Flexible Hydrazone-Linked Metal-Covalent Organic Frameworks with Copper Clusters for Efficient Electrocatalytic Oxygen Evolution Reaction.

Despite the challenges associated with the synthesis of flexible metal-covalent organic frameworks (MCOFs), these offer the unique advantage of maximizing the atomic utilization efficiency. However, the construction of flexible MCOFs with flexible building units or linkages has rarely been reported. In this study, novel flexible MCOFs are constructed using flexible building blocks and copper clusters with hydrazone linkages. The heterometallic frameworks (Cu, Co) are prepared through the hydrazone linkage coordination method and evaluated as catalysts for the oxygen evolution reaction (OER). Owing to the spatial separation and functional cooperation of the heterometallic MCOF catalysts, the as-synthesized MCOFs exhibited outstanding catalytic activities with an overpotential of 268.8 mV at 10 mA cm-2 for the OER in 1 M KOH, which is superior to those of the reported covalent organic frameworks (COFs)-based OER catalysts. Theoretical calculations further elucidated the synergistic effect of heterometallic active sites within the linkages and frameworks, contributing to the enhanced OER activity. This study thus introduces a novel approach to the fundamental design of flexible MCOF catalysts for the OER, emphasizing their enhanced atomic utilization efficiency.

Size and Morphology Dependent Activity of Cu Clusters for CO2 Activation and Reduction: A First Principles Investigation.

Various Cu-based materials in diverse forms have been investigated as efficient catalysts for electrochemical reduction of CO2; however, they suffer from issues such as higher over potential and poor selectivity. The activity and selectivity of CO2 electro reduction have been shown to change significantly when the surface morphology (steps, kinks, and edges) of these catalysts is altered. In light of this, size and morphology dependent activity of selected copper clusters, Cun (n=2-20) have been evaluated for the activation and reduction of CO2 molecule. The phase-space of these copper clusters is rich in conformations of distinct morphologies starting from planar, 2D geometries to prolate-shaped geometries and also high-symmetry structures. The binding efficiency and the activation of CO2 are highest for medium sized clusters (n=9-17) with prolate-morphologies as compared to small or larger sized CunCO2 clusters that are existing mainly as planar (triangular, tetragonal etc.) or highly-symmetric geometries (icosahedron, capped-icosahedron etc.), respectively. The best performing (prolate-shaped) CunCO2 conformations are quite fluxional and also they are thermally stable, as demonstrated by the molecular dynamics simulations. Furthermore, on these CunCO2 conformations, the step-by-step hydrogenation pathways of CO2 to produce value-added products like methanol, formic acid, and methane are exceptionally favorable and energy-efficient.

Promoting CO2 Electroreduction to Hydrocarbon Products via Sulfur-Enhanced Proton Feeding in Atomically Precise Thiolate-Protected Cu Clusters.

Thiolate-protected Cu clusters with well-defined structures and stable low-coordinated Cu+ species exhibit remarkable potential for the CO2RR and are ideal model catalysts for establishing structure-electrocatalytic property relationships at the atomic level. However, extant Cu clusters employed in the CO2RR predominantly yield 2e- products. Herein, two model Cu4(MMI)4 and Cu8(MMI)4(tBuS)4 clusters (MMI = 2-mercapto-1-methylimidazole) are prepared to investigate the synergistic effect of Cu+ and adjacent S sites on the CO2RR. Cu4(MMI)4 can reduce CO2 to deep-reduced products with a 91.0% Faradaic efficiency (including 53.7% for CH4) while maintaining remarkable stability. Conversely, Cu8(MMI)4(tBuS)4 shows a remarkable preference for C2+ products, achieving a maximum FE of 58.5% with a C2+ current density of 152.1 mA∙cm-2. In situ XAS and ex situ XPS spectra reveal the preservation of Cu+ species in Cu clusters during CO2RR, extensively enhancing the adsorption capacity of *CO intermediates. Moreover, kinetic analysis and theoretical calculations confirm that S sites facilitate H2O dissociation into *H species, which directly participate in the protonation process on adjacent Cu sites for the protonation of *CO to *CHO. This study highlights the important role of Cu-S dual sites in Cu clusters and provides mechanistic insights into the CO2RR pathway at the atomic level.

Benchmark Models for Elucidating Ligand Effects: Thiols Ligated Isostructural Cu6 Nanoclusters.

Atomically precise copper nanoclusters (Cu NCs) have attracted tremendous attention for their huge potential in many applications. However, the uncertainty of the growth mechanism and complexity of the crystallization process hinder the in-depth understanding of their properties. In particular, the ligand effect has been rarely explored at the atomic/molecular level due to the lack of feasible models. Herein, three isostructural Cu6 NCs ligated with diverse mono-thiol ligands (2-mercaptobenzimidazole, 2-mercaptobenzothiazole, and 2-mercaptobenzoxazole, respectively) are successfully synthesized, which provide an ideal platform to unambiguously address the intrinsic role of ligands. The overall atom-by-atom structural evolution process of Cu6 NCs is mapped out with delicate mass spectrometry (MS) for the first time. It is intriguingly found that the ligands, albeit only atomic difference (NH, O, and S), can profoundly affect the building-up processes, chemical properties, atomic structures, as well as catalytic activities of Cu NCs. Furthermore, ion-molecule reactions combined with density functional theory (DFT) calculations demonstrate that the defective sites formed on ligand can significantly contribute to the activation of molecular oxygen. This study provides fundamental insights into the ligand effect, which is vital for the delicate design of high-efficient Cu NCs-based catalysts.

A copper nanocluster-based multifunctional nanoplatform for augmented chemo/chemodynamic/photodynamic combination therapy of breast cancer.

With the development of nano drug delivery system, the treatment mode that can overcome the shortcomings of chemotherapy drugs and integrate combined therapy remains to be explored. Herein, a nano drug system was designed to achieve the combined effect of chemo/chemodynamic/photodynamic therapy on cancer. Specifically, copper clusters (CuNCs) were used as the carrier, hyaluronic acid (HA) and doxorubicin (DOX) were coupled on CuNCs and then and chlorin e6 (Ce6) was introduced to form the self-assembled HA-CuNCs@DC nanoparticles. In this system, the HA-CuNCs@DC was involved in the reaction to the acidic tumor microenvironment (TME)-release of DOX, which could not only inhibit tumor growth through chemotherapy, but enhance the generation of hydrogen peroxide. CuNCs carriers had the properties of Fenton-like activity to realize chemodynamic therapy (CDT) and oxidase-like activity to deplete intracellular glutathione (GSH). Additionally, the chemotherapy drug susceptibility increased owing to the GSH depletion and the outbreak of reactive oxygen species, indicating the enhanced CDT efficacy and amplified chemotherapy efficacy. It was also noteworthy that Ce6 could be activated by 660 nm light to produce abundant singlet oxygen for photodynamic therapy. Overall, our platform demonstrated excellent biosafety and tumor suppression capabilities. This multimodal theranostic strategy provided new insights into cancer therapy.

Cluster-enabled patterning of copper nanostructures from aqueous solution using a femtosecond laser.

A one-step method for patterning low-resistivity nanoscale copper wire is proposed herein to solve the challenging issues of using common metals rather than noble metal nanostructures fabricated by direct laser writing in solution. A complexing and a reducing agent were introduced for the single-photon absorption of copper solution in the visible range and to enable two-photon absorption with a femtosecond laser. Copper clusters were generated prior to direct laser writing to decrease induced laser energy during two-photon absorption and accelerate copper nanowire patterning to avoid the boiling of copper solution. A surfactant was used to restrain the overgrowth of copper clusters to obtain written nanowires with high uniformity. By controlling the laser writing parameters, the obtained copper wire had a minimum width of 230 nm and a resistivity of 1.22 × 10-5Ω·m. Our method paves the way for the fabrication of common metal nanodevices by direct laser writing.

Mediating CO2 Electroreduction Activity and Selectivity over Atomically Precise Copper Clusters.

Atomically precise copper clusters are highly desirable catalysts for electrocatalytic CO2 reduction reaction (CO2 RR) and provide an ideal platform for elaborating structure-activity relationships. However, systematic comparative studies of Cu cluster isomers for electrocatalytic CO2 RR are lacking because they are challenging to synthesize. A group of structurally precise Cu8 cluster isomers with different core structures (cube- and ditetrahedron-shaped) were developed and investigated for highly active and selective CO2 reduction. Electrocatalytic measurements showed that the ditetrahedron-shaped Cu8 cluster exhibited a higher FEHCOOH (≈92 %) at -1.0 V and higher selectivity than the cube-shaped cluster. Theoretical investigations revealed different levels of competitiveness with the hydrogen evolution reaction on the respective core-shaped Cu8 clusters and decreased free energies for the adsorbed HCOO* intermediates on the ditetrahedron-shaped Cu8 clusters.

Restriction of Intramolecular Vibration in Aggregation-Induced Emission Luminogens: Applications in Multifunctional Luminescent Metal-Organic Frameworks.

Butterfly-like molecules of oxacalix[2]arene[2]pyrazine (OAP) are reported, which exhibit the typical characteristics of aggregation-induced emission (AIE) via the restriction of intramolecular vibration (RIV) mechanism. Unlike any of the reported RIV-type AIE molecules, the synthetic procedures of which are complicated and have associated high costs, OAP AIEgens can be synthesized in a facile manner by a one-step catalyst-free reaction using commercially available materials. Notably, OAP AIEgens are ideal ligands for constructing metal-organic frameworks (MOFs) due to their built-in pyrazine coordination sites. OAP-based MOFs exhibit multiple potential applications in reversible gas response, encrypted information storage, and construction of white light-emitting devices. This work builds on RIV-type AIEgens, offers additional selections of bridging ligands for constructing luminescent MOFs and provides a visualized prototype to understand the effect of the RIV process on the luminescence properties of MOFs.

Construction of dendritic mesoporous SiO2 supported Cu catalyst for hydrodeoxidation of lignin derivatives.

Chemoselective hydrodeoxygenation (HDO) of lignin derivatives is of great importance in converting biomass into high value-added chemicals. Herein, we report a simple hydrothermal pathway to fabricate a highly active, chemically selective, and reusable catalyst (Cu/DMSN) by loading copper clusters on "dendritic" mesoporous silica nanospheres. Cu/DMSN exhibits exceptional catalytic activity (conversion > 99 %, selectivity > 99 %) in the HDO of vanillin toward 2-methoxy-4-methylphenol under mild conditions (140 °C, 0.5 MPa, 3 h), along with good scalability and a wide range of substrates. The excellent catalytic performance can be owed to the combination of suitable acid site and more exposed metal site. This study provides a new strategy for designing supported metal catalysts for hydrodeoxidation of biomass and its derivatives.

DNA Framework-Templated Synthesis of Copper Cluster Nanozyme with Enhanced Activity and Specificity.

Nanozymes have been developed to overcome the inherent limitations of natural enzymes, such as their low stability and high cost. However, their efficacy has been hindered by their relatively low specificity and activity. Here, we demonstrate the self-assembly of individual copper nanoclusters (CuNCs) via a simple yet fast (10 min) DNA nanosheet (DNS)-templated method, enhancing the peroxidase-like activity and specificity of CuNCs. Furthermore, we demonstrate the successful assembly of CuNCs on different DNA nanostructures by atomic force microscopy (AFM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The resulting micron-scale ultrathin DNA nanosheet-templated CuNCs (DNS@CuNCs) exhibit exceptional catalytic activity, with a specific activity reaching 1.79 × 103 U mg-1. Investigation into the catalytic process reveals that the enhanced activity and specificity arise from disparities in active intermediate content before and after CuNCs assembly. Significantly, the DNS@CuNCs-based biosensor demonstrates remarkable anti-interference capabilities, enabling the detection of H2O2 in undiluted human serum for the first time with a detection limit of 0.99 µM.

Boosting 2000-Fold Hypergolic Ignition Rate of Carborane by Substitutes Migration in Metal Clusters.

Hypergolic propellants rely on fuel and oxidizer that spontaneously ignite upon contact, which fulfill a wide variety of mission roles in launch vehicles and spacecraft. Energy-rich carboranes are promising hypergolic fuels, but triggering their energy release is quite difficult because of their ultrastable aromatic cage structure. To steer the development of carborane-based high-performance hypergolic material, carboranylthiolated compounds integrated with atomically precise copper clusters are presented, yielding two distinct isomers, Cu14B-S and Cu14C-S, both possessing similar ligands and core structures. With the migration of thiolate groups from carbon atoms to boron atoms, the ignition delay (ID) time shortened from 6870 to 3 ms when contacted with environmentally benign oxidizer high-test peroxide (HTP, with a H2O2 concentration of 90%). The extraordinarily short ignition ID time of Cu14B-S is ranking among the best of HTP-active hypergolic materials. The experimental and theoretical findings reveal that benefitting from the migration of thiolate groups, Cu14B-S, characterized by an electron-rich metal kernel, displays enhanced reducibility and superior charge transfer efficiency. This results in exceptional activation rates with HTP, consequently inducing carborane combustion and the simultaneous release of energy. This fundamental investigation shed light on the development of advanced green hypergolic propulsion systems.

The Role of a Confined Space on the Reactivity and Emission Properties of Copper(I) Clusters.

Metal clusters have gained a lot of interest for their remarkable photoluminescence and catalytic properties. However, a major drawback of such materials is their poor stability in air and humidity conditions. Herein we describe a versatile method to synthesize luminescent Cu(I) clusters inside the pores of zeolites, using a sublimation technique with the help of high vacuum and high temperature. The porous materials play an essential role as a protecting media against the undesirable and easy oxidation of Cu(I). The obtained clusters show fascinating luminescence properties, and their reactivity can be triggered by insertion in the pores of organic monodentate ligands such as pyridine or triphenylphosphine. The coordinating ligands can lead to the formation of Cu(I) complexes with completely different emission properties. In the case of pyridine, the final compound was characterized and identified as a cubane-like structure. A thermochromism effect is also observed, featuring, for instance, a hypsochromic effect for a phosphine derivative at 77K. The stability of the encapsulated systems in zeolites is rather enthralling: they are stable and emissive even after several months in the air.

Copper Nanocluster Enables Simultaneous Photodynamic and Chemo Therapy for Effective Cancer Diagnosis and Treatment in Vitro.

Metal-nanocluster-mediated cancer diagnosis and therapy has drawn considerable attention in recent years due to the unique optical and photophysical properties of metal clusters. This type of material is highly useful for the diagnosis, treatment, and further follow-up of disease. However, a single treatment modality is not sufficient for a complete cure. The use of a multi-therapeutic strategy is among the most promising methods for effective treatment, along with an early-stage diagnosis. To address the multiple therapeutic modalities in a single nanomaterial, a copper nanocluster was synthesized using glutathione, having inherent singlet oxygen generation and emission at 674 nm. A tumor-targeting agent (folic acid) and an anticancer drug (doxorubicin) was conjugated to the copper cluster for cancer diagnosis via targeted imaging and further double therapy (photodynamic and chemotherapy) in vitro. 10.5 µg (18.1 nmol) of drug conjugated copper cluster shows 56 % cell death for 30 second laser irradiation in HeLa cells. Effective cancer cell imaging and therapeutic efficacy are demonstrated in vitro.

First-principles modelling of the new generation of subnanometric metal clusters: Recent case studies.

The very recent development of highly selective techniques making possible the synthesis and experimental characterization of subnanometric (subnanometer-sized) metal clusters (even single atoms) is pushing our understanding far beyond the present knowledge in materials science, driving these clusters as a new generation of quantum materials at the lower bounds of nanotechnology. When the size of the metal cluster is reduced to a small number of atoms, the d-band of the metal splits into a subnanometric d-type molecular orbitals network in which all metal atoms are inter-connected, with the inter-connections having the length of a chemical bond (1-2 Å). These molecular characteristics are at the very core of the high stability and novel properties of the smallest metal clusters, with their integration into colloidal materials interacting with the environment having the potential to further boost their performance in applications such as luminescence, sensing, bioimaging, theranostics, energy conversion, catalysis, and photocatalysis. Through the presentation of very recent case studies, this Feature Article is aimed to illustrate how first-principles modelling, including methods beyond the state-of-the-art and an interplay with cutting-edge experiments, is helping to understand the special properties of these clusters at the most fundamental level. Moreover, it will be discussed how superfluid helium droplets can act both as nano-reactors and carriers to achieve the synthesis and surface deposition of metal clusters. This concept will be illustrated with the quantum simulation of the helium droplet-assisted soft-landing of a single Au atom onto a titanium dioxide (TiO2) surface. Next, it will be shown how the application of first-principles methods have disclosed the fundamental reasons why subnanometric Cu5 clusters are resistant to irreversible oxidation, and capable of increasing and extending into the visible region the solar absorption of TiO2, of augmenting its efficiency for photo-catalysis beyond a factor of four, also considering the decomposition and photo-activation of CO2 as a prototypical (photo-) catalytic reaction. Finally, I will discuss how the modification of the same material with subnanometric Ag5 clusters has converted it into a "reporter" of a surface polaron property as well as a novel two-dimensional polaronic material.

Synthesis of Highly Fluorescent Copper Clusters Using Living Polymer Chains as Combined Reducing Agents and Ligands.

We present the synthesis of colloidally stable ultrasmall (diameter of 1.5 ± 0.6 nm) and fluorescent copper clusters (Cu-clusters) exhibiting outstanding quantum efficiencies (up to 67% in THF and approximately 30% in water). For this purpose, an amphiphilic block copolymer poly(ethylene glycol)-block-poly(propylene sulfide) (MPEG-b-PPS) was synthesized by living anionic ring-opening polymerization. When CuBr is mixed with the living polymer chains in THF, the formation of Cu-clusters is detected by the appearance of the fluorescence. The cluster growth is quenched by the addition of water, followed by THF removal. The structural features of the MPEG-b-PPS copolymer control the cluster formation and the stabilization: the poly(propylene sulfide) segment acts as coordinating and reducing agent for the copper ions in THF, and imparts a hydrophobic character. This hydrophobic block protects the Cu-clusters from water exposure, thus allowing to obtain a stable emission in water. The PEG segment instead provides the hydrophilicity, rendering the Cu-clusters water-soluble. To obtain fluorescent and stable Cu-clusters exhibiting outstanding quantum efficiencies, the removal of the excess of free polymer and copper salt was crucial. The Cu-clusters are also colloidally and optically stable in physiological media and showed bright fluorescence even when taken up by HeLa cells, being noncytotoxic when administered at a Cu dose between 10 nM and 1.6 µM. Given the very small size of the Cu-clusters, localization and fluorescent staining of cell nucleus is achieved, as demonstrated by confocal cell imaging performed at different Cu-cluster doses and at different incubation temperatures.

Synthesis of Cu/CuO nanoparticles in mesoporous material by solid state reaction.

The Mobil Composition of Matter No. 41 (MCM-41) containing 1.0 and 5.0 wt.% of Cu was synthesized under solid state reaction. The calcinations of samples were done at two different temperatures, 500 and 300°C. X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM) were used for samples characterization. Powder X-ray diffraction showed that when Cu(CH3COO)2 content is about 1.0 wt.% in Cu/MCM-41, the guest CuO-NPs and copper ions is formed on the silica channel wall, and more exists in the crystalline state. When Cu(CH3COO)2 content exceeds this value (5.0 wt.%), CuO nanoparticles and Cu(2+) ions can be observed in low crystalline state. From the diffuse reflectance spectra it was confirmed that 5 wt.% Cu/MCM-41 sample calcined at 500°C show plasmon resonance band due to Cu nanoparticles in the range between 500 and 600 nm and small copper clusters Cun in 450 nm. It also shows that some of the Cu(2+) ions are present octahedrally in extraframework position in all samples. Both fourier transform infrared and diffuse reflectance spectra indicate that some of Cu(2+) ions are tetrahedrally within the framework position in 1 wt.% Cu/MCM-41 samples. TEM images indicated that nanoparticles size of CuO is in range of 30-40 nm.