiolate monolayer-protected nanoparticles were synthesized and used as interfacial layers on chemiresistor sensors for the analysis of violate organic compounds (VOCs). Toluene, ethanol, acetone and ethyl acetate were chosen as the target vapors. Both the resistance and capacitance were measured as the function of analyte concentrations. The effect of humidity on the sensor sensitivity to VOCs was investigated. The sensitivity decreases with humidity increasing, depending on the hydrophobicity of the target compounds. Less effect was observed on the higher hydrophobic compounds. While the relative humidity (RH) increased from 0 to 60%, the sensitivity to acetone decreased by 39 and 37%, respectively on the Au-octanethiol (C₈Au) and Au-2-phenylethanethiol (BC₂Au) coated sensors, while the sensitivity to toluene decreased by 12 and 14%, respectively. These results show that the sensors coated with hydrophobic compounds protected-metal nanoparticles can be employed in high humidity for hydrophobic compounds analysis. The resistance responses to VOCs are rapid, reversible, and linear, while the capacitance response is not sensitive and consequently not applicable for VOCs analysis. The response mechanism was also discussed based on the sensor response to water vapor. The capacitance response is not sensitive to the film swelling in dry environment.     

Synthesis of nanoparticle-cored dendrimers by convergent dendritic functionalization of monolayer-protected nanoparticles

This article presents a synthesis method for nanoparticle-cored dendrimers (NCDs), which have dendritic architectures around a monolayer-protected gold nanoparticle. The synthesis method is based on a strategy in which the synthesis of monolayer-protected nanoparticles is followed by adding dendrons on functionalized nanoparticles by a single coupling reaction. NMR spectroscopy, IR spectroscopy, and thermogravimetric analysis (TGA) characterizations confirmed the successful coupling reaction between dendrons with different generations ([G1], [G2], and [G3]) and COOH-functionalized nanoparticles ( approximately Au201L71). The dendrimer wedge density also could be controlled by reacting nanoparticles having different loading of COOH groups ( approximately 60 and approximately 10% COOH of the 71 ligands per gold nanoparticle) with functionalized dendrons. Transmission electron microscope results showed that this synthesis strategy maintains the average size of the nanoparticle core during dendron coupling reactions. This control over the composition and core size makes the systematic study of NCDs with different generations possible. The chemical stability of NCDs was found to be affected by dendron generation around the nanoparticle core. The current-potential response of NCD films on microelectrode arrays exhibited better electrical conductivity for NCDs with lower dendron generation.     

Formation and plasmonic response of self-assembled layers of colloidal gold nanorods and branched gold nanoparticles

The plasmonic properties of self-assembled layers of rod- and branched-shaped gold nanoparticles were investigated using optical techniques. Nanoparticles were synthesized by a surfactant-guided, seed-mediated growth method. The layers were obtained by gradual assembly of nanoparticles at the interface between a polar and a nonpolar solvent and were transferred to a glass slide. Polarization and angle-dependent extinction measurements showed that the layers made of gold nanorods were governed by an effective medium response. The response of the layers made by branched gold particles was characterized by random light scattering. Microscopic mapping of the spatial mode structure demonstrates a uniform optical response of the nanoparticle layers down to a submicrometer length scale.     

Chemometric and microscopic analyses for the size growth of monolayer-protected gold nanoparticles during their superlattice formation

Superlattices of gold nanoparticles have been produced at an air/solution interface under a highly acidic condition. The nanoparticle surface is protected by N-acetylglutathione (NAG). During the course of the superlattice formation, size growth of nanoparticles was observed: The superlattices were composed of nanoparticles of 6.6 nm in core diameter, whereas the as-prepared nanoparticles had the core diameter of 1.4 nm. The growth kinetics was pursued by the time evolution of the UV-vis absorption spectra for the sample solution. The change in the absorption spectral profiles was so small that we conducted principal-component analysis (PCA), which is known as a chemometric technique to resolve (or extract) spectra of minute chemical species submerged in the original spectra. Scanning transmission electron microscopy (STEM) corroborated the PCA results, yielding a successful explanation of the growth scheme of the NAG-protected gold nanoparticles.     

Organic reactions of monolayer-protected metal nanoparticles

This paper presents a concise review of various organic reactions of monolayer-protected metal nanoparticles, with an emphasis on their current applications. Organic reactions of monolayer-protected nanoparticles lead to the functionalized nanoparticles, which exhibit interesting properties such as catalytic, electrochemical, photoresponsive, chemical sensing, and biocompatible properties. To cite this article: D.K.P. Ng, C. R. Chimie 6 (2003).     

Second-harmonic spectroscopy of surface immobilized gold nanospheres above a gold surface supported by self-assembled monolayers

We have investigated linear and nonlinear optical properties of surface immobilized gold nanospheres (SIGNs) above a gold surface with a gap distance of a few nanometers. The nanogap was supported by amine or merocyanine terminated self-assembled monolayers (SAMs) of alkanethiolates. A large second-harmonic generation (SHG) was observed from the SIGN systems at localized surface plasmon resonance condition. The maximum enhancement factor of SHG intensity was found to be 3 x 10(5) for the SIGN system of nanospheres 100 nm in diameter with a gap distance of 0.8 nm. The corresponding susceptibility was estimated to be chi((2))=750 pmV (1.8 x 10(-6) esu). In the SIGN system supported with the merocyanine terminated SAMs, the SHG response was also resonant to the merocyanine in the nanogap. It was found that the SHG response of the SIGN systems is strongly frequency dependent. This leads us to conclude that the large chi((2)) is caused by enhanced electric fields at the localized surface plasmon resonance condition and is not due to an increase of the surface susceptibility following from the presence of the gold nanospheres. The observed SHG was consistent with the theoretical calculations involving Fresnel correction factors, based on the quasistatic approximation.