The monolayer thickness dependence of quantized double-layer capacitances of monolayer-protected gold clusters

This report describes how the electrochemical double-layer capacitances of nanometer-sized alkanethiolate monolayer-protected Au clusters (MPCs) dissolved in electrolyte solution depend on the alkanethiolate chain length (C4 to C16). The double-layer capacitances of individual MPCs (C(CLU)) are sufficiently small (sub-attoFarad, aF) that their metal core potentials change by >0.1 V increments for single electron transfers at the electrode/solution interface. Thus, the current peaks observed are termed "quantized double layer charging peaks", and their spacing on the potential axis varies with C(CLU). Differential pulse voltammetric measurements of C(CLU) in solutions of core-size-fractionated (i.e., monodisperse) MPCs are compared to a simple theoretical model, which considers the capacitance as governed by the thickness of a dielectric material (the monolayer, whose chain length is varied) between concentric spheres of conductors (the Au core and the electrolyte solution). The experimental results fit the simple model remarkably well. The prominent differential pulse voltammetric charging peaks additionally establish this method, along with high-resolution transmission electron microscopy and laser ionization-desorption mass spectrometry, as a tool for evaluating the degree of monodispersity of MPC preparations. We additionally report on a new tactic for the preparation of monodisperse MPCs with hexanethiolate monolayers.     

Temperature-dependent quantized double layer charging of monolayer-protected gold clusters

This paper describes low-temperature voltammetry of purified hexanethiolate-coated monolayer-protected Au140 clusters (C6 MPCs). Lowered temperatures enhance the resolution of quantized double layer (QDL) charging peaks in differential pulse voltammetry (DPV) observations. As many as 13 resolved peaks are seen in illustrative voltammetry at 263 K in CH2Cl2 solvent, and the concept of voltammetric peak capacity is introduced. For the one-electron MPC charge steps surrounding the E(PZC) of the MPC (small numbers of electrons added or removed from the core), the capacitance C(CLU) of the MPCs (measured from the voltage spacing between charging peaks) increases by approximately 15% as the solvent temperature is lowered from 273 to 210 K. The experimental C(CLU) temperature dependency (d[ln(C(CLU))]/dT approximately -0.0025, in 0.1 M electrolyte) is discussed in light of temperature dependencies of the compact and diffuse double layer capacitances. It is concluded that the observed temperature dependence is probably a mixed diffuse, compact dependence. The regular voltage spacing of MPC charging peaks near the potential of zero charge is generally consistent with electrical double layer properties, but the irregular pattern of charging of the nanoparticles seen at higher charge states suggests intervention of the incipient molecular behavior of Au140 cores in the spacing of energies at which further electrons are added or removed.     

HPLC of monolayer-protected gold nanoclusters

Polydisperse samples of Au nanoparticles protected with monolayers of hexanethiolate ligands (C6 MPCs) and with mixed monolayers of hexanethiolate and mercaptoundecanoic acid (C6/MUA MPCs) have been chromatographically separated using C8 120-A columns and acetone/ toluene mobile phase. The spectral details of eluted peaks and of quantized double-layer charging features in the differential pulse voltammetry of collected fractions were used to show that the elution orders of C6 MPC mixtures and of C6/MUA MPC mixtures were different. For C6 MPCs, the smallest MPCs were eluted first, whereas the smallest C6/MUA MPCs were eluted last. The reversal of order of elution was rationalized in terms of intermolecular interactions with the stationary phase, dominant for the C6 MPC, being suppressed by the heightened polarity of the monolayer surface of the C6/MUA MPCs, making a size exclusion mechanism dominant. The range of apparent core diameters of the separated nanoparticles was 1.3-2 nm.     

Redox and double-layer charging of phenothiazine functionalized monolayer-protected clusters

Monolayer-protected Au clusters (MPCs) have been prepared with mixed monolayers of alkanethiolates and alkanethiolates terminally omega-functionalized with phenothiazine. The mixed monolayer MPCs can contain as many as 10 phenothiazines/MPC; these electron donors are electroactive in rapid, successive one-electron reactions. Surface adsorption of the functionalized MPCs is evident in cyclic voltammetry. Double-potential-step chronocoulometry with incremented potential steps was applied to unfunctionalized hexanethiolate-coated MPCs and to those functionalized with phenothiazine to analyze the coupling between the diffusion-controlled double-layer charging of the MPC cores and the oxidation of the phenothiazine centers. Apparent changes in ordering of the MPC alkanethiolate chains were observed with infrared spectroscopy in solutions of MPCs where alcohol, carboxylic acid, or phenothiazine moieties had been incorporated into the monolayer.     

DNA binding of an ethidium intercalator attached to a monolayer-protected gold cluster

Ethidium intercalation has been investigated as a means of inducing binding of Au nanoparticles to DNA. The ethidium sites are attached to the nanoparticles as thiolate ligands, using 3,8-diamino-5-mercaptododecyl-6-phenylphenanthridinium (ethidium thiolate). Each nanoparticle bears only one or two ethidium thiolate ligands. The rest of the thiolate monolayer ligands on the monolayer-protected Au clusters (MPCs) were either N-(2-mercaptopropionyl)glycine (tiopronin/ethidium MPC) or trimethyl(mercaptoundecyl)ammonium (TMA/ethidium MPC). In solution mixtures of DNA and MPCs, the energy-transfer quenching of the ethidium ligands by the metal-like MPC core is partially released by ethidium binding to DNA, as observed by an increase in the intensity of ethidium fluorescence. Binding of the cationic TMA/ethidium MPC to DNA was efficient and rapid. The negatively charged tiopronin/ethidium MPC, in contrast, exhibits slow intercalation kinetics, relative to ethidium cation not attached to an MPC. The slow kinetics were analyzed as two competing binding interactions. The tiopronin/ethidium MPC binding to DNA was imaged by AFM.     

Chemiresistive vapor sensing with microscale films of gold monolayer protected clusters

Here we report the stability, conductivity, and vapor-sensing properties of microcontact-printed films of 1.6-nm average diameter hexanethiolate-coated gold monolayer protected clusters (C6 Au MPCs). The C6 Au MPCs were stamped into parallel lines (approximately 1.2 microm wide and 400 nm thick) across two Au electrodes separated by a 1-microm gap. The chemiresistive vapor-sensing properties were measured for saturated toluene and 2-propanol vapors. As-prepared patterned Au MPC films were unstable in the presence of saturated toluene vapor, and their current response was irreversible. Chemically linking the films with vapor-phase hexanedithiol greatly improves their stability and leads to reversible responses. The extent of Au MPC cross-linking and vapor response to organic vapors varies with different exposure times to dithiol vapor. The response to toluene changed from 61 to 8% for exposures of 1 and 60 min, respectively, which is likely due to greater film flexibility with less dithiol exposure. The current measured through the films varies from 10(-11) to 10(-3) Angstroms as a function of the temperature between 250 and 320 degrees C, which correlates with the loss of organic material as measured by FT-IR spectroscopy and the change in thickness and width of the film as measured by atomic force microscopy. The vapor-sensing properties vary with temperature, current, and organic content in the film, which are all interrelated. Response to toluene decreased with increasing temperature and conductivity, while the response to 2-propanol was less predictable. Reducing the size of vapor-sensing devices based on Au MPCs is important for creating highly portable devices that can simultaneously detect multiple analytes. This work demonstrates a simple method for reducing the size of such devices down to the microscale and describes methods for maximizing response, stability, and reversibility.     

Supporting electrolyte and solvent effects on single-electron double layer capacitance charging of hexanethiolate-coated Au140 nanoparticles

Sequential injections of single electrons (or holes) into the cores of Au(140) hexanethiolate monolayer-protected clusters (MPCs) occur at measurably different electrochemical potentials owing to the extremely small (subattofarad) values of the single MPC capacitance (C(MPC)) of the nanoparticle. The potential increment for each sequential injection is DeltaV = e/C(MPC). The dependence of DeltaV on the concentration of supporting electrolyte (from 1 to 100 mM), measured using square wave voltammetry, is shown to be caused, primarily, by changes in the diffuse double layer component (C(DIFFUSE)) of C(MPC). The dependence of C(DIFFUSE) on r(core), the radius of the nanoparticle, is considered. Additionally, significant changes in the magnitude of the compact double layer component (C(COMPACT), equivalent to the Stern layer) of C(MPC) were induced by adding hydrophobic solvent components such as hexane or dodecane or by introducing hydrophobic electrolyte ions (tetrabutyl-, tetrahexyl-, and tetraoctylammonium, perchlorate, and tetraphenylborate). These changes are interpreted as specific solvation and ion penetration of the hexanethiolate monolayer. For brevity we will refer to these phenomena as solvation/penetration phenomena.     

Continuous free-flow electrophoresis of water-soluble monolayer-protected clusters

There has recently been a surge of interest in the properties and applications of monolayer protected clusters (MPCs). MPCs are metal nanoparticles that have unique optical, chemical, and electrochemical properties resulting from their small size. Because the size defines their properties, MPC particle size fractionation is important for control of the MPC characteristics for use in many potential applications. This paper explores the use of continuous free-flow electrophoresis (CFE) for the size fractionation of N-(2-mercaptopropionyl)glycine (tiopronin) monolayer protected gold clusters into monodisperse nanoparticle samples. CFE is a fractionation technique that isolates monodisperse particle sizes into several different collection vials on the tens of milligrams scale. This allows the MPCs to be separated based on their electrophoretic mobilities into isolated, monodisperse particles across a wide range of sizes. CFE separation of water-soluble tiopronin MPCs yielded fractions that varied in color, UV-visible spectra, transmission electron microscopy (TEM) size histograms, and solubility, indicating narrow size dispersity in the isolated fractions. UV-visible spectrophotometry verified the separation of the tiopronin MPCs through the inspection of surface plasmon resonance peak sizes for the different fractions. TEM was also used to verify the narrowed dispersity of MPC samples. The ability to separate water-soluble nanoparticles into 30 or more fractions in a continuous flow process will enable future studies on their size dependent properties.     

Characterization of pyridine-octanethiolated gold nanoparticles: desorption kinetics by thermal analysis mass spectrometry

We describe a synthetic pathway to the formation of stable pyridine-functionalized octanethiolate mixed monolayer-protected Au clusters (MPCs). The spectroscopic characterization data of MPCs using NMR, UV-Vis, TEM, XPS, and thermal-analysis-mass techniques are discussed. TEM analysis showed that spherical nanoclusters of 3-5 nm were produced. Furthermore, the particle sizes are uniform with a narrow size distribution. The pyridine-functionalized MPCs formed 2D superlattices with hexagonal packing covering on the carbon-coated copper grids during the toluene evaporation. For all samples, the S 2p(3/2) and 2p(1/2) components that appeared at approximately 162 and approximately 163 eV, respectively, in the XPS spectra compare very well with the typical value of chemisorbed S species. Thermal analysis mass spectrometer was used to analyze desorption behavior of octanethiolated MPCs or pyridine-functionalized mixed MPCs. The TA-mass spectra have revealed that MCPs exist monomer and dimer desorption behavior from monomeric thiolate adsorbed on the surface.     

Fabrication and AFM investigation of the temperature-dependent surface morphology of Au (100) single crystal ultramicroelectrodes

A previously reported method for preparation of gold (111) single-crystal utramicroelectrodes (scumes) has been used to fabricate gold (100) scumes with effective diameters from 20 to 50 microm. Cyclic voltammograms for these ultramicroelectrodes obtained in perchloric acid show similar features to gold (100) single-crystal electrodes of more conventional sizes, but with differences that are a result of different surface preparation methods used before the measurement. The gold crystals used to prepare Au (100) scumes were grown at room temperature so that the cyclic voltammetric characteristics reflect the unique properties of a room temperature-ordered surface. The AFM images of the (100) facets of these crystals are also presented. The effects of annealing Au (100) at 800 degrees C for a short time were studied both electrochemically and using AFM and are discussed with respect to the data obtained for crystals grown at room temperature.     

High-sensitivity NO2 detection with carbon nanotube-gold nanoparticle composite films

Composite films of single-walled carbon nanotube mesh doped with alkanethiol monolayer protected gold clusters (MPCs) have been investigated for ultrahigh sensitivity detection of nitrogen dioxide. The response to NO2 (measured as increased conductance) of the composite materials increased with MPC loading until a threshold MPC loading level was achieved, after which no further enhancement of sensor response is observed. The total of about ten droplets of MPC solution had been cast atop the SWNT mesh. The detection limit for NO2 has been improved 9.6-fold, to 4.6 ppb, compared with that obtained with pure SWNT sensors. Ultraviolet illumination helps to speed up the sensor recovery. All tests were done under ambient conditions.     

Crown ether-metal “sandwiches” as linking mechanisms in assembled nanoparticle films

Crown ether ligands attached to monolayer-protected clusters (MPCs) were assembled as films and the linking mechanism between the crown ether–metal ion–crown ether bridges between nanoparticles was examined. Thicker films exhibited a red shift in the absorbance maximum for the surface plasmon band which was attributed to the increasing aggregation and cross linking within the film. Quantized double layer charging peaks suggest that film growth is selective toward a specific core size or exchange rate, either of which affect the number of potential linking ligands in the periphery of the MPCs. Multi-layer growth of films was only achieved with metal ions capable of coordinating within the cavity of the 15-crown-5 ether. Our exchange reaction parameters are in stark contrast to other types of MPC film assemblies.     

Electron hopping dynamics in monolayer-protected au cluster network polymer films by rotated disk electrode voltammetry

Electrons are transported within polymeric films of alkanethiolate monolayer-protected Au clusters (MPCs) by electron hopping (self-exchange) between the metal cores. The surrounding monolayers, the molecular linkers that generate the network polymer film, or both, presumably serve as tunneling bridges in the electron transfers. This paper introduces a steady-state electrochemical method for measuring electron hopping rates in solvent-wetted and swollen, ionically conductive MPC films. The films are network polymer films of nanoparticles, coated on a rotated disk electrode that is contacted by a solution of a redox species (decamethylferrocene, CpFe). Controlling the electrode potential such that the film mediates oxidation of the redox probe can force control of the overall current onto the rate of electron hopping within the film, which is characterized as the apparent electron diffusion coefficient D(E). D(E) is translated into an apparent electron hopping rate k(ET) by a cubic lattice model. The experiment is applied to MPC network polymer films linked by alpha,omega-alkanedithiolates and by metal ion-carboxylate connections. We evaluate the dependencies of apparent hopping rate on CpFe concentration, film thickness, electrode potential relative to the CpFe formal potential, film-swelling solvent, and temperature. The apparent hopping rates are in the 10(4)-10(5) s(-)(1) range, which is slower than those for the same kind of MPC films, but in a dry (nonswollen) state measured by electronic conductivities.     

Analytical evidence for the monolayer-protected cluster Au225[(S(CH2)5CH3)]75

The Brust synthesis of thiolate-protected gold clusters has been modified to produce identifiable proportions of a hexanethiolate-protected Au225 core nanoparticle that display quantized double layer charging voltammetry consistent with a Au225 core dimension. Transmission electron microscopy (TEM) and thermogravimetric results indicate an average nanoparticle formula of Au225[(S(CH2)5CH3)]75. A simulated pulse voltammogram that accounts for the TEM nanoparticle dispersity matches reasonably well with that of the polydisperse synthetic sample containing the Au225 component. In confirmation of the size determination, an HPLC analysis using ratiometric absorbance and electrochemical detectors gives a core radius of 1.0 nm for the Au225 nanoparticle.     

Incorporation of multipoint constraints into the balancing domain decomposition method and its parallel implementation

The domain decomposition method (DDM) is a major solution algorithm that is used to parallelize the finite element method. In the case of implicit structural analysis using the DDM, the substructuring‐based iterative linear solver is a powerful tool when an effective preconditioner such as the balancing domain decomposition (BDD) method is used. In the present study, a method by which to incorporate a set of linear multipoint constraints (MPC) into the BDD method is proposed. In this method, when an MPC is enforced on the internal degrees of freedom (DOFs) in some subdomains, the DOFs are converted into interface DOFs, that is, all of the DOFs constrained by MPCs become interface DOFs. Then, the interface problem with the set of MPCs for the interface DOFs is solved by the conjugate projected gradient method. In order to combine the above procedure with the preconditioner used in the BDD method, the effect of the MPCs for the interface DOFs is also imposed on the coarse grid problem of the BDD method using the penalty method. A parallel implementation of the present method is also described. Some illustrative examples are solved and good convergence and parallel performance of the present method are demonstrated. Copyright © 2006 John Wiley & Sons, Ltd.     

Electrochemical oxidation of histamine and serotonin at highly boron-doped diamond electrodes

The electrochemistry of histamine and serotonin in neutral aqueous media (pH 7.2) was investigated using polycrystalline, boron-doped diamond thin-film electrodes. Cyclic voltammetry, hydrodynamic voltammetry, and flow injection analysis (FIA) with amperometric detection were used to study the oxidation reactions. Comparison experiments were carried out using polished glassy carbon (GC) electrodes. At diamond electrodes, highly reproducible and well-defined cyclic voltammograms were obtained for histamine with a peak potential at 1.40 V vs SCE. The voltammetric signal-to-background ratios obtained at diamond were 1 order of magnitude higher than those obtained for GC electrodes at and above 100 microM analyte concentrations. A linear dynamic range of 3-4 orders of magnitude and a detection limit of 1 microM were observed in the voltammetric measurements. Well-defined sweep rate-dependent voltammograms were also obtained for 5-hydroxytryptamine (5-HT). The characteristics of the voltammogram indicated lack of adsorption of its oxidation products on the surface. No fouling or deactivation of the electrode was observed within the experimental time of several hours. A detection limit of 0.5 microM (signal-to-noise ratio 13.8) for histamine was obtained by use of the FIA technique with a diamond electrode. A remarkably low detection limit (10 nM) was obtained for 5-HT on diamond by the same method. Diamond electrodes exhibited a linear dynamic range from 10 nM to 100 microM for 5-HT determination and a range of 0.5-100 microM for histamine determination. The FIA response was very reproducible from film to film, and the response variability was below 7% at the actual detection limits.     

前列腺特异性抗体电化学免疫传感器的研制

利用自组装的方法在金电极上制得巯基丁二胺铜(Ⅱ)/纳米金胶/前列腺特异性抗体(抗PSA)免疫修饰电极。用该修饰电极对PSA进行检测,发现其循环伏安图的氧化还原峰电流都随PSA浓度的增高而降低,峰电位没有变化。其最佳实验条件包括:pH5.2的0.1mol/L磷酸盐缓冲液作为底液,以及用示差脉冲方式进行定量测定。结果显示:该传感器的氧化峰电流减少值与PSA浓度在0.005-0.48μg/mL范围内成线性关系,检测下限为2ng/mL.在40 ng/mLPSA浓度下八次测量相对标准偏差为2.9%,该免疫传感器的稳定性和抗干扰性都较好。对血清中的PSA进行检测,获得满意的结果。     

Direct writing of large-area plasmonic photonic crystals using single-shot interference ablation

We report direct writing of metallic photonic crystals (MPCs) through a single-shot exposure of a thin film of colloidal gold nanoparticles to the interference pattern of a single UV laser pulse before a subsequent annealing process. This is defined as interference ablation, where the colloidal gold nanoparticles illuminated by the bright interference fringes are removed instantly within a timescale of about 6 ns, which is actually the pulse length of the UV laser, whereas the gold nanoparticles located within the dark interference fringes remain on the substrate and form grating structures. This kind of ablation has been proven to have a high spatial resolution and thus enables successful fabrication of waveguided MPC structures with the optical response in the visible spectral range. The subsequent annealing process transforms the grating structures consisting of ligand-covered gold nanoparticles into plasmonic MPCs. The annealing temperature is optimized to a range from 250 to 300?°C to produce MPCs of gold nanowires with a period of 300 nm and an effective area of 5 mm in diameter. If the sample of the spin-coated gold nanoparticles is rotated by 90° after the first exposure, true two-dimensional plasmonic MPCs are produced through a second exposure to the interference pattern. Strong plasmonic resonance and its coupling with the photonic modes of the waveguided MPCs verifies the success of this new fabrication technique. This is the simplest and most efficient technique so far for the construction of large-area MPC devices, which enables true mass fabrication of plasmonic devices with high reproducibility and high success rate.     

Multilayered assembly of dendrimers with enzymes on gold: thickness-controlled biosensing interface

A new approach to construct a multilayered enzyme film on the Au surface for use as a biosensing interface is described. The film was prepared by alternate layer-by-layer depositions of G4 poly(amidoamine) dendrimers and periodate-oxidized glucose oxidase (GOx). The cyclic voltammograms obtained from the Au electrodes modified with the GOx/dendrimer multilayers revealed that bioelectrocatalytic response is directly correlated to the number of deposited bilayers, that is, to the amount of active enzyme immobilized on the Au electrode surface. From the analysis of voltammetric signals, the coverage of active enzyme per GOx/dendrimer bilayer during the multilayer-forming steps was estimated, which demonstrates that the multilayer is constructed in a spatially ordered manner. Also, with the ellipsometric measurements, a linear increment of the film thickness was registered, supporting the formation of the proposed multilayered structure. The E5D5 electrode showed the sensitivity of 14.7 microA x mM(-1) glucose x cm(-2) and remained stable over 20 days under day-by-day calibrations. The proposed method is simple and would be applicable to the constructions of thickness- and sensitivity-controllable biosensing interfaces composed of multienzymes as well as a single enzyme.