Anionic Gold Research Articles

Self-Assembled Gold Nanoparticle Cluster Array Onto Seeded Optical Fiber for Chemical Sensors

IntroductionGold nanoparticles (Au NP) exhibit interesting optical, physical and chemical features that can be tuned by modifying their morphology or by functionalizing them [1]. Therefore, a wide variety of sensing applications based on optical fiber have been developed using them, such as the colorimetric detection of toxic metal ions, cysteine or hydrogen peroxide. For those purposes, Au NP are typically embedded into polyelectrolyte multilayers or polymeric matrices, in order to be deposited onto optical fibers. Recent studies showed the possibility of depositing Au NP onto substrates seeded with block copolymers forming nanoparticle clusters [2]. The resulting array gave rise to a localized surface plasmon resonance, whose wavelength can be tailored, for instance, by modifying the amount of nanoparticles of each cluster or the distribution of the clusters. In order to check out the feasibility of the array as sensing structure, citrate-stabilized Au NPs have been self-assembled onto a polymeric template deposited along an optical fiber. The utilization of this device as a pH sensor opens the doors for the fabrication of sensors for different applications by controlling the seeding process and the morphology of Au NP clusters onto the surface of the optical fiber. Self-assembly of patterned gold nanoparticle cluster array along the optical fiber Au NP clusters were self-assembled by electrostatic interactions with a polymeric template [2] along the core of a 200 μm-core optical fiber. The optical fiber was first cleaned and activated by immersing it for 20 minutes in each one of the following solutions: soap, ultrapure water, potassium hydroxide (KOH) and ultrapure water. Subsequently, in order to build the template and so seeding the substrate, the fiber was dipped for 30 seconds in a 5 mg/mL solution of the cationic copolymer poly(styrene-b-2-vinylpyridine) in o-xylene. Finally, gold nanoparticles were self-assembled onto the template by immersing the fiber for 60 minutes into the anionic citrate-stabilized gold nanoparticles solution. The resulting coating was analyzed with images obtained by a Scanning Electronic Microscope (SEM).The deposition of the Au NP cluster array along the fiber was controlled by analyzing the absorption spectra during the different steps of the process explained above. The experimental set up used to study the processes consisted of the connection of the optical fiber between a white light source in the Visible Spectra (VIS) and a spectrometer (see Figure 1). Results The averaged diameter of the clusters is 120nm whereas the distance between the centers is 140nm (see Figure 2). Due to their formation on the seeded surface of the optical fiber, an absorption peak was observed in the transmitted spectra: as the formation of the cluster array moved forward, the absorption peak broadened and red-shifted from 630 nm up to 700 nm, while the absorbance increased, as it is shown in Figure 3. These changes in the absorption spectra are characteristic of the formation of the cluster array along the surface of the optical fiber.In order to evaluate the utilization of this Au NP cluster array as a possible sensing structure, the as-patterned optical fiber was exposed to different pH values ranging from 2 to 6. For each distinct pH value, the absorption peak was located at a different wavelength: as it can be observed in Figure 4, the absorption peak was red-shifted as the pH value decreased, showing a repetitive and reversible response. Conclusions The results exposed previously showed how the formation of a patterned gold nanoparticle cluster array yield to a localized surface plasmon resonance that could be used for measuring pH variations. The utilization of this array as sensing structure opens the possibility of utilizing this pattern for other sensing purposes by employing modified of functionalized Au NP. These parameters are currently been studied, together with different modifications of the patterned array, such as the distribution of the clusters and the amount of gold nanoparticles adsorbed in each cluster. Acknowledgements This research was supported by the Spanish State Research Agency (AEI) through the TEC2016-79367-C2-2-R project.

One-pot synthesis of an anionic cyclodextrin-stabilized bifunctional gold nanoparticles for visual chiral sensing and catalytic reduction

A facile one-pot synthetic method for preparing gold nanoparticles by employing sugammadex (SUG), a carboxylic acid functionalized γ-cyclodextrin derivative, as reducing-cum-stabilizing agent herein was reported for the first time. The SUG protected gold nanoparticles (SUG-AuNPs) can work as a colorimetric sensor for visual chiral recognition of α-amino acids enantiomers, especially for lysine (Lys) and asparagine (Asn) enantiomers. The chiral recognition assay was successfully applied to determining the enantiometric excess of L-Lys and L-Asn ranging from −100 % to 100 % respectively. Moreover, the prepared SUG-AuNPs was found to exhibit efficient catalytic activity towards the reduction of toxic 4-nitrophenol by NaBH4 and the efficiency of the system was further demonstrated through the reduction of other typical nitroaromatics under mild condition. The as-synthesized SUG-AuNPs shows good performance for both chiral sensing and reduction activity and thus may facilitate the practical application in the area of both chiral discrimination and catalysis.

Adsorption and reactions of NO molecules on anionic gold clusters in the size range below 1 nm: effects of clusters' global electronic properties.

We systemically studied adsorption and reactions of NO on Aun- (n≤ 80) using a mini flow-tube reactor running at 150 K. For Aun- (n≤ 11), their reactions with NO mainly formed cluster complexes containing various numbers of NO units; for Aun- (n≥ 12), most active sizes eventually formed specific complexes Aun(NO)3-. The relative rates of the reactions with the first NO were measured. Correlations between these relative rates and the adiabatic detachment energies (ADEs) of Aun- revealed the dominant effect of the clusters' spins and a more complicated electron transfer mechanism than that of reactions with O2. Au20- as well as previously reported Au4,6,8- is an exceptional size, which eventually formed the disproportionate product Au20NO2-, and all these four sizes have very low ADEs. The effects of the clusters' global electronic properties on adsorption and reactions of NO on anionic gold are helpful to understand catalytic mechanisms of gold-based catalysts in NO removal reactions.

Facile Synthesis of Triangular and Hexagonal Anionic Gold Nanoparticles and Evaluation of Their Cytotoxicity.

Comprehension of the shape-dependent properties of gold nanoparticles (AuNPs) could benefit the advancements in cellular uptake efficiency. Spherical AuNPs have generally been used for drug delivery, and recent research has indicated that the cellular uptake of triangular AuNPs was higher than that of spherical ones. Previous reports have also revealed that chemically synthesized AuNPs were cytotoxic. Therefore, we have developed a facile, cost-effective, and environmentally friendly method for synthesizing triangular and hexagonal anionic AuNPs. The zeta potential of the synthesized AuNPs was negative, which indicated that their surface could be easily functionalized with positively charged molecules to upload drugs or biomolecules. Transmission electron microscopy (TEM) images illustrated that the largest particle size of the synthesized quasi-hexagonal AuNPs was 61 nm. The TEM images also illustrated that two types of equilateral-triangular AuNPs were synthesized: One featured sharp and the other rounded corners. The sides of the smallest and largest triangular AuNPs were 23 and 178 nm, respectively. Energy-dispersive X-ray spectra of the green-synthesized AuNPs indicated that they consisted entirely of elemental Au. The cytotoxicity of the green-synthesized AuNPs was evaluated using 3T3-L1 adipocytes. Using cell viability data, we determined that the green-synthesized AuNPs did not exhibit any cytotoxic effects on 3T3-L1 adipocytes.

Radiofrequency and Near-Infrared Responsive Core-Shell Nanostructures Using Layersome Templates for Cancer Treatment.

We report a multifunctional nanotherapeutic platform based on liposomes loaded with drug and iron oxide nanoparticles (IONs) coated with a gold nanoshell synthesized using a polyelectrolyte (layersome) soft templating technique. IONs and gold nanoshells were used to provide combined hyperthermia and triggered drug release via radio frequency (RF) or near-infrared (NIR) stimulation. IONs and the anticancer drug doxorubicin (DOX) were coencapsulated inside liposomes composed of zwitterionic phosphatidylcholine, anionic phosphatidylglycerol, and cholesterol lipids. Coating the magneto-liposomes with positively charged poly-l-lysine enriched the interface with gold anions to form a dense gold nanoshell and protected the structure against deformation and DOX cargo release during shell formation. After modification with thiol-terminated polyethylene glycol, intracellular delivery and release of DOX from the nanostructures was examined in A549 human lung cancer cells. The nanostructures retained their DOX cargo and remained in the cytosol after cellular uptake. Only when triggered by RF or NIR stimuli did the nanostructures release DOX, which then entered the cell nucleus. Compared to the single photothermal therapy or radio frequency treatment, the carriers with combined DOX and RF or NIR stimulation displayed higher therapeutic effect on A549 cells.

Synergistical Use of Electrostatic and Hydrophobic Interactions for the Synthesis of a New Class of Multifunctional Nanohybrids: Plasmonic Magneto-Liposomes.

By carefully controlling the electrostatic interactions between cationic liposomes, which already incorporate magnetic nanoparticles in the bilayers, and anionic gold nanoparticles, a new class of versatile multifunctional nanohybrids (plasmonic magneto-liposomes) that could have a major impact in drug delivery and controlled release applications has been synthesized. The experimental results confirmed the successful synthesis of hydrophobic superparamagnetic iron oxide nanoparticles (SPIONs) and polyethylene glycol functionalized (PEGylated) gold nanoparticles (AuNPs). The SPIONs were incorporated in the liposomal lipidic bilayers, thus promoting the formation of cationic magnetoliposomes. Different concentrations of SPIONs were loaded in the membrane. The cationic magnetoliposomes were decorated with anionic PEGylated gold nanoparticles using electrostatic interactions. The successful incorporation of SPIONs together with the modifications they generate in the bilayer were analyzed using Raman spectroscopy. The plasmonic properties of the multifunctional nanohybrids were investigated using UV-Vis absorption and (surface-enhanced) Raman spectroscopy. Their hyperthermic properties were recorded at different frequencies and magnetic field intensities. After the synthesis, the nanosystems were extensively characterized in order to properly evaluate their potential use in drug delivery applications and controlled release as a result of the interaction with an external stimulus, such as an NIR laser or alternating magnetic field.

Gold-coated plant virus as computed tomography imaging contrast agent.

Chemical modification of the surface of viruses, both the interior and the exterior, imparts new functionalities, that have potential applications in nanomedicine. In this study, we developed novel virus-based nanomaterials as a contrast agent for computed tomography (CT) imaging in vitro. The gold-coated cowpea mosaic virus (Au-CPMV) particles were generated by the electrostatic adsorption of positively charged electrolyte on the virus capsid with the subsequent incubation and reduction of anionic gold complexes. Au-CPMV particles as a CT contrast agent offer a fast scan time (less than 2 min), low cost, and biocompatibility and allow for high-resolution imaging with ca. 150 Hounsfield units (HU). The Au-CPMV surface was further modified allowing for the incorporation of targeting molecules of specific cell types.

High Ambipolar Mobility in a Neutral Radical Gold Dithiolene Complex

AbstractA new anionic gold dithiolene complex NBu4·[1] is synthesized from the (1‐((1,1‐biphenyl)‐4‐yl‐)‐ethylene‐1,2‐dithiolene ligand 1, and the cis and trans isomers are separated by recrystallization. The trans isomer is oxidized via electrocrystallisation to the neutral gold dithiolene complex 2. Complex 2 crystalizes in 1D chains, held together by short (3.30–3.37 Å) S–S contacts, which are packed in a herringbone arrangement in the ab‐plane. The complex exhibits semiconductor behavior (σRT = 1.5 × 10−4 S cm−1) at room temperature with a small activation energy (Ea = 0.11 eV), with greater conductivity along the stacking direction. The charge transport behavior of complex 2 is further investigated in single‐crystal field‐effect transistor (FET) measurements, the first such measurements reported for gold dithiolene complexes. Complex 2 shows incredibly balanced ambipolar behavior in the single‐crystal field‐effect transistor (SC‐FET), with high charge‐carrier mobilities of 0.078 cm2 V−1 s−1, the highest ambipolar mobilities reported for metal dithiolene complexes. This well‐balanced behavior, along with the activated conductivity and band structure calculations, suggests that 2 behaves as a Mott insulator. The magnetic properties are also studied by superconducting quantum interference device (SQUID) magnetometry and solid state 1H NMR, with evidence of a nonmagnetic ground state at low temperature.

Preferential Binding of Cytochrome c to Anionic Ligand-Coated Gold Nanoparticles: A Complementary Computational and Experimental Approach.

Membrane-bound proteins can play a role in the binding of anionic gold nanoparticles (AuNPs) to model bilayers; however, the mechanism for this binding remains unresolved. In this work, we determine the relative orientation of the peripheral membrane protein cytochrome c in binding to a mercaptopropionic acid-functionalized AuNP (MPA-AuNP). As this is nonrigid binding, traditional methods involving crystallographic or rigid molecular docking techniques are ineffective at resolving the question. Instead, we have implemented a computational assay technique using a cross-correlation of a small ensemble of 200 ns long molecular dynamics trajectories to identify a preferred nonrigid binding orientation or pose of cytochrome c on MPA-AuNPs. We have also employed a mass spectrometry-based footprinting method that enables the characterization of the stable protein corona that forms at long time-scales in solution but remains in a dynamic state. Through the combination of these computational and experimental primary results, we have established a consensus result establishing the identity of the exposed regions of cytochrome c in proximity to MPA-AuNPs and its complementary pose(s) with amino-acid specificity. Moreover, the tandem use of the two methods can be applied broadly to determine the accessibility of membrane-binding sites for peripheral membrane proteins upon adsorption to AuNPs or to determine the exposed amino-acid residues of the hard corona that drive the acquisition of dynamic soft coronas. We anticipate that the combined use of simulation and experimental methods to characterize biomolecule-nanoparticle interactions, as demonstrated here, will become increasingly necessary as the complexity of such target systems grows.

Effect of Surface Ligand and Temperature on Lipid Vesicle-Gold Nanoparticle Interaction: A Spectroscopic Investigation.

We herein investigate the interactions of differently functionalized anionic and cationic gold nanoparticles (AuNPs) with zwitterionic phosphocholine (PC) as well as inverse phosphocholine (iPC) lipid bilayers via spectroscopic measures. In this study, we used PC lipids with varying phase-transition temperatures, i.e., DMPC( Tm = 24 °C), DOPC( Tm = -20 °C), and iPC lipid DOCP ( Tm = -20 °C) to study their interactions with AuNPs functionalized with anionic ligands citrate, 3-mercaptopropionic acid, glutathione, and cationic ligand cysteamine. We studied the interactions by steady-state and time-resolved spectroscopic studies using membrane-sensitive probes 6-propionyl-2-dimethylaminonaphthalene (PRODAN)and 8-anilino-1 naphthalenesulfonate(ANS), as well as by confocal laser scanning microscopy (CLSM) imaging and dynamic light scattering (DLS) measurements. We observe that AuNPs bring in stability to the lipid vesicle, and the extent of interaction differs with the different surface ligands on the AuNPs. We observe that AuNPs functionalized with citrate effectively increase the phase-transition temperature of the vesicles by interacting with them. Our study reveals that the extent of interaction depends on the bulkiness of the ligands attached to the AuNPs. The bulkier ligands exert less van der Waals force, resulting in a weaker interaction. Moreover, we find that the interactions are more strongly pronounced when the vesicles are near the phase-transition temperature of the lipid.The CLSMimaging and DLSmeasurements demonstrate the surface modifications in the vesicles as a result of these interactions.

Nonlocal effects on the structural transition of gold clusters from planar to three-dimensional geometries.

Conventional density functional theory calculations heavily bias planar structures in gold clusters, failing to predict the structural transition from planar to three-dimensional geometries in experimentally detected gold species. Inspired by recent progress in calculating the defect energies of coinage metals with nonlocal effect-enhanced hybrid functionals, we have studied nonlocal effects in gold clusters. Although the hybrid functional was accurate for bulk gold, it heavily biased the planar structure for gold clusters. By including dispersive interactions into semilocal density functional calculations, we obtained an accurate vacancy formation energy of 0.72 eV for bulk gold along with the correct structural transition for gold clusters. The transition was found to occur at Au12− for gold anions and at Au8+ for gold cations, agreeing very well with the experimental results. For neutral gold clusters, we found the transition to occur at Au10, indicating the need for experimental verification. The results show the importance of nonlocal effects in the study of gold clusters, calling for further comprehensive theoretical and experimental studies to evaluate nonlocal effects in Au and other precious metals.

Abstraction of the I Atom from CH3I by Gas-Phase Au n - (n = 1-4) via Reductive Activation of the C-I Bond.

Gas-phase reactions of atomic gold anions and small gold cluster anions, Aun– (n = 1–4), with CH3I were investigated to clarify the effect of the cluster size on C–I bond activation and to elucidate the key properties of Au clusters that govern the reactivity. AunI– identified by mass spectrometry was observed as a common reaction product. Photoelectron spectroscopy and density functional theory calculations revealed that Au2I– has a linear structure in which the I atom is bonded to Au2, and Au3I– and Au4I– take a two-dimensional structure in which the I atom is bonded to triangular Au3 moieties. Pseudo-first-order kinetic analyses of the reaction revealed the inverse correlation of the reactivity of Aun– toward CH3I and the electron affinity of Aun, indicating the reductive activation of the C–I bond. Especially, Au2– showed the highest reactivity to form Au2I– as the main product, whereas the adduct compound Au2CH3I– was hardly formed, in sharp contrast to the reaction of Au– reported previously. On the basis of theoretical calculations, we propose that the reaction proceeded dominantly via the I atom abstraction pathway (attack of Au2– from the I atom side), which is highly preferential from the viewpoint of both the energetics and a steric factor. This study demonstrates that not only the reactivity but also the reaction mechanisms and products are governed by the cluster size in C–I bond activation by Au clusters at the smallest size region.

F‐bridged Anions of Bromine and Gold: Predictions of Unexpected Behavior

The significant similarity between MF3, MF4–, and M2F7– (M = Au, Br) is studied using quantum chemical methods. It is expected that compounds containing Au3F10– anions are likely to be stable. A theoretical background for the ongoing attempts of their synthesis is provided by calculations on the stabilities and molecular structures of various bromine and gold anions with the general composition of MnF3n+1– (n = 1, 2, 3, 4). The anions show unexpected differences and peculiarities in their energetically preferable molecular structures. Different chain‐like structures are predicted as the global minima for the hypothetical Au3F10– and Br4F13– anions.

Precise Modulation of Molecular Building Blocks from Tweezers to Rectangles for Recognition and Stimuli-Responsive Processes.

Alkynylplatinum(II) terpyridine complexes have been increasingly explored since the previous decades, mainly arising from their intriguing photophysical properties and aggregation affinities associated with their extensive Pt(II)···Pt(II) and π-π stacking interactions. Through molecular engineering, one can modulate their fundamental properties and assembly behavior by introduction of various functional groups and structural features. They can therefore serve as ideal candidates to construct metal complex-based molecular architectures to provide an alternative to organic compounds. The metal-based framework can be simultaneously built from predetermined building blocks, giving rise to their well-defined, unique, and discrete natures for molecular recognition. The individual constituents can contribute to molecular architectures with their integrated properties, allowing the manipulation of the various noncovalent intermolecular forces and interactions for selective guest capture. In this Account, our recent progress in the development of these metallomolecular frameworks based on the alkynylplatinum(II) terpyridine system and their recognition properties toward different guest molecules will be presented. Phosphorescent molecular tweezers have been constructed from the alkynylplatinum(II) terpyridine moiety to demonstrate host-guest interactions with cationic, charge-neutral and anionic platinum(II), palladium(II), gold(I), and gold(III) complexes and their binding affinities were found to be perturbed by different metal···metal, π-π and electrostatic interactions. The host-guest assembly process has also resulted in dramatic color changes, together with the turning on of near-IR (NIR) emissions as a result of extensive Pt(II)···Pt(II) interactions. Further work has also been performed to demonstrate that the tweezers can selectively recognize π-surfaces of different planar π-conjugated organic guests. The framework of molecular tweezers has been extended to a double-decker tweezers structure, or a triple-decker structure, which can bind two equivalents of square-planar platinum(II) guests cooperatively to induce a significant color change in solution, representing rare examples of discrete Magnus' green-like salts. By the approaches of structural modifications, we have further modulated the host architecture from molecular tweezers to molecular rectangles. The rectangles have been found to show selective encapsulation of different transition metal complex guests based on the size and steric environment of the host cavity. The molecular rectangles also exhibit reversible host-guest association, in which guest capture and ejection processes can be manipulated by the pH environment, illustrating a potential approach for precise and smart delivery of therapeutic reagents to the slightly more acidic cancer cells.

Evolution of Spin-Crossover Transition in Hybrid Crystals Involving Cationic Iron Complexes [Fe(III)(3-OMesal2-trien)]+ and Anionic Gold Bis(dithiolene) Complexes Au(dmit)2 and Au(dddt)2

Hybrid ion-pair crystals involving hexadentate [Fe(III)(3-OMesal2-trien)]+ spin-crossover (SCO) cationic complexes and anionic gold complexes [Au(dmit)2]− (1) (dmit = 4,5-dithiolato-1,3-dithiole-2-thione) and [Au(dddt)2]− (2) (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) were synthesized and studied by single-crystal X-ray diffraction, P-XRD, and SQUID magnetometry. Our study shows that both complexes have similar 1:1 stoichiometry but different symmetry and crystal packing. Complex 1 has a rigid structure in which the SCO cations are engaged in strong π-interplay with molecular surrounding and does not show SCO transition while 2 demonstrates a reversible transition at Tsco = 118 K in a much “softer”, hydrogen-bonded structure. A new structural indicator of spin state in [Fe(sal2-trien)]+ complexes based on conformational analysis has been proposed. Aging and thermocycling ruined the SCO transition increasing the residual HS fraction from 14 to 41%. Magnetic response of 1 is explained by the AFM coupled dimers S = 5/2 with J1 = −0.18 cm−1. Residual high-spin fraction of 2, apart from a contribution of the weak dimers with J12 = J34 = −0.29 cm−1, is characterized by a stronger interdimer coupling of J23 = −1.69 cm−1, which is discussed in terms of possible involvement of neutral radicals [Au(dddt)2].

Synthesis of an anionic Au(I) hydroamination precatalyst supported by charged hydrido-carboranyl phosphine ligands

Herein is reported the synthesis of an anionic gold complex bearing two charged hydrido carboranyl phosphine ligands. The compound is fully characterized by multinuclear NMR spectroscopy, high resolution mass spectrometry, as well as a single crystal X-ray diffraction study. In addition, we probe this compound’s ability to catalyze the hydroamination of alkynes and compare its behavior to a related highly active catalyst supported by a perchlorinated carboranyl phosphine.

Peripheral Membrane Proteins Facilitate Nanoparticle Binding at Lipid Bilayer Interfaces.

Molecular understanding of the impact of nanomaterials on cell membranes is critical for the prediction of effects that span environmental exposures to nanoenabled therapies. Experimental and computational studies employing phospholipid bilayers as model systems for membranes have yielded important insights but lack the biomolecular complexity of actual membranes. Here, we increase model membrane complexity by incorporating the peripheral membrane protein cytochrome c and studying the interactions of the resulting membrane systems with two types of anionic nanoparticles. Experimental and computational studies reveal that the extent of cytochrome c binding to supported lipid bilayers depends on anionic phospholipid number density and headgroup chemistry. Gold nanoparticles functionalized with short, anionic ligands or wrapped with an anionic polymer do not interact with silica-supported bilayers composed solely of phospholipids. Strikingly, when cytochrome c was bound to these bilayers, nanoparticles functionalized with short anionic ligands attached to model biomembranes in amounts proportional to the number of bound cytochrome c molecules. In contrast, anionic polymer-wrapped gold nanoparticles appeared to remove cytochrome c from supported lipid bilayers in a manner inversely proportional to the strength of cytochrome c binding to the bilayer; this reflects the removal of a weakly bound pool of cytochrome c, as suggested by molecular dynamics simulations. These results highlight the importance of the surface chemistry of both the nanoparticle and the membrane in predicting nano-bio interactions.

Ultrarapid Cationization of Gold Nanoparticles via a Single-Step Ligand Exchange Reaction.

We propose a novel method for the ultrarapid cationization of gold nanoparticles with diameters ranging from several to a hundred nanometers via a single-step ligand exchange reaction of citrate-protected anionic gold nanoparticles with cationic thiol ligand. This reaction was performed only in an aqueous medium via a single step from citrate to cationic thiol and therefore enables a rapid preparation of cationic Au nanoparticles without a contamination of organic solvent or lipid-soluble molecules. The cationization was successfully completed within 20 min without changes in the core diameter and optical characteristic.

Adsorption of O2 on Anionic Gold Clusters in the 0-1 nm Size Range: An Insight into the Electron Transfer Dynamics from Kinetic Measurements.

We systematically studied the adsorption of O2 on Au n- in the size range of 0-1 nm at low temperatures and determined new active sizes with n = 22, 24, 34, and 36. The kinetic measurements more clearly showed the correlation between the reactivity of Au n- with O2 and their electronic properties: the sizes with a closed electron shell are always inert, and the sizes with an unpaired electron can chemically adsorb one O2 molecule if their adiabatic detachment energies (ADEs) are lower than a threshold around 3.5 eV. This ADE threshold dividing the active and inert Au n- is independent of the clusters' sizes, global geometries, and local adsorption sites. According to the widely accepted electron transfer mechanism, this threshold could stand for the case in which the total energy of the Au n- and an O2 roughly equals that of the spin crossover point of the potential surfaces of Au n-O2- and Au n-···O2.

Functionalization of Fe<sub>3</sub>O<sub>4</sub>/SiO<sub>2</sub> with N-(2-Aminoethyl)-3-aminopropyl for Sorption of [AuCl<sub>4</sub>]<sup>-</sup>

Synthesis of Fe3O4/SiO2 modified with N-(2-aminoethyl)-3-aminopropyl group (Fe3O4/SiO2/ED) via coating method and its application for adsorption-desorption of anionic gold in aqueous solution have been conducted. The synthesized product was characterized with an X-ray diffractometer (XRD), a Fourier transform infrared (FT-IR) spectrophotometer and a transmission electron microscopy (TEM). Adsorption of Au(III) was conducted in a batch system and the variables included pH, contact time, and initial concentration were investigated. Results showed that magnetite/silica has been successfully functionalized with N-(2-aminoethyl)-3-aminopropyl in a homogeneous system. Kinetics study showed that adsorption of Au(III) followed the pseudo-second order model with rate constant of 0.710 g mmol L-1min-1. Furthermore, the experimental data fitted well with the Langmuir isotherm model with the maximum adsorption capacity for Au(III) of 142.9 mg g-1 and the energy of 25.0 kJ mol-1. Gold loaded on the Fe3O4/SiO2/ED could be easily desorbed with 0.2 mol L-1 HCl containing 2 wt.% of thiourea with recovery of 99.8%. Fe3O4/SiO2/ED was reusable and stable in 5 cycles of adsorption-desorption with recovery more than 90%. Fe3O4/SiO2/ED showed high selectivity towards Au(III) in the multimetal system Au(III)/Cu(II)/Cr(VI) with the coefficient selectivity for αAu-Cu of 227.5and for αAu-Cr of 12.3.