Kinetic control and thermodynamic selection in the synthesis of atomically precise gold nanoclusters

This work presents a combined approach of kinetic control and thermodynamic selection for the synthesis of monodisperse 19 gold atom nanoclusters protected by thiolate groups. The step of kinetic control allows the formation of a proper size distribution of initial size-mixed Au(n)(SR)(m) nanoclusters following the reduction of a gold precursor. Unlike the synthesis of Au(25)(SR)(18) nanoclusters, which involves rapid reduction of the gold precursor by NaBH(4) followed by size focusing, the synthesis of 19-atom nanoclusters requires slow reduction effected by a weaker reducing agent, borane-tert-butylamine complex. The initially formed mixture of nanoclusters then undergoes size convergence into a monodisperse product by means of a prolonged aging process. The nanocluster formula was determined to be Au(19)(SC(2)H(4)Ph)(13). This work demonstrates the importance of both kinetic control of the initial size distribution of nanoclusters prior to size focusing and subsequent thermodynamic selection of stable nanoclusters as the final product.     

Interstaple dithiol cross-linking in Au25(SR)18 nanomolecules: a combined mass spectrometric and computational study

A systematic study of cross-linking chemistry of the Au(25)(SR)(18) nanomolecule by dithiols of varying chain length, HS-(CH(2))(n)-SH where n = 2, 3, 4, 5, and 6, is presented here. Monothiolated Au(25) has six [RSAuSRAuSR] staple motifs on its surface, and MALDI mass spectrometry data of the ligand exchanged clusters show that propane (C3) and butane (C4) dithiols have ideal chain lengths for interstaple cross-linking and that up to six C3 or C4 dithiols can be facilely exchanged onto the cluster surface. Propanedithiol predominately exchanges with two monothiols at a time, making cross-linking bridges, while butanedithiol can exchange with either one or two monothiols at a time. The extent of cross-linking can be controlled by the Au(25)(SR)(18) to dithiol ratio, the reaction time of ligand exchange, or the addition of a hydrophobic tail to the dithiol. MALDI MS suggests that during ethane (C2) dithiol exchange, two ethanedithiols become connected by a disulfide bond; this result is supported by density functional theory (DFT) prediction of the optimal chain length for the intrastaple coupling. Both optical absorption spectroscopy and DFT computations show that the electronic structure of the Au(25) nanomolecule retains its main features after exchange of up to eight monothiol ligands.     

Robust Synthesis of Gold‐Based Multishell Structures as Plasmonic Catalysts for Selective Hydrogenation of 4‐Nitrostyrene

A robust self‐template strategy is used for facile and large‐scale synthesis of porous multishell gold with controllable shell number, sphere size, and in situ surface modification. The process involved the rapid reduction of novel Au‐melamine colloidal templates with a great amount of NaBH₄ in presence of poly(sodium‐p‐styrenesulfonate) (PSS). After soaking the templates in other metal salt solution, the obtained bimetallic templates could also be generally converted into bimetallic multishell structures by same reduction process. In the hydrogenation of 4‐nitrostyrene using NH₃BH₃ as a reducing agent, the porous triple‐shell Au with surface modification (S‐PTSAu) exhibited excellent selectivity (97 %) for 4‐aminostyrene in contrast with unmodified triple‐shell Au. Furthermore, it also showed higher enhancement of catalytic activity under irradiation of visible light as compared to similar catalysts with fewer shells.     

Robust Synthesis of Gold-Based Multishell Structures as Plasmonic Catalysts for Selective Hydrogenation of 4-Nitrostyrene

A robust self-template strategy is used for facile and large-scale synthesis of porous multishell gold with controllable shell number, sphere size, and in situ surface modification. The process involved the rapid reduction of novel Au-melamine colloidal templates with a great amount of NaBH₄ in presence of poly(sodium-p-styrenesulfonate) (PSS). After soaking the templates in other metal salt solution, the obtained bimetallic templates could also be generally converted into bimetallic multishell structures by same reduction process. In the hydrogenation of 4-nitrostyrene using NH₃BH₃ as a reducing agent, the porous triple-shell Au with surface modification (S-PTSAu) exhibited excellent selectivity (97 %) for 4-aminostyrene in contrast with unmodified triple-shell Au. Furthermore, it also showed higher enhancement of catalytic activity under irradiation of visible light as compared to similar catalysts with fewer shells.     

Organic-soluble fluorescent Au8 clusters generated from heterophase ligand-exchange induced etching of gold nanoparticles and their electrochemiluminescence

A unique heterophase ligand exchange induced etching process was used to transform gold nanoparticles into organic-soluble fluorescent gold clusters which were assigned to Au(8) by optical spectroscopy and MALDI-TOF mass spectrometry. Both the annihilation electrochemiluminescence of fluorescent Au(8) clusters in organic solution and the coreactant electrochemiluminescence of Au(8) cluster film in aqueous solution were studied.     

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.     

Mixed dithiolate durene-DT and monothiolate phenylethanethiolate protected Au130 nanoparticles with discrete core and core-ligand energy states

A new type of gold nanoparticle with interesting energetics has been created by employing a mixture of dithiol durene (Durene-DT) and monothiol phenylethanethiol (PhC2S) in the synthesis. The average composition of these mixed thiolate clusters is characterized to be Au(130)(Durene-DT)(29)(PhC2S)(22). Continuous quantized core charging behaviors were observed at lower potentials in voltammetric measurements, while ligand reaction and core-ligand interactions were observed at higher potentials. The absorbance spectrum displays discrete absorption bands at ca. 355, 490, 584, and 718 nm. The electrochemical and absorbance features are correlated through the determined energy states and charging energy. Broad near-IR luminescence was observed, associated with significant relaxation of excitation energy. Such interesting optical and electrochemical properties are attributed to the nanoparticle core size, ligand composition, and core-ligand charge delocalization determined by the dithiolate molecular structure.     

D-glucose-derived polymer intermediates as templates for the synthesis of ultrastable and redispersible gold colloids

A two-stage hydrothermal process was developed for the synthesis of highly dispersed Au colloids. In the first stage, a novel glucose-derived polymer template was prepared by the hydrothermal treatment of glucose at 160 degrees C. This template was then further used in the next step to synthesize highly dispersed gold (Au) colloids by hydrothermal treatment with HAuCl(4.) The templates treated at 160 degrees C with changing reaction times had different templating effects toward Au species. The 3-h treated template was able tightly adhere to the Au colloids. As a result, an unusual stability was observed for the prepared Au particles that could be repeatedly precipitated and redispersed with the template in H(2)O and were also stable against heating (below 160 degrees C) and aging. Meanwhile, the 5-h and 7-h treated templates had much poorer templating effects to Au species, leading to severe aggregation of the Au colloids immobilized on them. The various templating effects were correlated to the different structural features of the templates. Compared to the 5- or 7-h treated templates that were deeply carbonized, the 3-h treated template was only slightly carbonized, thus possessing a lot of functional and hydrophilic O-containing groups that could bind to Au species. These differences in templating ability were also observed in the Au samples prepared by the sonication-assisted method. The highly dispersed Au colloids immobilized on the 3-h treated template were tested for CO oxidation, and a good catalytic activity and stability for CO oxidation was observed.     

Polymer-induced synthesis of stable gold and silver nanoparticles and subsequent ligand exchange in water

A simple, inexpensive, single-step synthesis of gold and silver nanoparticles using poly(allylamine) (PAAm) as a reducing and stabilizing agent is reported. The synthetic process was carried out in aqueous solution, making the method versatile and environmentally friendly. The synthesized polymer-stabilized nanoparticles are stable in water without particle aggregation at room temperature for at least a month. We demonstrate successful ligand exchange on the polymer-stabilized gold nanoparticles (AuNPs) with a variety of omega-functionalized acid-, alcohol-, amine-, and biotin-terminated alkylthiols. The methodologies, including ligand exchange, also are applicable for the generation of finely dispersed silver nanoparticles. The synthesized gold and silver nanoparticles are characterized by UV-visible absorption spectroscopy and transmission electron microscopy (TEM). The different ligand-stabilized AuNPs are also analyzed by Fourier transform infrared (FTIR) spectroscopy.     

Structure and bonding issues at the interface between gold and self-assembled conjugated dithiol monolayers

Organic thiols have received extensive attention recently because of their relative stability and ease of examination compared to other potential molecular electronic materials. In this work, scanning probe microscopy (SPM) is used to study (i) the structural properties of self-assembled monolayers (SAMs) containing conjugated dithiols and (ii) the formation of the upper molecule-metal interface on dithiol SAMs. The top gold film is deposited either by thermal evaporation or by nano-transfer printing (nTP). Generally, the utility of thermal evaporation is limited because of Au diffusion through the SAMs. However, several dithiol SAMs are identified in this work that bond well to Au overlayers and act as satisfactory diffusion barriers. Coassembly of conjugated dithiols and alkanemonothiols is suggested as a route to obtain dithiols that are denser packed and more vertical (than is obtained from pure dithiol routes). High-yield nTP is demonstrated on coassembled SAMs. Advantages and limitations of different Au deposition and transfer techniques are compared on a variety of length scales.     

Gold-copolymer nanoparticles: Poly(ε-caprolactone)/poly(N-vinyl-2-pyrrolydone) Biodegradable triblock copolymer as stabilizer and reductant

Block copolymers have been extensively used in the synthesis of many types of nanoparticles, where generally are considered as stabilizer and protective agent. In this work a double function of the biodegradable triblock copolymer poly(N-vinyl-2-pyrrolidone)-b-poly(ε-caprolactone)-b-poly(N-vinyl-2-pyrrolidone), (PVP-PCL-PVP) in the gold nanoparticle-copolymer synthesis is reported.Gold-copolymer composed nanoparticles were synthesized using the triblock copolymer (PVP-PCL-PVP) and potassium tetrachloro aurate (III), both in aqueous solution. The copolymer work as both, reductant and stabilizer agent. The obtained nanoparticles were characterized by FT-IR, dynamic light scattering (DLS), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The shape and the size of the obtained nanoparticles are dependent on the copolymer/salt of gold concentration ratio used in the synthesis.To complement the experimental results about the copolymer role in the nanoparticles synthesis, computational tools were used to characterize the reactivity of the reactant species.     

Reaction mechanism governing formation of 1,3-bis(diphenylphosphino)propane-protected gold nanoclusters

This report outlines the determination of a reaction mechanism that can be manipulated to develop directed syntheses of gold monolayer-protected clusters (MPCs) prepared by reduction of solutions containing 1,3-bis(diphenylphosphino)propane (L(3)) ligand and Au(PPh(3))Cl. Nanocluster synthesis was initiated by reduction of two-coordinate phosphine-ligated [Au(I)LL'](+) complexes (L, L' = PPh(3), L(3)), resulting in free radical complexes. The [Au(0)LL'](?) free radicals nucleated, forming a broad size distribution of ligated clusters. Timed UV-vis spectroscopy and electrospray ionization mass spectrometry monitored the ligated Au(x), 6 ≤ x ≤ 13, clusters, which comprise reaction intermediates and final products. By employing different solvents and reducing agents, reaction conditions were varied to highlight the largest portion of the reaction mechanism. We identified several solution-phase reaction classes, including dissolution of the gold precursor, reduction, continuous nucleation/core growth, ligand exchange, ion-molecule reactions, and etching of colloids and larger clusters. Simple theories can account for the reaction intermediates and final products. The initial distribution of the nucleation products contains mainly neutral clusters. However, the rate of reduction controls the amount of reaction overlap occurring in the system, allowing a clear distinction between reduction/nucleation and subsequent solution-phase processing. During solution-phase processing, the complexes undergo core etching and core growth reactions, including reactions that convert neutral clusters to cations, in a cyclic process that promotes formation of stable clusters of specific metal nuclearity. These processes comprise "size-selective" processing that can narrow a broad distribution into specific nuclearities, enabling development of tunable syntheses.     

Ligand exchange reactions on Au(38) and Au(40) clusters: a combined circular dichroism and mass spectrometry study

The thiolate-for-thiolate ligand exchange reaction between the stable Au(38)(2-PET)(24) and Au(40)(2-PET)(24) (2-PET: 2-phenylethanethiol) clusters and enantiopure BINAS (BINAS: 1,1'-binaphthyl-2,2'-dithiol) was investigated by circular dichroism (CD) spectroscopy in the UV/vis and MALDI mass spectrometry (MS). The ligand exchange reaction is incomplete, although a strong optical activity is induced to the resulting clusters. The clusters are found to be relatively stable, in contrast to similar reactions on [Au(25)(2-PET)(18)](-) clusters. Maximum anisotropy factors of 6.6 × 10(-4) are found after 150 h of reaction time. During the reaction, a varying ratio between Au(38) and Au(40) clusters is found, which significantly differs from the starting material. As compared to Au(38), Au(40) is more favorable to incorporate BINAS into its ligand shell. After 150 h of reaction time, an average of 1.5 and 4.5 BINAS ligands is found for Au(38) and Au(40) clusters, respectively. This corresponds to exchange of 3 and 9 monodentate 2-PET ligands. To show that the limited exchange with BINAS is due to the bidentate nature of the ligand, exchange with thiophenol was performed. The monodentate thiophenol exchange was found to be faster, and more ligands were exchanged when compared to BINAS.     

Does core size matter in the kinetics of ligand exchanges of monolayer-protected Au clusters?

This paper compares the kinetics of exchanges of phenylethanethiolate ligands (PhC2S-) of the monolayer-protected clusters (MPCs) Au(38)(SC2Ph)(24) and Au(140)(SC2Ph)(53) with p-substituted arylthiols (p-X-PhSH), where X = NO(2), Br, CH(3), OCH(3), and OH. First-order rate constants at 293 K for exchange of the first ca. 25% of the ligands on the molecule-like Au(38)(SC2Ph)(24) MPC, measured using (1)H NMR, vary linearly with the in-coming arythiol concentration; ligand exchange is an overall second-order reaction. Remarkably, the second-order rate constants for ligand exchange on Au(38)(SC2Ph)(24) are very close to those of corresponding exchange reactions on the larger nanoparticle Au(140)(SC2Ph)(53) MPCs. These are the first results that quantitatively show that the chemical reactivity of different sized nanocrystals is almost independent of size; presumably, this is because the locus of the initial ligand exchanges is a common kind of site, thought to be the nanocrystal vertexes. The rates of later stages of exchange (beyond ca. 25%) differ for Au(38) and Au(140) cores, the latter being much slower presumably due to its larger terrace-like surface atom content. The reverse exchange reaction was studied for Au(38)(p-X-arylthiolate)(24) MPCs (X = NO(2), Br, and CH(3)), where the in-coming ligand is now phenylethanethiol. Remarkably, the rate constants of both forward and reverse exchanges display identical substituent effects, which implies a concurrent bonding of both in-coming and leaving ligands to the Au core in the rate-determining step, as in an associative mechanism. X = NO(2) gives the fastest rates, and the ratio of forward and reverse rate constants gives an equilibrium constant of K(EQ,PE) = 4.0 that is independent of X.     

Large-scale synthesis of thiolated Au25 clusters via ligand exchange reactions of phosphine-stabilized Au11 clusters

Phosphine-stabilized Au11 clusters in chloroform were reacted with glutathione (GSH) in water under a nitrogen atmosphere. The resulting Au:SG clusters exhibit an optical absorption spectrum similar to that of Au25(SG)18, which was isolated as one of the major products from chemically prepared Au:SG clusters (Negishi, Y. et al. J. Am. Chem. Soc. 2005, 127, 5261). Rigorous characterization by optical spectroscopy, electrospray ionization mass spectrometry, and polyacrylamide gel electrophoresis confirms that the Au25(SG)18 clusters were selectively obtained on the sub-100 mg scale by ligand exchange reaction under aerobic conditions. The ligand exchange strategy offers a practical and convenient method of synthesizing thiolated Au25 clusters on a large scale.     

Synthesis and optical characterization of submicrometer gold nanotubes grown on goethite rods

Goethite (FeOOH) rods were used as templates for growing gold nanotubes with a length of a few hundred nanometers and an aspect ratio between 3 and 4. Successful uniform growth required surface modification, followed by the attachment of small Au seeds and one-step seeded growth using formaldehyde as a reducing agent, as previously reported for the growth of Au shells on silica spheres and hematite spindles. The thickness and surface roughness of the obtained shells could be adjusted by simply varying the concentration ratio between seeds (modified goethite rods) and growth reagents (HAuCl 4 and formaldehyde). The morphology of the synthesized gold nanotubes was thoroughly characterized by TEM, SEM, and AFM/MFM. The resulting gold nanotubes display well-defined plasmon resonances, with a strong longitudinal mode centered around ca. 1400 nm and a broad band in the visible resulting from the overlap of a transverse mode and a multipolar mode, as was found from theoretical modeling using the boundary element method, which provides reasonable agreement with the experimental results.     

Sub-nanometer Au monolayer-protected clusters exhibiting molecule-like electronic behavior: quantitative high-angle annular dark-field scanning transmission electron microscopy and electrochemical characterization of clusters with precise atomic stoichiometry

The synthesis and characterization of the clusters Au13[PPh3]4[S(CH2)11CH3]2Cl2 (1) and Au13[PPh3]4[S(CH2)11CH3]4 (2) are described. These mixed-ligand, sub-nanometer clusters, prepared via exchange of dodecanethiol onto phosphine-halide gold clusters, show enhanced stability relative to the parent. The characterization of these clusters features the precise determination of the number of gold atoms in the cluster cores using high-angle annular dark-field scanning transmission electron microscopy, allowing the assignment of 13 gold atoms (+/-3 atoms) to the composition of both cluster molecules. Electrochemical and optical measurements reveal discrete molecular orbital levels and apparent energy gaps of 1.6-1.7 eV for the two cluster molecules. The electrochemical measurements further indicate that the Au13[PPh3]4[S(CH2)11CH3]2Cl2 cluster undergoes an overall two-electron reduction. The electrochemical and spectroscopic properties of the two Au13 cluster molecules are compared with those of a secondary synthetic product, which proved to be larger Au thiolate-derivatized monolayer-protected clusters with an average core of Au180. The latter shows behavior fully consistent with the adoption of metallic-like properties.     

Microfluidic synthesis and catalytic application of PVP-stabilized, approximately 1 nm gold clusters

Small PVP-stabilized gold clusters were successfully prepared by the homogeneous mixing of continuous flows of aqueous AuCl 4 (-) and BH 4 (-) in a micromixer. Spectroscopic characterization revealed that microfluidic synthesis could yield monodisperse Au:PVP clusters with an average diameter of approximately 1 nm, which is smaller than clusters produced by conventional batch methods. These approximately 1 nm Au:PVP clusters exhibited higher catalytic activity for the aerobic oxidation of p-hydroxybenzyl alcohol than did Au:PVP clusters prepared by batch methods.     

Mechanism of gold adsorption by persimmon tannin gel.

Gold adsorption by persimmon tannin (PT) gel from a solution containing hydrogen tetrachloroaurate(III) was examined. A flow-rate examination in a column system indicated the reduction of Au(III) ion to Au(0). XRD patterns clarified the existence of Au(0) on the gel which adsorbed gold. The gel could also adsorb colloidal Au(0) prepared independently. A model consisting of ligand exchange, Au(III) reduction to Au(0), and resulting Au(0) adsorption by PT gel was presented for the gold adsorption mechanism.     

A direct and practical approach for the synthesis of Au(I)-NHC complexes from commercially available imidazolium salts and Au(III) salts

A direct and practical approach for the synthesis of Au(I)-NHC complexes from imidazolium salts and commercially available aurate salt (MAuCl₄·2H₂O) is described. The reaction proceeded without sacrificing carbene transfer agent (Ag₂O) or using highly sensitive free NHC.