Gold surfaces and nanoparticles are protected by Au(0)-thiyl species and are destroyed when Au(I)-thiolates form

Dual-functional gold nanoparticles with antimicrobial and proangiogenic activities improve the healing of multidrug-resistant bacteria-infected wounds in diabetic mice

Gold nanoparticles (Au NPs) are conjugated with the vascular endothelial growth factor-A₁₆₅ (VEGF-A₁₆₅) and (11-mercaptoundecyl)-N,N,N-trimethylammonium (11-MTA) cation to form dual-functional gold nanoparticles (11-MTA/VEGF-Au NPs) that possess antimicrobial and proangiogenic activities for wound healing in diabetic (db/db) mice. VEGF-A₁₆₅ is a popular proangiogenic growth factor that stimulates multiple components in the wound-healing cascade. On the other hand, 11-MTA possesses antibacterial activity and can be bound to Au NPs easily through Au-S bonding. We have found that the surface density of VEGF-A₁₆₅ plays a vital role in promoting the proliferation, migration, and tube formation of human umbilical vein endothelial cells. 11-MTA tethered on the VEGF-modified Au NPs enables the nanocomposites (i.e., 11-MTA/VEGF-Au NPs) to exhibit a strong antimicrobial activity against multidrug-resistant bacteria [methicillin-resistant S. aureus (MRSA)]. The minimal inhibition concentration of 11-MTA/VEGF-Au NPs is ∼450-fold lower than that of 11-MTA, revealing their high antibacterial efficiency. 11-MTA/VEGF-Au NPs exhibit high biocompatibility. 11-MTA/VEGF-Au NPs as dressing materials to treat MRSA-infected wounds in diabetic mice not only show strong in vivo bactericidal activities but also enhance the healing process of the formation of collagen fibers and epithelialization. Our results show that dual-functional 11-MTA/VEGF-Au NPs are promising agents for clinical applications like treating chronic wound infections.

Controlling the stereospecific bonding motif of Au-thiolate links

Over the last decades, organosulfur compounds at the interface of noble metals have proved to be extremely versatile systems for both fundamental and applied research. However, the anchoring of thiols to gold remained an object of controversy for a long time. The RS-Au-SR linkage, in particular, is a robust bonding configuration that displays interesting properties. It is generated spontaneously at room temperature and can be used for the production of extended molecular nanostructures. In this work we explore the behavior of 1,4-bis(4-mercaptophenyl)benzene (BMB) on the Au(111) surface, which results in the formation of 2D crystalline metal-organic assemblies stabilized by this type of Au-thiolate bonds. We show how to control the thiolate's stereospecific bonding motif and thereby choose whether to form ordered arrays of Au₃BMB₃ units with embedded triangular nanopores or linearly stacked metal-organic chains. The former turn out to be thermodynamically favored structures and display confinement of the underneath Au(111) surface state. The electronic properties of single molecules as well as of the 2D crystalline self-assemblies have been characterized both on the metal-organic backbone and inside the associated pores.

Photoassisted Electrochemical Micropatterning of Gold Film

Electrochemical etching is a powerful and popular method for fabricating micropatterns on metal substrates for use in electronic devices, electrochemical sensors, and plasmonic substrates. In order to achieve micropatterning, either a prepatterned insulating layer (mask) or a scanning microelectrode is often required to selectively trigger electrochemical etching at the desired locations. In the present work, we employed a well-focused light beam to enable the photoassisted electrochemical etching of gold film with a spatial resolution close to the optical diffraction limit (∼300 nm). It was found that the simultaneous application of light irradiation and appropriate potential were critical for the oxidative dissolution (i.e., etching) of gold to occur. Superior controllability of light beam allowed for the direct-write micropatterning without the need of mask or probe. Etching kinetics and mechanism were also studied by monitoring the dynamic evolution of optical transparency with a conventional transmission bright-field microscope, together with characterizations on the as-obtained patterns with atomic force microscopy and electron microscopy. This study is anticipated to contribute a feasible method for the micropatterning of gold film with implications for nanoelectronics and electrochemical sensors.

Transition from stochastic events to deterministic ensemble average in electron transfer reactions revealed by single-molecule conductance measurement

Electron transfer reactions can now be followed at the single-molecule level, but the connection between the microscopic and macroscopic data remains to be understood. By monitoring the conductance of a single molecule, we show that the individual electron transfer reaction events are stochastic and manifested as large conductance fluctuations. The fluctuation probability follows first-order kinetics with potential dependent rate constants described by the Butler-Volmer relation. Ensemble averaging of many individual reaction events leads to a deterministic dependence of the conductance on the external electrochemical potential that follows the Nernst equation. This study discloses a systematic transition from stochastic kinetics of individual reaction events to deterministic thermodynamics of ensemble averages and provides insights into electron transfer processes of small systems, consisting of a single molecule or a small number of molecules.

Intracellular speciation of gold nanorods alters the conformational dynamics of genomic DNA

Gold nanorods are one of the most widely explored inorganic materials in nanomedicine for diagnostics, therapeutics and sensing¹. It has been shown that gold nanorods are not cytotoxic and localize within cytoplasmic vesicles following endocytosis, with no nuclear localization^2,3, but other studies have reported alterations in gene expression profiles in cells following exposure to gold nanorods, via unknown mechanisms⁴. In this work we describe a pathway that can contribute to this phenomenon. By mapping the intracellular chemical speciation process of gold nanorods, we show that the commonly used Au-thiol conjugation, which is important for maintaining the noble (inert) properties of gold nanostructures, is altered following endocytosis, resulting in the formation of Au(I)-thiolates that localize in the nucleus⁵. Furthermore, we show that nuclear localization of the gold species perturbs the dynamic microenvironment within the nucleus and triggers alteration of gene expression in human cells. We demonstrate this using quantitative visualization of ubiquitous DNA G-quadruplex structures, which are sensitive to ionic imbalances, as an indicator of the formation of structural alterations in genomic DNA.

Cluster Core Isomerism Induced by Crystal Packing Effects in the [HCo15Pd9C3(CO)38]2- Molecular Nanocluster

This article describes a rare case of cluster core isomerism in a large molecular organometallic nanocluster. In particular, two isomers of the [HCo15Pd9C3(CO)38]2- nanocluster, referred as TP-Pd9 and Oh-Pd9, have been structurally characterized by single-crystal X-ray crystallography as their [NMe3(CH2Ph)]2[HCo15Pd9C3(CO)38]·CH2Cl2 (ca. 1:1 TP-Pd9 and Oh-Pd9 mixture), [NMe3(CH2Ph)]2[HCo15Pd9C3(CO)38]·2CH2Cl2 (mainly TP-Pd9), [NEt3(CH2Ph)]2[HCo15Pd9C3(CO)38]·CH2Cl2 (mainly TP-Pd9), [MePPh3]2[HCo15Pd9C3(CO)38]·2.5CH2Cl2 (mainly TP-Pd9), and [MePPh3]2[HCo15Pd9C3(CO)38] (Oh-Pd9) salts. The cluster core of TP-Pd9 is a tricapped trigonal prism, whereas this is a tricapped octahedron in Oh-Pd9. The presence in the solid state of the Oh-Pd9 or TP-Pd9 isomers depends on the cation employed and/or the number and type of co-crystallized solvent molecules. Often, mixtures of the two isomers, within the same single crystal or as mixtures of different crystals within the same crystallization batch, are obtained. Structural isomerism in organometallic nanoclusters is discussed and compared to that in Au-thiolate nanoclusters.

Faraday cage screening reveals intrinsic aspects of the van der Waals attraction

General properties of the recently observed screening of the van der Waals (vdW) attraction between a silica substrate and silica tip by insertion of graphene are predicted using basic theory and first-principles calculations. Results are then focused on possible practical applications, as well as an understanding of the nature of vdW attraction, considering recent discoveries showing it competing against covalent and ionic bonding. The traditional view of the vdW attraction as arising from pairwise-additive London dispersion forces is considered using Grimme's "D3" method, comparing results to those from Tkatchenko's more general many-body dispersion (MBD) approach, all interpreted in terms of Dobson's general dispersion framework. Encompassing the experimental results, MBD screening of the vdW force between two silica bilayers is shown to scale up to medium separations as 1.25 de/d, where d is the bilayer separation and de is its equilibrium value, depicting antiscreening approaching and inside de Means of unifying this correlation effect with those included in modern density functionals are urgently required.

van der Waals forces control ferroelectric-antiferroelectric ordering in CuInP2S6 and CuBiP2Se6 laminar materials

We show how van der Waals (vdW) forces outcompete covalent and ionic forces to control ferroelectric ordering in CuInP2S6 nanoflakes as well as in CuInP2S6 and CuBiP2Se6 crystals. While the self-assembly of these 2D layered materials is clearly controlled by vdW effects, this result indicates that the internal layer structure is also similarly controlled. Using up to 14 first-principles computational methods, we predict that the bilayers of both materials should be antiferroelectric. However, antiferroelectric nanoflakes and bulk materials are shown to embody two fundamentally different types of inter-layer interactions, with vdW forces strongly favouring one and strongly disfavouring the other compared to ferroelectric ordering. Strong specific vdW interactions involving the Cu atoms control this effect. Thickness-dependent significant cancellation of these two large opposing vdW contributions results in a small net effect that interacts with weak ionic contributions to control ferroelectric ordering.

Photosensitiser-gold nanoparticle conjugates for photodynamic therapy of cancer

Gold nanoparticles (AuNPs) have been extensively studied within biomedicine due to their biocompatibility and low toxicity. In particular, AuNPs have been widely used to deliver photosensitiser agents for photodynamic therapy (PDT) of cancer. Here we review the state-of-the-art for the functionalisation of the gold nanoparticle surface with both photosensitisers and targeting ligands for the active targeting of cancer cell surface receptors. From the initial use of the AuNPs as a simple carrier of the photosensitiser for PDT, the field has significantly advanced to include: the use of PEGylated modification to provide aqueous compatibility and stealth properties for in vivo use; gold metal-surface enhanced singlet oxygen generation; functionalisation of the AuNP surface with biological ligands to specifically target over-expressed receptors on the surface of cancer cells and; the creation of nanorods and nanostars to enable combined PDT and photothermal therapies. These versatile AuNPs have significantly enhanced the efficacy of traditional photosensitisers for both in vitro and in vivo cancer therapy. From this review it is apparent that AuNPs have an important future in the treatment of cancer.

Cyanobacteria as Nanogold Factories: Chemical and Anti-Myocardial Infarction Properties of Gold Nanoparticles Synthesized by Lyngbya majuscula

To the best of our knowledge, cyanobacterial strains from the Arabian Gulf have never been investigated with respect to their potential for nanoparticle production. Lyngbya majuscula was isolated from the AlOqair area, Al-Ahsa Government, Eastern Province, Kingdom of Saudi Arabia. The cyanobacterium was initially incubated with 1500 mg/mL of HAuCl₄ for two days. The blue-green strain turned purple, which indicated the intracellular formation of gold nanoparticles. Prolonged incubation for over two months triggered the extracellular production of nanogold particles. UV-visible spectroscopy measurements indicated the presence of a resonance plasmon band at ~535 nm, whereas electron microscopy scanning indicated the presence of gold nanoparticles with an average diameter of 41.7 ± 0.2 nm. The antioxidant and anti-myocardial infarction activities of the cyanobacterial extract, the gold nanoparticle solution, and a combination of both were investigated in animal models. Isoproterenol (100 mg/kg, SC (sub cutaneous)) was injected into experimental rats for three days to induce a state of myocardial infarction; then the animals were given cyanobacterial extract (200 mg/kg/day, IP (intra peritoneal)), gold nanoparticles (200 mg/kg/day, IP), ora mixture of both for 14 days. Cardiac biomarkers, electrocardiogram (ECG), blood pressure, and antioxidant enzymes were determined as indicators of myocardial infarction. The results showed that isoproterenol elevates ST and QT segments and increases heart rate and serum activities of creatine phosphokinase (CPK), creatine kinase-myocardial bound (CP-MB), and cardiac troponin T (cTnT). It also reduces heart tissue content of glutathione peroxidase (GRx) and superoxide dismutase (SOD), and the arterial pressure indices of systolic arterial pressure (SAP), diastolic arterial pressure (DAP), and mean arterial pressure (MAP). Gold nanoparticles alone or in combination with cyanobacterial extract produced an inhibitory effect on isoproterenol-induced changes in serum cardiac injury markers, ECG, arterial pressure indices, and antioxidant capabilities of the heart.

The structural isomerism in gold nanoclusters

The isomerism in thiolate-protected gold (Au) nanoclusters is important for the understanding of structure-property correlations and the design of Au nanoclusters with specific structures and properties. Although recent studies have identified stereoisomerism, the understanding of structural isomerism is still lacking. Herein, we identified three distinct mechanisms of structural isomerism (i.e., core isomerism, staple isomerism, and complex isomerism) based on the crystallized isomers of thiolate-protected Au nanoclusters, and these mechanisms can be viewed as analogous to those of the structural isomerism in organic molecules (i.e., chain isomerism, point isomerism, and functional isomerism). Using the discovered core isomerism and staple isomerism, two Au28(SR)20 isomers are predicted and their synthesis feasibilities are illuminated. These new insights into the structural isomerism can facilitate rational design of new isomers of thiolate-protected Au nanoclusters and guide future experimental synthesis.

Avoiding Thiol Compound Interference: A Nanoplatform Based on High-Fidelity Au-Se Bonds for Biological Applications

Gold nanoparticles (Au NPs) assembled through Au-S covalent bonds have been widely used in biomolecule-sensing technologies. However, during the process, detection distortions caused by high levels of thiol compounds can still significantly influence the result and this problem has not really been solved. Based on the higher stability of Au-Se bonds compared to Au-S bonds, we prepared selenol-modified Au NPs as an Au-Se nanoplatform (NPF). Compared with the Au-S NPF, the Au-Se NPF exhibits excellent anti-interference properties in the presence of millimolar levels of glutathione (GSH). Such an Au-Se NPF that can effectively avoid detection distortions caused by high levels of thiols thus offers a new perspective in future nanomaterial design, as well as a novel platform with higher stability and selectivity for the in vivo application of chemical sensing and clinical therapies.

Redox Potentials and Electronic States of Iron Porphyrin IX Adsorbed on Single Crystal Gold Electrode Surfaces

Metalloporphyrins are active sites in metalloproteins and synthetic catalysts. They have also been studied extensively by electrochemistry as well as being prominent targets in electrochemical scanning tunneling microscopy (STM). Previous studies of FePPIX adsorbed on graphite and alkylthiol modified Au electrodes showed a pair of reversible Fe(III/II)PPIX peaks at about -0.41 V (vs NHE) at high solution pH. We recently used iron protoporphyrin IX (FePPIX) as an intercalating probe for long-range electrochemical electron transfer through a G-quadruplex oligonucleotide (DNAzyme); this study disclosed two, rather than a single pair of voltammetric peaks with a new and dominating peak, shifted 200 mV positive relative to the ≈-0.4 V peak. Prompted by this unexpected observation, we report here a study of the voltammetry of FePPIX itself on single-crystal Au(111), (100), and (110) and polycrystalline Au electrode surfaces. In all cases the dominating pair of new Fe(III/II)PPIX redox peaks, shifted positively by more than 200 mV compared to those of previous studies appeared. This observation is supported by density functional theory (DFT) which shows that strong dispersion forces in the FePPIX/Au electronic interaction drive the midpoint potential toward positive values. The FePPIX spin states depend on interaction with the Au(111) interface, converting all the Fe(II)/(III)PPIX species into low-spin states. These results support electrochemical evidence for the nature of the electronic coupling between FePPIX and Au-surfaces, and the electronic states of adsorbate molecules, with a bearing also on recent reports of magnetic FePPIX/Au(111) interactions in ultrahigh vacuum (UHV).

Enhanced Sensitivity to Detection Nanomolar Level of Cu2+ Compared to Spectrophotometry Method by Functionalized Gold Nanoparticles: Design of Sensor Assisted by Exploiting First-order Data with Chemometrics

A simple, sensitive and efficient colorimetric assay platform for the determination of Cu²⁺ was proposed with the aim of developing sensitive detection based on the aggregation of AuNPs in presence of a histamine H2-receptor antagonist (famotidine, FAM) as recognition site. This study is the first to demonstrate that the molar extinction coefficients of the complexes formed by FAM and Cu²⁺ are very low (by analyzing the chemometrics methods on the first order data arising from different metal to ligand ratio method), leading to the undesirable sensitivity of FAM-based assays. To resolve the problem of low sensitivity, the colorimetry method based on the Cu²⁺-induced aggregation of AuNPs functionalized with FAM was introduced. This procedure is accompanied by a color change from bright red to blue which can be observed with the naked eyes. Detection sensitivity obtained by the developed method increased about 100 fold compared with the spectrophotometry method. This sensor exhibited a good linear relation between the absorbance ratios at 670 to 520nm (A₆₇₀/₅₂₀) and the concentration in the range 2-110nM with LOD=0.76nM. The satisfactory analytical performance of the proposed sensor facilitates the development of simple and affordable UV-Vis chemosensors for environmental applications.

The long and successful journey of electrochemically active amino acids. From fundamental adsorption studies to potential surface engineering tools

Proteins have been the subject of electrochemical studies. It is possible to apply electrochemical techniques to obtain information about their structure due to the presence of five electroactive amino acids that can be oriented to the outside of the peptidic chain. These amino acids are L-Tryptophan (L-Trp), L-Tyrosine (L-Tyr), L-Histidine (L-His), L-Methionine (L-Met) and L-Cysteine (L-Cys); their electrochemical behavior being subject of extensive research, but it is still controversial. No spectroscopic investigations have been reported on L-Trp, and due to the short life time of the intermediates, ex situ techniques cannot be employed, leading to a never-ending discussion about possible intermediates. In the L-Tyr and L-His cases, spectroelectrochemical studies were performed and different intermediates were observed, suggesting that some intermediates may be observed under specific conditions, as proposed for L-Cys. This amino acid is the most interesting among the electroactive ones because of the presence of a thiol moiety at its side chain, leading to a wide range of oxidation states. It can adsorb onto surfaces of different crystallographic orientation in stereoselective conformation, modifying the surface for different applications.as a surface engineering tool since it plays the role of as an anchor for the growing of nanocrystals inside proteic templates.

Quantifying the Selective Modification of Au(111) Facets via Electrochemical and Electroless Treatments for Manipulating Gold Nanorod Surface Composition

Manipulating the composition of a mixed alkylthiol self-assembled monolayer (SAM) modified gold surface using both electrochemical and electroless methods is demonstrated. Through the use of fluorophore labeled thiolated DNA and in situ fluorescence microscopy with a gold single crystal bead electrode, a procedure was developed to study and quantify the selective desorption of an alkylthiolate SAM. This method enabled a self-consistent measurement of the removal of the SAM from the 111 surface compared to the 100 surface region at various potentials. A 20-fold increase in the electrochemical removal and replacement of the SAM from the 111 surface over the 100 surface was realized at -0.8 V/AgAgCl. A related procedure was developed for the solution-based electroless removal of the SAM using NaBH₄ achieving a similar selectivity at the same potential. Unfortunately, in the electroless process fine control over the reducing potential was difficult to achieve. In addition, working in the presence of O₂ complicates the solution potential measurement due to depolarization by the reduction of O₂, resulting in a less clear relationship between selectivity and measured solution potential. Interestingly, the electrochemical method was not disturbed by the presence of O₂. In preparation for work with Au nanorods, electrochemical measurements were performed in electrolyte that included 1 mM CTAB and was found to not interfere with this method. Preliminary results are promising for using this methodology for treatment of acid-terminated alkylthiol modified Au nanorods.

Thermodynamic stability of ligand-protected metal nanoclusters

Despite the great advances in synthesis and structural determination of atomically precise, thiolate-protected metal nanoclusters, our understanding of the driving forces for their colloidal stabilization is very limited. Currently there is a lack of models able to describe the thermodynamic stability of these 'magic-number' colloidal nanoclusters as a function of their atomic-level structural characteristics. Herein, we introduce the thermodynamic stability theory, derived from first principles, which is able to address stability of thiolate-protected metal nanoclusters as a function of the number of metal core atoms and thiolates on the nanocluster shell. Surprisingly, we reveal a fine energy balance between the core cohesive energy and the shell-to-core binding energy that appears to drive nanocluster stabilization. Our theory applies to both charged and neutral systems and captures a large number of experimental observations. Importantly, it opens new avenues for accelerating the discovery of stable, atomically precise, colloidal metal nanoclusters.

The role of the crystalline face in the ordering of 6-mercaptopurine self-assembled monolayers on gold

Well-ordered molecular films play an important role in nanotechnology, from device fabrication to surface patterning. Self-assembled monolayers (SAMs) of 6-mercaptopurine (6MP) on the Au(100)-(1 × 1) and Au(111)-(1 × 1) have been used to understand the interplay of molecule-substrate interactions for heterocyclic thiols capable of binding to the surface by two anchors, which spontaneously form a highly disordered film on Au(111). Our results reveal that for the same surface coverage the simple change of the substrate from Au(111)-(1 × 1) to Au(100)-(1 × 1) eliminates molecular disorder and yields well-ordered SAMs. We discuss these findings in terms of differences in the surface mobility of 6MP species on these surfaces, the energetics of the adsorption sites, and the number of degrees of freedom of these substrates for a molecule with reduced surface mobility resulting from its two surface anchors. These results reveal the presence of subtle molecule-substrate interactions involving the heteroatom that drastically alter SAM properties and therefore strongly impact on our ability to control physical properties and to build devices at the nanoscale.

Determining the composition of gold nanoparticles: a compilation of shapes, sizes, and calculations using geometric considerations

ABSTRACT

Size, shape, overall composition, and surface functionality largely determine the properties and applications of metal nanoparticles. Aside from well-defined metal clusters, their composition is often estimated assuming a quasi-spherical shape of the nanoparticle core. With decreasing diameter of the assumed circumscribed sphere, particularly in the range of only a few nanometers, the estimated nanoparticle composition increasingly deviates from the real composition, leading to significant discrepancies between anticipated and experimentally observed composition, properties, and characteristics. We here assembled a compendium of tables, models, and equations for thiol-protected gold nanoparticles that will allow experimental scientists to more accurately estimate the composition of their gold nanoparticles using TEM image analysis data. The estimates obtained from following the routines described here will then serve as a guide for further analytical characterization of as-synthesized gold nanoparticles by other bulk (thermal, structural, chemical, and compositional) and surface characterization techniques. While the tables, models, and equations are dedicated to gold nanoparticles, the composition of other metal nanoparticle cores with face-centered cubic lattices can easily be estimated simply by substituting the value for the radius of the metal atom of interest.

GRAPHICAL ABSTRACT

Understanding the Phase Diagram of Self-Assembled Monolayers of Alkanethiolates on Gold

Alkanethiolate monolayers on gold are important both for applications in nanoscience as well as fundamental studies of adsorption and self-assembly at metal surfaces. While considerable experimental effort has been put into understanding the phase diagram of these systems, theoretical work based on density functional theory (DFT) has long been hampered by the inability of conventional exchange-correlation functionals to describe dispersive interactions. In this work, we combine dispersion-corrected DFT calculations using the new vdW-DF-CX functional with the ab initio thermodynamics method to study the stability of dense standing-up and low-coverage lying-down phases on Au(111). We demonstrate that the lying-down phase has a thermodynamic region of stability starting from thiolates with alkyl chains consisting of n ≈ 3 methylene units. This phase emerges as a consequence of a competition between dispersive chain-chain and chain-substrate interactions, where the strength of the latter varies more strongly with n. A phase diagram is derived under ultrahigh-vacuum conditions, detailing the phase transition temperatures of the system as a function of the chain length. The present work illustrates that accurate ab initio modeling of dispersive interactions is both feasible and essential for describing self-assembled monolayers.

Putting David Craig’s Legacy to Work in Nanotechnology and Biotechnology

David Craig (1919–2015) left us with a lasting legacy concerning basic understanding of chemical spectroscopy and bonding. This is expressed in terms of some of the recent achievements of my own research career, with a focus on integration of Craig’s theories with those of Noel Hush to solve fundamental problems in photosynthesis, molecular electronics (particularly in regard to the molecules synthesized by Maxwell Crossley), and self-assembled monolayer structure and function. Reviewed in particular is the relation of Craig’s legacy to: the 50-year struggle to assign the visible absorption spectrum of arguably the world’s most significant chromophore, chlorophyll; general theories for chemical bonding and structure extending Hush’s adiabatic theory of electron-transfer processes; inelastic electron-tunnelling spectroscopy (IETS); chemical quantum entanglement and the Penrose–Hameroff model for quantum consciousness; synthetic design strategies for NMR quantum computing; Gibbs free-energy measurements and calculations for formation and polymorphism of organic self-assembled monolayers on graphite surfaces from organic solution; and understanding the basic chemical processes involved in the formation of gold surfaces and nanoparticles protected by sulfur-bound ligands, ligands whose form is that of Au0-thiyl rather than its commonly believed AuI-thiolate tautomer.