A Tray‐Shaped,PdII‐Clipped Au3 Complex as a Scaffold for the Modular Assembly of [3×n] Au Ion Clusters

A tray‐shaped Pd^II₃Au^I₃ complex ( 1 ) is prepared from 3,5‐bis(3‐pyridyl)pyrazole by means of tricyclization with Au^I followed by Pd^II clipping. Tray 1 is an efficient scaffold for the modular assembly of [3×n] Au^I clusters. Treatment of 1 with the Au^I₃ tricyclic guest 2 in H₂O/CH₃CN (7:3) or H₂O results in the selective formation of a [3×2] cluster ( 1 ? 2 ) or a [3×3] cluster ( 1 ? 2 ? 1 ), respectively. Upon subsequent addition of Ag^I ions, these complexes are converted to an unprecedented Au₃–Au₃–Ag–Au₃–Au₃ metal ion cluster.     

A Tray‐Shaped,PdII‐Clipped Au3 Complex as a Scaffold for the Modular Assembly of [3×n] Au Ion Clusters

A tray‐shaped Pd^II₃Au^I₃ complex ( 1 ) is prepared from 3,5‐bis(3‐pyridyl)pyrazole by means of tricyclization with Au^I followed by Pd^II clipping. Tray 1 is an efficient scaffold for the modular assembly of [3×n] Au^I clusters. Treatment of 1 with the Au^I₃ tricyclic guest 2 in H₂O/CH₃CN (7:3) or H₂O results in the selective formation of a [3×2] cluster ( 1 ⋅ 2 ) or a [3×3] cluster ( 1 ⋅ 2 ⋅ 1 ), respectively. Upon subsequent addition of Ag^I ions, these complexes are converted to an unprecedented Au₃–Au₃–Ag–Au₃–Au₃ metal ion cluster.     

Efficient Near-Infrared Electrochemiluminescence from Au18 Nanoclusters

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

Production of cationic 198Au3+ and nonionic 198Au0 for radionuclide therapy applications via the natAu(n,γ)198Au reaction

¹⁹⁸Au (??_max?=?0.96?MeV (98.6?%), ??_max?=?0.412?MeV (95.5?%) and T _1/2 ?=?2.7?days) is a radionuclide with very appealing characteristics. ¹⁹⁸Au has been widely used to treat the uterus, bladder, cervix, prostate, melanoma, breast, skin and other cancers. In the present study, cationic ¹⁹⁸Au⁺³ and nonionic ¹⁹⁸Au⁰ are prepared following thermal neutron irradiation of commercially available natural gold compounds in Tehran Research Reactor via the ^natAu(n,??)¹⁹⁸Au reaction. The prospects in the production of pure ¹⁹⁸Au⁰ and ¹⁹⁸Au⁺³ for radionuclide therapy are discussed and effect of reduction on the activity of radioactive gold is evaluated. Au⁰ particles were synthesized via NaBH₄ reduction of aqueous solutions of hydrogen tetrachloroaurate trihydrate. Then two quartz tubes were charged with cationic ¹⁹⁸Au³⁺ and nonionic ¹⁹⁸Au⁰. After irradiation by thermal neutrons, the samples were analyzed for a period of 1?month by liquid scintillation counter and high purity germanium detector. As a result, ^natAu³⁺ reduction process had no significant effect on the activity of the ¹⁹⁸Au sample. In conclusions, natural gold thermal neutron activation cross section is reasonably high for medical application.     

Luminescent Di‐ and Polynuclear Organometallic Gold(I)–Metal (Au2, {Au2Ag}n and {Au2Cu}n) Compounds Containing Bidentate Phosphanes as Active Antimicrobial Agents

The reaction of new dinuclear gold(I) organometallic complexes containing mesityl ligands and bridging bidentate phosphanes [Au₂(mes)₂(μ‐LL)] (LL=dppe: 1,2‐bis(diphenylphosphano)ethane 1 a , and water‐soluble dppy: 1,2‐bis(di‐3‐pyridylphosphano)ethane 1 b ) with Ag⁺ and Cu⁺ lead to the formation of a family of heterometallic clusters with mesityl bridging ligands of the general formula [Au₂M(μ‐mes)₂(μ‐LL)][A] (M=Ag, A=ClO₄^?, LL=dppe 2 a , dppy 2 b ; M=Ag, A=SO₃CF₃^?, LL=dppe 3 a , dppy 3 b ; M=Cu, A=PF₆^?, LL=dppe 4 a , dppy 4 b ). The new compounds were characterized by different spectroscopic techniques and mass spectrometry The crystal structures of [Au₂(mes)₂(μ‐dppy)] ( 1 b ) and [Au₂Ag(μ‐mes)₂(μ‐dppe)][SO₃CF₃] ( 3 a ) were determined by a single‐crystal X‐ray diffraction study. 3 a in solid state is not a cyclic trinuclear Au₂Ag derivative but it gives an open polymeric structure instead, with the {Au₂(μ‐dppe)} fragments “linked” by {Ag(μ‐mes)₂} units. The very short distances of 2.7559(6) Å (Au? Ag) and 2.9229(8) Å (Au? Au) are indicative of gold–silver (metallophilic) and aurophilic interactions. A systematic study of their luminescence properties revealed that all compounds are brightly luminescent in solid state, at room temperature (RT) and at 77 K, or in frozen DMSO solutions with lifetimes in the microsecond range and probably due to the self‐aggregation of [Au₂M(μ‐mes)₂(μ‐LL)]⁺ units (M=Ag or Cu; LL=dppe or dppy) into an extended chain structure, through Au? Au and/or Au? M metallophilic interactions, as that observed for 3 a . In solid state the heterometallic Au₂M complexes with dppe ( 2 a – 4 a ) show a shift of emission maxima (from ca. 430 to the range of 520‐540 nm) as compared to the parent dinuclear organometallic product 1 a while the complexes with dppy ( 2 b–4 b ) display a more moderate shift (505 for 1 b to a max of 563 nm for 4 b ). More importantly, compound [Au₂Ag(μ‐mes)₂(μ‐dppy)]ClO₄ ( 2 b ) resulted luminescent in diluted DMSO solution at room temperature. Previously reported compound [Au₂Cl₂(μ‐LL)] (LL dppy 5 b ) was also studied for comparative purposes. The antimicrobial activity of 1–5 and Ag[A] (A=ClO₄^?, SO₃CF₃^?) against Gram‐positive and Gram‐negative bacteria and yeast was evaluated. Most tested compounds displayed moderate to high antibacterial activity while heteronuclear Au₂M derivatives with dppe ( 2 a – 4 a ) were the more active (minimum inhibitory concentration 10 to 1 μg mL^?1). Compounds containing silver were ten times more active to Gram‐negative bacteria than the parent dinuclear compound 1 a or silver salts. Au₂Ag compounds with dppy ( 2 b , 3 b ) were also potent against fungi.     

Thermodynamic and Kinetic Origins of Au250 Nanocluster Electrochemiluminescence

Au clusters with protecting organothiolate ligands and core diameters less than 2 nm are molecule‐like structures, suitable for catalysis, optoelectronics and biology applications. The spectroscopy and electrochemistry of Au₂₅⁰ (Au₂₅[(SCH₂CH₂Ph)₁₈], SCH₂CH₂Ph=2‐phenylethanethiol) allowed us to construct a Latimer‐type diagram for the first time, which revealed a rich photoelectrochemistry of the cluster and the unique relationship to its various oxidation states and corresponding excited states. The occurrence of cluster electrochemiluminescence (ECL) was examined in the presence of tri‐n‐propylamine (TPrA) as a co‐reactant and was discovered to be in the near‐infrared (NIR) region with peak wavelengths of 860, 865, and 960 nm, emitted by Au₂₅^+*, Au₂₅^0*, and Au₂₅^?*, respectively. The light emissions, with an efficiency up to 103 % relative to that of the efficient Ru(bpy)₃²⁺/TPrA system, depended on the kinetics of the reactions between the electrogenerated TPrA radical and Au₂₅^z (z=2+, 1+, 1?, and 2?) in the vicinity of the electrode or the bulk Au₂₅⁰. These thermodynamic and kinetic origins were further explored by means of spooling ECL and photoluminescence spectroscopy during a sweep of the potential or at a constant potential applied to the working electrode. NIR‐ECL emissions of the cluster can be tuned in wavelength and intensity by adjusting the applied potential and TPrA concentration based on the above discoveries.     

Über den thermischen Abbau einiger Alkalimetall- und Ammoniumhalogenoaurate(III) und die Kristallstruktur der Zersetzungsprodukte Rb2Au2Br6, Rb3Au3Cl8 und Au(NH3)Cl3

Thermal Decomposition of Some Alkali Metal and Ammonium Halogenoaurates(III), and the Crystal Structure of the Decomposition Products, Rb₂Au₂Br₆, Rb₃Au₃Cl₈, and Au(NH₃)Cl₃ The thermal decomposition of the salts MAuX₄ and M₂Au₂I₆, with M = K, NH₄, Rb; and X = Cl, Br; has been investigated. With the exception of NH₄AuCl₄ and KAuCl₄, all decompose with loss of halogen to the mixed valence compound M₃Au₃X₈, sometimes via the intermediate M₂Au₂X₆. Tetraiodoaurates are not stable at room temperature. In the decomposition of NH₄AuCl₄, HCl, and Au(NH₃)Cl₃ are formed. KAuCl₄ decomposes directly into Au und KCl. The crystal lattices of the salts Rb₂Au₂Br₆ and Rb₃Au₃Cl₈ are monoclinic and built up from Rb⁺. AuX₂^?, and AuX₄^? ions. There exists a close structural relationship between Rb₂Au₂Br₆, Cs₂Au₂Cl₆, and the perovskite structure. Rb₂Au₂I₆ is isotypic with Rb₂Au₂Br₆. The Rb₃Au₃Cl₈ structure type is also observed for the M₃Au₃X₈ salts with M = NH₄, K, Rb; and X = Br, I. In the structure of Au(NH₃)Cl₃ there are discrete molecules in which Au(III) is surrounded by 3 Cl and 1 N atoms in square coplanar coordination. The infrared spectra of these compounds are discussed.     

Heteroctanuclear Au4Ag4 Cluster Complexes of 4,5-Diethynylacridin-9-One with Luminescent Mechanochromism

Two heteroctanuclear Au₄Ag₄ cluster complexes of 4,5-diethynylacridin-9-one (H₂L) were prepared through the self-assembly reactions of [Au(tht)₂](CF₃SO₃), Ag(tht)(CF₃SO₃), H₂L and PPh₃ or PPh₂Py (2-(diphenylphosphino)pyridine). The Au₄Ag₄ cluster consists of a [Au₄L₄]⁴⁻ and four [Ag(PPh₃)]⁺ or [Ag(PPh₂Py)]⁺ units with Au₄L₄ framework exhibiting a twisted paper clip structure. In CH₂Cl₂ solutions at ambient temperature, both compounds show ligand fluorescence at ca. 463 nm as well as phosphorescence at 650 nm for 1 and 630 nm for 2 resulting from admixture of ³IL (intraligand) of L ligand, ³LMCT (from L ligand to Au₄Ag₄) and ³MC (metal-cluster) triplet states. Crystals or crystalline powders manifest bright yellow-green phosphorescence with vibronic-structured emission bands at 530 (568sh) nm for complex 1 and 536 (576sh) nm for complex 2. Upon mechanical grinding, yellow-green emission in the crystalline state is dramatically converted to red luminescence centered at ca. 610 nm with a drastic redshift of the emission after crystal packing is destroyed.     

Au21 cage structures and their magic number tri-cations

We examine the stability and properties of three Au₂₁ cage structures, one with D₃ symmetry and denoted as Au₂₁ (D₃), which is novel, and the other two with C_2v symmetry. One, denoted as Au₂₁ (C_2v-1), has been previously reported but the other, denoted as Au₂₁ (C_2v-2), is novel. As reference Au₂₁ structures, we also examine a sheet isomer and a compact isomer, Au₂₁ (C_s-Tetra), formed by adsorbing an Au atom on Au₂₀ (T_d). For all structures, we consider charge ranging from −1 to +4. For the Au₂₁ cage structures, a primary property of interest is their spherical aromaticity, as measured by their nucleus independent chemical shift. Our focus is on charge +3 since each gold atom is assumed to contribute one (6s) valence electron, and 18 is a magic number for shell closing. We find that, although Au₂₁ (D₃)⁺³ has the largest aromaticity, it is not the most stable cage species. Surprisingly, Au₂₁ (C_2v-2), which is not even stable as a neutral cage species, is the most stable tri-cation cage species. We also examine the stability of (neutral) Au₂₁X_n cage structures relative to Au₂₁X_n structures derived from Au₂₁ (C_s-Tetra) (with X = F, Cl, Br, I and n = 1, 3, 5). We find that, although Au₂₁F₃ derived from Au₂₁ (D₃) has the largest aromaticity, it is not the most stable Au₂₁F₃ cage structure. Nonetheless, all cage structures are stabilized relative to Au₂₁ (C_s-Tetra) and, remarkably, for the trichlorinated, tribrominated, and triiodineated clusters, at least one cage structure is more stable than its Au₂₁ (C_s-Tetra) counterpart.     

Bright,NIR‐Emitting Au23 from Au25: Characterization and Applications Including Biolabeling

A novel interfacial route has been developed for the synthesis of a bright‐red‐emitting new subnanocluster, Au₂₃, by the core etching of a widely explored and more stable cluster, Au₂₅SG₁₈ (in which SG is glutathione thiolate). A slight modification of this procedure results in the formation of two other known subnanoclusters, Au₂₂ and Au₃₃. Whereas Au₂₂ and Au₂₃ are water soluble and brightly fluorescent with quantum yields of 2.5 and 1.3 %, respectively, Au₃₃ is organic soluble and less fluorescent, with a quantum yield of 0.1 %. Au₂₃ exhibits quenching of fluorescence selectively in the presence of Cu²⁺ ions and it can therefore be used as a metal‐ion sensor. Aqueous‐ to organic‐phase transfer of Au₂₃ has been carried out with fluorescence enhancement. Solvent dependency on the fluorescence of Au₂₃ before and after phase transfer has been studied extensively and the quantum yield of the cluster varies with the solvent used. The temperature response of Au₂₃ emission has been demonstrated. The inherent fluorescence of Au₂₃ was used for imaging human hepatoma cells by employing the avidin–biotin interaction.     

CO Oxidation Promoted by the Gold Dimer in Au2VO3− and Au2VO4− Clusters

Investigations on the reactivity of atomic clusters have led to the identification of the elementary steps involved in catalytic CO oxidation, a prototypical reaction in heterogeneous catalysis. The atomic oxygen species O^.? and O^2? bonded to early‐transition‐metal oxide clusters have been shown to oxidize CO. This study reports that when an Au₂ dimer is incorporated within the cluster, the molecular oxygen species O₂^2? bonded to vanadium can be activated to oxidize CO under thermal collision conditions. The gold dimer was doped into Au₂VO₄^? cluster ions which then reacted with CO in an ion‐trap reactor to produce Au₂VO₃^? and then Au₂VO₂^?. The dynamic nature of gold in terms of electron storage and release promotes CO oxidation and O? O bond reduction. The oxidation of CO by atomic clusters in this study parallels similar behavior reported for the oxidation of CO by supported gold catalysts.     

The Structure and Optical Properties of the [Au18(SR)14] Nanocluster

Decreasing the core size is one of the best ways to study the evolution from Au^I complexes into Au nanoclusters. Toward this goal, we successfully synthesized the [Au₁₈(SC₆H₁₁)₁₄] nanocluster using the [Au₁₈(SG)₁₄] (SG=L ‐glutathione) nanocluster as the starting material to react with cyclohexylthiol, and determined the X‐ray structure of the cyclohexylthiol‐protected [Au₁₈(C₆H₁₁S)₁₄] nanocluster. The [Au₁₈(SR)₁₄] cluster has a Au₉ bi‐octahedral kernel (or inner core). This Au₉ inner core is built by two octahedral Au₆ cores sharing one triangular face. One transitional gold atom is found in the Au₉ core, which can also be considered as part of the Au₄(SR)₅ staple motif. These findings offer new insight in terms of understanding the evolution from [Au^I(SR)] complexes into Au nanoclusters.     

Core Charge Density Dominated Size-Conversion from Au6P8 to Au8P8Cl2

The stimulus-response of metal nanoclusters is crucial to their applications in catalysis and bio-clinics, etc. However, its mechanistic origin has not been well studied. Herein, the mechanism of the Au^IPPh₃Cl-induced size-conversion from [Au₆(DPPP)₄]²⁺ to [Au₈(DPPP)₄Cl₂]²⁺ (DPPP is short for 1,3-bis(diphenylphosphino)propane) is theoretically investigated with density functional theory (DFT) calculations. The optimal size-growth pathway, and the key structural parameters were elucidated. The Au−P bond dissociation steps are key to the size-growth, the easiness of which was determined by the charge density of the metallic core of the cluster precursors (i.e., “core charge density”). This study sheds light on the inherent structure–reactivity relationships during the size-conversion, and will benefit the deep understanding on the kinetics of more complex systems.     

Gold clusters: synthesis and characterization of [Au8(PPh3)7(CNR)]2+, [Au9(PPh36(CNR2]3+ and [Au11(PPH37(CNR)2O]2+ and their reactivity towards amines. The crystal structure of [Au11(PPh3)7(CN-i-Pr)2I](PF6)2

In the reactions of [Au₈(PPh₃)₇]²⁺, [Au₈(PPh₃)₈]²⁺ and [Au₉(PPh₃)₈]³⁺ with RNC (R = isopropyl and t-butyl) in dichloromethane [Au₈(PPh₃)₇CNR]²⁺ is initially, and is then converted into [Au₉(PPh₃)₆(CNR)₂]³⁺ via various intermediates. [Au₉(PPh₃)₆(CNR)₂]³⁺ reacts with I⁻ at low temperature (−78°C) in methanol to yield [Au₁₁(PPh₃)₇(CNR)₂I]²⁺, but when the reaction is carried out at room temperature Au₁₁ (PPh₃)₆(CNR)I₃ is formed. The cluster compounds have been characterised by elemental analysis, ³¹P{¹H} NMR, conductivity measurements, IR and ¹⁹⁷Au Mössbauer spectroscopy. The reactions of the clusters with amines to form carbene clusters are very slow, and the reasons for this are considered.The structure of [Au₁₁C₁₃₄H₁₁₂IN₂P₇](PF₆ was determined by X-ray diffraction. M_r = 3796.39 cubic, space group 143_d, a 37.955(12) Å, V 54677.2 ų, Z = 16, D_c = 2.21 Mg m⁻³, Mo-K_α radiation (graphite crystal monochromator, λ 0.71069 Å), μ(Mo-K_α) 125.2 cm⁻¹, F(000) = 33510.3, T 293 K. Final conventional R-factor = 0.048, R_w = 0.062 ofr 1867 unique reflections and 198 variables. The Au-skeleton is the same as in Au₁₁(PPh₃)₈I₃ having C_3v symmetry with one central and 10 peripheral Au atoms.     

Bare Clusters Derived from Protein Templates: Au25+, Au38+ and Au102+

A discrete sequence of bare gold clusters of well‐defined nuclearity, namely Au₂₅⁺, Au₃₈⁺ and Au₁₀₂⁺, formed in a process that starts from gold‐bound adducts of the protein lysozyme, were detected in the gas phase. It is proposed that subsequent to laser desorption ionization, gold clusters form in the gas phase, with the protein serving as a confining growth environment that provides an effective reservoir for dissipation of the cluster aggregation and stabilization energy. First‐principles calculations reveal that the growing gold clusters can be electronically stabilized in the protein environment, achieving electronic closed‐shell structures as a result of bonding interactions with the protein. Calculations for a cluster with 38 gold atoms reveal that gold interaction with the protein results in breaking of the disulfide bonds of the cystine units, and that the binding of the cysteine residues to the cluster depletes the number of delocalized electrons in the cluster, resulting in opening of a super‐atom electronic gap. This shell‐closure stabilization mechanism confers enhanced stability to the gold clusters. Once formed as stable magic number aggregates in the protein growth medium, the gold clusters become detached from the protein template and are observed as bare Au_n⁺ (n=25, 38, and 102) clusters.     

Monitoring single Au38 nanocluster reactions via electrochemiluminescence

Herein, we report for the first time single Au₃₈ nanocluster reaction events of highly efficient electrochemiluminescence (ECL) with tri-n-propylamine radicals as a reductive co-reactant at the surface of an ultramicroelectrode (UME). The statistical analyses of individual reactions confirm stochastic single ones influenced by the applied potential.

Herein, we report for the first time single Au₃₈ nanocluster reaction events of highly efficient electrochemiluminescence (ECL) with tri-n-propylamine radicals as a reductive co-reactant at the surface of a Pt ultramicroelectrode (UME).

Single entity measurements have been introduced by Bard and Wightman based on the collisions/reactions of individual nanoparticles and molecules at an ultramicroelectrode (UME).^1–9 Since then, the field of single entity electrochemistry has gradually attracted several research groups and has become a frontier field of nanoelectrochemistry and electroanalytical chemistry.^8,10–14 For instance, it has been shown that the chemistry of the electrode surface plays an important role in the collision/reaction events and the kinetics of reaction processes.^15–21 Dasari et al. reported that hydrazine oxidation and proton reduction can be detected using single Pt nanoparticles on the surface of a mercury or bismuth modified Pt UME, and the material of the electrode was found to affect the shape of current–time transients.^22,23 Fast scan cyclic voltammetry provides better chemical information about transient electrode–nanoparticle interactions, which is otherwise difficult to obtain with constant-potential techniques.²⁴ There are only a few reports on photoelectrochemical systems including semiconductor nanoparticles designed to detect single nanoparticles in the course of photocatalysis processes.^25–28 More importantly, owing to the nature of stochastic processes of single entity reactions, statistical analyses have shown substantial influences on the understanding of the underlying processes.Electrochemiluminescence or electrogenerated chemiluminescence (ECL),²⁹ as a background-free technique,^30–32 was also utilized to detect individual chemical reactions and single Pt nanoparticle collisions based on the reaction between the Ru(bpy)₃²⁺ complex and tri-n-propylamine (TPrA) radicals on the surface of an ITO electrode.^2,33,34 It was found that the size of the nanoparticles, the origin of the interaction between particles and the electrode surface, the concentration of species generation, and the lifetime of individual electrogenerated nanocluster species (i.e., Au₃₈²⁺, Au₃₈³⁺, and Au₃₈⁴⁺) in conjunction with the reactivity of those oxidized species with co-reactant radical intermediates (i.e., TPrA radical) play crucial roles in the frequency of the ECL reaction events leading to individual ECL responses. More strikingly, a higher ECL reaction frequency is directly proportional to the amount of collected ECL light.²¹ Chen and co-workers also employed ECL to study stationary single gold-platinum nanoparticle reactivity on the surface of an ITO electrode.³⁵ Lin and co-workers monitored the hydrogen evolution reaction in the course of “ON” and “OFF” ECL signals.³⁶ Recently, we performed a systematic and mechanistic ECL study of a series of gold nanoclusters, with the general formula of Au_n(SC₂H₄Ph)_m^z (n = 25, 38, 144, m = 18, 24, 60 and z = −1, 0, +1), where near-infrared (NIR) ECL emission was observed.³⁷ There are several enhancement factors, such as catalytic loops^38,39 that improve the signal to noise ratio. The Wightman group was able to report single ECL reactions based on the capability of ECL.⁷ Furthermore, thus far, we have explored ECL mechanisms and reported the ECL efficiency of five different gold nanoclusters i.e., Au₂₅(SR)₁₈¹⁻, Au₂₅(SR)₁₈⁰, Au₂₅(SR)₁₈¹⁺, Au₃₈(SR)₂₄⁰ and Au₁₄₄(SR)₆₀⁰, among which the Au₃₈(SR)₂₄⁰/TPrA system revealed outstanding ECL efficiency, ca. 3.5 times higher than that of Ru(bpy)₃²⁺/TPrA as a gold standard. Therefore, we decided to focus on the Au₃₈ (SR)₂₄⁰/TPrA system. It was discovered that the ECL emission of these nanomaterials can be tuned through varying the applied potential and local concentration of the desired co-reactant.Herein, for the first time we report on ECL via a single Au₃₈(SC₂H₄Ph)₂₄ nanocluster (hereafter denoted as Au₃₈ NC) reaction (eq. (1)) in the vicinity of an UME in the presence of TPrA radicals as a reductive co-reactant.1where x is the oxidation number that can be either 0, 1, 2, 3 or 4. Single ECL spikes (Fig. 1A) along with ECL spectroscopy were used for elucidating individual reaction events. Indeed, each single ECL spike demonstrates a single Au₃₈^(x−1)* reaction product. Au₃₈ NCs were synthesized according to procedures reported by us and others, and fully characterized using UV-Visible-NIR, photoluminescence, ¹HNMR spectroscopy and MALDI mass spectrometry to confirm the Au₃₈ nanocluster synthesis (details are provided in ESI, Sections 1–3, Fig. S1–S4^†).^38,40,41Fig. 2 (left) shows a differential pulse voltammogram (DPV) in an anodic scan of a 2 mm Pt disc electrode immersed in 0.1 mM Au₃₈ acetonitrile/benzene solution containing 0.1 M TBAPF₆ as the supporting electrolyte. There are five discrete electrochemical peaks at which Au₃₈⁰ was oxidized to Au₃₈⁺ (E°′ = 0.39 V), Au₃₈²⁺ (E°′ = 0.60 V), and Au₃₈^3+/4+ (E°′ = 0.99 V) and reduced to Au₃₈⁻ (E°′ = −0.76 V) and Au₃₈²⁻ (E°′ = −1.01 V).^38,40,41Open in a separate windowFig. 1(A) An example of the reaction event transient of 10 μM Au₃₈ in benzene/acetonitrile (1 : 1) containing 0.1 M TBAPF₆ in the presence of 20 mM TPrA at 0.9 V vs. SCE, acquired at 15 ms time intervals using a 10 μm Pt UME. The white dashed-line indicates the threshold to identify single ECL spikes. (B) Illustration of a single nanocluster ECL spike. (C) ECL instrumentation with an inset showing ECL spike generation in the vicinity of the Pt UME.Open in a separate windowFig. 2Anodic DPV for Au₃₈ (left), reaction energy diagram of Au₃₈²⁺ and TPrA· (middle) along with the ECL–voltage curve (right) in an anodic potential scan at a 2 mm Pt disk electrode immersed in a solution of 10 μM Au₃₈ with 20 mM TPrA.The rich electrochemistry of Au₃₈ NCs is well-matched with that of co-reactants such as TPrA to generate near infrared-ECL (NIR-ECL), and the ECL emission efficiency of the Au₃₈/TPrA system is 3.5 times larger than that of the Ru(bpy)₃²⁺/TPrA co-reactant ECL system.²⁷Thus, it is of utmost interest to investigate the ECL generation of the above co-reactant system in single reactions, which improves the ECL signal detection sensitivity. To perform the ECL experiment a solution of 10 μM Au₃₈ NC with 20 mM TPrA was prepared. We first confirmed the ECL light generation of such solution along with its blank solution containing only TPrA using a typical 2 mm diameter Pt disk electrode (Fig. 2, S5 and S6^†).A 10 μm Pt UME electrode, which is electrochemically inert (Fig. S7^†), was utilized to investigate the ECL of single NC reactions under potentiostatic conditions, at which a specific positive bias potential was applied to oxidize both Au₃₈ and TPrA. Fig. 1A shows a typical ECL–time transient current curve (ECL intensity versus time) at 0.90 V vs. SCE, which was acquired using a photomultiplier tube (PMT, R928) for a duration of 1800 s at data acquisition time intervals of 15 ms (Fig. 1C and ESI, Section 3^†). Fig. 1B represents an exemplary event of a single ECL spike with a sharp increase followed by a decay in the ECL intensity. It is observed from the many spikes in Fig. 1B that this process can reoccur with a high probability in the vicinity of the UME, probably due to an electrocatalytic reaction loop (Fig. 1C). Indeed, ECL intensity was enhanced in this way as an already relaxed species, i.e., Au₃₈^z+1*, participates in an oxidation step to regenerate Au₃₈^z+1 to react with the TPrA radical (TPrA˙).Once photons resulting from the excited state relaxation in the vicinity of the UME are captured by the PMT, individual reaction events can be observed (Fig. 1A with the instrumentation schematic shown in Fig. 1C). As shown in Fig. 3A, there are many ECL spikes during 1800 s of measurement, each of which represents an individual ECL generation reaction in the vicinity of the UME surface. It is worth noting that there are several spikes with various intensities. This is most likely due to the Brownian motion which is random movement due to the diffusion of individual nanocluster species such as Au₃₈⁰, Au₃₈¹⁺, Au₃₈²⁺, etc., electrogenerated at the local applied potentials. Long and co-workers⁴² proposed that silver nanoparticle collision on the surface of a gold electrode follows Brownian motion, leading to several types of surface-nanoparticle response peak shapes. In fact, the observed ECL spikes, shown in Fig. 1C, with a rise and an exponential decay suggested that Au₃₈ nanocluster species diffuse directly through the electrode double-layer, move towards the tunneling region of the electrode surface, collide⁴² and become oxidized, react with TPrA radicals thereafter to produce excited states, and emit ECL. It is worth emphasizing that this path could be partially different for each individual nanocluster owing to the angle and direction relative to the electrode surface. The single Au₃₈ NC reaction behaviour at various bias potentials was investigated following the electrochemical energy diagram shown in Fig. 2, middle. For example, at a bias potential of 0.70 V (the green spot on the DPV in Fig. 2), Au₃₈⁰ undergoes two successive oxidation reactions to Au₃₈²⁺ and TPrA oxidation and deprotonation start to generate TPrA·. In fact, at a very close oxidation potential to Au₃₈²⁺, TPrA is also oxidized to its corresponding cation radical (ca. 0.80 V vs. SCE) Fig. S6,^† followed by deprotonation to form the TPrA radical.³⁸ The TPrA· with a very high reduction power (E°′ = −1.7 eV)⁴³ injects one electron to the LUMO orbital of the nanocluster and forms excited state Au₃₈⁺*, as illustrated in the reaction energy diagram in Fig. 2, middle.³⁸ Then, Au₃₈⁺* emits ECL light while relaxing to the ground state. For another instance, at 1.10 V vs. SCE (the red spot on the DPV in Fig. 2), Au₃₈⁰ is oxidized to Au₃₈^3/4+ feasibly. At this potential, the TPrA radical is generated massively in the vicinity of the electrode. The efficient electron transfer between the TPrA radical and Au₃₈^3/4+ generates both Au₃₈²⁺* and Au₃₈³⁺* that emit light at the same wavelength of 930 nm.³⁸ The results of such interactions produced a transient composed of many ECL events (Fig. 3A), which is an indication of bias potential enforcement on the nanocluster light emission.Open in a separate windowFig. 3Single-nanocluster ECL photoelectron spectroscopy of Au₃₈. ECL–time transients (A), statistics of the number of photons (B), histogram of the single reaction time between sequential spikes (C) and accumulated ECL spectrum (D) for a 10 μm Pt UME at 1.1 V immersed in a 10 μM Au₃₈ nanocluster solution in benzene/acetonitrile (1 : 1) containing 0.1 M TBAPF₆ in the presence of 20 mM TPrA. (E)–(H) The counterpart plots to (A)–(D) for the UME biased at 0.7 V. # represents the number.We further tried to collect the current–time traces of such events; however, owing to the high background current originating from the high concentration of TPrA relative to that of the nanocluster, no noticeable spikes in the current were observed.In order to study the photochemistry and understand deeply the single nanocluster reactions, ECL–time transients were collected at different applied potentials (i.e., 0.7, 0.8, 0.9 and 1.1 V vs. SCE) as labelled in green, brown, purple, and red on the DPV in Fig. 2, respectively. The transients were further analysed using our home-written MATLAB algorithm adapted from that for nanopore electrochemistry.⁴⁴ The population of individual events was identified by applying an appropriate threshold to discriminate ECL spikes from the noise as demonstrated in Fig. 1A. In fact, the applied algorithm also assisted us to learn about the raising time and intensity of each spike, as well as photons of individual spikes. For instance, Fig. 3A shows another typical transit for 1800 s at an UME potential bias of 1.1 V for the ECL events. Indeed, the integrated area of each peak, the charge of the photoelectrons at the PMT, is directly proportional to the number of photons emitted from individual reactions (see ESI, Section 5^†). Basically, the PMT amplifies the collected single photon emitted in the course of light-to-photoelectron conversion (see ESI, Section 6 and Fig. S8^†) and translates a single photon into photoelectrons. The extracted charge of each ECL reaction, Q_ECL, was then converted to the corresponding number of photons by dividing by the gain factor, g, which is 1.55 × 10⁶ (Fig. S8^†), following eqn (2):2The histograms of the number of photons show a Gaussian distribution (Fig. 3B) with a reaction frequency of 53.5 ± 2.9 at E = 1.1 V, whereas at a lower potential of 0.7 V the reaction frequency drops to 18.5 ± 1.7 (Fig. 3F). This indicates that there is a three-fold lower reaction occurrence at the lower potential. The integration of the Gaussian fitting at 1.1 V and 0.7 V also reveals a three-fold drop from 3.3 × 10⁵ to 1.2 × 10⁵ photons over 1800 s.To further explore the effect of electrode potential bias on the single Au₃₈ NCs ECL reaction, potentials lower than 1.1 and higher than 0.7 V, ca. 0.8 and 0.9 V (brown and purple labels in Fig. 2), were applied. In fact, the resulting ECL–time transients show a lower population of single spikes (Fig. S12A and ESI,^†). The integrated Gaussian curve values support the ECL–time transient observations with ∼4.1 × 10⁴ and ∼6.5 × 10⁴ photons, respectively. In fact, it is unlikely that the PMT would get more than two events in the duration, owing to the following reasons: (i) it has been shown that only 5.5% of incoming photons can be effectively converted to photoelectron signals by our R928 PMT during our absolute efficiency calibration, ESI Section 6 and Fig. S8–S19^†;⁴⁵ (ii) spherical ECL emission is proven to be detected for a substantial small part upon examination of our detection system for the absolute ECL quantum efficiency;⁴⁵ (iii) Au₃₈ nanocluster ECL emissions occur at 930 nm, which is almost at the wavelength detection limit of our PMT response curve.^38,45In addition, we evaluated the stochastic (a series of random events at various probability distributions) nature of the observed events and extracted the reaction time interval (τ) at various potentials. The resulting graph shows an exponential decay (Fig. 3C) as expressed in eqn (3):3where frequency (λ) gives the mean rate of the event and A represents the fitting amplitude. One can expect to obtain the distribution of the number of emitted photons and spatial brightness function. In fact, the exponential decay is a clear indication of random single reaction events as Whiteman and co-workers described for a 9,10-diphenylanthracene (DPA) ECL system in the annihilation pathway.^7,46 At a potential of 1.1 V, λ and A are found to be 4.98 ± 0.02 ms⁻¹ and 80.4 ± 3.2, whereas at 0.7 V, λ and A turned out to be 32.9 ± 1.6 ms⁻¹ and 9.5 ± 0.1 (Fig. 3C and G). Indeed, the lower potential of 0.70 V vs. SCE is high enough to generate the TPrA radical along with Au₃₈²⁺, thereby leading to excited Au₃₈⁺*, Fig. 3E. One can conclude that at the applied potentials of 0.7 V and 1.1 V, Au₃₈⁰ is oxidized to Au₃₈²⁺ and Au₃₈⁴⁺, resulting in the generation of Au₃₈⁺* and Au₃₈³⁺* under static conditions. Thus, there are higher populations of ECL spikes with no discrepancy in the number of collected photon distributions. However, at two intermediate potentials, i.e., 0.8 and 0.9 V, a dynamic behaviour which is due to the mixed oxidation of Au₃₈ species, in the vicinity of the UME, is observed. In fact, at these two applied potentials, the local concentration of the corresponding gold nanoclusters (i.e., Au₃₈³⁺ and Au₃₈⁴⁺) is not sufficient to produce significant ECL spikes. We also attempted to collect the ECL spectrum using a charge-coupled device (CCD) camera, which is relatively more sensitive in the NIR region (e.g., λ > 900 nm, Fig. S16^†). Fig. 3D and H display an accumulated spectrum at 1.1 and 0.7 V vs. SCE, which is collected for 30 minutes. The fitted accumulated ECL spectrum indicates an ECL peak emission at 930 nm and supports higher reactivity at 1.1 V than that at 0.7 V.³⁸ To confirm that the observed ECL spikes and accumulated spectra are generated based on the oxidation of Au₃₈ nanoclusters in the presence of TPrA radicals, ECL–time transients were recorded upon holding an applied potential at which no faradaic process occurs. Fig. S11^† represents ECL–time curves and accumulated ECL spectra at 0.0 V and 0.4 V. One can notice that no appreciable ECL signal can be observed.In addition, we investigate the Pearson cross-correlation (ρ) between the intensities of ECL spikes with τ as shown in Fig. S14^† in which there is a positive correlation at 0.7 and 1.0 V and a negative correlation at 0.8 and 0.9 V. In fact, ρ evaluates whether there is a stationary random process between the two defined parameters (see ESI, Section 6^†). Interestingly, the frequency of the reaction at different applied potentials revealed decay from 0.7 to 0.8 V, followed by an upward trend to 0.9 and 1.1 V vs. SCE (Fig. S15^†). This could be additional support for the transition stage at 0.8 and 0.9 V, where the applied potential as the major driving force to generate oxidized forms (e.g., Au₃₈³⁺ and Au₃₈⁴⁺) governs the flux of the nanocluster species that reach the vicinity of the electrode. Furthermore, the effectiveness of electron transfer reaction kinetics between the radical species, i.e., Au₃₈^z+1 and TPrA radical, competes with the flow of the incoming nanoclusters. It is worth mentioning that each of the ECL single event experiments was repeated three times, and very similar results were obtained. Moreover, lower (5 μM) and higher (20 μM) concentrations of Au₃₈ in the presence of 20 mM were tested. In fact, the former shows a smaller number of single reactions; however the later revealed a larger number of multiple reactions (Fig. S13^†).In summary, in this communication we demonstrated that Au₃₈ NC ECL at the single reaction level can be monitored using a simple photoelectrochemical setup following a straightforward protocol. Indeed, we have rich basic knowledge about the ECL mechanisms of various gold nanoclusters with different charge states (Au₂₅(SR)₁₈¹⁺, Au₂₅(SR)₁₈⁰, Au₂₅(SR)₁₈¹⁻) and various sizes (Au₂₅(SR)₁₈⁰, Au₃₈(SR)₂₄⁰, Au₁₄₄(SR)₆₀⁰) in fine detail. Thus, the ECL emission mechanisms of gold clusters, including the contribution of each charge state and influence of various concentrations of co-reactants, are well known. For instance, in our previous studies^{38,39,47–49} we clearly identified three charge states of an Au₂₅(SR)₁₈¹⁻/TPrA system and we discovered that at a high concentration of TPrA the reduction in the bulk solution of gold nanoclusters influences the ECL emission wavelength. We also have learnt that the Au₃₈/TPrA system is a co-reactant independent of co-reactant concentration. Furthermore, an extensively higher concentration of TPrA provides a dominant reaction over any unknown decomposition reaction at higher oxidation states of Au₃₈. It was discovered that the population of ECL reactions is directly governed by the applied bias potential on a Pt UME. This work is a strong indication of the high sensitivity of the ECL technique in detecting single ECL reactions in a simple solution, which complements those reported by the Bard group using rubrene, for instance, embedded in an organic emulsion in the presence of TPrA or oxalate as a co-reactant.^50,51 These systems needed a substantial ECL enhancement in the presence of an ionic liquid as the supporting electrolyte and emulsifier. The current approach can be further extended to investigate other molecules and nanomaterials'' electrocatalytic processes at the single reaction level.     

Crystal structures and new perspectives on Y3Au4 and Y14Au51

Y₃Au₄ (triyttrium tetragold) and Y₁₄Au₅₁ (tetradecayttrium henpentacontagold), two binary representatives of Au‐rich rare earth (R ) systems crystallize with the space groups R and P 6/m , adopting the Pu₃Pd₄ and Gd₁₄Ag₅₁ structure types, respectively (Pearson symbols hR 42 and hP 65). A variety of binary R –Au compounds have been reported, although only a few have been investigated thoroughly. Many reports lack information or misinterpret known compounds reported elsewhere. The Pu₃Pd₄ type is fairly common for group 10 elements Ni, Pd, and Pt, while Au representatives are restricted to just five examples, i.e. Ca₃Au₄, Pr₃Au₄, Nd₃Au₄, Gd₃Au₄, and Th₃Au₄. Sm₆Au₇ is suspected to be Sm₃Au₄ due to identical symmetry and close unit‐cell parameters. The Pu₃Pd₄ structure type allows for full substitution of the position of the rare earth atom by more electronegative and smaller elements, i.e. Ti and Zr. The Gd₁₄Ag₅₁ type instead is more common for the group 11 metals, while rare representatives of group 12 are known. Y₃Au₄ can be represented as a tunnel structure with encapsulated cations and anionic chains. Though tunnels are present in Y₁₄Au₅₁, this structure is more complex and is best described in terms of polyhedral `pinwheels' around the tunnel forming polyhedra along the c axis.     

Bonding and Acidity of the Formal Hydride in the Prototypical Au9(PPh3)8H2+ Nanocluster

The role of hydrogen atoms as surface ligands on metal nanoclusters is of profound importance but remains difficult to directly study. While hydrogen atoms often appear to be incorporated formally as hydrides, evidence suggests that they donate electrons to the cluster's delocalized superatomic orbitals and may consequently behave as acidic protons that play key roles in synthetic or catalytic mechanisms. Here we directly test this assertion for the prototypical Au₉(PPh₃)₈H²⁺ nanocluster, formed by addition of a hydride to the well-characterized Au₉(PPh₃)₈³⁺. Using gas-phase infrared spectroscopy, we were able to unambiguously isolate Au₉(PPh₃)₈H²⁺ and Au₉(PPh₃)₈D²⁺, revealing an Au−H stretching mode at 1528 cm⁻¹ that shifts to 1038 cm⁻¹ upon deuteration. This shift is greater than the maximum expected for a typical harmonic potential, suggesting a potential governing cluster-H bonding that has some square-well character consistent with the hydrogen nucleus behaving as a metal atom in the cluster core. Complexing this cluster with very weak bases reveals a redshift of 37 cm⁻¹ in the Au−H vibration, consistent with those typically seen for moderately acidic groups in gas phase molecules and providing an estimate of the acidity of Au₉(PPh₃)₈H²⁺, at least with regard to its surface reactivity.     

Stuffed Cage Structures and Properties of Neutral and Charged Au38 Nanocluster

Taking Au₃₈ as a prototype, hollow cages with different arrangements and those stuffed by a different number of atoms have been studied by using the scalar relativistic density functional theory. The global minimum structures of Au ₃₈ ⁱ (i = 0, ±1) are found to exhibit low symmetry core-shell structures: Au₃₈ and Au ₃₈ ^? feature a four-atom tetrahedral core and Au ₃₈ ⁺ possesses a six-atom octahedron core. For the neutral Au₃₈, the tube-like structure has the highest chemical stability. The obtained information forms the basis for future chemisorption studies to unravel the catalytic effects of gold nanoparticles.     

The Magic Au60 Nanocluster: A New Cluster‐Assembled Material with Five Au13 Building Blocks

Herein, we report the synthesis and atomic structure of the cluster‐assembled [Au₆₀Se₂(Ph₃P)₁₀(SeR)₁₅]⁺ material. Five icosahedral Au₁₃ building blocks from a closed gold ring with Au–Se–Au linkages. Interestingly, two Se atoms (without the phenyl tail) locate in the center of the cluster, stabilized by the Se‐(Au)₅ interactions. The ring‐like nanocluster is unprecedented in previous experimental and theoretical studies of gold nanocluster structures. In addition, our optical and electrochemical studies show that the electronic properties of the icosahedral Au₁₃ units still remain unchanged in the penta‐twinned Au₆₀ nanocluster, and this new material might be a promising in optical limiting material. This work offers a basis for deep understanding on controlling the cluster‐assembled materials for tailoring their functionalities.