Selective determination of dopamine using quantum-sized gold nanoparticles protected with charge selective ligands

On the variation of cluster core characteristics by an endohedral atom. Shape variation in 8-ce [EAu4(PPh3)4]2+ (E = N, P, As, Sb) clusters

Atomically precise gold superatoms have attracted interest owing to their suitable use as building blocks for cluster-assembled materials, favoring ordered structures with advanced properties. In this sense, expanding their versatility is a relevant issue for controlling their properties and retaining a specific nuclearity. Interestingly, the reported structure for isoelectronic [Au4N(PPh3)4]+ and [Au4Sb(PPh3)4]+ clusters denotes two contrasting shapes featuring a tetrahedral and square pyramidal structure, respectively. Herein, we further explore the [Au4E(PPh3)4]+ (E = N, P, As, Sb) series in order to evaluate energetic and structural factors determining the overall shape. Our results show a favorable [Au4(PPh3)4]4+/E3- interaction energy, predicting particular patterns in their UV-vis spectrum. Thus, the use of dopant atoms is enabled to vary the core shape and, in turn, to modify the cluster properties, which serve as a structural control, in addition to ligand-based and size approaches.

Intermediate Intercluster Bond Orders. Electronic Communication in Au38(SR)24 Superatomic Molecules

Ligand-protected gold clusters remain potential building blocks for envisaged molecular materials. The archetypal Au38(SR)24 cluster can be viewed as a robust template for the fusion of two Au25(SR)18 - cluster units, retaining a bi-icosahedral Au23 core. Via electrochemical properties, the overall charge state can be selectively tuned, enabling the access of 14 valence electron (ve) species featuring a single intercluster bond and nearby charge from -1 to +3, achieving related species bearing 15- to 11-ve with variable intercluster bond orders. Here, we explore the characteristics of intermediate intercluster bond orders in order to provide insights into the plausible electron communication between the constituent building blocks, with Au38(SR)24, as a representative template. Our results denote a small structural variation along -1 to +3 charge states, provided by the core-protecting ligand interaction, which is enhanced towards more oxidized species. The remaining unpaired electron from intermediate intercluster bond orders of 1.5 for Au38(SR)24 1-, 1.5 for Au38(SR)24 1+, and 2.5 for Au38(SR)24 3+, holds delocalized characteristics between the building block units, favoring electron communication for conductive and cooperative cluster aggregates. Such features are relevant for the formation of molecular electronic device applications, favoring the rationalization prior to engaging in explorative synthesis of larger ligand-protected cluster aggregates.

Intermediate Silver Doping of Au25(SR)18: Variation of Electronic, Optical, and Chiroptical Properties along Au25-xAgx(SH)18- (x = 0-12) Stoichiometry from DFT Calculations

The silver analogue of the prominent Au₂₅(SR)₁₈ nanocluster reveals the possibility of finding "gold"-like behavior despite their different nature, in addition to the common features among molecular AgNP. Herein, we explore the effect of successive additions of silver atoms reaching an intermediate Ag/Au doping ratio where the parent gold cluster exhibits properties from both elements. Our results show a more favorable situation as the Ag/Au ratio increases along the Au_25-xAg_x(SH)₁₈^- (x = 0-12) clusters, with structural distortions mainly centered at the ligand-protected shell. The calculated optical spectrum shows that from the Au₁₉Ag₆ species, a plasmon-like peak appears along species with a doping ratio above 25%, where all the silver atoms are located within the M₁₂ icosahedron. In addition, the chiral properties were explored, showing mild optical activity from the calculated circular dichroism spectra due to the distorted ligand-shell avoiding a centrosymmetric structure. Thus, an intermediate doping ratio ascribed to a specific structural layer can recover inherent properties to both elements in the binary Au_25-xAg_x(SH)₁₈^- series, suggesting the possibility of having clusters with dual properties at a certain degree of element exchange. This can be useful for further exploration theoretically and synthetically toward different and larger-nuclearity clusters.

Novel delipidated chicken feather waste-derived carbon-based molybdenum oxide nanocomposite as efficient electrocatalyst for rapid detection of hydroquinone and catechol in environmental waters

Chicken poultry industry produces a vast amount of feather waste and is often disposed into landfills, creating environmental pollution. Therefore, we explored the valorization of chicken feather waste into lipids and keratinous sludge biomass. This study demonstrates the successful utilization of keratinous sludge biomass as a unique precursor for the facile preparation of novel keratinous sludge biomass-derived carbon-based molybdenum oxide (KSC@MoO₃) nanocomposite material using two-step (hydrothermal and co-pyrolysis) processes. The surface morphology and electrochemical properties of as-prepared nanocomposite material were analyzed using HR-SEM, XRD, XPS, and cyclic voltammetric techniques. KSC@MoO₃ nanocomposite exhibited prominent electrocatalytic behavior to simultaneously determine hydroquinone (HQ) and catechol (CC) in environmental waters. The as-prepared electrochemical sensor showed excellent performance towards the detection of HQ and CC with broad concentration ranges between 0.5-176.5 μM (HQ and CC), and the detection limits achieved were 0.063 μM (HQ) and 0.059 μM (CC). Furthermore, the developed modified electrode has exhibited excellent stability and reproducibility and was also applied to analyze HQ and CC in environmental water samples. Results revealed that chicken feather waste valorization could result in sustainable biomass conversion into a high-value nanomaterial to develop a cost-effective electrochemical environmental monitoring sensor and lipids for biofuel.

Toward the Formation of N-Heterocyclic-Carbene-Protected Gold Clusters of Various Nuclearities. A Comparison with Their Phosphine-Protected Analogues from Density Functional Theory Calculations

The structure and bonding of a series of selected phosphine-protected gold clusters (Au_n-P) of nuclearity varying from n = 6 to 13 were investigated by density functional theory (DFT) calculations and compared to those of the hypothetical homologues in which phosphines were replaced by N-heterocyclic carbene (NHC) analogues (Au_n-C). Both the Au_n-P and Au_n-C series exhibit similar stabilities and structural features, except for n = 6, where some differences are noted. The NHC ligands are found to be even slightly more strongly bonded to the gold core (by a few kilocalories per mole per ligand) than phosphines. Investigation of the optical properties of both series using time-dependent DFT calculations indicates similarities in the nature and energies of the UV-vis optical transitions and, consequently, relatively similar shapes of the simulated spectra, with a general blue-shift tendency when going from Au_n-P to Au_n-C. The fluorescence behavior observed experimentally for some of the Au_n-P species is expected to occur also for their Au_n-C analogues, which can be extended to other carbene-ligand-protected nanoclusters. Our results show that it should be possible to stabilize gold clusters with NHC ligands, in relation to the seminal Au₁₃-ligand-protected core, offering novel building blocks for the design of nanostructured materials with various properties.

Triple 1D[triple bond, length as m-dash]1D superatomic bonding. Au22(dppo)6 as a Π4- and Δ2-triply bonded cluster based on Au11 assembled units

δ-Bonds have been intimately related to metal-metal bonds of d-elements since the archetypal d⁴-d⁴ [Re₂Cl₈]^2- ion with a σ²π⁴δ² bond. Currently, the notion of multiple superatom arrangements as initial steps toward molecular materials within the building-up approach is dominated by P-shell characteristics, as given in the well-described Au₂₅(SR)₁₈ ligand protected cluster. In this work we rationalize the Au₂₂(dppo)₆ cluster as a triple-bonded 22-valence electron (ve) supermolecule, featuring a bonding scheme based on 1D + 1D shell combinations, which largely contrasts with the 14-ve Au₃₈(SR)₂₄ with mainly 1P + 1P patterns mimicking a F₂ molecule. The resulting Π⁴Δ²-bonding pattern shows an unprecedented superatomic counterpart of a d-shell based bond inherently related to transition-metal dimers, adding useful key aspects to the understanding of species based on cluster-assembly.

In-situ graft-crosslinked gold nanoparticles with high-density surface defects and coated with a polytaurine membrane for the voltammetric determination of dopamine

Well-dispersed and graft-crosslinked gold nanoparticles (AuNPs) were synthesized by the reduction of tetrachloroaurate with hydrazine at room temperature. The AuNPs possess a high density of surface defects which is due to grafting of n-octanoic acid to polyvinylpyrrolidone. The physical and chemical properties of the resulting AuNPs were characterized by UV-vis, XRD, TEM/HRTEM, SAED, and XPS, respectively. The modified AuNPs were placed on a glassy carbon electrode (GCE) in an electropolymerized taurine layer to obtain a sensitive, selective, stable and rapid electrochemical dopamine sensor. The peak current, typically measured at 0.17 V (vs. SCE), increases linearly in the 1.0 to 120 μM dopamine concentration range, and the limit of detection (at S/N = 3) is 0.16 μM with a sensitivity of 2.94 μA·μM^-1·cm^-2. The sensor was successfully applied to the determination of dopamine in injections and spiked serum samples. The recoveries from spiked serum samples range from 97.5 to 102.4%, with RSDs ranging between 2.8 and 3.4%. Graphical abstract Schematic representation of a glassy carbon electrode modified with in-situ graft-crosslinked gold nanoparticles combined with an electropolymerized polytaurine membrane. The sensor exhibits excellent features towards dopamine determination.

Electrochemistry of Atomically Precise Metal Nanoclusters

Thiolate-protected metal nanoparticles containing a few to few hundred metal atoms are interesting materials exhibiting unique physicochemical properties. They encompass the bulk-to-molecule transition region, where discrete electronic states emerge and electronic band energetics yield to quantum confinement effects. Recent progresses in the synthesis and characterization of ultrasmall gold nanoparticles have opened up new avenues for the isolation of extremely monodispersed nanoparticles with atomically precision. These nanoparticles are also called nanoclusters to distinguish them from other regular metal nanoparticles with core diameter >2 nm. These nanoclusters are typically identified by their actual molecular formulas; prominent among these are Au₂₅(SR)₁₈, Au₃₈(SR)₂₄, and Au₁₀₂(SR)₄₄, where SR is organothiolate. A number of single crystal structures of these nanoclusters have been disclosed. Researchers have effectively utilized density functional theory (DFT) calculations to predict their atomic and electronic structures, as well as their physicochemical properties. The atomically precise metal nanoclusters have been the focus of recent studies owing to their novel size-specific electrochemical, optical, and catalytic properties. In this Account, we highlight recent advances in electrochemistry of atomically precise metal nanoclusters and their applications in electrocatalysis and electrochemical sensing. Compared with gold nanoclusters, much less progress has been made in the electrochemical studies of other metal nanoclusters, and thus, we mainly focus on the electrochemistry and electrochemical applications of gold-based nanoclusters. Voltammetry has been extremely powerful in investigating the electronic structure of metal nanoclusters, especially near HOMO and LUMO levels. A sizable opening of HOMO-LUMO gap observed for Au₂₅(SR)₁₈ gradually decreases with increasing nanocluster size, which is in line with the change in the optical gap. Heteroatom-doping has been a powerful strategy to modify the optical and electrochemical properties of metal nanoclusters at the atomic level. While the superatom theory predicts 8-electron configuration for [Au₂₅(SR)₁₈]^- and many doped nanoclusters thereof, Pt- and Pd-doped [PtAu₂₄(SR)₁₈]⁰ and [PdAu₂₄(SR)₁₈]⁰ nanoclusters show dramatically different electronic structures, as manifested in their optical spectra and voltammograms, suggesting the occurrence of the Jahn-Teller distortion in these doped nanoclusters. Furthermore, metal-doping may alter their surface binding properties, as well as redox potentials. Metal nanoclusters offer great potential for attaining high activity and selectivity in their electrocatalytic applications. The well-defined core-shell structure of a metal nanocluster is of special advantage because the core and shell can be independently engineered to exhibit suitable binding properties and redox potentials. We discuss recent progress made in electrocatalysis based upon metal nanoclusters tailored for water splitting, CO₂ conversion, and electrochemical sensing. A well-defined model nanocatalyst is absolutely necessary to reveal the detailed mechanism of electrocatalysis and thereby to lead to the development of a new efficient electrocatalyst. We envision that atomically controlled metal nanoclusters will enable us to systematically optimize the electrochemical and surface properties suitable for electrocatalysis, thus providing a powerful platform for the discovery of finely tuned nanocatalysts.

Single, double, and triple intercluster bonds: analyses of M2Au36(SR)24 (M = Au, Pd, Pt) as 14-, 12- and 10-ve superatomic molecules

Single, double, and triple-bonds can be found in fused superatomic clusters.

Aromatic character of [Au13]5+ and [MAu12]4+/6+ (M = Pd, Pt) cores in ligand protected gold nanoclusters – interplay between spherical and planar σ-aromatics

Ligand-protected superatoms are able to behave as both spherical and planar aromatic species, providing a strong link between spherical and planar σ-aromatics, which can be controlled selectively by tuning their redox charge states.

Potential of N-heterocyclic carbene derivatives from Au13(dppe)5Cl2gold superatoms. Evaluation of electronic, optical and chiroptical properties from relativistic DFT

N-heterocyclic carbene (NHC) introduction into well-defined atomically precise gold superatoms allows efficient control of structural, optical, chiroptical and emission features of the Au₁₃Cl₂core, related to the classical chiral [Au₁₃Cl₂(dppe)₅]³⁺nanocluster.

[Au12 (SR)6 ]2- , As Smaller 8-Electron Gold Nanocluster Retaining an SP3 -Core. Evaluation of Bonding and Optical Properties from Relativistic DFT Calculations

Exploring the versatility of atomically precise clusters is a relevant issue in the design of functional nanostructures. Superatomic clusters offer an ideal framework to gain further understanding of the different distinctive size-dependent physical and chemical properties. Here, we propose [Au12 (SR)6 ]2- as a minimal 8-electron superatom related to the prototypical [Au25 (SR)18 ]- cluster, depicting half of its core-mass (2.3 kDa vs 5.0 kDa). The [Au12 (SMe)6 ]2- cluster fulfills a 1S2 1P6 electronic configuration, with a distorted tetrahedral Au8 core further viewed as an SP3 -hybridized superatom. The distinctive optical properties show a blue-shift for the first relevant 1P→1D transition, in comparison to [Au25 (SR)18 ]- . In addition, chiroptical activity is observed, denoting intrinsic core chirality. We expect that our results can shed light into the variation of the molecular properties according to the size-dependent properties, and serve as guidelines for further experimental exploration of minimal or ultrasmall nanoclusters.

Atomically Precise Gold Nanoclusters Accelerate Hydrogen Evolution over MoS2 Nanosheets: The Dual Interfacial Effect

Hydrogen generation via electrocatalytic water splitting holds great promise for future energy revolution. It is desirable to design abundant and efficient catalysts and achieve mechanistic understanding of hydrogen evolution reaction (HER). Here, this paper reports a strategy for improving HER performance of molybdenum disulfide (MoS₂ ) via introducing gold nanoclusters as a cocatalyst. Compared to plain MoS₂ nanosheets, the Au₂₅ (SR)₁₈ /MoS₂ nanocomposite exhibits enhanced HER activity with a small onset potential of -0.20 V (vs reversible hydrogen electrode) and a higher current density of 59.3 mA cm^-2 at the potential of -0.4 V. In addition to the interfacial interaction between nanoclusters and MoS₂ , the interface between the Au₂₅ core and the surface ligands (thiolate vs selenolate) is also discovered to distinctly affect the catalytic performance. This work highlights the promise of metal nanoclusters in boosting the HER performance via tailoring the interfacial electronic interactions between gold nanoclusters and MoS₂ nanosheets, as well as the interface between metal core and surface ligands.

Gold Nanoclusters Promote Electrocatalytic Water Oxidation at the Nanocluster/CoSe2 Interface

Electrocatalytic water splitting to produce hydrogen comprises the hydrogen and oxygen evolution half reactions (HER and OER), with the latter as the bottleneck process. Thus, enhancing the OER performance and understanding the mechanism are critically important. Herein, we report a strategy for OER enhancement by utilizing gold nanoclusters to form cluster/CoSe₂ composites; the latter exhibit largely enhanced OER activity in alkaline solutions. The Au₂₅/CoSe₂ composite affords a current density of 10 mA cm^-2 at small overpotential of ∼0.43 V (cf. CoSe₂: ∼0.52 V). The ligand and gold cluster size can also tune the catalytic performance of the composites. Based upon XPS analysis and DFT simulations, we attribute the activity enhancement to electronic interactions between nanocluster and CoSe₂, which favors the formation of the important intermediate (OOH) as well as the desorption of oxygen molecules over Au_n/CoSe₂ composites in the process of water oxidation. Such an atomic level understanding may provide some guidelines for design of OER catalysts.

Fabrication of bimetallic Cu/Au nanotubes and their sensitive, selective, reproducible and reusable electrochemical sensing of glucose

Herein, we report a facile two-step approach to produce gold-incorporated copper (Cu/Au) nanostructures through controlled disproportionation of the Cu(+)-oleylamine complex at 220 °C to form copper nanowires and the subsequent reaction with Au(3+) at different temperatures of 140, 220 and 300 °C. In comparison with copper nanowires, these bimetallic Cu/Au nanostructures exhibit their synergistic effect to greatly enhance glucose oxidation. Among them, the shape-controlled Cu/Au nanotubes prepared at 140 °C show the highest electrocatalytic activity for non-enzymatic glucose sensing in alkaline solution. In addition to high sensitivity and fast response, the Cu/Au nanotubes possess high selectivity against interferences from other potential interfering species and excellent reproducibility with long-term stability. By introducing gold into copper nanostructures at a low level of 3, 1 and 0.1 mol% relative to the initial copper precursor, a significant electrocatalytic enhancement of the resulting bimetallic Cu/Au nanostructures starts to occur at 1 mol%. Overall, the present fabrication of stable Cu/Au nanostructures offers a promising low-cost platform for sensitive, selective, reproducible and reusable electrochemical sensing of glucose.

Designed self-assembled hybrid Au@CdS core–shell nanoparticles with negative charge and their application as highly selective biosensors

Schematic illustration of the reaction mechanism of Au@CdS core–shell structure with DA in the presence of UA and AA.

Fast, sensitive and selective colorimetric gold bioassay for dopamine detection

A highly sensitive and selective colorimetric biosensor for dopamine has been developed by using double molecular recognition modified Au nanoparticles.

Ionic liquid of a gold nanocluster: a versatile matrix for electrochemical biosensors

Ionic liquids are room-temperature molten salts that are increasingly used in electrochemical devices, such as batteries, fuel cells, and sensors, where their intrinsic ionic conductivity is exploited. Here we demonstrate that combining anionic, redox-active Au25 clusters with imidazolium cations leads to a stable ionic liquid possessing both ionic and electronic conductivity. The Au25 ionic liquid was found to act as a versatile matrix for amperometric enzyme biosensors toward the detection of glucose. Enzyme electrodes prepared by incorporating glucose oxidase in the Au25 ionic liquid show high electrocatalytic activity and substrate affinity. Au25 clusters in the electrode were found to act as effective redox mediators as well as electronic conductors determining the detection sensitivity. With the unique electrochemical properties and almost unlimited structural tunability, the ionic liquids of quantum-sized gold clusters may serve as versatile matrices for a variety of electrochemical biosensors.

Redox-active gold nanoclusters immobilized ZnO nanorod electrodes for electrochemical sensing applications

Well-defined composite electrodes were fabricated by combining redox-active Au₂₅ nanoclusters with highly oriented ZnO nanorods.

Understanding the extraordinary ionic reactivity of aqueous nanoparticles

Nanoparticles (NPs) are generally believed to derive their high reactivity from the inherently large specific surface area. Here we show that this is just the trivial part of a more involved picture. Nanoparticles that carry electric charge are able to generate chemical reaction rates that are even substantially larger than those for similar molecular reactants. This is achieved by Boltzmann accumulation of ionic reactants and the Debye acceleration of their transport. The ensuing unique reactivity features are general for all types of nanoparticles but most prominent for soft ones that exploit the accelerating mechanisms on a 3D level. These features have great potential for exploitation in the catalysis of ionic reactions: the reactivity of sites can be enhanced by increasing the indifferent charge density in the NP body.

One-pot synthesis of phenylmethanethiolate-protected Au20(SR)16 and Au24(SR)20 nanoclusters and insight into the kinetic control

We report two synthetic routes for concurrent formation of phenylmethanethiolate (-SCH2Ph)-protected Au20(SR)16 and Au24(SR)24 nanoclusters in one-pot by kinetic control. Unlike the previously reported methods for thiolate-protected gold nanoclusters, which typically involve rapid reduction of the gold precursor by excess NaBH4 and subsequent size focusing into atomically monodisperse clusters of a specific size, the present work reveals some insight into the kinetic control in gold-thiolate cluster synthesis. We demonstrate that the synthesis of -SCH2Ph-protected Au20 and Au24 nanoclusters can be obtained through two different, kinetically controlled methods. Specifically, route 1 employs slow addition of a relatively large amount of NaBH4 under slow stirring of the reaction mixture, while route 2 employs rapid addition of a small amount of NaBH4 under rapid stirring of the reaction mixture. At first glance, these two methods apparently possess quite different reaction kinetics, but interestingly they give rise to exactly the same product (i.e., the coproduction of Au20(SCH2Ph)16 and Au24(SCH2Ph)20 clusters). Our results explicitly demonstrate the complex interplay between the kinetic factors that include the addition speed and amount of NaBH4 solution as well as the stirring speed of the reaction mixture. Such insight is important for devising synthetic routes for different sized nanoclusters. We also compared the photoluminescence and electrochemical properties of PhCH2S-protected Au20 and Au24 nanoclusters with the PhC2H4S-protected counterparts. A surprising 2.5 times photoluminescence enhancement was observed for the PhCH2S-capped nanoclusters when compared to the PhC2H4S-capped analogues, thereby indicating a drastic effect of the ligand that is merely one carbon shorter.

Monoplatinum doping of gold nanoclusters and catalytic application

We report single-atom doping of gold nanoclusters (NCs), and its drastic effects on the optical, electronic, and catalytic properties, using the 25-atom system as a model. In our synthetic approach, a mixture of Pt(1)Au(24)(SC(2)H(4)Ph)(18) and Au(25)(SC(2)H(4)Ph)(18) was produced via a size-focusing process, and then Pt(1)Au(24)(SC(2)H(4)Ph)(18) NCs were obtained by selective decomposition of Au(25)(SC(2)H(4)Ph)(18) in the mixture with concentrated H(2)O(2) followed by purification via size-exclusion chromatography. Experimental and theoretical analyses confirmed that Pt(1)Au(24)(SC(2)H(4)Ph)(18) possesses a Pt-centered icosahedral core capped by six Au(2)(SC(2)H(4)Ph)(3) staples. The Pt(1)Au(24)(SC(2)H(4)Ph)(18) cluster exhibits greatly enhanced stability and catalytic activity relative to Au(25)(SC(2)H(4)Ph)(18) but a smaller energy gap (E(g) ≈ 0.8 eV vs 1.3 eV for the homogold cluster).

Electrochemical Characterization of Water-Soluble Au25 Nanoclusters Enabled by Phase-Transfer Reaction

We report the synthesis and electrochemical characterization of a new water-soluble Au25 cluster protected with (3-mercaptopropyl)sulfonate. The presence of sulfonate terminal groups on the surface of the cluster enabled facile phase transfer of the water-soluble cluster to organic phase by ion-pairing with hydrophobic counterions. The phase-transferred form of the water-soluble Au25 cluster was found to retain its integrity and allowed investigation of its electrochemical properties in organic media. The voltammetric investigation of the phase-transferred Au25 cluster in mixtures of CH2Cl2 and toluene has revealed that the cluster exhibits the characteristic Au25 peak pattern, but the electrochemical HOMO-LUMO energy gap of the cluster varies from 1.39 to 1.66 V depending on the solvent polarity. The origin of the solvent dependence is explained by the electrostatic field effect of the sulfonate anion on the redox potentials of the Au25 cluster.