Molecular dynamics description of grafted monolayers: effect of the surface coverage

Molecular dynamics simulations of monolayers of metal-chelating ligands grafted onto a graphite surface in water are carried out to calculate structural (density profiles, radius of gyration, and asphericity coefficients), dynamical (diffusion coefficients), and energetical properties as a function of the surface coverage. The purpose is to provide a better understanding of the dependence of various properties of these monolayers on the surface coverage. A critical value of the surface coverage from which all structural properties derive a limiting value has been established. It also appears that the chains rather adopt an elongated conformation along the direction normal to the surface from this critical surface coverage. The hydrogen-bonding structure and dynamics of water molecules are reported. An ordered structure of water in the region close to the terminal groups of the grafted molecules is shown at a relatively high surface coverage. This ordering is similar to that observed in the case of water in interaction with a solid surface.     

Sensitive determination of herbicide trifluralin on the surface of copper nanowire electrochemical sensor

A novel voltammetric method for the determination of trifluralin is proposed based on a composit of carbon paste and copper nanowire as a sensitive sensor. The presence of copper nanowire in the composite film enhance the conductivity and as a result increased the electron transfer rate constant and so the current will increase. The composite exhibits a promising higher electrocatalytic activity towards the oxidation of trifluralin in pH 4.0 aqueous solution. The reduction peak currents of trifluralin increased remarkably and the reduction peak potential shifted positively at the nanostructuring electrode, compared with that at a bare CPE which show the enhanced effect of nanowire. The support electrolyte to provide a more defined and intense peak current for trifluraline determination was 0.05 mol L⁻¹ phosphate buffer at pH 4.0. The fast Fourier transform square wave voltammetry was used as a new electrochemical technique in flow injection system to abtain more sensitivity by application of discrete fast Fourier transform method by background subtraction and two-dimensional integration of the electrode response over a selected potential range and time window, the signal-to-noise ratio has significantly increased and made the sensivity to be higher than other methods. The effective parameters such as frequency, amplitude, and pH were optimized to get the best sensitivity. As a result, the sensor showed a valuable response in linear concentration range of 100–0.02 nmol L⁻¹ with a (limit of detection) LOD of 0.008 nmol L⁻¹ and (limit of quantification) LOQ of 0.15 nmol L⁻¹ for trifluralin. A good recovery was obtained for assay spiked urine samples and a good quantification of trifluralin was achieved in soil samples.     

New insight into the effect of interface supercapacitance on the performance of titanium dioxide/carbon nanowire array for photoelectrochemical water oxidation

The electrode/electrlyte interface is of great signifance to photoelectrochemical (PEC) water oxidation as the reaction mainly occur here. Herein, we focus on the effect of supercapactance of the electrode/electrlyte interface on the performance of PEC. It is discovered that the supercapacitor on the interface is crucial because it links the charge transport and solution ion adsorption on its two sides. In this study, we demonstrate an approach to promote the performance of TiO₂ nanowire array (TiO₂ NWs) photoanode in photoelectrochemical cells (PECs) by increasing its supercapacitance. A 2−5 nm carbon layer was coated and the interface supercapacitance increases by about 150 times. This enhances the separation rate of electron-hole pairs by collecting more holes. Meanwhile, it also promotes the water oxidation rate by adsorbing more OH⁻ on its surface. As a result, the photocurrent density of C-TiO₂ NWs was about 8 times higher than that of its carbon-free counterpart. This approach of increasing the supercapacitance of photoanodes would be attractive for enhancement of the efficiency of PECs and this work demonstrate the importance of supercapacitance of the interface for PECs.     

Adsorption of modified dextrins on talc: effect of surface coverage and hydration water on hydrophobicity reduction

The adsorption of three modified dextrins on the basal plane of talc has been studied using in situ tapping mode atomic force microscopy (TMAFM). The images have been used to determine the layer thickness and coverage of the adsorbed polymers. Adsorption isotherms of the polymers on talc particles were also determined using the depletion technique. Values of the adsorbed amount at equilibrium were compared with the volume of adsorbed material as determined using in situ TMAFM, revealing the presence of significant amounts of hydration water in the adsorbed layer structure. This deduction was confirmed by comparing in and ex situ TMAFM images of the adsorbed dextrins. The effect of layer thickness, coverage, and hydration water content on the contact angle of talc particles treated with polymer was investigated using the Washburn method and the equilibrium capillary pressure (ECP) method. Distinct correlations were observed between adsorbed layer properties and the measured contact angles, with the ECP measurements especially highlighting the effect of the adsorbed polymer layer hydration water. The implications for the performance of the modified dextrins in flotation are discussed.     

Molecular simulation of liquid crystal sensor based on competitive inclusion effect

With the progressive understanding of liquid crystal materials that rely on the interface interactions, optical properties of liquid crystal are attracting attention as a detector for chemicals and biomolecules. In this work, a recently reported liquid crystal sensing system based on the competitive inclusion effect of β-cyclodextrin (β-CD) was studied. Quantum mechanical calculations were applied to study different β-CD inclusion complexes of methyl blue (MB), 4-cyano-4′-pentyl biphenyl (5CB), sodium dodecyl sulfonate (SDS), dopamine (DA) and their inclusion processes. The work shows that DA cannot be an analyte for the liquid crystal sensor as it could not compete for the cavity of β-CD with SDS. However, MB molecule can push SDS out of the β-CD cavity so as to induce the change in optical appearance when MB forms a 1:2 inclusion complex. The simulated absorption spectrum is in agreement with experiment results, implying that MB molecule may exist in both 1:1 and 1:2 inclusion complexes in the system of liquid crystal sensor.     

The effect of surface coverage on conformation changes of bovine serum albumin molecules at the air-solution interface detected by sum frequency generation vibrational spectroscopy

The air-BSA solution interface has been investigated by various techniques for years. From these studies we know that BSA molecules segregate at the BSA solution-air interface, and the surface coverage increases with the increase of the bulk solution concentration. However, questions still remain as to whether the protein changes conformation, orientation, or a combination of the two upon adsorption. In this paper, by using sum frequency generation (SFG) vibrational spectroscopy we found that the conformation of interfacial BSA molecules changes dramatically at the solution-air interface, compared to that of the native BSA in solution. The hydrophobic methyl groups of BSA molecules at this interface tend to align along the surface normal. The degree of such conformational changes of surface BSA molecules depend on the surface coverage, indicating that the protein-protein interaction plays a very important role in determining the conformation of interfacial protein molecules. At very low surface concentration, the adsorbed BSA molecules unfold substantially. Our results can provide a molecular interpretation of results obtained from other studies such as protein layer thickness and surface tension measurements of protein solution.     

Boosting the electrochromic performance of TiO_2 nanowire film via successively evolving surface structure

Electrochromic transition metal oxides(ETMOs) are useful in energy saving devices and smart indicators. Among these ETMO candidates, titanium dioxide(TiO_2) is intriguing for its abundance and environmental safety, but the low color efficiency and slow coloring rate are still barricades to promote its electrochromic application. Herein, we demonstrate an amorphization strategy to comprehensively enhance the performance of TiO_2 nanowire film(TNF) via atomic layer deposition(ALD) of an additional TiO_2 conformal layer onto TNF surface, of which the layer structure evolves successively from ordered to disordered,achieving tunable electrochromism by controlling ALD cycles. Besides the remarkable increment of charging efficiency by～35% and color efficiency by ～40%, bleached transmittance rectified optical density(BTR density) and ion diffusion coefficient are boosted by ～90% and over 15 times, respectively for TNF deposited with 150 cycles. A large number of self-doped Ti~(3+) defects and hydroxyl units together with order-disorder interconnections in the ALD TiO_2 layer are responsible for the performance enhancement. The concept for successively evolving surface structure shares the feasibility of upgrading conventional ETMOs as well as designing new electrochromic materials.     

硅胶表面苯并噻吩分子印迹聚合物的分子识别与吸附性能

选用γ-氨丙基三乙氧基硅烷和α-甲基丙烯酸修饰的硅胶作为载体,以甲基丙烯酸为功能单体,二甲基丙烯酸乙二醇酯为交联剂,苯并噻吩为模板分子,合成一种具有选择性识别苯并噻吩分子的印迹聚合物。采用红外光谱、元素分析及N2吸附对其结构进行了表征,以模拟汽油通过静态吸附对其吸附性能进行了研究。结果表明,在硅胶载体表面成功地嫁接了多孔的分子印迹聚合物薄层。印迹聚合物对苯并噻吩具有良好的识别性能,对苯并噻吩的吸附动力学满足Langergren准一级反应动力学方程,吸附过程属于单分子层吸附。符合Langmuir吸附模型印迹聚合物对苯并噻吩的平衡吸附容量达57.4×10-3,而非印迹聚合物的吸附容量为33.1×10-3。印迹聚合物在经过多次再生后其吸附容量基本不变,从而为在汽油深度脱硫中有效脱除噻吩类硫化物提供了一种新技术途径。     

Synthesis and interface structures of zinc sulfide sheathed zinc-cadmium nanowire heterojunctions

Zinc sulfide (ZnS) sheathed zinc (Zn)-cadmium (Cd) nanowire heterojunctions have been prepared by thermal evaporating of ZnS and CdS powders in a vertical induction furnace at 1200 degrees C. Studies found that both the Zn and Cd subnanowires, within a single nanoheterojunction, are single-crystallines with the growth directions perpendicular to the [210] plane, whereas the sheathed ZnS is polycrystalline with a thickness of ca. 5 nm. The Zn/Cd interface structure in the ZnS sheathed Zn-Cd nanowire heterojunctions was thoroughly experimentally studied by high-resolution transmission electron microscopy and theoretically studied using a near-coincidence site lattice (NCSL) concept. The results show that the Cd and Zn have a crystalline orientation relationship as [0001]Zn//[0001]Cd, (10(-)10)Zn//(10(-)10)Cd, (01(-)10)Zn//(01(-)10)Cd, and ((-)1100)Zn//((-)1100)Cd.     

Hybridization efficiency of molecular beacons bound to gold nanowires: effect of surface coverage and target length

Surface-bound nucleic acid probes designed to adopt specific secondary structures are becoming increasingly important in a range of biosensing applications but remain less well characterized than traditional single-stranded probes, which are typically designed to avoid secondary structure. We report the hybridization efficiency for surface-immobilized hairpin DNA probes. Our probes are molecular beacons, carrying a 3' dye moiety and a 5' thiol for attachment to gold nanowires, which serve as both scaffolds for probe attachment and quenchers. Hybridization efficiency was dependent on probe surface coverage, reaching a maximum of ～90% at intermediate coverages of (1-2) × 10(12) probes/cm(2) and dropping to ≤20% at higher or lower coverages. Fluorescence intensity did not track with the number of target molecules bound, and was highest for high probe coverage despite the lower bound targets per square centimeter. Backfilling with short thiolated oligoethylene glycol spacers increased hybridization efficiency at low hairpin probe coverages (～(3-4) × 10(11) probes/cm(2)), but not at higher probe coverages (1 × 10(12)/cm(2)). We also evaluated the effect of target length by adding up to 50 nonhybridizing nucleotides to the 3' or 5' end of the complementary target sequence. Additional nucleotides on the 3' end of the complementary target sequence (i.e., the end near the nanowire surface) had a much greater impact on hybridization efficiency as compared to nucleotides added to the 5' end. This work provides guidance in designing sensors in which surface-bound probes designed to adopt secondary structures are used to detect target sequences from solution.     

Molecular dynamics simulations of ionic liquid-vapour interfaces: effect of cation symmetry on structure at the interface

The influence of alkyl chain symmetry of the imidazolium cation on the structure and properties of the ionic liquid-vapour interface has been addressed through molecular dynamics simulations. The anion chosen is bis(trifluoromethylsulfonyl)imide (NTf(2)). Profiles of number densities, orientation of cations, charge density, electrostatic potential, and surface tension have been obtained. At the interface, both cations and anions were present, and the alkyl chains of the former preferred to orient out into the vapour phase. A large fraction of cations preferred to be oriented with their ring-normal parallel to the surface and alkyl chains perpendicular to it. These orientational preferences are reduced in ionic liquids with symmetric cations. Although the charge densities at the interface were largely negative, an additional small positive charge density has been observed for systems with longer alkyl chains. The electrostatic potential difference developed between the liquid and the vapour phases were positive and decreased with increasing length of the alkyl group. The calculated surface tension of the liquids also decreased with increasing alkyl chain length, in agreement with experiment. The surface tension of an ionic liquid with symmetric cation was marginally higher than that of one with an asymmetric, isomeric cation.     

Molecular orientation and interface compatibility for high performance organic thin film transistor based on vanadyl phthalocyanine

Organic thin film field-effect transistors (OTFTs) with mobility up to 1.0 cm2 V(-1) s(-1) and on/off ratio of 10(6)-10(8) as well as good environmental stability were demonstrated by using vanadyl phthalocyanine (VOPc), a pyramid-like compound with an ultra closely pi-stacked structure. The high performance, remarkable stability, low price, easy availability and nontoxicity of VOPc enabled it to be a promising candidate for OTFTs. Furthermore, we found that the mobility of the devices on OTS-modified Si/SiO2 substrates was 2 orders of magnitude higher than that of devices on Si/SiO2 substrates. Significantly, the relationship between field effect property and insulator surface property was explained from two new aspects of distribution of molecular orientation and interface compatibility, which might provide not only a useful model to explain why the surface modification with OTS could largely improve the field-effect performance but also a guide for rational optimization of device structure for higher performance. In addition, the field effect property of VOPc devices under vacuum, i.e., the oxygen doping effect on the VOPc devices, was measured. We found that the hole mobility decreased by several orders of magnitude with decreasing pressure. At a pressure below 10(-2) Pa, the device on OTS-modified substrates exhibited ambipolar conduction. These results indicated that the oxygen doping exerted essential effect on the field-effect property of VOPc, which was clearly distinct from that observed for pentacene-based OFETs.     

In situ characterisation of the aqueous gold colloid interface using CO as a surface probe: IR spectroscopic studies

Fourier transform infrared (FTIR) spectra are presented of CO gas-treated protected gold colloids prepared from hydrazinium hydrate reduction of an Au(III) precursor which reproducibly feature a weak, shortlived peak at ca. 2169 cm(-1). When the gold colloid was treated with 99% isotopically enriched (13)CO gas, the IR peak shifted to a frequency of 2114 cm(-1) which indicated that it represented a simple gold monocarbonyl species. The value of 2169 cm(-1) for the CO stretching frequency suggests the peak represents CO physisorbed on oxidised gold atoms on the colloid surface. The peak is not observed when the concentration of the colloidally dispersed gold is reduced either by use of lower starting salt concentrations or by aggregation. It is also not observed when solutions of the protecting agent or reducing agent or the dispersion medium (water) or even the starting Au(III) salts are CO-treated individually. This confirms that the spectral feature is uniquely associated with colloidally dispersed gold. In general, the work has shown that the surfaces of Au colloids in situ have partially oxidised Au character which is of interest in systems where supported nanoparticulate gold derived from colloid preparations are considered for low temperature oxidation catalysts for CO.     

Molecular interface effect of heterogeneous molecular junctions consisting of nonfluorinated and fluorinated phthalocyanines

We report the tunneling behavior of homogeneous and heterogeneous molecular junctions using p-type molecules of iron phthalocyanine (FePc), phthalocyanine (H(2)Pc), and copper(II) octaalkoxyl substituted phthalocyanine (CuPcOC8) and n-type molecule of copper hexadecafluorophthalocyanine (F(16)CuPc). The molecular films formed on the electrode surfaces were inspected by X-ray photoelectron spectroscopy (XPS). The measured characteristic tunneling curves of single-component phthalocyanines revealed comparable energy gaps for homogeneous tunneling junctions using the photoemission method. In contrast, for the heterogeneous tunnel junctions of mixed phthalocyanines including fluorinated phthalocyanine a distinctive offset of the energy gaps to the positive bias voltage direction can be clearly identified. It is suggested that the substitution of phthalocyanines and surface affinity of phthalocyanines could contribute to the controlled phase separation within the heterogeneous tunneling junctions. The apparent shift of the tunneling spectra is attributed to the existence of an internal electric field originated with the phase separation of the binary mixture of p-type and n-type phthalocyanines within the tunneling junction.     

The effect of surface treatments of glass capillaries on column performance

Summary The effects of etching and/or silylation of glass capillaries on column efficiency were studied. Coated with apolar OV-101, only columns subjected to the combined treatment showed an increase of about 50% in the number of effective plates. Surface hydroxylation, prior to silylation, gave a small additional improvement for this stationary phase.     

Chemical modification of nanocrystalline metal oxides: effect of the real structure and surface chemistry on the sensor properties

The relationships between the composition, structure, chemistry of the surface, and sensor properties of nanocomposites SnO₂-M _n O _m (M _n O _m = Fe₂O₃, MoO₃, V₂O₅) obtained by chemical precipitation from solutions were analyzed. The relationships between the elemental and phase composition of the nanocomposites and the effect of the composition on the nanostructure and the acidic and oxidation properties of the nanocomposite surface were considered. The modification of the SnO₂ surface by other oxides makes it possible to control the type and density of the acid sites and the oxidation properties of the surface and to enhance the selectivity of the materials in the detection of various gases. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1086–1105, June, 2008.     

The effect of solid electrolyte interface formation conditions on the aging performance of Li-ion cells

The effect of formation temperatures and current densities on the aging performance of LiNi_1/3Co_1/3Mn_1/3O₂/artificial graphite Li-ion cells during storage and cycle was investigated using three-electrode electrochemical impedance spectroscopy and charge–discharge experiment. The higher formation temperature at 45 °C decreased the resistance of solid electrolyte interphase (SEI) film and the irreversible capacity loss of Li-ion cells during SEI formation process. After Li-ion cell storage at 60 °C for 10 weeks, the ohmic resistance of the negative electrodes and the irreversible capacity loss of the cells reduced 24% and 7.9%, respectively, accompanied by a significant decrease of SEI film resistance when the formation temperature increased from 25 to 45 °C. The higher temperature at 45 °C may facilitate the transformation of metastable ROCO₂Li to stable inorganics to form a stable SEI film. Three hundred cycling tests indicated that the capacity retention of the cell formation at 25 °C was only 87.5%, about 8% less than that of the cell formation at 45 °C. However, the SEI formation current density did not significantly affect the property of SEI film and the irreversible capacity loss of the aged cells.     

Ligand field effect at oxide-metal interface on the chemical reactivity of ultrathin oxide film surface

Ultrathin oxide film is currently one of the paramount candidates for a heterogeneous catalyst because it provides an additional dimension, i.e., film thickness, to control chemical reactivity. Here, we demonstrate that the chemical reactivity of ultrathin MgO film grown on Ag(100) substrate for the dissociation of individual water molecules can be systematically controlled by interface dopants over the film thickness. Density functional theory calculations revealed that adhesion at the oxide-metal interface can be addressed by the ligand field effect and is linearly correlated with the chemical reactivity of the oxide film. In addition, our results indicate that the concentration of dopant at the interface can be controlled by tuning the drawing effect of oxide film. Our study provides not only profound insight into chemical reactivity control of ultrathin oxide film supported by a metal substrate but also an impetus for investigating ultrathin oxide films for a wider range of applications.     

Molecular simulation study of the effect of pressure on the vapor-liquid interface of the square-well fluid

We examine a model system to study the effect of pressure on the surface tension of a vapor-liquid interface. The system is a two-component mixture of spheres interacting with the square-well (A-A) and hard-sphere (B-B) potentials and with unlike (A-B) interactions ranging (for different cases) from hard sphere to strongly attractive square well. The bulk-phase and interfacial properties are measured by molecular dynamics simulation for coexisting vapor-liquid phases for various mixture compositions, pressures, and temperatures. The variation of the surface tension with pressure compares well to values given by surface-excess formulas derived from thermodynamic considerations. We find that surface tension increases with pressure only for the case of an inert solute (hard-sphere A-B interactions) and that the presence of A-B attractions strongly promotes a decrease of surface tension with pressure. An examination of density and composition profiles is made to explain these effects in terms of surface-adsorption arguments.