运用功能化修饰的碳纳米管针尖测量配体-受体的作用力

电弧法自制碳纳米管原子力显微镜针尖,对其末端进行功能化修饰,然后测量配体-受体之间的作用力。运用没有功能化修饰的碳纳米管针尖与修饰有亲和素分子的基底进行接触测量时,没有粘滞力出现;而运用末端修饰生物素分子的碳纳米管针尖测量时,有粘滞力产生。功能化的碳纳米管针尖直接测得的粘滞力均大约200pN,此值符合一对配体生物素和受体亲和素之间的作用力。这一结果很难用传统的针尖获得,功能化修饰的碳纳米管针尖能够克服传统针尖在力测量中的局限,在生物学和化学领域有着广泛的应用前景。     

Probe-induced native oxide decomposition and localized oxidation on 6H-SiC (0001) surface: an atomic force microscopy investigation

We report, for the first time, the native oxide decomposition/etching and direct local oxide growth on 6H-SiC (0001) surface induced by atomic force microscopy (AFM). Surface native oxide was decomposed and assembled into protruded lines when the negatively biased AFM tip was scanned over surface areas. The mechanism of decomposition was found to be governed by the Fowler-Nordheim emission current enhanced by the negatively biased AFM tip. Direct oxide growth on the SiC surface was achieved when the AFM tip was immobilized and longer bias duration applied. In particular, the aspect ratio of oxide grown on SiC was found to be several times higher than that on the Si surface. The improved aspect ratio on SiC was attributed to the anisotropic OH(-) diffusion involved in vertical and lateral oxidation along the polar and nonpolar directions such as [0001] and [110] axis in SiC crystal. The electron transport in the above AFM grown oxide on SiC was further investigated by I-V characteristics. The dielectrical strength of AFM oxide against degradation and breakdown under electrical stressing was evaluated.     

Transport mechanisms in capillary condensation of water at a single-asperity nanoscopic contact

Transport mechanisms involved in capillary condensation of water menisci in nanoscopic gaps between hydrophilic surfaces are investigated theoretically and experimentally by atomic force microscopy (AFM) measurements of capillary force. The measurements showed an instantaneous formation of a water meniscus by coalescence of the water layers adsorbed on the AFM tip and sample surfaces, followed by a time evolution of meniscus toward a stationary state corresponding to thermodynamic equilibrium. This dynamics of the water meniscus is indicated by time evolution of the meniscus force, which increases with the contact time toward its equilibrium value. Two water transport mechanisms competing in this meniscus dynamics are considered: (1) Knudsen diffusion and condensation of water molecules in the nanoscopic gap and (2) adsorption of water molecules on the surface region around the contact and flow of the surface water toward the meniscus. For the case of very hydrophilic surfaces, the dominant role of surface water transportation on the meniscus dynamics is supported by the results of the AFM measurements of capillary force of water menisci formed at sliding tip-sample contacts. These measurements revealed that fast movement of the contact impedes on the formation of menisci at thermodynamic equilibrium because the flow of the surface water is too slow to reach the moving meniscus.     

碳纳米管原子力显微镜针尖对脱氧核糖核酸高分辨率成像研究

运用自制的碳纳米管原子力显微镜针尖,在液体中观察了脱氧核糖核酸（DNA）分子的精细结构。结果表明,运用碳纳米管针尖获得的DNA分子的高度与电子显微镜的结果非常接近,且没有造成样品的变形损伤;碳纳米管针尖得到的DNA分子的宽度与真实值相近,减小了原子力显微镜成像的增宽效应,这是用传统的硅针尖无法获得的。DNA分子精细结构的高分辨率图像的获得为研究其功能提供了有价值的信息。     

Direct AFM measurements of adhesion forces between calcium oxalate monohydrate and kidney epithelial cells in the presence of Ca2+ and Mg2+ ions

Adhesion forces between the calcium oxalate monohydrate (COM, whewellite) crystal and the layer of the epithelial kidney cells have been directly measured under buffer solutions by using atomic force microscope (AFM). Two renal epithelial lines, MDCK (a collecting duct line) and LLC-PK1 (a proximal tubular line), were used. All experiments were conducted in buffer solutions containing additional Ca(2+) and Mg(2+) ions in the various concentrations. For MDCK-cells, the obtained values of the adhesion force were in the range 0.12-0.51 nN and 0.12-0.20 nN for Ca(2+) and Mg(2+), respectively. No adhesion force (larger than 0.05 nN) has been found for LLC-PK1 cells. The "critical" concentrations of ions, near which the adhesion force (for MDCK-cells) was maximal, were found to be 100 mM. The "critical" concentration of ions and the tendency of the adhesion forces with the changing ions concentration, confirm earlier results of Lieske et al. [J.C. Lieske, G. Farell, S. Deganello, Urol. Res. 32 (2004) 117-123], in which the affinity (rather than the adhesion force) between the COM micro-crystals and the layer of the MDCK-cells were measured, calculating the radioactive signal of radioactive (14)C COM-crystals stuck to the cells. We believe that the aggregation of the COM crystals does not occur in the bulk urine due to short travel time through the nephron. If so, the kidney stone formation is determined by COM-seeding on the tubules walls. The further growth of the stone on the seed can take practically unlimited time because the COM crystal is practically is not soluble in water or urine solutions. The value of the adhesion force can be useful for evaluation of the adhesion energy or probability of the COM-aggregates to stick to the kidney epithelium under the urine flow. This probability is calculated taking into account the adhesion force, F(ad), and hydrodynamic driving force of the flow. This probability reflects the opportunity of the small aggregates to grow and form the kidney stones.     

Scanning-induced growth on single crystal calcite with an atomic force microscope

We report observations of localized growth on the (1014) surface of single-crystal CaCO3 in supersaturated solutions while scanning with the tip of an atomic force microscope (AFM). At low contact forces, AFM scanning strongly enhances deposition along preexisting steps. This enhancement increases rapidly with increasing solution supersaturation, and is capable of filling in multilayer etch pits to produce defect-free surfaces at the resolution of the AFM. Attempts to achieve similar deposition rates in the absence of scanning require high supersaturations that produce three-dimensional crystal nuclei, which are important defects. Localized deposition produced by drawing the AFM tip back and forth across step edges can produce monolayer deposits extending well over a micron from the scanned area. These tip-induced deposits provide convincing evidence for the importance of ledge diffusion in calcite crystal growth.     

Modulation of calcium oxalate crystallization by linear aspartic acid-rich peptides

Calcium oxalate monohydrate (COM) kidney stone formation is prevented in most humans by urinary crystallization inhibitors. Urinary osteopontin (OPN) is a prototype of the aspartic acid-rich proteins (AARP) that modulate biomineralization. Synthetic poly(aspartic acids) that resemble functional domains of AARPs provide surrogate molecules for exploring the role of AARPs in biomineralization. Effects of linear aspartic acid-rich peptides on COM growth kinetics and morphology were evaluated by the combination of constant composition (CC) analysis and atomic force microscopy (AFM). A spacer amino acid (either glycine or serine) was incorporated during synthesis after each group of 3 aspartic acids (DDD) in the 27-mer peptide sequences. Kinetic CC studies revealed that the DDD peptide with serine spacers (DDDS) was more than 30 times more effective in inhibiting COM crystal growth than the DDD peptide with glycine spacers (DDDG). AFM revealed changes in morphology on (010) and (-101) COM faces that were generally similar to those previously described for OPN and citrate, respectively. At comparable peptide levels, the effects of step pinning and reduced growth rate caused by DDDS were remarkably greater. In CC nucleation studies, DDDS caused a greater prolongation of induction periods than DDDG. Thus, nucleation studies link changes in interfacial energy caused by peptide adsorption to COM to the CC growth and AFM results. These studies indicate that, in addition to the number of acidic residues, the contributions of other amino acids to the conformation of DDD peptides are also important determinants of the inhibition of COM nucleation and growth.     

In-situ atomic force microscopy (AFM) imaging: influence of AFM probe geometry on diffusion to microscopic surfaces

The effect of AFM probe geometry on diffusion to micrometer-scale reactive (electrode) interfaces is considered. A disk-shaped substrate electrode was held at a potential to reduce a species of interest (aqueous Ru(NH 3) 6 (3+)) at a diffusion-controlled rate and the current response during AFM imaging provided information on local mass transport to the interface. This approach reveals how the AFM probe influences diffusion to a reactive surface, which is of importance in more clearly delineating the conditions under which in-situ AFM can be treated as a noninvasive probe of surface processes involving mass transport (e.g., electrode reactions and crystal dissolution and growth). An assessment has been made of three types of probes: V-shaped silicon nitride contact mode probes; single beam silicon probes; and batch-fabricated scanning electrochemical-atomic force microscopy (SECM-AFM) probes. Two disk electrodes, (6.1 microm and 1.6 microm diameter) have been considered as substrates. The results indicate that conventional V-shaped contact mode probes are the most invasive and that the batch-fabricated SECM-AFM probes are the least invasive to diffusion at both of the substrates used herein. The experimental data are complemented by the development of simulations based on a simple 2D model of the AFM probe and active surface site. The importance of probe parameters such as the cantilever size, tip cone height, and cone angle is discussed, and the implications of the results for studies in other areas, such as growth and dissolution processes, are considered briefly.     

Adhesion between molecules and calcium oxalate crystals: critical interactions in kidney stone formation

Kidney stones are crystal aggregates, most commonly containing calcium oxalate monohydrate (COM) crystals as the primary constituent. Notably, in vitro studies have suggested that anionic molecules or macromolecules with substantial anionic functionality (e.g., carboxylate) play an important role in crystal aggregation and crystal attachment to renal epithelial cells. Furthermore, kidney stones contain measurable amounts of carboxylate-rich proteins that may serve as adhesives and promote aggregation of COM crystals. Atomic force microscopy (AFM) measurements of adhesion forces between tip-immobilized molecules and the COM (100) surface in aqueous media, described herein, reveal the effect of functional groups on adhesion and support an important role for the carboxylate group in processes responsible for kidney stone formation, specifically macromolecule-mediated adhesion of COM crystals to cells and crystal aggregation. The presence of poly(aspartic acid) during force measurements results in a reduction in the adhesion force measured for carboxylate-modified tips, consistent with the blocking of binding sites on the COM (100) surface by the carboxylate-rich polymer. This competitive binding behavior mimics the known reduction in attachment of COM crystals to renal epithelial cells in the presence of carboxylate-rich urinary macromolecules. These results suggest a feasible methodology for identifying the most important crystal surface-macromolecule combinations related to stone formation.     

In situ hydrodynamic lateral force calibration of AFM colloidal probes

Lateral force microscopy (LFM) is an application of atomic force microscopy (AFM) to sense lateral forces applied to the AFM probe tip. Recent advances in tissue engineering and functional biomaterials have shown a need for the surface characterization of their material and biochemical properties under the application of lateral forces. LFM equipped with colloidal probes of well-defined tip geometries has been a natural fit to address these needs but has remained limited to provide primarily qualitative results. For quantitative measurements, LFM requires the successful determination of the lateral force or torque conversion factor of the probe. Usually, force calibration results obtained in air are used for force measurements in liquids, but refractive index differences between air and liquids induce changes in the conversion factor. Furthermore, in the case of biochemically functionalized tips, damage can occur during calibration because tip-surface contact is inevitable in most calibration methods. Therefore, a nondestructive in situ lateral force calibration is desirable for LFM applications in liquids. Here we present an in situ hydrodynamic lateral force calibration method for AFM colloidal probes. In this method, the laterally scanned substrate surface generated a creeping Couette flow, which deformed the probe under torsion. The spherical geometry of the tip enabled the calculation of tip drag forces, and the lateral torque conversion factor was calibrated from the lateral voltage change and estimated torque. Comparisons with lateral force calibrations performed in air show that the hydrodynamic lateral force calibration method enables quantitative lateral force measurements in liquid using colloidal probes.     

Nanotribology under electrochemical conditions: influence of a copper (sub)monolayer deposited on single crystal electrodes on friction forces studied with atomic force microscopy

Friction force measurements performed by means of an atomic force microscope (AFM) under electrochemical conditions on a pure Au(111) electrode surface and one modified with a foreign metal are presented; after deposition of a (sub)monolayer copper on a Au(111) single crystal electrode a large increase of the friction force is observed compared to the pure Au(111) electrode surface; the extent of the increase not only depends on the copper coverage, but also on the normal load and may be explained by a higher energy dissipation due to motion of the sulfate anions adsorbed on the copper atoms induced by the AFM tip.     

Probing crystallization of calcium oxalate monohydrate and the role of macromolecule additives with in situ atomic force microscopy

Kidney stones are crystal aggregates, most commonly containing calcium oxalate monohydrate (COM) microcrystals as the primary constituent. Macromolecules, specifically proteins rich with anionic side chains, are thought to play an important role in the regulation of COM growth, aggregation, and attachment to cells, all key processes in kidney stone formation. The microscopic events associated with crystal growth on the [010], [121], and [100] faces have been examined with in situ atomic force microscopy (AFM). Lattice images of each face reveal two-dimensional unit cells consistent with the COM crystal structure. Each face exhibits hillocks with step sites that can be assigned to specific crystal planes, enabling direct determination of growth rates along specific crystallographic directions. The rates of growth are found to depend on the degree of supersaturation of calcium oxalate in the growth medium, and the growth rates are very sensitive to the manner in which the growth solutions are prepared and introduced to the AFM cell. The addition of macromolecules with anionic side chains, specifically poly(acrylic acid), poly(aspartic acid), and poly(glutamic acid), results in inhibition of growth on the hillock step planes. The magnitude of this effect depends on the macromolecule structure, macromolecule concentration, and the identity of the step site. Poly(acrylic acid) was the most effective inhibitor of growth. Whereas poly(aspartic acid) inhibited growth on the (021) step planes of the (100) hillocks more than poly(glutamic acid), the opposite was found for the same step planes on the (010) hillocks. This suggests that growth inhibition is due to macromolecule binding to both planes of the step site or pinning of the steps due to binding to the (100) and (010) faces alone. The different profiles observed for these three macromolecules argue that local binding of anionic side chains to crystal surface sites governs growth inhibition rather than any secondary polymer structure. Growth inhibition by cationic macromolecules is negligible, further supporting an important role for proteins rich in anionic side chains in the regulation of kidney stone formation.     

Cu(II) Sorption Mechanism on Montmorillonite: An Electron Paramagnetic Resonance Study

The mass of gamma-globulin fouling an Anodisc alumina membrane with a nominal pore diameter of 0.1 μm has been measured at several concentrations and pHs. This fouling resulted from filtering through the membrane in a continuous recirculation device. The low-concentration fouling can be attributed mainly to adsorption. The complete concentration dependence of fouling mass has been obtained and fitted to a Freundlich heterogeneous isotherm, from which the pH dependence of active fouling sites and energies has been also obtained. Adsorption is studied as a function of the electrostatic forces between the solute and the membrane. A sharp maximum in the adsorbed mass for zero electrostatic force is observed. At high concentrations, accumulation plays a relevant role at alkaline pH, as confirmed by flux decay experiments, retention measurements, and AFM (atomic force microscopy) pictures. Copyright 2000 Academic Press.     

Higher Acenes by On‐Surface Dehydrogenation: From Heptacene to Undecacene

A unified approach to the synthesis of the series of higher acenes up to previously unreported undecacene has been developed through the on‐surface dehydrogenation of partially saturated precursors. These molecules could be converted into the parent acenes by both atomic manipulation with the tip of a scanning tunneling and atomic force microscope (STM/AFM) as well as by on‐surface annealing. The structure of the generated acenes has been visualized by high‐resolution non‐contact AFM imaging and the evolution of the transport gap with the increase of the number of fused benzene rings has been determined on the basis of scanning tunneling spectroscopy (STS) measurements.     

Distribution of olfactory marker protein on a tissue section of vomeronasal organ measured by AFM

Distribution of olfactory marker protein (OMP) on a tissue section of vomeronasal organ (VNO) was successfully measured by atomic force microscopy (AFM). Anti-OMP antibodies were covalently crosslinked with the tip of the AFM and were used as a probe to observe the distribution of OMP on a tissue section. First, force measurements were performed using a glass surface on which OMP was covalently immobilized to verify the success of tip modification. Clear differences of interaction forces were observed between a specific pair and the control experiments, indicating that the tip preparation succeeded. Next, distributions of OMP on the tissue section were observed by AFM and were compared with immunohistochemical observations. For large scale observation, a microbead was used as a probe in the AFM measurements. The results of the AFM measurements were well overlapped with that of immunohistochemistry, confirming the reliability of our method. A mapping of the AFM measurement with high resolution was also successfully obtained, which showed an advantage of the application of the AFM measurement in analysis of proteins on the tissue section.     

Imaging and manipulation of adsorbed lipid vesicles by an AFM tip: experiment and Monte Carlo simulations

Single lipid vesicles adsorbed on SiO(2) were manipulated using an atomic force microscope (AFM) operated in contact mode. For large force setpoints, single vesicles were either pushed sideways or ruptured by the tip, depending on the tip type (sharp or blunt) used, while for small force setpoints the vesicles were imaged by the tip. To extend the interpretation of and to guide the experiment, we have developed a generic model of the vesicle-tip-substrate system and performed Monte Carlo simulations, addressing the influence of force setpoint and tip speed and shape on the type of imaging or manipulation observed. Specifically, we have explored AFM-image height and width variations versus force setpoint, typical AFM images for small and large force setpoints, tip-induced vesicle strain versus force setpoint, typical vesicle shapes during pushing for different tip speeds, and the details of vesicle rupture induced by the tip.     

Ferrocenylundecanethiol self-assembled monolayer charging correlates with negative differential resistance measured by conducting probe atomic force microscopy

Electrical and mechanical properties of metal-molecule-metal junctions formed between Au-supported self-assembled monolayers (SAMs) of electroactive 11-ferrocenylundecanethiol (FcC(11)SH) and a Pt-coated atomic force microscope (AFM) tip have been measured using a conducting probe (CP) AFM in insulating alkane solution. Simultaneous and independent measurements of currents and bias-dependent adhesion forces under different applied tip biases between the conductive AFM probe and the FcC(11)SH SAMs revealed reversible peak-shaped current-voltage (I-V) characteristics and correlated maxima in the potential-dependent adhesion force. Trapped positive charges in the molecular junction correlate with high conduction in a feature showing negative differential resistance. Similar measurements on an electropassive 1-octanethiol SAM did not show any peaks in either adhesion force or I-V curves. A mechanism involving two-step resonant hole transfer through the occupied molecular orbitals (MOs) of ferrocene end groups via sequential oxidation and subsequent reduction, where a hole is trapped by the phonon relaxation, is proposed to explain the observed current-force correlation. These results suggest a new approach to probe charge-transfer involving electroactive groups on the nanoscale by measuring the adhesion forces as a function of applied bias in an electrolyte-free environment.     

Unbinding of the streptavidin-biotin complex by atomic force microscopy: a hybrid simulation study

A hybrid molecular simulation technique, which combines molecular dynamics and continuum mechanics, was used to study the single-molecule unbinding force of a streptavidin-biotin complex. The hybrid method enables atomistic simulations of unbinding events at the millisecond time scale of atomic force microscopy (AFM) experiments. The logarithmic relationship between the unbinding force of the streptavidin-biotin complex and the loading rate (the product of cantilever spring constant and pulling velocity) in AFM experiments was confirmed by hybrid simulations. The unbinding forces, cantilever and tip positions, locations of energy barriers, and unbinding pathway were analyzed. Hybrid simulation results from this work not only interpret unbinding AFM experiments but also provide detailed molecular information not available in AFM experiments.     

Investigations of the Growth of Self-Assembled Octadecylsiloxane Monolayers with Atomic Force Microscopy

 Self-assembled monolayers of octadecylsiloxane were prepared and characterized by atomic force microscopy and ellipsometry. Parameters, like the residual water concentration of the solvent and the solution age, that affect both the surface coverage and the order of the film were investigated. Besides ex-situ measurements, also in-situ atomic force microscopy was used to characterize the growth and the kinetics of the adsorption process. Furthermore, self-assembly of organic films was used as a model system for studying the influence of the measurement process on in-situ experiments in the AFM liquid cell.     

草酸钾诱导的LB膜缺陷及对草酸钙堆积图形的影响

采用扫描电子显微镜(SEM)和原子力显微镜(AFM)研究了经不同浓度草酸钾(K2C2O4)处理后二棕榈酰磷酯酰胆碱(DPPC)的缺陷LB膜及其对一水草酸钙(COM)成核和生长的影响. K2C2O4的处理可进一步破坏LB膜中圆形畴区内的分子列阵, 尤其是使处在液态扩张相(LE)/液态凝集相(LC)边界的分子列阵无序程度增加, 从而促进了COM晶体在此处的成核和生长, 最终诱导圆形堆积的COM晶体图形出现. 随着损伤LB膜的K2C2O4浓度c(K2C2O4)从0.3 mmol/L增加到5.0 mmol/L, 其对LB膜畴区有序结构的破坏作用逐渐增强, 圈状堆积的晶体图形数量增多. 当c(K2C2O4)为0.3 mmol/L时, 主要诱导实心的圆形堆积的COM晶体图形, 而当c(K2C2O4)增加至5.0 mmol/L时, 生成圈状COM晶体图形, 且图形的半径减小. 这一研究结果将有助于从分子和超分子水平上了解肾小管上皮细胞膜损伤后的微结构变化及其与肾结石形成的关系.