Functionalized Au22 clusters: synthesis, characterization, and patterning

Functionalization of Polymer-Wrapped Silver Nanoclusters and Potential Applications as Antimicrobial Mask Materials

The poly(methacrylic acid) (PMAA) polymer stabilized silver nanoclusters Agn (n = 2-9), synthesized in aqueous solution by the selected light wavelength irradiation photolysis approach, have been functionalized with thiol and amine ligands and successfully transferred from aqueous to organic media. Low- or high-resolution positive mass spectra showed constant species composites with the molecular formula AgnLn-1 [n = 2 to ∼9, L = butylmercaptan (C4H9S), thiolphenol (C6H5S), or dodecanethiol (C12H25S)] and proved that the molecules consist of deprotonated sulfur ligands in each species with one positive charge. Fourier transform infrared and X-ray photoelectron spectroscopy are consistent, indicating deprotonated sulfur, while silver has a zero valence value. The composition of the functionalized silver clusters is in agreement with that observed from polymer-wrapped "naked" silver clusters, which strongly indicates their real existence. For the silver cluster amine systems (heptylamine, dodecylamine, and oleylamine), only "naked" silver cluster species were detected from mass spectroscopy, similar to the polymer-wrapped case, indicating they are not stable enough in the gas phase. The development of a new antibacterial mask material is very important. The dodecylamine-capping silver nanoclusters were selected by coating the coffee filter surface to conduct antibacterial tests with Staphylococcus aureus and Escherichia coli, demonstrating very efficient antimicrobial properties even with organic capping ligands. Experiments also show that they work on mask material. One nanowire assembly with polystyrene and dodecylamine-capping silver nanoclusters was prepared, showing uniform nanofibers generated via the electrospray technique.

Self-Assembly of Precision Noble Metal Nanoclusters: Hierarchical Structural Complexity, Colloidal Superstructures, and Applications

Ligand protected noble metal nanoparticles are excellent building blocks for colloidal self-assembly. Metal nanoparticle self-assembly offers routes for a wide range of multifunctional nanomaterials with enhanced optoelectronic properties. The emergence of atomically precise monolayer thiol-protected noble metal nanoclusters has overcome numerous challenges such as uncontrolled aggregation, polydispersity, and directionalities faced in plasmonic nanoparticle self-assemblies. Because of their well-defined molecular compositions, enhanced stability, and diverse surface functionalities, nanoclusters offer an excellent platform for developing colloidal superstructures via the self-assembly driven by surface ligands and metal cores. More importantly, recent reports have also revealed the hierarchical structural complexity of several nanoclusters. In this review, the formulation and periodic self-assembly of different noble metal nanoclusters are focused upon. Further, self-assembly induced amplification of physicochemical properties, and their potential applications in molecular recognition, sensing, gas storage, device fabrication, bioimaging, therapeutics, and catalysis are discussed. The topics covered in this review are extensively associated with state-of-the-art achievements in the field of precision noble metal nanoclusters.

Chiral Au22(SR)17-: a new ligand-binding strategy for structural prediction of thiolate-protected gold nanocluster

A new atomic structure of chiral thiolate-protected gold nanocluster Au22(SR)17- is predicted on the basis of the new ligand-binding strategy, namely, redistributing the Au-S "staple" motifs on the well-known Au10 core from previously laboratory-determined Au21(SR)15 crystal structure. Density functional theory calculations show that this structure is very likely the realistic structure for the synthesized Au22(SR)17-.

Structural insights into DNA-stabilized silver clusters

Despite their great promise as fluorescent biological probes and sensors, the structure and dynamics of Ag complexes derived from single stranded DNA (ssDNA) are less understood than their double stranded counterparts. In this work, we seek new insights into the structure of single AgNssDNA clusters using analytical ultracentrifugation (AUC), nuclear magnetic resonance spectroscopy, infrared spectroscopy and molecular dynamics simulations (MD) of a fluorescent (AgNssDNA)8+ nanocluster. The results suggest that the purified (AgNssDNA)8+ nanocluster is a mixture of predominantly Ag15 and Ag16 species that prefer two distinct long-lived conformational states: one extended, the other approaching spherical. However, the ssDNA strands within these clusters are highly mobile. Ag(i) interacts preferentially with the nucleobase rather than the phosphate backbone, causing a restructuring of the DNA strand relative to the bare DNA. Infrared spectroscopy and MD simulations of (AgNssDNA)8+ and model nucleic acid homopolymers suggest that Ag(i) has a higher affinity for cytosine over guanine bases, little interaction with adenine, and virtually none with thymine. Ag(i) shows a tendency to interact with cytosine N3 and O2 and guanine N7 and O6, opening the possibility for a Ag(i)-base bifurcated bond to act as a nanocluster nucleation and strand stabilizing site. This work provides valuable insight into nanocluster structure and dynamics which drive stability and optical properties, and additional studies using these types of characterization techniques are important for the rational design of single stranded AgDNA nanocluster sensors.

Tailoring the photoluminescence of atomically precise nanoclusters

Due to their atomically precise structures and intriguing chemical/physical properties, metal nanoclusters are an emerging class of modular nanomaterials. Photo-luminescence (PL) is one of their most fascinating properties, due to the plethora of promising PL-based applications, such as chemical sensing, bio-imaging, cell labeling, phototherapy, drug delivery, and so on. However, the PL of most current nanoclusters is still unsatisfactory-the PL quantum yield (QY) is relatively low (generally lower than 20%), the emission lifetimes are generally in the nanosecond range, and the emitted color is always red (emission wavelengths of above 630 nm). To address these shortcomings, several strategies have been adopted, and are reviewed herein: capped-ligand engineering, metallic kernel alloying, aggregation-induced emission, self-assembly of nanocluster building blocks into cluster-based networks, and adjustments on external environment factors. We further review promising applications of these fluorescent nanoclusters, with particular focus on their potential to impact the fields of chemical sensing, bio-imaging, and bio-labeling. Finally, scope for improvements and future perspectives of these novel nanomaterials are highlighted as well. Our intended audience is the broader scientific community interested in the fluorescence of metal nanoclusters, and our review hopefully opens up new horizons for these scientists to manipulate PL properties of nanoclusters. This review is based on publications available up to December 2018.

Efficient One-Pot Synthesis and pH-Dependent Tuning of Photoluminescence and Stability of Au18(SC2H4CO2H)14 Cluster

Developing efficient ways to control the nanocluster properties and synthesize atomically precise metal nanoclusters are the foremost goals in the field of metal nanocluster research. In this article, we demonstrate that the direct synthesis of atomically precise, hydrophilic metal nanoclusters as well as tuning of their properties can be achieved by an appropriate selection of reactants, binding ligand, and their proportions. Thus, a facile, single-step method has been developed for the direct synthesis of Au₁₈(SC₂H₄CO₂H)₁₄ nanocluster in an aqueous medium under ambient conditions. The synthesis does not require any pH or temperature control and postsynthesis size-separation step. The use of a hydrophilic, bifunctional short carbon-chain capping ligand, HSC₂H₄CO₂H, allows tuning of cluster properties such as the photoluminescence and stability in an aqueous medium via the variation of pH of the cluster solution. By using a phase transfer catalyst, the nanoclusters can also be transferred into toluene solvent, which further enhances the nanocluster photoluminescence. The formation, composition, and purity of the product clusters have been characterized by using a number of methods such as the polyacrylamide gel electrophoresis, UV-visible and FTIR spectroscopies, transmission electron microscopy, energy dispersive X-ray analysis, thermogravimetric analysis, X-ray photoelectron spectroscopy, and matrix-assisted laser desorption ionization-time-of-flight mass spectrometry. Gold nanoclusters with properties such as water solubility, water-to-organic phase-transfer ability, and tunable stability and photoluminescence are promising for various studies and applications. The work reveals a few principles that can be helpful in the development of a general toolbox for the rational design of size-selective synthesis and properties tuning of the metal nanoclusters.

Switching the subcellular organelle targeting of atomically precise gold nanoclusters by modifying the capping ligand

Here, we have achieved the target switching fluorescence imaging and photodynamic activity of hydrosoluble AuNCs from lysosomes to mitochondria through ligand exchange.

Formation and optical properties of fluorescent gold nanoparticles obtained by matrix sputtering method with volatile mercaptan molecules in the vacuum chamber and consideration of their structures

This paper proposes a novel methodology to synthesize highly fluorescent gold nanoparticles (NPs) with a maximum quantum yield of 16%, in the near-infrared (IR) region. This work discusses the results of using our (previously developed) matrix sputtering method to introduce mercaptan molecules, α-thioglycerol, inside the vacuum sputtering chamber, during the synthesis of metal NPs. The evaporation of α-thioglycerol inside the chamber enables to coordinate to the "nucleation stage" very small gold nanoclusters in the gas phase, thus retaining their photophysical characteristics. As observed through transmission electron microscopy, the size of the Au NPs obtained with the addition of α-thioglycerol varied from approximately 2-3 nm to approximately 5 nm. Plasmon absorption varied with the size of the resultant nanoparticles. Thus, plasmon absorption was observed at 2.4 eV in the larger NPs. However, it was not observed, and instead a new peak was found at approximately 3.4 eV, in the smaller NPs that resulted from the introduction of α-thioglycerol. The Au NPs stabilized by the α-thioglycerol fluoresced at approximately 1.8 eV, and the maximum wavelength shifted toward the red, in accordance with the size of the NPs. A maximum fluorescent quantum yield of 16% was realized under the optimum conditions, and this value is extremely high compared to values previously reported on gold NPs and clusters (generally ∼1%). To our knowledge, however, Au NPs of size >2 nm usually do not show strong fluorescence. By comparison with results reported in previous literature, it was concluded that these highly fluorescent Au NPs consist of gold-mercaptan complexes. The novel method presented in this paper therefore opens a new door for the effective control of size, photophysical characteristics, and structure of metal NPs. It is hoped that this research contributes significantly to the science in this field.

Thiolate-protected Ni39 and Ni41 nanoclusters: synthesis, self-assembly and magnetic properties

Thiolate-protected soluble nickel clusters, Ni(39)(SC(2)H(4)Ph)24 and Ni(41)(SC(2)H(4)Ph)25, were synthesized via a wet chemical method. The cluster formulae were identified by MALDI-TOF. Possible structures of the clusters were discussed. These clusters exhibit ferromagnetism with hysteresis loops in the 1.8-300 K range. By solvent evaporation, the clusters can self-assemble into simple cubic structured crystals with a width in the range of 1-10 μm and length up to 300 μm. These properties shed light on their application potentials in nanomagnetics working at room temperature.

Tuning of silver cluster emission from blue to red using a bio-active peptide in water

Blue, green, and red emitting silver quantum clusters have been prepared through green chemical approach by using a bio-active peptide glutathione (reduced) in a 50 mM phosphate buffer at pH 7.46. This study describes fluorescence emission tuning of the silver clusters by making different sized Ag clusters using slightly different reaction conditions keeping the same stabilizing ligand, reducing agent, solvent system, and silver salt precursor. The preparation procedure of these silver quantum clusters is new and highly reproducible. Each of these clusters shows very interesting fluorescence properties with large stokes shifts, and the quantum yields of blue, green, and red clusters are 2.08%, 0.125%, and 1.39%, respectively. These silver quantum clusters have been characterized by using different techniques including fluorescence spectroscopy, UV-vis spectroscopy, field-emission gun transmission electron microscopic (FEG-TEM) imaging and MALDI-TOF MS analyses. MALDI-TOF MS analyses show that the size of these blue, green and red emitting silver clusters are Ag5 (NC1, nanoclusters 1), Ag8 (NC2, nanoclusters 2) and Ag13 (NC3, nanoclusters 3), respectively, by using 2,5-dihydroxybenzoic acid as a matrix. These clusters are stable in broad ranges of pH. The NC3 (red emitting) has been successfully utilized for selective and sensitive detection of toxic Hg(II) ions in water by using even naked eyes, fluorometric, and calorimetric studies. The lower limit of detection of Hg(II) ions in water has been estimated to be 126 and 245 nM from fluorometric and UV-vis analyses, respectively. Enthalpy change (ΔH) during this Hg(II) sensing process is 2508 KJ mol(-1).

Rapid synthesis of highly luminescent and stable Au20 nanoclusters for active tumor-targeted imaging in vitro and in vivo

Rapid synthesis of protein-stabilized Au20 nanoclusters (Au20NCs) with high fluorescence quantum yield (QY) up to ∼15% is successfully achieved by manipulating the reaction kinetics. The as-obtained Au20NCs, identified by mass spectrometry, have an average size of 2.6 nm, with strong fluorescence emission at 620 nm (2.00 eV) upon excitation at either 370 nm (3.35 eV) or 470 nm (2.64 eV). The advantages of the as-obtained Au20NCs, including small sizes, high fluorescence QY, excellent photostability, non-toxicity, and good stability in biological media, make them ideal candidates as good luminescent probes for optical imaging in vitro and in vivo. Our results demonstrate that the uptake of Au20NCs by both cancer cells and tumor-bearing nude mice can be improved by receptor-mediated internalization, compared with that by passive targeting. Because of their selective accumulation at the tumor sites, the Au20NC probes can be used as potential indicators for cancer diagnosis. This work not only provides a new understanding of the rapid synthesis of highly luminescent Au20NCs but also demonstrates that the functionalized-Au20NCs are excellent probes for active tumor-targeted imaging in vitro and in vivo.

Fuel mediated solution combustion synthesis of ZnO supported gold clusters and nanoparticles and their catalytic activity and in vitro cytotoxicity

Well characterized atomic gold clusters and nanoparticles on the surface of ZnO are found to be effective against cancer cells.

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.

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.

Evidence for rod-shaped DNA-stabilized silver nanocluster emitters

Fluorescent DNA-stabilized silver nanoclusters contain both cationic and neutral silver atoms. The absorbance spectra of compositionally pure solutions follow the trend expected for rod-shaped silver clusters, consistent with the polarized emission measured from individual nanoclusters. The data suggest a rod-like assembly of silver atoms, with silver cations mediating attachment to the bases.

Functionalized gold magic clusters: Au25(SPhNH2)17

New Au(25) nanoclusters stabilized by heterotopic 4-aminothiophenolate ligands (HSPhNH(2)) have been isolated with a yield of ~70%. The nanoclusters formula determined by ESI-MS is Au(25)(SPhNH(2))(17), with the 18(th) position occupied by an amine or DMF molecules to close their electronic shell.

Protein-protected luminescent noble metal quantum clusters: an emerging trend in atomic cluster nanoscience

Noble metal quantum clusters (NMQCs) are the missing link between isolated noble metal atoms and nanoparticles. NMQCs are sub-nanometer core sized clusters composed of a group of atoms, most often luminescent in the visible region, and possess intriguing photo-physical and chemical properties. A trend is observed in the use of ligands, ranging from phosphines to functional proteins, for the synthesis of NMQCs in the liquid phase. In this review, we briefly overview recent advancements in the synthesis of protein protected NMQCs with special emphasis on their structural and photo-physical properties. In view of the protein protection, coupled with direct synthesis and easy functionalization, this hybrid QC-protein system is expected to have numerous optical and bioimaging applications in the future, pointers in this direction are visible in the literature.

Luminescent, freestanding composite films of Au15 for specific metal ion sensing

A highly luminescent freestanding film composed of the quantum cluster, Au(15), was prepared. We studied the utility of the material for specific metal ion detection. The sensitivity of the red emission of the cluster in the composite to Cu(2+) has been used to make a freestanding metal ion sensor, similar to pH paper. The luminescence of the film was stable when exposed to several other metal ions such as Hg(2+), As(3+), and As(5+). The composite film exhibited visual sensitivity to Cu(2+) up to 1 ppm, which is below the permissible limit (1.3 ppm) in drinking water set by the U.S. environmental protection agency (EPA). The specificity of the film for Cu(2+) sensing may be due to the reduction of Cu(2+) to Cu(1+)/Cu(0) by the glutathione ligand or the Au(15) core. Extended stability of the luminescence of the film makes it useful for practical applications.

Mass spectrometry in India

This review emphasizes the mass spectrometry research being performed at academic and established research institutions in India. It consists of three main parts covering the work done in organic, atomic and biological mass spectrometry. The review reveals that the use of mass spectrometry techniques started in the middle of the 20th century and was applied to research in the fields of organic, nuclear, geographical and atomic chemistry. Later, with the advent of soft and atmospheric ionization techniques it has been applied to pharmaceutical and biological research. In due course, several research centers with advanced mass spectrometry facilities have been established for specific areas of research such as gas-phase ion chemistry, ion-molecule reactions, proscribed chemicals, pesticide residues, pharmacokinetics, protein/peptide chemistry, nuclear chemistry, geochronological studies, archeology, petroleum industry, proteomics, lipidomics and metabolomics. Day-by-day the mass spectrometry centers/facilities in India have attracted young students for their doctoral research and other advanced research applications.

One-pot synthesis of near-infrared fluorescent gold clusters for cellular fluorescence lifetime imaging

A facile strategy to synthesize water-soluble fluorescent gold nanoclusters (Au NCs) stabilized with the bidentate ligand dihydrolipoic acid (DHLA) is reported. The DHLA-capped Au NCs are characterized by UV-vis absorption spectroscopy, fluorescence spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The Au NCs possess many attractive features including ultrasmall size, bright near-infrared luminescence, high colloidal stability, and good biocompatibility, making them promising imaging agents for biomedical and cellular imaging applications. Moreover, their long fluorescence lifetime (>100 ns) makes them attractive as labels in fluorescence lifetime imaging (FLIM) applications. As an example, the internalization of Au NCs by live HeLa cells is visualized using the FLIM technique.

Patterned self-assembly of gold nanoparticles on chemical templates fabricated by soft UV nanoimprint lithography

Chemical templates for the patterned immobilization of gold nanoparticles were fabricated by soft UV nanoimprint lithography. The template structures were fabricated by means of the consecutively performed process steps of nanoimprint lithography, reactive ion etching, chemical functionalization with amino groups, and lift-off of imprint resist. These chemical templates were used for the defined assembly of 20 nm diameter citrate stabilized gold nanoparticles from aqueous solution. By reducing the ionic strength of the solution, one- and zero-dimensional particle assemblies were generated on sub-100-nm template structures. By this means, the pattern resolution predefined by the lithography process could be easily enhanced by dilution of the nanoparticle solution.

Synthesis and characterization of human transferrin-stabilized gold nanoclusters

Human transferrin has been biolabelled with gold nanoclusters (Au NCs) using a simple, fast and non-toxic method. These nanocrystals (<2 nm) are stabilized in the protein via sulfur groups and have a high fluorescence emission in the near infrared region (QY=4.3%; λem=695 nm). Structural investigation and photophysical measurements show a high population of clusters formed of 22-33 gold atoms covalently bound to the transferrin. In solutions with pH ranging from 5 to 10 and in buffer solutions (PBS, HEPES), those biolabelled proteins exhibit a good stability. No significant quenching effect of the fluorescent transferrin has been detected after iron loading of iron-free transferrin (apoTf) and in the presence of a specific polyclonal antibody. Additionally, antibody-induced agglomeration demonstrates no alteration in the protein activity and the receptor target ability. MTT and Vialight® Plus tests show no cytotoxicity of these labelled proteins in cells (1 µg ml(-1)-1 mg ml(-1)). Cell line experiments (A549) indicate also an uptake of the iron loaded fluorescent proteins inside cells. These remarkable data highlight the potential of a new type of non-toxic fluorescent transferrin for imaging and targeting.

Preparation of highly luminescent mannose-gold nanodots for detection and inhibition of growth of Escherichia coli

In this paper, we describe a novel, simple, and convenient method for preparing water-soluble biofunctional gold nanodots (Au NDs) for the sensitive and selective detection of Escherichia coli (E. coli) and the inhibition of its growth. We obtained luminescent mannose-capped Au NDs (Man-Au NDs) from as-prepared 2.9-nm Au nanoparticles (Au NPs) and 29,29'-dithio bis(3',6',9',12',15',18'-hexaoxa-nonacosyl α-D-mannopyranoside) (Man-RSSR-Man). To obtain improved quantum yield (>20%), luminescent Man-Au NDs (1.8 nm) were prepared from Au NPs (0.47 μM) and Man-RSSR-Man (2.5 mM) in the presence of sodium borohydride (NaBH(4); 1.0 mM). The highly luminescent properties of Man-Au NDs prepared by the NaBH(4)-assisted method were characterized by UV-vis absorption, photoluminescence, and X-ray photoelectron spectroscopies. The results supported the high-density coverage of the NDs surface by Man-RS ligands. Multivalent interactions between Man-Au NDs and FimH proteins located on the bacterial pili of E. coli resulted in the formation of aggregated cell clusters. After concentrating this agglutinative E. coli from a large-volume cell solution (5 mL), Man-Au NDs were displaced by mannose (100 mM) and stabilized by Man-RSSR-Man (5 mM). Monitoring the luminescence of Man-Au NDs allowed the detection of E. coli at levels as low as 150 CFU/mL. Man-Au NDs were also found to be efficient antibacterial agents, selectively inhibiting the growth of E. coli through Man-Au ND-induced agglutination. Our small-diameter Man-Au NDs, which provided an ultra high ligand density (local concentration) of mannose units for multivalent interactions with E. coli, have great potential for use as an antibacterial agent in other applications.

Quantum-sized gold nanoclusters: bridging the gap between organometallics and nanocrystals

This Concept article provides an elementary discussion of a special class of large-sized gold compounds, so-called Au nanoclusters, which lies in between traditional organogold compounds (e.g., few-atom complexes, <1 nm) and face-centered cubic (fcc) crystalline Au nanoparticles (typically >2 nm). The discussion is focused on the relationship between them, including the evolution from the Au⋅⋅⋅Au aurophilic interaction in Au(I) complexes to the direct Au-Au bond in clusters, and the structural transformation from the fcc structure in nanocrystals to non-fcc structures in nanoclusters. Thiolate-protected Au(n)(SR)(m) nanoclusters are used as a paradigm system. Research on such nanoclusters has achieved considerable advances in recent years and is expected to flourish in the near future, which will bring about exciting progress in both fundamental scientific research and technological applications of nanoclusters of gold and other metals.

Supported quantum clusters of silver as enhanced catalysts for reduction

Quantum clusters (QCs) of silver such as Ag7(H2MSA)7, Ag8(H2MSA)8 (H2MSA, mercaptosuccinic acid) were synthesized by the interfacial etching of Ag nanoparticle precursors and were loaded on metal oxide supports to prepare active catalysts. The supported clusters were characterized using high resolution transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and laser desorption ionization mass spectrometry. We used the conversion of nitro group to amino group as a model reaction to study the catalytic reduction activity of the QCs. Various aromatic nitro compounds, namely, 3-nitrophenol (3-np), 4-nitrophenol (4-np), 3-nitroaniline (3-na), and 4-nitroaniline (4-na) were used as substrates. Products were confirmed using UV-visible spectroscopy and electrospray ionization mass spectrometry. The supported QCs remained active and were reused several times after separation. The rate constant suggested that the reaction followed pseudo-first-order kinetics. The turn-over frequency was 1.87 s-1 per cluster for the reduction of 4-np at 35°C. Among the substrates investigated, the kinetics followed the order, SiO2 > TiO2 > Fe2O3 > Al2O3.

On the ligand's role in the fluorescence of gold nanoclusters

The fluorescence of metal nanoparticles (such as gold and silver) has long been an intriguing topic and has drawn considerable research interest. However, the origin of fluorescence still remains unclear. In this work, on the basis of atomically monodisperse, 25-atom gold nanoclusters we present some interesting results on the fluorescence from [Au(25)(SR)(18)](q) (where q is the charge state of the particle), which has shed some light on this issue. Our work explicitly shows that the surface ligands (-SR) play a major role in enhancing the fluorescence of gold nanoparticles. Specifically, the surface ligands can influence the fluorescence in two different ways: (i) charge transfer from the ligands to the metal nanoparticle core (i.e., LMNCT) through the Au-S bonds, and (ii) direct donation of delocalized electrons of electron-rich atoms or groups of the ligands to the metal core. Following these two mechanisms, we have demonstrated strategies to enhance the fluorescence of thiolate ligand-protected gold nanoparticles. This work is hoped to stimulate more experimental and theoretical research on the atomic level design of luminescent metal nanoparticles for promising optoelectronic and other applications.