Properties of a Ubiquitous 29 kDa Au:SR Cluster Compound

Miles D.T., Murray R.W.

Monolayer-protected Au clusters (MPCs) have been prepared with mixed monolayers of alkanethiolates and alkanethiolates terminally omega-functionalized with phenothiazine. The mixed monolayer MPCs can contain as many as 10 phenothiazines/MPC; these electron donors are electroactive in rapid, successive one-electron reactions. Surface adsorption of the functionalized MPCs is evident in cyclic voltammetry. Double-potential-step chronocoulometry with incremented potential steps was applied to unfunctionalized hexanethiolate-coated MPCs and to those functionalized with phenothiazine to analyze the coupling between the diffusion-controlled double-layer charging of the MPC cores and the oxidation of the phenothiazine centers. Apparent changes in ordering of the MPC alkanethiolate chains were observed with infrared spectroscopy in solutions of MPCs where alcohol, carboxylic acid, or phenothiazine moieties had been incorporated into the monolayer.

Wuelfing W.P., Zamborini F.P., Templeton A.C., Wen X., Yoon H., Murray R.W.

Monolayer-protected clusters (MPCs) are used to prepare solid, continuous metal films containing a single white metal or an alloy thereof. MPCs consist of nanoscopic metal cores coated with monolayers of thiolate ligands. In one method, multilayer films of carboxylate-functionalized alkanethiolate MPCs are assembled using Cu2+ coordinative bridges. The MPC film can be thermally decomposed at moderate temperature (

Gittins D.I., Bethell D., Schiffrin D.J., Nichols R.J.

So-called bottom-up fabrication methods aim to assemble and integrate molecular components exhibiting specific functions into electronic devices that are orders of magnitude smaller than can be fabricated by lithographic techniques. Fundamental to the success of the bottom-up approach is the ability to control electron transport across molecular components. Organic molecules containing redox centres—chemical species whose oxidation number, and hence electronic structure, can be changed reversibly—support resonant tunnelling1,2 and display promising functional behaviour when sandwiched as molecular layers between electrical contacts3,4, but their integration into more complex assemblies remains challenging. For this reason, functionalized metal nanoparticles have attracted much interest5,6,7: they exhibit single-electron characteristics8,9,10 (such as quantized capacitance charging) and can be organized11,12,13 through simple self-assembly methods into well ordered structures, with the nanoparticles at controlled locations. Here we report scanning tunnelling microscopy measurements showing that organic molecules containing redox centres can be used to attach metal nanoparticles to electrode surfaces and so control the electron transport between them. Our system consists of gold nanoclusters a few nanometres across and functionalized with polymethylene chains that carry a central, reversibly reducible bipyridinium moiety14,15. We expect that the ability to electronically contact metal nanoparticles via redox-active molecules, and to alter profoundly their tunnelling properties by charge injection into these molecules, can form the basis for a range of nanoscale electronic switches.

Schaaff T.G., Whetten R.L.

A series of giant metal-cluster compounds, each composed of a gold core and a glutathione (GSH) adsorbate layer, have been prepared from Au(I)SG polymers and separated by gel electrophoresis, using methods reported in a recent Letter [J. Phys. Chem. B 1998, 102, 10643−6]. Identification of the separated compounds by core mass is accomplished through laser desorption mass spectrometry of matrix-diluted films. Three principal compounds have core masses of ca. 4.3, 5.6, and 8.2 kDa (in the range of ∼20−40 Au atoms), and show structured optical absorption spectra with clear optical absorption onsets near 1.7, 1.3, and 1.0 eV, respectively. Each of these shows unusually strong chiroptical activity in the metal-based electronic transitions across the near-infrared, visible, and near-ultraviolet regions, whereas neither the crude (unseparated) mixture nor its higher molecular weight components possess such strong optical activity. The location and strength of the optical activity suggest a metal electronic struc...

Gittins D.I., Bethell D., Nichols R.J., Schiffrin D.J.

Novel redox active dithiols containing the viologen (4,4′-bipyridinium group) have been synthesised. Monolayers and multilayers consisting of alternating layers of the dithiol and gold nanoparticles have been self-assembled on a gold substrate and visualised using SPM. Such materials have been investigated for their electroactivity and their ability to mediate electron transfer to an external redox couple, [Ru(NH3)6]3+/2+, in aqueous solution. Viologen moieties in these multilayers are addressable electrochemically provided they are within 6–8 layers of the external surface and it is shown that electron transfer through the deeper layers is very facile as a consequence of the presence of partially filled viologen π-orbitals. When the viologen containing dithiol is present in the outer layer, a diode effect is observed; electron transfer from the multilayer to the external couple takes place, but the reverse electron transport is inhibited.

Gutiérrez E., Powell R.D., Furuya F.R., Hainfeld J.F., Schaaff T.G., Shafigullin M.N., Stephens P.W., Whetten R.L.

We report the isolation of a previously elusive giant gold-cluster compound with unusual electronic structure and excellent solid-state ordering properties. Greengold is a water-soluble Au:PR3 compound (R=p-C6H4-CONHCH3) formed in high yield in the solution-phase reduction of R3PAuCl. Following chromatographic isolation, it has been investigated by electron microscopy (STEM), optical spectroscopy, mass spectrometry (MALDI), and X-ray diffraction, from which emerges a consistent picture of Greengold’s intrinsic characteristics as (i) a single compound of ∼22000 amu molecular weight, containing a strongly bound globular metal core of mass 14800 (Au75, 1.3-nm equivalent diameter); (ii) a highly ordered solid, exhibiting diffraction through 21 orders of a 2.47-nm spacing; and (iii) a carrier of a highly structured optical absorption across the entire visible and near-infrared spectrum (to ∼1.0 eV), including relative transparency in the blue and yellow regions, giving rise to its eponymous green tint.

Schaaff T.G., Whetten R.L.

When Au:SR cluster compounds (R = (CH2)5CH3) with an Au metallic cluster core mass of ∼14 kDa (∼75 atoms, 1.1 nm equivalent diameter), are heated in neat dodecane−thiol solution under inert atmosphere, species with smaller Au core masses are formed, presumably by the removal of Au atoms from the outermost surface layer of the cluster's Au core. This process was monitored through laser desorption mass spectrometry, optical absorption spectroscopy, and X-ray diffraction, where all three methods indicate a substantial decrease (>50%, by mass or number of Au atoms) in the size of the cluster compound's inorganic core. The optical absorption spectra and laser desorption mass spectra of the compounds generated by etching are strikingly similar to compounds previously separated without an etching step. The dual function of the thiol as both stabilizing adsorbate and efficient etchant sets this cluster (or nanocrystal) system apart from other metallic and semiconductor systems.

Hicks J.F., Templeton A.C., Chen S., Sheran K.M., Jasti R., Murray R.W., Debord J., Schaaff T.G., Whetten R.L.

This report describes how the electrochemical double-layer capacitances of nanometer-sized alkanethiolate monolayer-protected Au clusters (MPCs) dissolved in electrolyte solution depend on the alkanethiolate chain length (C4 to C16). The double-layer capacitances of individual MPCs (C(CLU)) are sufficiently small (sub-attoFarad, aF) that their metal core potentials change by >0.1 V increments for single electron transfers at the electrode/solution interface. Thus, the current peaks observed are termed "quantized double layer charging peaks", and their spacing on the potential axis varies with C(CLU). Differential pulse voltammetric measurements of C(CLU) in solutions of core-size-fractionated (i.e., monodisperse) MPCs are compared to a simple theoretical model, which considers the capacitance as governed by the thickness of a dielectric material (the monolayer, whose chain length is varied) between concentric spheres of conductors (the Au core and the electrolyte solution). The experimental results fit the simple model remarkably well. The prominent differential pulse voltammetric charging peaks additionally establish this method, along with high-resolution transmission electron microscopy and laser ionization-desorption mass spectrometry, as a tool for evaluating the degree of monodispersity of MPC preparations. We additionally report on a new tactic for the preparation of monodisperse MPCs with hexanethiolate monolayers.

Pietron J.J., Hicks J.F., Murray R.W.

Monolayer-protected clusters (MPCs) of gold with a majority composition of [CH3(CH2)6S]50Au145 have been synthesized. Solutions of this material in a toluene−acetonitrile solvent, containing a supporting electrolyte, are shown to display quantized double layer capacitance charging in differential pulse voltammetry experiments. These solutions can also be electrolytically charged to controllable potentials, and the average number of electrons (or holes) stored on the metal-like MPC cores can be estimated from the pattern of single-electron quantized charging peaks. The charged solutions are stable for hours. The charged MPCs can also be isolated in dried form, and retain most of their charge upon redissolution. The electronic charge on the MPC cores can be employed to carry out redox reactions with electron acceptor and donor molecules, such as ethylferrocene and TCNQ. These reactions proceed in a predictable and quantifiable way. Charged MPCs also undergo electron transfer reactions when mixed with soluti...

Whetten R.L., Shafigullin M.N., Khoury J.T., Schaaff T.G., Vezmar I., Alvarez M.M., Wilkinson A.

Hostetler M.J., Templeton A.C., Murray R.W.

Monolayer-protected gold clusters (Au MPCs) are stable, easily synthesized, organic solvent-soluble, nanoscale materials. MPCs with protecting monolayers composed of alkanethiolate ligands (RS) can be functionalized (R‘S) by ligand place-exchange reactions, i.e., x(R‘SH) + (RS)mMPC → x(RSH) + (R‘S)m(RS)m-xMPC, where x is the number of ligands place-exchanged (1 to 108) and m is the original number (ca. 108) of alkanethiolate ligands per Au314 cluster. The dynamics and mechanism of this reaction were probed by determining its kinetic order and final equilibrium position relative to incoming (R‘S) and initial (RS) protecting thiolate ligands. The reactions were characterized by 1H NMR and IR spectroscopy, and the dispersity of place-exchange reaction products was preliminarily inspected by chromatography. The results of these experiments show that ligand exchange is an associative reaction and that the displaced thiolate becomes a thiol solution product. Disulfides and oxidized sulfur species are not involv...

Templeton A.C., Chen S., Gross S.M., Murray R.W.

Isolable, water-soluble gold clusters protected by monolayers of tiopronin (tiopronin−MPCs) or coenzyme A (CoA−MPCs) were synthesized by a procedure of comparable simplicity to the Brust synthesis for alkanethiolate monolayer-protected gold clusters. High-resolution transmission electron microscopy shows that, like their alkanethiolate−MPC counterparts, the average core diameters of tiopronin−MPCs can be systematically controlled by varying the tiopronin:Au mole ratio employed in the reaction. The UV−vis spectra of tiopronin−MPCs exhibit pH and core size dependency of the surface plasmon band position and intensity, respectively. Thermogravimetric analysis of the tiopronin−MPCs gave average numbers of tiopronin ligands per cluster; for example, tiopronin−MPCs with an average core size of 1.8 nm (∼Au201) are protected with an average of 85 tiopronin ligands. 1H NMR reveals a size-dependent evolution of spectral features interpreted as reflecting differences in attachment sites (terrace, defects) and/or res...

Schaaff T.G., Knight G., Shafigullin M.N., Borkman R.F., Whetten R.L.

An unprecedented small thioaurite cluster compound (with metallic Au0 core) has been isolated in high yield by decomposition of polymeric Au(I)SG compounds, where GSH is the ubiquitous tripeptide glutathione, N-γ-glutamyl-cysteinyl-glycine. The Au:SG clusters appear to share the high stability and robustness of their hydrophobic n-alkyl analogues but are highly water soluble. The most abundant cluster produced by these methods can be easily separated from its homologues by gel electrophoresis. Its total molecular weight is ca. 10.4 kDa, and the mass of its strongly bound inorganic core is 5.6 kDa, suggesting the composition Au28(SG)16. This composition is also consistent with the X-ray diffraction pattern of the crystalline molecular solid. Distinct features in the optical absorption spectroscopy are inherently different from either larger clusters or smaller gold cluster compounds. The compound is optically active, as evidenced by circular dichroism in the near-IR, visible, and near-UV regions. The 13C N...

Zhang Q.M., Bharti V., Zhao X.

An exceptionally high electrostrictive response ( approximately 4 percent) was observed in electron-irradiated poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer. The material exhibits typical relaxor ferroelectric behavior, suggesting that the electron irradiation breaks up the coherent polarization domain (all-trans chains) in normal ferroelectric P(VDF-TrFE) copolymer into nanopolar regions (nanometer-size, all-trans chains interrupted by trans and gauche bonds) that transform the material into a relaxor ferroelectric. The expanding and contracting of these polar regions under external fields, coupled with a large difference in the lattice strain between the polar and nonpolar phases, generate an ultrahigh strain response.

Shin H.K.

The possibility of producing highly excited OD radicals in the reaction of gas-phase atomic oxygen with deuterium chemisorbed on a tungsten surface has been explored using classical trajectory procedures. The many-body nature of the gas−surface reaction is explicitly considered in constructing a potential energy surface. Nearly all reactive events occur in a direct collision on a subpicosecond scale. The vibrational population distribution of OD radicals is found to be inverted, with the highest population appearing at vibrational level 6. The Eley−Rideal mechanism can account for these results. The calculations are carried out at the gas and surface temperatures of 1000 and 300 K, respectively, where the probability of OD formation is 0.12.

Peng B., Zhang K.

AbstractThe debate about water as an emitter has spanned nearly a century, but how it emits bright colors remains elusive. In this report, using the widely used Au(I)‐alkanethiolate complex (Au(I)‐SRs, R=−(CH2)12H) with AIE properties as a model system, by carefully manipulating the delicate surface‐ligand interactions at the nanoscale interface, together with a careful spectral investigation and an isotopic diagnostic experiment of heavy water (D2O), we demonstrated that the structural water molecules (SWs) trapped in the nanoscale interface or space are the true emission centers of metal nanoclusters (NCs) and the aggregates of Au(I)‐SRs complexes, instead of a well‐organized metal core dominated by quantum confinement mechanics. Unlike conventional hydrogen‐bonded water molecules, due to interfacial adsorption or spatial confinement, the p‐orbitals of two O atoms in SWs can form strong electronic interactions through spatial overlap, thus constructing a set of interfacial states, one of which is characterized by π‐bonding, thus providing alternative channels (or paths) for the relaxation decay of the excited electrons. Using the one‐dimensional free‐electron gas model, the energy levels calculated by the Schrödinger equation are in perfect agreement with the experimental observations, further validating the SWs model.

McCandler C.A., Pihlajamäki A., Malola S., Häkkinen H., Persson K.A.

Jiang Y., Li S., Wang Y., Sanvito S., Hou S.

Kuda‐Singappulige G.U., Window P.S., Hosier C.A., Anderson I.D., Aikens C.M., Ackerson C.J.

AbstractThe crystal structures of 4 ligand‐rotational isomers of Au25(PET)18 are presented. Two new ligand‐rotational isomers are revealed, and two higher‐quality structures (allowing complete solution of the ligand shell) of previously solved Au25(PET)18 clusters are also presented. One of the structures lacks an inversion center, making it the first chiral Au25(SR)18 structure solved. These structures combined with previously published Au25(SR)18 structures enable an analysis of the empirical ligand conformation landscape for Au25(SR)18 clusters. This analysis shows that the dihedral angles within the PET ligand are restricted to certain observable values, and also that the dihedral angle values are interdependent, in a manner reminiscent of biomolecule dihedral angles such as those in proteins and DNA. The influence of ligand conformational isomerism on optical and electronic properties was calculated, revealing that the ligand conformations affect the nanocluster absorption spectrum, which potentially provides a way to distinguish between isomers at low temperature.

Gu W., Wu Z.

Metal nanoclusters (NCs) are ultrasmall-sized nanoparticles and stable entities composed of 10s–100s of metal atoms in the size range. Compared with conventional nanocrystals, metal NCs show some attractive advantages, such as atomic monodispersity and precise compositions/structures. To date, with the continuous progress of wet chemistry, various metal NCs with atomic precision have been successfully prepared and isolated, including gold, silver, and copper NCs. This chapter discusses the gold nanoclusters (Au NCs) with the most abundant species, which can be dated back to the 1960s. It focuses on the thiolated Au NCs with well-defined compositions and structures. The chapter reviews general synthesis strategies, facile synthesis methods, and common synthesis parameters that influence the structures and properties of NCs, basing on the results of the literature.

Du X., Ma H., Zhang X., Zhou M., Liu Z., Wang H., Wang G., Jin R.

Alloying is an important strategy in tailoring the functionality of materials. In metal nanoclusters (NCs), the introduction of a heterometal leads to alloy nanoclusters that often outperform the homometal ones in terms of the physical and chemical properties. In this work, a series of four M144(PET)60 alloy NCs (where, M = Au/Ag, PET = −SCH2CH2Ph) are synthesized and characterized. The silver doping into the homogold template (Au144) leads to more prominent optical absorption features in the steady-state spectrum in the visible range. Femtosecond transient absorption spectroscopy reveals the effect of Ag doping on the electronic relaxation dynamics compared to Au144 and the pump fluence independent dynamics. Electrochemical results reflect a narrowing of HOMO—LUMO gap (Eg) induced by Ag doping. A temperature dependence of the single-electron charging is also observed for the series of alloy NCs, in which the Eg values of the alloy NCs enlarge as the temperature decreases, which is characteristic of semiconducting behavior.

Eswaramoorthy S.K., Dass A.

Dainese T., Antonello S., Bonacchi S., Morales-Martinez D., Venzo A., Black D.M., Mozammel Hoque M., Whetten R.L., Maran F.

We isolated and characterized the thiolate-protected Au145 nanocluster. Au145 exhibits a behavior similar to that of Au144, but for its much more intense luminescence.

Kawawaki T., Shimizu N., Funai K., Mitomi Y., Hossain S., Kikkawa S., Osborn D.J., Yamazoe S., Metha G.F., Negishi Y.

This work established a simple method for the size-selective synthesis of a series of ligand-protected platinum nanoclusters with superior oxygen reduction reactivity.

Abou-Omar M.N., Attia M.S., Afify H.G., Amin M.A., Boukherroub R., Mohamed E.H.

The urge for sensitive, facile, minimally invasive, and fast detection method of CA-125, a significant and crucial biomarker in ovarian malignancy, is currently substantial. This paper describes the detailed construction and characterization of a newly designed optical nano-biosensor to detect CA-125 accurately and sensitively. The fabricated sensor consists of a nano-gold thin film doped into a matrix of sol–gel, exhibiting a centered fluorescence band at 423 nm when excited at 340 nm. The quantification of CA-125 relies on its quenching ability of this fluorescence signal. The sensor was challenged to evaluate its sensitivity and specificity in detecting CA-125 present in samples collected from ovarian cancer diagnosed patients and compared to samples from healthy women as a control. Our findings revealed that the developed biosensor had a sensitivity of 97.35% and a specificity of 94.29%. Additionally, a wide linearity range over 2.0–127.0 U mL–1 for CA-125 was achieved with a detection limit of 1.45 U mL–1. Furthermore, the sensor could successfully discriminate samples between healthy and diseased people, which demonstrates its suitability in CA-125 assessment.

Wang Z., Pan X., Qian S., Yang G., Du F., Yuan X.

• An overview of the two-phase strategies for ultrasmall metal nanoclusters. • The two-phase synthesis, processing, functionalization, and application of metal nanoclusters. • The challenges and prospects related to the two-phase strategies for metal nanoclusters. Ultrasmall metal nanoclusters (NCs) are a sub-class of metal nanoparticles (NPs) with a metal core size of ≤3 nm, and display interesting molecule-like properties. Atomically precise metal NCs can be synthesized by many solution chemistry methods in polar and non-polar reaction environments. Among these methods, two-phase based strategies involving a water-oil binary reaction system have emerged as a facile means to synthesize, process, functionalize, and apply metal NCs. This Review firstly presents an overview of the two-phase based strategies in the synthesis of hydrophobic and hydrophilic metal NCs, and aims to elucidate the underlying working principles of these strategies. Then, how a two-phase system could process metal NCs for size/composition tailoring, luminescence generation, sorting and purification is discussed. Subsequently, several examples that utilizing a two-phase system to efficiently functionalize metal NCs with different surface chemistry are highlighted. Finally, two-phase based applications of metal NCs in catalysis and self-assembly are also reviewed. The fundamentals and the advances of the two-phase based strategies in the synthesis, processing, functionalization, and application of metal NCs summarized in this Review may increase the acceptance of two-phase based strategy in the metal NC field.

Li C., Xu S., Jin M., Wan D.

A-few-thioether-integrated polyamine patches on a support directly mediate uniform gold nanoclusters to afford highly catalytically active and well recyclable catalysts.

Sakthivel N.A., Jupally V.R., Eswaramoorthy S.K., Wijesinghe K.H., Nimmala P.R., Kumara C., Rambukwella M., Jones T., Dass A.

Highly monodisperse and pure samples of atomically precise gold nanomolecules (AuNMs) are essential to understand their properties and to develop applications using them. Unfortunately, the synthetic protocols that yield a single-sized nanomolecule in a single-step reaction are unavailable. Instead, we observe a polydisperse product with a mixture of core sizes. This product requires post-synthetic reactions and separation techniques to isolate pure nanomolecules. Solvent fractionation based on the varying solubility of different sizes serves well to a certain extent in isolating pure compounds. It becomes tedious and offers less control while separating AuNMs that are very similar in size. Here, we report the versatile and the indispensable nature of using size exclusion chromatography (SEC) as a tool for separating nanomolecules and nanoparticles. We have demonstrated the following: (1) the ease of separation offered by SEC over solvent fractionation; (2) the separation of a wider size range (∼5-200 kDa or ∼1-3 nm) and larger-scale separation (20-100 mg per load); (3) the separation of closely sized AuNMs, demonstrated by purifying Au137(SR)56 from a mixture of Au329(SR)84, Au144(SR)60, Au137(SR)56, and Au130(SR)50, which could not be achieved using solvent fractionation; (4) the separation of AuNMs protected by different thiolate ligands (aliphatic, aromatic, and bulky); and (5) the separation can be improved by increasing the column length. Mass spectrometry was used for analyzing the SEC fractions.

Sholokhova A.Y., Malkin A.I., Buryak A.K.

Powders of boron, magnesium and tungsten before and after mechanochemical activation are studied via laser desorption/ionization. It is found that the activation process removes low molecular weight hydrocarbon impurities and high molecular weight clusters of oxides and hydroxides. It is shown that this technique allows determination of clusters in a wide range of masses.