Small Gold Clusters Research Articles

Adsorption of O2 on the Preferred -O-Au Sites of Small Gold Oxide Clusters: Charge-dependent Interaction and Activation.

Decades of research have illuminated the significant roles of gold/gold oxide clusters in small molecule catalytic oxidation. However, many fundamental questions, such as the actual sites to adsorb and activate O2 and the impact of charge, remain unanswered. Here, we have utilized an improved genetic algorithm program coupled with the DFT method to systematically search for the structures of Au1-5Ox-/+/0 (x = 1-4) and calculated binding interactions between Au1-5Ox-/+/0 (x = 1-2) and O2, aiming to determine the active sites and to elucidate the impact of different charge states in gold oxide systems. The results revealed that the reactivity of all three kinds of small gold oxide clusters toward O2 is strongly site-dependent, with clusters featuring an -O-Au site exhibiting a preference for adsorption. The charges on small gold oxide clusters significantly impact the interaction strength and the activation degree of adsorbed O2: in the case of anionic cluster, the interaction between O2 and the -O-Au sites leads to a chemical reaction involving electron transfer, thereby significantly activating O2; in neutral and cationic clusters, the adsorption of O2 on their -O-Au sites can be viewed as an electrostatic interaction. Pointedly, for cationic clusters, the highly concentrated positive charge on the Au atom of the -O-Au sites can strongly adsorb but hardly activate the adsorbed O2. These results have certain reference points for understanding the gold oxide interfaces and the improved catalytic oxidation performance of gold-based systems in the presence of atomic oxygen species.

Orchestration of ferro- and anti-ferromagnetic ordering in gold nanoclusters.

The unpaired electron in the gold clusters (Aun, n = no. of Au atoms) with an odd number of total electrons is solely responsible for the magnetic properties in the small-sized Au nano-clusters. However, no such unpaired electron is available due to pairing in the even number of atom gold clusters and behaving as a diamagnetic entity similar to bulk gold. In this work, we unveiled the spin-density distribution of odd Aun clusters with n = 1 to 19 that reveals that a single unpaired electron gets distributed non-uniformly among all Au-atoms depending on the cluster size and morphology. The delocalization of the unpaired electron leads to the spin dilution approaching a value of ∼1/n spin moments on each atom for the higher clusters. Interestingly, small odd-numbered gold clusters possess spin-magnetic moments similar to the delocalized spin moments as of organic radicals. Can cooperative magnetic properties be obtained by coupling these individual magnetic gold nanoparticles? In this work, by applying state-of-the-art computational methodologies, we have demonstrated ferromagnetic or anti-ferromagnetic couplings between such magnetic nanoclusters upon designing suitable organic spacers. These findings will open up a new avenue of nanoscale magnetic materials combining organic spacers and odd-electron nano-clusters.

The cyanidation of sub microscopic gold: An experimental and molecular modeling study

Sub microscopic refractory gold ore is an important resource of gold with the depletion of free-milling gold deposits in this century. It has been generally accepted that submicroscopic gold mainly in the form of solid-solution and colloidal particles are physically locked inside mineral phases and impervious to cyanide solution. However, some researches on synthetic nanoparticles show they have very different physical and chemical properties which can affect their dissolution behavior.This research has been conducted to investigate the dissolution behavior of gold sub microscopic particles experimentally and theoretically using density functional theory. For this purpose, 20 nm gold particles have been synthesized and dissolved in cyanide solution under controlled condition. The rate of dissolution has been monitored during cyanidation and compared with a gold disk. The results depict the rate of dissolution per surface area is significantly reduced in gold nanoparticles (30 times, the surface ratio of 14/9 m2/g and 0.006 m2/g in samples is 2500 times smaller while the dissolution rate in nanoparticles is 83 times faster). Also, the smaller nanoparticles have much slower dissolution kinetics than large nanoparticles (5.485 × 10−5 and 4.280 × 10−6 g/m2.s for 16 and 2 nm particles, respectively). Two different dissolution regimes can be identified for >3 nm and < 3 nm nanoparticles. In larger particles, by increasing the number of surface atoms with weaker bond energy and also lower oxidation potential, the dissolution rate increases compared to bulk gold. In nanoparticles smaller than 3 nm, the electrochemical reaction in the cyanide medium is disrupted by the reduction of oxidation potential and decreasing the conductivity as the requirement for electrochemical dissolution of gold in cyanide.The molecular modeling study on the gold clusters of 1 to 20 atom confirm that the oxidation of small clusters of gold is more difficult than bulk gold due to the high ionization energy (9.66 vs. 5.4 eV). Also, the monoatomic gold has a much weaker interaction with cyanide (−4.36 vs. -1.6 eV) and higher interaction with oxygen (−0.64 vs. -0.15 eV) compared to the bulk gold. The HOMO of monoatomic and bulk gold (−9.22 and − 4.5 eV) as well as LUMO of cyanide and oxygen (1.734 and − 4.657 eV), as two reactants in cyanidation process confirm the results based on HSAB theory. As a result, oxygen competes with cyanide for adsorption on the surface of gold nanoparticles, disrupting the first step of electrochemical dissolution of gold, as the adsorption of cyanide and the formation of the Au-CN− complex. This is possible by deactivating the surface by creating a resistant layer of gold oxide or electrostatic adsorption of oxygen on the gold surface and passivating the surface. The modeling data are very well in agreement with the experimental results indicating the inherent inactivity of free surface gold sub microscopic particles during the cyanidation process.

Theoretical study of the binding mechanism between anticancerous drug mercaptopurine and gold nanoparticles using a cluster model.

Mercaptopurine is an effective anticancer medicine yet known with serious adverse reactions, thus requiring further attempts to enhance its biological targeting. Small gold clusters Aun (n = 2-10) were used as model reactants to simulate the surface of gold nanoparticles. The computed results show that the drug molecules tend to anchor on the gold clusters at the S atom with the associated binding energies varying from -50 to -34 kcal mol-1 (in vacuum) and from -42 to -28 kcal mol-1 (in aqueous solution). Furthermore, the adsorption of the drug onto the gold surface is considered as a reversible process, and the mechanism of drug releasing was found to be triggerable by internal factors, such as a pH change or the concentrated presence of thiol amino acids in cancerous protein structures. Calculations based on density functional theory (DFT) were performed to probe the nature of interactions between the drug and gold nanoparticles. Structural features, thermodynamic parameters, bonding characteristics, and electronic properties of the resulting complexes were investigated at the PBE//cc-pVTZ/cc-pVDZ-PP level.

Gold cluster encapsulated liposomes: theranostic agent with stimulus triggered release capability

Cancer is a major cause of death worldwide. Cancer-resistant to chemo or radiotherapy treatment is a challenge that could be overcome by a nanotechnology approach. Providing a theranostic nano-platform for different cancer treatment strategies could be revolutionary. Here we introduce a multifunctional theranostic nanostructure which has the capacity for improving cancer diagnosis and treatment through better chemo and radiotherapy and current x-ray imaging systems through co-encapsulation of a small gold cluster and anticancer drug doxorubicin. 2nm gold clusters represent good heating under radio frequency electric field (RF-EF) exposure and have been used for in vitro hyperthermia treatment of cancerous cells. Liposomal doxorubicin (169 ± 19.8nm) with gold clusters encapsulation efficiency of 13.2 ± 3.0% and doxorubicin encapsulation efficiency of 64.7 ± 0.7% were prepared and studied as a theranostic agent with a high potential in different cancer treatment modalities. Exposure to a radiofrequency electric field on prepared formulation caused 20.2 ± 2.1% drug release and twice decreasing of IC50 on colorectal carcinoma cells. X-ray attenuation efficiency of the liposomal gold cluster was better than commercial iohexol and free gold clusters in different concentrations. Finally, treatment of gold clusters on cancerous cells results in a significant decrease in the viability of irradiated cells to cobalt-60 beam. Based on these experiments, we concluded that the conventional liposomal formulation of doxorubicin that has been co-encapsulated with small gold clusters could be a suitable theranostic nanostructure for cancer treatment and merits further investigation.

Solvent field regulated superhalogen in pure and doped gold cluster anions

Condensed-phase synthesis of atomically precise clusters has become a vital branch of cluster science, where solvents are indispensable in the synthesis process. Herein, by employing the density functional theory (DFT) calculations and molecular dynamics (MD) simulations, we demonstrated that polar solvents not only provide an important environment to stabilize clusters, but they can also dramatically alter the electronic property of cluster anions forming novel superhalogen anions. Such a regulation effect was first verified in small model gas-phase pure and doped gold cluster anions, which was further evidenced in a real experimentally synthesized Au18 nanocluster. Different solvation models reveal that the solvent field, which is a noninvasive methodology different from conventional electron-counting rules, can be considered as a novel external field to remarkably increase the electron-binding capability of cluster anions while maintaining their geometrical and electronic structures. Considering the indispensability and convenient availability of the solvents, present findings may boost the potential applications of superatoms in constructing super oxidizers in the condensed phase.

Atomistic modeling of thermal effects in focused electron beam-induced deposition of Me$$_2$$Au(tfac)

The role of thermal effects in the focused electron beam induced deposition (FEBID) process of Me$_2$Au(tfac) is studied by means of irradiation-driven molecular dynamics simulations. The FEBID of Me$_2$Au(tfac), a commonly used precursor molecule for the fabrication of gold nanostructures, is simulated at different temperatures in the range of $300-450$ K. The deposit's structure, morphology, growth rate, and elemental composition at different temperatures are analyzed. The fragmentation cross section for Me$_2$Au(tfac) is evaluated on the basis of the cross sections for structurally similar molecules. Different fragmentation channels involving the dissociative ionization (DI) and dissociative electron attachment (DEA) mechanisms are considered. The conducted simulations of FEBID confirm experimental observations that deposits consist of small gold clusters embedded into a carbon-rich organic matrix. The simulation results indicate that accounting for both DEA- and DI-induced fragmentation of all the covalent bonds in Me$_2$Au(tfac) and increasing the amount of energy transferred to the system upon fragmentation increase the concentration of gold in the deposit. The simulations predict an increase in Au:C ratio in the deposit from 0.18 to 0.25 upon the temperature increase from 300 K to 450 K, being within the range of experimentally reported values.

Binding mechanism and SERS spectra of 5-fluorouracil on gold clusters.

The adsorption behaviour of the 5-fluorouracil (5FU) on small gold clusters Au N with N = 6, 8, 20 was evaluated by means of density functional theory using the PBE-D3 functional in combination with a mixed basis set, i.e. cc-pVDZ-PP for gold atoms and cc-pVTZ for non-metal elements. The binding energies between 5FU and gold clusters were determined in the range of 16-24 and 11-19kcal/mol in gas-phase and aqueous media, respectively. The corresponding Gibbs energies were found to be around -7 to -10kcal/mol in vacum and sigificantly reduced to -1 to -6kcal/mol in water solution, indicating that both the association and dissociation processes are likely spontaneous. An analysis on the charge density difference tends to confirm the existence of a charge transfer from the 5FU molecule to Au atoms. Analysis of the surface-enhanced Raman scattering (SERS) spectra of 5FU adsorbed on the Au surfaces shows that the stretching vibrations of N-H and C=O bonds play a major role in the SERS phenomenon. A mechanism for the drug releasing from the gold surfaces is also proposed. The process is triggered by either the low pH in cancerous tumors or the presence of cysteine residues in protein matrices.

The Au12 Gold Cluster: Preference for a Non-Planar Structure

The transition point from a two-dimensional (2D) to a three-dimensional (3D) structure in a series of small gold clusters remains a topic of continuing debate. In the present study, coupled-cluster CCSD(T) and DFT calculations are performed to re-examine the relative energies of several low-lying isomers of Au12, aiming to shed new light on this issue. At odds with many previous reports on the preference of a planar di-capped elongated-hexagon structure, the Au12 size is found to energetically prefer a globular cup-like form with C2v symmetry. While DFT results are not able to assign the most stable form of Au12 as the relative energies between the lowest-lying isomers are strongly functional-dependent, coupled-cluster theory calculations point out the preference of a 3D structure for having a D3h symmetry. Such a prediction is further supported by a comparison of the vibrational spectra computed using the revTPSS density functional with the available experimental infrared ones that were previously recorded from the far-IR multiple photon dissociation (FIR-MPD) experiment.

Kinetics of Propylene Epoxidation over Extracrystalline Gold Active Sites on AU/TS-1 Catalysts

To determine whether the catalytic roles of extracrystalline and intracrystalline gold nanoparticles supported on titanosilicate-1 (TS-1) for direct propylene epoxidation are intrinsically different, the kinetics of direct propylene epoxidation were measured in a gas-phase continuous stirred tank reactor (CSTR) over polyvinylpyrrolidone-coated (PVP) gold nanoparticles (Au-PVP/TS-1) deposited on TS-1. The as-made PVP-coated gold nanoparticles were too large to fit into the micropores of TS-1, even after ligands were removed in situ by a series of pretreatments, as confirmed by both TEM and TGA-DSC. The activation energy (51 kJ mol–1) and reaction orders for H2 (1.3), O2 (0.4), propylene (0.4), propylene oxide (−0.6), carbon dioxide (0), and water (0) for propylene epoxidation measured on Au-PVP/TS-1 were consistent with those reported for Au/TS-1 prepared via deposition-precipitation (Au-DP/TS-1) (52 kJ mol–1, H2, 1; O2, 0.4; C3H6, 0.4; C3H6O, −0.6; CO2, 0; H2O, 0). However, while the reaction orders for hydrogen oxidation on Au-PVP/TS-1 (H2, 0.8; O2, 0; C3H6, −0.3; C3H6O, −0.1; CO2, 0; H2O, −0.2) were similar to those measured on Au-DP/TS-1 (H2, 0.9; O2, 0.3; C3H6, −0.3; C3H6O, 0; CO2, 0; H2O, −0.1), a decrease in activation energy from approximately 30 kJ mol–1 for Au-DP/TS-1 to 4–5 kJ mol–1 for Au-PVP/TS-1 suggests there is a change in rate-limiting step or active site for hydrogen oxidation. Additionally, an active site model was developed which determines the number of Ti within an interaction range of the perimeter of extracrystalline Au nanoparticles (i.e., the number of Au–Ti active site pairs). This model was used to estimate catalytic turnover frequencies over solely proximal Au–Ti pairs, assuming that hydrogen peroxide does not desorb and migrate from Au sites to Ti sites and instead propylene oxide forms in a concerted mechanism previously termed the “simultaneous” mechanism. Turnover frequencies estimated for this active site model for a data set containing both Au-DP/TS-1 and Au-PVP/TS-1 were ∼20× higher than the highest previous reported estimates (∼80 s–1 vs 1–5 s–1 at 473 K) for catalytic oxidation on noble metals, suggesting that the simultaneous mechanism occurring over proximal Au–Ti sites alone is incapable of explaining the observed rate of propylene epoxidation and that short-range migration of hydrogen peroxide is kinetically relevant. The agreement of reaction orders, activation energy, and active site model for propylene epoxidation on both Au-DP/TS-1 and Au-PVP/TS-1 suggests a common mechanism for propylene epoxidation on both catalysts containing small intraporous gold clusters and catalysts with exclusively larger extracrystalline gold nanoparticles. Rates of hydrogen oxidation were found to vary proportionally to the amount of surface gold atoms. This is also consistent with the hypothesis that the observed decrease in hydrogen efficiency and PO site-time-yield per gold mass with increasing gold loading are driven primarily by the gold dispersion in Au/TS-1 catalysts.

Solvation of small gold clusters in supercritical water

Solvation in supercritical water features fascinating properties such as the selective dissolution or precipitation of non-polar versus polar species. This is known since long to open up astonishing possibilities in organic synthesis, catalysis, or engineering. Less known is the role that supercritical aqueous solutions play close to hydrothermal vents in the deep sea and in ore deposit formation in Earth’s crust. Within this realm, we explore here the solvation of small uncharged gold species, namely the gold dimer and tetramer, in supercritical water in terms of structural and electronic properties using ab initio molecular dynamics simulations. Our simulations reveal that a highly anisotropic first solvation shell is formed around these gold clusters which consists of less than two water molecules on average. Importantly, we find very significant charge transfer and polarization effects between these water molecules and the neutral gold particles, which cannot be captured when using non-polarizable force fields. These electronic many-body effects lead not only to enormous dipole moment fluctuations of the gold clusters themselves, but also to an unexpectedly strong increase of the average effective dipole moments in their first solvation shell. Thus, supercritical solutions of gold particles in water turn out to be surprisingly complex.

Bonding Nature between Noble Gases and Small Gold Clusters.

Noble gases are usually seen as utterly inert, likewise gold, which is typically conceived as the noblest of all metals. While one may expect that noble gases bind to gold via dispersion interactions only, strong bonds can be formed between noble gas atoms and small gold clusters. We combine mass spectrometry, infrared spectroscopy, and density functional theory calculations to address the bonding nature between Aun+ (n ≤ 4) clusters and Ar, Kr, and Xe. We unambiguously determine the geometries and quantitatively uncover the bonding nature in AunNgm+ (Ng = Ar, Kr, Xe) complexes. Each Au cluster can form covalent bonds with atop bound noble gas atoms, with strengths that increase with the noble gas atomic radius. This is demonstrated by calculated adsorption energies, Bader electron charges, and analysis of the electron density. The covalent bonding character, however, is limited to the atop-coordinated Ng atoms.

Isomer dynamics of the [Au6(NHC-S)4]2+ nanocluster.

The use of metal nanoclusters is strongly reliant on their size and configuration; hence, studying the potential isomers of a cluster is extremely beneficial in understanding their performance. In general, the prediction and identification of isomer structures and their properties can be challenging and computationally expensive. Our work describes an investigation to find local isomers for the previously experimentally characterized small gold cluster [Au6(NHC-S)4]2+ protected by bidentate mixed carbene-thiolate ligands. We employ the molecular dynamics simulation method where the interatomic forces are calculated from density functional theory. We find several isomers that are more stable than the isomer corresponding to the experimental crystal structure, as well as a significant impact of the finite-temperature atom dynamics on the electronic structure and optical properties. Our work highlights the growing need to investigate ligand-stabilized metal clusters to uncover isomerism and temperature effects on their properties.

Exploring the Effects of a Doping Silver Atom on Anionic Gold Clusters' Reactivity with O2.

Reactivities of AgAun-1- (n = 3-10) with O2 at a low temperature were studied using an instrument combining a magnetron sputter cluster source, a microflow reactor, and a time-of-flight mass spectrometer. Their reaction products as well as size-dependent kinetic rates were nearly identical to those of corresponding Aun- (n = 3-10). Previous experiments showed that the Ag atom in AgAun-1- (n = 3-10) was fully or partially enclosed by the gold atoms. We studied the adsorption of O2 on these reported structures using the B3LYP theory with relatively large basis sets. The theoretical results indicate that the adsorption sites as well as the adsorption energies of O2 on AgAun-1- (n = 3-10) are nearly identical to those on the corresponding Aun- (n = 3-10). The O2 adsorption on a series of proposed isomers of AgAun-1- (denoted as Aun-1Ag-), in which the silver atom was on the protruding site, was explored using the same theoretical methods. The O2 tends to bond with the protruding Ag atoms, and the binding energies are apparently higher than those on the corresponding Aun- and AgAun-1-. The adsorption and activation of O2 on Aun-, AgAun-1-, and Aun-1Ag- were correlated with their global electron detachment energies (VDEs) as well as the element types of the adsorption sites. Generally, low VDE values and silver sites facilitate the O2 adsorption, and these two factors separately dominate in various cluster species. The revealed effects of a doping silver atom in small gold clusters are helpful to understand the role of the residual silver components in many nano gold catalysts.

The Diffusion and Clustering Formation of Gold Atoms on Alpha-Graphyne

Motivated by the experiment, high mobility of gold atoms on two-dimensional carbon sheets, we examine the ground-state structures, mobility, and clustering formations of small gold clusters (Au_n, n = 1-4) on monolayer alpha-graphyne using first-principles DFT calculations and finite temperature MD simulations. We reveal that Au_n cluster prefers to locate at the center of a hexagon in alpha-graphyne. The binding energy of Au_n on alpha-graphyne increases with increasing the number (n) of gold atoms. Moreover, we predict the step-wise formation of Au_2 out of two pre-adsorbed Au_1 ad-atoms. Likewise, the formation of Au_3 and Au_4 is also considered in the same way. The diffusion energy barrier of Au_1 on alpha-graphyne is found to be only 0.26 eV, indicating the high mobility of gold atoms on alpha-graphyne. Remarkably, the energy required for the cluster formation of gold atoms on alpha-graphyne is about less than 0.2 eV. According to our MD simulations at room temperature (RT), the Au_n cluster is subsequently formed on alpha-graphyne. Considering the high mobility of a single gold atom, the strong binding energy of small gold clusters, and the easy clustering of Au_n at RT on alpha-graphyne, we suggest that alpha-graphyne is a suitable substrate for gold cluster formation.

Theoretical study of Au20/WS2 composite material as a potential candidate for the capture of XO (X=C, N, S) gases

In this study, density functional theory (DFT) calculations were carried out to understand the structural stability of the interaction of Au20 pyramidal cluster on the tungsten disulfide (WS2) monolayer. According to the adsorption energy values, the predominant interaction in this system (Au20/WS2) corresponds to chemisorption (−4.68 eV). This behavior is also reproduced by small gold clusters (Aun, n = 3, 5, 9, 13 and 16). The Au20/WS2 composite system presents a mechanism of interaction influenced by the charge transfer from the Au20 cluster to the WS2 monolayer. The structural stability was corroborated by ab-initio molecular dynamics (AIMD) calculations, which exhibits how the WS2 monolayer is a good support for transition metals such as Au clusters. According to the analysis of the electronic and optical properties, the Au20/WS2 composite system has a semi-metallic behavior, based on to the band spectrum (0.65 eV). The optical properties of the Au20/WS2 composite system exhibits a broad absorption spectrum of visible light, in contrast to the isolated Au20 cluster and the WS2 monolayer. The performance of this composite material in the adsorption process of the XO (X = C, N, and S) molecule was also evaluated. The interaction between the XO molecule and the Au atoms shows a favorable formation of bonding with a van der Waals (vdW) type interaction, according to the critical points analysis. Our results show high adsorption energy values in all active sites of the Au20 cluster. This kind of composite system provide a structural idea, about the use of the WS2 monolayer as a support of transition metals at the nanoscale level for the capture of polluting gases.

Spectral tuning of 11-cis retinal in conjugation with Au14 cluster and concomitant effect on isomerization: A theoretical outlook

The present study explores the optical properties of retinal molecules in presence of small gold cluster (Au 14 ), where the cluster motifs are varied through O h , D 2h and C 4v symmetries. The constructive interference of the transition dipole moments (TDMs) in retinal-gold nanoconjugates is responsible for most intense electronic transitions. The charge transfer transition from the occupied Au cluster orbital to the unoccupied retinal orbital is playing significant role in the spectral shift of the conjugates. The vibrational frequency analysis of 11-cis retinal and its conjugate (Au 14 in D 2h symmetry) reveal that the different vibrational modes associated with the isomerization process are red shifted in the conjugate. The conjugation of isolated retinal with gold probes makes the process of 11-cis to all-trans isomerization thermodynamically favorable. The entire evaluation reflects the effect of gold cluster in the spectral tuning of retinal molecules and reveals its relevant consequences on photoisomerization and hence the process of vision.

Double σ-Aromaticity in a Planar Zinc-Doped Gold Cluster: Au9Zn.

The strong relativistic effects result in many interesting chemical and physical properties for gold and gold compounds. One of the most surprising findings has been that small gold clusters prefer planar structures. Dopants can be used to tune the electronic and structural properties of gold nanoclusters. Here we report an experimental and theoretical investigation of a Zn-doped gold cluster, Au9Zn-. Photoelectron spectroscopy reveals that Au9Zn- is a highly stable electronic system with an electron binding energy of 4.27 eV. Quantum chemical studies show that the global minimum of Au9Zn- has a D3h structure with a closed-shell electron configuration (1A1'), which can be viewed as replacing the central Au atom by Zn in the open-shell parent Au10- cluster. The high electronic stability of Au9Zn- is corroborated by its extremely large HOMO-LUMO gap of 3.3 eV. Chemical bonding analyses revealed that the D3h Au9Zn- are bonded by two sets of delocalized σ bonds, giving rise to double σ aromaticity and its remarkable stability. Two planar low-lying isomers are also observed, corresponding to a similar triangular structure with the Zn atom on the edge and another one with one of the corner Au atoms moved to the edge of the triangle.

Optical Properties and Surface Morphologies of Silica–Gold Nanoshells Depending on Buffer Solutions and Reducing Agents

Silica–gold nanoshells (SGNSs) were prepared at different concentrations of reducible gold salts (K+AuCl4−) with a toxic formaldehyde (FAH) reducing agent. FAH-induced SGNSs (F-SGNSs) showed not only a distinct red-shift of the plasmon resonance peak but also an increase in the thickness of the gold shell in proportion to the concentration of gold salts (0.38–1.90 mM). However, the excessive addition of reducible gold salts resulted in agglomeration of the SGNSs. The F-SGNSs showed enhanced colloidal stability after the addition of buffer solutions containing different oxyanions in the following order: K2CO3 > citrate > citrate + PBS > PBS. In addition, biocompatible ascorbic acid (ASA) was used to avoid the problem of toxic FAH agents for in vivo applications. The ASA-induced SGNSs showed a smoother surface morphology and more red-shift of the surface plasmon resonance band in comparison to F-SGNSs, probably because of the coalescence of small gold clusters formed under the mild reducing power of ASA. The surface morphologies of the SGNSs and their optical properties were characterized by transmission electron microscopy, scanning electron microscopy, zeta-potential measurement, and UV–visible spectroscopy.

QM study of interaction between arginine amino acid and Au clusters and the effects on arginine acidity

Understanding the various structures of gold clusters and the interaction modes between Au clusters and biomolecules is an important issue in material science such as biosensors and catalysts. The binding of small gold clusters (Aun n = 2–5) with neutral and anionic forms of arginine (Arg) amino acid is investigated in this study using density functional theory (DFT) and B3LYP level. The relative stability among different forms of Au clusters including linear, zigzag, planar, and three-dimensional Au clusters was estimated, initially. The calculated findings show that the zigzag structure for Au3 and the planar structure for Au4 and Au5 are the best form. Furthermore, the different modes of interaction were taken into account from thermodynamic view point between the most stable conformers of Arg and Arg− with gold clusters. Finally, the arginine is considered as a weak organic acid to investigate the impact of Au clusters on the gas phase acidity. The acidity of isolated arginine and the acidity of [Aun/Arg] complexes were also compared. Based on the obtained results, upon the complexation with Au clusters at 298 K, for the interaction of Au, Au2, Au3, Au4, and Au5 clusters with arginine, the gas phase acidity (GPA) of arginine alters from 342.12 to 314.17, 303.04, 299.42, 303.41, and 331.66 kcal/mol respectively. These calculated values predict that when a weak organic acid is complexed with Au clusters, it will be altered to super acid. Furthermore, for isolated and complexed species of Arg, pKa values were evaluated in water solvent.