Gold nanoparticles with tailored size through ligand modification for catalytic applications

Fundamentals and Challenges of Ligand Modification in Heterogeneous Electrocatalysis

The development of efficient catalytic materials in the energy field could promote the structural transformation from traditional fossil fuels to sustainable energy. In heterogeneous catalytic reactions, ligand modification is an effective way to regulate both electronic and steric structures of catalytic sites, thus paving a prospective avenue to design the interfacial structures of heterogeneous catalysts for energy conversion. Although great achievements have been obtained for the study and applications of heterogeneous ligand-modified catalysts, the systematical refinements of ligand modification strategies are still lacking. Here, we reviewed the ligand modification strategy from both the mechanistic and applicable scenarios by focusing on heterogeneous electrocatalysis. We elucidated the ligand-modified catalysts in detail from the perspectives of basic concepts, preparation, regulation of physicochemical properties of catalytic sites, and applications in different electrocatalysis. Notably, we bridged the electrocatalytic performance with the electronic/steric effects induced by ligand modification to gain intrinsic structure-performance relations. We also discussed the challenges and future perspectives of ligand modification strategies in heterogeneous catalysis.

Rapid Size Determination of Quasispherical Gold Nanoparticles by Electrocatalysis Efficiency-Regulated Electrochemiluminescence

The size of gold nanoparticles (AuNPs) largely decides their properties and applications, making the rapid screening of AuNP size important. Despite the fact that AuNP-amplified electrochemiluminescence (ECL) is widely used in various ECL sensing applications, the mechanism of ECL enhancement remains elusive, especially the quantitative relationship between the enhanced ECL intensity and the size of AuNPs. In this work, taking quasispherical and citrate-stabilized AuNPs as model nanoparticles, we have reported that the ECL intensity of the S2O82--O2 system enhanced significantly with the increasing AuNP size. AuNPs acted as bielectrocatalysts for reducing the S2O82- and O2. The further study of enhancement mechanism demonstrates that AuNPs with increasing size facilitate the electron transfer and promote the generation of radicals required for the ECL emission, which produces more emitters-singlet oxygen. Meanwhile, the high surface density of citrate on small AuNPs suppresses the ECL signal by forming an electrostatic barrier. On the basis of the above phenomena, an ECL-based rapid AuNP size screening approach has been established. The accuracy of this platform is verified by the consistent results in comparison to transmission electron microscopy (TEM) measurements. This work not only provides deep insight into the correlation between the AuNP size and the ECL enhancement but also contributes an alternative to the TEM technique for the rapid AuNP size screening. Additionally, this study also extends the exploration of ECL-based structure analysis techniques toward nanomaterials through clarifying the structure-electrocatalytic activity correlation.

Plasma-based post-processing of colloidal nanocrystals for applications in heterogeneous catalysis

This review summarizes the work on the use of plasmas to post-process nanostructures, in particular colloidal nanocrystals, as promising candidates for applications of heterogeneous catalysis. Using plasma to clean or modify the surface of nanostructures is a more precisely controlled method compared to other conventional methods, which is preferable when strict requirements for nanostructure morphology or chemical composition are necessary. The ability of plasma post-processing to create mesoporous materials with high surface areas and controlled microstructure, surfaces, and interfaces has transformational potential in catalysis and other applications that leverage surface/interface processes.

Effect of Gold Nanoparticle Size on Regulated Catalytic Activity of Temperature-Responsive Polymer-Gold Nanoparticle Hybrid Microgels

Gold nanoparticles (AuNPs) possess attractive electronic, optical, and catalytic properties, enabling many potential applications. Poly(N-isopropyl acrylamide) (PNIPAAm) is a temperature-responsive polymer that changes its hydrophilicity upon a slight temperature change, and combining PNIPAAm with AuNPs allows us to modulate the properties of AuNPs by temperature. In a previous study, we proposed a simpler method for designing PNIPAAm-AuNP hybrid microgels, which used an AuNP monomer with polymerizable groups. The size of AuNPs is the most important factor influencing their catalytic performance, and numerous studies have emphasized the importance of controlling the size of AuNPs by adjusting their stabilizer concentration. This paper focuses on the effect of AuNP size on the catalytic activity of PNIPAAm-AuNP hybrid microgels prepared via the copolymerization of N-isopropyl acrylamide and AuNP monomers with different AuNP sizes. To quantitatively evaluate the catalytic activity of the hybrid microgels, we monitored the reduction of 4-nitrophenol to 4-aminophenol using the hybrid microgels with various AuNP sizes. While the hybrid microgels with an AuNP size of 13.0 nm exhibited the highest reaction rate and the apparent reaction rate constant (kapp) of 24.2 × 10-3 s-1, those of 35.9 nm exhibited a small kapp of 1.3 × 10-3 s-1. Thus, the catalytic activity of the PNIPAAm-AuNP hybrid microgel was strongly influenced by the AuNP size. The hybrid microgels with various AuNP sizes enabled the reversibly temperature-responsive on-off regulation of the reduction reaction.

A Uni-Micelle Approach for the Controlled Synthesis of Monodisperse Gold Nanocrystals

Small-size gold nanoparticles (AuNPs) are showing large potential in various fields, such as photothermal conversion, sensing, and medicine. However, current synthesis methods generally yield lower, resulting in a high cost. Here, we report a novel uni-micelle method for the controlled synthesis of monodisperse gold nanocrystals, in which there is only one kind micelle containing aqueous solution of reductant while the dual soluble Au (III) precursor is dissolved in oil phase. Our synthesis includes the reversible phase transfer of Au (III) and "uni-micelle" synthesis, employing a Au (III)-OA complex as an oil-soluble precursor. Size-controlled monodisperse AuNPs with a size of 4-11 nm are synthesized by tuning the size of the micelles, in which oleylamine (OA) is adsorbed on the shell of micelles and enhances the rigidity of the micelles, depressing micellar coalescence. Monodisperse AuNPs can be obtained through a one-time separation process with a higher yield of 61%. This method also offers a promising way for the controlled synthesis of small-size alloy nanoparticles and semiconductor heterojunction quantum dots.

Plasma-Enabled Ligand Removal for Improved Catalysis: Furfural Conversion on Pd/SiO2

A nonthermal, atmospheric He/O2 plasma (NTAP) successfully removed polyvinylpyrrolidone (PVP) from Pd cubic nanoparticles supported on SiO2 quickly and controllably. Transmission electron microscopy (TEM) revealed that the shape and size of Pd nanoparticles remain intact during plasma treatment, unlike mild calcination, which causes sintering and polycrystallinity. Using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS), we demonstrate the quantitative estimation of the PVP plasma removal rate and control of the nanoparticle synthesis. First-principles calculations of the XPS and CO FTIR spectra elucidate electron transfer from the ligand to the metal and allow for estimates of ligand coverages. Reactivity testing indicated that PVP surface crowding inhibits furfural conversion but does not alter furfural selectivity. Overall, the data demonstrate NTAP as a more efficient method than traditional calcination for organic ligand removal in nanoparticle synthesis.

Gold nanoparticles aggregation on graphene using Reactive force field: A molecular dynamic study

We examine the aggregation behavior of AuNPs of different sizes on graphene as function of temperature using molecular dynamic simulations with Reax Force Field. In addition, the consequences of such aggregation on the morphology of AuNPs and the charge transfer behavior of AuNP-Graphene hybrid structure are analyzed. The aggregation of AuNPs on graphene is confirmed from the center of mass distance calculation. The simulation results indicate that the size of AuNPs and temperature significantly affect the aggregation behavior of AuNPs on graphene. The strain calculation showed that shape of AuNPs changes due to the aggregation and the smaller size AuNPs on graphene exhibit more shape changes than larger AuNPs at all the temperatures studies in this work. The charge transfer calculation reveals that, the magnitude of charge transfer is higher for larger AuNPs-graphene composite when compared with smaller AuNPs-graphene composite. The charge transfer trend and the trends seen in the number of Au atoms directly in touch with graphene are identical. Hence, our results conclude that, quantity of Au atoms directly in contact with graphene during aggregation is primarily facilitates charge transfer between AuNPs and graphene. Our results on the size dependent strain and charge transfer characteristics of AuNPs will aid in the development of AuNPs-graphene composites for sensor applications.

Polydopamine-coated i-motif DNA/Gold nanoplatforms for synergistic photothermal-chemotherapy

The combination of photothermal therapy with chemotherapy has gradually developed into promising cancer therapy. Here, a synergistic photothermal-chemotherapy nanoplatform based on polydopamine (PDA)-coated gold nanoparticles (AuNPs) were facilely achieved via the in situ polymerization of dopamine (DA) on the surface of AuNPs. This nanoplatform exhibited augmented photothermal conversion efficiency and enhanced colloidal stability in comparison with uncoated PDA shell AuNPs. The i-motif DNA nanostructure was assembled on PDA-coated AuNPs, which could be transformed into a C-quadruplex structure under an acidic environment, showing a characteristic pH response. The PDA shell served as a linker between the AuNPs and the i-motif DNA nanostructure. To enhance the specific cellular uptake, the AS1411 aptamer was introduced to the DNA nanostructure employed as a targeting ligand. In addition, Dox-loaded NPs (DAu@PDA-AS141) showed the pH/photothermal-responsive release of Dox. The photothermal effect of DAu@PDA-AS141 elicited excellent photothermal performance and efficient cancer cell inhibition under 808 nm near-infrared (NIR) irradiation. Overall, these results demonstrate that the DAu@PDA-AS141 nanoplatform shows great potential in synergistic photothermal-chemotherapy.

Surfactant- and Ligand-Free Synthesis of Platinum Nanoparticles in Aqueous Solution for Catalytic Applications

The synthesis of surfactant-free and organic ligand-free metallic nanoparticles in solution remains challenging due to the nanoparticles’ tendency to aggregate. Surfactant- and ligand-free nanoparticles are particularly desirable in catalytic applications as surfactants, and ligands can block access to the nanoparticles’ surfaces. In this contribution, platinum nanoparticles are synthesized in aqueous solution without surfactants or bound organic ligands. Pt is reduced by sodium borohydride, and the borohydride has a dual role of reducing agent and weakly interacting stabilizer. The 5.3 nm Pt nanoparticles are characterized using UV-visible spectroscopy and transmission electron microscopy. The Pt nanoparticles are then applied as catalysts in two different reactions: the redox reaction of hexacyanoferrate(III) and thiosulfate ions, and H2O2 decomposition. Catalytic activity is observed for both reactions, and the Pt nanoparticles show up to an order of magnitude greater activity over the most active catalysts reported in the literature for hexacyanoferrate(III)/thiosulfate redox reactions. It is hypothesized that this enhanced catalytic activity is due to the increased electron density that the surrounding borohydride ions give to the Pt nanoparticle surface, as well as the absence of surfactants or organic ligands blocking surface sites.

Electronic Tuning of Gold Nanoparticle Active Sites for Reduction Catalysis

Electronic tuning of active sites in heterogeneous catalysis with organic ligands remains challenging since the ligands are often bound to the most active sites on the catalysts' surfaces. In this work, gold nanoparticles, which are on average less than 2 nm in diameter, are synthesized with strongly binding thiol and phosphine ligands and have measurable quantities of accessible sites on their surfaces in both cases. Triphenylphosphine (TPP) is used as the phosphine ligand, while triphenylmethyl mercaptan (TPMT) serves as the thiol ligand. Phosphines are chosen because they are electron-donating ligands when bound to Au, and thiols are selected because they are electron-withdrawing on the Au surface. X-ray photoelectron spectroscopy (XPS) results show differences in the Au 4f binding energies between the TPP- and TPMT-bound Au nanoparticles. Fourier transform infrared spectroscopy (FTIR) measurements of bound CO indicate that the TPP-bound Au nanoparticles are more electron-rich than the TPMT-bound Au nanoparticles. The number of binding sites on the surface is quantified using 2-naphthalenethiol titration experiments. It is observed that the number of binding sites on the thiol and phosphine-bound Au nanoparticles is similar in both cases. The Au nanoparticles are used for three different reactions: resazurin reduction, CO oxidation, and benzyl alcohol oxidation. For both CO oxidation and benzyl alcohol oxidation, which are performed with the ligands attached, TPP- and TPMT-bound nanoparticles are both catalytically active. However, for resazurin reduction, the TPMT-bound Au nanoparticles are not active, while the TPP-bound Au nanoparticles are catalytically active. These results illustrate that the catalytic activity can be tuned using bound organic ligands with different electronic properties for reduction reactions using Au nanoparticle catalysts.

Novel biogenic gold nanoparticles stabilized on poly(styrene-co-maleic anhydride) as an effective material for reduction of nitrophenols and colorimetric detection of Pb(II)

Biogenic gold nanoparticles (AuNPs) have been extensively studied for the catalytic conversion of nitrophenols (NP) into aminophenols and the colorimetric quantification of heavy metal ions in aqueous solutions. However, the high self-agglomeration ability of colloidal nanoparticles is one of the major obstacles hindering their application. In the present study, we offered novel biogenic AuNPs synthesized by a green approach using Cistanche deserticola (CD) extract as a bioreducing agent and stabilized on poly(styrene-co-maleic anhydride) (PSMA). The prepared Au@PSMA nanoparticles were characterized by various techniques (HR-TEM, SEAD, FE-SEM, DLS, TGA, XRD, and FTIR) and studied for two applications: the catalytic reduction of 3-NP by NaBH₄ and the sensing detection of Pb²⁺ ions. The optimal conditions for the synthesis of AuNPs were investigated and established at 60 °C, 20 min, pH of 9, and 0.5 mM Au³⁺. Morphological studies showed that AuNPs synthesized by CD extract were mostly spherical with a mean diameter of 25 nm, while the size of polymer-integrated AuNPs was more than two-fold larger. Since PSMA acted as a matrix keeping the nanoparticles from coagulation and maintaining the optimal surface area, AuNPs integrated with PSMA showed higher catalytic efficiency with a faster reaction rate and lower activation energy than conventional nanoparticles. Au@PSMA could completely reduce 3-NP within 10 min with a rate constant of 0.127 min^-1 and activation energy of 9.96 kJ/mol. The presence of PSMA also improved the stability and recyclability of AuNPs. Used as a sensor, Au@PSMA exhibited excellent sensitivity and selectivity for Pb²⁺ ions with a limit of detection of 0.03 μM in the linear range of 0-100 μM. The study results suggested that Au@PSMA could be used as a promising catalyst for the reduction of NP and the colorimetric sensor for detection of Pb²⁺ ions in aqueous environmental samples.

Support, composition, and ligand effects in partial oxidation of benzyl alcohol using gold–copper clusters

The effect of metallic composition, support, and ligands on catalytic performance using AuCu clusters in benzyl alcohol oxidation is investigated.