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

In Situ TEM Observation of the Atomic Transport Process during the Coalescence of Au Nanoparticles

In practical applications, the coalescence of metal nanoparticles (NPs) is a major factor affecting their physical chemistry properties. Currently, due to a lack of understanding of the atomic-level mechanisms during the nucleation and growth stages of coalescence, the correlation between the different dynamic factors in the different stages of NP coalescence is unclear. In this study, we used advanced in situ characterization techniques to observe the formation of atomic material transport channels (Au chains) during the initiation of coalescence nucleation. We focused on the movement and migration states of Au atoms and discovered an atomic ordered arrangement growth mechanism that occurs after the completion of nucleation. Simultaneously, we used density functional theory to reveal the formation principle of Au chains. These findings improve our understanding of the atomic-scale coalescence process, which can provide a new perspective for further research on coalescence atomic dynamics.

Atomically Resolved Characterization of Optically Driven Ligand Reconfiguration on Nanoparticle Catalyst Surfaces

Dynamic ligand layers on nanoparticle surfaces could prove to be critically important to enhance the functionality of individual materials. Such capabilities could complement the properties of the inorganic component to provide multifunctionality or the ability to be remotely actuated. Peptide-based ligands have demonstrated the ability to be remotely responsive to structural changes when adsorbed to nanoparticle surfaces via incorporation of photoswitches into their molecular structure. In this contribution, direct spectroscopic evidence of the remote actuation of a photoswitchable peptide adsorbed onto Au nanoparticles is demonstrated using X-ray absorption fine structure spectroscopic methods. From this analysis, Au-X (X = C or N) coordination numbers confirm the changes before and after photoswitching in the surface ligand conformation, which was correlated directly to variations in the catalytic application of the materials for nitrophenol reduction processes. In addition, the catalytic application of the materials was demonstrated to be significantly sensitive to the structure of the nitrophenol substrate used in the reaction, suggesting that changes in the reactivity are likely based upon the peptide conformation and substrate structure. Such results confirm that surface ligands can be remotely reconfigured on nanoparticle surfaces, providing pathways to apply such capabilities to a variety of applications beyond catalysis ranging from drug delivery to sensing.

Multifunctional polymeric micellar nanomedicine in the diagnosis and treatment of cancer

Polymeric micelles are a prevalent topic of research for the past decade, especially concerning their fitting ability to deliver drug and diagnostic agents. This delivery system offers outstanding advantages, such as biocompatibility, high loading efficiency, water-solubility, and good stability in biological fluids, to name a few. The multifunctional polymeric micellar architect offers the added capability to adapt its surface to meet the looked-for clinical needs. This review cross-talks the recent reports, proof-of-concept studies, patents, and clinical trials that utilize polymeric micellar family architectures concerning cancer targeted delivery of anticancer drugs, gene therapeutics, and diagnostic agents. The manuscript also expounds on the underlying opportunities, allied challenges, and ways to resolve their bench-to-bedside translation for allied clinical applications.

In Situ/Operando Electrocatalyst Characterization by X-ray Absorption Spectroscopy

During the last decades, X-ray absorption spectroscopy (XAS) has become an indispensable method for probing the structure and composition of heterogeneous catalysts, revealing the nature of the active sites and establishing links between structural motifs in a catalyst, local electronic structure, and catalytic properties. Here we discuss the fundamental principles of the XAS method and describe the progress in the instrumentation and data analysis approaches undertaken for deciphering X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra. Recent usages of XAS in the field of heterogeneous catalysis, with emphasis on examples concerning electrocatalysis, will be presented. The latter is a rapidly developing field with immense industrial applications but also unique challenges in terms of the experimental characterization restrictions and advanced modeling approaches required. This review will highlight the new insight that can be gained with XAS on complex real-world electrocatalysts including their working mechanisms and the dynamic processes taking place in the course of a chemical reaction. More specifically, we will discuss applications of in situ and operando XAS to probe the catalyst's interactions with the environment (support, electrolyte, ligands, adsorbates, reaction products, and intermediates) and its structural, chemical, and electronic transformations as it adapts to the reaction conditions.

In-situ synthesis of ultrasmall Au nanoparticles on bimetallic metal-organic framework with enhanced electrochemical activity for estrone sensing

In this work, ultrasmall Au nanoparticles decorated bimetallic metal-organic framework (US Au NPs@AuZn-MOF) hybrids were facilely prepared by a sequential ion exchange and in-situ chemical reduction strategy. Numerous of Au nanoparticles with size less than 5 nm was homogeneously dispersed on the surface of the whole bimetallic AuZn-MOF polyhedrons. The integration of ultrasmall Au nanoparticles greatly enhanced the electron transfer capacity and electrochemical active surface area of the metal-organic framework host. Compared with the pristine Zn-MOF, bimetallic AuZn-MOF, the as-synthesized US Au NPs@AuZn-MOF hybrids exhibited remarkably promoted electrochemical activity toward the oxidation and sensing of endocrine-disrupting chemical (EDC) estrone. As a result, a highly sensitive electrochemical sensing platform was developed for the detection of estrone in the range of 0.05 μM-5 μM with limit of detection of 12.3 nM (S/N = 3) and sensitivity of 101.3 μA^-1 μM^-1 cm^-2. Considering the structural diversity of MOFs and superior property of ultrasmall Au nanoparticles, the strategy proposed here may open a new avenue for the design and synthesis of other high-activity nanomaterials for electrochemical sensing or other challenging fields.

Free-standing Monatomic Thick Two-dimensional Gold

Monolayer metal membranes have attracted research attention owing to their fascinating physical properties. Unlike layered materials with weak interlayer van der Waals bonding, metallic monolayer membranes are difficult to exfoliate due to strong metallic bonding between layers. Here, we fabricate free-standing monatomic-thick Au membranes and nanoribbons framed in bulk crystals using in situ dealloying inside transmission electron microscope. The Au membranes are robust under high energy electron beam. Monatomic-thick nanoribbons with a minimal width of 0.6 nm are observed. First-principles calculations reveal that zigzag-edged nanoribbons are ferromagnetic with magnetic moments ranging 0.38-0.51 μB per unit-cell for a width less than 0.9 nm. In addition, a linear relationship between the bond length and the coordination number of atoms is directly investigated using atomic resolution images of monolayer and bilayer Au membranes. This work provides a pathway for direct fabrication of metal membranes and nanoribbons and to achieve novel physical properties.

Ligand Migration from Cluster to Support: A Crucial Factor for Catalysis by Thiolate-protected Gold Clusters

Thiolate protected metal clusters are valuable precursors for the design of tailored nanosized catalysts. Their performance can be tuned precisely at atomic level, e. g. by the configuration/type of ligands or by partial/complete removal of the ligand shell through controlled pre-treatment steps. However, the interaction between the ligand shell and the oxide support, as well as ligand removal by oxidative pre-treatment, are still poorly understood. Typically, it was assumed that the thiolate ligands are simply converted into SO2, CO2 and H2O. Herein, we report the first detailed observation of sulfur ligand migration from Au to the oxide support upon deposition and oxidative pre-treatment, employing mainly S K-edge XANES. Consequently, thiolate ligand migration not only produces clean Au cluster surfaces but also the surrounding oxide support is modified by sulfur-containing species, with pronounced effects on catalytic properties.

Surface-confined fluorescence enhancement of Au nanoclusters anchoring to a two-dimensional ultrathin nanosheet toward bioimaging

Gold nanoclusters (Au NCs) as ultrasmall fluorescent nanomaterials possess discrete electronic energy and unique physicochemical properties, but suffer from relatively low quantum yield (QY) which severely affects their application in displays and imaging. To solve this conundrum and obtain highly-efficient fluorescent emission, 2D exfoliated layered double hydroxide (ELDH) nanosheets were employed to localize Au NCs with a density as high as 5.44 × 10(13) cm(-2), by virtue of the surface confinement effect of ELDH. Both experimental studies and computational simulations testify that the excited electrons of Au NCs are strongly confined by MgAl-ELDH nanosheets, which results in a largely promoted QY as well as prolonged fluorescence lifetime (both ∼7 times enhancement). In addition, the as-fabricated Au NC/ELDH hybrid material exhibits excellent imaging properties with good stability and biocompatibility in the intracellular environment. Therefore, this work provides a facile strategy to achieve highly luminescent Au NCs via surface-confined emission enhancement imposed by ultrathin inorganic nanosheets, which can be potentially used in bio-imaging and cell labelling.

A combined theoretical and experimental EXAFS study of the structure and dynamics of Au147 nanoparticles

We demonstrated the capability of combined EXAFS and DFT calculations for characterizing the structural and thermal properties of Au₁₄₇ clusters.

Atomic-scale identification of Pd leaching in nanoparticle catalyzed C-C coupling: effects of particle surface disorder

C-C coupling reactions are of great importance in the synthesis of numerous organic compounds, where Pd nanoparticle catalyzed systems represent new materials to efficiently drive these reactions. Despite their pervasive utility, the catalytic mechanism of these particle-based reactions remains highly contested. Herein we present evidence of an atom leaching mechanism for Stille coupling under aqueous conditions using peptide-capped Pd nanoparticles. EXAFS analysis revealed Pd coordination changes in the nanoparticle consistent with Pd atom abstraction, where sizing analysis by SAXS confirmed particle size changes associated with a leaching process. It is likely that recently discovered highly disordered surface Pd atoms are the favored catalytic active sites and are leached during oxidative addition, resulting in smaller particles. Probing the mechanism of nanoparticle-driven C-C coupling reactions through structural analyses provides fundamental information concerning these active sites and their reactivity at the atomic-scale, which can be used to improve catalytic performance to meet important sustainability goals.

Probing the Limits of Conventional Extended X-ray Absorption Fine Structure Analysis Using Thiolated Gold Nanoparticles

We present a method for quantifying the accuracy of extended X-ray absorption fine structure (EXAFS) fitting models. As a test system, we consider the structure of bare Au147 nanoparticles as well as particles bound with thiol ligands, which are used to systematically vary disorder in the atomic structure of the nanoparticles. The accuracy of the fitting model is determined by comparing two distributions of bond lengths: (1) a direct average over a molecular dynamics (MD) trajectory using forces and energies from density functional theory (DFT) and (2) a fit to the theoretical EXAFS spectra generated from that same trajectory. Both harmonic and quasi-harmonic EXAFS fitting models are used to characterize the first-shell Au-Au bond length distribution. The harmonic model is found to significantly underestimate the coordination number, disorder, and bond length. The quasi-harmonic model, which includes the third cumulant of the first-shell bond length distribution, yields accurate bond lengths, but incorrectly predicts a decrease in particle size and little change in the disorder with increasing thiol ligands. A direct analysis of the MD data shows that the particle surfaces become much more disordered with ligand binding, and the high disorder is incorrectly interpreted by the EXAFS fitting models. Our DFT calculations compare well with experimental EXAFS measurements of Au nanoparticles, synthesized using a dendrimer encapsulation technique, showing that systematic errors in EXAFS fitting models apply to nanoparticles 1-2 nm in size. Finally we show that a combination of experimental EXAFS analysis with candidate models from DFT is a promising strategy for a more accurate determination of nanoparticle structures.

A facile, one-pot synthesis of highly branched Au nanocorals and their enhanced electrocatalytic activity for ethanol oxidation

Dendritic Au nanocorals were prepared using a facile one-pot strategy, and exhibit excellent catalytic activity and stability for the ethanol oxidation reaction.