Dodecanethiol-protected copper/silver bimetallic nanoclusters and their surface properties

Dodecanethiol-protected copper/silver bimetallic nanoclusters were prepared by a liquid-phase method using different copper to silver feed ratios. The morphology and size of the prepared nanoclusters were analyzed with X-ray diffraction (XRD) and transmission electron microscopy (TEM), while their spectroscopic and surface properties were characterized by infrared (IR) and X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), and 13C cross-polarization magic angle spinning NMR (13C CPMAS NMR). TEM analysis indicated that all the bimetallic clusters prepared are approximately 4-6 nm in size. On the other hand, the results of XRD, XPS, and Fourier transform infrared (FTIR) spectroscopy suggested that the surfaces of the alloy nanoclusters are mostly enriched with the less noble metal copper atoms. This surface enrichment of copper may be attributed to a galvanic exchange process during preparation, and the extent of enrichment is directly related to the copper feed ratio used. Interestingly, DSC studies showed two melting transitions in some of these alloy samples, suggesting different packing behavior of the dodecanethiol chains onto the heterogeneously intercalated silver- and copper-rich surfaces.     

Scanning tunneling microscopy of template-stripped Au surfaces and highly ordered self-assembled monolayers

Template stripping of Au films in ultrahigh vacuum (UHV) produces atomically flat and pristine surfaces that serve as substrates for highly ordered self-assembled monolayer (SAM) formation. Atomic resolution scanning tunneling microscopy of template-stripped (TS) Au stripped in UHV confirms that the stripping process produces a flat, predominantly 111 textured, atomically clean surface. Octanethiol SAMs vapor deposited in situ onto UHV TS Au show a c(4 x 2) superlattice with (square root 3 x square root 3) R30 degrees basic molecular structure having an ordered domain size up to 100 nm wide. These UHV results validate the TS Au surface as a simple, clean and high-quality surface preparation method for SAMs deposited from both vapor phase and solution phase.     

Single-electron charging features of larger, dodecanethiol-protected gold nanoclusters: electrochemical and scanning tunneling microscopy studies

In this report, we demonstrate the single-electron charging features of larger-sized (ca. 3.72 nm) Au nanoclusters protected with dodecanethiol [approximate composition, Au1415(RS)328] using combined electrochemical and scanning tunneling microscopic (STM) studies. In particular, these nanoclusters show a highly populated single-electron charging peak in voltammetric experiments, where the calculated capacitance is in good agreement with the experimentally obtained value of 1.6 aF. In comparison to the voltammetric studies, STM measurements over a single Au particle on the highly oriented pyrolytic graphite surface reveal nonlinear current-voltage (I-V) characteristics with a large central gap, signifying single-electron-transfer features. The I-V results demonstrate a clear Coulomb blockade effect with a central gap of around 0.2 eV, which is in good agreement with the orthodox theory for the double barrier tunnel junction system. The standard heterogeneous electron-transfer rate constant estimated from impedance measurements is found to be of 7.97 x 10(-6) cm.s(-1), suggesting that the process is very sluggish. Furthermore, diffusion coefficient (Dc) values calculated from chronoamperometry and impedance measurements are in good agreement with theoretically calculated values using the modified Stokes-Einstein equation. The electron-transfer rate constant estimated from cyclic voltammograms of adsorbed monolayer protected Au nanoclusters is found to be about 2 s(-1), which is slower than that reported for its smaller analogues.     

Low-temperature solution synthesis of the non-equilibrium ordered intermetallic compounds Au3Fe, Au3Co, and Au3Ni as nanocrystals

Alloys and intermetallic compounds of Au with the 3d transition metals Fe, Co, and Ni are nonequilibrium phases that have many useful potential applications as catalytic, magnetic, optic, and multifunctional magneto-optic materials. However, the atomically ordered Au-M (M = Fe, Co, Ni) intermetallics are particularly elusive from a synthetic standpoint. Here we report the low-temperature solution synthesis of the L12 (Cu3Au-type) intermetallic compounds Au3Fe, Au3Co, and Au3Ni using n-butyllithium as a reducing agent. Reaction pathway studies for the Au3Co system indicate that Au nucleates first, followed by Co incorporation to form the intermetallic. The nonequilibrium intermetallic nanocrystals have been characterized by powder XRD, TEM, EDS, selected area electron diffraction, and nanobeam electron diffraction, which collectively confirm the compositions and superlattice structures.     

Formation of Cu and Cu2O nanoparticles by variation of the surface ligand: preparation, structure, and insulating-to-metallic transition

Copper and copper (I) oxide nanoparticles protected by self-assembled monolayers of thiol, carboxyl, and amine functionalities [X(CH(2))(n)-CH(3), where X can be -COOH, -NH(2), or -SH] have been prepared by the controlled reduction of aqueous copper salts using Brust synthesis. The optical absorption spectrum (lambda(max)=289 nm) is found to be invariant with the nature of the capping molecule while the particle shape and distribution are found to depend strongly on it. A comparison of the protection efficiency for different capping agents such as dodecanethiol (DDT), tridecylamine (TDA), and lauric acid (LA) suggests that although zerovalent Cu is initially formed for dodecanethiol, all other cases allow oxidation to Cu(2)O nanoparticles. Despite the variation in particle size and relative stability, nanoparticles have been found to form oxides after a few days, especially for the case of LA and TDA surface capping. For all the samples studied, the size has been found to be 4-8 nm by high-resolution transmission electron microscopy. The protective ability is found to be better for dodecanethiol SAM (similar to the case of Au and Ag nanoparticles), while the order of capping efficiency varies as Cu-DDT>Cu-TDA>Cu-LA. In the present study we also demonstrate a reversible metal-insulator transition (MIT) in capped nanoparticles of Cu using temperature-dependent electrical resistivity measurement. However, the LA-capped sample does not show any such transition, possibly due to the oxide formation.     

Synthesis and assembly of monodisperse spherical Cu2S nanocrystals

High-quality monodisperse Cu₂S nanocrystals (sizes from 2 nm to 20 nm) have been successfully synthesized by the reaction of copper stearate (CuSt₂) and dodecanethiol (DDT) in 1-octadecene (ODE). The nanocrystals were characterized using X-ray diffraction (XRD), X-ray photoelectron spectrum (XPS), and transmission electron microscopy (TEM). These as-prepared Cu₂S nanocrystals with certain sizes have been found with good self-assembly behaviors, and they were easily to assemble into two-dimensional and three-dimensional superlattice structures. DDT served as both sulfur source and capping ligand, and was found a key factor to affect the growth and the self-assembly behaviors of the Cu₂S nanocrystals.     

Gold nanoclusters protected by conformationally constrained peptides

The preparation and properties of a series of gold nanoclusters protected by thiolated peptides based on the alpha-aminoisobutyric acid (Aib) unit are described. The peptides were devised to form 0-3 C=O...H-N intramolecular hydrogen bonds, as required by their 3(10)-helical structure. The monolayer-protected clusters (MPCs) were prepared, using a modified version of the two-phase Brust-Schiffrin preparation, and fully characterized with (1)H NMR spectrometry, IR and UV-vis absorption spectroscopies, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). The MPCs were obtained with core diameters in the range of 1.1-2.3 nm, depending on the reaction conditions. Structured peptides formed smaller clusters. The smallest MPC obtained is in agreement with the average formula Au(38)Pep(18). The results showed that the chemical integrity of the peptide is maintained upon monolayer formation and that the average number of peptide ligands per gold cluster is typically 75-85% the value calculated for alkanethiolate MPCs of similar sizes. The IR and NMR spectra indicated that in the monolayer the peptides are involved in both intra- and interligand C=O...H-N hydrogen bonds.     

Comparative investigation of underpotential deposition of Ag from aqueous and ionic electrolytes: An electrochemical and in situ STM study

Underpotential deposition (UPD) of Ag on Au(111) has been studied with two different electrolytes: aqueous 0.1 M H2SO4 solution in comparison with the ionic liquid 1-butyl-3-methylimidazolium chloride BMICl + AlCl3. Of particular interest is the distinct behavior of 2D phase formation at both interfaces, which has been investigated by cyclic and linear sweep voltammetry in combination with in situ electrochemical scanning tunneling microscopy (STM). It is found that one monolayer (ML) of Ag is formed in the UPD region in both electrolytes. In aqueous solution, atomically resolved STM images at 500 mV versus Ag/Ag+ show a (3 x 3) adlayer of Ag, whereas after sweeping the potential just before the commencement of the bulk Ag deposition, a transition from expanded (3 x 3) to pseudomorphic ML of Ag on Au(111) occurs. In BMICl-AlCl3, the first UPD process of Ag exhibits two peaks at 410 and 230 mV indicating that two distinct processes on the surface take place. For the first time, STM images with atomic resolution reveal a transition from an inhomogeneous to an ordered phase with a (square root of 3 x square root of 3)R30 degrees structure and an adsorption of AlCl4- anions having a superlattice of (1.65 x square root of 3)R30 degrees preceding the deposition of Ag.     

Water-soluble gold nanoparticles protected by fluorinated amphiphilic thiolates

The preparation and the properties of gold nanoparticles (Au NPs) protected by perfluorinated amphiphiles are described. The thiols were devised to form a perfluorinated region close to the gold surface and to have a hydrophilic portion in contact with the bulk solvent to impart solubility in water. The monolayer protected clusters were prepared, in an homogeneous phase using sodium thiolates because of the low nucleophilicity of the alpha-perfluorinated thiols, and fully characterized with (1)H, (19)F NMR spectrometry, IR and UV-vis absorption spectroscopies, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). Au NPs with core diameters ranging from 1.6 to 2.9 nm, depending on the reaction conditions, were obtained. Water-soluble NPs (MPC-F8-PEGs) were obtained with the thiol HS-F8-PEG ending with a short poly(ethylene glycol) unit (PEG-OMe 550), whereas thiols with shorter PEG chains give rise to NPs insoluble in water. MPC-F8-PEGs undergo an exchange reaction with amphiphilic alkyl thiols. ESR investigations, using a hydrophobic radical probe, indicate that the MPC-F8-PEG monolayer shows a greater hydrophobicity compared to the analogous hydrogenated monolayer.     

Phase change materials of n-alkane-containing microcapsules: observation of coexistence of ordered and rotator phases

In the present investigation, the crystallization and phase transition behaviours of normal alkane (n-docosane) in microcapsules with a mean diameter of 3.6 μm were studied by the combination of differential scanning calorimetry (DSC), temperature-dependent X-ray diffraction (XRD) and variable-temperature solid-state nuclear magnetic resonance (VT solid-state (13)C NMR). The DSC and VT solid-state (13)C NMR results reveal that a surface freezing monolayer is formed prior to the bulk crystallization of the microencapsulated n-docosane. More interestingly, it is confirmed that after the bulk crystallization, the ordered triclinic phase coexists with the rotator phase I (RI) for the microencapsulated n-docosane. We argue that the reduction of the free energy difference between the two phases, resulting from the microencapsulation process, leads to the coexistence of the ordered triclinic and rotator phases of the normal alkanes.     

选择性控制的配体工程在金团簇催化中的作用

金团簇表面的硫醇配体影响着团簇的催化性质,尤其是选择性.我们采用在真空条件下通过程序升温的方法逐渐剥除金团簇表面的硫醇配体来制备催化剂,利用透射电镜,红外光谱对催化剂结构进行表征,以硝基化合物催化还原反应为模型反应,详细研究了配体对催化活性和选择性的影响.研究发现因配体被剥离导致底物更容易接近团簇表面,最终使得反应转换率大幅升高.实验结果还表明金团簇催化剂催化不同官能团取代的底物显示了良好的官能团兼容性,有吸电子效应的硫配体使团簇表面带正电荷,进而避免苯胺衍生物的产生.     

Formation of a regular fullerene nanochain lattice

A vicinal Au(11 12 12) surface, naturally patterned into a rectangular superlattice, has been used as a template to prepare C60 nanostructures with long-range order and uniform size. At a coverage of 0.1 monolayer and at room temperature, a two-dimensional long-range ordered superlattice of molecular nanochains is achieved, which perfectly replicates the periodicity of the template surface. The fullerene nanochains are found to be located exclusively on the face-centered cubic stacking domains at the lower step edges. Our experiments demonstrate that highly periodic molecular nanochains can be fabricated through a site-selective anchoring method.     

Effect of peptide ligand dipole moments on the redox potentials of Au38 and Au140 nanoparticles

Phenylethanethiolate monolayer-protected Au38 and Au140 nanoclusters were modified by ligand place exchange with a series of thiolated peptides. The peptides were homooligomers based on the alpha-aminoisobutyiric acid unit. The effects of changing the peptide concentration and the peptide length in the capping monolayer were studied by differential pulse voltammetry. The results showed that the redox behavior of the nanoparticles can be affected very significantly by such modifications. For example, the first oxidation peak of Au38, a cluster displaying molecule-like behavior, could be shifted positively by as much as 0.7-0.8 V. Detectable redox shifts were noted even when one single oriented peptide was in the Au140 monolayer. These effects were attributed to the molecular dipole moments of the peptide ligands.     

Preparation and drug release behavior of temperature-responsive mesoporous carbons

A temperature-responsive composite based on poly (N-isopropylacrylamide) (PNIPAAm) and ordered mesoporous carbons (OMCs) has been successfully prepared by a simple wetness impregnation technique. The structures and properties of the composite were characterized by infrared spectroscopy (IR), X-ray diffraction (XRD), transmission electron microscopy (TEM), N₂ sorption, thermogravimetric analysis (TG) and differential scanning calorimetry (DSC). The results showed that the inclusion of PNIPAAm had not greatly changed the basic ordered pore structure of the OMCs. Ibuprofen (IBU) was selected as model drug, and in vitro test of IBU release exhibited a temperature-responsive controlled release delivery.     

不同固醇与DPPC二元体系的液态有序相

应用同步辐射X射线衍射和差示扫描量热法研究了由不同结构的固醇(胆固醇、脱氢胆固醇、豆固醇、谷固醇、麦角固醇以及固醇核)和二棕榈酰磷脂酰胆碱(DPPC)二元体系形成的液态有序相. 研究表明, 胆固醇比植物固醇(豆固醇和谷固醇)和真菌固醇(麦角固醇)能更有效地与DPPC形成液态有序相(Lo); 有胆固醇或者脱氢胆固醇参与的液态有序相能够在较宽的温度范围内保持稳定, 而由植物固醇和真菌固醇参与的液态有序相对温度有较强的依赖性, 在DPPC主相变温度附近有明显的热致相变过程, 因此这一液态有序相应该进一步区分为Loβ和Loα相. 研究结果有助于阐明固醇尾链在液态有序相以及脂筏中的作用, 也有助于理解在进化过程中动物细胞膜为何选择胆固醇作为主要固醇.     

Development and evaluation of gold-centered monolayer protected nanoparticle stationary phases for gas chromatography

The current status for the development of novel open-tubular gas chromatography (GC) stationary phases consisting of thin films of gold-centered monolayer protected nanoparticles (MPNs) is reported. Dodecanethiol MPNs, in which the monolayer is dodecanethiol linked to the gold nanoparticle, have shown great promise as a GC stationary phase with efficient columns having been produced in a variety of capillary i.d.'s with stationary phase film depths ranging from 10 to 60 nm, +/-2 nm at a given film depth. Stationary phase operational parameters are discussed including maximum operating temperature, sample capacity, and stationary phase lifetime and robustness. An overview of the general method employed for column production is also included. The sample capacity was determined for a 2.5 m, 250 microm i.d. column with a stationary phase film thickness of 40 nm, at 50 degrees C using anisole (k' = 1.86) as the probe analyte. The sample capacity was experimentally found to be 2.3 ng under these conditions, similar to values reported for thicker, polymer stationary phases. The efficiency of the dodecanethiol MPN stationary phase was determined with a 100 microm i.d. capillary and found to have a reduced plate height hmin value of 0.95 for octane (k' = 0.68). Areas of application illustrated and discussed utilizing the dodecanethiol MPN stationary phase include complementary separations such as two-dimensional GC (GC x GC), potential utilization within a model system for a micro-fabricated GC (microGC), as well as efficient single dimension high-speed separations. Initial development of polar stationary phases utilizing 4-chlorobenzenethiol MPNs and 4-(trifluoromethyl)benzenethiol MPNs is discussed. Included is a selectivity comparison of the retention behavior of the 4-chlorobenzenethiol MPN stationary phase and the dodecanethiol MPN stationary phase.     

Phytosynthesis of Au, Ag and Au-Ag bimetallic nanoparticles using aqueous extract and dried leaf of Anacardium occidentale

Present study reports a green chemistry approach for the biosynthesis of Au, Ag, Au-Ag alloy and Au core-Ag shell nanoparticles using the aqueous extract and dried powder of Anacardium occidentale leaf. The effects of quantity of extract/powder, temperature and pH on the formation of nanoparticles are studied. The nanoparticles are characterized using UV-vis and FTIR spectroscopies, XRD, HRTEM and SAED analyses. XRD studies show that the particles are crystalline in the cubic phase. The formation of Au core-Ag shell nanoparticles is evidenced by the dark core and light shell images in TEM and is supported by the appearance of two SPR bands in the UV-vis spectrum. FTIR spectra of the leaf powder before and after the bioreduction of nanoparticles are used to identify possible functional groups responsible for the reduction and capping of nanoparticles. Water soluble biomolecules like polyols and proteins are expected to bring about the bio-reduction.     

Crystallization behaviors of n-nonadecane in confined space: observation of metastable phase induced by surface freezing

Crystallization and phase transition behaviors of n-nonadecane in microcapsules with a diameter of about 5 mum were studied with the combination of differential scanning calorimetry (DSC) and synchrotron radiation X-ray diffraction (XRD). As evident from the DSC measurement, a surface freezing monolayer, which is formed in the microcapsules before the bulk crystallization, induces a novel metastable rotator phase (R(II)), which has not been reported anywhere else. We argue that the existence of the surface freezing monolayer decreases the nucleating potential barrier of the R(II) phase and induces its appearance, while the lower free energy in the confined geometry turns the transient R(II) phase to a "long-lived" metastable phase.     

Size‐Controlled Synthesis of Highly Monodisperse Gold Nanoparticles without a Size‐Selection and Long Range Ordered 2‐D Arrangement

A simple method is used to control the size of cetyltrimethylammoniumbromide‐protected Au nanoparticles by a reversal micelle in safe organic solvent. These Au nanoparticles can be evolved to highly monodisperse Au nanoparticles capped 1‐dodecanthiol in the 2, 3, and 5 nm diameter by refluxing at～160°C for 7 hours. Their ultraviolet visible spectroscopy (UV‐vis), x‐ray diffraction (XRD, transmission electron microscopy (TEM) showed that all the three different size gold nanoparticles(NPs) displayed high size homogenous properties and easy formed large areas of long ordered two‐dimensional arrangement at the air/solid interface.     

A simple method for measuring the size of metal nanoclusters in solution

The connection between quantum size effects and the surface plasmon resonance of metal nanoclusters is introduced and the pros and cons of in situ and ex situ cluster analysis methods are outlined. A new method for estimating the size of nanoclusters is presented. This method combines core/shell cluster synthesis, UV-visible spectroscopy, and Mie theory. The core/shell approach enables the estimation of metal cluster sizes directly from the UV-visible spectra, even for transition metal nanoclusters such as Pd that have no distinct surface-plasmon peak in UV-visible region. Pd/Au and Au/Pd core/shell clusters as well as Au-Pd alloy clusters are synthesized and used as test cases for simulations and spectroscopic measurements. The results of the simulations and UV-visible spectroscopy experiments are validated with transmission electron microscopy.