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

Cationic Polymer Coating Increases the Catalytic Activity of Gold Nanoparticles toward Anionic Substrates

Organic coatings on catalytic metal nanoparticles (NPs) typically hinder their activity due to the blocking of active sites. Therefore, considerable effort is made to remove organic ligands when preparing supported NP catalytic materials. Here, cationic polyelectrolyte coatings are shown to increase the catalytic activity of partially embedded gold nanoislands (Au NIs) toward transfer hydrogenation and oxidation reactions with anionic substrates compared to the activity of identical but uncoated Au NIs. Any potential steric hindrance caused by the coating is countered by a decrease in the activation energy of the reaction by half, resulting in overall enhancement. The direct comparison to identical but uncoated NPs isolates the role of the coating and provides conclusive evidence of enhancement. Our findings show that engineering the microenvironment of heterogeneous catalysts, creating hybrid materials that cooperatively interact with the reactants involved, is a viable and exciting path to improving their performance.

Features of Hydrogen Reduction of Fe(CN)63- Ions in Aqueous Solutions: Effect of Hydrogen Dissolved in Palladium Nanoparticles

Preliminary saturation of 2.6 nm palladium nanoparticles with hydrogen accelerates the reduction of Fe(CN)₆³⁻ ions in aqueous solution three to four-fold. An analytical equation was derived describing the hydrogen saturation of palladium nanoparticles and the dependence of their catalytic activity on the hydrogen content in the metal. The specific rate constants of reduction do not depend on the content of palladium nanoparticles in the solution. A change in the temperature and pH or stirring of the solution do not affect the rate of catalytic reaction. Approaches to optimization of palladium-catalyzed reactions involving hydrogen are substantiated.

Revealing the Active Site of Gold Nanoparticles for the Peroxidase-Like Activity: The Determination of Surface Accessibility

Despite the fact that the enzyme-like activities of nanozymes (i.e., nanomaterial-based artificial enzymes) are highly associated with their surface properties, little is known about the catalytic active sites. Here, we used the sulfide ion (S2−)-induced inhibition of peroxidase-like activity to explore active sites of gold nanoparticles (AuNPs). The inhibition mechanism was based on the interaction with Au(I) to form Au2S, implying that the Au(I) might be the active site of AuNPs for the peroxidase-like activity. X-ray photoelectron spectroscopy (XPS) analysis showed that the content of Au(I) on the surface of AuNPs significantly decreased after the addition of S2−, which might be contributed to the more covalent Au–S bond in the formation of Au2S. Importantly, the variations of Au(I) with and without the addition of S2− for different surface-capped AuNPs were in good accordance with their corresponding peroxidase-like activities. These results confirmed that the accessible Au(I) on the surface was the main requisite for the peroxidase-like activity of AuNPs for the first time. In addition, the use of S2− could assist to determine available active sites for different surface modified AuNPs. This work not only provides a new method to evaluate the surface accessibility of colloidal AuNPs but also gains insight on the design of efficient AuNP-based peroxidase mimics.

Simulation studies on structural and thermal properties of alkane thiol capped gold nanoparticles

The structural and thermal properties of the passivated gold nanoparticles were explored employing molecular dynamics simulation for the different surface coverage densities of the self-assembled monolayer (SAM) of alkane thiol. The structural properties of the monolayer protected gold nanoparticles such us overall shape, organization and conformation of the capping alkane thiol chains were found to be influenced by the capping density. The structural order of the thiol capped gold nanoparticles enhances with the increase in the surface coverage density. The specific heat capacity of the alkane thiol capped gold nanoparticles was found to increase linearly with the thiol coverage density. This may be attributed to the enhancement in the lattice vibrational energy. The present simulation results suggest, that the structural and thermal properties of the alkane thiol capped gold nanoparticles may be modified by the suitable selection of the SAM coverage density.

Surface Ligand-Mediated Plasmon-Driven Photochemical Reactions

Contrary to the general expectation that surface ligands reduce the reactivity of surfaces by blocking the active sites, we present experimental evidence that surface ligands can in fact increase reactivity and induce important reaction pathways in plasmon-driven surface photochemistry. The remarkable effect of surface ligands is demonstrated by comparing the photochemistry of p-aminothiophenol (PATP) on resonant plasmonic gold nanorods (AuNRs) in the presence of citrate, hexadecyltrimethylammonium bromide (CTAB), and no surface ligands under visible light irradiation. The use of AuNRs with citrate and no surface ligand results in the usual azo-coupling reaction. In contrast, CTAB-coated AuNRs oxidize PATP primarily to p-nitrothiophenol (PNTP). Strong correlation has been observed between the N-O and Au-Br vibration band intensities, suggesting that CTAB influences the reaction pathway through the Br^- counterions that can minimize the electron-hole recombination rate by reacting with the hole and hence increasing the concentration of hot electrons that drive the oxidation reaction.

Tailoring Enzyme-Like Activities of Gold Nanoclusters by Polymeric Tertiary Amines for Protecting Neurons Against Oxidative Stress

The cytotoxicity of nanozymes has drawn much attention recently because their peroxidase-like activity can decompose hydrogen peroxide (H2 O2 ) to produce highly toxic hydroxyl radicals (•OH) under acidic conditions. Although catalytic activities of nanozymes are highly associated with their surface properties, little is known about the mechanism underlying the surface coating-mediated enzyme-like activities. Herein, it is reported for the first time that amine-terminated PAMAM dendrimer-entrapped gold nanoclusters (AuNCs-NH2 ) unexpectedly lose their peroxidase-like activity while still retaining their catalase-like activity in physiological conditions. Surprisingly, the methylated form of AuNCs-NH2 (i.e., MAuNCs-N(+) R3 , where R = H or CH3 ) results in a dramatic recovery of the intrinsic peroxidase-like activity while blocking most primary and tertiary amines (1°- and 3°-amines) of dendrimers to form quaternary ammonium ions (4°-amines). However, the hidden peroxidase-like activity is also found in hydroxyl-terminated dendrimer-encapsulated AuNCs (AuNCs-OH, inside backbone with 3°-amines), indicating that 3°-amines are dominant in mediating the peroxidase-like activity. The possible mechanism is further confirmed that the enrichment of polymeric 3°-amines on the surface of dendrimer-encapsulated AuNCs provides sufficient suppression of the critical mediator •OH for the peroxidase-like activity. Finally, it is demonstrated that AuNCs-NH2 with diminished cytotoxicity have great potential for use in primary neuronal protection against oxidative damage.

The adsorbed state of a thiol on palladium nanoparticles

Imaging, spectroscopy and computation show that 1-dodecanethiol forms largely ordered 1-dodecanethiolate on the surface of palladium nanoparticles.

The induction phenomenon and catalytic deactivation of thiolate-stabilized raspberry-like polymer composites coated with gold nanoparticles

Alkylthiolate ligands play dual roles in metal nanoparticles-coated polymer composite catalysts: stabilizer and deactivator. Herein, individual raspberry-like polymer composite spheres coated with gold nanoparticles were separated from each other in the presence of 6-mercaptohexanoic acid or 3-mercaptopropionic acid ligands. Effects of thiolate ligands on the induction time and the catalytic activity of such non-aggregated polymer composites were investigated experimentally and theoretically in the 4-nitrophenol/NaBH4 model reaction from the following aspects: ligand surface coverage, chain order and chain length. With the increase in alkylthiolate surface coverage and chain order on composite particles, the induction time increases first and then decreases, which can be explained based on spontaneous dynamic surface restructuring and electron injection from borohydride ions to the gold nanoparticle surface. The catalytic activity is compromised with the existence of thiolate ligands, but is enhanced with increasing alkylthiolate ligand coverage, which can be ascribed to sulfur-induced electronic charge depletion of the gold nanoparticles. The increment of CH2 in alkylthiolate chains results in the increase of induction time and the decrease of the catalytic activity, which can be attributed to the steric hindrance effect. The reactant addition sequence was also found to affect the induction time and the catalytic activity, which can be partially credited to NaBH4 reductant-induced desorption of thiolate ligands.

In situ synthesis of ultra-small platinum nanoparticles using a water soluble polyphenolic polymer with high catalytic activity

Ultra-small platinum nanoparticles are generated by in situ polymer reduction technique which shows high catalytic activity in water and in organic solvent.

Interfacial polygonal patterning via surfactant-mediated self-assembly of gold nanoparticles

In this work, we explored the formation processes of interfacial polygonal patterning via surfactant-mediated self-assembly of gold nanoparticles (AuNPs). We found that a balance between DDT-capped AuNPs and PVP-passivated AuNPs is a key to making these inorganic-organic thin films. The interfacial polygonal patterning possesses many processing advantages and flexibilities, such as controllable interfacial shape and inter-AuNP distance, tuning of particle sizes, thiol population, chain lengths, and other new properties by introducing functional groups to thiol chains. In principle, self-assembly of AuNPs via well-designed interfaces may be useful for fabrications of other complex architectures.

Role of gold nanoparticles capping density on stability and surface reactivity to design drug delivery platforms

Five-nanometer sized gold nanoparticles (Au NPs) stabilized with citrate ions have been reacted with various amounts of dihydrolipoic acid (DHLA) (×28, ×56, ×140, ×222, relative to Au NPs). Ligand exchange between citrate and the dithiol resulted in DHLA-capped Au NPs, whose degree of inertia was found to be related to the density of capping. The results revealed the importance of DHLA coating density to enhance the colloidal stability and modulate the reactivity toward free radicals and proteins of biological relevance. Thus, Au NPs capped with the highest amount of DHLA were found to be the ones that were, first, the most resistant to environmental changes, then characterized by the lowest residual catalytic reactivity of their metallic core, and finally the lowest interacting with proteins through nonspecific adsorption. The physicochemical properties conferred to Au NPs prepared with the ×222 excess should be valuable for further pharmaceutical development of nanoparticle platforms.

Suspended hybrid films assembled from thiol-capped gold nanoparticles

In this work, we explored the formation processes of suspended hybrid thin films of thiol-capped Au nanoparticles (AuNPs) inside metal oxide tubular structures. We found that a balance between in-film interactions of the AuNPs and boundary interactions with metal oxides is a key in making these special organic-inorganic thin films. The hybrid films process many processing advantages and flexibilities, such as controllable film thickness, interfacial shape and inter-AuNPs distance, tuning of particle sizes, thiol population, chain lengths, and other new properties by introducing functional groups to thiol chains. Among their many unique features, the assembly-disassembly property may be useful for future on-off or store-release applications.