Nanoparticulate Au Research Articles

Interlaboratory comparison of centrifugal ultrafiltration with ICP-MS detection in a first-step towards methods to screen for nanomaterial release during certification of drinking water contact materials

A key requirement for evaluating the safety of nano-enabled water treatment devices is measuring concentrations of insoluble nanomaterials released from devices into water that may be ingested by consumers. Therefore, there is a need for simple technique that uses commonly available commercial laboratory techniques to discriminate between nanoparticles and dissolved by-products of the nanomaterial (e.g., ionic metals). Such capabilities would enable screening for particulate or dissolved metals released into water from nanomaterial-containing drinking water contact materials (e.g., paint coatings) or devices (e.g., filters). This multi-laboratory study sought to investigate the use of relatively inexpensive centrifugal ultrafilters to separate nanoparticulate from ionic metal in combination with inductively-coupled plasma mass spectrometry (ICP-MS) detection. The accuracy, precision, and reproducibility for the proposed method were assessed using mixtures of nanoparticulate and ionic gold (Au) in a standard and widely utilized model water matrix (NSF International Standard 53/61). Concentrations for both ionic and nanoparticulate gold based upon measurements of Au mass in the initial solutions and Au permeating the centrifugal ultrafilters. Results across different solution compositions and different participating labs showed that ionic and nanoparticulate Au could be consistently discriminated with ppb concentrations typically resulting in <10 % error. A mass balance was not achieved because nanoparticles were retained on membranes embedded in plastic holders inside the centrifuge tubes, and the entire apparatus could not be acid and/or microwave digested. This was a minor limitation considering the ultrafiltration method is a screening tool, and gold concentration in the permeate indicates the presence of ionic metal rather than nanoforms. With further development, this approach could prove to be an effective tool in screening for nanomaterial release from water-system or device materials as part of third-party certification processes of drinking water compatible products.

Oxidation Catalysis of Au Nano-Particles Immobilized on Titanium(IV)- and Alkylthiol-Functionalized SBA-15 Type Mesoporous Silicate Supports

Novel Au nano-particle catalysts immobilized on both titanium(IV)- and alkylthiol-functionalized SBA-15 type ordered mesoporous silicate supports were developed. The bi-functionalized SBA-15 type support could be synthesized by a one-pot method. To the synthesized supports, Au was immobilized by the reaction of the alkylthiol groups on the supports with AuCl4−, following reduction with NaBH4. The immobilized amount and the formed structures and the electronic property of the Au species depended on the loading of alkylthiol. The moderate size (2–3 nm) nano particulate Au sites formed on Ti(0.5)-SBASH(0.5) were negatively charged. The aerobic alcohol oxidation activity of the catalysts depended on the loading of alkylthiol and the structure of the Au nano-particles. The non-thiol-functionalized catalyst (Au/Ti(0.5)-SBASH(0)) composed of the large (5–30 nm) and the higher thiol-loaded catalyst (Au/Ti(0.5)-SBASH(8)) composed of the small cationic Au species were almost inactive. The most active catalyst was Au/Ti(0.5)-SBASH(0.5) composed of the electron-rich Au nano-particles formed by the electron donation from the highly dispersed thiol groups. Styrene oxidation activity in the presence of 1-phenylethanol with O2 depended on the loadings of titanium(IV) on the Au/Ti(x)-SBASH(0.5). The titanium(IV) sites trapped the H2O2 generated through the alcohol oxidation reaction, and also contributed to the alkene oxidation by activating the trapped H2O2.

Measurements of Ion Transfer Kinetics for the Study of Electrocatalyst Corrosion in Gas Diffusion Electrode Microenvironments

Nanoparticulate Au, Ag, and Cu films with percolating ion-conducting polymers are common within devices for the electroreduction of carbon dioxide (CO2) to carbon monoxide, hydrocarbons, and alcohols. The kinetics of ion-transfer are influential on the rate of corrosion and catalyst sintering, but direct measurements of ion-transfer rates are challenging at bulk metal electrodes. We present data on the corrosion kinetics of metal nanoclusters and monolayers deposited via underpotential deposition on well-oriented and/or single-crystal metal surfaces within suspended ionomer electrolytes. Our results support the important role of nucleation and growth in corrosion processes as well as the influence of structural anions on reactive intermediates present during corrosion reactions. Figure 1

Redox Cycling within Nanoparticle-Nucleated Protein Superstructures: Electron Transfer between Nanoparticulate Gold, Molecular Reductant, and Cytochrome c.

We previously described how thousands of the heme protein cytochrome c (cyt.c) self-organize into multilayered, roughly spherical superstructures as initiated by nucleation around one colloidal gold or silver nanoparticle. Within these superstructures, the protein is stabilized to unfolding in buffered media and survives superstructure encapsulation within silica gels and processing to form bioaerogels. We now report that Au∼cyt.c superstructures in buffered media are not simply static groupings of proteins, but that the Au core and protein corona exhibit dynamic electron-transfer reactions within the superstructure as verified by UV-visible and resonance Raman spectroscopy. Within the superstructure, hundreds to thousands of ferricytochrome c (FeIII-cyt.c) are reduced to ferrocytochrome c (FeII-cyt.c) following first-order kinetics with an average apparent forward rate constant of 1.9 ±0.4 × 10-5 s-1. The reducing power in the microheterogeneous medium is derived from two multielectron reductants: tannic acid used to stabilize the commercial gold sol and the Au nanoparticle at the center of the protein superstructure. Fluorescence monitoring of guanidinium chloride-induced unfolding reveals that superstructure-associated cyt.c is stabilized to unfolding before and after chemical reduction of FeIII-cyt.c to form FeII-cyt.c, indicating that the superstructures remain intact during microheterogeneous redox reactions. Smaller nucleating Au nanoparticles or lower ionic strength in the buffered medium yields a greater extent of cyt.c reduction. Partial oxidation of the cyt.c-associated nanoparticulate Au is verified by X-ray photoelectron spectroscopy. The Au nanoparticle at the heart of the superstructure functions as a direct electron donor to the heme with oxidized Au atoms being recycled back to Au(0) as long as residual tannic acid, derived from the Au sol mother liquor, is present in the aqueous microheterogeneous medium.

Formation of geochemical anomalies in loess-paleosol sequence at the Dahu Gold Mine, central China: A clastic-hydromorphic-gaseous dispersion model

Modern mineral exploration requires a comprehensive understanding of how geochemical anomalies form, particularly where the mineral deposits are covered by unconsolidated sediments. However, the mechanisms that form anomalies vary as a function of cover type and are not well understood. The Dahu Gold Mine in central China is characterized by a lode gold deposit, overlain by a younger loess-paleosol sequence (including saprolite). Thus, the site provides an opportunity to examine how elements associated with primary mineralization have been dispersed through the loess to form anomalies in this poorly-investigated cover type. Fine-grained (<74 μm) samples were collected to determine the concentrations of Au, Cu, Pb, Zn, As, Mo, Co, Ni and Mn in the loess-paleosol sequence. The results from two vertical profiles over the deposit show that anomalies of Au, As, Mo, Pb, Zn and Cu were developed in three layers. From the bottom up, it includes (1) the saprolite, (2) the lower Wucheng Loess and (3) the upper Lishi Loess. In contrast, non-mineralized Co, Ni and Mn show a uniform distribution along the profiles. The vertical profile outside the context of primary mineralization, however, shows no anomalous zones of metal enrichment. Generally, the anomalies in the lower Wucheng Loess and upper Lishi Loess are 2 to 5 times higher than the background, for instance, >70 μg/g of Pb vs. the average 30 μg/g of Pb in the Chinese loess. Anomalies in the saprolite are interpreted to have formed by in-situ weathering of the primary mineralized rocks. In contrast, anomalies in the lower Wucheng Loess which correspond to the calcrete horizon were probably formed by the precipitation of metals from ascending Ca (HCO3)2-rich mineralized groundwater. The SN directed horizontal profile shows that anomalies of Au, Cu, Mo and Pb in the upper Lishi Loess which are not in direct contact with the mineralization, spatially coincide with major faults (i.e. the Huashan Frontal Fault) and fractures in the loess-paleosol sequence. Moreover, Pb isotopic ratios documented for the sulphide ores and anomalous and background loess suggest that they are partially derived from the sulphide ores. The anomalies in the upper Lishi Loess are interpreted to be formed by a possible gaseous dispersion process. Elements, probably in metallic form (e.g. nanoparticulate Au), were carried by gases that flowed up faults and fractures to the surface and were precipitated in carbonate-rich clayey loess. Therefore, the formation of anomalies in the loess-paleosol sequence is a synthesis of clastic, hydromorphic and gaseous dispersion processes.

(Invited) Nanoscale Design and Modification of Plasmonic Aerogels for Photocatalytic Hydrogen Generation

Composite catalytic aerogels comprise a highly-flexible design motif for the creation of solar fuels photocatalysts. We exploit the compositional and interfacial design flexibility of catalytic aerogels to couple surface plasmon resonant (SPR) guests to nanometric oxidation and reduction catalysts in one hierarchical photocatalytic composite architecture. In our composite aerogels, the nanoscale TiO2 aerogel acts as a 3D-interconnected network of nanowires that couples all of the functional elements required to photogenerate molecular hydrogen from visible light and water: visible light sensitization, electron and ion transport, and oxidation and reduction catalysis. We investigate the effects of synthetically modifying the TiO2 aerogel network and the nanoparticulate Au||TiO2 interfaces in plasmonic Au–TiO2 aerogels on light sensitization, carrier (electron–hole pair) generation, and photocatalytic H2 evolution under both broadband (i.e., UV + visible) and visible excitation. We also introduce oxidation and reduction co-catalyst nanoparticles into the plasmonic aerogels, creating composite aerogels that perform visible light SPR–driven photocatalytic reduction of water to generate H2. The nanostructured high surface–area network in the aerogel spatially and effectively separates charge while electrochemically connecting plasmonic nanoparticle sensitizers and metal nanoparticle all in one mesoscale architecture and at length scales compatible with the kinetics of each reaction. Reference: "Plasmonic aerogels as a 3D nanoscale platform for solar fuels photocatalysis.” P. A. DeSario, J. J. Pietron, A. Dunkelburger, T.H. Brintlinger, O. Baturina, R. M. Stroud, J. C. Owrutsky, and D. R. Rolison, Langmuir, 2017, 33, 9444–9454; doi: 10.1021/acs.langmuir.7b01117 This work is supported by the U.S. Office of Naval Research.

Effects of Nanoscale Interfacial Design on Photocatalytic Hydrogen Generation Activity at Plasmonic Au–TiO2 and Au–TiO2/Pt Aerogels

We demonstrate that composite catalytic aerogels represent a superior materials design motif for the creation of solar fuels photocatalysts. We couple surface plasmon resonant (SPR) guests to the inherent compositional and interfacial design flexibility of catalytic aerogels to photogenerate molecular hydrogen (H2). We investigate the effects of synthetically modifying the TiO2 aerogel network and the nanoparticulate Au||TiO2 interfaces in plasmonic Au–TiO2 aerogels on H2 evolution under both broadband (i.e., UV + visible light) and visible excitation. We also introduce non-plasmonic Pt co-catalyst nanoparticles into our composite aerogels, creating Au–TiO2/Pt aerogels that perform visible light SPR-driven photocatalytic reduction of water to generate H2. The fuels production achieved with this multicomponent photocatalytic nanoreactor demonstrates that the nanostructured high-surface-area network in the aerogel can spatially and effectively separate charge while electrochemically connecting plasmonic nanoparticle sensitizers and metal nanoparticle. In doing so, we prove several crucial concepts: (1) integration of a plasmonic sensitizer with a separate water reduction co-catalyst within the ultraporous aerogel nanoarchitecture; (2) wiring the electron–hole pairs generated under visible light at the plasmonic Au||TiO2 interface to the co-catalyst via the nanoscale TiO2 network; and (3) combining both the photocatalytic oxidation and reduction reactions critical to solar fuels photocatalysis into one composite material at length scales compatible with the reaction kinetics. This work is supported by the Office of Naval Research.

Application of Surface Click Reactions to Localized Surface Plasmon Resonance (LSPR) Biosensing.

Localized surface plasmon resonance (LSPR) spectroscopy is an effective tool for sensitive, affordable, and label-free biosensing. LSPR transducers based on nanoparticulate Au films have been applied to biosensing of receptor-analyte interactions, employing primarily thiolated receptors for constructing biorecognition interfaces on nanostructured Au surfaces. This popular method suffers from a major drawback, that is, the need to prepare a thiolated receptor for each system used, which is typically synthetically complex and time-consuming. Herein, we present an alternative approach based on the click reaction between azide and terminal alkyne, which avoids the need to synthesize thiol-derivatized receptors and is applicable to the heterogeneous morphology of LSPR transducers. The receptors are tethered with an alkyne group, which is considerably simpler than thiolation, while producing a stable product. The transducer surface is modified with a layer of a commercial long-chain thiol-azide molecule, then clicked with an alkyne-dertivatized receptor to produce the biorecognition interface. This method is employed for immobilization of four different alkyne-bearing receptor molecules on Au nano-island film based LSPR transducers, followed by testing of their performance in biorecognition of specific analytes using LSPR and FTIR spectroscopies. The results establish the usefulness of click chemistry for the preparation of biorecognition interfaces on nanostructured LSPR transducers.

Pt Submonolayers on Au Nanoparticles: Coverage-Dependent Atomic Structures and Electrocatalytic Stability on Methanol Oxidation

Deposition of platinum monolayers on Au substrate (denoted as Au@PtML) has been shown an efficient catalyst design strategy for the electrocatalysis of alcohol oxidation due to presumed 100% utilization of Pt atoms and substrate-enhanced catalytic activities. However, the atomic structure and stability of Pt (sub)monolayers on realistic nanoparticulate Au surface still remains elusive. Here, we reveal coverage-dependent atomic structures and electrocatalytic stabilities of Pt submonolayers (sML) on Au nanoparticles for methanol oxidation reaction (MOR) by using high-resolution transmission electron microscopy combined with energy dispersive X-ray spectrum imaging and electrochemical techniques. At lower Pt coverages, the PtsML more resembled monatomic-thick layers, whereas higher Pt coverages above 0.5 ML resulted in 3D subnanometer Pt nanoclusters leading to lower Pt utilization efficiencies. Moreover, the Au@PtsML catalysts with Pt coverage below 0.5 ML showed higher structural and electrocatalytic stab...

Resonant photothermal laser processing of hybrid gold/titania nanoparticle films

Photothermal processing of thin anatase TiO2 and hybrid Au/anatase TiO2 nanoparticle films on glass supports is investigated using continuous-wave microfocused lasers at λ=355nm and λ=532nm. UV/Vis spectroscopy, Raman spectroscopy, optical microscopy, atomic force microscopy and scanning electron microscopy are used for characterization. Processing of TiO2 nanoparticle films is feasible at λ=355nm only. In contrast, the addition of Au nanoparticles enhances the overall absorbance of the material in the visible range and enables processing at both wavelengths, i.e. at λ=355nm and λ=532nm. Generally, laser heating induces a transition from anatase to rutile. The modification degree increases with increasing laser power and laser irradiation time. Resonant laser processing of hybrid Au/TiO2-mesoporous films provide promising perspectives in various applications, e.g. in photovoltaics, where embedded nanoparticulate Au could be exploited to enhance light trapping.

Understanding interactions between manganese oxide and gold that lead to enhanced activity for electrocatalytic water oxidation.

To develop active nonprecious metal-based electrocatalysts for the oxygen evolution reaction (OER), a limiting reaction in several emerging renewable energy technologies, a deeper understanding of the activity of the first row transition metal oxides is needed. Previous studies of these catalysts have reported conflicting results on the influence of noble metal supports on the OER activity of the transition metal oxides. Our study aims to clarify the interactions between a transition metal oxide catalyst and its metal support in turning over this reaction. To achieve this goal, we examine a catalytic system comprising nanoparticulate Au, a common electrocatalytic support, and nanoparticulate MnOx, a promising OER catalyst. We conclusively demonstrate that adding Au to MnOx significantly enhances OER activity relative to MnOx in the absence of Au, producing an order of magnitude higher turnover frequency (TOF) than the TOF of the best pure MnOx catalysts reported to date. We also provide evidence that it is a local rather than bulk interaction between Au and MnOx that leads to the observed enhancement in the OER activity. Engineering improvements in nonprecious metal-based catalysts by the addition of Au or other noble metals could still represent a scalable catalyst as even trace amounts of Au are shown to lead a significant enhancement in the OER activity of MnOx.

Capabilities of Single Particle Inductively Coupled Plasma Mass Spectrometry for the Size Measurement of Nanoparticles: A Case Study on Gold Nanoparticles

The increasing application of engineered nanomaterials (ENMs) in consumer and medical products has motivated the development of single-particle inductively coupled plasma mass spectrometry (spICP-MS) for characterizing nanoparticles under realistic environmental exposure conditions. Recent studies have established a set of metrological criteria and evaluated the feasibility of spICP-MS for sizing or quantifying various highly commercialized ENMs. However, less is known about the performance of spICP-MS for detecting nanoparticles with sizes greater than 80 nm. This paper presents a systematic study on spICP-MS for accurate size measurement of gold nanoparticles from 10 to 200 nm. We show that dwell time contributes significantly to the quality of data, with the optimal dwell time that limits split particle events, particle coincidences and false positives being 10 ms. A simple approach to correct for split particle events is demonstrated. We show that transient features of single particle events can be temporally resolved on a conventional quadrupole ICP-MS system using a sufficiently short dwell time (0.1 ms). We propose an intensity-size diagram for estimating the linear dynamic size range and guiding the selection of ICP-MS operating conditions. The linear dynamic size range of the ICP-MS system under standard (highest) sensitivity conditions is 10 to 70 nm but can be further extended to 200 nm by operating in less sensitive modes. Finally, the ability of spICP-MS to characterize heterogeneous forms of metal containing nanoparticles is evaluated in mixtures containing both dissolved and poly disperse nanoparticulate Au.

Enhanced Photocatalytic Activity: Macroporous Electrospun Mats of Mesoporous Au/TiO2 Nanofibers

AbstractAn electrospinning technique coupled with sol–gel chemistry is applied for the one‐pot preparation of macroporous–mesoporous titanium dioxide nanofibrous mats embedded with Au nanoparticles. This facile approach produces nanofibers that are thermally treated to simultaneously 1) remove the organics, 2) reduce AuIII to Au0 to form Au nanoparticles, and 3) crystallize titania to the anatase phase. This methodology allows high‐temperature treatment (650 °C) for well‐crystallized titania without agglomeration of the nanoparticulate Au (13±3 nm), which thereby significantly advances the existing methods for the synthesis of Au/TiO2 materials for which multiple steps are required. Additionally, high Au yields (≈100 %) were achieved in the final structure, which demonstrates another merit of this technique. The X‐ray diffraction patterns of the nanofibers were monitored by using a synchrotron beamline as the sample was heated to determine the optimum calcination temperature required to maximize the anatase phase and minimize the formation of rutile. Both the material properties (which include morphology, pore size distribution, surface area, Au particle size, and TiO2 crystal phase and size) and photocatalytic activity can be readily tuned as a function of the calcination temperature and Au content. The material calcined at 550 °C with 2.0 wt % Au shows the highest photocatalytic activity. This material is anatase in phase (which maximizes the electron‐transfer activity) and has the highest loading of small Au nanoparticles (which act as electron sinks to decrease the recombination of photo‐excited electrons and holes). These Au/TiO2 nanofibrous mats have the advantage of easy recovery and, therefore, reuse after the photocatalytic reaction, which addresses the issue of photocatalyst separation when using nanoparticulate powders as photocatalysts.

Supergene gold transformation: Biogenic secondary and nano-particulate gold from arid Australia

Biofilms on Au grains from sub-tropical and tropical sites promote the dispersion of gold (Au) by continuously recycling coarse Au and releasing nano-particulate Au. In this study we investigated the influences of (bio)geochemical processes on the transformation of Au grains in arid environments. At these sites water and nutrient availability are limited and episodic, hence abiogenic evaporative mechanisms were thought to control the formation of secondary and especially nano-particulate Au. Gold grains were collected from eight arid sites in three Australian Au provinces, i.e., Lawlers (Western Australia), Tanami (Northern Territory), and Flinders Ranges (South Australia). Sites were chosen based on contrasting deposit styles, i.e., primary underground and epithermal deposits as well as secondary eluvial-, colluvial- and alluvial placers at increasing distances from primary mineralization. Gold grains were studied using optical microscopy, FEG-SEM, FIB–SEM–EDXA, EPMA and PIXE. Gold grains from all surface environments displayed supergene transformation features, i.e., morphotypes indicative of Au and Ag dissolution, Au aggregation and Au neoformation. The latter included spheroidal and bacteriomorphic Au, which increased in abundance with distance to source. Viable biofilms containing abundant Au nano-particles and spheroidal Au μ-crystals were detected on all grains from the Flinders Ranges. Gold grains from the Lawlers and the Tanami provinces are covered by polymorphic layers containing abundant nano-particulate, spheroidal and bacteriomorphic Au. The polymorphic layers consist of vermiform clays and organic matter, suggestive of remnant biofilms. These results demonstrate that biofilms capable of transforming Au grains develop episodically on Au grains in arid environments. In conclusion, this study shows that microbial processes play a critical role for the transformation of Au grains and contribute to the dispersion of Au and the formation of geochemical anomalies in arid environments.

Supergene gold transformation: Secondary and nano-particulate gold from southern New Zealand

Biogeochemical processes drive the transformation of gold (Au) in surface environments. In this study we assess the link between surface morphologies of Au grains and supergene transformation processes, with a focus on the formation of nano-particulate Au in temperate settings. Gold grains were collected from six localities across the South Island of New Zealand. Deposit styles vary from eluvial-, alluvial-, and beach placer deposits in areas of moderate to very high levels of precipitation. Gold grains were assessed using optical microscopy (OM), field emission scanning electron microscopy (FEG-SEM), focused ion beam-scanning electron microscopy (FIB-SEM) coupled with X-ray dispersive analysis (EDXA) and electron microprobe analyses (EPMA). Morphologies indicative of Au- and Ag dissolution, e.g., grain boundary dissolution, as well as abundant Au neoformation- and aggregation morphologies were observed on all grains. The latter include a variety of secondary Au morphotypes, in particular nano-particulate- and μ-crystalline forms as well as bacteriomorphic Au, sheet-Au and porous, branched Au networks. Pervasive dissolution features on grains from an outcropping quartz-vein system as well as extensive nano-particle formation on weathered quartz-vein- and placer grains from the west coast of New Zealand's South Island, which is subject to very heavy orographic precipitation, suggest that these climatic conditions enhance the transformation of Au grains. At these sites, Au nano-particles are most abundant in the polymorphic layer (i.e., a coating of biofilms, secondary Au, siliceous and carbonaceous materials on the surface of transforming Au grains), and in soil materials associated with the grains. Nano-particulate Au is also highly abundant in carbonaceous, likely exopolymeric, coatings on Au grains from Orepuki Beach, suggesting that Au dissolution in seawater and microbial biomineralization are important contributors to Au alteration in beach placer deposits. In conclusion, surface morphologies of Au grains from New Zealand are the result of supergene transformations occurring in current environments. The formation of nano-particulate Au, which was previously thought to be evaporation-driven, is in these high-rainfall environments likely due to other mechanisms, such as biomineralization.

Photocatalytic Degradation of 2-Propanol and Phenol Using Au Loaded MnWO4 Nanorod Under Visible Light Irradiation

Single crystalline MnWO4 nanorod has been prepared by low temperature hydrothermal reaction at 180 °C. The prepared MnWO4 possesses band gap of 2.63 eV. Photochemical decomposition method has been followed to disperse Au nanoparticles onto MnWO4 nanorod. The prepared Au loaded MnWO4 nanorod demonstrated greatly enhanced photocatalytic activity in decomposing 2-propanol and evolving CO2 in gas phase and phenol in aqueous phase compared to bare MnWO4 and commercial TiO2 nanoparticles (Degussa P25) under visible light (λ ≥ 420 nm) irradiation. The Au loading was optimized to 3.79 wt% for the highest efficiency. The enhanced photocatalytic activity originates from the absorption of visible light by MnWO4 as well as the introduction of nanoparticulate Au on the surface of MnWO4 as cocatalyst to impede the recombination of photogenerated charge-carriers.

Nanoparticle factories: Biofilms hold the key to gold dispersion and nugget formation

Biofilms living on gold (Au) grains play a key role in the biogeochemical cycle of Au by promoting the dispersion of Au via the formation of Au nanoparticles as well as the formation of secondary biomorphic Au. Gold grains from Queensland, Australia, are covered by a polymorphic, organic-inorganic layer that is up to 40 μm thick. It consists of a bacterial biofilm containing Au nanoparticles associated with extracellular polymeric substances as well as bacterioform Au. Focused ion beam (FIB) sectioning through the biofilm revealed that aggregates of nanoparticulate Au line open spaces beneath the active biofilm layer. These aggregates (bacterioform Au type 1) resulted from the reprecipitation of dissolved Au, and their internal growth structures provide direct evidence for coarsening of the Au grains. At the contact between the polymorphic layer and the primary Au, bacterioform Au type 2 is present. It consists of solid rounded forms into which crystal boundaries of underlying primary Au extend, and is the result of dealloying and Ag dissolution from the primary Au. This study demonstrates that (1) microbially driven dissolution, precipitation, and aggregation lead to the formation of bacterioform Au and contribute to the growth of Au grains under supergene conditions, and (2) the microbially driven mobilization of coarse Au into nanoparticles plays a key role in mediating the mobility of Au in surface environments, because the release of nanoparticulate Au upon biofilm disintegration greatly enhances environmental mobility compared to Au complexes only.

Metal−Organic Framework Supported Gold Nanoparticles as a Highly Active Heterogeneous Catalyst for Aerobic Oxidation of Alcohols

The use of metal−organic frameworks (MOFs), an emerging class of porous materials, as supports for gold nanoparticles (NPs) may bring new opportunities in the development of highly active heterogeneous gold catalysts for a variety of catalytic reactions. Although currently a few examples of MOF-supported nanoparticulate Au have been reported, the preparation of an active Au NPs deposited MOF catalyst in solution is still a challenging research target. In this study, we report a highly efficient heterogeneous gold catalyst, which was deposited on a zeolite-type MOF (MIL-101) by a simple colloidal method with polyvinylpyrrolidone (PVP) as protecting agent using HAuCl4 as the Au precursor. The resulting Au/MIL-101 (CD/PVP) catalyst exhibited extremely high catalytic activities in liquid-phase aerobic oxidation of a wide range of alcohols, which could even efficiently catalyze the oxidation under ambient conditions in the absence of water or base. Moreover, the catalyst was easily recoverable and could be reused several times without leaching of metals or debasing of activity. The high dispersion of Au NPs and the electron donation effects of aryl rings to the Au NPs within the large cages of the MIL-101 support are suggested to be the main reasons for the observed high activities of the Au/MIL-101 (CD/PVP) catalyst in aerobic oxidation of alcohols under base-free conditions.

Thermal stability and reconstruction of nanoparticulate Au film on model molecular surfaces

Thermally activated morphological reconstruction of nanoparticulate gold films deposited onto model molecular surfaces was investigated at 200 °C as a function of the annealing time. Results show a strong correlation of the spatial reorganization of the metallic particles to the surface chemistry of the underlying substrate. On the nonpolar surface, the thin nanoparticulate Au film dewets by the formation of randomly distributed nucleation holes. Two morphological reconstruction kinetics were observed. The first kinetics characterized by a sigmoid growth describes the time-evolution of the dewetted spots and particle size inside the spots, and the second corresponds to the number density of particles and displays an exponential decay with time. Ultimately our results show that even at temperatures well below the bulk metal melting point, nanoparticulate metal structures can undergo drastic morphological reconstructions which can irreversibly affect their functional properties and performance (catalysis, electronic).

Synthesis of Au nanoparticles supported on metal oxides (H3PMo12O40 and TiO2) by PS-PVP block copolymer encapsulation method

Au supported on metal oxides (P-25, HPA) was successfully synthesized by PS-PVP block copolymer encapsulation method. The size of nanoparticles was strongly dependent on calcination conditions (time and temperature). The particle size was also controlled by selection of the appropriate length of PVP part in block copolymer. When particulate gold was doped on the surface of P-25, it strongly improved visible light absorption by plasma resonance. Compared with pure TiO2, Au supported on P-25 demonstrated much higher photocatalytic activity for acetaldehyde decomposition under visible light illumination. The enhanced photocatalytic activity could be attributed to the increase of visible light absorption and catalytic activity of nanoparticulate Au.