Probing nanotribological and electrical properties of organic molecular films with atomic force microscopy

Structural aspects of organic molecular films, such as disordering, packing density, molecular bending or tilts, and phase separation, influence electrical properties as well as friction and adhesion. This indicates a correlation between nanomechanical and charge transport properties of molecular films at the molecular scale. In this review, we highlight the recent studies on correlations between charge transport and nanomechanical properties probed with atomic force microscopy. We discuss the key issues that determine charge transport and nanomechanical properties on several organic molecular films, including self-assembled monolayers formed by saturated hydrocarbon molecules conjugated molecules, and hybrid molecules as well as polymer and polymer blend films. We address the role of molecular deformation and bending in friction and conductance measurements.     

Molecular dynamics study of optimal packing structure of OTS self-assembled monolayers on SiO2 surfaces

Optimal packing structure of Octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) adsorbed on a SiO₂ (1 0 0) surface with a Si substrate was studied performing molecular dynamics (MD) computational simulations. Molecular substitution, substitution pattern and molecular orientation of the OTS molecules on the SiO₂ (1 0 0) are the main factors studied in order to determine the optimal packing structure taking into account energetic balance. We have used the optimal packing structure to study other properties usually used to characterize SAMs as molecular and system tilt angles, film thickness and gauche defects. These properties and monolayer stability were studied performing MD simulations in a temperature range from 100 to 600 K and we found that results obtained agree with those from experimental measurements. We found that OTS films are stable up to 500 K. The optimal structure obtained could be used in further MD simulations studies in order to determine tribological properties of OTS–SiO₂ systems.     

Three-dimensional molecular distribution in single cells analysed using the digital imaging microscope

Cellular changes in molecular distribution are believed to underly a wide range of cell functions. In order to investigate changes in molecular distribution in single cells utilizing fluorescent probes we have developed a digital imaging microscope. The system, consisting of both hardware and software, automatically acquires 3-D data sets consisting of optical sections and then processes such data to facilitate the analysis of molecular distribution in single cells. The first major step in processing reverses distortion introduced principally by the optics of the fluorescent microscope. Various procedures for accomplishing this task are compared and a method based on regularization theory is shown to give superior results for several different 3-D images. Following this step features of interest are automatically extracted from 3-D images utilizing an artificial 3-D visual system. This artificial visual system utilizes a system of spatial filters to identify regional characteristics of images, the information obtained from these filters being used to identify and characterize clusters of molecules within the image. This information is then utilized to construct a 3-D graphical model of molecular distribution in single cells. Such models are displayed in 3-D and may be further analysed utilizing interactive 3-D computer graphics. These methods are illustrated by results obtained regarding alpha-actinin distribution in single smooth muscle cells.     

Real-time and label-free detection of chloramphenicol residues with specific molecular interaction

Molecular measurements with specific or non-specific molecular interaction are usually reported by using different methods. This paper shows a method for small-molecular label-free measurements in real time with specific and non-specific molecular interaction based on surface plasmon resonance. The quantitative measurement of glucose with non-specific molecular interaction was performed simply to validate our self-built angle-dependent surface plasmon resonance system, which showed a good sensitivity and stability, such as that of Fourier transform infrared spectrometer. As application, we present the measurement of chloramphenicol residues with specific molecular interaction employing an indirect competitive immunoreaction by using the same surface plasmon resonance system with a limit of detection about 0.5 ng mL⁻¹, which shows a potential application for food safety.     

香附挥发油的提取和GC/MS分析

采用中药专用提取剂(PR溶剂)超声波振荡从香附中提取挥发油，用GC/MS法测定其化学成分和相对含量，主要成分为 α-可巴烯（1.97%）、香附烯（15.73%）、α-红没药烯（2.14%）、α-古芸烯（1.29%）、2-甲氧基-8-甲基-1,4-萘二酮（4.01%）、β-蛇床烯（17.99%）、氧化-α-依兰烯（3.00%）、4,4α,5,6,7,8-六氢-4α,5-二甲基-3-(1-甲基乙烯基)-2(3H)-萘酮（8.11%）、α-香附酮（26.15%）、长叶松香芹酮（1.11%）等，这些含量较多的成分占香附挥发油总量的81.50%,为建立香附挥发油质量标准提供了科学依据。     

STM observation of a single diarylethene flickering

We report the STM observation of single diarylethene derivatives (DD) embedded into alkylthiol self-assembled monolayers (SAM) on Au(111). Telegraph noise in the data shows that the molecular conductance oscillates between two states. Comparing our results to the ones obtained by other teams observing conductance flickering with systems in the same geometry, we relate the two observed states to the two isomeric configurations of the molecule under study.     

Construction of a low-temperature thermodynamic measurement system for single crystal of molecular compounds under pressures

An apparatus to obtain low-temperature thermodynamic information under high pressures for a tiny single crystal of molecular compounds was developed based on the ac technique. To detect small temperature oscillation of a sample inside the cramp-type pressure cell, we have used a small ruthenium oxide chip sensor as a thermometer. The adoption of the four-terminal method by the ac resistance bridge has made high-resolution detection of thermal anomaly possible in the low-temperature region. The constructed high-pressure thermodynamic system was mounted on a 3He refrigerator and we have succeeded to detect the thermal anomaly in relevant to magnetic order of single crystal sample of Mn4-cluster complex up to 1.05 GPa. A distinct peak of the heat capacity and its upward shift with increasing pressures was observed using a tiny crystal of about 100 microg. The high-pressure behavior of the discontinuity of heat capacity at the superconductive transition of 6 mg of metal indium has also been detected by this apparatus. The details and performance of the technique are reported.     

Crystallography of dispersed liquid crystalline phases studied by cryo-transmission electron microscopy

Low molecular weight surfactants, for example monoglycerides and phospholipids, form a multitude of self‐assembled structures, such as inverted cubic or hexagonal mesophases, if brought into contact with water/oil. These mesophases can be dispersed in water using adequate surface‐active materials such as low molecular weight surfactants or surface active polymers. In order to use such mesophase particles for incorporating drugs and aromas, it is essential to determine their internal crystallographic structure and to understand their mechanism of stabilization. Cryo‐transmission electron microscopy was used to investigate the internal structure of different dispersed particles at various temperatures and oil contents. It is shown here that cryo‐transmission electron microscopy, in combination with fast Fourier transform and tilting experiments, is effective in obtaining information on crystallographic structure, space group and morphology of particles with reversed bicontinuous cubic and hexagonal structures. In particular, using the presence or the absence of the {111} reflections and viewing the same particle under different axes of observation allows one to discriminate between the Im3m and Pn3m space groups. A major advantage of cryo‐transmission electron microscopy is the ability to analyse single particles. This allows the identification of particles present at very low concentrations and the coexistence of particles with different internal self‐assembly structures. With this technique we have obtained strong evidence for the presence of two cubic internal self‐assembly structures with different space groups within the same dispersion. In addition, we found that cryo‐transmission electron microscopy combined with tilting experiments enables the analysis of internal particle morphology, allowing the discussion of mechanisms for hexosome stabilization.     

Atomic force microscopy to characterize the molecular size of prion protein

The conformational transition of α‐helix‐rich cellular prion protein (PrP^C) to an isomer with high β‐sheet content is associated with transmissible spongiform encephalopathies. With the ultimate long‐term goal of using imaging techniques to study PrP aggregation, we report the results of initial experiments to determine whether PrP molecules could be visualized as single molecules, and if the observed size corresponded to the calculated size for PrP. The investigation of single molecules, and not those embedded into larger aggregates, was the key in our experimental approach. Using atomic force microscopy (AFM) as an imaging method, the immobilization of recombinant histidine (His)₁₀‐tagged PrP on mica was performed in the presence of different heavy metal ions. The addition of Cu²⁺ resulted in an enhanced PrP immobilization, whereas Ni²⁺ reduced coverage of the surface by PrP. High‐resolution data from dried PrP preparations provided a first approximation to geometrical parameters of PrP precipitates, which indicated that the volume of a single PrP molecule was 30 nm³. Molecular dynamics simulations performed to complement the structural aspects of the AFM investigation yielded a calculated molecular volume of 33 nm³ for PrP. These experimentally observed and theoretically expected values provide basic knowledge for further studies on the size and composition of larger amyloidal PrP aggregates, PrP isoforms or mutants such as PrP molecules without octarepeats.     

Performance evaluation experiments on a laser spot triangulation probe

Laser triangulation probes are increasingly used for dimensional measurements in a variety of applications. At the National Institute of Standards and Technology, we have recently explored the use of laser spot triangulation probes to determine dimensional features such as height and width of channels in a fuel cell plate. To assess the suitability of the probes for performing these dimensional measurements, we designed several experiments that highlighted different error sources in the probes. This report is a summary of those experiments. Numerous studies have been reported in the literature on error sources in laser triangulation probes utilizing artifacts of varying shape (form), color, reflectivity, surface finish, etc. However, our experiments are targeted towards establishing bounds on errors when measuring simple linear dimensions such as height and width on prismatic objects of a single color and material. Our scope is indeed narrow, but intentionally so; it is our objective to highlight the influence of a number of “hidden performance attributes” [1] that impact accuracy of even simple linear dimensional measurements so that it may be of use to others who perform similar measurements.     

Time-resolved confocal analysis of antibody penetration into living,solid tumor spheroids

The in vivo function of a biologically active molecule is governed in part by the dynamics of its distribution within its target tissue. To enhance our ability to probe living cells, we have endeavored to improve live confocal microscopy methods and to develop analytical methods that simplify the handling of the resulting complex data sets. To do this we attached a recently developed micro-incubation system to the stage of a Leica confocal laser scanning microscope and were able to maintain physiologic culture conditions over several hours. Axial stability was achieved by modifying the room air conditioning. Laser illumination was low enough to retain cell viability through several hours of continuous scanning. With this setup, planar, time-resolved data sets (xyt) were produced by continuously rescanning a single xy plane at the rate of one scan/min. As an alternative, volumetric data sets (xyz) were acquired by stepping the scanned plane through the z axis. In both types of data sets, a semi-quantitative determination of the concentration of a fluorescent reporter molecule (e.g., FITC) over a gray level range of 0--255 was recorded along with the positional information. Thus, concentration (as intensity of fluorescence, or i) gave a fourth variable by either scan method, resulting in high-density xyti or xyzi data sets. The biological model we used to examine these methods was the penetration of a FITC-labeled, anti-carcinoma monoclonal antibody into cultured spheroids of tumor cells bearing the antibody-binding epitope. In one case, the distribution of antibody-FITC conjugate was compared with that of a long wavelength membrane dye. DiIC₁₈(5). Several different software analyses were compared, including examining xyt data sets as “volumes.” We observed that by increasing the displayed resolution of one variable, the demonstrable resolution of the other variables was reduced. For example, with high temporal resolution, either quantitative or positional resolution had to be sacrificed. Thus, we needed to perform several different analyses of a single data set to compare all of the variables properly. In these experiments, the dynamic aspects of the changes in antibody-FITC distribution were examined. Along with comparison of antibody-FITC penetration with that of DiI, these data suggest an as yet unexplained biological transport of antibody into a tumor spheroid, which is not consistent with mere passive diffusion through the fluid of extracellular clefts. Using this model system, we have performed and analyzed highly time-resolved confocal microscopy on living specimens maintained under physiologic conditions.     

A study of factors influencing the accuracy of quantitative X-ray analysis of thin foil specimens

Needle-shaped specimens were used to determine experimental values of X-ray intensity ratios for L/K and M/L lines. These ratios can be used to determine k_AB factors for L and M lines. They also provide necessary data for absorption correction with the X-ray intensity ratio method. Recent theoretical calculations of k_AB factors are evaluated and new constants in the parametrization of ionization cross-sections for L and M lines are presented. The average X-ray production per unit thickness has been measured as a function of mass thickness for a stainless steel specimen using acceleration voltages of 100 and 200 kV. An increase in X-ray production by more than 20% was found at 100 kV for mass thicknesses up to 1 mg/cm² while at 200 kV the increase was about 5%. The contamination spot method for thickness measurements has also been investigated using needle-shaped specimens. The reported overestimate of thickness using this method can be explained by the formation of broad contamination layers around the deposited spots.     

Comparison of different methods for estimating muscle forces in human movement

The aim of this study was to compare muscle-force estimates derived for human locomotion using three different methods commonly reported in the literature: static optimisation (SO), computed muscle control (CMC) and neuromusculoskeletal tracking (NMT). In contrast with SO, CMC and NMT calculate muscle forces dynamically by including muscle activation dynamics. Furthermore, NMT utilises a time-dependent performance criterion, wherein a single optimisation problem is solved over the entire time interval of the task. Each of these methods was used in conjunction with musculoskeletal modelling and experimental gait data to determine lower-limb muscle forces for self-selected speeds of walking and running. Correlation analyses were performed for each muscle to quantify differences between the various muscle-force solutions. The patterns of muscle loading predicted by the three methods were similar for both walking and running. The correlation coefficient between any two sets of muscle-force solutions ranged from 0.46 to 0.99 (p < 0.001 for all muscles). These results suggest that the robustness and efficiency of static optimisation make it the most attractive method for estimating muscle forces in human locomotion.     

Development of a diagnostic system using LPC/cepstrum analysis in machine vibration

Based on the linear predictive coding (LPC) and cepstrum analysis coefficients (LPC/Cepstrum), the implementation of LPC procedure to detect faults in engine assembly line diagnostics systems is proposed. In addition, with the purpose to gain more accurate results in the implementation of LPC procedure instead the use of Euclidean distance, Mahalanobis distance is applied. In order to prove the effectiveness of the procedure, we retrieved data sets obtained from diesel engines under normal and abnormal conditions with different RPMs. During experiments, it was possible to detect the system status and it also was proved that the Mahalanobis distance overcomes the limitations of Euclidean distance where the statistical distribution of the variables are taken into account. In this way, we achieved a clear and accurate diagnosis of the system even when the differences between system status were not possible to detect through conventional methods. It was proved that the consistent patterns of each system status can be established and also possible to determine the system status successfully. The differences and advantages of LPC/Cepstrum approach compared with common analysis are discussed in this paper.     

A two-color tunable infrared/vacuum ultraviolet spectrometer for high-resolution spectroscopy of molecules in molecular beams

We describe here the key technical elements of a two-color tunable IR/VUV photoionization TOF mass spectrometer system which allows a wide-range of high-resolution experiments to be performed on a diverse range of cold molecules and clusters in a molecular beam. In particular we highlight the methods we have applied to provide efficient wavelength separation of the VUV radiation from the longer wavelength components used to generate it and discuss a number of systems that we have studied with the instrument which highlight its flexibility for use in the study of molecular spectroscopy.     

PEG/壳聚糖相变材料储热性能研究

用物理共混法将PEG和壳聚糖共混物改性,得到具有固-固相变性能的储能材料,用DSC具体研究了PEG的质量分数及PEG的相对分子质量对共混材料的相变温度和相变焓的影响。结果表明,不同相对分子质量的PEG在63～68℃之间具有较大的相变焓,且相变焓在180～220J/g之间,均可做为相变储能材料;相对分子质量为8000的PEG相对于其他的PEG更适宜做相变储能材料,共混物中PEG的质量分数低于85%时才表现为固-固相变材料。     

低频扭转振动攻丝基本工艺参数组研究

描述振动攻丝运动形式的参数组很多，不同参数组中影响攻丝扭矩和攻丝效率的参数个数不同，选择对攻丝扭矩和攻丝效率影响因素少的参数组可以大大减少振动攻丝工艺参数选择和优化的难度，在给定条件下，通过试验得到，当以纯切削角，角振幅和振动频率三元素组成工艺参数组时，可以使影响攻丝扭矩的参数由原来的3个下降到2个，因此认为该工艺参数组及其等价的参数组都可以作为低频扭转振动攻丝的基本工艺参数组。     

Achieving uniform field emission from carbon nanotube composite cold cathode with different carbon nanotube contents: Effects of conductance and plasma treatment

A carbon nanotube (CNT) composite cold cathode was studied for field emission display application. The CNT composite cold cathode was composed of CNTs and silicon dioxide binder. Field emission from CNT composite cold cathode with different CNT contents was studied. It was found that with increase in CNT contents, the threshold field decreased. The conductance of the composite cathode was measured and with increasing CNT content, there was a critical CNT content where the conductance increased several orders of magnitude. Plasma etching using SF₆ as the etchant was adopted to treat the cathode. Improvement in emission uniformity was achieved. It was also found that after post-treatment the threshold field of the cathode decreased. The morphology of the etched cathode was analyzed and the improvement of uniformity and lowering of the threshold field was attributed to the exposure of CNTs after etching.     

The measurement of deformation rate of flexible container based on constant conductance element in molecular regime

An innovative method using constant conductance element to measure the deformation rate of flexible container was proposed based on the theoretical model of dynamic micro-flow detection. The measurement system of dynamic deformation rate was built by using the semi-flexible container encapsulated with polyimide film, and a constant conductance element was installed between the flexible container and the pressure stabilizing chamber to provide a constant micro-flow rate. The dynamic differential pressure variation between the inlet and the outlet of the constant conductance element was measured and then the deformation rate of flexible container could be obtained through the iterative calculation of the measured differential pressure change over time. The results show that the deformation rate gradually decreases with the increase of differential pressure. In addition, Boyle's law was used to verify the experimental results of dynamic micro-flow detection method, and it is revealed that the measurement error is less than 11%. Finally, a standard leak element was applied to simulate the leak source of flexible container. The conductance of standard leak element and the leak rate of flexible container have been successfully predicted by using the measured deformation rate.