Total Structure Determination of Thiolate-Protected Au38 Nanoparticles

Noble metal alloy clusters in the gas phase derived from protein templates: unusual recognition of palladium by gold

Matrix assisted laser desorption ionization of a mixture of gold and palladium adducts of the protein lysozyme (Lyz) produces naked alloy clusters of the type Au24Pd(+) in the gas phase. While a lysozyme-Au adduct forms Au18(+), Au25(+), Au38(+) and Au102(+) ions in the gas phase, lysozyme-Pd alone does not form any analogous cluster. Addition of various transition metal ions (Ag(+), Pt(2+), Pd(2+), Cu(2+), Fe(2+), Ni(2+) and Cr(3+)) in the adducts contributes to drastic changes in the mass spectrum, but only palladium forms alloys in the gas phase. Besides alloy formation, palladium enhances the formation of specific single component clusters such as Au38(+). While other metal ions like Cu(2+) help forming Au25(+) selectively, Fe(2+) catalyzes the formation of Au25(+) over all other clusters. Gas phase cluster formation occurs from protein adducts where Au is in the 1+ state while Pd is in the 2+ state. The creation of alloys in the gas phase is not affected whether a physical mixture of Au and Pd adducts or a Au and Pd co-adduct is used as the precursor. The formation of Au cores and AuPd alloy cores of the kind comparable to monolayer protected clusters implies that naked clusters themselves may be nucleated in solution.

Au103(SR)45, Au104(SR)45, Au104(SR)46 and Au105(SR)46 nanoclusters

High resolution ESI mass spectrometry of the "22 kDa" nanocluster reveals the presence of a mixture containing Au103(SR)45, Au104(SR)45, Au104(SR)46, and Au105(SR)46 nanoclusters, where R = -CH2CH2Ph. MALDI TOF MS data confirm the purity of the sample and a UV-vis spectrum shows minor features. Au102(SC6H5COOH)44, whose XRD crystal structure was recently reported, is not observed. This is due to ligand effects, because the 102 : 44 composition is produced using aromatic ligands. However, the 103-, 104- and 105-atom nanoclusters, protected by -SCH2CH2Ph and -SC6H13 ligands, are at or near 58 electron shell closing.

Ultrathin nanostructures: smaller size with new phenomena

Ultrathin nanostructures possess the very essential features of nanomaterials, including quantum-confinement effects and unconventional reactivities, which are determined by the significant structure variations from the bulk material. More and more isolated reports on ultrathin nanostructures and various new phenomena have appeared in recent years but a comprehensive review on their typical features and future development has not followed. Here we aim to present a well-organized review which comments on the most important characteristics of non-carbon ultrathin nanostructures, in an attemp to reveal the underlying relationship between their reactivity, stability and transformation law, and their structures.

Reversible chirality control in peptide-functionalized gold nanoparticles

We report the induction of chiroptical properties in 2 nm diameter gold nanoparticles passivated with short peptides characterized by the Aib-l-Ala repetition in their sequence. The nanoparticles present relevant ECD signals in the 300-650 nm wavelength region, corresponding to the gold nanoparticle's quantized electronic structure. Although the only chiral amino acid present in the peptide sequences is l-Ala, the particles show mirror image spectra like those of enantiomers according to the number of amino acids in the main chain (odd or even). Such a behavior appears to be strongly influenced by the secondary structure assumed by the peptides when passivating the nanoparticles and vanishes when the sequence is long enough to assume a 310-helix conformation. Moreover, chirality control is a reversible process and can be deactivated or reactivated by increasing or decreasing the temperature.

Structures and chiroptical properties of the BINAS-monosubstituted Au38(SCH3)24 cluster

The structure and optical properties of a set of R-1,1'-binaphthyl-2,2'-dithiol (R-BINAS) monosubstituted A-Au38(SCH3)24 clusters are studied by means of time dependent density functional theory (TD-DFT). While it was proposed earlier that BINAS selectively binds to monomer motifs (SR-Au-SR) covering the Au23 core, our calculations suggest a binding mode that bridges two dimer (SR-Au-SR-Au-RS) motifs. The more stable isomers show a negligible distortion induced by BINAS adsorption on the Au38(SCH3)24 cluster which is reflected by similar optical and Circular Dichroism (CD) spectra to those found for the parent cluster. The results furthermore show that BINAS adsorption does not enhance the CD signals of the Au38(SCH3)24 cluster.

On the flexibility of the gold-thiolate interface: racemization of the Au40(SR)24 cluster

The two enantiomers of the Au40(2-PET)24 cluster were collected using HPLC and analyzed by MALDI-TOF mass spectrometry, UV-vis- and CD-spectroscopy. The flexibility of the cluster surface allows racemization of the intrinsically chiral cluster at elevated temperatures (80-130 °C) which was monitored following the optical activity. The determined activation energy (25 kcal mol(-1)) lies in the range of previously reported values for Au38 nanoclusters whereas the activation entropy deviates significantly from the one in Au38. The latter may indicate that the racemization can take place via different mechanisms.

Mesostructural Bi-Mo-O catalyst: correct structure leading to high performance

Structure-activity relationship has been one of the main topics of research on catalysts all the time. Component and structure are the two moieties governing the performance of solid materials as catalysts. Multicomponent bismuth molybdates are well known catalysts for propene oxidation but pure crystalline phases of bismuth molybdate are inactive for the reaction. We have designed mesostructural Bi-Mo-O catalyst with pure bismuth molybdate nanocrystals attached to molybdenum oxide nanobelts and found it is a high performance catalyst for the reaction, though the two domains themselves are inactive. The strongly expitaxial interaction between the two domains causes the lattice shrinkage and distortion of the bismuth molybdate nanocrystals and extremely promotes their catalytic activity toward propene oxidation while keeping high selectivity at the same time. The results are instructive for design of nano oxide catalysts with mesostructures leading to high performance.

Thiolate-protected Ag₃₂ clusters: mass spectral studies of composition and insights into the Ag-thiolate structure from NMR

Clusters composed of a 32 silver atom core, protected with thiolates of glutathione (GSH) and N-(2-mercaptopropionyl)glycine (MPGH), were synthesized by a solid-state route in milligram scale. They do not exhibit surface plasmon resonance unlike their larger sized nanoparticle analogues but show molecule-like features in absorption and luminescence spectra, falling in the visible window. The compositions Ag₃₂SG₁₉ (SG: thiolate of glutathione) and Ag₃₂MPG₁₉ (MPG: thiolate of MPGH) were identified from electrospray ionization mass spectrometry (ESI MS). Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) was not successful for -SG protected clusters as reported before, but for Ag₃₂MPG₁₉ a peak at 6.1 kDa was seen at a threshold laser intensity. This peak shifted to low mass region with increasing laser intensity due to systematic losses of Ag₂S. Further confirmation of the composition Ag₃₂SG₁₉ was made using various studies such as XPS and EDAX. One-dimensional (1D) and two-dimensional (2D) NMR spectroscopic investigations of Ag₃₂SG₁₉ provided interesting spectral features which indicated the dominant -[SR-Ag-SR]- structural motif. This structural motif as the predominant entity is found for the first time in silver clusters.

Unique Bonding Properties of the Au36(SR)24 Nanocluster with FCC-Like Core

The recent discovery on the total structure of Au36(SR)24, which was converted from biicosahedral Au38(SR)24, represents a surprising finding of a face-centered cubic (FCC)-like core structure in small gold-thiolate nanoclusters. Prior to this finding, the FCC feature was only expected for larger (nano)crystalline gold. Herein, we report results on the unique bonding properties of Au36(SR)24 that are associated with its FCC-like core structure. Temperature-dependent X-ray absorption spectroscopy (XAS) measurements at the Au L3-edge, in association with ab initio calculations, show that the local structure and electronic behavior of Au36(SR)24 are of more molecule-like nature, whereas its icosahedral counterparts such as Au38(SR)24 and Au25(SR)18 are more metal-like. Moreover, site-specific S K-edge XAS studies indicate that the bridging motif for Au36(SR)24 has different bonding behavior from the staple motif from Au38(SR)24. Our findings highlight the important role of "pseudo"-Au4 units within the FCC-like Au28 core in interpreting the bonding properties of Au36(SR)24 and suggest that FCC-like structure in gold thiolate nanoclusters should be treated differently from its bulk counterpart.

Ag44(SeR)30: A Hollow Cage Silver Cluster with Selenolate Protection

Selenolate protected, stable and atomically precise, hollow silver cluster was synthesized using solid state as well as solution state routes. The optical absorption spectrum shows multiple and sharp features similar to the thiolated Ag44 cluster, Ag44(SR)30 whose experimental structure was reported recently. High-resolution electrospray ionization mass spectrometry (HRESI MS) shows well-defined molecular ion features with two, three, and four ions with isotopic resolution, due to Ag44(SePh)30. Additional characterization with diverse tools confirmed the composition. The closed-shell 18 electron superatom electronic structure, analogous to Ag44(SR)30 stabilizes the dodecahedral cage with a large HOMO-LUMO gap of 0.71 eV. The time-dependent density functional theory (TDDFT) prediction of the optical absorption spectrum, assuming the Ag44(SR)30 structure, matches the experimental data, confirming the structure.

Electronic structure and optical properties of the thiolate-protected Au28(SMe)20 cluster

The recently reported crystal structure of the Au28(TBBT)20 cluster (TBBT: p-tert-butylbenzenethiolate) is analyzed with (time-dependent) density functional theory (TD-DFT). Bader charge analysis reveals a novel trimeric Au3(SR)4 binding motif. The cluster can be formulated as Au14(Au2(SR)3)4(Au3(SR)4)2. The electronic structure of the Au14(6+) core and the ligand-protected cluster were analyzed, and their stability can be explained by formation of distorted eight-electron superatoms. Optical absorption and circular dichroism (CD) spectra were calculated and compared to the experiment. Assignment of handedness of the intrinsically chiral cluster is possible.

Emerging chirality in nanoscience

Chirality in nanoscience may offer new opportunities for applications beyond the traditional fields of chirality, such as the asymmetric catalysts in the molecular world and the chiral propellers in the macroscopic world. In the last two decades, there has been an amazing array of chiral nanostructures reported in the literature. This review aims to explore and categorize the common mechanisms underlying these systems. We start by analyzing the origin of chirality in simple systems such as the helical spring and hair vortex. Then, the chiral nanostructures in the literature were categorized according to their material composition and underlying mechanism. Special attention is paid to highlight systems with original discoveries, exceptional structural characteristics, or unique mechanisms.

The expanding universe of thiolated gold nanoclusters and beyond

Thiolated gold nanoclusters form a universe of their own. Researchers in this field are constantly pushing the boundary of this universe by identifying new compositions and in a few "lucky" cases, solving their structures. Such solved structures, even if there are only few, provide important hints for predicting the many identified compositions that are yet to be crystallized or structure determined. Structure prediction is the most pressing issue for a computational chemist in this field. The success of the density functional theory method in gauging the energetic ordering of isomers for thiolated gold clusters has been truly remarkable, but to predict the most stable structure for a given composition remains a great challenge. In this feature article from a computational chemist's point of view, the author shows how one understands and predicts structures for thiolated gold nanoclusters based on his old and new results. To further entertain the reader, the author also offers several "imaginative" structures, claims, and challenges for this field.

Visible light induced synthesis of fluorescent silver clusters in reverse micelles

A light-induced reverse micelle mediated method was developed for the preparation of stable Ag clusters in high yield. The clusters exhibit a molecule-like UV-Vis spectrum consisting of five absorption bands, emit red fluorescence and have chiroptical properties.

A comparison of the chiral counterion, solvent, and ligand used to induce a chiroptical response from Au25(-) nanoclusters

A 25-atom gold nanocluster capped with an achiral thiolate exhibits no chiroptical signals in circular dichroism (CD) measurements. Herein, we report a systematic study on the effects of the chiral environment on the CD response from the Au25 metal core. We found that Au25(SC2H4Ph)18(-)TOA(+) dissolved in a chiral solvent did not give rise to a CD response, nor did Au25(SC2H4Ph)18(-) when associated with a chiral counterion (e.g., (-)-N-dodecyl-N-methylephedrinium, DME(+)). Both scenarios imply that the interaction of the chiral counterion (or chiral solvent molecules) with the achiral Au25(SC2H4Ph)18(-) nanocluster is not strong enough to induce CD signals from the metal core. In contrast, when the metal core is capped with chiral ligands (i.e., Au25(SCH2C*H(NH2)CH2Ph)18), strong CD signals in the visible wavelength range were observed. Thus, the induction of CD signals by surface chiral ligands is much stronger than that by the external chiral environment (including the chiral solvent or counterion). This work reveals some further insight into the origin of the chiroptical response of the Au nanoclusters. These chiral nanoclusters hold potential for practical applications in bioconjugation, sensing, and chiral catalysis.

Atomically precise gold nanoclusters as new model catalysts

Many industrial catalysts involve nanoscale metal particles (typically 1-100 nm), and understanding their behavior at the molecular level is a major goal in heterogeneous catalyst research. However, conventional nanocatalysts have a nonuniform particle size distribution, while catalytic activity of nanoparticles is size dependent. This makes it difficult to relate the observed catalytic performance, which represents the average of all particle sizes, to the structure and intrinsic properties of individual catalyst particles. To overcome this obstacle, catalysts with well-defined particle size are highly desirable. In recent years, researchers have made remarkable advances in solution-phase synthesis of atomically precise nanoclusters, notably thiolate-protected gold nanoclusters. Such nanoclusters are composed of a precise number of metal atoms (n) and of ligands (m), denoted as Aun(SR)m, with n ranging up to a few hundred atoms (equivalent size up to 2-3 nm). These protected nanoclusters are well-defined to the atomic level (i.e., to the point of molecular purity), rather than defined based on size as in conventional nanoparticle synthesis. The Aun(SR)m nanoclusters are particularly robust under ambient or thermal conditions (<200 °C). In this Account, we introduce Aun(SR)m nanoclusters as a new, promising class of model catalyst. Research on the catalytic application of Aun(SR)m nanoclusters is still in its infancy, but we use Au₂₅(SR)₁₈ as an example to illustrate the promising catalytic properties of Aun(SR)m nanoclusters. Compared with conventional metallic nanoparticle catalysts, Aun(SR)m nanoclusters possess several distinct features. First of all, while gold nanoparticles typically adopt a face-centered cubic (fcc) structure, Aun(SR)m nanoclusters (<2 nm) tend to adopt different atom-packing structures; for example, Au₂₅(SR)₁₈ (1 nm metal core, Au atomic center to center distance) has an icosahedral structure. Secondly, their ultrasmall size induces strong electron energy quantization, as opposed to the continuous conduction band in metallic gold nanoparticles or bulk gold. Thus, nanoclusters become semiconductors and possess a sizable bandgap (e.g., ~1.3 eV for Au₂₅(SR)₁₈). In addition, Aun(SR)m can be doped with a single atom of other metals, which is of great interest for catalysis, because the catalytic properties of nanoclusters can be truly tuned on an atom-by-atom basis. Overall, atomically precise Aun(SR)m nanoclusters are expected to become a promising class of model catalysts. These well-defined nanoclusters will provide new opportunities for achieving fundamental understanding of metal nanocatalysis, such as insight into size dependence and deep understanding of molecular activation, active centers, and catalytic mechanisms through correlation of behavior with the structures of nanoclusters. Future research on atomically precise nanocluster catalysts will contribute to the fundamental understanding of catalysis and to the new design of highly selective catalysts for specific chemical processes.

Thiol ligand-induced transformation of Au38(SC2H4Ph)24 to Au36(SPh-t-Bu)24

We report a disproportionation mechanism identified in the transformation of rod-like biicosahedral Au38(SCH2CH2Ph)24 to tetrahedral Au36(TBBT)24 nanoclusters. Time-dependent mass spectrometry and optical spectroscopy analyses unambiguously map out the detailed size-conversion pathway. The ligand exchange of Au38(SCH2CH2Ph)24 with bulkier 4-tert-butylbenzenethiol (TBBT) until a certain extent starts to trigger structural distortion of the initial biicosahedral Au38(SCH2CH2Ph)24 structure, leading to the release of two Au atoms and eventually the Au36(TBBT)24 nanocluster with a tetrahedral structure, in which process the number of ligands is interestingly preserved. The other product of the disproportionation process, i.e., Au40(TBBT)m+2(SCH2CH2Ph)24-m, was concurrently observed as an intermediate, which was the result of addition of two Au atoms and two TBBT ligands to Au38(TBBT)m(SCH2CH2Ph)24-m. The reaction kinetics on the Au38(SCH2CH2Ph)24 to Au36(TBBT)24 conversion process was also performed, and the activation energies of the structural distortion and disproportionation steps were estimated to be 76 and 94 kJ/mol, respectively. The optical absorption features of Au36(TBBT)24 are interpreted on the basis of density functional theory simulations.

Photoelectron velocity-map imaging spectroscopic and theoretical study on the reactivity of the gold atom toward CH3SH, CH3OH, and H2O

Photoelectron velocity-map imaging spectroscopy has been used to study the reaction of the anionic gold atom with the HR (R = SCH3, OCH3, OH) molecules. The solvated [Au···HR](-) and inserted [HAuR](-) products have been experimentally observed for R = SCH3, whereas only solvated [Au⋯HR](-) products were found for R = OCH3 and OH. This significant difference in the photoelectron spectra suggests the different reactivity of the Au(-) toward the CH3SH, CH3OH, and H2O molecules. Second order Møller-Plesset perturbation theory and coupled-cluster single double triple excitation calculations have been performed to aid the structural assignment of the spectra and to explore the reaction mechanism. Activation energies for the isomerizations of the solvated structures to the inserted ones in the Au(-)∕Au + HR reactions (R = OCH3 and OH) are predicted to be much higher than those for the Au(-)∕Au + CH3SH reactions, supporting the experimental observation. Theoretical calculations provide the evidence that the intriguing [HAuSCH3](-) product may be formed by the attachment of the electron onto the neutral HAuSCH3 species or the isomerization from the anionic [Au···HSCH3](-) one. These findings should be helpful for understanding the feature that the thiols are able to form the staple motifs, whereas CH3OH and H2O are not.

One-pot synthesis of phenylmethanethiolate-protected Au20(SR)16 and Au24(SR)20 nanoclusters and insight into the kinetic control

We report two synthetic routes for concurrent formation of phenylmethanethiolate (-SCH2Ph)-protected Au20(SR)16 and Au24(SR)24 nanoclusters in one-pot by kinetic control. Unlike the previously reported methods for thiolate-protected gold nanoclusters, which typically involve rapid reduction of the gold precursor by excess NaBH4 and subsequent size focusing into atomically monodisperse clusters of a specific size, the present work reveals some insight into the kinetic control in gold-thiolate cluster synthesis. We demonstrate that the synthesis of -SCH2Ph-protected Au20 and Au24 nanoclusters can be obtained through two different, kinetically controlled methods. Specifically, route 1 employs slow addition of a relatively large amount of NaBH4 under slow stirring of the reaction mixture, while route 2 employs rapid addition of a small amount of NaBH4 under rapid stirring of the reaction mixture. At first glance, these two methods apparently possess quite different reaction kinetics, but interestingly they give rise to exactly the same product (i.e., the coproduction of Au20(SCH2Ph)16 and Au24(SCH2Ph)20 clusters). Our results explicitly demonstrate the complex interplay between the kinetic factors that include the addition speed and amount of NaBH4 solution as well as the stirring speed of the reaction mixture. Such insight is important for devising synthetic routes for different sized nanoclusters. We also compared the photoluminescence and electrochemical properties of PhCH2S-protected Au20 and Au24 nanoclusters with the PhC2H4S-protected counterparts. A surprising 2.5 times photoluminescence enhancement was observed for the PhCH2S-capped nanoclusters when compared to the PhC2H4S-capped analogues, thereby indicating a drastic effect of the ligand that is merely one carbon shorter.

CeO2-supported Au38(SR)24 nanocluster catalysts for CO oxidation: a comparison of ligand-on and -off catalysts

The catalytic properties of atomically precise, thiolate-protected Au38(SR)24 (R = CH2CH2Ph) nanoclusters supported on CeO2 were investigated for CO oxidation in a fixed bed quartz reactor. Oxygen (O2) thermal pretreatment of Au38(SR)24/CeO2 at a temperature between 100 and 175 °C largely enhanced the catalytic activity, while pretreatment at higher temperatures (>200 °C) for removing thiolate instead gave rise to a somewhat lower activity than that for 175 °C pretreatment, and the ligand-off clusters were also found to be less stable. The CO conversion in the case of wet feed-gas (i.e. the presence of H2O vapor) was appreciably higher than the case of dry feed-gas when the reaction temperature was kept relatively low (between 60 and 80 °C), and interestingly the ligand-on and ligand-off catalysts exhibited opposite response to water vapor. Finally, we discussed some insights into the catalytic reaction involving the well-defined gold nanocluster catalyst.

One-step fabrication of intense red fluorescent gold nanoclusters and their application in cancer cell imaging

A one-step method for successfully fabrication of water-soluble and alkanethiol-stabilized Au nanoclusters (NCs) was demonstrated. The novel and facile method was based on simply placing histidine (His), HAuCl4 and 11-mercaptoundcanoic acid (MUA) together at room temperature. The resulting Au NCs were exclusively composed of Au17MUA4His22 (AMH), as demonstrated by the photoluminescence, UV-Vis absorption, electrospray ionization mass and X-ray photoelectron spectroscopy. AMH exhibited intense red fluorescence (λem = 600 nm), a long fluorescence lifetime (7.11 μs), considerable stability, and a large Stoke's shift (320 nm). Based on the excellent properties of the AMH, cell experiments were conducted. Cytotoxicity studies showed that the Au NCs exhibited negligible effects in altering cell proliferation or triggering apoptosis. Cancer cell imaging of HeLa cell lines indicated that the obtained AMH could serve as a promising fluorescent bioprobe for bioimaging. This strategy, based on the one-step method, may offer a novel approach to fabricate other water-soluble and alkanethiol-stabilized metal nanoclusters for application in biolabelling and bioimaging.

Chiral structure of thiolate-protected 28-gold-atom nanocluster determined by X-ray crystallography

We report the crystal structure of a new nanocluster formulated as Au28(TBBT)20, where TBBT = 4-tert-butylbenzenethiolate. It exhibits a rod-like Au20 kernel consisting of two interpenetrating cuboctahedra. The kernel is protected by four dimeric "staples" (-SR-Au-SR-Au-SR-) and eight bridging thiolates (-SR-). The unit cell of Au28(TBBT)20 single crystals contains a pair of enantiomers. The origin of chirality is primarily rooted in the rotating arrangement of the four dimeric staples as well as the arrangement of the bridging thiolates (quasi-D2 symmetry). The enantiomers were separated by chiral HPLC and characterized by circular dichroism spectroscopy.

Facile and rapid synthesis of a dithiol-protected Ag7 quantum cluster for selective adsorption of cationic dyes

We report a facile and rapid (less than 15 min) synthesis of atomically precise, dithiol-protected, silver quantum cluster, Ag7(DMSA)4 (DMSA: meso-2,3-dimercaptosuccinic acid), through a modified solid state route. The as-synthesized cluster exhibits molecular optical absorption features with a prominent λmax at ~500 nm. Composition of the cluster was confirmed using various spectroscopic and microscopic techniques such as electrospray ionization mass spectrometry (ESI MS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive analysis of X-rays (EDAX). Clusters supported on neutral alumina have been shown as better adsorbents for selective adsorption of cationic dyes (over anionic dyes) from water. This selectivity for cationic dyes was evaluated by zeta potential (ζ) measurements. The efficiency of clusters for removal of dyes is very high when compared to nanoparticles (NPs) protected with ligands (citrate and mercaptosuccinic acid (MSA)) possessing similar chemical structures as that of DMSA. The higher efficiency of clusters for the removal of dyes is attributed to their smaller size and large surface area compared to the NPs in addition to favorable electrostatic interactions between the clusters and cationic dyes. Adsorption of dyes (cationic and anionic) was enhanced when dye molecules contain hydrogen bond forming functional groups. Supported clusters have been reused up to five cycles without the loss of activity once the adsorbed dye is extracted using suitable solvents.

Oxidation of gold clusters by thiols

The formation of gold-thiolate nanoparticles via oxidation of gold clusters by thiols is examined in this work. Using the BP86 density functional with a triple ζ basis set, the adsorption of methylthiol onto various gold clusters Aun(Z) (n = 1-8, 12, 13, 20; Z = 0, -1, +1) and Au38(4+) is investigated. The rate-limiting step for the reaction of one thiol with the gold cluster is the dissociation of the thiol proton; the resulting hydrogen atom can move around the gold cluster relatively freely. The addition of a second thiol can lead to H2 formation and the generation of a gold-thiolate staple motif.

Nanometallic chemistry: deciphering nanoparticle catalysis from the perspective of organometallic chemistry and homogeneous catalysis

Nanoparticle (NP) catalysis is traditionally viewed as a sub-section of heterogeneous catalysis. However, certain properties of NP catalysts, especially NPs dispersed in solvents, indicate that there could be benefits from viewing them from the perspective of homogeneous catalysis. By applying the fundamental approaches and concepts routinely used in homogeneous catalysis to NP catalysts it should be possible to rationally design new nanocatalysts with superior properties to those currently in use.

Precursor engineering and controlled conversion for the synthesis of monodisperse thiolate-protected metal nanoclusters

In very recent years, thiolate-protected metal nanoclusters (or thiolated MNCs) with core sizes smaller than 2 nm have emerged as a new direction in nanoparticle research due to their discrete and size dependent electronic structures and molecular-like properties, such as HOMO-LUMO transitions in optical absorptions, quantized charging, and strong luminescence. Synthesis of monodisperse thiolated MNCs in sufficiently large quantities (up to several hundred micrograms) is necessary for establishing reliable size-property relationships and exploring potential applications. This Feature Article reviews recent progress in the development of synthetic strategies for the production of monodisperse thiolated MNCs. The preparation of monodisperse thiolated MNCs is viewed as an engineerable process where both the precursors (input) and their conversion chemistry (processing) may be rationally designed to achieve the desired outcome - monodisperse thiolated MNCs (output). Several strategies for tailoring the precursor and the conversion process are analyzed to arrive at a unifying understanding of the processes involved.

Emergence of large chiroptical responses by ligand exchange cross-linking of monolayer-protected gold clusters with chiral dithiol

We here present a study of cross-linking chemistry of optically inactive monothiol-protected gold clusters by chiral bidentate dithiol with two stereogenic centers, (2R,3R)-1,4-dimercapto-2,3-butanediol (L-dithiothreitol; L-DTT), and explore the impacts of the cross-linking on their chiroptical responses. The pristine protective ligand is racemic penicillamine (rac-Pen), and the products of the ligand exchange reactions include clusters containing both rac-Pen and L-DTT (partial exchange). Electrophoresis using polyacrylamide gel with a very low gel concentration (3%) can make the products separable into two components, each of which has the similar mean core diameter of 0.78 and 0.83 nm, so the difference in the relative mobility is mainly ascribed to the size of the cluster assembly. In addition, very large optical activity with the maximum anisotropy factors of about 1.0 × 10(-3) is found for the assemblies. In comparison with chiral 1,3-dithiol protection incapable of cross-linking between gold clusters, we propose that the observed optical activity is due to surface intrinsic handedness caused by a cyclic cross-linking with at least two L-DTT molecules.

Selective synthesis of the [Ni36Co8C8(CO)48]6- octa-carbide carbonyl cluster by thermal decomposition of the [H2Ni22Co6C6(CO)36]4- hexa-carbide

The thermal decomposition in thf solution of [H2Ni22Co6C6(CO)36](4-) results in the new [HNi36Co8C8(CO)48](5-) bimetallic Ni-Co octa-carbide, which can be converted into the closely related [H6-nNi36Co8C8(CO)48](n-) (n = 3-6) polyhydrides by means of acid-base reactions. The structure of the [Ni36Co8C8(CO)48](6-) hexa-anion has been established via X-ray crystallography, showing that the eight interstitial carbide atoms are lodged within different metal cages. Thus, two C-atoms are enclosed within regular square anti-prismatic Ni8C cages, four within irregular Ni8C square anti-prismatic cages, and the last two within mono-capped trigonal prismatic Ni5Co2C cages. The structure of [Ni36Co8C8(CO)48](6-) is non-compact and closely related to [Ni32C6(CO)38](6-) and [HNi38C6(CO)44](5-). [Ni36Co8C8(CO)48](6-) approaches the nanosize regime and the whole molecular ion has a diameter (measured from the outer oxygen atoms) of ca. 1.61 nm.

Atomically precise Au25 superatoms immobilized on CeO2 nanorods for styrene oxidation

Atomically precise Au25 superatoms with electron-deficient Au12 shells and electron-rich Au13 cores immobilized on the surface of CeO2 nanorods achieved novel catalytic activity for styrene oxidation.

New Protocols for the Synthesis of Stable Ag and Au Nanocluster Molecules

"Catching" metals in the nonmetallic form in solution, as they grow to bulk, is one of the most exciting areas of contemporary materials research. A new kind of stabilization to catch the nonmetallic form of noble metals with small thiols has evolved as an exciting area of synthesis during the past decade. Gold clusters stay in the frontline of this research, yielding new "molecules" composed of a few to several hundreds of atoms. By taking guidelines from gold cluster research, various new protocols for silver nanoclusters were developed. In this Perspective, we highlight the recent advances on the synthesis of atomically precise silver, gold, and their alloy clusters with a special emphasis on silver. As a result of intense efforts of the recent past, clusters such as Ag7,8(SR)7,8, Ag7(-S-R-S-)4, Ag9(SR)7, Ag32(SR)19, Ag44(SR)30, Ag140(SR)53, Ag280(SR)140, and Ag152(SR)60 (SR and S-R-S refer to thiolate and dithiolate ligands, respectively) were added to the literature. Moreover, "silver-covered" and "gold-covered" alloy clusters have also been synthesized. Early reports of the crystallization of such clusters are available. Several of these clusters are shown to act as sensors, catalysts, and pesticide degradation agents, which suggests that these materials may find applications in daily life in the foreseeable future.

Stabilizing subnanometer Ag(0) nanoclusters by thiolate and diphosphine ligands and their crystal structures

The combined use of thiolate and diphosphine as surface ligands helps to stabilize subnanometer Ag(0) nanoclusters, resulting in the successful crystallization of two Ag(0)-containing nanoclusters (Ag16 and Ag32) for X-ray single crystal analysis. Both clusters have core-shell structures with Ag8(6+) and Ag22(12+) as their cores, which are not simply either fragments of face-centered cubic metals or their five-fold twinned counterparts. The clusters display UV-Vis absorption spectra consisting of molecule-like optical transitions.

Passive tumor targeting of renal-clearable luminescent gold nanoparticles: long tumor retention and fast normal tissue clearance

Glutathione-coated luminescent gold nanoparticles (GS-AuNPs) with diameters of ∼2.5 nm behave like small dye molecules (IRDye 800CW) in physiological stability and renal clearance but exhibit a much longer tumor retention time and faster normal tissue clearance, indicating that the well-known enhanced permeability and retention effect, a unique strength of conventional NPs in tumor targeting, still exists in such small NPs. These merits enable the AuNPs to detect tumor more rapidly than the dye molecules without severe accumulation in reticuloendothelial system organs, making them very promising for cancer diagnosis and therapy.

Bare clusters derived from protein templates: Au25(+), Au38(+) and Au102(+)

A discrete sequence of bare gold clusters of well-defined nuclearity, namely Au25(+), Au38(+) and Au102(+), formed in a process that starts from gold-bound adducts of the protein lysozyme, were detected in the gas phase. It is proposed that subsequent to laser desorption ionization, gold clusters form in the gas phase, with the protein serving as a confining growth environment that provides an effective reservoir for dissipation of the cluster aggregation and stabilization energy. First-principles calculations reveal that the growing gold clusters can be electronically stabilized in the protein environment, achieving electronic closed-shell structures as a result of bonding interactions with the protein. Calculations for a cluster with 38 gold atoms reveal that gold interaction with the protein results in breaking of the disulfide bonds of the cystine units, and that the binding of the cysteine residues to the cluster depletes the number of delocalized electrons in the cluster, resulting in opening of a super-atom electronic gap. This shell-closure stabilization mechanism confers enhanced stability to the gold clusters. Once formed as stable magic number aggregates in the protein growth medium, the gold clusters become detached from the protein template and are observed as bare Au(n)(+) (n=25, 38, and 102) clusters.

Protein-encapsulated gold cluster aggregates: the case of lysozyme

We report the evolution and confinement of atomically precise and luminescent gold clusters in a small protein, lysozyme (Lyz) using detailed mass spectrometric (MS) and other spectroscopic investigations. A maximum of 12 Au(0) species could be bound to a single Lyz molecule irrespective of the molar ratio of Lyz : Au(3+) used for cluster growth. The cluster-encapsulated protein also forms aggregates similar to the parent protein. Time dependent studies reveal the emergence of free protein and the redistribution of detached Au atoms, at specific Lyz to Au(3+) molar ratios, as a function of incubation time, proposing inter-protein metal ion transfer. The results are in agreement with the studies of inter-protein metal transfer during cluster growth in similar systems. We believe that this study provides new insights into the growth of clusters in smaller proteins.

STEM Electron Diffraction and High Resolution Images Used in the Determination of the Crystal Structure of Au144(SR)60 Cluster

Determination of the total structure of molecular nanocrystals is an outstanding experimental challenge that has been met, in only a few cases, by single-crystal X-ray diffraction. Described here is an alternative approach that is of most general applicability and does not require the fabrication of a single crystal. The method is based on rapid, time-resolved nanobeam electron diffraction (NBD) combined with high-angle annular dark field scanning/transmission electron microscopy (HAADF-STEM) images in a probe corrected STEM microscope, operated at reduced voltages. The results are compared with theoretical simulations of images and diffraction patterns obtained from atomistic structural models derived through first-principles density functional theory (DFT) calculations. The method is demonstrated by application to determination of the structure of the Au₁₄₄(SCH₂CH₂Ph)₆₀ cluster.