Tailoring the Asymmetric Structure of NH2-UiO-66 Metal-Organic Frameworks for Light-promoted Selective and Efficient Gold Extraction and Separation

Designing adsorption materials with high adsorption capacities and selectivities is highly desirable for precious metal recovery. Desorption performance is also particularly crucial for subsequent precious metal recovery and adsorbent regeneration. Herein, a metal–organic framework (MOF) material (NH₂-UiO-66) with an asymmetric electronic structure of the central zirconium oxygen cluster has an exceptional gold extraction capacity of 2.04 g g⁻¹ under light irradiation. The selectivity of NH₂-UiO-66 for gold ions is up to 98.8 % in the presence of interfering ions. Interestingly, the gold ions adsorbed on the surface of NH₂-UiO-66 spontaneously reduce in situ, undergo nucleation and growth and finally achieve the phase separation of high-purity gold particles from NH₂-UiO-66. The desorption and separation efficiency of gold particles from the adsorbent surface reaches 89 %. Theoretical calculations indicate that -NH₂ functions as a dual donor of electrons and protons, and the asymmetric structure of NH₂-UiO-66 leads to energetically advantageous multinuclear gold capture and desorption. This adsorption material can greatly facilitate the recovery of gold from wastewater and can easily realize the recycling of the adsorbent.     

Deposition of Prussian blue on nanoporous gold film electrode and its electrocatalytic reduction of H<Subscript>2</Subscript>O<Subscript>2</Subscript>

Prussian blue-modified nanoporous gold film (PB-NPGF) electrode was fabricated in this study. The fabrication was realized through electrodeposition of Prussian blue nanoparticles on the skeleton of a nanoporous gold film electrode without destroying the porous structure of NPGF electrode. The resulting PB-NPGF composite electrode showed very high electrocatalytic activity, repeatability, and stability to the reduction of H₂O₂. For instance, its activity was about twenty times that of the PB-modified polished gold electrode. More importantly, the sensitivity of the PB-NPGF composite electrode reaches as high as 10.6 μA μM⁻¹ cm⁻². This PB-NPGF composite electrode is very promising in the fields of catalysis, analysis, and so on.     

Quartz crystal microbalance monitoring of density changes in mesoporous TiO2 phytate films during redox and ion exchange processes

Nanofilm deposits of TiO₂ nanoparticle phytates are formed on gold electrode surfaces by ‘directed assembly’ methods. Alternate exposure of a 3-mercapto-propionic acid modified gold surface to (i) a TiO₂ sol and (ii) an aqueous phytic acid solution (pH 3) results in layer-by-layer formation of a mesoporous film. Ru(NH₃)₆³⁺ is shown to strongly adsorb/accumulate into the mesoporous structure whilst remaining electrochemically active. Scanning the electrode potential into a sufficiently negative potential range allows the Ru(NH₃)₆³⁺ complex to be reduced to Ru(NH₃)₆²⁺ which undergoes immediate desorption. When applied to a gold coated quartz crystal microbalance (QCM) sensor, electrochemically driven adsorption and desorption processes in the mesoporous structure become directly detectable as a frequency response, which corresponds directly to a mass or density change in the membrane. The frequency response (at least for thin films) is proportional to the thickness of the mass-responsive film, which suggests good mechanical coupling between electrode and film. Based on this observation, a method for the amplified QCM detection of small mass/density changes is proposed by conducting measurements in rigid mesoporous structures.     

Solubility-Driven Isolation of a Metastable Nonagold Cluster with Body-Centered Cubic Structure

The conventional synthetic methodology for atomically precise gold nanoclusters by using reduction in solution offers only the thermodynamically most stable nanoclusters. Herein, a solubility-driven isolation strategy is reported to access a metastable gold cluster. The cluster, with the composition of [Au₉(PPh₃)₈]⁺ ( 1 ), displays an unusual, nearly perfect body-centered cubic (bcc) structure. As revealed by ESI-MS and UV/Vis measurements, the cluster is metastable in solution and converts to the well-known [Au₁₁(PPh₃)₈Cl₂]⁺ ( 2 ) within just 90 min. DFT calculations revealed that although both 1 and 2 are eight-electron superatoms, there is a driving force to convert 1 to 2 as shown by the increased cohesion and larger HOMO–LUMO energy gap of 2 . The isolation and crystallization of the metastable gold cluster were achieved in a biphasic reaction system in which reduction of gold precursors and crystallization of 1 took place concurrently. This synthetic protocol represents a successful strategy for investigations of other metastable species in metal nanocluster chemistry.     

Theoretical study of the electronic spectrum of binuclear gold(I) complexes

The electronic structure and the spectroscopic properties of [Au₂(CS₃)₂]^?2, [Au₂(pym‐2‐S)₂] (pym = pyrimidethiolate), [Au₂(dpm)₂]⁺² (dpm = bis(diphosphino)methane) were studied using density functional theory (DFT) at the B3LYP level. The absorption spectrum of these binuclear gold(I) complexes was calculated by single excitation time‐dependent (TD) method. All complexes showed a ¹(5dσ* → 6pσ) transition associated with a metal–metal charge transfer, which is strongly interrelated with the gold–gold distance. Furthermore, we have calculated the frequency of the gold–gold vibration (ν_Au2) on the above complexes. The values obtained are theoretically in agreement with experimental range. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

CP/MAS NMR (13C, 15N, and 113Cd), thermal behavior, and sorption properties of cadmium Di-iso-butyldithiocarbamate. The gold(III)-binding species sorbed from solutions

The structure and thermal behavior of cadmium di-iso-butyldithiocarbamate (I) have been studied by solid-state ¹³C and ¹⁵N CP/MAS NMR and simultaneous thermal analysis (STA). The cadmium complex forms two crystalline polymorphs, ?? and ??. The structure of each of the polymorphs contains two structurally nonequivalent binuclear [Cd₂{S₂CN(iso-C₄H₉)₂}4] molecules. The STA data have shown the possibility of the ??-I ?? ??-I phase transition. Studying the thermal behavior of the complex has demonstrated that the end product of thermodestruction is a fine yellow-orange powder of CdS. The sorption properties of I have been studied with respect to gold(III) chloride solutions in 2 M HCl. It has been found that gold(III) is efficiently sorbed from strong acid solutions in the Au³⁺ concentration range 0.032?C3.220 mg/mL. On the basis of multinuclear (¹³C, ¹⁵N, and ¹¹³Cd) MAS NMR data, it has been demonstrated that the individual gold(III)-binding species in the sorption system is the ionic gold(III) cadmium-complex of composition [Au{S₂CN(iso-C₄H₉)₂}₂]₂[CdCl₄].     

Gold nanoparticle-templated assembly of oligothiophenes: preparation and film properties

Terthiophene-appended gold nanoparticles were prepared by the reduction of AuCl₄⁻(C₈H₁₇)₄N⁺ with NaBH₄ in the presence of bis[2,5-di(3-hexylthiophen-2-yl)thiophene-3-carboxyloxyhexanyl]disulfide. A hexagonal self-assembly of particles with gold core diameters (1.9±0.1 nm) was detected by high-angle annular dark-field scanning transmission electron microscopy. The electric conductivity of the iodine-doped film was 9.1×10⁻⁶ S cm⁻¹, which was ascribable to the terthiophene-based inter-ligand π-π interactions. The Au/terthiophene hybrid spin-coated film consisted of a highly three-dimensional assembled structure of terthiophenes, as inferred from grazing-incidence small-angle X-ray scattering, indicating that such monodispersed and small-sized gold nanoparticles can serve as a template for this organization. In this study, a gold nanoparticle-templated assembly of oligothiophenes has been fabricated for proposing a method to develop tailor-made organizations of π-conjugated oligomers.     

Molecular orbital analysis of the bonding in high nuclearity gold cluster compounds

Extended Hückel molecular orbital calculations on high nuclearity gold clusters of the general type [Au(AuPH₃)_n]^x+ have demonstrated that they can be classified into two broad topological classes according to the three-dimensional disposition of the peripheral gold atoms. If they lie approximately on a sphere they are characterised by a total of 12n + 18 valence electrons, but if they adopt a toroidal or eliptical arrangement the total electron count is 12n + 16. The computed energy differences between alternative polyhedral geometries is generally small and accounts for the stereochemical non-rigidity of the gold cluster compounds in solution. Detailed aspects of the structures of the high nuclearity gold cluster compounds have been interpreted in terms of molecular orbital calculations on clusters derived from the centred chair [Au₇(PH₃)₆]⁺ by edge- and face-capping with Au(PH₃)⁺ fragments.     

Synthesis of GoldMag particles with assembled structure and their applications in immunoassay

Micrometer-sized Fe3O4 particles and nano-sized gold particles were first synthesized by methods of self-aggregation of surface-chemically modified Fe3O4 nanoparticles and citrate reduction of the Au3 to Au0, respectively. Interaction between these two types of particles resulted in the assembly of nano-sized gold particles on the surface of the micrometer-sized Fe3O4 particles, forming an assembled structure with the Fe3O4 core particles around which are attached nano-sized gold parti- cles. The Fe3O4/Au structure is named GoldMag particles with assembled structure. The synthetic process, structure, and magnetic property of the GoldMag particles were analyzed. GoldMag particles with assembled structure have an irregular shape, rough surface with a diameter of 2―3 μm. These particles exhibit the superparamagnetic property with saturated magnetization of 41 A·m2/kg. In a single step, antibodies could be readily immobilized onto the surface of the particles with a high binding capacity. The GoldMag particles can be used as a novel carrier in immunoassays. The maximum quantity of human IgG immobilized onto GoldMag particles was 330 μg/mg. In order to validate the quality of the GoldMag particles as immunoassay carriers, an immunoassay system was used. The relative amount of immobilized human IgG was measured by HRP-labeled anti human IgG. The coefficient of variation within parallel samples of each group was below 6% and the coefficient of variation of means between five groups carried out separately was below 7%. Based on the sandwich method, the Hepatitis B surface antigen (HBsAg) and interleukin-8 (IL-8) were also analyzed by qualitative and quantitative detection, respectively. The result indicated that the GoldMag particles with assembled structure were an ideal carrier in immunoassay.     

Volatile dimethylgold(III) complex with di-iso-butyldithiophosphinate ligand: synthesis,structure, and thermal behavior in condensed and gas phase

Synthesis and molecular structure of air stable, low-melting dimethylgold(III) complex with dithiophosphinate (CH₃)₂AuS₂PⁱBu² (ⁱBu=CH₂CH(CH₃)₂), its thermal properties, and the features as precursor for the metal–organic chemical vapor deposition of gold films are reported. Thermal behavior of the compound in the condensed and gas phase was studied by thermogravimetric analysis and mass spectrometry. Pathways of heterogeneous thermolysis of the compound to elemental gold are discussed. It was found that α-P–H elimination followed by coupling of two alkenyl groups from the coordinated ligand is one of the main thermolysis pathways in condensed and gas phase.     

Efficient Near-Infrared Electrochemiluminescence from Au18 Nanoclusters

Bright, near-infrared electrochemiluminescence (NIR–ECL) of Au₁₈ nanoclusters is reported herein. Spooling ECL and photoluminescence spectroscopy were used to track and link NIR emissions at 832 and 848 nm to three emissive species, Au₁₈⁰*, Au₁₈¹⁺* and Au₁₈²⁺*, with a considerably high ECL efficiency of 5.5 relative to that of the gold standard Ru(bpy)₃²⁺/TPrA (with 5–6 % reported ECL efficiency). The unprecedentedly high efficiency is due to the overlapped oxidation potentials of Au₁₈⁰ and tri-n-propylamine as co-reactant, the exposed facets of Au₁₈⁰ gold core, and electrocatalytic loops. These discoveries will add a new member to the efficient NIR-ECL gold nanoclusters family and bring more potential applications.     

Cadmium-Doped and Pincer Ligand-Modified Gold Nanocluster for Catalytic KA2 Reaction

A gold nanocluster Au₁₇Cd₂(PNP)₂(SR)₁₂ (PNP=2,6-bis(diphenylphosphinomethyl)pyridine, SR=4-MeOPhS) consisting of an icosahedral Au₁₃ kernel, two Au₂CdS₆ staple motifs, and two PNP pincer ligands has been designed, synthesized and well characterized. This cadmium and PNP pincer ligand co-modified gold nanocluster showed high catalytic efficiency in the KA² reaction, featuring high TON, mild reaction conditions, broad substrate scope as well as catalyst recyclability. Comparison of the catalytic performance between Au₁₇Cd₂(PNP)₂(SR)₁₂ and the structurally similar single cadmium (or PNP) modified gold nanoclusters demonstrates that the co-existence of the cadmium and PNP on the surface is crucial for the high catalytic activity of the gold nanocluster. This work would be enlightening for developing efficient catalysts for cascade reactions and discovering the catalytic potential of metal nanoclusters in organic transformations.     

Synthesis of gold nanoparticles by reduction of HAuCl4 under UV irradiation

Gold nanoparticles were prepared in surfactant solutions by reduction of HAuCl₄ under UV irradiation without adding extra reductants or other organic substances. The effect of the structure and the property of surfactant on the size and the optical properties of prepared gold nanoparticles were studied. It was found that the longer the alkyl chain of the surfactant, the larger gold particles are obtained. On the other hand, lengthen the geminis spacer benefits the formation of smaller gold particles. The formation of adduct micelles composed of the charged surface active portion of the surfactant molecule and the (Au^IIICl₄)⁻ ion in cationic surfactant solution serves as the gold source and favors the formation of gold particles with larger sizes. While the repulsion between the (Au^IIICl₄)⁻ ion and the negative charged surface of anionic surfactant micelle is in favor of the formation of gold nanoparticles with smaller sizes. The nonionic surfactants can also assist the formation of dispersed gold nanoparticles.     

Predicting Stable Molecular Structures for (RNC)2AuIX Complexes

Calculations have been performed at the MP2 and DFT levels for investigating the reasons for the difficulties in synthesizing bis(isocyanide)gold(I) halide complexes. Three‐coordinated gold(I) complexes of the type (R₃P)₂Au^IX ( 1 ) can be synthesized, whereas the analogous isocyanide complexes (RNC)₂Au^IX ( 2 ) are not experimentally known. The molecular structures of (R₃P)₂Au^IX (X = Cl, Br, and I) and (RNC)₂Au^IX with X = halide, cyanide, nitrite, methylthiolate, and thiocyanate are compared and structural differences are discussed. Calculations of molecular properties elucidate which factors determine the strength of the gold‐ligand interactions in (RNC)₂Au^IX. The linear bonding mode of RNC favors a T‐shaped geometry instead of the planar Y‐shaped trigonal structure of (R₃P)₂Au^IX complexes that have been synthesized. An increased polarity of the Au–X bond in 2 leads to destabilization of the Y‐shaped structure. Chalcogen‐containing ligands or cyanide appear to be good X‐ligand candidates for synthesis of (RNC)₂Au^IX complexes.     

Optical Spectroscopy of the Au4+ Cluster: The Resolved Vibronic Structure Indicates an Unexpected Isomer

Knowledge of the geometric and electronic structure of gold clusters and nanoparticles is vital for understanding their catalytic and photochemical properties at the molecular level. In this study, we report the vibronic optical photodissociation spectrum of cold and mass‐selected Au₄⁺ clusters measured at a resolution high enough to allow for comparison with Franck–Condon simulations of the excited state transitions based on time‐dependent density functional theory calculations. The three vibrational frequencies identified for the lowest‐lying optically accessible excited state at 2.17 eV stem from the Y‐shaped isomer (C_2v) and not from the rhombic isomer (D_2h) considered to be the ground state structure of Au₄⁺. This study demonstrates that an analysis of low‐resolution electronic spectra by calculations of vertical transitions alone is not sufficient for a reliable isomer assignment of such metal clusters.     

Anomalous Property of Ag(BO2)2 Hyperhalogen: Does Spin–Orbit Coupling Matter?

Hyperhalogens were recently identified as a new class of highly electronagative species which are composed of metals and superhalogens. In this work, high‐level theoretical calculations and photoelectron spectroscopy experiments are systematically conducted to investigate a series of coinage‐metal‐containing hyperhalogen anions, Cu(BO₂)₂^?, Ag(BO₂)₂^?, and Au(BO₂)₂^?. The vertical electron detachment energy (VDE) of Ag(BO₂)₂^? is anomalously higher than those of Au(BO₂)₂^? and Cu(BO₂)₂^?. In quantitative agreement with the experiment, high‐level ab initio calculations reveal that spin–orbit coupling (SOC) lowers the VDE of Au(BO₂)₂^? significantly. The sizable magnitude of about 0.5 eV of SOC effect on the VDE of Au(BO₂)₂^? demonstrates that SOC plays an important role in the electronic structure of gold hyperhalogens. This study represents a new paradigm for relativistic electronic structure calculations for the one‐electron‐removal process of ionic Au^IL₂ complexes, which is characterized by a substantial SOC effect.     

Continuous Electrochemical Detection of Gold Nanoparticles in Flow

Nanoparticles have already found numerous applications and their global production is still increasing. Therefore, the engineered nanoobjects of uncertain toxicity become ubiquitous in the environment and a continuous monitoring of their presence is highly desirable. Here, we demonstrate a continuous electrochemical detection of gold nanoparticles (AuNPs) based on synchronous processes of their electrodissolution and electrocatalysis. This approach is realized by the injection of nanoparticles suspension into the Flow Injection Analysis (FIA) system. The modular structure of FIA system is particularly applicable for carrying out of sequential operations: AuNPs passivation, oxidation of aqueous SO₂ and gold. It enables continuous, fast and reproducible gold nanoparticles determination in a wide concentration range: 10⁻¹⁰– 10⁻⁷ mol nanoobjects L⁻¹.     

Metal substrate effects upon the kinetics of simple electrochemical reactions: The reduction of cobalt(III) ammines at single-crystal gold faces

The one-electron electroreduction kinetics of Co(NH₃)³⁺₆, Co(NH₃)₅F²⁺, Co(NH₃)₅OSO⁺₃, Co(NH₃)₅NCS²⁺, and c-Co(en)₂(NCS)⁺₂ (en = ethylenediamine) in aqueous perchlorate have been examined at gold (111), (100), (110), and polycrystalline surfaces and compared with those at mercury, silver (110), and polycrystalline silver surfaces. The first three, nominally outer-sphere, processes all exhibit rate constants that are substantially (ca. 10³ − 10⁴-fold) larger at each of the gold faces than at mercury and silver. This rate enhancement is well beyond that attributable to diffuse-layer effects. In contrast, the latter two, thiocyanate-bridged, reactions exhibited smaller variations with the electrode material which are attributable chiefly to differences in the extent of reactant adsorption. A stronger sensitivity of both the extent of adsorption and the electron-transfer kinetics within the adsorbed state was found, however, for c-Co(en)₂(NCS)⁺₂ reduction at the various single-crystal gold surfaces using very low (≲10 μM) reactant concentrations, the adsorption being particularly extensive at gold (111). The observed rate variations with the substrate are discussed in terms of current models of heterogeneous kinetics and double-layer structure.     

Fast atom bombardment mass spectrometry of ionic gold(I) and gold(III) carbene derivatives: An investigation of the formation of [M – H]+ species

Fast atom bombardment (FAB) mass spectrometry of the gold(I) and gold(III) derivatives, {Au[C(Y)–NHAr]₂}⁺X^? and {Au[C(Y)–NHAr]₂I₂} ⁺ X^? (Y =? OC₂H₅ or ? NHAr; X^? = CIO₄^? or BF₄^?; Ar = p-CH₃? C₆H₄) has led to the detection, for the alkoxyamino derivatives only, of [M–H]⁺˙ molecular species. The mechanism of the formation of these unusual species has been studied with respect to the oxidation state of gold, nature of the matrix, matrix acidity and ligand structure. The energetics of two possible alternative mechanisms has been studied by means of ab initio theoretical calculations. Both experimental and theoretical data indicate that [M–H]⁺˙ formation is due to the reaction of M⁺ with H⁺-philic and/or H˙-philic species produced from the matrix by FAB. Whatever the operative mechanism, the [M–H]⁺˙ formation is to be considered a FAB-induced oxidative process.