Magic-number gold nanoclusters are atomically precise nanomaterials that have enabled unprecedented insight into structure-property relationships in nanoscience. Thiolates are the most common ligand, ...binding to the cluster via a staple motif in which only central gold atoms are in the metallic state. The lack of other strongly bound ligands for nanoclusters with different bonding modes has been a significant limitation in the field. Here, we report a previously unknown ligand for gold(0) nanoclusters-N-heterocyclic carbenes (NHCs)-which feature a robust metal-carbon single bond and impart high stability to the corresponding gold cluster. The addition of a single NHC to gold nanoclusters results in significantly improved stability and catalytic properties in the electrocatalytic reduction of CO
. By varying the conditions, nature and number of equivalents of the NHC, predominantly or exclusively monosubstituted NHC-functionalized clusters result. Clusters can also be obtained with up to five NHCs, as a mixture of species.
Silver nanoparticles (Ag NPs) have attracted much attention in the past decade because of their unique physicochemical properties and notable antibacterial activities. In particular, thiol-protected ...Ag NPs have come to the forefront of metal nanoparticle studies, as they have been shown to possess high stability and interesting structure–property relationships. However, a clear correlation between thiol-protecting ligands, the resulting Ag NP surface structure, and their antibacterial properties has yet to be determined. Here, a multielement (Ag and S), multi-edge (Ag K-edge, Ag L3-edge, S K-edge) X-ray absorption spectroscopy (XAS) methodology was used to identify the structure and composition of Ag NPs protected with cysteine. XAS characterization was carried out on similar-sized Ag NPs protected with poly(vinylpyrrolidone) (PVP), in order to provide a valid comparison of the ligand effect on surface structure. The PVP-Ag NPs showed a metallic Ag surface and composition, consistent with metal NPs protected by weak protecting ligands. On the other hand, the Cys-Ag NPs exhibited a distinct surface shell of silver sulfide, which is remarkably different than previously studied Cys-Ag NPs. The minimum inhibitory concentration (MIC) of both types of Ag NPs against Gram-positive (+) and Gram-negative (−) bacteria were tested, including Staphylococcus aureus (+), Escherichia coli (−), and Pseudomonas aeruginosa (−). It was found that the MICs of the Cys-Ag NPs were significantly lower than the PVP-Ag NPs for each bacteria, implicating the influence of the sulfidized surface structure. Overall, this work shows the effect of ligand on the surface structure of Ag NPs, as well as the importance of surface structure in controlling antibacterial activity.
Silver nanoparticles (AgNPs) have gained much attention in biomedical research because of their antibacterial properties. However, they have also exhibited cytotoxicity toward certain mammalian ...cells. In order to improve therapeutic efficacy, the incorporation of gold (Au) and Ag into bimetallic Ag–Au NPs is a promising strategy, as it has the potential to increase biocompatibility and maintain antibacterial activity. Toward this end, we prepared a series of bimetallic Ag–Au NPs and studied them with X-ray absorption spectroscopy (XAS) in order to elucidate the correlation of atomic structure to their bioactivities. The addition of Au was found to drastically change the atomic structure of the Ag NPs; namely, the Ag core of the NPs was gradually replaced with Au, while Ag was found mostly on the surface. Next, NP antibacterial activity toward S. aureus and cytotoxicity toward NIH-3T3 fibroblast cells were assessed. It was found that the antibacterial activity of the bimetallic NPs was lower than pure Ag NPs and dependent on the Ag location within the NPs. On the other hand, the cytotoxicity of bimetallic NPs was much lower than the pure Ag NPs and dependent on the overall Au concentration. Using the structural information garnered from XAS, we were able to rationalize the bioactivity results of the NPs based on their atomic structure and provide guiding principles to design Au–Ag NPs with balanced antibacterial and cytotoxic activities. This work represents an important step toward engineering the atomic structure of bimetallic Au–Ag NPs for biomedical applications.
The characterization of nanometer-scale interactions between carbon-containing substrates and alumina surfaces is of paramount importance to industrial and academic catalysis applications, but it is ...also very challenging. Here, we demonstrate that dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP SENS) allows the unambiguous description of the coordination geometries and conformations of the substrates at the alumina surface through high-resolution measurements of 13C–27Al distances. We apply this new technique to elucidate the molecular-level geometry of 13C-enriched methionine and natural abundance poly(vinyl alcohol) adsorbed on γ-Al2O3-supported Pd catalysts, and we support these results with element-specific X-ray absorption near-edge measurements. This work clearly demonstrates a surprising bimodal coordination of methionine at the Pd–Al2O3 interface.
Benzimidazolium hydrogen carbonate salts have been shown to act as N‐heterocyclic carbene precursors, which can remove oxide from copper oxide surfaces and functionalize the resulting metallic ...surfaces in a single pot. Both the surfaces and the etching products were fully characterized by spectroscopic methods. Analysis of surfaces before and after NHC treatment by X‐ray photoelectron spectroscopy demonstrates the complete removal of copper(II) oxide. By using 13C‐labelling, we determined that the products of this transformation include a cyclic urea, a ring‐opened formamide and a bis‐carbene copper(I) complex. These results illustrate the potential of NHCs to functionalize a much broader class of metals, including those prone to oxidation, greatly facilitating the preparation of NHC‐based films on metals other than gold.
Monolayers: For the first time, a simultaneous etch‐functionalize protocol has been developed for oxidized copper surfaces using N‐heterocyclic carbenes. These results demonstrate the dual functionality of N‐heterocyclic carbenes when applied to more reactive metal surfaces that are prone to oxidation and offers a new strategy for chemical etching that avoids dangerous reagents. This work also provides an opportunity to replace precious metals, such as gold, with more abundant metals, such as copper, in organic‐on‐metal devices.
Thiolate-protected Ag nanoparticles (NPs) exhibit interesting physical and chemical properties which may lead to various sensing, diagnostic, and therapeutic applications. Further, understanding ...structure–property relationships of Ag NPs is of great interest to optimize their application. Herein, we used TEM, UV–vis, and a series of synchrotron X-ray spectroscopy techniques to probe the local structure and chemical bonding properties of thiolate-stabilized Ag NPs. Compared with other Ag nanostructures prepared under slightly modified conditions, the Ag NPs were found to have pronounced structural changes, which led to immensely different optical properties. Notably, the NPs were also found to have similar surface structure to recently elucidated Ag nanoclusters prepared with different thiolates. These findings suggest that the NP structure and optical properties can be sensitively tailored by controlling the synthetic conditions. The multi-element, multi-core excitation approach (i.e., Ag K-, Ag L
3
-, and S K-edges) employed in the X-ray absorption spectroscopy measurements was also demonstrated as an effective tool to uncover the NP structure from both the metal core and the ligand shell perspectives.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Surface plasmon resonance (SPR)-based biosensing is a powerful tool to study the recognition processes between biomolecules in real-time without need for labels. The use of thiol chemistry is a ...critical component in surface functionalization of various SPR biosensor surfaces on gold. However, its use is hampered by the high propensity for oxidation of the gold–thiol linkage even in ambient atmosphere, resulting in a short lifetime of SPR sensor chips unless strict precautions are taken. Herein, we describe an approach to overcome this limitation by employing highly robust self-assembled monolayers (SAMs) of alkylated N-heterocyclic carbenes (NHCs) on gold. An alkylated NHC sensor surface was developed and its biosensing capabilities were compared to a commercial thiol-based analoguea hydrophobic association (HPA) chipin terms of its ability to act as a reliable platform for biospecific interaction analysis under a wide range of conditions. The NHC-based SPR sensor outperforms related thiol-based sesnsors in several aspects, including lower nonspecific binding capacity, better chemical stability, higher reproducibility, shorter equilibration time, and longer life span. We also demonstrate that the NHC-based sensor can be used for rapid and efficient formation of a hybrid lipid bilayer for use in membrane interaction studies. Overall, this work identifies the great promise in designing NHC-based surfaces as a new technology platform for SPR-based biosensing.
Following the recent breakthrough of total structural determination of a Au-thiolate nanocluster P. Jadzinsky et al., Science 318, 430 (2007), extensive interests have been stimulated to unveil (or ...revisit) the structure-property relationship of various thiolate-Au nanostructures in light of the new finding of -SR-(Au-SR)(x)- "staple" motif. Here, we present experimental x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS) results on the local structure and electronic properties of thiolate-protected Au nanocluster encapsulated in bovine serum albumin (Au-BSA) together with theoretical calculation of projected local density of states (l-DOS) of Au(25)(SR)(18) model cluster. Analysis of the Au L(3)-edge extended x-ray absorption fine structure (EXAFS) of Au-BSA suggested that the nanocluster is Au(25) with Au-thiolate "staple" motif. X-ray absorption near-edge structure (XANES) and Au 4f XPS were used to probe the electronic behavior of Au-BSA. The Au d-electron density of Au-BSA was found to decrease by 0.047 e(-) relative to that of the bulk. A self-consistent real space Green's function approach implemented in ab initio FEFF8 program was used to calculate the l-DOS of Au(25)(SR)(18) and other model clusters from a site-specific perspective. The theoretical results are in good agreement with the experimental d-DOS data of Au-BSA and, importantly, systematically illustrate the effect of Au-thiolate "staple" motif on the electronic behavior of Au(25)(SR)(18). The present work sheds light on the structure-property relationship of thiolate-protected Au(25) from both experimental and theoretical perspectives and illustrates the usefulness of XAS/l-DOS method in such studies.
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.
N-Heterocyclic carbenes (NHCs) have been the focus of renewed interest in recent years for their ability to form extremely stable monolayers on noble metals. However, their ability to bond to ...non-noble metals is less understood. Magnesium is among the most notorious of these, as the native oxide layer is too porous to be passive but too protective to be catalytic. Through X-ray photoelectron spectroscopy (XPS) and potentiodynamic polarization curves (PDP), we find that NHCs with a wide group of side group functionalities increase the electrochemical activity of magnesium, potentially improving its potential for hydrogen storage.
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