Direct measurement of interatomic force gradients using an ultra-low-amplitude atomic force microscope

Interatomic force gradients between a W tip and a 7 × 7 reconstructed Si(111) surface were measured using an off-resonance, ultra-low-amplitude atomic force microscope (AFM) technique. The amplitudes used were less than 1 Å (peak-to-peak), which allowed direct measurement of the interaction force gradients as a function of separation. The force gradient curves are shown to consist of an attractive van der Waals part and short-range attractive and repulsive interactions. The van der Waals background can be subtracted, leaving a short-range interaction with an energy parameter of 1.9-3.4 eV and an interaction length-scale of 0.54-1.26 Å, characteristic of a single atomic bond. This correlates well with our observation of single-atom resolved force gradient images. In general, the interaction is reversible up to the zero intercept of the force gradient (inflection point of the energy). Beyond this point hysteresis tends to be observed and the onset of inelastic deformation can be clearly discerned. An analysis of the atomic scale contact gives reasonable values for the interfacial energy, yield strength, and the energy per atom needed to initiate plastic deformation

The influence of protein malnutrition on the production of GM-CSF and M-CSF by macrophages

ABSTRACT It is well established that protein malnutrition (PM) impairs immune defenses and increases susceptibility to infection. Macrophages are cells that play a central role in innate immunity, constituting one of the first barriers against infections. Macrophages produce several soluble factors, including cytokines and growth factors, important to the immune response. Among those growth factors, granulocyte-macrophage colony-stimulating factor (GM-CSF) and macrophage colony-stimulating factor (M-CSF). GM-CSF and M-CSF are important to monocyte and macrophage development and stimulation of the immune response process. Knowing the importance of GM-CSF and M-CSF, we sought to investigate the influence of PM on macrophage production of these growth factors. Two-month-old male BALB/c mice were subjected to PM with a low-protein diet (2%) and compared to a control diet (12%) mouse group. Nutritional status, hemogram and the number of peritoneal cells were evaluated. Additionally, peritoneal macrophages were cultured and the production of GM-CSF and M-CSF and mRNA expression were evaluated. To determine if PM altered macrophage production of GM-CSF and M-CSF, they were stimulated with TNF-α. The PM animals had anemia, leukopenia and a reduced number of peritoneal cells. The production of M-CSF was not different between groups; however, cells from PM animals, stimulated with or without TNF-α, presented reduced capability to produce GM-CSF. These data imply that PM interferes with the production of GM-CSF, and consequently would affect the production and maturation of hematopoietic cells and the immune response

High-sensitivity noncontact atomic force microscope/scanning tunneling microscope (ne AFM/STM) operating at subangstrom oscillation amplitudes for atomic resolution imaging and force spectroscopy

A highly sensitive noncontact atomic force microscope/scanning tunneling microscope (STM) operating in ultrahigh vacuum (UHV) with subangstrom oscillation amplitudes for atomic resolution imaging and force-distance spectroscopy was described. A novel fiber interferometer with very low noise levels was employed to detect cantilever displacements. The subangstrom oscillation amplitudes allow the force-distance measurements which reveal very short range force interactions

Synthesis of isomaltooligosaccharides and oligodextrans by the combined use of dextransucrase and dextranase

Batch syntheses of isomaltooligosaccharides (IMO) from sucrose, using the enzymes dextransucrase and dextranase were performed with the aim of understanding the reaction mechanism and the parameters which affect product characteristics and molecular size. Both activities described for dextransucrase (dextran formation and acceptor reaction) achieved synthesis whilst the hydrolytic activity of dextranase regulated the product molecular size and acceptor availability. Depending on the reaction conditions, the product oligosaccharide mixtures contained mainly sugars (up to 36%) with degrees of polymerization (DP) varying between 10 and 60 together with lower concentrations of both lower and higher molecular weight sugars. Alterations in substrate and dextranase concentrations (50-400 mg ml(-1) and 2.5-46 U ml(-1), respectively) affected the molecular weight of IMO, the reaction rate and the formation of leucrose. This permitted manipulation of the product characteristics. It was found that higher substrate and dextranase concentrations gave rise to products with lower molecular sizes and a dextransucrase:dextranase ratio of 1: 1 or 1:2 appeared to produce a polymer with a molecular weight which is desirable for prebiotic use. (C) 2004 Elsevier Inc. All rights reserved

Quantitative atom-resolved force gradient imaging using noncontact atomic force microscopy

Quantitative force gradient images are obtained using a sub-angström amplitude, off-resonance lever oscillation method during scanning tunneling microscopy imaging. We report the direct observation of short-range bonds, and the measured short-range force interaction agrees well in magnitude and length scale with theoretical predictions for single bonds. Atomic resolution is shown to be associated with the presence of a prominent short-range contribution to the total force interaction. It is shown that the background longer-range interaction, whose relative magnitude depends on the tip structure, has a significant effect on the contrast observed at the atomic scale. © 2001 American Institute of Physics

Nanomechanics using an ultra-small amplitude AFM

A new type of AFM is presented which allows for direct measurements of nanomechanical properties in ultra-high vacuum and liquid environments. The AFM is also capable to atomic-scale imaging of force gradients. This is achieved by vibrating a stiff lever at very small amplitudes of less than 1 Å (peak-to-peak) at a sub-resonance amplitude. This linearizes the measurement and makes the interpretation of the data straight-forward. At the atomic scale, interaction force gradients are measured which are consistent with the observation of single atomic bonds. Also, atomic scale damping is observed which rapidly rises with the tip-sample separation. A mechanism is proposed to explain this damping in terms of atomic relaxation in the tip. We also present recent results in water where we were able to measure the mechanical response due to the molecular ordering of water close to an atomically flat surface

Nanomechanics using an ultra-small amplitude AFM

A new type of AFM is presented which allows for direct measurements of nanomechanical properties in ultra-high vacuum and liquid environments. The AFM is also capable to atomic-scale imaging of force gradients. This is achieved by vibrating a stiff lever at very small amplitudes of less than 1 Å (peak-to-peak) at a sub-resonance amplitude. This linearizes the measurement and makes the interpretation of the data straight-forward. At the atomic scale, interaction force gradients are measured which are consistent with the observation of single atomic bonds. Also, atomic scale damping is observed which rapidly rises with the tip-sample separation. A mechanism is proposed to explain this damping in terms of atomic relaxation in the tip. We also present recent results in water where we were able to measure the mechanical response due to the molecular ordering of water close to an atomically flat surface