Electron Dose-Controlled Formation, Growth, and Assembly of Nanoclusters and Nanoparticles from Aurophilic Au(I)-Thiolate Ensemble on Surfaces.

The ability to precisely control electron irradiation-induced formation, growth, and assembly of nanoclusters or nanoparticles on a solid surface is important for design and creation of catalytically or chemically active surface sites and interfaces free from chemical reducing agents. Here, we show the results of an investigation of the electron dose-controlled formation, growth, and assembly of nanoclusters and nanoparticles in a molecularly assembled thin film of Au(I)-thiolate motifs on a substrate, highlighting an in situ monitoring of the evolution of morphology under controlled electron dose. With aurophilic motifs of Au(I)-thiolate being confined by electrostatic interactions, the sizes of Au nanoclusters and nanoparticles were shown to increase with electron dose, revealing a propensity of a string alignment of the grown nanoclusters and nanoparticles. This growth preference to one-dimensional assembly is supported by the analysis of the surface reaction kinetics in terms of the surface density of electron dose for the growth of the nanoclusters and nanoparticles. The electron dose-controlled size-focusing and directional assembly of nanoclusters and nanoparticles may be exploited as new strategy for the precise control of nanoclusters or nanoparticles and their assemblies on solid surfaces not only free from chemical reducing agent but also with the ability of visual monitoring of the morphological evolution during growth.

Nanoscale Lacing by Electrons.

The ability to harness the optical or electrical properties of nanoscale particles depends on their assembly in terms of size and spatial characteristics which remains challenging due to lack of size focusing. Electrons provide a clean and focusing agent to initiate the assembly of nanoclusters or nanoparticles. Here an intriguing route is demonstrated to lace gold nanoclusters and nanoparticles in string assembly through electron-initiated nucleation and aggregative growth of Au(I)-thiolate motifs on a thin film substrate. This size-focused assembly is demonstrated by controlling the electron dose under transmission electron microscopic imaging conditions. The Au(I)-thiolate motifs, in combination with the molecularly mediated alignment, facilitate the interstring electrostatic and intrastring aurophilic interactions, which functions as a molecular template to aid electron-initiated 1D lacing. The findings demonstrate a hierarchical route for the 1D assemblies with size and spatial tunable catalytic, optical, sensing, and diagnostic properties.

Nanocontainer-Enhanced Self-Healing for Corrosion-Resistant Ni Coating on Mg Alloy.

The ability to manipulate the functionalization of Ni coating is of great importance in improving the corrosion resistance of magnesium (Mg) alloy for many industrial applications. In the present work, MCM-41 type mesoporous silica nanocontainers (MSNs) loaded with corrosion inhibitor (NaF) were synthesized and employed as smart reinforcements to enhance the integrity and corrosion inhibition of the Ni coating. The incorporation of the F-loaded MSNs (F@MSNs) to enhance the corrosion resistant capacity of a metallic coating is reported for the first time. The mesoporous structures of the as-prepared MSNs and F@MSNs were confirmed by transmission electron microscopy (TEM), small angle X-rays scattering (SAXS), and N2 adsorption-desorption isotherms. The X-ray photoelectron spectroscopy (XPS) data demonstrated the successful immobilization of fluoride ion on the MSNs and formation of a magnesium fluoride (MgF2) protective film at the corrosion sites of the Mg alloy upon soaking in a F@MSNs-containing NaCl solution. The results from potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) for both bare Mg alloy and Ni coatings with and without F@MSNs have revealed a clear decrease in corrosion rate in a corrosive solution for a long-time immersion due to the introduction of F@MSNs. These findings open new opportunities in the exploration of self-healing metallic coatings for highly enhanced anticorrosion protection of Mg alloy.

Chemiresistive properties regulated by nanoscale curvature in molecularly-linked nanoparticle composite assembly.

Interparticle spatial properties influence the electrical and functional properties of nanoparticle-structured assemblies. This report describes the nanoscale curvature-induced change in chemiresistive properties of molecularly-linked assemblies of gold nanoparticles on multiwalled carbon nanotubes, which are exploited for sensitive detection of volatile organic compounds. In addition to using linking/capping molecules to define interparticle spatial distances, the nanoscale curvature radius of the carbon nanotubes provides intriguing tunability of the interparticle spatial properties to influence electrical properties, which contrast with those observed for nanoparticle thin films assembled directly on chemiresistor devices. The electrical characteristics of the nanoparticle-nanotube composite give positive response profiles for the vapor molecules that are distinctively different to those observed for conventional nanoparticle thin-film assemblies. The dominant effect of electron coupling on overall chemiresistive properties is shown in relation to that of nanoscale curvature radius on the nanoparticle thin-film sensing properties. Sensing data are also further assessed in correlation with the solubility parameters of the vapor molecule. These findings have significant implications for the design of sensitive interfaces with nanocomposite-structured sensing materials and microfabricated chemiresistor devices.

Composition Tunability and (111)-Dominant Facets of Ultrathin Platinum-Gold Alloy Nanowires toward Enhanced Electrocatalysis.

The ability for tuning not only the composition but also the type of surface facets of alloyed nanomaterials is important for the design of catalysts with enhanced activity and stability through optimizing both ensemble and ligand effects. Herein we report the first example of ultrathin platinum-gold alloy nanowires (PtAu NWs) featuring composition-tunable and (111) facet-dominant surface characteristics, and the electrocatalytic enhancement for the oxygen reduction reaction (ORR). PtAu NWs of different bimetallic compositions synthesized by a single-phase and surfactant-free method are shown to display an alloyed, parallel-bundled structure in which the individual nanowires exhibit Boerdijk-Coxeter helix type morphology predominant in (111) facets. Results have revealed intriguing catalytic correlation with the binary composition, exhibiting an activity maximum at a Pt:Au ratio of ∼3:1. As revealed by high-energy synchrotron X-ray diffraction and atomic pair distribution function analysis, NWs of this ratio exhibit a clear shrinkage in interatomic bonding distances. In comparison with PtAu nanoparticles of a similar composition and degree of shrinking of atomic-pair distances, the PtAu NWs display a remarkably higher electrocatalytic activity and stability. The outperformance of NWs over nanoparticles is attributed to the predominant (111)-type facets on the surface balancing the contribution of ensemble and ligand effects, in addition to the composition synergy due to optimal adsorption energies for molecular and atomic oxygen species on the surface as supported by DFT computation of models of the catalysts. The findings open up a new pathway to the design and engineering of alloy nanocatalysts with enhanced activity and durability.

Composition- and Structure-Tunable Gold-Cobalt Nanoparticles and Electrocatalytic Synergy for Oxygen Evolution Reaction.

The increasing energy crisis constitutes an inspiring drive seeking alternative energies such as hydrogen from water splitting which is clean and abundant, but a key challenge for water splitting is the need of highly efficient catalysts for oxygen evolution reaction (OER). This report describes findings of an investigation of the synthesis of gold-cobalt (AuCo) nanoparticles by a facile one-pot and injection method and their use as highly efficient catalysts for OER. While particle size depends on the synthesis method, the composition of the nanoparticles is controlled by feeding ratio of Au and Co precursors in the synthesis. Depending on Co content, the nanoparticles exhibit largely phase-segregated domains with a core (Au)-shell (Co) type of structure at a high level of Co. Upon the thermochemical treatment of carbon-supported AuCo nanoparticles, the redox activity of Co species in the nanoparticles with cycle number is shown to decrease which changes the surface oxidation state of Co species without changing the composition significantly. The electrocatalytic activity for OER in alkaline electrolytes is shown to depend on the bimetallic composition, displaying a maximum activity for an Au:Co ratio of ∼2:3. This dependence is also shown to correlate with the surface oxidation state and redox activities, providing an insight into the electrocatalytic activity. Mechanistic aspects of the electrocataltytic properties are discussed in terms of the bifunctional synergy of Co and Au in the nanoparticle catalysts.

'Squeezed' interparticle properties for plasmonic coupling and SERS characteristics of duplex DNA conjugated/linked gold nanoparticles of homo/hetero-sizes.

The formation of interparticle duplex DNA conjugates with gold nanoparticles constitutes the basis for interparticle plasmonic coupling responsible for surface-enhanced Raman scattering signal amplification, but understanding of its correlation with interparticle spatial properties and particle sizes, especially in aqueous solutions, remains elusive. This report describes findings of an investigation of interparticle plasmonic coupling based on experimental measurements of localized surface plasmon resonance and surface enhanced Raman scattering characteristics for gold nanoparticles in aqueous solutions upon introduction of interparticle duplex DNA conjugates to define the interparticle spatial properties. Theoretical simulations of the interparticle optical properties and electric field enhancement based on a dimer model have also been performed to aid the understanding of the experimental results. The results have revealed a 'squeezed' interparticle spatial characteristic in which the duplex DNA-defined distance is close or shorter than A-form DNA conformation, which are discussed in terms of the interparticle interactions, providing fresh insight into the interparticle double-stranded DNA-defined interparticle spatial properties for the design of highly-sensitive nanoprobes in solutions for biomolecular detection.

Surface Enhanced Raman Scattering Detection of Cancer Biomarkers with Bifunctional Nanocomposite Probes.

This report describes new findings of an investigation of a bifunctional nanocomposite probe for the detection of cancer biomarkers, demonstrating the viability of magnetic focusing and SERS detection in a microfluidic platform. The nanocomposite probe consists of a magnetic nickel-iron core and a gold shell. Upon bioconjugation, the nanoprobes are magnetically focused on a specific spot in a microfluidic channel, enabling an enrichment of "hot spots" for surface enhanced Raman scattering detection of the targeted carcinoembryonic antigen. The detection sensitivity, with a limit of detection of ∼0.1 pM, is shown to scale with the magnetic focusing time and the nanoparticle size. The latter is also shown to exhibit an excellent agreement between the experimental data and the theoretical simulation. Implications of the findings to the development of rapid and sensitive microfluidic detection of cancer biomarkers are also discussed.

Harnessing the interparticle J-aggregate induced plasmonic coupling for surface-enhanced Raman scattering.

This report demonstrates that both surface plasmon resonance absorption and surface-enhanced Raman scattering work in concert with plasmonic coupling. The kinetic correlation between the two spectroscopic signatures highlights an effective pathway for harnessing the nanoparticles in solution for a broad range of applications by exploiting the plasmonic and spectroscopic properties.

Catalytic activity of bimetallic catalysts highly sensitive to the atomic composition and phase structure at the nanoscale.

The ability to determine the atomic arrangement in nanoalloy catalysts and reveal the detailed structural features responsible for the catalytically active sites is essential for understanding the correlation between the atomic structure and catalytic properties, enabling the preparation of efficient nanoalloy catalysts by design. Herein we describe a study of CO oxidation over PdCu nanoalloy catalysts focusing on gaining insights into the correlation between the atomic structures and catalytic activity of nanoalloys. PdCu nanoalloys of different bimetallic compositions are synthesized as a model system and are activated by a controlled thermochemical treatment for assessing their catalytic activity. The results show that the catalytic synergy of Pd and Cu species evolves with both the bimetallic nanoalloy composition and temperature of the thermochemical treatment reaching a maximum at a Pd : Cu ratio close to 50 : 50. The nanoalloys are characterized structurally by ex situ and in situ synchrotron X-ray diffraction, including atomic pair distribution function analysis. The structural data show that, depending on the bimetallic composition and treatment temperature, PdCu nanoalloys adopt two different structure types. One features a chemically ordered, body centered cubic (B2) type alloy consisting of two interpenetrating simple cubic lattices, each occupied with Pd or Cu species alone, and the other structure type features a chemically disordered, face-centered cubic (fcc) type of alloy wherein Pd and Cu species are intermixed at random. The catalytic activity for CO oxidation is strongly influenced by the structural features. In particular, it is revealed that the prevalence of chemical disorder in nanoalloys with a Pd : Cu ratio close to 50 : 50 makes them superior catalysts for CO oxidation in comparison with the same nanoalloys of other bimetallic compositions. However, the catalytic synergy can be diminished if the Pd50Cu50 nanoalloys undergo phase segregation into distinct chemically-ordered (B2-type) and disordered (fcc-type) domains. This finding is significant since it provides a rational basis for streamlining the design and preparation of Pd-based nanoalloy catalysts in terms of atomic structure and phase state.

Design of functional nanoparticles and assemblies for theranostic applications.

Nanostructured materials have found increasing applications in medical therapies and diagnostics (theranostics). The main challenge is the ability to impart the nanomaterials with structurally tailored functional properties which can effectively target biomolecules but also provide signatures for effective detection. The harnessing of functional nanoparticles and assemblies serves as a powerful strategy for the creation of the structurally tailored multifunctional properties. This article highlights some of the important design strategies in recent investigation of metals (especially gold and silver), and magnetically functionalized nanoparticles, and molecularly assembled or biomolecularly conjugated nanoparticles with tunable optical, spectroscopic, magnetic, and electrical properties for applications in several areas of potential theranostic interests. Examples include colorimetric detection of amino acids and small peptides, surface-enhanced Raman scattering detection of biomolecular recognition of proteins and DNAs, delivery in cell transfection and bacteria inactivation, and chemiresistive detection of breath biomarkers. A major emphasis is placed on understanding how the control of the nanostructures and the molecular and biomolecular interactions impact these biofunctional properties, which has important implications for bottom-up designs of theranostic materials.

DNA assembly and enzymatic cutting in solutions: a gold nanoparticle based SERS detection strategy.

The ability to monitor biomolecular recognition such as DNA hybridization and enzymatic reactivity in solutions with high sensitivity is important for developing effective bioassay strategies. Surface enhanced Raman scattering (SERS) based on use of solid substrates to produce the SERS effect for the detection often requires substrate preparation which is ineffective for rapid monitoring. This report describes a new strategy exploiting a gold nanoparticle (AuNP) based interparticle "hot-spot" for SERS monitoring of DNA mediated assembly and enzyme induced cleavage of the assembly in solution phase. The DNAs consist of two different complementary DNA strands with a thiol modification for attachment to AuNPs of selected sizes. In a solution containing AuNPs conjugated with one of the single-stranded (ss) DNA and other AuNPs labeled with a Raman reporter molecule, 4-mercaptobenzoic acid (MBA), the introduction of the complementary DNA strand leads to a linkage of the two types of AuNPs, producing double-stranded (ds) DNA-AuNP assembly (ds-DNA-AuNPs) with an interparticle "hot-spot" for SERS detection of the diagnostic bands of the reporter. Upon introducing a restriction enzyme (e.g. MspI) into the ds-DNA-AuNP assembly solution, the removal of the interparticle "hot-spot" due to restriction enzyme cleavage of the ds-DNA leads to a decrease of the SERS signals. While the detailed cleavage process may depend on the reaction time and the amount of enzyme, the viability of using gold nanoparticle "hot-spot" based SERS monitoring of DNA assembly and enzyme cleavage is clearly demonstrated, which has important implications for developing new strategies for bioassays.

Bifunctional nanoparticles for SERS monitoring and magnetic intervention of assembly and enzyme cutting of DNAs.

The ability to harness the nanoscale structural properties is essential for the exploration of functional properties of nanomaterials. This report demonstrates a novel strategy exploring bifunctional nanoparticles for spectroscopic detection and magnetic intervention of DNA assembly, disassembly, and enzyme cutting processes in a solution phase. In contrast to existing single-function based approaches, this strategy exploits magnetic MnZn ferrite nanoparticles decorated with gold or silver on the surface to retain adequate magnetization while producing sufficient plasmonic resonance features to impart surface-enhanced Raman scattering (SERS) functions. The decoration of MnZn ferrite nanoparticles with Au or Ag (MZF/Au or MZF/Ag) was achieved by thermally activated deposition of Au or Ag atoms/nanoparticles on MZF nanoparticles. Upon interparticle double-stranded DNA linkage of the MZF/Au (or MZF/Ag) nanoparticles with gold nanoparticles labeled with a Raman reporter, the resulting interparticle "hot spots" are shown to enable real time SERS monitoring of the DNA assembly, disassembly, or enzyme cutting processes, where the magnetic component provides an effective means for intervention of the biomolecular processes in the solution. The unique bifunctional combination of the SERS "hot spots" and the magnetic separation capability serves as the first example of bifunctional nanoprobes for biomolecular recognition and intervention.