Monolayer-protected Clusters Research Articles

Optical absorption of (Ag-Au)133(SCH3)52 bimetallic monolayer-protected clusters

The evolution of the optical absorption spectrum of bimetallic Ag-Au monolayer-protected clusters (MPC) obtained by progressively doping Ag into the experimentally known structure of Au133(SR)52 was predicted via rigorous time-dependent density-functional theory (TDDFT) calculations. In addition to monometallic Au133(SR)52 and Ag133(SR)52 species, 5 different (Ag-Au)133(SR)52 homotops were considered with varying Ag content and site positioning, and their electronic structure and optical response were analyzed in terms of Projected Density Of States (PDOS), the induced or transition electron density, and Transition Component Maps (TCM) at selected excitation energies. It was found that Ag doping led to the effects rather different from those encountered in bare metal clusters. And it was also observed that Ag doping could produce structured spectral features, especially in the 3–4eV range but also in the optical region if Ag atoms were located in the sub-staple region, as rationalized by the accompanying electronic analysis. Additionally, Au doping into the staples of Ag-rich MPC also gave rise to a more homogeneous induced electron density. These findings show the great sensitivity of the electronic response of MPC nanoalloy systems to the exact location of the alloying sites.

Monolayer‐Protected Clusters: Versatile Materials of Electrochemical Importance

Monolayer‐Protected Clusters: Versatile Materials of Electrochemical Importance

Chemiresistor Arrays Prepared by Simple and Fast Vapor‐Phase Thiol Place‐Exchange Functionalization of Gold Monolayer‐Protected Cluster Films

AbstractWe describe a simple, versatile, and relatively fast method to alter the sensor selectivity of chemiresistors based on films of Au monolayer‐protected clusters (MPCs) by varying their functionalization via vapor‐phase thiol place‐exchange reactions. Drop‐cast deposited films of hexanethiolate (C6S)‐coated Au MPCs are exposed to volatile thiol liquids, such as mercaptoethanol (HOC2S), mercaptopropionic acid (HOOCC2S), and mercaptopropyltrimethoxysilane (H3COSiC3S), which leads to thiol place‐exchange and new functionalities in the film. Generally, the film is about 10–20 % exchanged during the first 4 to 5 h and 80–90 % exchanged after 24 h. The extent and the rate of exchange decreases as the thickness of the film of C6S Au MPCs increases, leading to irreproducibility issues if film thickness is not carefully controlled. Importantly, the chemiresistive sensing response ratio of 2‐propanol relative to toluene vapor increased significantly from about 0.3 prior to exchange to 2.0 after 24 to 48 h of exchange with HOC2S, showing that the selectivity of the sensor could be significantly altered by this method, which is simpler, faster, and more amenable to a wide variety of ligands compared to solution‐based functionalization methods. The chemiresistive response of an unexchanged C6S Au MPC films and those exchanged with HOC2S, HOOCC2S, and H3COSiC3S demonstrates potential electronic nose applications.

Layered Xerogel Films Incorporating Monolayer‐Protected Cluster Networks on Platinum‐Black‐Modified Electrodes for Enhanced Sensitivity in First‐Generation Uric Acid Biosensing

AbstractAmperometric uric acid (UA) biosensing schemes incorporating networks of alkanethiolate‐protected gold nanoparticles, monolayer protected clusters (MPCs), and platinum black (Pt‐B) electrode modification through the layer‐by‐layer construction of xerogels are investigated. MPC doping and Pt‐B augmentation are implemented within hydroxymethyltriethoxysilane xerogel bilayers at platinum electrodes. The first xerogel adlayer is doped with an MPC network and houses uricase for the enzymatic reaction required for first‐generation schemes. Polyluminol–aniline and polyurethane are used as selective/stabilizing interfacial layers. The sensing performance with and without Pt‐B and/or MPC doping is assessed by amperometry with standardized UA injections. The use of each individual material results in an enhancement of UA sensitivity compared with analogous films without these materials. The use of Pt‐B and MPC doping in concert results in a biosensor design with the highest observed UA sensitivity (0.97 μA mm−1) and fast, linear responses over physiologically relevant UA concentrations. Enhancement is attributed to Pt‐B providing increased electrode surface area and integration into the xerogel for greater electronic coupling of the MPC network and more efficient reporting of H2O2 oxidation. The findings have implications for advancing clinical in vivo sensing devices that require scalability or additional biocompatibility layering, both of which would benefit from signal enhancement strategies.

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).

Superatom State-Resolved Dynamics of the Au25(SC8H9)18(-) Cluster from Two-Dimensional Electronic Spectroscopy.

Superatom state-resolved dynamics of the Au25(SC8H9)18(-) monolayer-protected cluster (MPC) were examined using femtosecond two-dimensional electronic spectroscopy (2DES). The electronic ground state of the Au25(SC8H9)18(-) MPC is described by an eight-electron P-like superatom orbital. Hot electron relaxation (200 ± 15 fs) within the superatom D manifold of lowest-unoccupied molecular orbitals was resolved from hot hole relaxation (290 ± 20 fs) in the superatom P states by using 2DES in a partially collinear pump-probe geometry. Electronic relaxation dynamics mediated by specific superatom states were distinguished by examining the time-dependent cross-peak amplitudes for specific excitation and detection photon energy combinations. Quantification of the time-dependent amplitudes and energy positions of cross peaks in the 2.21/1.85 eV (excitation/detection) region confirmed that an apparent energetic blue shift observed for transient bleach signals results from rapid hot electron relaxation in the superatom D states. The combination of structurally precise MPCs and state-resolved 2DES can be used to examine directly the influence of nanoscale structural modifications on electronic carrier dynamics, which are critical for developing nanocluster-based photonic devices.

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.

The Only Way is Up!

In the past year, ChemElectroChem has grown at a good clip. For this, we thank our authors and reviewers, whose support is essential for our success. A new feature that we have introduced during the course of 2015 is the Accepted Article—an option that enables you to have your article published online within a day or two after acceptance, in the form in which it has been accepted. This is done on an opt-in basis, so that you can be assured that your article will only be published in its unedited format with your explicit permission. We have also put in much effort to streamline our post-acceptance process, with the result that proofs are now ready within one or two days after sending a manuscript to the typesetters in most cases. Of course, we still do the careful scientific editing that the ChemPubSoc Europe journals are known for. We are planning a few special issues this year that you can look forward to. Issue 3 will be on Bipolar Electrochemistry, which is a fast-growing field due to advances in the miniaturisation of bipolar electrodes and the various applications that are now possible. Later issues will cover the electrochemistry of monolayer-protected clusters and electrocatalytic nanomaterials. For the new year, we wish all out authors and reviewers success with their research and we encourage you to submit your best work to ChemElectroChem. After all, we are society-owned, so if you support us, you also support your society! G. Heydenrych ChemElectroChem Wiley-VCH Boschstraße 12, 69469 Weinheim (Germany)

Nonlinear Optical Properties of Thiolate-Protected Gold Clusters: A Theoretical Survey of the First Hyperpolarizabilities

A series of thiolate-protected gold clusters has been successfully crystallized in recent years, and on the basis of these crystal structures, we investigate the static first hyperpolarizabilities β0 of eight model clusters [Aum(SH)n]z (m = 18–38) by means of density-functional theory. We used simplified ligands −SH, which may lead to higher symmetries than in actual systems (e.g., with −SCH2CH2Ph or −SPh ligands). A dependence of the obtained values on the exchange-correlation functional during geometry optimization was found. No correlation between cluster size and β0 was identified. Instead, the symmetry of the clusters seems to dominate the NLO properties. Our survey predicts strong NLO responses in the chiral Au38(SR)24 cluster, whereas centrosymmetric structures such as the [Au25(SR)18)]− yield hyperpolarizabilities close to zero. This is in line with recent experimental results obtained by second-harmonic generation. The centrosymmetry of the Au25 cluster is efficiently destroyed by ligand exchange, as demonstrated by the inclusion of chiral, bidentate ligands (1,1′-binaphthyl-2,2′-dithiol, 1,1′-biphenyl-2,2′-dithiol) and two thiophenolate ligands. This induces significant hyperpolarizabilities, surpassing those of intrinsically chiral clusters (e.g., Au38(SH)24). Our results are of significance for the use of monolayer-protected noble metal clusters as contrast agents in NLO imaging applications.

Facile synthesis and characterization of PTA stabilized hydrophilic Au55 nanoparticles via a DEN–MPC method

The synthesis of hydrophilic gold nanoparticles using a combined dendrimer templated (DEN) and monolayer protected cluster (MPC) method is reported. Initially, using the dendrimer as a template leads to well-ordered nanoparticle crystal growth (in the case of geometrical shell structure configurations), which can be controlled by using the desired dendrimer:gold ratio. The nanoparticles are then further stabilized by extraction with a hydrophilic ligand to form MPC’s. Using this combined DEN–MPC method, we managed to synthesize Au55 atom cluster nanoparticles with an average particle size of 1.5±0.9nm. In our attempts to synthesize nanoparticles of different sizes, it was clear that the dendrimer micelle to metal ratio not only plays a role in the formation of stable nanoparticles, but also in the specific atom cluster size. Complete geometrical shell configuration numbers, such as Au55 and Au13 (average size of 3.3±1.0nm), form stable monodisperse nanoparticles, whereas the attempted Au31 atom clusters were unstable, resulting in polydispersed nanoparticles with an average size of 9.4±4.7nm. The mechanism by which these nanoparticles form is discussed.

Optical Properties of Monolayer-Protected Aluminum Clusters: Time-Dependent Density Functional Theory Study

We examine the electronic and optical properties of experimentally known monolayer-protected aluminum clusters Al4(C5H5)4, Al50(C5Me5)12, and Al69(N(SiMe3)2)183– using time-dependent density functional theory. By comparing Al4(C5H5)4 and the theoretical Al4(N(SiMe3)2)4 cluster, we observe significant changes in the optical absorption spectra caused by different hybridization between metal core and ligands. Using these initial observations, we explain the calculated spectra of Al50(C5Me5)12 and Al69(N(SiMe3)2)183–. Al50(C5Me5)12 shows a structured spectrum with clear regions of low-intensity core-to-core transitions followed by high-intensity ligand-to-core transitions due to its high symmetry and π-bonding to the Cp ligands. The spectrum of Al69(N(SiMe3)2)183– is rather featureless as the core-to-core and ligand-to-core regions partially merge because of the lower symmetry found in the metal core and differences in the ligand–core hybridization. Though there are minimal features in the spectra, the most i...

Electrochemical characterization of self-assembled monolayers on gold substrates derived from thermal decomposition of monolayer-protected cluster films

Networked films of monolayer-protected clusters (MPCs), alkanethiolate-stabilized gold nanoparticles, can be thermally decomposed to form stable gold on glass substrates that are subsequently modified with self-assembled monolayers (SAMs) for use as modified electrodes. Electrochemical assessment of these SAM-modified gold substrates, including double-layer capacitance measurements, linear sweep desorption of the alkanethiolates, and diffusional redox probing, all show that SAMs formed on gold supports formed from thermolysis of MPC films possess substantially higher defect density compared to SAMs formed on traditional evaporated gold. The density of defects in the SAMs on thermolyzed gold is directly related to the strategies used to assemble the MPC film prior to thermolysis. Specifically, gold substrates formed from thermally decomposing MPC films formed with electrostatic bridges between carboxylic acid-modified MPCs and metal ion linkers are particularly sensitive to the degree of metal exposure during the assembly process. While specific metal dependence was observed, metal concentration within the MPC precursor film was determined to be a more significant factor. Specific MPC film linking strategies and pretreatment methods that emphasized lower metal exposure resulted in gold films that supported SAMs of lower defect density. The defect density of a SAM-modified electrode is shown to be critical in certain electrochemical experiments such as protein monolayer electrochemistry of adsorbed cytochrome c. While the thermal decomposition of nanoparticle film assemblies remains a viable and interesting technique for coating both flat and irregular shaped substrates, this study provides electrochemical assessment tools and tactics for determining and controlling SAM defect density on this type of gold structure, a property critical to their effective use in subsequent electrochemical applications.

Insights into the Distance Dependence of the Electron Transfer Rate through the Monolayer Protecting Au25 Nanoclusters

The monolayer that protects small gold nanoparticles is often represented as the 3D equivalent of 2D self-assembled monolayers (SAMs) on extended gold surfaces. However, despite the growing relevance of monolayer-protected clusters (MPCs) in important applied areas, such as catalysis and nanomedicine, our knowledge of the structure of these 3D SAMs is quite limited. This is particularly true for the MPCs displaying molecule-like properties (core diameters of less than 1.6 nm). We prepared a large series of monodisperse Au25(SCnH2n+1)18 clusters and studied how electrons tunnel through these monolayers. Heterogeneous electron-transfer results, nicely supported by 1H NMR spectroscopy and molecular dynamics results, provide an interesting picture of the effective structure of the 3D monolayers protecting ultrasmall gold clusters. The distance dependence of the electron-transfer rate in solution is also compared with that of electron transport in the solid state.

Formic Acid Electrochemical Oxidation on Au25 and Pt@Au24 Nanocatalysts: A DFT Approach

Electrooxidation of formic acid has been the focus of intensive investigations due to its importance in the development of low temperature fuel cells. In this work, first principles density functional theoretical calculations were carried out to examine the reaction path for the electrocatalytic oxidation of formic acid on the monolayer-protected Au25 and Pt@Au24 nanoclusters. The Au25-nanocluster oxidizes HCOOH via an indirect pathway; first, forming CO, which can then be reoxidized to form CO2. On the Pt@Au24 nanocluster, it proceeds either through a formate and/or direct path at lower onset potentials than Au25. The calculations present a mechanism for electrochemical experimental observations for formic acid oxidation on monolayer-protected cluster nanocatalysts.

Charge-Transfer Effects in Ligand Exchange Reactions of Au25 Monolayer-Protected Clusters.

Reported here are second-order rate constants of associative ligand exchanges of Au25L18 nanoparticles (L = phenylethanethiolate) of various charge states, measured by proton nuclear magnetic resonance at room temperature and below. Differences in second-order rate constants (M(-1) s(-1)) of ligand exchange (positive clusters ∼1.9 × 10(-5) versus negative ones ∼1.2 × 10(-4)) show that electron depletion retards ligand exchange. The ordering of rate constants between the ligands benzeneselenol > 4-bromobenzene thiol > benzenethiol reveals that exchange is accelerated by higher acidity and/or electron donation capability of the incoming ligand. Together, these observations indicate that partial charge transfer occurs between the nanoparticle and ligand during the exchange and that this is a rate-determining effect in the process.

Structure–function relationships affecting the sensing mechanism of monolayer-protected cluster doped xerogel amperometric glucose biosensors

A systematic study of the structure–function relationships critical to understanding the sensing mechanism of 1st generation amperometric glucose biosensors with an embedded nanoparticle (NP) network is presented. Xerogel-based films featuring embedded glucose oxidase enzyme and doped with alkanethiolate-protected gold NPs, known as monolayer protected clusters (MPCs), exhibit significantly enhanced performance compared to analogous systems without NPs including higher sensitivity, faster response time, and extended linear/dynamic ranges. The proposed mechanism involves diffusion of the glucose to glucose oxidase within the xerogel, enzymatic reaction production of H2O2 with subsequent diffusion to the embedded network of MPCs where it is oxidized, an event immediately reported via fast electron transfer (ET) through the MPC system to the working electrode. Various aspects of the film construct and strategy are systematically probed using amperometry, voltammetry, and solid-state electronic conductivity measurements, including the effects of MPC peripheral chain length, MPC functionalization via place-exchange reaction, MPC core size, and the MPC density or concentration within the xerogel composite films. The collective results of these experiments support the proposed mechanism and identify interparticle spacing and the electronic communication through the MPC network is the most significant factor in the sensing scheme with the diffusional aspects of the mechanism that may be affected by film/MPC hydrophobicity and functionality (i.e., glucose and H2O2 diffusion) shown to be less substantial contributors to the overall enhanced performance. Understanding the structure–function relationships of effective sensing schemes allows for the employment of the strategy for future biosensor design toward clinically relevant targets.

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.

Collision-induced dissociation of monolayer protected clusters Au144 and Au130 in an electrospray time-of-flight mass spectrometer.

Gas-phase reactions of larger gold clusters are poorly known because generation of the intact parent species for mass spectrometric analysis remains quite challenging. Herein we report in-source collision-induced dissociation (CID) results for the monolayer protected clusters (MPCs) Au144(SR)60 and Au130(SR)50, where R- = PhCH2CH2-, in a Bruker micrOTOF time-of-flight mass spectrometer. A sample mixture of the two clusters was introduced into the mass spectrometer by positive mode electrospray ionization. Standard source conditions were used to acquire a reference mass spectrum, exhibiting negligible fragmentation, and then the capillary-skimmer potential difference was increased to induce in-source CID within this low-pressure region (∼4 mbar). Remarkably, distinctive fragmentation patterns are observed for each MPC[3+] parent ion. An assignment of all the major dissociation products (ions and neutrals) is deduced and interpreted by using the distinguishing characteristics in the standard structure-models for the respective MPCs. Also, we propose a ring-forming elimination mechanism to explain R-H neutral loss, as separate from the channels leading to RS-SR or (AuSR)4 neutrals.

Temperature-dependent photoluminescence of structurally-precise quantum-confined Au25(SC8H9)18 and Au38(SC12H25)24 metal nanoparticles.

Temperature-dependent photoluminescence of structurally precise Au25(SC8H9)18 and Au38(SC12H25)24 monolayer-protected cluster (MPC) nanoparticles were studied using energy-resolved, intensity-integrated, and time-resolved spectroscopy. Measurements were carried out at sample temperatures spanning the range from 4.5 to 200 K following electronic excitation using 3.1 eV pulsed lasers. The integrated PL intensity for Au25(SC8H9)18 increased sharply by 70% as the sample temperature was increased from 4.5 to 45 K. The PL intensity was statistically invariant for temperatures between 45 and 65 K but was quenched when the sample temperature was raised above 65 K. For both MPC samples, the global PL emission included several components. Each PL component exhibited an increase in emission energy when the sample temperature was increased from 4.5 to 40 K. This unexpected behavior may imply that MPCs in the 1 nm domain have negative expansion coefficients. Quantitative analysis of PL emission energies and peak widths obtained at sample temperatures greater than 45 K indicated MPC nonradiative relaxation dynamics are mediated by coupling to low-frequency vibrations associated with the ligand shell that passivated the nanoclusters, which accounted for the low emission yields at high sample temperatures. Contributions from two different vibrational modes were identified: Au(I)-S stretching (200 cm(-1)) and Au(0)-Au(I) stretching (90 cm(-1)). Analysis of each PL component revealed that the magnitude of electronic-vibration coupling was state-specific, and consistently larger for the high-energy portions of the PL spectra. The total integrated PL intensity of the Au25(SC8H9)18 MPC was correlated to the relative branching ratios of the emission components, which confirmed decreased emission for recombination channels associated with strong electron-vibration coupling and high emission yields for low emission energies at low temperature. The efficient low-energy emission was attributed to a charge-transfer PL transition. This conclusion was reached based on the strong correlation between temperature-dependent intensity-integrated and time-resolved emission measurements that revealed an ∼3.5-5.5 meV activation barrier to nonradiative decay. These findings suggest that nanoscale structure and composition can be modified to tailor the optical and mechanical properties and electronic relaxation dynamics of MPC nanostructures.