Atomically Monodisperse Gold Nanoclusters Catalysts with Precise Core-Shell Structure

Photoluminescent Characterization of Metal Nanoclusters: Basic Parameters, Methods, and Applications

Metal nanoclusters (MNCs) can be synthesized with atomically precise structures and molecule formulae due to the rapid development of nanocluster science in recent decades. The ultrasmall size range (normally < 2 nm) endows MNCs with plenty of molecular-like properties, among which photoluminescent properties have aroused extensive attention. Tracing the research and development processes of luminescent nanoclusters, various photoluminescent analysis and characterization methods play a significant role in elucidating luminescent mechanism and analyzing luminescent properties. In this review, it is aimed to systematically summarize the normally used photoluminescent characterizations in MNCs including basic parameters and methods, such as excitation/emission wavelength, quantum yield, and lifetime. For each key parameter, first its definition and meaning is introduced and then the relevant characterization methods including measuring principles and the revelation of luminescent properties from the collected data are discussed. Then, it is discussed in details how to explore the luminescent mechanism of MNCs and construct NC-based applications based on the measured data. By means of these characterization strategies, the luminescent properties of MNCs and NC-based designs can be explained quantitatively and qualitatively. Hence, this review is expected to provide clear guidance for researchers to characterize luminescent MNCs and better understand the luminescent mechanism from the measured results.

Nanohybrids of atomically precise metal nanoclusters

Atomically precise metal nanoclusters (NCs) with molecule-like structures are emerging nanomaterials with fascinating chemical and physical properties. Photoluminescence (PL), catalysis, sensing, etc., are some of the most intriguing and promising properties of NCs, making the metal NCs potentially beneficial in different applications. However, long-term instability under ambient conditions is often considered the primary barrier to translational research in the relevant application fields. Creating nanohybrids between such atomically precise NCs and other stable nanomaterials (0, 1, 2, or 3D) can help expand their applicability. Many such recently reported nanohybrids have gained promising attention as a new class of materials in the application field, exhibiting better stability and exciting properties of interest. This perspective highlights such nanohybrids and briefly explains their exciting properties. These hybrids are categorized based on the interactions between the NCs and other materials, such as metal-ligand covalent interactions, hydrogen-bonding, host-guest, hydrophobic, and electrostatic interactions during the formation of nanohybrids. This perspective will also capture some of the new possibilities with such nanohybrids.

Characterization of Pt-doping effects on nanoparticle emission: a theoretical look at Au24Pt(SH)18 and Au24Pt(SC3H7)18

Developments in nanotechnology have made the creation of functionalized materials with atomic precision possible. Thiolate-protected gold nanoclusters, in particular, have become the focus of study in literature as they possess high stability and have tunable structure-property relationships. In addition to adjustments in properties due to differences in size and shape, heteroatom doping has become an exciting way to tune the properties of these systems by mixing different atomic d characters from transition metal atoms. Au₂₄Pt(SR)₁₈ clusters, notably, have shown incredible catalytic properties, but fall short in the field of photochemistry. The influence of the Pt dopant on the photoluminescence mechanism and excited state dynamics has been investigated by a few experimental groups, but the origin of the differences that arise due to doping has not been clarified thoroughly. In this paper, density functional theory methods are used to analyze the geometry, optical and photoluminescent properties of Au₂₄Pt(SR)₁₈ in comparison with those of [Au₂₅(SR)₁₈]^1-. Furthermore, as these clusters have shown slightly different geometric and optical properties for different ligands, the analysis is completed with both hydrogen and propyl ligands in order to ascertain the role of the passivating ligands.

Master key to coinage metal nanoclusters treasure chest: 38-metal clusters

Atomically precise metal nanoclusters with specific chemical compositions have become a popular research topic due to their precise structures, attractive properties, and wide range of applications in various fields. Currently, among more than 100 reported metal nanoclusters with precise formulas, 38-atom coinage metal nanoclusters stand out due to their unique structural diversities, such as face-centered cubic (FCC) and body-centered cubic (BCC) arrangements. Among them, the formation of the metal cores includes vertex-sharing, face-fusion, and FCC cubes fusion. Due to their geometrical features, 38-atom coinage metal nanoclusters exhibit attractive properties, making them an ideal model for exploring structure-property relationships. Therefore, 38-atom coinage metal nanoclusters are a universal key to the treasure trove of nanoclusters, which can open almost all fields and are of great research significance. This paper focuses on the structure of 38-atom coinage metal nanoclusters and reviews the preparation and crystallization methods, excellent properties, and practical applications. Finally, future research prospects and development opportunities are provided.

Modulation of the Activity of Gold Clusters Immobilized on Functionalized Mesoporous Materials in the Oxidation of Cyclohexene via the Functional Group. The Case of Aminopropyl Moiety

Gold nanoclusters and isolated gold atoms have been produced in a two-liquid phase procedure that involves a solution of gold in aqua regia and rosemary essential oil as organic layer. These gold entities have been immobilized on the ordered mesoporous silica material SBA-15 functionalized with different amounts of aminopropyl groups. The resulting materials have been characterized by XRD, N₂ adsorption, chemical analysis, TGA, ²⁹Si MAS NMR, ¹³C CP/MAS NMR, UV-vis spectroscopy, XPS, and STEM. The Au-containing materials retain the ordering and porosity of the pristine support. Gold content varies in the range of 0.07–0.7 wt% as a function of the specific immobilization conditions, while STEM evidences the presence of isolated gold atoms. XPS shows a shift of the Au 4f BE toward values lower than those of metallic gold. The catalytic activity in the oxidation of cyclohexene with molecular oxygen at atmospheric pressure parallels the Au content of the aminopropyl-SBA-15 supports. This activity is higher than that of analogous Au entities immobilized on SBA-15 functionalized with thiol or sulfonate groups, the activity decreasing in the order Au-NH₂ > Au-SO₃⁻ > Au-SH. This behavior has been attributed to differences in the interaction strength between the functional group and the Au entities, which is optimum for the aminopropyl groups.

Atomically precise alloy nanoclusters: syntheses, structures, and properties

Metal nanoclusters fill the gap between discrete atoms and plasmonic nanoparticles, providing unique opportunities for investigating the quantum effects and precise structure-property correlations at the atomic level. As a versatile strategy, alloying can largely improve the physicochemical performances compared to the corresponding homo-metal nanoclusters, and thus benefit the applications of such nanomaterials. In this review, we highlight the achievements of atomically precise alloy nanoclusters, and summarize the alloying principles and fundamentals, including the synthetic methods, site-preferences for different heteroatoms in the templates, and alloying-induced structure and property changes. First, based on various Au or Ag nanocluster templates, heteroatom doping modes are presented. The templates with electronic shell-closing configurations tend to maintain their structures during doping, while the others may undergo transformation and give rise to alloy nanoclusters with new structures. Second, alloy nanoclusters of specific magic sizes are reviewed. The arrangement of different atoms is related to the symmetry of the structures; that is, different atoms are symmetrically located in the nanoclusters of smaller sizes, and evolve into shell-by-shell structures at larger sizes. Then, we elaborate on the alloying effects in terms of optical, electrochemical, electroluminescent, magnetic and chiral properties, as well as the stability and reactivity via comparisons between the doped nanoclusters and their homo-metal counterparts. For example, central heteroatom-induced photoluminescence enhancement is emphasized. The applications of alloy nanoclusters in catalysis, chemical sensing, bio-labeling, and other fields are further discussed. Finally, we provide perspectives on existing issues and future efforts. Overall, this review provides a comprehensive synthetic toolbox and controllable doping modes so as to achieve more alloy nanoclusters with customized compositions, structures, and properties for applications. This review is based on publications available up to February 2020.

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.

Au25(SR)18: the captain of the great nanocluster ship

Noble metal nanoclusters are in the intermediate state between discrete atoms and plasmonic nanoparticles and are of significance due to their atomically accurate structures, intriguing properties, and great potential for applications in various fields. In addition, the size-dependent properties of nanoclusters construct a platform for thoroughly researching the structure (composition)-property correlations, which is favorable for obtaining novel nanomaterials with enhanced physicochemical properties. Thus far, more than 100 species of nanoclusters (mono-metallic Au or Ag nanoclusters, and bi- or tri-metallic alloy nanoclusters) with crystal structures have been reported. Among these nanoclusters, Au25(SR)18-the brightest molecular star in the nanocluster field-is capable of revealing the past developments and prospecting the future of the nanoclusters. Since being successfully synthesized (in 1998, with a 20-year history) and structurally determined (in 2008, with a 10-year history), Au25(SR)18 has stimulated the interest of chemists as well as material scientists, due to the early discovery, easy preparation, high stability, and easy functionalization and application of this molecular star. In this review, the preparation methods, crystal structures, physicochemical properties, and practical applications of Au25(SR)18 are summarized. The properties of Au25(SR)18 range from optics and chirality to magnetism and electrochemistry, and the property-oriented applications include catalysis, chemical imaging, sensing, biological labeling, biomedicine and beyond. Furthermore, the research progress on the Ag-based M25(SR)18 counterpart (i.e., Ag25(SR)18) is included in this review due to its homologous composition, construction and optical absorption to its gold-counterpart Au25(SR)18. Moreover, the alloying methods, metal-exchange sites and property alternations based on the templated Au25(SR)18 are highlighted. Finally, some perspectives and challenges for the future research of the Au25(SR)18 nanocluster are proposed (also holding true for all members in the nanocluster field). This review is directed toward the broader scientific community interested in the metal nanocluster field, and hopefully opens up new horizons for scientists studying nanomaterials. This review is based on the publications available up to March 2018.

Instant room temperature synthesis of self-assembled emission-tunable gold nanoclusters: million-fold emission enhancement and fluorimetric detection of Zn2+

Facile synthesis of luminescent metal nanoclusters (NCs) accompanied by emission color tuning is currently an active area of research. In this work we describe a rapid (1 s) room temperature synthesis of luminescent Au NCs from completely nonluminescent NCs through the incorporation of Zn²⁺. The nanoclusters are initially stabilized by mercaptopropionate, and the coordination of Zn²⁺ with the carboxylate groups of the ligands rigidifies the Au(i) thiolates restricting the intramolecular rotation-vibrational motion. This significantly reduces the nonradiative relaxation of the excited state to produce yellow luminescent NCs (λ_em = 580 nm, QY: 6%, τ = 0.2 ms) with almost a million-fold emission enhancement. The enhanced luminescence is due to the self-assembly mediated aggregation induced emission (AIE) of NCs. These NCs on aging for 24 hours transform to highly ordered green emitting NCs (λ_em = 500 nm, QY: 20%, τ = 20 ns). The blue shift in emission is due to the dominance of inter Au(i)-Au(i) interaction and inter-NC Zn²⁺ interaction over the intra modes. TEM images show this distinct transition, a decrease in inter NC distance with increased self-assembly. Excited state relaxation dynamics associated with Au(i) thiolate shell dynamics in yellow and green emitting NCs is explained based on the time resolved fluorescence study. The rapid formation of luminescent NCs from nl-NCs has been used for efficient visual and fluorimetric detection of Zn²⁺.

Photoassisted photoluminescence fine-tuning of gold nanodots through free radical-mediated ligand-assembly

In this study, we have developed a simple photoassisted ligand assembly to fine-tune the photoluminescence (PL) of (11-mercaptoundecyl)-N,N,N-trimethylammonium bromide-capped gold nanodots (11-MUTAB-Au NDs). The 11-MUTAB-Au NDs (size: ca. 1.8 nm), obtained from the reaction of gold nanoparticles (ca. 3 nm) and 11-MUTAB, exhibited weak, near-infrared (NIR) PL at 700 nm with a quantum yield (QY) of 0.37% upon excitation at 365 nm. The PL QY of the Au NDs increased to 11.43% after reaction with 11-mercaptoundecanoic acid (11-MUA) for 30 min under ultraviolet (UV) light, which was accompanied by a PL wavelength shift to the green region (∼520 nm). UV-light irradiation accelerates 11-MUA assembly on the 11-MUTABAu NDs (11-MUA/11-MUTAB-Au NDs) through a radical-mediated reaction. Furthermore, the PL wavelength of the 11-MUA/11-MUTAB-Au NDs can be switched to 640 nm via cysteamine under UV-light irradiation. We propose that the PL of the Au NDs with NIR and visible emissions was originally from the surface thiol-Au complexes and the Au core, respectively. These dramatically different optical properties of the Au NDs were due to variation in the surface ligands, as well as the densities and surface oxidant states of the surface Au atoms/ions. These effects can be controlled by assembling surface thiol ligands and accelerated by UV irradiation.

Radicals Are Required for Thiol Etching of Gold Particles

Etching of gold with an excess of thiol ligand is used in both synthesis and analysis of gold particles. Mechanistically, the process of etching gold with excess thiol is unclear. Previous studies have obliquely considered the role of oxygen in thiolate etching of gold. Herein, we show that oxygen or a radical initiator is a necessary component for efficient etching of gold by thiolates. Attenuation of the etching process by radical scavengers in the presence of oxygen, and the restoration of activity by radical initiators under inert atmosphere, strongly implicate the oxygen radical. These data led us to propose an atomistic mechanism in which the oxygen radical initiates the etching process.

Isolation and Tandem Mass Spectrometric Identification of a Stable Monolayer Protected Silver-Palladium Alloy Cluster

A selenolate-protected Ag-Pd alloy cluster was synthesized using a one-pot solution-phase route. The crude product upon chromatographic analyses under optimized conditions gave three distinct clusters with unique optical features. One of these exhibits a molecular peak centered at m/z 2839, in its negative ion mass spectrum assigned to Ag5Pd4(SePh)12(-), having an exact match with the corresponding calculated spectrum. Tandem mass spectrometry of the molecular ion peak up to MS(9) was performed. Complex isotope distributions in each of the mass peaks confirmed the alloy composition. We find the Ag3Pd3(-) core to be highly stable. The composition was further supported by scanning electron microscopy, energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy.

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

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