Unexpected reactivity of Au25(SCH2CH2Ph)18 nanoclusters with salts

We report some interesting results of the chemical reactivity of thiolate-protected [Au(25)(SCH(2)CH(2)Ph)(18)](0) nanoclusters with two types of salts, including tetraoctylammonium halide (TOAX) and NaX. At the early stage of the reaction, [Au(25)(SCH(2)CH(2)Ph)(18)](0) was found to spontaneously convert to its anionic form ([Au(25)(SCH(2)CH(2)Ph)(18)](-)) in the presence of either type of salt. However, a large difference was observed in the second stage of the reaction. With NaX, we observed decomposition of anionic clusters, while with TOAX, the clusters show excellent stability. We have gained some insight into the reaction mechanism. The X(-) ions seem to attack [Au(25)(SCH(2)CH(2)Ph)(18)](q) surface and displace some thiolates, evidenced by the observation of halide-bound clusters such as Au(25)(SCH(2)CH(2)Ph)(18-x)Br(x) in mass spectrometry analysis. These halide-bound clusters show a reduced stability, and their decomposition into Au(I) complexes leads to the release of gold valence electrons of the clusters; concurrently, the non-halide-bound [Au(25)(SCH(2)CH(2)Ph)(18)](0) clusters are reduced into [Au(25)(SCH(2)CH(2)Ph)(18)](-). For the second stage of reaction with organic salts such as TOA-Br, after [Au(25)(SCH(2)CH(2)Ph)(18)](0) clusters are converted to [Au(25)(SCH(2)CH(2)Ph)(18)](-)) the TOA(+) counterions surprisingly protect the anionic clusters from further attack by halide ions, hence, TOA(+) cations can stabilize [Au(25)(SCH(2)CH(2)Ph)(18)](-) clusters. In contrast, with NaX salts the Na(+) ions do not provide any steric stabilization of the [Au(25)(SCH(2)CH(2)Ph)(18)](-) clusters, hence, degradation occurs when being further attacked by halide ions, especially Br(-) and I(-).     

A photoresponsive Au25 nanocluster protected by azobenzene derivative thiolates

An Au(25) cluster protected by azobenzene derivative thiolates (S-Az) ([Au(25)(S-Az)(18)](-)) was synthesized with the aim of producing a photoresponsive Au(25) cluster. The matrix-assisted laser desorption/ionization mass spectrum of the product revealed that [Au(25)(S-Az)(18)](-) was synthesized in high purity. Optical absorption spectra of [Au(25)(S-Az)(18)](-) obtained before and after photoirradiation suggest that the azobenzenes in the ligands of Au(25)(S-Az)(18) isomerize with an efficiency of nearly 100%, both from the trans to cis conformation and from the cis to trans conformation. Furthermore, the redox potential and optical absorption of Au(25)(S-Az)(18) were found to change reversibly due to photoisomerization of azobenzenes.     

The halogen analogs of thiolated gold nanoclusters

Is it possible to replace all the thiolates in a thiolated gold nanocluster with halogens while still maintaining the geometry and the electronic structure? In this work, we show from density functional theory that such halogen analogs of thiolated gold nanoclusters are highly likely. Using Au(25)X(18)(-) as an example, where X = F, Cl, Br, or I replaces -SR, we find that Au(25)Cl(18)(-) demonstrates a high similarity to Au(25)(SR)(18)(-) by showing Au-Cl distances, Cl-Au-Cl angles, band gap, and frontier orbitals similar to those in Au(25)(SR)(18)(-). DFT-based global minimization also indicates the energetic preference of staple formation for the Au(25)Cl(18)(-) cluster. The similarity between Au(m)(SR)(n) and Au(m)X(n) could be exploited to make viable Au(m)X(n) clusters and to predict structures for Au(m)(SR)(n).     

Investigating the structural evolution of thiolate protected gold clusters from first-principles

Unlike bulk materials, the physicochemical properties of nano-sized metal clusters can be strongly dependent on their atomic structure and size. Over the past two decades, major progress has been made in both the synthesis and characterization of a special class of ligated metal nanoclusters, namely, the thiolate-protected gold clusters with size less than 2 nm. Nevertheless, the determination of the precise atomic structure of thiolate-protected gold clusters is still a grand challenge to both experimentalists and theorists. The lack of atomic structures for many thiolate-protected gold clusters has hampered our in-depth understanding of their physicochemical properties and size-dependent structural evolution. Recent breakthroughs in the determination of the atomic structure of two clusters, [Au(25)(SCH(2)CH(2)Ph)(18)](q) (q = -1, 0) and Au(102)(p-MBA)(44), from X-ray crystallography have uncovered many new characteristics regarding the gold-sulfur bonding as well as the atomic packing structure in gold thiolate nanoclusters. Knowledge obtained from the atomic structures of both thiolate-protected gold clusters allows researchers to examine a more general "inherent structure rule" underlying this special class of ligated gold nanoclusters. That is, a highly stable thiolate-protected gold cluster can be viewed as a combination of a highly symmetric Au core and several protecting gold-thiolate "staple motifs", as illustrated by a general structural formula [Au](a+a')[Au(SR)(2)](b)[Au(2)(SR)(3)](c)[Au(3)(SR)(4)](d)[Au(4)(SR)(5)](e) where a, a', b, c, d and e are integers that satisfy certain constraints. In this review article, we highlight recent progress in the theoretical exploration and prediction of the atomic structures of various thiolate-protected gold clusters based on the "divide-and-protect" concept in general and the "inherent structure rule" in particular. As two demonstration examples, we show that the theoretically predicted lowest-energy structures of Au(25)(SR)(8)(-) and Au(38)(SR)(24) (-R is the alkylthiolate group) have been fully confirmed by later experiments, lending credence to the "inherent structure rule".     

Water-soluble Au25(Capt)18 nanoclusters: synthesis, thermal stability, and optical properties

This work was motivated by the unsatisfactory stability of Au(25)(SG)(18) in solution under thermal conditions (e.g. 70-90 °C for DNA melting). Thus, we searched for a better, water-soluble thiol ligand. Herein, we report a one-pot synthesis and investigation of the stability and optical properties of captopril (abbreviated Capt)-protected Au(25)(Capt)(18) nanoclusters. The Au(25)(Capt)(18) (anionic, counterion: Na(+)) nanoclusters were formed via size focusing under ambient conditions. Significantly, Au(25)(Capt)(18) nanoclusters exhibit largely improved thermal stability compared to the glutathione (HSG) capped Au(25)(SG)(18). Both Au(25)(Capt)(18) and Au(25)(SG)(18) nanoclusters show fluorescence centered at ～700 nm. The chiral ligands (Capt, SG, as well as chirally modified phenylethanethiol (PET*)) give rise to distinct chiroptical features. The high thermal stability and distinct optical properties of Au(25)(Capt)(18) nanoclusters render this material quite promising for biological applications.     

Ligand-exchange synthesis of selenophenolate-capped Au25 nanoclusters

We report the synthesis and characterization of selenophenolate-capped 25-gold-atom nanoclusters via a ligand-exchange approach. In this method, phenylethanethiolate (PhCH(2)CH(2)S) capped Au(25) nanoclusters are utilized as the starting material, which is subject to ligand-exchange with selenophenol (PhSeH). The as-obtained cluster product is confirmed to be selenophenolate-protected Au(25) nanoclusters through characterization by electrospray ionization (ESI) and matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS), thermogravimetric analysis (TGA), elemental analysis (EA), UV-Vis and (1)H/(13)C NMR spectroscopies. The ligand-exchange synthesis of [Au(25)(SePh)(18)](-)[(C(8)H(17))(4)N](+) nanoclusters demonstrates that the core size of gold nanoclusters is retained in the thiolate-to-selenolate exchange process and that the 18 surface thiolate ligands can be completely exchanged by selenophenolate, rather than giving rise to a mixed ligand shell on the cluster. The two types of Au(25)L(18) (L = thiolate or selenolate) nanoclusters also show some differences in stability and optical properties.     

N,N-Dimethylformamide-stabilized gold nanoclusters as a catalyst for the reduction of 4-nitrophenol

In this study, we investigated the catalytic properties of N,N-dimethylformamide (DMF)-stabilized gold nanoclusters (AuNCs) in the reduction of 4-nitrophenol (PNP) to 4-aminophenol by NaBH(4), a well known model reaction to be catalyzed by metal surfaces. The DMF-stabilized AuNCs were prepared in DMF by a surfactant-free method. The DMF-stabilized AuNCs showed high catalytic activity even when used in small quantities (～10(-7) g). The pseudo-first-order rate constant (k(app)) and activation energy were estimated to be 3 × 10(-3) s(-1) and 31 kJ mol(-1), respectively, with 1.0 μM of the gold catalyst at 298 K. The catalytic activity of the DMF-stabilized AuNCs was strongly influenced by the layer of adsorbed DMF on the Au NCs. This layer of adsorbed DMF prohibited the reactants from penetrating to the surface of the AuNCs via the diffusion at the beginning of the reaction, resulting in an induction time (t(0)) before PNP reduction began. Restructuring of the DMF layer (essentially a form of activation) was the key to achieving high catalytic activity. In addition, atomically monodisperse Au(25)(SG)(18)NCs (SG: glutathione) showed higher catalytic activity in the PNP reduction (k(app) = 8 × 10(-3) s(-1)) even with a low catalyst concentration (1.0 μM), and there was no induction time (t(0)) in spite of the strongly binding ligand glutathione. This suggested that the catalytically active surface sites of the Au(25)(SG)(18)NCs were not sterically hindered, possibly because of the unique core-shell-like structure of the NCs. Retaining these open sites on AuNCs may be the key to making the NCs effective catalysts.     

Facile synthesis and optical properties of magic-number Au13 clusters

Synthesis of molecular gold clusters through a post-synthetic scheme involving HCl-promoted nuclearity convergence was examined with various phosphine ligands. Systematic studies with a series of bis(diphenylphosphino) ligands (Ph(2)P-(CH(2))(m)-PPh(2)) using electrospray ionization mass spectrometry (ESI-MS) and electronic absorption spectroscopy demonstrated that the use of dppp (m = 3), dppb (m = 4) and dpppe (m = 5) as the ligands resulted in the formation of [Au(13)P(8)Cl(4)](+) type clusters, whereas the [Au(13)P(10)Cl(2)](3+) type cluster was formed with dppe (m = 2). The cluster species did not survive the HCl treatment step when monophosphines PPh(3), PMe(2)Ph, and POct(3) were employed, but [Au(13)(POct(3))(8)Cl(4)](+) was isolated as a minor product in the NaBH(4) reduction of Au(POct(3))Cl in aqueous THF. Electronic absorption and photoluminescence studies of a series of Au(13) clusters revealed that their optical properties are highly dependent on the phosphine/chloride composition ratio, but are far less so on the phosphine structure.     

Quantum sized gold nanoclusters with atomic precision

Gold nanoparticles typically have a metallic core, and the electronic conduction band consists of quasicontinuous energy levels (i.e. spacing δ ? k(B)T, where k(B)T is the thermal energy at temperature T (typically room temperature) and k(B) is the Boltzmann constant). Electrons in the conduction band roam throughout the metal core, and light can collectively excite these electrons to give rise to plasmonic responses. This plasmon resonance accounts for the beautiful ruby-red color of colloidal gold first observed by Faraday back in 1857. On the other hand, when gold nanoparticles become extremely small (<2 nm in diameter), significant quantization occurs to the conduction band. These quantum-sized nanoparticles constitute a new class of nanomaterial and have received much attention in recent years. To differentiate quantum-sized nanoparticles from conventional plasmonic gold nanoparticles, researchers often refer to the ultrasmall nanoparticles as nanoclusters. In this Account, we chose several typical sizes of gold nanoclusters, including Au(25)(SR)(18), Au(38)(SR)(24), Au(102)(SR)(44), and Au(144)(SR)(60), to illustrate the novel properties of metal nanoclusters imparted by quantum size effects. In the nanocluster size regime, many of the physical and chemical properties of gold nanoparticles are fundamentally altered. Gold nanoclusters have discrete electronic energy levels as opposed to the continuous band in plasmonic nanoparticles. Quantum-sized nanoparticles also show multiple optical absorption peaks in the optical spectrum versus a single surface plasmon resonance (SPR) peak at 520 nm for spherical gold nanocrystals. Although larger nanocrystals show an fcc structure, nanoclusters often have non-fcc atomic packing structures. Nanoclusters also have unique fluorescent, chiral, and magnetic properties. Due to the strong quantum confinement effect, adding or removing one gold atom significantly changes the structure and the electronic and optical properties of the nanocluster. Therefore, precise atomic control of nanoclusters is critically important: the nanometer precision typical of conventional nanoparticles is not sufficient. Atomically precise nanoclusters are represented by molecular formulas (e.g. Au(n)(SR)(m) for thiolate-protected ones, where n and m denote the respective number of gold atoms and ligands). Recently, major advances in the synthesis and structural characterization of molecular purity gold nanoclusters have made in-depth investigations of the size evolution of metal nanoclusters possible. Metal nanoclusters lie in the intermediate regime between localized atomic states and delocalized band structure in terms of electronic properties. We anticipate that future research on quantum-sized nanoclusters will stimulate broad scientific and technological interests in this special type of metal nanomaterial.     

One-Pot Formation of a Metallosupramolecularly Assembled and Redox-Active Adlayer at the Solid–Liquid Interface

A redox-active adlayer consisting of cobalt ions and terpyridine ligands, 4′,4′′′′-(1,4-phenylene)bis(2,2′:6′,2′′-terpyridine), was prepared on a Au(111) surface by a stepwise coordination method. The obtained adlayer produced a well-defined, stable redox wave associated with cobalt ions coordinated to 4′,4′′′′-(1,4-phenylene)bis(2,2′:6′,2′′-terpyridine), as compared to a tetra-2-pyridinyl-pyrazine adlayer, for which no redox wave was observed. In situ scanning tunneling microscopy revealed a structural change in the redox-active adlayer consisting of 4′,4′′′′-(1,4-phenylene)bis(2,2′:6′,2′′-terpyridine) and cobalt ions. It was found that the ability of cobalt ions to coordinate on Au(111) was clearly dependent on the chemical structure of the ligand, suggesting that ligand coordination with metal ions on the Au surface is determined by the molecular orientation and configuration of the ligand when the ligand is adsorbed on a Au substrate.     

Ligand effects on the stability of thiol-stabilized gold nanoclusters: Au25(SR)18(-), Au38(SR)24, and Au102(SR)44

We have studied the electrochemical and thermodynamic stability of Au(25)(SR)(18)(-), Au(38)(SR)(24), and Au(102)(SR)(44), R = CH(3), C(6)H(13), CH(2)CH(2)Ph, Ph, PhF, and PhCOOH, in order to examine ligand effects on the stability of thiol-stabilized gold nanoclusters, Au(m)(SR)(n). Aliphatic thiols, in general, have higher electrochemical and thermodynamic stability than aromatic thiols, and the -SCH(2)CH(2)Ph thiol is particularly appealing because of its high electrochemical and thermodynamic stability. The stabilization of Au(m) by nSR for Au(m)(SR)(n) can be rationalized by the stabilization of an Au atom by an SR for the simple molecule AuSR, regardless of interligand interaction and system size and shape. Thiol moieties play a strong role in the electron oxidation and reduction of Au(m)(SR)(n). Accounting for the characteristics of thiol ligands is essential for understanding the electronic and thermodynamic stability of thiol-stabilized gold nanoclusters.     

Recommendations on reporting electrode potentials in nonaqueous solvents: IUPC commission on electrochemistry

This report recommends that the redox couples ferrocene/ferricenium ion [ferrocene: bis(η-cyclopentadienyl) iron(II)] and bis(biphenyl)chromium(I)/bis(biphenyl)chromium(O) be employed as reference redox systems. Procedures for measuring and reporting electrode potentials in nonaqueous solvents versus these reference redox systems are presented. The difference in electrode potentials for ferrocene/ferricenium ion and bis(biphenyl)chromium(O)/bis(biphenyl)chromium(I) ion for 22 solvents obtained by polarography and cyclic voltammetry are listed. Both of the recommended redox systems approach a solvent independent redox system as closely as presently known. Electrode potential data reported versus these systems permit the comparison of electrode potentials for a given system in different nonaqueous solvents and thus allow studies of solvent effects on electrode potentials in nonaqueous systems by compiling and comparing data obtained in different solvents.     

AuAg alloy nanomolecules with 38 metal atoms

Au(38-n)Ag(n)(SCH(2)CH(2)Ph)(24) alloy nanomolecules were synthesized, purified and characterized by MALDI TOF mass spectrometry. Similar to 25 and unlike 144 metal atom count AuAg alloy nanomolecules, incorporation of Ag atoms here results in loss or smearing out of distinct UV-vis features. We propose that the short and long staples contain Au atoms, while the inner core consists of both Au and Ag atoms.     

Au25(SG)18 as a fluorescent iodide sensor

The recently emerging gold nanoclusters (GNC) are of major importance for both basic science studies and practical applications. Based on its surface-induced fluorescence properties, we investigated the potential use of Au(25)(SG)(18) (GSH: glutathione) as a fluorescent iodide sensor. The current detection limit of 400 nM, which can possibly be further enhanced by optimizing the conditions, and excellent selectivity among 12 types of anion (F(-), Cl(-), Br(-), I(-), NO(3)(-), ClO(4)(-), HCO(3)(-), IO(3)(-), SO(4)(2-), SO(3)(2-), CH(3)COO(-) and C(6)H(5)O(7)(3-)) make Au(25)(SG)(18) a good candidate for iodide sensing. Furthermore, our work has revealed the particular sensing mechanism, which was found to be affinity-induced ratiometric and enhanced fluorescence (abbreviated to AIREF), which has rarely been reported previously and may provide an alternative strategy for devising nanoparticle-based sensors.     

Strong non-linear effects in the chiroptical properties of the ligand-exchanged Au38 and Au40 clusters

Ligand exchange reactions on size-selected Au(38)(2-PET)(24) and Au(40)(2-PET)(24) clusters (2-PET: 2-phenylethylthiol) with mono- and bi-dentate chiral thiols were performed. The reactions were monitored with MALDI mass spectrometry and the arising chiroptical properties were compared to the number of incorporated chiral ligands. Only a small fraction of chiral ligands is needed to induce significant optical activity to the clusters. The use of bidentate 1,1'-binaphthyl-2,2'-dithiol (BINAS) leads to slow exchange, but the optical activity measured is strong. Moreover, a non-linear behaviour between optical activity and the number of chiral ligands is found in the BINAS case for both Au(38) and Au(40), which may indicate different exchange rates of enantiopure BINAS with the enantiomers of inherently chiral (but racemic) clusters. This is ascribed to effects arising from the bidentate nature of BINAS. In contrast, the use of monodentate camphor-10-thiol (CamSH) leads to comparably fast exchange on both clusters. The arising optical activity is weak. This is the first study where chiroptical effects are directly correlated with the composition of the ligand shell.     

Gold(I)‐Catalyzed Tandem Alkoxylation/Lactonization of γ‐Hydroxy‐α,β‐Acetylenic Esters

The formation of 4‐alkoxy‐2(5H)‐furanones was achieved via tandem alkoxylation/lactonization of γ‐hydroxy‐α,β‐acetylenic esters catalyzed by 2 mol% of [2,6‐bis(diisopropylphenyl)imidazol‐2‐ylidine]gold bis(trifluoromethanesulfonyl)imidate [Au(IPr)(NTf₂)]. The economic and simple procedure was applied to a series of various secondary propargylic alcohols allowing for yields of desired product of up to 95%. In addition, tertiary propargylic alcohols bearing mostly cyclic substituents were converted into the corresponding spiro derivatives. Both primary and secondary alcohols reacted with propargylic alcohols at moderate temperatures (65–80 °C) in either neat reactions or using 1,2‐dichloroethane as a reaction medium allowing for yields of 23–95%. In contrast to [Au(IPr)(NTf₂)], reactions with cationic complexes such as [2,6‐bis(diisopropylphenyl)imidazol‐2‐ylidine](acetonitrile)gold tetrafluoroborate [Au(IPr)(CH₃CN)][BF₄] or (μ‐hydroxy)bis{[2,6‐bis(diisopropylphenyl)imidazol‐2‐ylidine]gold} tetrafluoroborate or bis(trifluoromethanesulfonyl)imidate – [{Au(IPr)}₂(μ‐OH)][X] (X=BF₄, NTf₂) – mostly stop after the alkoxylation. Analysis of the intermediate proved the exclusive formation of the E‐isomer which allows for the subsequent lactonization.     

Hydrogenolysis of nitrosodimethyl amine in gas phase over Au/γ-Al2O3 nanocatalyst

Nitrosodimethyl amine (NDMA), as a carcinogenic byproduct in production of unsymmetrical dimethyl hydrazine (UDMH) in space industries, should be decomposed in the vapor phase. A suitable method for this purpose is selective catalytic hydrogenolysis of NDMA over Au/γ-Al₂O₃ nanocatalyst. We synthesized and characterized the Au/γ-Al₂O₃ nanocatalyst by homogeneous deposition-precipitation (HDP)/DP-urea method. Activity of the catalyst was influenced by nanosized Au particles, Au loading and the bed temperature. The optimum parameters for the catalyst were: Au particles <5 nm, Au loading at 1.5 wt% and bed temperature of 35–45 °C. The reaction was strongly sensitive to the Au particle size. The reaction occurred over the catalyst to produce dimethyl amine (DMA) and nitroxyl in a selective manner. The kinetics of NDMA hydrogenolysis over the nanocatalyst was studied in an integral fixed bed reactor. There existed a consistency with the Langmuir-Hinshelwood mechanism involving dissociative adsorption of H₂ and NDMA.     

Multifunctional peroxide as alternative crosslink agents for dynamically vulcanized epoxidized natural rubber/polypropylene blends

Commonly used dicumyl peroxide (DCP) in combination with coagent, triallyl cyanurate (TAC), as a crosslinking agent is well acceptable for dynamically vulcanized rubber phase of thermoplastic vulcanizates (TPVs). However, it generally produces volatile decomposition products, which cause a typical unpleasant smell and a blooming phenomenon. In this work, influence of two types of multifunctional peroxides: 2,4‐diallyloxy‐6‐tert‐butylperoxy‐1,3,5‐triazine (DTBT) and 1‐(2‐tert‐butylperoxyisopropyl)‐3‐isopropenyl benzene (TBIB), on properties of TPVs based on epoxidized natural rubber (ENR)/polypropylene (PP) blends were investigated. The conventional peroxide/coagent combinations, i.e., DCP/TAC and tert‐butyl cumyl peroxide (TBCP)/α‐methyl styrene (α‐MeS) were also used to prepare the TPVs for a comparison purpose. The TPVs with multifunctional peroxide, DTBT, provided good mechanical properties and phase morphology of small dispersed vulcanized rubber domains in the PP matrix which were comparable with the DCP/TAC cured TPVs. However, the TPVs with TBIB/α‐MeS and TBCP/α‐MeS showed comparatively low values of the tensile properties as well as rather large phase morphology. The results were interpreted by three main factors: the kinetic aspects of the various peroxides, solubility parameters of respective peroxide/coagent combinations in the ENR and PP phases, and the tendency to form unpleasantly smelling byproducts. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Highly efficient and fast adsorption of Au(III) and Pd(II) by crosslinked polyethyleneimine-glutaraldehyde

An adsorbent (PEI-GA) is prepared by crosslinking polyethyleneimine with glutaraldehyde. PEI-GA shows outstanding adsorption performance towards Au(III) and Pd(II). PEI-GA presents large adsorption capacity towards Au(III) in a wide application pH range from 1 to 9. The adsorption capacities of PEI-GA for Au(III) and Pd(II) at 25°C reach 2575 and 497 mg/g, respectively. Au(III) and Pd(II) can be adsorbed completely within 10 min for 8.3 mg/L Au(III) and 20 min for 9.7 mg/L Pd(II). The adsorption equilibrium time required for 523.9 mg/L Au(III) and for 565.6 mg/L Pd(II) is 2 and 9 h, respectively. The Sips model is the most suitable to describe the adsorption isotherms which leads to more realistic adsorption capacities for both metals. PEI-GA also exhibits high selectivity and repeatability towards Au(III) and Pd(II). The adsorption mechanism involves redox, chelation coordination, and electrostatic interactions for Au(III), and coordination and electrostatic interactions for Pd(II).