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.     

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(-).     

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).     

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.     

Controlled reduction for size selective synthesis of thiolate-protected gold nanoclusters Aun (n = 20, 24, 39, 40)

ABSTRACT: This work presents a controlled reduction method for the selective synthesis of different sized gold nanoclusters protected by thiolate (SR = SC2H4Ph). Starting with Au(III) salt, all the syntheses of Aun(SR)m nanoclusters with (n, m) = (20, 16), (24, 20), (39, 29), and (40, 30) necessitate experimental conditions of slow stirring and slow reduction of Au(I) intermediate species. By controlling the reaction kinetics for the reduction of Au(I) into clusters by NaBH4, different sized gold nanoclusters are selectively obtained. Two factors are identified to be important for the selective growth of Au20, Au24, and Au39/40 nanoclusters, including the stirring speed of the Au(I) solution and the NaBH4 addition speed during the step of Au(I) reduction to clusters. When comparing with the synthesis of Au25(SC2H4Ph)18 nanoclusters, we further identified that the reduction degree of Au(I) by NaBH4 also plays an important role in controlling cluster size. Overall, our results demonstrate the feasibility of attaining new sizes of gold nanoclusters via a controlled reduction route.     

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.     

Ligand K-edge X-ray absorption spectroscopy: a direct probe of ligand-metal covalency

Ligand K-edge X-ray absorption spectroscopy (XAS) is a new experimental probe of the covalency of a metal-ligand bond. The intensity of the ligand pre-edge feature is proportional to the mixing of ligand orbitals into the metal d orbitals. The methodology to determine covalencies in one-electron (hole) and many-electron systems is described and demonstrated for a series of metal tetrachlorides [MCl(4)](n)(-), metal tetrathiolates [M(SR)(4)](n)(-), and dimeric iron-sulfur (Fe-S) clusters [Fe(2)S(2)(SR)(4)](2-). It is then applied to blue Cu proteins, the Cu(A) site, hydrogen bonding in Fe-S clusters, and the delocalization behavior in [2Fe-2S] vs [4Fe-4S] clusters. The covalencies determined in these studies provide important electronic structure insight into function.     

Surface decorated platinum carbonyl clusters

Four molecular Pt-carbonyl clusters decorated by Cd-Br fragments, i.e., [Pt(13)(CO)(12){Cd(5)(μ-Br)(5)Br(2)(dmf)(3)}(2)](2-) (1), [Pt(19)(CO)(17){Cd(5)(μ-Br)(5)Br(3)(Me(2)CO)(2)}{Cd(5)(μ-Br)(5)Br(Me(2)CO)(4)}](2-) (2), [H(2)Pt(26)(CO)(20)(CdBr)(12)](8-) (3) and [H(4)Pt(26)(CO)(20)(CdBr)(12)(PtBr)(x)](6-) (4) (x = 0-2), have been obtained from the reactions between [Pt(3n)(CO)(6n)](2-) (n = 2-6) and CdBr(2)·H(2)O in dmf at 120 °C. The structures of these molecular clusters with diameters of 1.5-2 nm have been determined by X-ray crystallography. Both 1 and 2 are composed of icosahedral or bis-icosahedral Pt-CO cores decorated on the surface by Cd-Br motifs, whereas 3 and 4 display a cubic close packed Pt(26)Cd(12) metal frame decorated by CO and Br ligands. An oversimplified and unifying approach to interpret the electron count of these surface decorated platinum carbonyl clusters is suggested, and extended to other low-valent organometallic clusters and Au-thiolate nanoclusters.     

Electronic and steric effects in gold(i) phosphine thiolate complexes

The unusual yellow color of Au(2)(dppm)(SR)(2) (R = 4-tolyl; dppm = diphenylphosphinomethane) is attributed to a red-shift in the S-->Au charge transfer caused by destabilization of the sulfur highest occupied molecular orbital (HOMO). Variable temperature experiments show two broad bands at -80 degrees C in the (31)P{(1)H} NMR spectrum of Au(2)(dppm)(SR)(2) and the activation energy for interconversion is 10 kcal/mol. Only one sharp band is observed down to -80 degrees C in the spectrum of the white complex, Au(2)(dppe)(SR)(2) (dppe = diphenylphosphinoethane). Molecular mechanics calculations on Au(2)(dppm)(SR)(2) and Au(2)(dppe)(SR)(2) reveal that, for Au(2)(dppe)(SR)(2), a series of maxima and minima, separated by 2.5 kcal/mol, occur every 120 degrees which is consistent with rotation around an unhindered carbon-phosphorus single bond. The Au atoms are not within bonding distance in any conformation. Computational results for Au(2)(dppm)(SR)(2) indicate one minimum energy structure in which the Au-P bonds are anti. There is a high energy conformation (9 kcal/mol above the global minimum) where overlap between golds is maximized. The implications of gold-gold bonding in this complex are discussed. The steric influence of the thiolate ligand has been examined by synthesizing a series of dinuclear gold(I) complexes in which the steric properties of the thiolate are varied: Au(2)(dppm)(SR)(2) (R = 2,6-dichlorophenyl; 2,6-dimethylphenyl; 3,5-dimethylphenyl). The 2,6-disubstituted complexes are white, while the 3,5-dimethyl complex is yellow. These results, along with VT-NMR experiments, are consistent with the conclusion that the more sterically-bulky thiolates hinder the close approach of the golds in the dinuclear complexes.     

Magnetically separable Prussian blue analogue Mn₃[Co(CN)₆]₂·nH₂O porous nanocubes as excellent absorbents for heavy metal ions

The application of Prussian blue analogue (PBA) Mn(3)[Co(CN)(6)](2)·nH(2)O porous nanocubes as absorbents for heavy metal ions has been demonstrated. The result indicates that Mn(3)[Co(CN)(6)](2)·nH(2)O porous nanocubes with average diameter of 240 nm possess excellent adsorption efficiency for Pb(2+) ions (94.21% at initial Pb(2+) concentration of 10 mg L(-1)). Moreover, Mn(3)[Co(CN)(6)](2)·nH(2)O porous nanocubes can also show high adsorption efficiency on heavy metal ions even in a strong acidic solution due to its chemical stability. Notably, an external magnet could be used to accelerate the separation of Mn(3)[Co(CN)(6)](2)·nH(2)O from the treated solution. It is suggested that the high adsorption efficiency may derive from the large surface area, M(3)(II)[M(III)(CN)(6)](2)·nH(2)O porous framework structure and affinity between polarizable π-electron clouds of the cyanide bridges and heavy metals ions.     

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.     

Atomic Resolution Imaging of Au Nanocluster Dispersed in TiO2, SrTiO3, and MgO

Gold nanoclusters dispersed in single crystal TiO₂, MgO, and SrTiO₃ have been prepared by ion implantation at 300–975 K and subsequent annealing at 1275 K for 10 h. High-resolution transmission electron microscopy and high-angle annular dark field (HAADF) imaging in aberration corrected scanning transmission electron microscope (STEM) have been used to characterize the microstructure of the gold nanoclusters dispersed materials. STEM-HAADF imaging with atomic resolution has directly revealed for all three materials that Au atoms partially occupy cation lattice positions. Cavities up to several tens of nanometers were observed in MgO and SrTiO₃. The cavities and gold clusters are spatially associated in MgO and SrTiO₃, indicating a strong interaction between the Au cluster and cavities. For MgO and SrTiO₃, the faceting planes appear to be the same for both nanometer-sized cavity and the Au cluster, demonstrating that both the surface energy and the interfacial energy between Au cluster and the matrix are lowest on these planes.     

(AuAg)144(SR)60 alloy nanomolecules

(Au-Ag)(144)(SR)(60) alloy nanomolecules were synthesized and characterized by ESI mass spectrometry to atomic precision. The number of Ag atoms can be varied by changing the incoming metal ratio and plateaus at ～60. UV-vis data demonstrates that the electronic structure of the nanomolecules can be tuned by incorporation of silver atoms. Based on the proposed 3-shell structure of Au(144)(SR)(60), we hypothesize that the Ag atoms are selectively incorporated in to the symmetry equivalent 60-atom shell-having Au(12), Au(42), Ag(60) concentric shells with 30 -SR-Au-SR- protecting units.     

Relaxation dynamics of Au25L18 nanoclusters studied by femtosecond time-resolved near infrared transient absorption spectroscopy

The relaxation dynamics of electronically excited [Au(25)(SR)(18)](q), where q = 0 or -1 and SR = S(CH(2))(2)Ph, were studied using femtosecond time-resolved transient absorption spectroscopy. Nanoclusters excited by 400 nm light were probed using temporally delayed broad-bandwidth continuum probe pulses. Continuum pulses were generated in both the visible and near infrared (NIR) spectral regions, providing access to a wide range of transient spectral features. The use of NIR probe pulses allowed the relaxation dynamics of the excited states located near the HOMO-LUMO energy gap to be monitored in the probe step via the sp ← LUMO and sp ← LUMO+1 transitions. These NIR measurements yielded excited state absorption (ESA) data that were much less congested than the typical visible transient spectrum. For the neutral nanocluster, the time-domain data were composed of three components: (1) a few-picosecond decay, (2) a slower decay taking a few hundred picoseconds and (3) a non-decaying plateau function. Component 1 reflected energy relaxation to semi-ring ligand states; component 2 was attributed to relaxation via a manifold of states located near the HOMO-LUMO energy gap. Component 3 arose from slow radiative recombination. The dynamics of the anion depended upon the identity of the excited state from which the particle was relaxing. The LUMO+1 state of the anion exhibited relaxation dynamics that were similar to those observed for the neutral nanocluster. By comparison, the time-domain data observed for the LUMO state contained only two components: (1) a 3.3 ± 0.2 ps decay and (2) a 5 ± 1 ns decay. The amplitude coefficients of each component were also analyzed. Taken together, the amplitude coefficients and lifetimes were indicative of an activation barrier located approximately 100 meV above the HOMO-LUMO energy gap, which mediated a previously unobserved excited state decay process for [Au(25)(SR)(18)](0). These data suggested that NIR ESA measurements will be instrumental in describing the relaxation processes of quantum-confined nanoclusters.     

Myochrysine solution structure and reactivity

We have determined the framework structure of Myochrysine (disodium gold(I)thiomalate) in the solid state and extremely concentrated aqueous solution, previously. It consists of an open chain polymer with linear gold coordination to two thiolates from the thiomalic acid moieties which bridge between pairs of gold atoms providing an Au-S-Au angle of 95 degrees . The question remained: was this structure relevant to the dilute solutions of drugs administered and the still lower concentrations of gold found in the bodies of patients (typically 1 ppm Au in blood and urine or 5 muM in Au). We have provided an answer to that question using extended X-ray absorption spectroscopy (EXAFS) and capillary zone electrophoresis (CZE). EXAFS studies confirm that the polymeric structure with two sulfur atoms per gold atom persists from molar concentrations down to millimolar concentrations. CZE is able to separate and detect Myochrysine at millimolar levels. More importantly, at micromolar levels Myochrysine solutions exhibit identical CZE behavior to that measured at millimolar levels. Thus, aqueous solutions of the drug remain oligomeric at concentrations commensurate with those found in patient blood and urine.The reactivity of Myochrysine with cyanide, a species especially prevalent in smoking patients, was explored using CZE. Cyanide freely replaces thiomalic acid to form [Au(CN)(2)](-) and thiomalic acid via a mixed ligand intermediate. The overall apparent equilibrium constant (K(app)) for the reaction is 6x10(-4)M(-1). Further reaction of [Au(CN)(2)](-) with a large excess of L, where L is cysteine, N-acetylcysteine, or glutathione, shows that these amino acids readily replace cyanide to form [AuL(2)](-). These species are thus potential metabolites and could possibly be active forms of gold in vivo. That all of these species are readily separated and quantified using CZE demonstrates that capillary electrophoresis is an accessible and powerful tool to add to those used for the study of gold-based antiarthritis drugs.     

Reaction of Au(111) with Sulfur and Oxygen: Scanning Tunneling Microscopic Study

The reaction of sulfur and oxygen with the gold surface is important in many technological applications, including heterogeneous catalysis, corrosion, and chemical sensors. We have studied reactions on Au(111) using scanning tunneling microscopy (STM) in order to better understand the surface structure and the origin of gold’s catalytic activity. We find that the Au(111) surface dynamically restructures during deposition of sulfur and oxygen and that these changes in structure promote the reactivity of Au with respect to SO₂ and O₂ dissociation. Specifically, the Au(111) herringbone reconstruction lifts when either S or O is deposited on the surface. We attribute this structural change to the reduction of tensile surface stress via charge redistribution by these electronegative adsorbates. This lifting of the reconstruction was accompanied by the release of gold atoms from the herringbone structure. At high coverage, clusters of gold sulfides or gold oxides form by abstraction of gold atoms from regular terrace sites of the surface. Concomitant with the restructuring is the release of gold atoms from the herringbone structure to produce a higher density of low-coordinated Au sites by forming serrated step edges or small gold islands. These undercoordinated Au atoms may play an essential role in the enhancement of catalytic activity of gold in reactions such as oxygen dissociation or SO₂ decomposition. Our results further elucidate the interaction between sulfur and oxygen and the Au(111) surface and indicate that the reactivity of Au nanoclusters on reducible metal oxides is probably related to the facile release of Au from the edges of these small islands. Our results provide insight into the sintering mechanism which leads to deactivation of Au nanoclusters and into the fundamental limitation in the edge definition in soft lithography using thiol-based self-assembled monolayers (SAMs) on Au. Furthermore, the enhanced reactivity of Au after release of undercoordinated atoms from the surface indicate a relatively insignificant role of an oxide support for high reactivity.     

Formation of Gold Clusters on TiO2 from Adsorbed Au(CH3)2(C5H7O2): Characterization by X-ray Absorption Spectroscopy

Gold nanoclusters on TiO₂ powder were prepared from adsorbed Au^III(CH₃)₂(C₅H₇O₂) (dimethyl acetylacetonate gold(III)) and characterized by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies. The samples were tested as catalysts for CO oxidation at 298 K and atmospheric pressure and characterized by EXAFS and XANES with the catalysts in the working state. The XANES results identify Au(III) in the initially prepared sample, and the EXAFS data indicate mononuclear gold complexes as the predominant surface gold species in this sample, consistent with the lack of Au–Au contributions in the EXAFS spectrum. The mononuclear gold complex is bonded to two oxygen atoms of the TiO₂ surface at an Au–O distance of 2.16 Å. Treatment of this complex in He or in H₂ at increasing temperatures led to formation of metallic gold clusters of increasing size, ultimately those with an average diameter of about 15 Å. The data demonstrate the presence of metallic gold clusters in the working catalysts and also show these clusters alone are not responsible for the catalytic activity.     

First-principles investigation of ag-doped gold nanoclusters

Gold nanoclusters have the tunable optical absorption property, and are promising for cancer cell imaging, photothermal therapy and radiotherapy. First-principle is a very powerful tool for design of novel materials. In the present work, structural properties, band gap engineering and tunable optical properties of Ag-doped gold clusters have been calculated using density functional theory. The electronic structure of a stable Au(20) cluster can be modulated by incorporating Ag, and the HOMO-LUMO gap of Au(20-) (n)Ag(n) clusters is modulated due to the incorporation of Ag electronic states in the HOMO and LUMO. Furthermore, the results of the imaginary part of the dielectric function indicate that the optical transition of gold clusters is concentration-dependent and the optical transition between HOMO and LUMO shifts to the low energy range as the Ag atom increases. These calculated results are helpful for the design of gold cluster-based biomaterials, and will be of interest in the fields of radiation medicine, biophysics and nanoscience.