Substituent effects on the exchange dynamics of ligands on 1.6 nm diameter gold nanoparticles

The kinetics of exchange ofphenylethanethiolate ligands (PhC2S) of monolayer-protected clusters (MPCs, average formula Au140(PhC2S)53) by para-substituted arylthiols (p-X-ArSH) are described. 1H NMR measurements of thiol concentrations show that the exchange reaction is initially rapid and gradually slows almost to a standstill. The most labile ligands, exchanging at the shortest reaction times, are thought to be those at defect sites (edges, vertexes) on the nanoparticle core surface. The pseudo-first-order rate constants derived from the first 10% of the exchange reaction profile vary linearly with in-coming arylthiol concentration, meaning that the labile ligands exchange in a second-order process, which is consistent with ligand exchange being an associative process. A linear Hammett relationship with slope p = 0.44 demonstrates a substituent effect in the ligand place exchange reaction, in which the bimolecular rate constants increase for ligands with electron-withdrawing substituents (1.4 x 10-2 and 3.8 x 10(-3) M(-1) s(-1) for X = NO2 and 4-OH, respectively). This is interpreted as the more polar Au-S bonds at the defect sites favoring bonding with more electron deficient sulfur moieties. At longer reaction times, where ligands exchange on nondefect (terrace) as well as defect sites, the extent of ligand exchange is higher for thiols with more electron-donating substituents. The difference between short-time kinetics and longer-time pseudoequilibria is rationalized based on differences in Au-S bonding at defect vs nondefect MPC core sites. The study adds substance to the mechanisms of exchange of protecting ligands on nanoparticles. The scope and limitations of 1H NMR spectroscopy for determining rate data are also discussed.     

Dynamic and static quenching of fluorescence by 1-4 nm diameter gold monolayer-protected clusters

How the efficiency of molecular quenching by Au nanoparticles depends on nanoparticle size is reported for (a) dynamic (collisional) quenching of four different fluorophores by three Au nanoparticles having similar protective layers but differing core diameters (1.1, 1.6, and 2.0 nm) and (b) static quenching in the electrostatic association between [Ru(bpy)3]2+ and five tiopronin-protected Au nanoparticles having core diameters from 1.3 to 3.9 nm. The quenching constants systematically increase with core size. In (a), the dynamic constants scale with the molar absorbance coefficients of the nanoparticles, showing the essentially of the absorbance/emission spectral overlap, and the associated nanoparticle core density of electronic states, in energy-transfer quenching. In (b), the fluorescence of the Au nanoparticle itself was enhanced by energy transfer from the [Ru(bpy)3]2+ fluorophore.     

Size-controlled synthesis of near-monodisperse gold nanoparticles in the 1-4 nm range using polymeric stabilizers

We report here a simple one-step protocol for the preparation of near-monodisperse gold hydrosols in the small size regime (<5 nm). The particle size can be controlled by varying the concentration of the stabilizing polymer, which can be readily displaced by thiol ligands to yield monolayer protected clusters of the usual type.     

Equilibrium morphology of face-centered cubic gold nanoparticles >3 nm and the shape changes induced by temperature

Many of the unique properties of metallic nanoparticles are determined not only by their finite size but also by their shape, defined by the crystallographic orientation of the surface facets. These surfaces (and therefore the nanoparticles themselves) may differ in a number of ways, including surface atom densities, electronic structure, bonding, chemical reactivities, and thermodynamic properties. In the case of gold, it is known that the melting temperature of nanoparticles strongly depends on the crystal size and that the shape may alter considerably (and yet somewhat unpredictably) during annealing. In this work we use first principle calculations and a thermodynamic model to investigate the morphology of gold nanoparticles in the range 3-100 nm. The results predict that the equilibrium shape of gold nanoparticles is a modified truncated octahedron and that the (size-dependent) melting of such particles is preceded by a significant change in the nanoparticle's morphology.     

Synthesis of hybrid nanoparticles based on magnetic Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> nanoparticles and luminescent CdS nanoparticles

A new approach to the synthesis of hybrid nanoparticles based on magnetic Fe₃O₄ nanoparticles and CdS quantum dots, combining magnetic and luminescence properties, has been suggested. Conditions for preparation of their stable aqueous suspensions have been found, and their optical properties have been studied. Nanocomposites produced at the molar ratio Fe₃O₄: CdS = 5: 1, which exhibited the luminescence properties) and gave stable aqueous suspensions, have turned out to be most promising. The results are evidence that the synthesized nanoparticles can be used for the development of visualizing agents for in vitro biomedical research.     

Size control in the synthesis of 1-6 nm gold nanoparticles via solvent-controlled nucleation

We report a facile synthetic route for size-controlled preparation of gold nanoparticles. Nearly monodisperse gold nanoparticles with core diameters of 1-6 nm were obtained by reducing AuP(Phenyl)(3)Cl with tert-butylamine borane in the presence of dodecanethiol in the solvent mixture of benzene and CHCl(3). Mechanism studies have shown that the size control is achieved by the solvent-controlled nucleation in which the nuclei concentration increases with increasing the fraction of CHCl(3), leading to smaller particles. It was also found that, following the solvent-controlled nucleation, particle growth occurs via ligand replacement of PPh(3) on the nuclei by Au(I)thiolate generated by the digestive etching of small particles. This synthetic strategy was successfully demonstrated with other alkanethiols of different chain length with which size-controlled, monodisperse gold nanoparticles were prepared in remarkable yield without requiring any postsynthesis treatments.     

Effect of surface oxidation on the interaction of 1-methylaminopyrene with gold nanoparticles

The effect of the surface chemistry of gold nanoparticles (GNPs) on the GNP-amine (-NH(2)) interaction was investigated via conjugating an amine probe--1-methylaminopyrene (MAP) chromophore--with three Au colloidal samples of the same particle size yet different surface chemistry. The surface of laser-irradiated and ligand-exchanged-irradiated GNPs is covered with acetonedicarboxylic ligands (due to laser-introduced citrate oxidization) and citrate ligands, respectively, and both surfaces contain oxidized Au species which are essentially lacking for the citrate-capped GNPs prepared by the pure chemical approach. Both laser-irradiated samples show inferior adsorption capacity of MAP as compared with the purely chemically prepared GNPs. Detailed investigations indicate that MAP molecules mainly complex directly with Au atoms via forming Au-NH(2)R bonds, and the oxidization of the GNP surface strongly influences the ratio of this direct bonding to the indirect bonding originating from the electrostatic interaction between protonated amine (-NH(3)(+)) and negatively charged surface ligands. The impact of the oxidized GNP surface associated with the laser treatment is further confirmed by aging experiment on GNP-MAP conjugation systems, which straightforwardly verifies that the surface oxidation leads to the decrease in the MAP adsorption on GNPs.     

Microwave-induced synthesis of highly dispersed gold nanoparticles within the pore channels of mesoporous silica

Highly dispersed gold nanoparticles have been incorporated into the pore channels of SBA-15 mesoporous silica through a newly developed strategy assisted by microwave radiation (MR). The sizes of gold are effectively controlled attributed to the rapid and homogeneous nucleation, simultaneous propagation and termination of gold precursor by MR. Diol moieties with high dielectric and dielectric loss constants, and hence a high microwave activation, were firstly introduced to the pore channels of SBA-15 by a simple addition reaction between amino group and glycidiol and subsequently served as the reduction centers for gold nanoparticles. Extraction of the entrapped gold from the nanocomposite resulted in milligram quantities of gold nanoparticles with low dispersity. The successful assembly process of diol groups and formation of gold nanoparticles were monitored and tracked by solid-state NMR and UV-vis measurements. Characterization by small angle X-ray diffraction (XRD) and transmission electron microscopy (TEM) indicated that the incorporation of gold nanoparticles would not breakup the structural integrity and long-range periodicity of SBA-15. The gold nanoparticles had a narrow size distribution with diameters in the size range of 5-10 nm through TEM observation. The average particles size is 7.9 nm via calculation by the Scherrer formula and TEM measurements. Nitrogen adsorption and desorption isotherms gave further evidence that the employed method was efficient and gold nanoparticles were successfully incorporated into the pore channels of SBA-15.     

Concomitant synthesis of polyaniline and highly branched gold nanoparticles in the presence of DNA

The reduction of chloroauric acid using aniline adsorbed on DNA produces highly branched dendritic gold nanoparticles with concomitant formation of polyaniline (PANI) in contrast to the formation of spherical Au nanoparticles in the absence of DNA. The conformation of DNA remains intact in the process as evident from circular dichroism (CD) spectra. The UV-Vis spectrum exhibits a broad absorption peak at 520-900 nm, for a combined effect of the gold surface plasmon and π band to localized polaron band transition of DNA-doped PANI. Both the dendritic Au-PANI-DNA and the spherical Au-PANI systems emit two peaks for excitation with radiation of 300 nm and the intensity ratio of the emission and FRET peak is higher in the dendritic Au-PANI than that in the spherical Au-PANI system. The dc-conductivity values of spherical Au-PANI and dendritic Au-PANI-DNA systems are 1.2×10(-10) and 1.7×10(-8) S/cm at 30°C, respectively.     

Using micellar electrokinetic chromatography for the highly efficient preconcentration and separation of gold nanoparticles

In this paper, we report the use of micellar electrokinetic chromatography (MEKC) for the highly efficient preconcentration and separation of gold nanoparticles (Au NPs). We used the reversed electrode polarity stacking mode (REPSM) of the MEKC system for the on-line enhancement and separation of the Au NPs. Several parameters had dramatic effects on the systems’ performance, including the concentration of sodium dodecylsulfate (SDS) surfactant, the presence of salts in the NP solution, the pH of the running electrolyte, and the temperature of the capillary. Under the optimized conditions [buffer: SDS (70 mM) and 3-cyclohexylamino-1-propanesulfonic acid (CAPS; 10 mM) at pH 10.0; applied voltage: 20 kV; operating temperature: 25 °C; additive: sodium dihydrogenphosphate (NaH₂PO₄, 10 mM); REPSM strategy for sample preconcentration], the number of theoretical plates for the 5.3- and 40.1-nm-diameter Au NPs were 3000 and (an ultrahigh) 2.1 × 10⁶, respectively; in addition, the detection sensitivities toward the Au NPs were enhanced ca. 20- and 380-fold, respectively, relative to those obtained using standard MEKC analysis conditions. Furthermore, monitoring the electropherograms using diode-array detection allowed us to identify and characterize the sizes of the separated NPs from their UV–vis spectra. Our findings suggest that MEKC is a highly efficient tool for both the preconcentration and separation of NPs.     

Thioether moiety functionalization of mesoporous silica films for the encapsulation of highly dispersed gold nanoparticles

Highly dispersed gold nanoparticles within mesoporous thin films (MTFs) have been synthesized through a newly developed controllable strategy, in which (1,4)-bis(triethoxysilyl)propane tetrasufide (BPTS) organosiloxane coupling agent was co-assembled with tetraethyl orthosilicate (TEOS) to form organic groups functionalized mesoporous composite films followed with oxidization, ion-exchange with Au(en)₂Cl₃ (en: 1,2-ethanediamine) compound and calcination under hydrogen/nitrogen mixing atmosphere. Small-angle X-ray diffraction (XRD) characterization indicated that up to 10 mol% of BPTS could be incorporated into mesoporous hybrid films, and that would not breakup the structural integrity and long-range periodicity. The loaded gold nanoparticles were uniformly distributed due to the molecular level homogenous mixing of the BPTS precursor with TEOS, and its concentration could be controlled via the original ratio of BPTS to TEOS. The nanoparticles had a narrow size distribution with diameters in the size range of 3-7 nm through transmission electron microscopy (TEM) observation and underwent a slight size increase with the higher gold load level. An overall increase in the absorption intensity, a red shift of absorption peak, together with a comparatively narrower bandwidth could be observed at higher gold concentration within composite films from UV-vis spectra. Wide-angle XRD, TEM, X-ray photoelectron spectroscopy (XPS) and UV-vis spectra characterizations all agreed on the fact that the gold loading level could be controlled by the amount of BPTS in the starting sol for preparing MTFs.     

Reductant-directed formation of PS-PAMAM-supported gold nanoparticles for use as highly active and recyclable catalysts for the aerobic oxidation of alcohols and the homocoupling of phenylboronic acids

Polystyrene-polyamidoamine-supported gold nanoparticles were prepared using a reductant-directed formation strategy. The resulting catalysts exhibited excellent activities in the aerobic oxidation of benzyl alcohols and the homocoupling of phenylboronic acids under mild conditions and can be recycled at least 14 times without significant loss of activity.     

Preparation of highly loaded platinum nanoparticles on silica by intercalation of [Pt(NH<Subscript>3</Subscript>)<Subscript>4</Subscript>]<Superscript>2+</Superscript> ions into layered sodium silicate ilerite

Highly loaded Pt nanoparticles (20 wt%) on silica were prepared by intercalation of [Pt(NH₃)₄]²⁺ ions into layered sodium silicate ilerite at room temperature, followed by calcination of the intercalated ilerite ([Pt(NH₃)₄]²⁺ ilerite) in air at 380 °C for 5 h. Transmission electron microscopy revealed that the size of the Pt particles was in the range 2-5 nm.     

Enhancement of the electrocatalytic activity of gold nanoparticles via NaBH4 treatment

We report on the enhancement of the electrocatalytic activity of Au nanoparticles after NaBH(4) treatment and its application to H(2)O(2) detection.     

Dendritic effects on the ordered assembly and the interfacial one-electron oxidation of redox-active dendron-functionalized gold nanoparticles

A higher generation dendron with a long-alkyl chain thiol (DA2-SH) induced the formation of nanoparticles with a small core with quite a narrow size distribution (1.5 +/- 0.1 nm), the self-assembly of one-dimensional arrays of these gold nanoparticles (DA2-Au), and the stabilization for the formation of the radical cation of the phenothiazine of DA2-Au nanoparticles from the interfacial one-electron oxidation of the nanoparticles with NOBF4.     

pH-Sensitive assembly/disassembly gold nanoparticles with the potential of tumor diagnosis and treatment

Due to the ability to combine the separately unique characteristics of assembled and disassembled nanoparticles(NPs), the stimuli-responsive self-assembly of NPs has attracted considerable interest in functional material applications especially biomaterials. Here we demonstrate a facile and versatile approach to regulate the self-assembly process and transition pH of Au NPs by fine-tuning the co-modified pH-responsive compounds and poly(ethylene glycol)(PEG). Importantly the transition pH(ΔpH=0.4) of the system can be predetermined in the range of 8.2–5.8(assembled to disassembled) and 8.2–4.2(disassembled to assembled), which ideally covers the pH of normal tissue, tumor tissue milieu and organelles. The results of fluorescence imaging, Raman spectroscopy and photothermal conversion of the stimuli-responsive Au NPs shows the potential application for tumor specificity theranostics. In a nutshell this study provides a useful toolkit to design tumor-activatable self-assembled NPs with high specificity and universality.     

Colorimetric determination of hypochlorite with unmodified gold nanoparticles through the oxidation of a stabilizer thiol compound

In this article, we report a colorimetric approach for the determination of hypochlorite (OCl(-)) with gold nanoparticles (Au NPs). The test proceeds as two individual steps and selectivity is developed based on the strong oxidizing ability of hypochlorite. In concentrated phosphate buffer (PB), the red solution of citrate-capped Au NPs could be stabilized by the chemisorption of 11-mercaptoundecanoic acid (MUA), without which the colloidal suspension turned blue because of salt-induced particles aggregation. However, by its oxidizing power, OCl(-) converted the alkanethiol to a sulfonate derivative, which could not protect Au NPs from aggregation, thereby a blue solution was observed after the subsequent addition of Au suspension. With this method and under the optimal conditions (28 nm Au NP, 50 mM PB, pH 7.0, and 10 min for the colorimetric response), 1.5 μM of OCl(-) can be easily visualized by the naked eye. This sensitive and selective colorimetric assay opens up a fresh insight of facile, rapid, and reliable detection of OCl(-), and may find its future application in the monitoring of OCl(-)/HOCl in waters sanitized by chlorine or hypochlorite compounds.     

Aerobic oxidation of alcohols over hydrotalcite-supported gold nanoparticles: the promotional effect of transition metal cations

Chromium (III)-containing hydrotalcites show strong synergy with gold nanoparticles in achieving high activity in the aerobic oxidation of alcohols.