Monolayer-protected Gold Clusters Research Articles

Cesium ion-guided detection of trichloroethylene in air

Whereas the close associations of cesium ion with organochlorine compounds have been previously documented, the present report is the first attempt to exploit these interactions to create a trichloroethylene (TCE)-selective sensor. Gold monolayer-protected clusters peripherally functionalized with Cs+ ions were used to prepare a chemiresistance film on MEMS-fabricated interdigitated electrodes. Vapor sensing properties of the cesium-rich chemiresistor were determined using a panel of chlorinated hydrocarbons including TCE as well as polar and non-polar VOCs for comparison. The sensor was selective and highly sensitive toward VOCs containing a 1,2-dichloro group at concentrations as low as 0.1 ppm. The results suggest the key interaction contributing to sensor response is a bidentate, metallocycle-like coordination of the 1,2-dichloro group to the cesium cations at the sensor surface.

Single-Electron Charging of Thioctic Acid Monolayer-Protected Gold Clusters.

There is great interest in the use of Monolayer-Protected Gold Clusters (AuMPCs) as nanoscale capacitors in aqueous media for nanobiotechnological applications, such as bioelectrocatalysts, biofuel cells, and biosensors. However, AuMPCs exhibiting subattofarad double-layer capacitance at room temperature, and the resolution of single-electron charging, has been mainly obtained in an organic medium with nonfunctional capping ligands. We report here the synthesis of Thioctic Acid Monolayer-Protected Au Clusters (TA-AuMPCs) showing electrochemical single electron quantized capacitance charging in organic and aqueous solutions and when immobilized onto different self-assembled monolayer-modified gold electrodes. The presence of functional carboxylic groups opens a simple strategy for interfacing a nanoparticle assembly to biomolecules for their use as electron donors or acceptors in biological electron transfer reactions.

Harnessing the cation-π interactions of metalated gold monolayer-protected clusters to detect aromatic volatile organic compounds

The strong, non-covalent interactions between π-systems and cations have been the focus of numerous studies on biomolecule structure and catalysis. These interactions, however, have yet to be explored as a sensing mechanism for detecting trace levels of volatile organic compounds (VOCs). In this article, we provide evidence that cation-π interactions can be used to elicit sensitive and selective chemiresistor responses to aromatic VOCs. The chemiresistors are fitted with carboxylate-linked alkali metals bound to the surface of gold monolayer-protected clusters formulated on microfabricated interdigitated electrodes. Sensor responses to aromatic and non-aromatic VOCs are consistent with a model for cation-π interactions arising from association of electron-rich aromatic π-systems to metal ions with the relative strength of attraction following the order K+ > Na+ > Li+. The results point toward cation-π interactions as a promising research avenue to explore for developing aromatic VOC-selective sensors.

(Digital Presentation) Fabrication of Monolayer-Protected Gold Cluster Thin Films: Electrochemical and Optical Properties

Quantum-sized monolayer-protected gold clusters have captivated the imagination of scientists over the last two decades and show exciting optical, electrochemical and catalytic properties when compared to larger-sized plasmonic gold nanoparticles, making them useful for myriad new applications in photonics, catalysis and sensors. One interesting property is the photoluminescence (PL) of gold clusters making them suitable as biological imaging labels. However, the low PL Quantum Yields (QY) are a concern for these clusters and research is focused on improving the PL QYs. One strategy, our research group has worked on is the shell-gold rigidification via the use of surfactants and proteins. Shell ligand rigidification was achieved by ion pairing with bulky tertiary alkyl ammonium groups and aromatic chromophores. Another strategy to reduce the non-radiative deactivation pathways is by making the gold cluster thin films. In a recent study, it was shown that solid state films of clusters not only have manifold increase in PL quantum yield but also their luminescent lifetimes and two photon absorption cross-sections are greatly enhanced in comparison to their solution analogues. Researchers have worked on various strategies to make gold cluster thin films for device applications. One problem with thin films of gold clusters is that the clusters lose their interesting optical and electrochemical properties that were observed in solution phase. For making the clusters useful by enhancing PL for photonic device applications, it is important for the clusters to retain their properties in solid state, which is the main objective of the study carried out here. One problem is that drying intrinsic gold clusters results in crystallization whose optical properties are difficult to characterize. In this study, we have used Nafion and polyvinylpyrrolidone (PVP) to make thin films of water-soluble glutathione-protected gold clusters. Water soluble cluster Au22(SG)18 with reported PLQY of 8% were synthesized and characterized by UV-VIS absorption and PL measurements and thin films of these clusters were made using water soluble PVP and Nafion polymers via drop casting. Enhanced PL and electrochemical properties were observed for clusters in solid state. Similarly, non-polar gold cluster films are fabricated with Polystyrene and optical absorption and PL properties were retained in the solid state.Although drop casting with polymers was found to retain the optical properties, the uniformity of films is a problem. The uniformity of the film is dependent on solvent’s surface tension thus diminishing the quality of the film. Therefore, there is a need of depositing films of gold clusters in a controlled manner which governs their thickness and uniformity. Electropolymerization technique is known to make films during the polymerization of monomers possessing functional groups whose oxidation or reduction via anti-bonding or non-boding electrons on a heteroatom. In this study, we have used electropolymerization strategy to fabricate thin films of gold clusters on conducting substrates. The film is deposited at the working electrode during cyclic voltammetric (CV) sweep when passed through the required reduction or oxidation potential of the monomer under suitable conditions. This technique makes a uniform film throughout the exposed area of the working electrode active surface and the thickness of the film is often proportional to the number of sweep cycles employed. Therefore, introduction of gold clusters in this process with monomers which are previously known to bond with each other will incorporate clusters in the film during the polymerization process and will ease their film formation with uniform thickness. Amongst various size gold clusters, the one which shows good stability as a result of following the magic number series is Au25(SR)18 - . The stability of clustersis one of the important parameter required for practical applications. Aniline and other amine based monomers are used as precursors to make thin films of Au25 on conducting substrate such as Indium tin oxide. By solubilizing the gold clusters with the monomers they are deposited along with the polymers.

Harnessing the Cation-Π Interactions of Metalated Gold Monolayer-Protected Clusters to Detect Aromatic Volatile Organic Compounds

Harnessing the Cation-Π Interactions of Metalated Gold Monolayer-Protected Clusters to Detect Aromatic Volatile Organic Compounds

Development of monolayer-protected gold clusters-based hydrolytic nanozymes containing catalytic units

<p indent=0mm>Nanozymes can catalyze the biochemical reactions mediated by natural enzymes under physiological conditions, and show natural enzymes-like reaction kinetics and catalytic mechanism. Monolayer-protected gold clusters (Au MPCs) containing catalytic units can greatly improve the catalytic abilitys of nanozymes, so it has been developed rapidly in recent years. In this review, the progress of Au MPCs hydrolytic nanozymes containing catalytic units in recent years is summarized. The design of surface catalytic units, supramolecular assembly and application of Au MPCs hydrolytic nanozymes are discussed.

Electron Transfer in Films of Atomically Precise Gold Nanoclusters

In applications of monolayer-protected gold clusters, assessing the dynamic behavior of the capping monolayer is crucial, as this determines the cluster’s effective size and electron-transfer (ET) properties. Here, we describe a systematic study on the effect of the monolayer thickness on the ET between molecular Au25(SR)180 nanoclusters in films. The length of the ligands protecting the Au core was varied by using a series of linear-chain thiolate (SCnH2n+1) protected Au25 clusters, where n = 3, 4, 5, 6, 7, 8, and 10. The effect of branching was assessed by using the 2-methyl-1-butanethiolate-protected Au25 cluster. Conductivity measurements were carried out on dry films obtained by drop-casting Au25(SR)180 solutions onto interdigitated gold electrodes. Changing the alkyl chain from C3 to C10 induces a smooth decrease in the film conductivity by 3.5 orders of magnitude. By using electrochemically determined Stokes radii, the conductivity results were transformed into the corresponding ET rate constants (kETs) between neighboring clusters. kET exponentially depends on the distance, and the data show that the average Au center-to-center distance in the film is not larger than the sum of two Stokes radii. This indicates that linear alkyl chains hold a detectable degree of fluidity in films, although not as much as in solution, as shown by the trend of the corresponding heterogeneous standard rate-constant values. For both physical states, these conclusions are precisely confirmed by the much slower ET rate determined for the Au25 cluster protected by the hindered thiolate. ET in films was also studied as a function of temperature, and the results were analyzed in the framework of the ET theory. We found that the outer and inner reorganization energies increase with n. For these ETs, the electronic coupling resonance between the reactant and product states, HRP, ranges from 12 (n = 3) to 0.7 cm–1 (n = 10) and therefore ETs are nonadiabatic. The data also show that 3/4 of the effect of distance on kET is related to HRP, while the rest is associated with the intrinsic barrier and thus the distance dependence of the outer and inner reorganization energies.

Tunable Aminooxy-Functionalized Monolayer-Protected Gold Clusters for Non-Polar or Aqueous Oximation Reactions.

Aminooxy (-ONH2) groups are well known for their chemoselective reactions with carbonyl compounds, specifically aldehydes and ketones. The versatility of aminooxy chemistry has proven to be an attractive feature that continues to stimulate new applications. This work describes application of aminooxy 'click chemistry' on the surface of gold nanoparticles. We present here a trifunctional amine-containing aminooxy alkane thiol ligand for use in the functionalization of gold monolayer protected clusters (Au MPCs). Diethanolamine is readily transformed into an organic-soluble aminooxy thiol (AOT) ligand using a short synthetic path. The synthesized AOT ligand was coated on ≤ 2 nm diameter hexanethiolate (C6S)-capped Au MPCs using a ligand exchange protocol to afford organic-soluble AOT/C6S (1:1 ratio) Au mixed monolayer protected clusters (MMPCs). This work describes the synthesis of Au(C6S)(AOT) MMPCs and representative oximation reactions with various types of aldehyde-containing molecules, highlighting the ease and versatility of the chemistry and how amine protonation can be used to switch solubility characteristics.

HPLC of Monolayer-Protected Gold Clusters with Baseline Separation.

The properties of ultrasmall metal nanoparticles (ca. 10-200 metal atoms), or monolayer-protected metal clusters (MPCs), drastically depend on their atomic structure. For systematic characterization and application, assessment of their purity is of high importance. Currently, the gold standard for purity control of MPCs is mass spectrometry (MS). Mass spectrometry, however, cannot always detect small impurities; MS of certain clusters, for example, ESI-TOF of Au40(SR)24, is not successful at all. We here present a simple reversed-phase HPLC method for purity control of a series of small alkanethiolate-protected gold clusters. The method allows the detection of small impurities with high sensitivity. Linear correlation between alkyl chain length of Au25(SC nH2 n+1)18 clusters ( n = 6, 8, 10, 12) and their retention time was noticed.

Resonance Enhancement of Nonlinear Optical Scattering in Monolayer-Protected Gold Clusters.

Monolayer-protected metal clusters (MPCs) have recently gained significant research interest, since they are promising candidates for various applications in bioimaging and catalysis. Besides this, MPCs promise to aid in understanding the evolution of the metallic state from bottom-up principles. MPCs can be prepared with atomic precision, and their nonscalable properties (indicating molecule-like behavior) have been studied with a variety of techniques both theoretically and experimentally. Here, we present spectrally resolved second-order nonlinear optical scattering experiments on thiolate-protected gold clusters (Au130(SR)50, Au144(SR)60, and Au500(SR)120). The three clusters share common resonance enhancement around 490 nm, which is ascribed to an interband transition. This indicates emerging metal-like properties, and we tentatively assign the onset of metal-like behavior somewhere between 102 and 130 gold atoms.

Magnetic Ordering in Gold Nanoclusters.

Several research groups have observed magnetism in monolayer-protected gold cluster samples, but the results were often contradictory, and thus, a clear understanding of this phenomenon is still missing. We used Au25(SCH2CH2Ph)180, which is a paramagnetic cluster that can be prepared with atomic precision and whose structure is known precisely. Previous magnetometry studies only detected paramagnetism. We used samples representing a range of crystallographic orders and studied their magnetic behaviors using electron paramagnetic resonance (EPR). As a film, Au25(SCH2CH2Ph)180 exhibits a paramagnetic behavior, but at low temperature, ferromagnetic interactions are detectable. One or few single crystals undergo physical reorientation with the applied field and exhibit ferromagnetism, as detected through hysteresis experiments. A large collection of microcrystals is magnetic even at room temperature and shows distinct paramagnetic, superparamagnetic, and ferromagnetic behaviors. Simulation of the EPR spectra shows that both spin−orbit (SO) coupling and crystal distortion are important to determine the observed magnetic behaviors. Density functional theory calculations carried out on single cluster and periodic models predict the values of SO coupling and crystal-splitting effects in agreement with the EPR-derived quantities. Magnetism in gold nanoclusters is thus demonstrated to be the outcome of a very delicate balance of factors. To obtain reproducible results, the samples must be (i) controlled for composition and thus be monodisperse with atomic precision, (ii) of known charge state, and (iii) well-defined in terms of crystallinity and experimental conditions.

Synthesis of Au clusters-redox centre hybrids by diazonium chemistry employing double layer charged gold nanoparticles

The experimental conditions in the two-phase synthesis of hexanethiolate gold monolayer-protected clusters (C6-Au) have been investigated by varying the thiolation and reduction times to give particles with an average core diameter of 1.66nm and a capacitance of 0.54–0.59aF. These clusters were further functionalised with anthraquinone by reaction between the diazonium derivative of 1-amino anthraquinone and electrochemically or chemically charged nanoparticles. Cyclic voltammetry and differential pulse voltammetry measurements demonstrate functionalisation by the redox couple and the presence of quantised double layer (QDL) charging events at the gold core. The feasibility of using a dispersion of charged nanoparticles as reagents for driving a diazotisation reaction to attach redox ligands is demonstrated.

Electrocrystallization of Monolayer-Protected Gold Clusters: Opening the Door to Quality, Quantity, and New Structures.

Thiolate-protected metal clusters are materials of ever-growing importance in fundamental and applied research. Knowledge of their single-crystal X-ray structures has been instrumental to enable advanced molecular understanding of their intriguing properties. So far, however, a general, reliable, chemically clean approach to prepare single crystals suitable for accurate crystallographic analysis was missing. Here we show that single crystals of thiolate-protected clusters can be grown in large quantity and very high quality by electrocrystallization. This method relies on the fact that charged clusters display a higher solubility in polar solvents than their neutral counterparts. Nucleation of the electrogenerated insoluble clusters directly onto the electrode surface eventually leads to the formation of a dense forest of millimeter-long single crystals. Electrocrystallization of three known Au25(SR)180 clusters is described. A new cluster, Au25(S-nC5H11)18, was also prepared and found to crystallize by forming bundles of millimeter-long Au25 polymers.

Exploring the atomic structure of 1.8 nm monolayer-protected gold clusters with aberration-corrected STEM

Monolayer-protected (MP) Au clusters present attractive quantum systems with a range of potential applications e.g. in catalysis. Knowledge of the atomic structure is needed to obtain a full understanding of their intriguing physical and chemical properties. Here we employed aberration-corrected scanning transmission electron microscopy (ac-STEM), combined with multislice simulations, to make a round-robin investigation of the atomic structure of chemically synthesised clusters with nominal composition Au144(SCH2CH2Ph)60 provided by two different research groups. The MP Au clusters were “weighed” by the atom counting method, based on their integrated intensities in the high angle annular dark field (HAADF) regime and calibrated exponent of the Z dependence. For atomic structure analysis, we compared experimental images of hundreds of clusters, with atomic resolution, against a variety of structural models. Across the size range 123–151 atoms, only 3% of clusters matched the theoretically predicted Au144(SR)60 structure, while a large proportion of the clusters were amorphous (i.e. did not match any model structure). However, a distinct ring-dot feature, characteristic of local icosahedral symmetry, was observed in about 20% of the clusters.

Extension of the Time-Dependent Density Functional Complex Polarizability Algorithm to Circular Dichroism: Implementation and Applications to Ag8and Au38(SC2H4C6H5)24

We detail the calculation of rotatory strengths as an extension of the complex polarizability algorithm of time dependent density functional theory (TDDFT) proposed in a recent publication (O. Baseggio, G. Fronzoni, M. Stener, J. Chem. Phys. 2015, 143, 024106). To demonstrate the generality and applicability of the proposed algorithm, we calculate the photoabsorption and circular dichroism (CD) spectra of Ag8 (as a validation case), Au38(SCH3)24, and Au38(SCH2CH2Ph)24 monolayer-protected gold clusters (as a system of great current interest for their optical properties). For Au38(SCH2CH2Ph)24, the computed CD spectrum agrees well with the experimental data from the literature. Furthermore, a comparison of the calculated CD spectra of the two thiolate-protected nanoclusters reveals that the most distinctive features of the spectra are rather insensitive to the nature of the thiolate tail groups, which, however, play a significant role in shaping optical and dichroic response of the systems, especially in th...

Capillary Liquid Chromatography Mass Spectrometry Analysis of Intact Monolayer-Protected Gold Clusters in Complex Mixtures

In some respects, large noble-metal clusters protected by thiolate ligands behave as giant molecules of definite composition and structure; however, their rigorous analysis continues to be quite challenging. Analysis of complex mixtures of intact monolayer-protected clusters (MPCs) by liquid chromatography mass spectrometry (LC-MS) could provide quantitative identification of the various components present. This advance is critical for biomedical and toxicological research, as well as in fundamental studies that rely on the identification of selected compositions. This work expands upon the separate LC and MS results previously achieved, by interfacing the capillary liquid chromatograph directly to the electrospray source of the mass spectrometer, in order to provide an extremely sensitive, quantitative, and rapid means to characterize MPCs and their derivatives far beyond that of earlier reports. Here, we show that nonaqueous reversed-phase chromatography can be coupled to mass-spectrometry detection to resolve complex mixtures in minute (∼100 ng) samples of gold MPCs, of molecular masses up to ∼40 kDa, and with single-species sensitivity easily demonstrated for components on the level of sub-10 ng or picomole (1 pmol).

Enhanced absorption and fluorescence of gold nanoclusters using initial alkali concentrations

Understanding carrier dynamics and electromagnetic interactions between emerging quantum-confined nanostructures and plasmonic structures is crucial for future biological applications. In this research, we fabricate gold monolayer-protected clusters (AuMPC). We demonstrate enhanced light absorption and fluorescence of AuMPCs by varying the initial alkali concentration. We measure absorption bands enhanced up to nine times with extended and distinct features centered at 3.33 eV, and fluorescence enhanced up to 3.9 times. An increased alkali concentration changes the charge transfer capability of the surface thiolate ligands through sulfur-gold bonds, which in turn enhance/reduce the fluorescence intensity.

A Unified Framework for Understanding the Structure and Modifications of Atomically Precise Monolayer Protected Gold Clusters

Atomically precise monolayer protected clusters are molecules comprising a few-atom cluster core of a noble metal, typically Au or Ag, surrounded by a protective layer of ligands, exhibiting many special optical, electrical, catalytic, and magnetic properties, and are emerging as important materials in biology, medicine, catalysis, energy conversion and storage, and sensing. The structural diversity of these clusters or aspicules, as we definitively term them, meaning shielded molecules, combining the Greek word aspis (shield) with molecule, is rapidly increasing due to new compositions and modification routes such as ligand-exchange, alloying, or supramolecular functionalization. We present a structural analysis of the most stable cluster of this kind, Au25(SR)18, and propose a Borromean rings diagram for the cluster, showing its topological configuration of three interlocked (Au8S6)-rings. This simplified two-dimensional diagram is used to represent its structure and modifications via ligand or metal atom substitution uniquely. We enumerate and name its isomers with two-ligand or metal atom substituents. Among the several structural insights obtained, the identification of the Borromean rings-interlocked configuration in Au25(SR)18 may explain its high geometric stability and indicate a possible general unified structural viewpoint for these clusters without the division between core and staple motifs. On the basis of our structural analysis, we developed a structure-based nomenclature system that can be applied to both describe and understand the structure and modifications of gold thiolate clusters, AuM(SR)N, and is adaptable to the general case of MM(X)N (M, metal and X, ligand). The application of structural analysis and diagrams to Au38(SR)24 and Au102(SR)44, revealing the possible formation of the cluster core by stacking or growth of rings of metal atoms, is also presented.

Nanometals: Identifying the Onset of Metallic Relaxation Dynamics in Monolayer-Protected Gold Clusters Using Femtosecond Spectroscopy

Electronic relaxation dynamics were studied for a series of gold monolayer-protected clusters (MPCs) whose sizes ranged from 1.5 to 2.4 nm. Au96(mMBA)42, Au102(pMBA)44, Au115(pMBA)49, Au117(mMBA)50, Au144(pMBA)60, Au250(pMBA)98, and Au459(pMBA)170 (pMBA = para-mercaptobenzoic acid; mMBA = meta-mercaptobenzoic acid) were selected for study because they bridged the expected transition from nonmetallic to metallic electron behavior. Excitation-pulse-energy-dependent measurements confirmed Au144(pMBA)60 (1.8 nm) as the smallest MPC to exhibit metallic behavior, with a quantifiable electron–phonon coupling constant of (1.63 ± 0.25) × 1016 W m–3 K–1. Smaller, nonmetallic MPCs exhibited nanocluster-specific transient extinction spectra characteristic of transitions between discrete quantum-confined electronic states. Volume-dependent electronic relaxation dynamics for ≤1.8 nm MPCs were observed and attributed to a combination of large energy differences between electronic states and phonon frequencies and spatia...

Flavin-Functionalized Gold Nanoparticles as an Efficient Catalyst for Aerobic Organic Transformations

Monolayer-protected gold clusters functionalized with synthetic flavins were synthesized and their catalytic activity in aerobic organic transformations investigated. Gold nanoparticles with 5-ethyl-3-(8-thiooctyl)lumiflavinium perchlorate acts as an efficient catalyst for the aerobic oxidation of organic sulfides to the corresponding sulfoxides upon treatment with hydrazine at room temperature and under atmospheric pressure in oxygen. With a catalytic amount of gold nanoparticles with 3-(8-thiooctyl)lumiflavin, diimide reduction of various olefins can be performed with hydrazine at room temperature under atmospheric pressure in air with greater yields of product alkanes than with non-supported 3-methyllumiflavin catalyst under the same conditions. Kinetic studies revealed that the monolayer-protected gold cluster-catalyzed reactions proceeded faster than those with non-supported catalysts over the full substrate concentration range for the hydrogenation of olefins and at lower substrate concentrations for sulfoxidation. This positive effect was rationalized by assuming a Michaelis–Menten-type mechanism in which the specific inclusion of substrates into the enzyme-like reaction cavities was a key factor in the high efficiency of the supported flavin catalysts.