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.     

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.     

Synthesis and surface structure of thymine-functionalized, self-assembled monolayer-protected gold nanoparticles

Thymine-functionalized SAM-protected gold nanoparticles with diameters of 2.2 +/- 0.3 nm and 7.0 +/- 1.0 nm were prepared via a modified two-phase transfer method. UV-vis spectra showed that particle size and solvent type, as well as surface charge, influenced the gold surface plasmon band absorption, along with the interaction between thymine terminal groups in the solution. Although the bulky thymine end groups interacted strongly on the particle surface, a well-ordered monolayer of thyminethiol derivatives with a long hydrocarbon chain was formed on the particle surface, exhibiting an ordered, all-trans conformation of the methylene backbone, similar to those of corresponding self-assembled monolayers (SAMs) generated from normal alkanethiols. A larger particle size and a longer reaction time facilitated the formation of more ordered thymine-terminated thiol SAMs. Thermal analysis indicated that reorientation of the SAMs during heat treatment occurred by two processes, caused possibly by the separate recrystallization of the hydrocarbon long chains and thymine units. More ordered SAMs with a higher thermal stability were formed on the larger particle surfaces when compared with those on the smaller ones. A greater density of molecular packing was found on the smaller particle surfaces. However, SAMs formed on the larger gold particles resembled 2D SAMs on the smooth, flat gold surfaces. XPS results confirmed the thymine structure as well as the chemical bond between gold and sulfur. One type of adsorbed sulfur species was observed for the smaller particles and two for the larger ones, but a slightly higher binding energy of thiolate was found for the smaller ones.     

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.     

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.     

Inclusion-water-cluster in a three-dimensional superlattice of gold nanoparticles

Three-dimensional particle crystals made of dicarboxylic thiolate (MSA)-stabilized gold nanoparticles were formed at an air/water interface. FTIR spectra of this supracrystal showed that three well-resolved peaks existed from 3400 to 3550 cm-1 just falling in a region of OH stretching vibrational mode. Precise analysis showed that all these peaks originated from the water cluster included in the interstice of a particle crystal.     

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.      

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…     

Fine-tuning size of gold nanoparticles by cooling during reverse micelle synthesis

By lowering the reaction temperature during metal ion reduction in a reverse micelle system, gold nanoparticle size can be subtly tuned from 6.6 to 2.2 nm in diameter. Under these reaction conditions, the water-to-surfactant ratio (W value) also plays an important role in controlling the particle size, enabling a wide range of products obtainable via a simple, quick, reproducible synthesis. Particle sizes were measured by HRTEM, and size trends were supported by UV-vis spectroscopy.     

Biocompatibility of gold nanoparticles and their endocytotic fate inside the cellular compartment: a microscopic overview

Macrophages are one of the principal immune effector cells that play essential roles as secretory, phagocytic, and antigen-presenting cells in the immune system. In this study, we address the issue of cytotoxicity and immunogenic effects of gold nanoparticles on RAW264.7 macrophage cells. The cytotoxicity of gold nanoparticles has been correlated with a detailed study of their endocytotic uptake using various microscopy tools such as atomic force microscopy (AFM), confocal-laser-scanning microscopy (CFLSM), and transmission electron microscopy (TEM). Our findings suggest that Au(0) nanoparticles are not cytotoxic, reduce the production of reactive oxygen and nitrite species, and do not elicit secretion of proinflammatory cytokines TNF-alpha and IL1-beta, making them suitable candidates for nanomedicine. AFM measurements suggest that gold nanoparticles are internalized inside the cell via a mechanism involving pinocytosis, while CFLSM and TEM studies indicate their internalization in lysosomal bodies arranged in perinuclear fashion. Our studies thus underline the noncytotoxic, nonimmunogenic, and biocompatible properties of gold nanoparticles with the potential for application in nanoimmunology, nanomedicine, and nanobiotechnology.     

Complex formation between cationically modified gold nanoparticles and DNA: an atomic force microscopic study

Atomic force microscopy has been used for direct visualization of the wrapping of DNA around 30-nm-sized functionalized gold nanoparticles for the first time. The morphology of the complexes seems to be dictated by the relative concentration of the nanoparticles and DNA. A higher concentration of the former leads to the formation of a network of nanoparticles assembled on DNA. This assembly pattern seems to be significantly different from the manner in which cationically modified gold nanoparticles of smaller size (< 5 nm) arrange linearly on DNA, as shown in the literature. A DNA-gold nanoparticle can be developed as a model system for in vitro studies on the mechanism of DNA condensation and also for developing novel methods of nanoparticle self-assembly on the DNA template.     

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.     

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.     

In situ formation and size control of gold nanoparticles into chitosan for nanocomposite surfaces with tailored wettability

The in situ formation of gold nanoparticles into the natural polymer chitosan is described upon pulsed laser irradiation. In particular, hydrogel-type films of chitosan get loaded with the gold precursor, chloroauric acid salt (HAuCl(4)), by immersion in its aqueous solution. After the irradiation of this system with increasing number of ultraviolet laser pulses, we observe the formation of gold nanoparticles with increasing density and decreasing size. Analytical studies using absorption measurements, atomic force microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy of the nanocomposite samples throughout the irradiation procedure reveal that under the specific irradiation conditions there are two competing mechanisms responsible for the nanoparticles production: the photoreduction of the precursor responsible for the rising growth of gold particles with increasing size and the subsequent photofragmentation of these particles into smaller ones. The described method allows the localized formation of gold nanoparticles into specific areas of the polymeric films, expanding its potential applications due to its patterning capability. The size and density control of the gold nanoparticles, obtained by the accurate increase of the laser irradiation time, is accompanied by the simultaneously controlled increase of the wettability of the obtained gold nanocomposite surfaces. The capability of tailoring the hydrophilicity of nanocomposite materials based on natural polymer and biocompatible gold nanoparticles provides new potentialities in microfluidics or lab on chip devices for blood analysis or drugs transport, as well as in scaffold development for preferential cells growth.     

Stabilization and size control of gold nanoparticles during laser ablation in aqueous cyclodextrins

Femtosecond laser radiation has been used to ablate a gold target in aqueous beta-cyclodextrin (CD) solutions to produce stable gold nanoparticle colloids with extremely small size (2 to 2.4 nm) and size dispersion (1 to 1.5 nm). On the basis of XPS and zeta-potential measurements, we propose a model involving chemical interactions between the gold and the CDs. The model includes both the hydrophobic interaction of the Au0 with the interior cavity of the CD and the hydrogen bonding of O- groups on the partially oxidized gold surface with -OH groups of the CDs.     

Carbosilane dendrons as stabilizing agents for the formation of gold nanoparticles

The synthesis and characterization of two new carbosilane dendrons functionalized in the focal point with a single thiol group is described. These molecules were used as stabilizing agents for the formation of gold nanoparticles. These materials were obtained by the reduction of hydrogen tetrachloroaurate phase-transferred into toluene in the presence of the carbosilane dendrons. The gold nanoparticle-cored dendrons (NCDs) were characterized by TEM, XPS, AFM, IR, UV, and NMR techniques. Average particle diameter ranged from 2.5 to 2.6 nm with narrow size distribution and remarkable stability; in particular, the elimination of the protecting shells to give naked gold, after NCDs deposition on a support, is more difficult using carbosilane dendrons than in the case of conventional dodecanethiol-protected gold nanoparticles.     

Spectroscopic and microscopic investigation of gold nanoparticle formation: ligand and temperature effects on rate and particle size

We report a spectroscopic and microscopic investigation of the synthesis of gold nanoparticles (AuNPs) with average sizes of less than 5 nm. The slow reduction and AuNP formation processes that occur by using 9-borabicyclo[3.3.1]nonane (9-BBN) as a reducing agent enabled a time-dependent investigation based on standard UV-vis spectroscopy and transmission electron microscopy (TEM) analyses. This is in contrast to other borohydride-based syntheses of thiolate monolayer protected AuNPs which form particles very rapidly. We investigated the formation of 1-octadecanethiol (ODT) protected AuNPs with average diameters of 1.5-4.3 nm. By studying the progression of nanoparticle formation over time, we find that the nucleation rate and the growth time, which are interlinked with the amount of ODT and the temperature, influence the size and the size dispersion of the AuNPs. High-resolution TEM (HRTEM) analyses also suggest that the nanoparticles are highly single crystalline throughout the synthesis and appear to be formed by a diffusion-controlled Ostwald-ripening growth mechanism.     

Surface-functionalized silica-coated gold nanoparticles and their bioapplications

Monodisperse Au at SiO₂ nanoparticles has been functionalized with carboxylic groups for further bioconjugation with amino-terminated oligonucleotides. The oligonucleotide-modified Au at SiO₂ nanoprobes have been applied in the fast colorimetric DNA based on the sequence-specific hybridization properties of DNA. Self-assembling behavior of Au at SiO₂ nanoparticles was also investigated.     

Extinction coefficients of gold nanoparticles and their dimers. Dependence of optical factor on particle size

The generalized Mie theory has been employed to calculate extinction coefficients κ for isolated gold nanoparticles of different sizes (4–80 nm) and their dimers (κ₂) at the maximum of the short-wave plasmon resonance band. It has been found that the value of κ₂ essentially depends on both interparticle distance s and particle sizes R. According to the character of variations in the κ₂(s) dependence, three ranges of the distances are distinguished, i.e., large, intermediate, and small. In the first range, the κ₂ values slightly differ from doubled κ values. Nevertheless a tendency toward an increase in κ₂ is distinctly seen as the particles approach each other, and, within some range of s values and for particles with radii R < 15 nm, κ₂ is higher than 2κ. For dimers of larger particles, κ₂ < 2κ, with its value gradually decreasing with a reduction in the s value. The behavior changes when the particle sizes are of about 50 nm. In the range of small interparticle distances, κ₂ values slightly vary with the distance between the particles somewhat decreasing or oscillating with a small amplitude about some mean value. In this range, as the sizes of the gold particles grow, the extinction coefficients of dimers increasingly deviate from the sum of the extinction coefficients of the particles composing the dimers. For 20–80 nm nanoparticles, the size-dependence plotted for the extinction efficiency of dimers in logarithmic coordinates within the range of small interparticle distances is described by a straight line, the slope of which (1.036 ± 0.039) appears to be somewhat smaller than the slope of a corresponding line for individual gold particles (1.274 ± 0.014). The ratio between κ₂ and κ predetermines the character of variations in the optical factor as depending on the particle size and the interparticle distance and governs the behavior of the sol turbidity at the stage of nanoparticle dimerization.