Ultrasmall Metal Nanoclusters Research Articles

Motif-to-Core Nucleation in a Decahedral Evolution Pattern.

The atomic precision of ultrasmall metal nanoclusters has opened the door to elucidating the structural evolution principles of metal nanomaterials at the molecular level. Here, we report a novel set of super-atomic Ag clusters, including [Ag19(TBBT)16(DPPP)4]+ (Ag19), [Ag22(DMAT)8(DPPM)4Cl8]2+ (Ag22), Ag26(SPh3,5-CF3)15(DPPF)4Cl5 (Ag26), and [Ag30(DMAT)12(DPPP)4Cl8]2+ (Ag30). The core structures of these clusters correspond to one decahedral Ag7, perpendicular bi-decahedrons, three-dimensional penta-decahedrons, and hexa-decahedrons, respectively. The Ag atoms in AgS2 blocks show a strong correlation with the decahedral cores: the five equatorial Ag atoms in the decahedral Ag7 core of Ag19 all adopt the AgS2 coordination, while the Ag atoms in AgS2 blocks of Ag22, Ag26, and Ag30 unexceptionally constitute additional decahedral structures with the core Ag atoms. Specifically, two and four core Ag atoms of Ag26 and Ag30 clusters occupy positions that highly resemble that of Ag (in AgS2 motifs) of Ag22. The strong structural correlation demonstrates the motif-to-core evolution of the surface Ag (on AgS2) to build extra-decahedral blocks. Density functional theory calculations indicate that the 2e, 4e, 6e, and 8e clusters (from Ag19 to Ag30) adopt 1S2, 1S21P2, 1S21P4, and 1S21P6 electron configurations, all of which feature excellent super-atomic characters.

Electrochemical Approach for the Synthesis of Ultrasmall Cu13 Clusters and Their Application in the Detection of Endotoxin.

Ultrasmall metal nanoclusters are attractive for their size-dependent optical and electrochemical properties. Here, blue-emitting copper clusters stabilized with cetyltrimethylammonium bromide (CTAB) are synthesized using an electrochemical approach. The electrospray ionization (ESI) analysis reveals that the cluster has 13 copper atoms in the core. The clusters are then used for the electrochemical detection of endotoxin─a bacterial toxin in Gram-negative bacteria. Differential pulse voltammetry (DPV) is used to detect endotoxins with high selectivity and sensitivity. It shows a lower detection limit of 100 ag mL-1 with a linear range of 100 ag mL-1 to 10 ng mL-1. The sensor is efficient for detecting endotoxins from human blood serum samples.

L-cysteine-protected ruthenium nanoclusters on CdS as efficient and reusable photocatalysts for hydrogen production

Nanocluster-modified semiconductor-based photocatalysts have been identified as a vital area of research in the area of photocatalytic hydrogen evolution from water. However, the existing ligand protection strategy for synthesizing ultrasmall metal nanoclusters remains confined to a few metals, including Au, Ag, Cu, and their alloys. In this investigation, we describe a facile solution-phase reduction synthesis method for the production of L-cysteine-protected Ru nanoclusters. Our findings demonstrate that these novel Ru nanoclusters function as cocatalysts, which notably increase the photocatalytic activity and photostability of CdS photocatalysts. Moreover, the hybrid CdS photocatalyst modified with Ru nanoclusters exhibits superior activity and stability relative to photoinduced Ru nanoparticles/CdS composite photocatalysts. The simplicity of the synthesized metal nanocluster cocatalyst and its effectiveness in enhancing photocatalyst activity, while reducing the use of precious metals, present new avenues for the development of advanced photocatalysts.

Depletion Driven Assembly of Ultrasmall Metal Nanoclusters: From Kinetically Arrested Assemblies to Thermodynamically Stable, Spherical Superclusters.

Nanoscale assembly of ultrasmall metal nanoclusters (MNCs) by means of molecular forces has proven to be a powerful strategy to engineer their molecule-like properties in multiscale dimensions. By leveraging depletion attraction as the guiding force, herein, we demonstrate the formation of kinetically trapped NCs assemblies with enhanced photoluminescence (PL) and excited state lifetimes and extend the principle to cluster impregnated cationic nanogels, nonluminescent Au(I)-thiolate complexes, and weakly luminescent CuNCs. We further demonstrate a thermal energy driven kinetic barrier breaking process to isolate these assemblies. These isolated assemblies are thermodynamically stable, built from a strong network among several discrete, ultrasmall AuNCs and exhibit several unusual properties such as high stability in various pH, strong PL, microsecond lifetimes, large Stocks shifts, and higher accumulation in the lysosome of cancer cells. We anticipate our strategy may find wider use in creating a large variety of MNC-based assemblies with many unforeseen arrangements, properties, and applications.

Zeolite-encapsulated Cu nanoparticles with enhanced performance for ethanol dehydrogenation

The selective dehydrogenation of ethanol to acetaldehyde is one of the pivotal reactions for biomass valorization, wherein the well-defined and robust Cu-based solid catalysts are highly desired for potential practical application. Herein, ultra-small Cu metal nanoclusters (∼1.8 nm), encapsulated inside Silicate-1 (S-1), have been successfully fabricated by an in-situ approach. The as-synthesized Cu@S-1 catalyst shows a near quantitative ethanol conversion, a good acetaldehyde selectivity up to ∼90 %, and more importantly an excellent stability with no apparent deactivation even after a TOS of 110 h in the continuous dehydrogenation of ethanol to acetaldehyde at 523 K. Comprehensive and thorough characterization studies reveal that the spatial constraint environment of the zeolite could not only efficiently restrict the movements of Cu entities, which thus prevents the metal sintering during catalysis, but also induce a predominant amount of well-retained Cu+ species (up to ∼70 %) that play an essential role for the enhanced performance.

Lethal Interactions of Atomically Precise Gold Nanoclusters and Pseudomonas aeruginosa and Staphylococcus aureus Bacterial Cells.

Ultrasmall metal nanoclusters (NCs) are employed in an array of diagnostic and therapeutic applications due to their tunable photoluminescence, high biocompatibility, polyvalent effect, ease of modification, and photothermal stability. However, gold nanoclusters' (AuNCs') intrinsically antimicrobial properties remain poorly explored and are not well understood. Here, we share an insight into the antimicrobial action of atomically precise AuNCs based on their ability to passively translocate across the bacterial membrane. Functionalized by a hydrophilic modified-bidentate sulfobetaine zwitterionic molecule (AuNC-ZwBuEt) or a more hydrophobic monodentate-thiolate, mercaptohexanoic acid (AuNC-MHA) molecule, 2 nm AuNCs were lethal to both Gram-negative Pseudomonas aeruginosa and Gram-positive Staphylococcus aureus bacteria. The bactericidal efficiency was found to be bacterial strain-, time-, and concentration-dependent. The direct visualizations of the translocation of AuNCs and AuNC-cell and subcellular interactions were investigated using cryo-soft X-ray nano-tomography, transmission electron microscopy (TEM), and scanning TEM energy-dispersive spectroscopy analyses. AuNC-MHA were identified in the bacterial cytoplasm within 30 min, without evidence of the loss of membrane integrity. It is proposed that the bactericidal effect of AuNCs is attributed to their size, which allows for efficient energy-independent translocation across the cell membrane. The internalization of both AuNCs caused massive internal damage to the cells, including collapsed subcellular structures and altered cell morphology, leading to the eventual loss of cellular integrity.

Ultrasmall Gold Nanoclusters-Enabled Fabrication of Ultrafine Gold Aerogels as Novel Self-Supported Nanozymes.

Metal aerogels represent an emerging type of functional porous materials with promising applications in diverse fields, but the fabrication of metal aerogels with specific structure and property still remains a challenge. Here, the authors report a new approach to fabricate metal aerogels by using ultrasmall metal nanoclusters (NCs) as functional building blocks. By taking D-penicillamine-stabilized gold NCs (AuNCs) with a diameter of 1.4nm as an example, Au aerogels with ultrafine ligament size (3.5nm) and good enzyme-mimic properties are synthesized. Detailed characterization shows that the obtained Au aerogels possess typical 3D self-supported porous network structure with high gold purity and surface area. Time-lapse spectroscopic and microscopic monitoring of the gelation process reveal that these ultrasmall AuNCs first grow into large nanoparticles before fusion into nanowire networks, during which both pH and the precursor concentration are identified to be the determining factor. Owing to their highly porous structure and abundant metal nodes, these self-supported Au aerogels display excellent peroxidase-like properties. This work provides a strategy for fabricating advanced metal aerogels by taking ultrasmall-sized metal NCs as building blocks, which also opens new avenues for engineering the structure and properties of metal aerogels for further advancing their applications.

Controlling the nucleation and growth of ultrasmall metal nanoclusters with MoS2 grain boundaries.

The stabilization of supported nanoclusters is critical for different applications, including catalysis and plasmonics. Herein we investigate the impact of MoS2 grain boundaries (GBs) on the nucleation and growth of Pt NCs. The optimum atomic structure of the metal clusters is obtained using an adaptive genetic algorithm that employs a hybrid approach based on atomistic force fields and density functional theory. Our findings show that GBs stabilize the NCs up to a cluster size of nearly ten atoms, and with larger clusters having a similar binding to the pristine system. Notably, Pt monomers are found to be attracted to GB cores achieving 60% more stabilization compared to the pristine surface. Furthermore, we show that the nucleation and growth of the metal seeds are facile with low kinetic barriers, which are of similar magnitude to the diffusion barriers of metals on the pristine surface. The findings highlight the need to engineer ultrasmall NCs to take advantage of enhanced stabilization imparted by the GB region, particularly to circumvent sintering behavior for high-temperature applications.

Ultrasmall AuPd nanoclusters on amine-functionalized carbon blacks as high-performance bi-functional catalysts for ethanol electrooxidation and formic acid dehydrogenation

The synthesis of ultrasmall metal nanoclusters (NCs) with high catalytic activities is of great importance for the development of clean and renewable energy technologies but remains a challenge. Here we report a facile wet-chemical method to prepare ∼1.0 nm AuPd NCs supported on amine-functionalized carbon blacks. The AuPd NCs exhibit a specific activity of 5.98 mA cmAuPd−2 and mass activity of 5.25 A mgAuPd−1 for ethanol electrooxidation, which are far better than those of commercial Pd/C catalysts (1.74 mA cmPd−2 and 0.54 A mgPd−1). For formic acid dehydrogenation, the AuPd NCs have an initial turn over frequency of 49339 h−1 at 298 K without any additive, which is much higher than those obtained for most of reported AuPd catalysts. The reported synthesis may represent a facile and low-cost approach to prepare other ultrasmall metal NCs with high catalytic activities for various applications.

Interactions of Metal Nanoclusters with Light: Fundamentals and Applications.

The interactions of materials with light determine their applications in various fields. In the past decade, ultrasmall metal nanoclusters (NCs) have emerged as a promising class of optical materials due to their unique molecular-like properties. Herein, the basic principles of optical absorption and photoluminescence of metal NCs, their interactions with polarized light, and light-induced chemical reactions, are discussed, highlighting the roles of the core and protecting ligands/motifs of metal NCs in their interactions with light. The metal core and protecting ligands/motifs determine the electronic structures of metal NCs, which are closely related to their optical properties. In addition, the protecting ligands/motifs of metal NCs contribute to their photoluminescence and chiral origin, further promoting the interactions of metal NCs with light through various pathways. The fundamentals of light-NC interactions provide guidance for the design of metal NCs in optical applications, which are discussed in the second part. In the last section, some strategies are proposed to further understand light-NC interactions, highlighting the challenges and opportunities. It is hoped that this work will stimulate more research on the optical properties of metal NCs and their applications in various fields.

Electrostatic adsorption of ultra-small silver nanoclusters on titanium dioxide modified mesoporous MCM-41as a high-performance photocatalyst for wastewater treatment

For the first time, monodispersed bare silver nanoclusters with an average particle size of 1.2 nm were synthesized without protecting ligand and deposited inside the pores of a titanium dioxide modified mesoporous MCM-41 utilizing the novel strong electrostatic adsorption (SEA) technique. The pH of the MCM-41 and TiO2/MCM-41 surfaces was adjusted at 11 to optimize the electrostatic adsorption of the silver metal complexes and MB dye. Where, the pH point of zero charges (pHPZC) of the MCM-41 and TiO2/MCM-41 are 4 and 4.9, respectively. For comparison, microwave assisted sol–gel and hydrothermal methods were used to load silver nanoparticles on a titanium dioxide modified mesoporous MCM-41, where, the average particle size of the silver nanoparticles is 2–5 nm and 4–5.5 nm, respectively. The texture properties of the synthesized photocatalysts were investigated using the liquid nitrogen sorption isotherms at _196 °C. The crystallinity of the photocatalysts was characterized by X-ray diffraction (XRD). The charge of the loaded silver nanoclusters and silver nanoparticles was investigated by X-ray photoelectron spectroscopy (XPS). The silver content was around 1.0 wt% over the TiO2/MCM-41 and MCM-41 supports that was measured by the XPS. The performance of the synthesized photocatalysts was tested by photocatalytic degradation of MB dye under visible light irradiation. Around 100 % decolorization of 50 ppm MB solution was archived after two hours of irradiation over Ag/TiO2/MCM-41 (SEA) photocatalyst. This catalyst shows stable efficiency for at least five cycles without significant leaching or loss of catalytic activity. The surface morphology of the Ag/TiO2/MCM-41 (SEA) photocatalyst was investigated using scanning electron microscopy (SEM). The excellent performance of the SEA catalyst is attributed to ultra-small Ag nanoclusters that uniformly dispersed in the mesopores of MCM-41 so that more Ag surface sites are available for the catalytic reaction. In conclusion, the SEA is an excellent synthetic strategy for the deposition of ultra-small bare metal nanoclusters on porous materials.

Zeolite-Tailored Active Site Proximity for the Efficient Production of Pentanoic Biofuels.

Biofuel production can alleviate reliance on fossil resources and thus carbon dioxide emission. Hydrodeoxygenation (HDO) refers collectively to a series of important biorefinery processes to produce biofuels. Here, well-dispersed and ultra-small Ru metal nanoclusters (ca. 1 nm), confined within the micropores of zeolite Y, provide the required active site intimacy, which significantly boosts the chemoselectivity towards the production of pentanoic biofuels in the direct, one-pot HDO of neat ethyl levulinate. Crucial for improving catalyst stability is the addition of La, which upholds the confined proximity by preventing zeolite lattice deconstruction during catalysis. We have established and extended an understanding of the "intimacy criterion" in catalytic biomass valorization. These findings bring new understanding of HDO reactions over confined proximity sites, leading to potential application for pentanoic biofuels in biomass conversion.

Size-Scalable Near-Infrared Photoluminescence in Gold Monolayer Protected Clusters.

Near-infrared photoluminescence of a series of three gold monolayer protected clusters (MPCs) with volumes spanning 50-200 Å3 was studied by using variable-temperature photoluminescence (VT-PL) spectroscopy. The three MPCs, which included Au20(SC8H9)15-diglyme, Au25(SC8H9)18, and Au38(SC12H25)24, all exhibited temperature-dependent intensities that reflected a few-millielectronvolt energy gap that separated bright emissive and dark nonradiative electronic states. All clusters showed increased PL intensities upon raising the sample temperature from 4.5 K to a cluster-specific value, upon which increased sample temperature resulted in emission quenching. The increased PL in the low-temperature range is attributed to thermally activated carrier transfer from dark to bright states. The quenching at elevated temperatures is attributed to nonradiative vibrational relaxation through Au-Au stretching of the MPCs metal core. Importantly, the results show evidence of a common and size scalable metal-centered intraband PL mechanism that is general for ultrasmall metal nanoclusters, which are expected to show nonscalable optical properties.

The beauty of binary phases: A facile strategy for synthesis, processing, functionalization, and application of ultrasmall metal nanoclusters

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

Induction of apoptosis in Trypanosoma brucei following endocytosis of ultra-small noble metal nanoclusters

Trypanosoma brucei, the causative agent of African trypanosomiasis, is a unicellular protozoan with an intricate life cycle involving an insect vector and mammalian hosts. Trypanosomiasis remains a threat to the health of human beings and animals. Despite the rapid development of nanomedicine, there are few valuable reports on the application of nanoscience in protozoa, especially the lack of understanding of the mechanism. In this study, we investigated the endocytosis of two noble metal nanoclusters (NM-NCs), Ag2[email protected] and [email protected] in T. brucei. Both types of NC can be efficiently up-taken by T. brucei via a clathrin-dependent endocytosis pathway, and displayed in a dose-dependent anti-parasitic manner by inducing pathological alterations to apoptosis-associated organelles. The Ag2[email protected] mainly interacted with functional proteins in the mitochondrion and endoplasmic reticulum, while the [email protected] predominantly interfered with the biological activity of cytoplasmic enzymes involved in mRNA maturation, protein degradation and signal transduction. These data not only reveal the clathrin-dependent endocytosis pathway in T. brucei, but also open up a new avenue for NM-NC-based drug development for trypanosomiasis.

High-Yield Synthesis of AIE-Type Au22(SG)18 Nanoclusters through Precursor Engineering and Its pH-Dependent Size Transformation

In recent years, ultrasmall metal nanoclusters (NCs) with aggregation-induced emission (AIE) characteristics have drawn enormous attention due to their strong photoluminescence (PL) and potential a...

Observing antimicrobial process with traceable gold nanoclusters

Understanding the interaction of nanomaterials with biological systems has always been of high concern and interest. An emerging type of nanomaterials, ultrasmall metal nanoclusters (NCs, < 2 nm in size), are promising in this aspect due to their well-defined molecular formulae and structures, as well as unique physical and chemical properties that are distinctly different from their larger counterparts (metal nanoparticles). For example, metal NCs possess intrinsic strong luminescence, which can be used for real-time tracking of their interactions with biological systems. Herein, luminescent gold (Au) NCs were used as traceable antimicrobial agents to study their interactions with the bacteria and to further understand their underlining antimicrobial mechanism. It is shown for the first time that the Au NCs would first attach on the bacterial membrane, penetrate, and subsequently accumulate inside the bacteria. Thereafter, the internalized Au NCs would induce reactive oxygen species (ROS) generation and damage the bacterial membrane, resulting in the leakage of bacterial contents, which can finally kill the bacteria. Traceable Au NCs (or other metal NCs) provide a promising platform to study the antimicrobial mechanisms as well as other fundamentals on the interfacing of functional nanomaterials with the biological systems, further increasing their acceptance in various biomedical applications.

Overcoming bacterial physical defenses with molecule-like ultrasmall antimicrobial gold nanoclusters

The size of metal nanoparticles (NPs) is crucial in their biomedical applications. Although abundant studies on the size effects of metal NPs in the range of 2–100 nm have been conducted, the exploration of the ultrasmall metal nanoclusters (NCs) of ~1 nm in size with unique features is quite limited. We synthesize three different sized gold (Au) NCs of different Au atom numbers and two bigger sized Au NPs protected by the same ligand to study the size influence on antimicrobial efficacy. The ultrasmall Au NCs can easily traverse the cell wall pores to be internalized inside bacteria, inducing reactive oxygen species generation to oxidize bacterial membrane and disturb bacterial metabolism. This explains why the Au NCs are antimicrobial while the Au NPs are non-antimicrobial, suggesting the key role of size in antimicrobial ability. Moreover, in contrast to the widely known size-dependent antimicrobial properties, the Au NCs of different atom numbers demonstrate molecule-like instead of size-dependent antimicrobial behavior with comparable effectiveness, indicating the unique molecule-like feature of ultrasmall Au NCs. Overcoming the bacterial defenses at the wall with ultrasmall Au NCs changes what was previously believed to harmless to the bacteria instead to a highly potent agent against the bacteria.

Pressure-Induced Optical Transitions in Metal Nanoclusters.

Currently, a comprehensive understanding of the relationship between atomic structures and optical properties of ultrasmall metal nanoclusters with diameters between 1 and 3 nm is lacking. To address this challenge, it is necessary to develop tools for perturbing the atomic structure and modulating the optical properties of metal nanoclusters beyond what can be achieved using synthetic chemistry. Here, we present a systematic high-pressure study on a series of atomically precise ligand-protected metal nanoclusters. A diamond anvil cell is used as a high-pressure chamber to gradually compress the metal nanoclusters, while their optical properties are monitored in situ. Our experimental results show that the photoluminescence (PL) of these nanoclusters is enhanced by up to 2 orders of magnitude at pressures up to 7 GPa. The absorption onset red-shifts with increasing pressure up to ∼12 GPa. Density functional theory calculations reveal that the red-shift arises because of narrowing of the spacing between discrete energy levels of the cluster due to delocalization of the core electrons to the carbon ligands. The pressure-induced PL enhancement is ascribed to (i) the enhancement of the near-band-edge transition strength, (ii) suppression of the nonradiative vibrations, and (iii) hindrance of the excited-state structural distortions. Overall, our results demonstrate that high pressure is an effective tool for modulating the optical properties and improving the luminescence brightness of metal nanoclusters. The insights into structure-property relations obtained here also contribute to the rational design of metal nanoclusters for various optical applications.

Ultrasmall Metal Nanoclusters As Bifuntional Electrocatalysts for Overall Water Splitting

Splitting water into hydrogen and oxygen is one of the key processes for renewable energy production. Electrocatalytic water splitting composed of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is considered to be a viable method due to its high efficiency and strong connectivity to energy-related electrodevices. Consequently, development of water splitting electrocatalysts is emerging field spanning from traditional Pt group metals to non-precious metal based catalysts. Ligand-protected metal nanoclusters could be one of the candidates because of their uniformity, stability and distinctive catalytic activity. Here we report electrocatalytic activity of atomically precise Mx(SR)y (M = metal, SR = thiolate) nanoclusters for water splitting reactions. HER activities were investigated by linear sweep voltammetry (LSV) and constant potential electrolysis (CPE). Metal nanoclusters exhibited distinctive HER activity with low Tafel slopes for universal pH range (pH 1 – 14). The activity could be modified by controlling the number of metals and ligands with atomic precision. OER activities of metal nanoclusters were examined by LSV and showed higher catalytic current than commercial IrO2 in alkaline media. Furthermore, metal nanoclusters were used as bifunctional water splitting (HER/OER) catalysts that showed high current density (100 mA cm-2) at a low electrochemical cell potential (2.5 V).