Quantifying the Effects of Contact Duration, Loading Rate, and Approach Velocity on P-Selectin-PSGL-1 Interactions Using AFM

Kinetics and its regulation by extrinsic physical factors govern selectin-ligand interactions that mediate tethering and rolling of circulating cells on the vessel wall under hemodynamic forces. While the force regulation of off-rate for dissociation of selectin-ligand bonds has been extensively studied, much less is known about how transport impacts the on-rate for association of these bonds and their stability. We used atomic force microscopy (AFM) to quantify how the contact duration, loading rate, and approach velocity affected kinetic rates and strength of bonds of P-selectin interacting with P-selectin glycoprotein ligand I (PSGL-1). We found a saturable relationship between the contact time and the rupture force, a biphasic relationship between the adhesion probability and the retraction velocity, a piece-wise linear relationship between the rupture force and the logarithm of the loading rate, and a threshold relationship between the approach velocity and the rupture force. These results provide new insights into how physical factors regulate receptor-ligand interactions.

Nonlinear Analysis of Oscillatory Indentation in Elastic and Viscoelastic Solids

Determining the mechanical properties at micro- and nanometer length scales using nanoindentation or atomic force microscopy is important to many areas of science and engineering. Here we establish equations for obtaining storage and loss modulus from oscillatory indentations by performing a nonlinear analysis of conical and spherical indentation in elastic and viscoelastic solids. We show that, when the conical indenter is driven by a sinusoidal force, the square of displacement is a sinusoidal function of time, not the displacement itself, which is commonly assumed. Similar conclusions hold for spherical indentations. Well-known difficulties associated with measuring contact area and correcting thermal drift may be circumvented using the newly derived equations. These results may help improve methods of using oscillatory indentation for determining elastic and viscoelastic properties of solids.

Micro- and macro-tribological study on a self-assembled dual-layer film

A novel self-assembled dual-layer film as apotential excellent lubricant for micromachines was successfully prepared on single-crystal silicon substrate by chemical adsorption of stearic acid (STA) molecules on self-assembled monolayer of 3-aminopropyltri

牛血清蛋白在亲水硅片表面吸附的原子力显微成像

本文用原子力显微术（AFM）研究了牛血清白蛋白（BSA）在亲水硅片表面的吸附，硅片表面经亲水处理后，将牛血清蛋白（BSA）吸附在表面，采用轻敲模式，可获得清晰的AFM图像，牛血清蛋白（BSA）的AFM图像表明：BSA在亲水硅片表面是单分子，水平吸附在硅片表面，且吸颗粒状；1mg/ml的BSA在吸附30min后为饱和吸附。BSA到达硅表面后，蛋白中可移动的带正电荷的基团可以趋向亲水表面，使BSA与硅表面的静电相互作用由斥力变为吸引力，BSA可以稳定地吸附在亲水硅片表面。

Equivalency and Locality in Nano-Scale Measurement

Two principal problems of equivalency and locality in nano-scale measurement are considered in this paper. The conventional measurements of force and displacement are always closely related to the equivalency problem between the measuremental results by experimental system and the real physical status of the sample, and the locality of the mechanical quantities to be measured. There are some noticeable contradictions in nano-scale measurements induced by the two problems. In this paper, by utilizing a coupled molecular-continuum method, we illustrate the important effects of the two principal problems in atomic force microscopy (AFM) measurements on nano-scale. Our calculations and analysis of these typical mechanical measurement problems suggest that in nano-meter scale measurements, the two principal problems must be carefully dealt with. The coupled molecular-continuum method used in this paper is very effective in solving these problems on nano-scale.

杜仲抗真菌蛋白晶体生长的原子力显微成像研究—快速生长与晶面生长速率

杜仲抗真菌蛋白(Eucommiaantifungalprotein,EAFP)的单晶体具有在几小时内就可长大的快速生长特性.用原子力显微成像(atomicforcemicroscope,AFM)技术,原位实时观测了EAFP单斜晶体生长过程中的｛10 0｝表面形貌动态变化,并分别在不同的过饱和度下测量了其生长速率.结果表明,EAFP晶体生长的速率与蛋白质溶液的过饱和度相关,在过饱和度高时(σ =1 78)晶面生长极快;在中等过饱和度(σ =1 5)下,其晶面台阶的生长速率沿b,c方向分别为 12nm/s和 2 4 2nm/s,比溶菌酶生长速率(6～ 7nm/s)快很多;在蛋白质浓度很低的情况下,其生长速率仍与其他蛋白质相当.EAFP晶体快速生长可能与该分子尺寸较小,内部结构紧凑,分子骨架呈刚性和分子表面性质等其固有特性密切相关.沉淀剂浓度对EAFP晶体生长也有影响.过饱和度很低时,提高沉淀剂浓度会干扰晶体生长.

Immunosensor Interface Based on Physical and Chemical Immunogylobulin G Adsorption onto Mixed Self-Assembled Monolayers

An immunosensor interface based on mixed hydrophobic self-assembled monolayers (SAMs) of methyl and carboxylic acid terminated thiols with covalently attached human Immunoglobulin G (hIgG), is investigated. The densely packed and organised SAMs were characterised by contact angle measurements and cyclic voltammetry. The effect of the non-ionic surfactant, Tween 20, in preventing nonspecific adsorption is addressed by ellipsometry during physical and covalent hIgG immobilization on pure and mixed SAMs, respectively. It is clearly demonstrated that nonspecific adsorption due to hydrophobic interactions of hIgG on methyl ended groups is totally inhibited, whereas electrostatic/hydrogen bonding interactions with the exposed carboxylic groups prevail in the presence of surfactant. Results of ellipsometry and Atomic Force Microscopy, reveal that the surface concentration of covalently immobilized hIgG is determined by the ratio of COOH/CH3-terminated thiols in SAM forming solution. Moreover, the ellipsometric data demonstrates that the ratio of bound anti-hIgG/hIgG depends on the density of hIgG on the surface and that the highest ratio is close to three. We also report the selectivity and high sensitivity achieved by chronoamperometry in the detection of adsorbed hIgG and the reaction with its antibody.

Md Simulation Of Dislocation Mobility During Cutting With Diamond Tip On Silicon

By means of Tersoff and Morse potentials, a three-dimensional molecular dynamics simulation is performed to study atomic force microscopy cutting on silicon monocrystal surface. The interatomic forces between the workpiece and the pin tool and the atoms of workpiece themselves are simulated. Two partial edge dislocations are introduced into workpiece Si, it is found that the motion of dislocations does not occur during the atomic force microscopy cutting processing. Simulation results show that the shear stress acting on dislocations is far below the yield strength of Si. (c) 2008 Elsevier Ltd. All rights reserved.

Piezoelectricity Of Zno Films Prepared By Sol-Gel Method

ZnO piezoelectric thin films were prepared on crystal substrate Si(111) by sol-gel technology, then characterized by scanning electron microscopy, X-ray diffraction and atomic force microscopy (AFM). The ZnO films characterized by X-ray diffraction are highly oriented in (002) direction with the growing of the film thickness. The morphologies, roughness and grain size of ZnO film investigated by AFM show that roughness and grain size of ZnO piezoelectric films decrease with the increase of the film thickness. The roughness dimension is 2.188-0.914 nm. The piezoelectric coefficient d(33) was investigated with a piezo-response force microscope (PFM). The results show that the piezoelectric coefficient increases with the increase of thickness and (002) orientation. When the force reference is close to surface roughness of the films, the piezoelectric coefficient measured is inaccurate and fluctuates in a large range, but when the force reference is big, the piezoelectric coefficient d(33) changes little and ultimately keeps constant at a low frequency.

Instability Of Crack Propagation In Brittle Bulk Metallic Glass

The instability of the crack tip in brittle Mg-based bulk metallic glass (BMG) is studied. The formation of various fractographic surfaces of the BMG is associated with the instability of the fluid meniscus, which is due to viscous fluid matter being present on the fracture process zone. Depending on the values of the wavelength of the initial perturbation of the fluid meniscus and the local stress intensity factor, different fracture surface profiles, i.e. a dimple-like structure, a periodic corrugation pattern and a pure mirror zone are formed. The fractographic evolution is significantly affected by the applied stress. A decreased fracture Surface roughness is observed under a low applied stress. An increased fracture surface roughness, which has frequently been reported by other researchers, is also observed in the present studies under a high applied stress. Unique fractographic features are attributed to the non-linear hyperelastic stiffening for less softening) mechanism. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Molecular Dynamics Study Of Mechanical Behaviour Of Screw Dislocation During Cutting With Diamond Tip On Silicon

By means of Tersoff and Morse potentials, a three-dimensional molecular dynamics simulation is performed to study atomic force microscopy cutting on silicon monocrystal surface. The interatomic forces between the workpiece and the pin tool and the atoms of workpiece themselves are calculated. A screw dislocation is introduced into workpiece Si. It is found that motion of dislocations does not occur during the atomic force microscopy cutting processing. Simulation results show that the shear stress acting on dislocation is far below the yield strength of Si.

Robust Nanoadhesion Under Torque

In this article, optimization of shear adhesion strength between an elastic cylindrical fiber and a rigid substrate under torque is studied. We find that when the radius of the fiber is less than a critical value, the bonding-breaking along the contact interface occurs uniformly, rather than by mode III crack propagation. Comparison between adhesion models under torque and tension shows that nanometer scale of fibers may have evolved to achieve optimization of not only the normal adhesive strength but also the shear adhesive strength in tolerance of possible contact flaws.

Mn implanted GaAs by low energy ion beam deposition

High dose Mn was implanted into semi-insulating GaAs substrate to fabricate embedded ferromagnetic Mn-Ga binary particles by mass-analyzed dual ion beam deposit system at room temperature. The properties of as-implanted and annealed samples were measured with X-ray diffraction, high-resolution X-ray diffraction to characterize the structural changes. New phase formed after high temperature annealing. Sample surface image was observed with atomic force microscopy. All the samples showed ferromagnetic behaviour at room temperature. There were some differences between the hysteresis loops of as-implanted and annealed samples as well as the cluster size of the latter was much larger than that of the former through the surface morphology. (C) 2004 Elsevier B.V. All rights reserved.

Growth of liquid bridge in AFM

Capillary forces are dominant in adhesive forces measured with an atomic force microscope (AFM) in ambient air, which are thought to be dependent on water film thickness, relative humidity and the free energy of the water film. In this paper, besides these factors, we study the nature of the 'pull-off' force on a variety of atmospheres as a function of the contact time. It is found that capillary forces strongly depend on the contact time. In lower relative humidity atmosphere, the adhesion force is almost independent of the contact time. However, in higher relative humidity, the adhesion force increases with the contact time. Based on the experiment and a model that we present in this paper, the growth of the liquid bridge can be seen as undergoing two processes: one is water vapour condensation; the other is the motion of the thin liquid film that is absorbed on the substrate. The experiment and the growth model presented in this paper have direct relevance to the working mechanism of AFM in ambient air.

Multi-scale analysis of AFM tip and surface interactions

Thoroughly understanding AFM tip-surface interactions is crucial for many experimental studies and applications. It is important to realize that despite its simple appearance, the system of tip and sample surface involves multiscale interactions. In fact, the system is governed by a combination of molecular force (like the van der Waals force), its macroscopic representations (such as surface force) and gravitational force (a macroscopic force). Hence, in the system, various length scales are operative, from sub-nanoscale (at the molecular level) to the macroscopic scale. By integrating molecular forces into continuum equations, we performed a multiscale analysis and revealed the nonlocality effect between a tip and a rough solid surface and the mechanism governing liquid surface deformation and jumping. The results have several significant implications for practical applications. For instance, nonlocality may affect the measurement accuracy of surface morphology. At the critical state of liquid surface jump, the ratio of the gap between a tip and a liquid dome (delta) over the dome height (y(o)) is approximately (n-4) (for a large tip), which depends on the power law exponent n of the molecular interaction energy. These findings demonstrate that the multiscale analysis is not only useful but also necessary in the understanding of practical phenomena involving molecular forces. (c) 2007 Elsevier Ltd. All rights reserved.