Correlation between catalytic activity and surface ligands of monolayer protected gold nanoparticles

Gold nanoparticles (GNPs) are known to be a very good catalyst. Also, the anchoring of GNPs with stabilizing ligands is essential for surface modification, tuning of size and shapes, and to prevent from aggregation in suspension. But the effect of ligand on the catalytic property of ligand-capped GNP is yet to be explored in detail. In this paper, we perform an in-depth study of effect of ligands on the catalytic activity of monolayer protected GNPs. For this study, a series of different ligand functionalized GNPs in suspension as well as functionalized GNPs' thin film on glass substrate are prepared and used as catalysts in two model reactions, viz. borohydride reduction of 4-nitrophenol and redox reaction between potassium ferricyanide and sodium thiosulfate. The functionalization of GNPs with any ligand reduces its virgin catalytic activity, no matter whether the GNPs are suspended or supported as thin film. An increase in alkyl chain length of alkanethiols and alkylamines ligands and their graft density to the surface of GNP reduces its catalytic activity. Interestingly, the capping of GNPs with 11-mercaptoundecanoic acid and 11-mercaptoundecanol ligands completely destroys its catalytic activity. The effect of anchoring group of ligand molecules on the catalytic activity of ligand-protected GNPs is also discussed.     

Self-assembly of an octanethiol monolayer on a gold-stepped surface

We investigate the influence of the native staircase nanostructure of a Au(111) vicinal surface upon the self-assembly of alkylthiols. Through a comparison with standard alkylthiol SAMs deposited on Au(111) flat surfaces, we show that on the vicinal surface the octanethiol monolayer (OT SAM) reproduces the nanopatterned staircase structure, giving rise to a new kind of molecular layer self-ordered on the nanometer scale. The SAM's structure is determined by UHV STM and PM-IRRAS measurements and exhibits a specific behavior relative to the nanostructured substrate. The differences from the film grown on Au(111) are attributed to the influence of step edges on the molecular packing, leading to a specific 2D crystallographic order through the step edges.     

Electrical property and water repellency of a networked monolayer film prepared from Au nanoparticles

Gold nanoparticles, modified with alkyl thiol, formed a film on polystyrene substrate, and it was found that the deposited film drastically changes its conductivity and hydrophobicity, depending on the alkyl chain length of the thiol used.     

Heterophase ligand exchange and metal transfer between monolayer protected clusters

This paper describes reactions in which ligands are exchanged and metals are transferred between monolayer-protected metal clusters (MPCs) that are in different phases (heterophase exchange) or are in the same phase. For example, contact of toluene solutions of alkanethiolate-coated gold MPCs with aqueous solutions of tiopronin-coated gold MPCs yields toluene-phase MPCs that have some tiopronin ligands and aqueous-phase MPCs that have some alkanethiolate ligands. In a second example, heterophase transfer reactions occur between toluene solutions of alkanethiolate-coated gold MPCs and aqueous solutions of tiopronin-coated silver MPCs, in which tiopronin ligands are transferred to the former and gold metal to the latter phase. These ligand and metal exchange reactions are inhibited when conducted under N(2). The results implicate participation of an oxidized form of Au (such as a Au(I) thiolate, Au(I)-SR) as both a ligand and metal carrier in the exchange reactions. Au(I)-SR is demonstrated to be an exchange catalyst.     

Mechanistic aspects of ligand exchange in Au nanoparticles

Ligand exchange reaction is a very important and useful tool for preparing functionalised metal nanoparticles. Understanding the mechanism of this process is essential for rational design of nanoparticle-based devices. However the underlying chemistry was found to be very rich. In this article, we summarise the research efforts of several groups to unravel the mechanisms of ligand exchange reaction, and discuss implications for future developments.     

Structural and theoretical basis for ligand exchange on thiolate monolayer protected gold nanoclusters

Ligand exchange reactions are widely used for imparting new functionality on or integrating nanoparticles into devices. Thiolate-for-thiolate ligand exchange in monolayer protected gold nanoclusters has been used for over a decade; however, a firm structural basis of this reaction has been lacking. Herein, we present the first single-crystal X-ray structure of a partially exchanged Au(102)(p-MBA)(40)(p-BBT)(4) (p-MBA = para-mercaptobenzoic acid, p-BBT = para-bromobenzene thiol) with p-BBT as the incoming ligand. The crystal structure shows that 2 of the 22 symmetry-unique p-MBA ligand sites are partially exchanged to p-BBT under the initial fast kinetics in a 5 min timescale exchange reaction. Each of these ligand-binding sites is bonded to a different solvent-exposed Au atom, suggesting an associative mechanism for the initial ligand exchange. Density functional theory calculations modeling both thiol and thiolate incoming ligands postulate a mechanistic pathway for thiol-based ligand exchange. The discrete modification of a small set of ligand binding sites suggests Au(102)(p-MBA)(44) as a powerful platform for surface chemical engineering.     

Electrochemical resolution of 15 oxidation states for monolayer protected gold nanoparticles

The first observation of 15 voltammetric quantized charging peaks for a solution of hexanethiol-capped gold nanoparticles (so-called monolayer protected clusters MPCs) at room temperature is reported where the variation in peak spacing with increasing charge stored in the metal core is discussed in terms of MPC capacitance.     

Synthesis and catalytic properties of soluble platinum nanoparticles protected by a thiol monolayer

Several new platinum monolayer protected clusters (MPCs) have been synthesized and characterized. Two methods of platinum reduction were used depending on the solubility of the thiol: sodium borohydride for the water-soluble thiols and lithium triethylborohydride for the organic soluble thiols. In general, reactant solutions containing a 1:1 thiol/Pt ratio yielded the best particles in a single-phase reaction. Higher thiol/Pt ratios produced lower yields of MPCs, while much lower ratios produced gray-black precipitates. The Pt MPCs were used as catalysts to hydrogenate allyl alcohol to propanol by reducing the carbon-carbon double bond. The Pt-mercaptoammonium MPCs were also used as catalysts in the hydrogenation of maleic acid to succinic acid. Differences in the catalytic hydrogenation rates among the various monolayer coatings for MPCs are attributed to the variations in ligand chain length, branching, charged functional groups, packing density, and core size.     

Electrochemical behaviors of novel electroactive Au nanoparticles protected by self-assembled monolayers

There has been substantial recent interest in studying monolayer-protected gold clusters (MPCs) owing to their diverse applications. The present work is an electrochemical study of novel gold nanoparticles covered with a monolayer of mercapto-dodecanol ended chloro-dicyano-quinone (HS-C12O-CDQ), which was adsorbed on the electrode (CDQ-MPCs film). Our findings reveal a redox behavior for CDQ-MPCs film similar to the solution electrochemistry of dichloro-dicyano-quinone. Furthermore, a diffusion-like mechanism was found for electron transfer, which may have occurred due to proton diffusion towards or outwards the electrode through the film casted. Chronoamperometry confirmed diffusion behavior of the ET process. Finally, EIS was used to find the rate constant of ET process for the redox reaction that occurred and the contribution of MPCs in total interfacial capacitance.     

Double hydrophilic block copolymer monolayer protected hybrid gold nanoparticles and their shell cross-linking

This paper describes the syntheses of core/shell gold nanoparticles stabilized with a monolayer of double hydrophilic block copolymer and their stimuli responsiveness before and after shell cross-linking. The hybrid nanoparticles consist of gold core, cross-linkable poly(2-(dimethylamino)ethyl methacrylate) (PDMA) inner shell, and poly(ethylene oxide) (PEO) corona. First, diblock copolymer PEO-b-PDMA was prepared via the reversible addition-fragmentation chain transfer (RAFT) technique using a PEO-based macroRAFT agent. The dithioester end group of PEO-b-PDMA diblock copolymer was reduced to a thiol end group. The obtained PEO-b-PDMA-SH was then used to prepare diblock copolymer stabilized gold nanoparticles by the "grafting-to" approach. 1,2-Bis(2-iodoethoxy)ethane (BIEE) was utilized to selectively cross-link the PDMA residues in the inner shell. The stimuli responsiveness and colloidal stability of core/shell gold nanoparticles before and after shell cross-linking were characterized by laser light scattering (LLS), UV-vis transmittance, and transmission electron microscopy (TEM). At pH 9, the average hydrodynamic radius Rh of non-cross-linked hybrid gold nanoparticles starts to increase above 35 degrees C due to the lower critical solution temperature (LCST) phase behavior of the PDMA blocks in the inner shell. In contrast, Rh of the shell cross-linked gold nanoparticles were essentially independent of temperature. Core/shell gold nanoparticles before and after shell cross-linking exhibit reversible swelling on varying the solution pH. Compared to non-cross-linked core/shell gold nanoparticles, shell cross-linking of the hybrid gold nanoparticles leads to permanent core/shell nanostructures with much higher colloidal stability and physically isolates the gold core from the external environment.     

Effects of the surface pressure on the formation of Langmuir-Blodgett monolayer of nanoparticles

The effects of the surface pressure on the particle arrangement of Langmuir-Blodgett (LB) monolayers of alkanethiol-capped gold nanoparticles were studied. The LB monolayers were prepared from a highly concentrated particle solution, which increases film fabrication efficiency but readily causes small particle voids in the particle array. Overcompressing the LB monolayer to a high surface pressure restructured the particles and eliminated the voids. When the gold particles capped by dodecanethiol were 8.5 nm in diameter, the particle arrangement was vastly improved and a wafer-scale LB monolayer was transferred onto a substrate at the surface pressure of 20 mN/m.     

Synthesis and characterization of Au and Ag nanoparticles protected by PAMAM and its derivative

1,8-naphthalimide-labelled polyamidoamine dendrimer (PAMAM-N) was synthesized and used to stabilize Au and Ag nanoparticles, which were characterized by UV-Vis absorption and fluorescence spectroscopies, and transmission electron microscopy. The dimensions of Au and Ag nanoparticles protected by PAMAM-N were smaller and the size distribution was narrower when compared to those of polyamidoamine dendrimer. The presence of chromophore fragment (1,8-naphthalimide) allowed the fluorescent labeling of Au and Ag nanoparticles.     

Understanding mercapto ligand exchange on the surface of FePt nanoparticles

Tailoring the surface of nanoparticles is essential for biological applications of magnetic nanoparticles. FePt nanoparticles are interesting candidates owing to their high magnetic moment. Established procedures to make FePt nanoparticles use oleic acid and oleylamine as the surfactants, which make them dispersed in nonpolar solvents such as hexane. As a model study to demonstrate the modification of the surface chemistry, stable aqueous dispersions of FePt nanoparticles were synthesized after ligand exchange with mercaptoalkanoic acids. This report focuses on understanding the surface chemistry of FePt upon ligand exchange with mercapto compounds by conducting X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) studies. It was found that the mercapto end displaces oleylamine on the Pt atoms and the carboxylic acid end displaces the oleic acid on the Fe atoms, thus exposing carboxylate and thiolate groups on the surface that provide the necessary electrostatic repulsion to form stable aqueous dispersions of FePt nanoparticles.     

Self-assembly of phospholipid molecules at a Au(111) electrode surface

We described the first scanning tunneling microscopy study of spreading unilamellar vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) at a Au(111) electrode surface. At the initial stage of the film formation, the molecular resolution images revealed that DMPC molecules are adsorbed flat with the acyl chains oriented parallel to the surface. The molecules assemble into double rows by aligning the acyl chains in the nearest neighbor direction of the reconstructed Au(111) surface and assuming a 90 +/- 10 degrees angle with respect to line of the molecular row. After approximately 30 min, this film is transformed into a hemimicellar state with long rows characteristic for the formation of hemicylindrical surface micelles. At hydrophilic surfaces such as glass, spreading of vesicles involves adsorption, rupture, and sliding of a single bilayer on a lubricating film of the solvent. We have provided the first evidence that a different mechanism is involved in spreading the vesicles at gold. The molecules released by rupture of vesicles self-assemble into an ordered film, and the assembly is controlled by the chain-substrate interaction.     

Synthesis of TiO2 nanoparticles on the Au(111) surface

The growth of titanium oxide nanoparticles on reconstructed Au(111) was investigated by scanning tunneling microscopy and x-ray photoelectron spectroscopy. Ti was deposited by physical-vapor deposition at 300 K. Regular arrays of titanium nanoparticles form by preferential nucleation of Ti at the elbow sites of the herringbone reconstruction. The titanium oxide nanoclusters were synthesized by subsequent exposure to O(2) at 300 K. Two-and three-dimensional titanium oxide nanocrystallites form during annealing in the temperature range from 600 to 900 K. At the same time, the Au(111) surface assumes a serrated 110-oriented step-edge morphology suggesting step-edge pinning by titanium oxide nanoparticles. The oxidation state of the titanium oxide nanoparticles varies with annealing temperature. Specifically, annealing to 900 K results in the formation of stoichiometric TiO(2) nanocrystals as judged by the Ti(2p) binding energies measured in the x-ray photoelectron data. The nanodispersed TiO(2) on Au(111) is an ideal system to test the various models proposed for the enhanced catalytic reactivity of supported Au nanoparticles.     

Characterization of surface water on Au core Pt-group metal shell nanoparticles coated electrodes by surface-enhanced Raman spectroscopy

We utilized the strategy of 'borrowing SERS activity', by chemically coating several atomic layers of a Pt-group metal on highly SERS-active Au nanoparticles, to obtain the first SERS (also Raman) spectra of surface water on Pt and Pd metals, and propose conceptual models for water adsorbed on Pt and Pd metal surfaces.     

Ethylene glycol monolayer protected nanoparticles: synthesis, characterization, and interactions with biological molecules

The usefulness of the hybrid materials of nanoparticles and biological molecules on many occasions depends on how well one can achieve a rational design based on specific binding and programmable assembly. Nonspecific binding between nanoparticles and biomolecules is one of the major barriers for achieving their utilities in a biological system. In this paper, we demonstrate a new approach to eliminate nonspecific interactions between nanoparticles and biological molecules by shielding the nanoparticle with a monolayer of ethylene glycol. A direct synthesis of di-, tri-, and tetra(ethylene glycol)-protected gold nanoparticles (Au-S-EGn, n = 2, 3, and 4) was achieved under the condition that the water content was optimized in the range of 9-18% in the reaction mixture. With controlled ratio of [HAuCl4]/[EGn-SH] at 2, the synthesized particles have an average diameter of 3.5 nm and a surface plasma resonance band around 510 nm. Their surface structures were confirmed by 1H NMR spectra. These gold nanoparticles are bonded with a uniform monolayer with defined lengths of 0.8, 1.2, and 1.6 nm for Au-S-EG2, Au-S-EG3, and Au-S-EG4, respectively. They have great stabilities in aqueous solutions with a high concentration of electrolytes as well as in organic solvents. Thermogravimetric analysis revealed that the ethylene glycol monolayer coating is ca. 14% of the total nanoparticle weight. Biological binding tests by using ion-exchange chromatography and gel electrophoresis demonstrated that these Au-S-EGn (n = 2, 3, or 4) nanoparticles are free of any nonspecific bindings with various proteins, DNA, and RNA. These types of nanoparticles provide a fundamental starting material for designing hybrid materials composed of metallic nanoparticles and biomolecules.     

Effects of nanoparticles on the ultrafiltration of surface water

The effects of nanoparticles on the fouling behavior of UF membranes were investigated by filtering river water containing natural organic matter (NOM). Self-dispersible carbon black (70–200 nm) was employed to model nanoparticles in natural water. The presence of nanoparticles transformed the mode of initial fouling from internal pore adsorption of NOM to intermediate pore blocking, which caused a significant flux reduction. The use of powdered activated carbon to adsorb organic micromolecules reduced internal pore fouling, but this effect on initial fouling mode did not much mitigate the overall flux decline. As filtration proceeded, cake filtration became the dominant fouling mode. The resistance-in-series model revealed that boundary-layer resistance contributed significantly to increased filtration resistance in the filtration of river water. The nanoparticles nullified boundary-layer resistance plausibly by removing organic macromolecules from river water, but aggravated cake resistance, which required chemical cleaning. Addition of calcium significantly increased the aggregate size of nanoparticles from 0.18–0.35 μm to 3.4 μm, and thus reduced pore blocking and total cake resistance.     

Accelerating the initial rate of hydrolysis of methyl parathion with laser excitation using monolayer protected 10 nm Au nanoparticles capped with a Cu(bpy) catalyst

Using a low power green laser, we have demonstrated a rate acceleration of ~2-fold for the hydrolysis of methyl parathion by irradiating the plasmon absorption band of Au nanoparticles capped with a Cu(bpy) catalyst.     

Size and surface effects of prussian blue nanoparticles protected by organic polymers

Prussian blue (PB) nanoparticles protected by organic polymers such as poly(vinylpyrrolidone) (PVP) and poly(diallyldimethylammonium chloride) (PDDA) were prepared. Different experimental conditions (concentrations of Fe ions and feed ratios of Fe to the polymers) have been investigated to control the size of the PB nanoparticles. For example, the averaged dimensions of the PB nanoparticles were tuned from 12 to 27 nm by use of PVP in the different conditions. Addition of PDDA produced the PB nanoparticles with very small dimensions (5-8 nm) by an effective electrostatic interaction. We found that the surface environments of the PB nanoparticles affect the inherent properties of PB. The shifts of charge transfer (CT) absorptions from Fe(2+) to Fe(3+) in the PB nanoparticles resulted from the surface-protecting polymers. Especially, the PB nanoparticles with the PVP protection show high solubility in a variety of organic solvents and a solvent-dependent CT absorption. Measurement of the magnetic properties of the PB nanoparticles showed unprecedented size-dependency, surface effect, and superparamagnetic properties.