Ultrasmall Metal Research Articles

Enhanced Photoacoustic Signal of Passion Fruit-Like Nanoarchitectures in a Biological Environment

Photoacoustic imaging (PAI) is a promising noninvasive technique for molecular and cellular characterization of cancer. Cancer detection by PAI is an area of active research, and the recent advent of contrast agents based on near-infrared (NIR)-absorbing dyes or organic/inorganic nanoparticles tremendously improved the specificity and the signal contrast. Here we report the photoacoustic (PA) signal enhancement produced by biodegradable passion fruit-like nanoarchitectures in phantoms and ex vivo. By this approach, the synergistic interaction between commercial NIR-fluorophores and ultrasmall metal nanoparticles is exploited to produce a PA signal enhancement in the biological window. Moreover, the degradation of the nanoarchitectures is investigated in physiological environment as reference matrix.

Passion fruit-like nano-architectures: a general synthesis route

Noble metal nanostructures have demonstrated a number of intriguing features for both medicine and catalysis. However, accumulation issues have prevented their clinical translation, while their use in catalysis has shown serious efficiency and stability hurdles. Here we introduce a simple and robust synthetic protocol for passion fruit-like nano-architectures composed by a silica shell embedding polymeric arrays of ultrasmall noble metal nanoparticles. These nano-architectures show interesting features for both oncology and catalysis. They avoid the issue of persistence in organism thanks to their fast biodegradation in renal clearable building blocks. Furthermore, their calcination results in yolk-shell structures composed by naked metal or alloy nanospheres shielded from aggregation by a silica shell.

Research Update: Interfacing ultrasmall metal nanoclusters with biological systems

Metal nanoclusters (NCs), a new type of nanomaterial with unique physicochemical properties, show great potential in many biomedical applications. Understanding their behavior in the complex biological environment is critical not only for designing highly efficient NC-based nanomedicines but also for elucidating the biological impact (e.g., toxicity) of these emerging nanomaterials. In this review, we give an overview of recent progress in exploring interactions of metal NCs with biological systems, including protein adsorption onto NCs, NC interactions with cells, and also the in vivo behavior of NCs. We also discuss the biological responses to the interactions, key parameters defining the interactions, and current challenges in the exploration of NCs in the complex biological environment.

Control of reactivity through chemical order in very small RuRe nanoparticles.

The choice of suitable organometallic precursors, [Re2(C3H5)4] and [Ru(Me-Allyl)2COD] or [Ru(COD)(COT)], allows us to synthesize polyvinylpyrrolidone (PVP) stabilized bimetallic RuRe nanoparticles of ca. 1.3 nm with narrow size dispersity, displaying the hcp crystal structure and to control their chemical order: an alloy or Re rich surface. The structural features of these NPs were determined using complementary characterization techniques (TEM, HRTEM, STEM-HAADF, EDX, WAXS, FT-IR, MAS-NMR and ICP). In particular, surface state investigation based on CO adsorption and oxidation reactions provided useful information of the chemical order in these nanoparticles. The RuRe NPs were obtained as stable colloidal solutions or powders. Surface reactivity studies demonstrated that the alloy type RuRe/PVP NPs show better resistance to oxidation than the ones displaying a Re enriched surface and are more active towards CO dissociation than monometallic Re/PVP NPs as a result of the synergic effect between Ru and Re. Interestingly, the dissociation of CO was not observed with RuRe/PVP NPs displaying a Re enriched surface. Besides the synthetic aspect, this work highlights the crucial influence of the chemical order resulting from the choice of the metal sources in the control of the reactivity of ultra-small metal nanoparticles.

Tailoring Renal Clearance and Tumor Targeting of Ultrasmall Metal Nanoparticles with Particle Density

AbstractIdentifying key factors that govern the in vivo behavior of nanomaterials is critical to the clinical translation of nanomedicines. Overshadowed by size‐, shape‐, and surface‐chemistry effects, the impact of the particle core density on clearance and tumor targeting of inorganic nanoparticles (NPs) remains largely unknown. By utilizing a class of ultrasmall metal NPs with the same size and surface chemistry but different densities, we found that the renal‐clearance efficiency exponentially increased in the early elimination phase while passive tumor targeting linearly decreased with a decrease in particle density. Moreover, lower‐density NPs are more easily distributed in the body and have shorter retention times in highly permeable organs than higher‐density NPs. The density‐dependent in vivo behavior of metal NPs likely results from their distinct margination in laminar blood flow, which opens up a new path for precise control of nanomedicines in vivo.

Tailoring Renal Clearance and Tumor Targeting of Ultrasmall Metal Nanoparticles with Particle Density.

Identifying key factors that govern the in vivo behavior of nanomaterials is critical to the clinical translation of nanomedicines. Overshadowed by size-, shape-, and surface-chemistry effects, the impact of the particle core density on clearance and tumor targeting of inorganic nanoparticles (NPs) remains largely unknown. By utilizing a class of ultrasmall metal NPs with the same size and surface chemistry but different densities, we found that the renal-clearance efficiency exponentially increased in the early elimination phase while passive tumor targeting linearly decreased with a decrease in particle density. Moreover, lower-density NPs are more easily distributed in the body and have shorter retention times in highly permeable organs than higher-density NPs. The density-dependent in vivo behavior of metal NPs likely results from their distinct margination in laminar blood flow, which opens up a new path for precise control of nanomedicines in vivo.

Reducing Capsule Based on Electron Programming: Versatile Synthesizer for Size-Controlled Ultra-Small Metal Clusters.

Controlled reducing capsules with a specific number of reducing electrons were achieved by appropriately placed BH3 units in the dendritic polyphenylazomethines (DPAs). Using the 1:1 coordination fashion on their basic branches with radius affinity gradient, the 4th generation DPA (DPAG4) possessing four BH3 units in the central positions was prepared as a template synthesizer for size-controlled ultra-small metal clusters. This was well-demonstrated by reduction of Ag, Pt, and other metal ions resulting in monodispersed ultra-small clusters.

Soluble Metal-Nanoparticle-Decorated Porous Coordination Polymers for the Homogenization of Heterogeneous Catalysis.

Ultrasmall metal nanoparticles (MNPs) were decorated on soluble porous coordination polymers (PCPs) with high metal loadings. The solubility of the composite and the size of the MNPs can be controlled by varying the ratio of the precursors to the supports. The soluble PCPs can serve as a platform to homogenize heterogeneous MNPs catalysts, which exhibited excellent activity and recyclability in C-H activation and Suzuki reactions. This strategy combines the advantages of homogeneous and heterogeneous catalysis and may bring new inspiration to catalysis.

Self-Printing on Graphitic Nanosheets with Metal Borohydride Nanodots for Hydrogen Storage.

Although the synthesis of borohydride nanostructures is sufficiently established for advancement of hydrogen storage, obtaining ultrasmall (sub-10 nm) metal borohydride nanocrystals with excellent dispersibility is extremely challenging because of their high surface energy, exceedingly strong reducibility/hydrophilicity and complicated composition. Here, we demonstrate a mechanical-force-driven self-printing process that enables monodispersed (~6 nm) NaBH4 nanodots to uniformly anchor onto freshly-exfoliated graphitic nanosheets (GNs). Both mechanical-forces and borohydride interaction with GNs stimulate NaBH4 clusters intercalation/absorption into the graphite interlayers acting as a ‘pen’ for writing, which is accomplished by exfoliating GNs with the ‘printed’ borohydrides. These nano-NaBH4@GNs exhibit favorable thermodynamics (decrease in ∆H of ~45%), rapid kinetics (a greater than six-fold increase) and stable de-/re-hydrogenation that retains a high capacity (up to ~5 wt% for NaBH4) compared with those of micro-NaBH4. Our results are helpful in the scalable fabrication of zero-dimensional complex hydrides on two-dimensional supports with enhanced hydrogen storage for potential applications.

Rational Design of Biomolecular Templates for Synthesizing Multifunctional Noble Metal Nanoclusters toward Personalized Theranostic Applications.

Biomolecule-templated or biotemplated metal nanoclusters (NCs) are ultrasmall (<2 nm) metal (Au, Ag) particles stabilized by a certain type of biomolecular template (e.g., peptides, proteins, and DNA). Due to their unique physiochemical properties, biotemplated metal NCs have been widely used in sensing, imaging, delivery and therapy. The overwhelming applications in these individual areas imply the great promise of harnessing biotemplated metal NCs in more advanced biomedical aspects such as theranostics. Although applications of biotemplated metal NCs as theranostic agents are trending, the rational design of biomolecular templates suitable for the synthesis of multifunctional metal NCs for theranostics is comparatively underexplored. This progress report first identifies the essential attributes of biotemplated metal NCs for theranostics by reviewing the state-of-art applications in each of the four modalities of theranostics, namely sensing, imaging, delivery and therapy. To achieve high efficacy in these modalities, we elucidate the design principles underlying the use of biomolecules (proteins, peptides and nucleic acids) to control the NC size, emission color and surface chemistries for post-functionalization of therapeutic moieties. We then propose a unified strategy to engineer biomolecular templates that combine all these modalities to produce multifunctional biotemplated metal NCs that can serve as the next-generation personalized theranostic agents.

CuO Quantum Dots Embedded in Carbon Nanofibers as Binder‐Free Anode for Sodium Ion Batteries with Enhanced Properties

The design of sodium ion batteries is proposed based on the use of flexible membrane composed of ultrasmall transition metal oxides. In this paper, the preparation of CuO quantum dots (≈2 nm) delicately embedded in carbon nanofibers (denoted as 2-CuO@C) with a thin film via a feasible electrospinning method is reported. The CuO content can be controlled by altering the synthetic conditions and is optimized to 54 wt%. As binder-free anode for sodium ion batteries, 2-CuO@C delivers an initial reversible capacity of 528 mA h g-1 at the current density of 100 mA g-1 , an exceptional rate capability of 250 mA h g-1 at 5000 mA g-1 , and an ultra-stable capacity of 401 mA h g-1 after 500 cycles at 500 mA g-1 . The enhancement of electrochemical performance is attributed to the unique structure of 2-CuO@C, which offers a variety of advantages: highly conductive carbon matrix suppressing agglomeration of CuO grains, interconnected nanofibers ensuring short transport length for electrons, well-dispersed CuO quantum dots leading to highly utilization rate, and good mechanical properties resulting in strong electrode integrity.

Experimental Challenges in Studying Hydrogen Absorption in Ultrasmall Metal Nanoparticles

Recent advances on synthesis, characterisation and hydrogen absorption properties of ultra-small metal nanoparticles (defined here as objects with average size ≤ 3 nm) are briefly reviewed in the first part of this work. The experimental challenges encountered in performing accurate measurements of hydrogen absorption in Mg- and noble metal-based ultra-small nanoparticles are addressed. The second part of this work reports original results obtained for ultra-small bulk immiscible Pd-Rh nanoparticles. Carbon supported Pd-Rh nanoalloys in the whole binary chemical composition range have been successfully prepared by liquid impregnation method followed by reduction at 300 °C. EXAFS investigations suggested that the local structure of these nanoalloys is partially segregated into Rh-rich core and Pd-rich surface coexisting within the same nanoparticles. Downsizing to ultra-small dimensions completely suppresses the hydride formation in Pd-rich nanoalloys at ambient conditions, contrary to bulk and larger nanosized (5-6 nm) counterparts. The ultra-small Pd90Rh10 nanoalloy can absorb hydrogen forming solid solutions under these conditions, as suggested by in situ XRD. Apart from this composition, common laboratory techniques such as, in situ XRD, DSC and PCI failed to clarify the hydrogen interaction mechanism : either adsorption on developed surfaces or both adsorption and absorption with formation of solid solutions. Concluding insights were brought by in situ EXAFS experiments at synchrotron: ultra-small Pd75Rh25 and Pd50Rh50 nanoalloys absorb hydrogen forming solid solutions at ambient conditions. Moreover, the hydrogen solubility in these solid solutions is higher with increasing Pd content and this trend can be understood in terms of hydrogen preferential occupation in the Pd-rich regions, as suggested by in situ EXAFS. The Rh-rich nanoalloys (Pd25Rh75 and Pd10Rh90) only adsorb hydrogen on the developed surface of ultra-small nanoparticles. In summary, in situ characterization techniques carried out at large scale facilities are unique and powerful tools for in-depth investigation of hydrogen interaction with ultra-small nanoparticles at local level.

Robust Polymer-Coated Diamond Supports for Noble-Metal Nanoparticle Catalysts

Much research has been done using polymer and silica particles as support materials for catalytically active noble metal nanoparticles, but these materials have limited stability in organic solvents or under extreme reaction conditions such as high pH. Here we present a robust and versatile composite polymer-diamond support for ultrasmall noble metal nanoparticles combining chemical and mechanical stability of diamond with the chemical versatility of a polymer. By exploiting the rich surface chemistry of nanodiamond and incorporating a reactive thiol–ene polymer, a thinly coated polymer-diamond composite was formed. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) confirmed the presence of the polymer. High resolution scanning transmission electron microscopy (S/TEM) analysis showed that in situ growth of gold, platinum and palladium nanoparticles produced high density coverage at the polymer-diamond support surface. Energy dispers...

Dumbbell-shaped metallothionein-templated silver nanoclusters with applications in cell imaging and Hg2+ sensing

Metallothionein (MT) is a cysteine-rich, low-molecular-weight protein, which adopts a unique dumbbell-shaped structure with a stable C-terminal α-domain and a reactive N-terminal β-domain. The specific configuration serves as a unique scaffold for the synthesis of ultra-small fluorescent metal nanoclusters (NCs). For the first time, MT-templated Ag NCs (MT-Ag NCs) with excellent antioxidant capacity and superior biocompatibility were facilely synthesized. The NCs were thoroughly characterized by various techniques. Zn2+ in the β-domain was preferentially replaced by Ag+, which was then reduced by NaBH4 to yield Ag NCs, while Zn2+ in the α-domain was intactly tetrahedrally-coordinated through thiolate ligands in MT. Fluorescent imaging of HeLa cells was achieved by attaching folic acid (FA) to MT-Ag NCs. Due to the strong binding capacity toward the thiolate ligands in the α-domain, Hg2+ was assayed via quenching the fluorescence of Ag NCs by an energy transfer process. The methodology described herein may be extended to the synthesis of other metal NCs with potential applications in biosensing and cell imaging.

Ligand-Free Noble Metal Nanocluster Catalysts on Carbon Supports via "Soft" Nitriding.

We report a robust, universal "soft" nitriding method to grow in situ ligand-free ultrasmall noble metal nanocatalysts (UNMN; e.g., Au, Pd, and Pt) onto carbon. Using low-temperature urea pretreatment at 300 °C, soft nitriding enriches nitrogen-containing species on the surface of carbon supports and enhances the affinity of noble metal precursors onto these supports. We demonstrated sub-2-nm, ligand-free UNMNs grown in situ on seven different types of nitrided carbons with no organic ligands via chemical reduction or thermolysis. Ligand-free UNMNs supported on carbon showed superior electrocatalytic activity for methanol oxidation compared to counterparts with surface capping agents or larger nanocrystals on the same carbon supports. Our method is expected to provide guidelines for the preparation of ligand-free UNMNs on a variety of supports and, additionally, to broaden their applications in energy conversion and electrochemical catalysis.

Charge and Compositional Effects on the 2D–3D Transition in Octameric AgAu Clusters

Abstract The unbiased density functional-based Birmingham Cluster Genetic Algorithm is employed to locate the global minima of all neutral and mono-ionic silver-gold octamer clusters. Structural, energetic and electronic trends are determined across the series, in order to clarify the role of composition and charge on the position of the 2D–3D transition in ultrasmall coinage metal systems. Our calculations indicate a preference for three dimensional structures at high silver concentrations, which varies significantly with charge. The minimum in composition dependent mixing energies is independent of the charge, however, with a preference for the maximally mixed clusters, Ag4Au4 ν for all charge states ν. The sensitivity of isomeric preference to ν is found to be greater for electron-rich and electron-deficient clusters, implying a complexity of unambiguous determination of cluster motifs in related experiments. Vertical ionization potentials and detachment energies are calculated to probe electronic behaviour, providing numerical predictions for future spectroscopic studies.

Doping Au25 nanoparticles using ultrasmall silver or copper nanoparticles as the metal source

The reaction between ultrasmall heterogeneous metal nanoparticles is demonstrated, which could provide a unique strategy to controllably dope metal nanoparticles.

Controllable synthesis of ultra-small metal-organic framework nanocrystals composed of copper(ii) carboxylates.

Controlling the size of metal-organic frameworks (MOFs) down to the nanoscale is of great importance for the optimization of their properties. Chemically synthesized MOF nanoparticles commonly range from tens of nanometers to several microns. Herein, a general two-ligand modulation strategy in an interface reaction system is developed to synthesize a series of ultra-small MOF nanocrystals composed of copper(ii) ions and carboxylate linkers. The size of these MOF nanocrystals can be tuned within 10 nm by the alkyl chain length, the ligand to metal ion ratio and the reaction time. The ion exchange and self-assembly of the MOF nanocrystals were also investigated. They are well dispersed in nonpolar solvents applicable for spin-coating, suggesting their application potentials in functional thin film devices.

All-thiolate-protected silver and silver-rich alloy nanoclusters with atomic precision: stable sizes, structural characterization and optical properties

Ultrasmall metal clusters (<2 nm), with dimensions between those of small molecules and plasmonic metal nanoparticles, have received significant attention due to their discrete electronic structure as well as unique physical and chemical properties. It remains to be a major dream for nanochemists to accomplish the controllable synthesis and structure analysis of clusters with atomic precision. This dream has been realized in gold clusters, especially all-thiolate (SR)-protected clusters. In contrast, despite the similarities between gold and silver, all-thiolate-protected silver clusters have been less reported due primarily to the low stability and synthetic bottlenecks. Nevertheless, there has been some success in Ag clusters recently. Since the total structure of [Ag44(SR)30]4− was solved by Bigioni's group and Zheng's group independently, a few more structures have been achieved including [Ag25(SR)18]− and [Ag29(SR)12(TPP)4]3− (TPP: triphenylphosphine) very recently. Moreover, some Ag-rich alloy clusters have also been reported, such as [MAg24(SR)18]2− (M = Pd, Pt), [AuAg24(SR)18]−, [Au12Ag32(SR)30]4− and [Ag46Au24(SR)32]2+. In this Highlight, some recent progress in developing strategies for the synthesis of high-quality all-thiolated Ag and Ag-rich alloy clusters will be emphasized. The stable sizes, structural characterization and optical properties will be discussed in detail. A brief outlook on the future development of Ag and Ag-rich alloy clusters from the viewpoint of controlled synthesis, stability enhancement and single-atom level manipulation will be given.

Monodispersed ultrasmall NiMo metal oxide nanoclusters as hydrodesulfurization catalyst

In this study, we report the synthesis of ultrasmall NiMo bimetallic metal oxide nanoclusters (∼2nm), supported on γ-Al2O3 for hydrodesulfurization reaction. The metal oxide nanoclusters were prepared by colloidal synthesis using oleic acid and oleylamine as ligands, followed by incorporation of nanoclusters into the pores of γ-Al2O3. The nanocluster supported γ-Al2O3 was calcined to remove the ligands followed by sulfidation. Compared to the catalyst prepared using conventional wet impregnation method, the nanoclusters prepared using colloidal synthesis showed enhanced catalytic activity for the hydrodesulfurization of dibenzothiophene because of the ultrasmall size of the metal oxide nanoclusters supported on γ-Al2O3. In addition, we have evaluated the performance of our catalyst with a diesel sample (containing ∼14,700ppm sulfur) obtained from a crude distillation unit. The sulfur content of the feed was reduced to ∼35ppm at 335°C and LHSV 2h−1 showing the good catalytic activity.