X-ray studies of the structure and electronic behavior of alkanethiolate-capped gold nanoparticles: the interplay of size and surface effects

Local Structural Distortion Induced Uniaxial Negative Thermal Expansion in Nanosized Semimetal Bismuth

The corrugated layer structure bismuth has been successfully tailored into negative thermal expansion along c axis by size effect. Pair distribution function and extended X-ray absorption fine structure are combined to reveal the local structural distortion for nanosized bismuth. The comprehensive method to identify the local structure of nanomaterials can benefit the regulating and controlling of thermal expansion in nanodivices.

Nanometer-Size Effect on Hydrogen Sites in Palladium Lattice

Nanometer-sized materials attract much attention because their physical and chemical properties are substantially different from those of bulk materials owing to their size and surface effects. In this work, neutron powder diffraction experiments on the nanoparticles of palladium hydride, which is the most popular metal hydride, have been performed at 300, 150, and 44 K to investigate the positions of the hydrogen atoms in the face-centered cubic (fcc) lattice of palladium. We used high-quality PdD0.363 nanocrystals with a diameter of 8.0 ± 0.9 nm. The Rietveld analysis revealed that 30% of D atoms are located at the tetrahedral (T) sites and 70% at the octahedral (O) sites. In contrast, only the O sites are occupied in bulk palladium hydride and in most fcc metal hydrides. The temperature dependence of the T-site occupancy suggested that the T-sites are occupied only in a limited part, probably in the subsurface region, of the nanoparticles. This is the first study to determine the hydrogen sites in metal nanoparticles.

Thermoelectric performance of restacked MoS2 nanosheets thin-film

MoS2 has been predicted to be an excellent thermoelectric material due to its large intrinsic band gap and high carrier mobility. In this work, we exfoliated bulk MoS2 by the assistance of lithium intercalation and fabricated the restacked MoS2 thin-film using a simple filtration technique. These MoS2 thin-films with different thickness showed different thermoelectric performance. It was found that with the increase of thickness, carrier concentration, electrical conductivity and Seebeck coefficient all showed an increasing trend. In particular, the maximum Seebeck coefficient was able to reach 93.5 μV K(-1). This high thermopower indicates that MoS2 will have ideal thermoelectric performance in the future through optimizing its structure. The highest figure of merit (ZT = 0.01) is calculated in this experiment.

Aqueous Growth of Gold Clusters with Tunable Fluorescence Using Photochemically Modified Lipoic Acid-Based Ligands

We report a one-phase aqueous growth of fluorescent gold nanoclusters (AuNCs) with tunable emission in the visible spectrum, using a ligand scaffold that is made of poly(ethylene glycol) segment appended with a metal coordinating lipoic acid at one end and a functional group at the other end. This synthetic scheme exploits the ability of the UV-induced photochemical transformation of LA-based ligands to provide DHLA and other thiol byproducts that exhibit great affinity to metal nanoparticles, obviating the need for chemical reduction of the dithiolane ring using classical reducing agents. The influence of various experimental conditions, including the photoirradiation time, gold precursor-to-ligand molar ratios, time of reaction, temperature, and the medium pH, on the growth of AuNCs has been systematically investigated. The photophysical properties, size, and structural characterization were carried out using UV-vis absorption and fluorescence spectroscopy, TEM, DOSY-NMR, and X-ray photoelectron spectroscopy. The hydrodynamic size (RH) obtained by DOSY-NMR indicates that the size of these clusters follows the trend anticipated from the absorption and PL data, with RH(red) > RH(yellow) > RH(blue). The tunable emission and size of these gold nanoclusters combined with their high biocompatibility would make them greatly promising for potential use in imaging and sensing applications.

Solvent-induced desorption of alkanethiol ligands from Au nanoparticles

Removing surfactants from a colloidal metal nanoparticle surface is necessary for their realistic applications, and how they could be stripped is a subject of active investigation. Here, we report a solvent-induced desorption of dodecanethiol ligands from the gold nanoparticle surface, and traced this desorption process using a combination of in situ X-ray absorption fine structure (XAFS) and Raman spectroscopic techniques. In situ analysis results reveal that the solvent exchange of ethanol with tetrahydrofuran (THF) can effectively remove dodecanethiol ligands while keeping the particle morphology unchanged. Upon increasing the THF/ethanol ratio from 0 : 1 to 5 : 1, the surface coverage of thiol on the Au surface is reduced from 0.47 to 0.07, suggesting the depletion of ligands first from the nanoparticle facet sites, then from the edge sites, while the ligands at the corner sites are intact. This work enriches our knowledge on surfactant removal and may pave the way towards preparing surface-clean nanoparticles for practical applications.

A Segregated, Partially Oxidized, and Compact Ag10 Cluster within an Encapsulating DNA Host

Silver clusters develop within DNA strands and become optical chromophores with diverse electronic spectra and wide-ranging emission intensities. These studies consider a specific cluster that absorbs at 400 nm, has low emission, and exclusively develops with single-stranded oligonucleotides. It is also a chameleon-like chromophore that can be transformed into different highly emissive fluorophores. We describe four characteristics of this species and conclude that it is highly oxidized yet also metallic. One, the cluster size was determined via electrospray ionization mass spectrometry. A common silver mass is measured with different oligonucleotides and thereby supports a Ag10 cluster. Two, the cluster charge was determined by mass spectrometry and Ag L3-edge X-ray absorption near-edge structure spectroscopy. Respectively, the conjugate mass and the integrated white-line intensity support a partially oxidized cluster with a +6 and +6.5 charge, respectively. Three, the cluster chirality was gauged by circular dichroism spectroscopy. This chirality changes with the length and sequence of its DNA hosts, and these studies identified a dispersed binding site with ∼20 nucleobases. Four, the structure of this complex was investigated via Ag K-edge extended X-ray absorption fine structure spectroscopy. A multishell fitting analysis identified three unique scattering environments with corresponding bond lengths, coordination numbers, and Debye-Waller factors for each. Collectively, these findings support the following conclusion: a Ag10(+6) cluster develops within a 20-nucleobase DNA binding site, and this complex segregates into a compact, metal-like silver core that weakly links to an encapsulating silver-DNA shell. We consider different models that account for silver-silver coordination within the core.

Au 4f spin–orbit coupling effects in supported gold nanoparticles

We reveal that the ratio of spin–orbit components in X-ray photoelectron Au 4f spectra of titania-supported gold nanoparticles deviates from the statistical ratio 4 : 3 and demonstrates an appreciable dependence on the concentration of Au atoms on the surface of TiO₂ support and size of Au nanoparticles.

Design and synthesis of model and practical palladium catalysts using atomic layer deposition

We investigated the “one-batch” synthesis of model and practical palladium catalysts using atomic layer deposition (ALD).

Localized surface plasmon resonance on Au nanoparticles: tuning and exploitation for performance enhancement in ultrathin photovoltaics

We report a detailed correlation analysis of the size, shape, and distribution of Au nanoparticles (NPs) on fine-tuning of localized surface plasmon resonance and optical absorption cross-section.

Structure, Optical Absorption, and Performance of Organic Solar Cells Improved by Gold Nanoparticles in Buffer Layers

11-Mercaptoundecanoic acid (MUA)-stabilized gold nanoparticles (AuNPs) embedded in copper phthalocyanine (CuPc) were used as a buffer layer between a poly(3-hexyl-thiophene) (P3HT)/[6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction and anodic indium-tin oxide (ITO) substrate. As systematic synchrotron-based grazing incidence X-ray diffraction (GIXRD) experiments demonstrated that the AuNPs present in the buffer layer can improve the microstructure of the active layer with a better lamella packing of P3HT from the surface to the interior, UV-visible absorption spectrum measurements revealed enhanced optical absorption due to the localized surface plasma resonance (LSPR) generated by the AuNPs. The device of ITO/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate/CuPc:MUA-stabilized AuNPs/P3HT:PCBM/LiF/Al was found with over 24% enhancement of power conversion efficiency (PCE) in comparison with reference devices without AuNPs. This remarkable improvement in PCE should be partially attributed to LSPR generated by the AuNPs and partially to improved crystallization as well as preferred orientation order of P3HT due to the presence of the AuNPs, which would promote more applications of metal NPs in the organic photovoltaic devices and other organic multilayer devices.

Magnetism and Mn Clustering in (In,Mn)Sb Magnetic Semiconductors

Previously, high-temperature ferromagnetism with a Curie temperature in excess of 400 K was reported in the magnetic semiconductor (In,Mn)Sb films grown by metal-organic vapor phase epitaxy (MOVPE). To determine the role of Mn distribution on its magnetic properties, the Mn 2p core-level X-ray photoelectron spectroscopy (XPS) of (In,Mn)Sb films was measured. For films grown on an InSb substrate, Mn composition is spatially inhomogeneous and its concentration increases with increasing deposition temperature. Spin-orbit splitting energy of the Mn 2p core-level was found to increase with increasing Mn concentration. From the dependence of the measured spin-orbit splitting energy on the Mn concentration, evidence of atomic-scale Mn cluster formation was observed. The measured magnetic moment per Mn atom decreases from 3.0 μB/Mn to 1.8 μB/Mn with increasing Mn concentration, which is attributed to atomic-scale clusters that are ferromagnetic or ferrimagnetic. This detailed investigation gives an insight into the Mn distribution, phase composition and origin of magnetism in MOVPE-grown (In,Mn)Sb magnetic thin films.

Development of Ferromagnetic Superspins in Bare Cu Nanoparticles by Electronic Charge Redistribution

We report on the results of investigating the ferromagnetic properties of bare Cu nanoparticles. Three sets of bare Cu nanoparticle assemblies with mean particle diameters of 6.6, 8.1, and 11.1 nm were fabricated, employing the gas condensation method. Curie-Weiss paramagnetic responses to a weak driving magnetic field were detected, showing the appearance of particle superspins that overcomes the diamagnetic responses from the inner core. The isothermal magnetization displays a Langevin field profile together with magnetic hysteresis appearing even at 300 K, demonstrating the existence of ferromagnetic superspins in the Cu nanoparticles. Shifting of a noticeable amount of electronic charge from being distributed near the lattice sites in bulk form toward their neighboring ions in nanoparticles was found. The extended 3d and 4s band mixture are the main sources for the development of localized 3d holes for the development of ferromagnetic particle superspins in Cu nanoparticles.

In situ studies on controlling an atomically-accurate formation process of gold nanoclusters

Knowledge of the molecular formation mechanism of metal nanoclusters is essential for developing chemistry for accurate control over their synthesis. Herein, the "top-down" synthetic process of monodisperse Au13 nanoclusters via HCl etching of polydisperse Aun clusters (15 ≤ n ≤ 65) is traced by a combination of in situ X-ray/UV-vis absorption spectroscopy and time-dependent mass spectrometry. It is revealed experimentally that the HCl-induced synthesis of Au13 is achieved by accurately controlling the etching process with two distinctive steps, in sharp contrast to the traditional thiol-etching mechanism through release of the Au(i) complex. The first step involves the direct fragmentation of the initial larger Aun clusters into metastable intermediate Au8-Au13 smaller clusters. This is a critical step, which allows for the secondary size-growth step of the intermediates toward the atomically monodisperse Au13 clusters via incorporating the reactive Au(i)-Cl species in the solution. Such a secondary-growth pathway is further confirmed by the successful growth of Au13 through reaction of isolated Au11 clusters with AuClPPh3 in the HCl environment. This work addresses the importance of reaction intermediates in guiding the way towards controllable synthesis of metal nanoclusters.

Formation of Mercury Sulfide from Hg(II)-Thiolate Complexes in Natural Organic Matter

Methylmercury is the environmental form of neurotoxic mercury that is biomagnified in the food chain. Methylation rates are reduced when the metal is sequestered in crystalline mercury sulfides or bound to thiol groups in macromolecular natural organic matter. Mercury sulfide minerals are known to nucleate in anoxic zones, by reaction of the thiol-bound mercury with biogenic sulfide, but not in oxic environments. We present experimental evidence that mercury sulfide forms from thiol-bound mercury alone in aqueous dark systems in contact with air. The maximum amount of nanoparticulate mercury sulfide relative to thiol-bound mercury obtained by reacting dissolved mercury and soil organic matter matches that detected in the organic horizon of a contaminated soil situated downstream from Oak Ridge, TN, in the United States. The nearly identical ratios of the two forms of mercury in field and experimental systems suggest a common reaction mechanism for nucleating the mineral. We identified a chemical reaction mechanism that is thermodynamically favorable in which thiol-bound mercury polymerizes to mercury-sulfur clusters. The clusters form by elimination of sulfur from the thiol complexes via breaking of mercury-sulfur bonds as in an alkylation reaction. Addition of sulfide is not required. This nucleation mechanism provides one explanation for how mercury may be immobilized, and eventually sequestered, in oxygenated surface environments.

The surface structure of silver-coated gold nanocrystals and its influence on shape control

Understanding the surface structure of metal nanocrystals with specific facet indices is important due to its impact on controlling nanocrystal shape and functionality. However, this is particularly challenging for halide-adsorbed nanocrystals due to the difficulty in analysing interactions between metals and light halides (for example, chloride). Here we uncover the surface structures of chloride-adsorbed, silver-coated gold nanocrystals with {111}, {110}, {310} and {720} indexed facets by X-ray absorption spectroscopy and density functional theory modelling. The silver–chloride, silver–silver and silver–gold bonding structures are markedly different between the nanocrystal surfaces, and are sensitive to their formation mechanism and facet type. A unique approach of combining the density functional theory and experimental/simulated X-ray spectroscopy further verifies the surface structure models and identifies the previously indistinguishable valence state of silver atoms on the nanocrystal surfaces. Overall, this work elucidates the thus-far unknown chloride–metal nanocrystal surface structures and sheds light onto the halide-induced growth mechanism of anisotropic nanocrystals.

Nanocrystal surface structure affects many properties but is tough to determine for halide-adsorbed materials. Here, the authors combine X-ray absorption measurements and computational modelling to elucidate the chloride metal surface structures for silver-coated gold nanocrystals with controlled shapes.

The induction phenomenon and catalytic deactivation of thiolate-stabilized raspberry-like polymer composites coated with gold nanoparticles

Alkylthiolate ligands play dual roles in metal nanoparticles-coated polymer composite catalysts: stabilizer and deactivator. Herein, individual raspberry-like polymer composite spheres coated with gold nanoparticles were separated from each other in the presence of 6-mercaptohexanoic acid or 3-mercaptopropionic acid ligands. Effects of thiolate ligands on the induction time and the catalytic activity of such non-aggregated polymer composites were investigated experimentally and theoretically in the 4-nitrophenol/NaBH4 model reaction from the following aspects: ligand surface coverage, chain order and chain length. With the increase in alkylthiolate surface coverage and chain order on composite particles, the induction time increases first and then decreases, which can be explained based on spontaneous dynamic surface restructuring and electron injection from borohydride ions to the gold nanoparticle surface. The catalytic activity is compromised with the existence of thiolate ligands, but is enhanced with increasing alkylthiolate ligand coverage, which can be ascribed to sulfur-induced electronic charge depletion of the gold nanoparticles. The increment of CH2 in alkylthiolate chains results in the increase of induction time and the decrease of the catalytic activity, which can be attributed to the steric hindrance effect. The reactant addition sequence was also found to affect the induction time and the catalytic activity, which can be partially credited to NaBH4 reductant-induced desorption of thiolate ligands.

Lithium intercalation compound dramatically influences the electrochemical properties of exfoliated MoS2

MoS2 and other transition metal dichalcogenides (TMDs) have recently gained a renewed interest due to the interesting electronic, catalytic, and mechanical properties which they possess when down-sized to single or few layer sheets. Exfoliation of the bulk multilayer structure can be achieved by a preliminary chemical Li intercalation followed by the exfoliation due to the reaction of Li with water. Organolithium compounds are generally adopted for the Li intercalation with n-butyllithium (n-Bu-Li) being the most common. Here, the use of three different organolithium compounds are investigated and compared, i.e., methyllithium (Me-Li), n-butyllithium (n-Bu-Li) and tert-butyllithium (t-Bu-Li), used for the exfoliation of bulk MoS2 . Scanning transmission electron microscopy (STEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV) are adopted for a comprehensive characterization of all materials under investigation. In addition, catalytic properties towards the hydrogen evolution reaction (HER) and capacitive properties are also tested. Different organolithium compounds exhibit different extent of Li intercalation resulting in different degrees of exfoliation. The inherent electrochemical behavior of MoS2 consisting of significant anodic and cathodic peaks as well as its capacitive behavior and catalytic properties towards hydrogen evolution reaction are strongly connected to the exfoliation compound used. This research significantly contributes to the development of large-scale synthesis of electrocatalytic MoS2 -based materials.

MoS₂ exhibits stronger toxicity with increased exfoliation

MoS₂ belong to a class of inorganic 2D nanomaterials known as transition metal dichalcogenides (TMDs) which have recently attracted a renewed and growing interest due to their interesting electronic and catalytic properties when scaled down to single or few layer sheets. Although exfoliated MoS₂ nanosheets have been proposed for numerous energy-related and biosensing applications, little is known about the toxicological impacts of using MoS₂ nanosheets. Here, we report about the in vitro toxicity of MoS₂ nanosheets that have been chemically exfoliated with different lithium intercalating agents and compared their respective cytotoxic influence. Methyllithium (Me-Li), n-butyllithium (n-Bu-Li) and tert-butyllithium (t-Bu-Li) were used for the exfoliation of bulk MoS₂ and we found the t-Bu-Li and n-Bu-Li exfoliated MoS₂ nanosheets to be more cytotoxic than MoS₂ exfoliated by Me-Li. t-Bu-Li and n-Bu-Li provide more efficient exfoliation over Me-Li, and we establish that the extent of exfoliation that MoS₂ undergo is a factor influencing their toxicity. Specifically, the more exfoliated the MoS₂ nanosheets, the stronger its cytotoxic influence, which may be due to an increase in surface area and active edge sites. The potential toxicity of these MoS₂ nanosheets should be taken into account before their employment in real world applications and we have shown the effect the amount of exfoliation can have on the toxicity of MoS₂ nanosheets, representing the first step towards a better understanding of their toxicological properties.

Structural and electronic properties of micellar Au nanoparticles: size and ligand effects

Gaining experimental insight into the intrinsic properties of nanoparticles (NPs) represents a scientific challenge due to the difficulty of deconvoluting these properties from various environmental effects such as the presence of adsorbates or a support. A synergistic combination of experimental and theoretical tools, including X-ray absorption fine-structure spectroscopy, scanning transmission electron microscopy, atomic force microscopy, and density functional theory was used in this study to investigate the structure and electronic properties of small (∼1-4 nm) Au NPs synthesized by an inverse micelle encapsulation method. Metallic Au NPs encapsulated by polystyrene 2-vinylpiridine (PS-P2VP) were studied in the solution phase (dispersed in toluene) as well as after deposition on γ-Al2O3. Our experimental data revealed a size-dependent contraction of the interatomic distances of the ligand-protected NPs with decreasing NP size. These findings are in good agreement with the results from DFT calculations of unsupported Au NPs surrounded by P2VP, as well as those obtained for pure (ligand-free) Au clusters of analogous sizes. A comparison of the experimental and theoretical results supports the conclusion that the P2VP ligands employed to stabilize the gold NPs do not lead to strong distortions in the average interatomic spacing. The changes in the electronic structure of the Au-P2VP NPs were found to originate mainly from finite size effects and not from charge transfer between the NPs and their environment (e.g., Au-ligand interactions). In addition, the isolated ligand-protected experimental NPs only display a weak interaction with the support, making them an ideal model system for the investigation of size-dependent physical and chemical properties of structurally well-defined nanomaterials.

Focused-ion-beam induced interfacial intermixing of magnetic bilayers for nanoscale control of magnetic properties

Modification of the magnetic properties in a thin-film ferromagnetic/non-magnetic bilayer system by low-dose focused ion-beam (FIB) induced intermixing is demonstrated. The highly localized capability of FIB may be used to locally control magnetic behaviour at the nanoscale. The magnetic, electronic and structural properties of NiFe/Au bilayers were investigated as a function of the interfacial structure that was actively modified using focused Ga(+) ion irradiation. Experimental work used MOKE, SQUID, XMCD as well as magnetoresistance measurements to determine the magnetic behavior and grazing incidence x-ray reflectivity to elucidate the interfacial structure. Interfacial intermixing, induced by low-dose irradiation, is shown to lead to complex changes in the magnetic behavior that are associated with monotonic structural evolution of the interface. This behavior may be explained by changes in the local atomic environment within the interface region resulting in a combination of processes including the loss of moment on Ni and Fe, an induced moment on Au and modifications to the spin-orbit coupling between Au and NiFe.

Palladium and gold nanotubes as oxygen reduction reaction and alcohol oxidation reaction catalysts in base

Palladium (PdNTs) and gold nanotubes (AuNTs) were synthesized by the galvanic displacement of silver nanowires. PdNTs and AuNTs have wall thicknesses of 6 nm, outer diameters of 60 nm, and lengths of 5-10 and 5-20 μm, respectively. Rotating disk electrode experiments showed that the PdNTs and AuNTs have higher area normalized activities for the oxygen reduction reaction (ORR) than conventional nanoparticle catalysts. The PdNTs produced an ORR area activity that was 3.4, 2.2, and 3.7 times greater than that on carbon-supported palladium nanoparticles (Pd/C), bulk polycrystalline palladium, and carbon-supported platinum nanoparticles (Pt/C), respectively. The AuNTs produced an ORR area activity that was 2.3, 9.0, and 2.0 times greater than that on carbon-supported gold nanoparticles (Au/C), bulk polycrystalline gold, and Pt/C, respectively. The PdNTs also had lower onset potentials than Pd/C and Pt/C for the oxidation of methanol (0.236 V), ethanol (0.215 V), and ethylene glycol (0.251 V). In comparison to Pt/C, the PdNTs and AuNTs further demonstrated improved alcohol tolerance during the ORR.

Efficient plasmonic dye-sensitized solar cells with fluorescent Au-encapsulated C-dots

A simple strategy to improve the efficiency of a ZnO-nanorod-based dye-sensitized solar cell (DSSC) by use of Au-encapsulated carbon dots (Au@C-dots) in the photoanode is presented. The localized surface plasmonic resonance of Au in the 500-550 nm range coupled with the ability of C-dots to undergo charge separation increase the energy-harvesting efficiency of the DSSC with ZnO/N719/Au@C-dots photoanodes. Charge transfer from N719 dye to Au@C-dots is confirmed by fluorescence and lifetime enhancements of Au@C-dots. Forster resonance energy transfer (FRET) from the gap states of ZnO nanorods to N719 dye is also ratified and the energy transfer rate is 4.4×10(8) s(-1) and the Forster radius is 1.89 nm. The overall power conversion efficiency of the plasmonic and FRET-enabled DSSC with ZnO/N719/Au@C-dots as the photoanode, I2/I(-) as the electrolyte and multiwalled carbon nanotubes as the counter electrode is 4.1%, greater by 29% compared to a traditional ZnO/N719 cell.

Real Time Determination of the Electronic Structure of Unstable Reaction Intermediates during Au2O3 Reduction

Chemical reactions are always associated with electronic structure changes of the involved chemical species. Determining the electronic configuration of an atom allows probing its chemical state and gives understanding of the reaction pathways. However, often the reactions are too complex and too fast to be measured at in situ conditions due to slow and/or insensitive experimental techniques. A short-lived Au2O compound has been detected for the first time under in situ conditions during the temperature-programmed reduction of Au2O3. A time-resolved resonant inelastic X-ray scattering experiment (RIXS) allowed the determination of changes in the Au electronic structure, enabling a better understanding of the reaction mechanism of Au(III) reduction. On the basis of time-resolved RIXS data analysis combined with genetic algorithm methodology, we determined the electronic structure of the metastable Au2O intermediate species. The data analysis showed a notably larger value for the lattice constant of the intermediate Au as compared to the theoretical predictions. With support of DFT calculations, we found that such a structure may indeed be formed and that the expanded lattice constant is due to the termination of Au2O on the Au2O3 structure.

Probing the dynamics of plasmon-excited hexanethiol-capped gold nanoparticles by picosecond X-ray absorption spectroscopy

The onset of disorder 100 ps after optical excitation of small Au NPs was revealed by X-ray absorption spectroscopy.

Adsorption kinetic process of thiol ligands on gold nanocrystals

Understanding the kinetic mechanism during ligand adsorption on gold nanocrystals is important for designing and fine-tuning their properties and implications. Here, we report a kinetic study on the adsorption process of dodecanethiol ligands on Au nanocrystals of 3.3 nm by an in situ time-resolved X-ray absorption fine structure technique. A two-step process of dodecanethiol adsorption on Au NC surfaces is proposed based on the obtained ligand coverage, which shows a quick increase from 0 to 0.40 within the first 20 min, followed by a much slower increase to the limiting value of 0.94. In-depth analysis suggests that the first stage involves the quick adsorption of dodecanethiol to the corner and edge sites of Au NCs surfaces, leading to remarkable surface Au-Au bond length relaxation (from 2.79 to 2.81 Å) and pronounced gold-to-ligand charge transfer. The second step that corresponds to the much slower adsorption process to the surface facets could be described by the Langmuir kinetics equation with an adsorption rate constant of 0.0132 min(-1) and an initial coverage of 0.41, in good agreement with the initially preferable adsorption of thiols to the most favorable sites.

Magnetism in nanoparticles: tuning properties with coatings

This paper reviews the effect of organic and inorganic coatings on magnetic nanoparticles. The ferromagnetic-like behaviour observed in nanoparticles constituted by materials which are non-magnetic in bulk is analysed for two cases: (a) Pd and Pt nanoparticles, formed by substances close to the onset of ferromagnetism, and (b) Au and ZnO nanoparticles, which were found to be surprisingly magnetic at the nanoscale when coated by organic surfactants. An overview of theories accounting for this unexpected magnetism, induced by the nanosize influence, is presented. In addition, the effect of coating magnetic nanoparticles with biocompatible metals, oxides or organic molecules is also reviewed, focusing on their applications.

Revealing the binding structure of the protein corona on gold nanorods using synchrotron radiation-based techniques: understanding the reduced damage in cell membranes

Regarding the importance of the biological effects of nanomaterials, there is still limited knowledge about the binding structure and stability of the protein corona on nanomaterials and the subsequent impacts. Here we designed a hard serum albumin protein corona (BSA) on CTAB-coated gold nanorods (AuNRs) and captured the structure of protein adsorption using synchrotron radiation X-ray absorption spectroscopy, microbeam X-ray fluorescent spectroscopy, and circular dichroism in combination with molecular dynamics simulations. The protein adsorption is attributed to at least 12 Au-S bonds and the stable corona reduced the cytotoxicity of CTAB/AuNRs. These combined strategies using physical, chemical, and biological approaches will improve our understanding of the protective effects of protein coronas against the toxicity of nanomaterials. These findings have shed light on a new strategy for studying interactions between proteins and nanomaterials, and this information will help further guide the rational design of nanomaterials for safe and effective biomedical applications.

The immobilization of gold from gold (III) chloride by a halophilic sulphate-reducing bacterial consortium

A consortium containing halophilic, dissimilatory sulphate-reducing bacteria was enriched from Basque Lake #1, located near Ashcroft, British Columbia, Canada to evaluate the role these bacteria have on the immobilization of soluble gold. The consortium immobilized increasing amounts of gold from gold (III) chloride solutions, under saline to hypersaline conditions, over time. Gold (III) chloride was reduced to elemental gold in all experimental systems. Salinity did not affect gold immobilization. Scanning electron microscopy and transmission electron microscopy demonstrated that reduced gold (III) chloride was immobilized as c. 3–10 nm gold colloids and c. 100 nm colloidal aggregates at the fluid–biofilm interface. The precipitation of gold at this organic interface protected cells within the biofilm from the ‘toxic effect’ of ionic gold. Analysis of these experimental systems using X-ray absorption near-edge spectroscopy confirmed that elemental gold with varying colloidal sizes formed within minutes. The immobilization of gold by halophilic sulphate-reducing bacteria highlights a possible role for the biosphere in ‘intercepting’ mobile gold complexes within natural, hydraulic flow paths. Based on the limited toxicity demonstrated in this experimental model, significant concentrations of elemental gold could accumulate over geological time in natural systems where soluble gold concentrations are more dilute and presumably ‘non-toxic’ to the biosphere.

Unique Bonding Properties of the Au36(SR)24 Nanocluster with FCC-Like Core

The recent discovery on the total structure of Au36(SR)24, which was converted from biicosahedral Au38(SR)24, represents a surprising finding of a face-centered cubic (FCC)-like core structure in small gold-thiolate nanoclusters. Prior to this finding, the FCC feature was only expected for larger (nano)crystalline gold. Herein, we report results on the unique bonding properties of Au36(SR)24 that are associated with its FCC-like core structure. Temperature-dependent X-ray absorption spectroscopy (XAS) measurements at the Au L3-edge, in association with ab initio calculations, show that the local structure and electronic behavior of Au36(SR)24 are of more molecule-like nature, whereas its icosahedral counterparts such as Au38(SR)24 and Au25(SR)18 are more metal-like. Moreover, site-specific S K-edge XAS studies indicate that the bridging motif for Au36(SR)24 has different bonding behavior from the staple motif from Au38(SR)24. Our findings highlight the important role of "pseudo"-Au4 units within the FCC-like Au28 core in interpreting the bonding properties of Au36(SR)24 and suggest that FCC-like structure in gold thiolate nanoclusters should be treated differently from its bulk counterpart.

Spin polarization and quantum spins in Au nanoparticles

The present study focuses on investigating the magnetic properties and the critical particle size for developing sizable spontaneous magnetic moment of bare Au nanoparticles. Seven sets of bare Au nanoparticle assemblies, with diameters from 3.5 to 17.5 nm, were fabricated with the gas condensation method. Line profiles of the X-ray diffraction peaks were used to determine the mean particle diameters and size distributions of the nanoparticle assemblies. The magnetization curves M(H_a) reveal Langevin field profiles. Magnetic hysteresis was clearly revealed in the low field regime even at 300 K. Contributions to the magnetization from different size particles in the nanoparticle assemblies were considered when analyzing the M(H_a) curves. The results show that the maximum particle moment will appear in 2.4 nm Au particles. A similar result of the maximum saturation magnetization appearing in 2.3 nm Au particles is also concluded through analysis of the dependency of the saturation magnetization M_P on particle size. The M_P(d) curve departs significantly from the 1/d dependence, but can be described by a log-normal function. Magnetization can be barely detected for Au particles larger than 27 nm. Magnetic field induced Zeeman magnetization from the quantum confined Kubo gap opening appears in Au nanoparticles smaller than 9.5 nm in diameter.

Single-atom Catalysis Using Pt/Graphene Achieved through Atomic Layer Deposition

Platinum-nanoparticle-based catalysts are widely used in many important chemical processes and automobile industries. Downsizing catalyst nanoparticles to single atoms is highly desirable to maximize their use efficiency, however, very challenging. Here we report a practical synthesis for isolated single Pt atoms anchored to graphene nanosheet using the atomic layer deposition (ALD) technique. ALD offers the capability of precise control of catalyst size span from single atom, subnanometer cluster to nanoparticle. The single-atom catalysts exhibit significantly improved catalytic activity (up to 10 times) over that of the state-of-the-art commercial Pt/C catalyst. X-ray absorption fine structure (XAFS) analyses reveal that the low-coordination and partially unoccupied densities of states of 5d orbital of Pt atoms are responsible for the excellent performance. This work is anticipated to form the basis for the exploration of a next generation of highly efficient single-atom catalysts for various applications.

Peptide-directed preparation and X-ray structural study of Au nanoparticles on titanium surfaces

We report the peptide-directed preparation and X-ray structural study of biofunctionalized Au nanoparticles (NPs) deposited on Ti surfaces. Au NPs were prepared by reduction of Au(3+) compound onto HCl-refreshed Ti in the presence of thiol-functionalized small peptides. A modified extended X-ray absorption fine-structure (EXAFS) technique, equipped with a rotating-stage and glancing-angle setup, was able to more sensitively detect the structure and bonding of Au NPs on Ti with low surface coverage. It was found that the use of the tripeptide glutathione (GSH) results in smaller NP size when compared to N-(2-mercapto-propionyl) glycine (MPG), a pseudodipeptide, over a wide range of Au/peptide molar ratios (20:1, 10:1, 5:1, and 2:1). By varying the ligand concentration, the Au NP structure in both systems can be controlled, generating nanocrystals, nanoclusters, and Au-thiolate polymer, which is unique for substrate-supported NP synthesis. This work presents a facile preparation of Au-peptide nanoparticles on biocompatible surfaces, and illustrates the high sensitivity of this modified EXAFS technique for structural studies of substrate-supported nanoparticles with low coverage.

Transformation of PVP coated silver nanoparticles in a simulated wastewater treatment process and the effect on microbial communities

Background

Manufactured silver nanoparticles (AgNPs) are one of the most commonly used nanomaterials in consumer goods and consequently their concentrations in wastewater and hence wastewater treatment plants are predicted to increase. We investigated the fate of AgNPs in sludge that was subjected to aerobic and anaerobic treatment and the impact of AgNPs on microbial processes and communities. The initial identification of AgNPs in sludge was carried out using transmission electron microscopy (TEM) with energy dispersive X-ray (EDX) analysis. The solid phase speciation of silver in sludge and wastewater influent was then examined using X-ray absorption spectroscopy (XAS). The effects of transformed AgNPs (mainly Ag-S phases) on nitrification, wastewater microbial populations and, for the first time, methanogenesis was investigated.

Results

Sequencing batch reactor experiments and anaerobic batch tests, both demonstrated that nitrification rate and methane production were not affected by the addition of AgNPs [at 2.5 mg Ag L^-1 (4.9 g L^-1 total suspended solids, TSS) and 183.6 mg Ag kg ^-1 (2.9 g kg^-1 total solids, TS), respectively].

The low toxicity is most likely due to AgNP sulfidation. XAS analysis showed that sulfur bonded Ag was the dominant Ag species in both aerobic (activated sludge) and anaerobic sludge. In AgNP and AgNO₃ spiked aerobic sludge, metallic Ag was detected (~15%). However, after anaerobic digestion, Ag(0) was not detected by XAS analysis. Dominant wastewater microbial populations were not affected by AgNPs as determined by DNA extraction and pyrotag sequencing. However, there was a shift in niche populations in both aerobic and anaerobic sludge, with a shift in AgNP treated sludge compared with controls. This is the first time that the impact of transformed AgNPs (mainly Ag-S phases) on anaerobic digestion has been reported.

Conclusions

Silver NPs were transformed to Ag-S phases during activated sludge treatment (prior to anaerobic digestion). Transformed AgNPs, at predicted future Ag wastewater concentrations, did not affect nitrification or methanogenesis. Consequently, AgNPs are very unlikely to affect the efficient functioning of wastewater treatment plants. However, AgNPs may negatively affect sub-dominant wastewater microbial communities.