Competitive protein adsorption studied with atomic force microscopy and imaging ellipsometry

The adsorption and competitive adsorption of collagen and bovine serum albumin (BSA) were directly visualized and quantified using atomic force microscopy (AFM) and imaging ellipsometry. Chemically modified silicon surfaces were used as hydrophilic and hydrophobic substrates. The results showed that collagen and BSA in single component solution adsorbed onto a hydrophobic surface two times more than that onto a hydrophilic surface. The competitive adsorption between collagen and BSA showed that serum albumin preferentially adsorbed onto a hydrophobic surface, while collagen on a hydrophilic surface. In the binary solution of BSA (1 mg/ml BSA) and collagen (0.1 mg/ml), nearly 100% of the protein adsorbed onto the hydrophobic surface was BSA, but on the hydrophilic surface only about 6% was BSA. Surface affinity was the main factor controlling the competitive adsorption.

Concentrated-Mass Cantilever Enhances Multiple Harmonics In Tapping-Mode Atomic Force Microscopy

The natural frequencies of a cantilever probe can be tuned with an attached concentrated mass to coincide with the higher harmonics generated in a tapping-mode atomic force microscopy by the nonlinear tip-sample interaction force. We provide a comprehensive map to guide the choice of the mass and the position of the attached particle in order to significantly enhance the higher harmonic signals containing information on the material properties. The first three eigenmodes can be simultaneously excited with only one carefully positioned particle of specific mass to enhance multiple harmonics. Accessing the interaction force qualitatively based on the high-sensitive harmonic signals combines the real-time material characterization with the imaging capability. (C) 2008 American Institute of Physics.

Atomic force microscopy observations of the topography of regenerated silk fibroin aggregations

How fibroin molecules fold themselves and further self-assemble into aggregations with specific structures when the solution concentration increases is the key to understanding the natural silk-forming process of the silkworm. A regenerated Bombyx mori silk fibroin solution was prepared, and serially diluted solutions were coated on aminated coverslips. Atomic force microscopy (AFM) observations of the topography of fibroin molecules revealed a transformation from rodlike aggregations 100-200 nm long to small globules 50 mn in diameter with decreasing concentrations. When the incubation duration increased, the aggregations of fibroin molecules showed a self-assembling process, which was measured with AFM. In particular, after the molecules were incubated for more than 20 min, rodlike micelles formed and were distributed evenly on the surface of the aminated slides. Flow chamber technology was used to study the effect of the shear loading on the topography of the fibroin molecular aggregations. After a shear loading was applied, larger rodlike particles formed at a higher incubation concentration in comparison with those at a lower concentration and were obviously oriented along the direction of fluid flow.

Nonlinear dynamics of atomic force microscopy with intermittent contact

When the atomic force microscopy (AFM) in tapping mode is in intermittent contact with a soft substrate, the contact time can be a significant portion of a cycle, resulting in invalidity of the impact oscillator model, where the contact time is assumed to be infinitely small. Furthermore, we demonstrate that the AFM intermittent contact with soft substrate can induce the motion of higher modes in the AFM dynamic response. Traditional ways of modeling AFM (one degree of freedom (DOF) system or single mode analysis) are shown to have serious mistakes when applied to this kind of problem. A more reasonable displacement criterion on contact is proposed, where the contact time is a function of the mechanical properties of AFM and substrate, driving frequencies/amplitude, initial conditions, etc. Multi-modal analysis is presented and mode coupling is also shown. (c) 2006 Published by Elsevier Ltd.

A thin liquid film and its effects in an atomic force microscopy measurement

Recently, it has been observed that a liquid film spreading on a sample surface will significantly distort atomic force microscopy (AFM) measurements. In order to elaborate on the effect, we establish an equation governing the deformation of liquid film under its interaction with the AFM tip and substrate. A key issue is the critical liquid bump height y(0c) at which the liquid film jumps to contact the AFM tip. It is found that there are three distinct regimes in the variation of y(0c) with film thickness H, depending on Hamaker constants of tip, sample and liquid. Noticeably, there is a characteristic thickness H* physically defining what a thin film is; namely, once the film thickness H is the same order as H* , the effect of film thickness should be taken into account. The value of H* is dependent on Hamaker constants and liquid surface tension as well as tip radius.

Interaction between single molecules of Mac-1 and ICAM-1 in living cells: An atomic force microscopy study

The interaction between integrin macrophage differentiation antigen associated with complement three receptor function (Mac-1) and intercellular adhesion molecule-1 (ICAM-1), which is controlled tightly by the ligand-binding activity of Mac-1, is central to the regulation of neutrophil adhesion in host defense. Several "inside-out" signals and extracellular metal ions or antibodies have been found to activate Mac-1, resulting in an increased adhesiveness of Mac-1 to its ligands. However, the molecular basis for Mac-1 activation is not well understood yet. In this work, we have carried out a single-molecule study of Mac-1/ICAM-1 interaction force in living cells by atomic force microscopy (AFM). Our results showed that the binding probability and adhesion force of Mac-1 with ICAM-1 increased upon Mac-1 activation. Moreover, by comparing the dynamic force spectra of different Mac-1 mutants, we expected that Mac-1 activation is governed by the downward movement of its alpha 7 helix. (c) 2007 Elsevier Inc. All rights reserved.

Femtosecond laser pulse irradiation of Sb-rich AgInSbTe films: Scanning electron microscopy and atomic force microscopy investigations

The single-layer and multilayer Sb-rich AgInSbTe films were irradiated by a single femtosecond laser pulse with the duration of 120 fs. The morphological feature resulting from the laser irradiation have been investigated by scanning electron microscopy and atom force microscopy. For the single-layer film, the center of the irradiated spot is a dark depression and the border is a bright protrusion; however, for the multilayer film, the center morphology changes from a depression to a protrusion as the energy increases. The crystallization threshold fluence of the single-layer and the multilayer films is 46.36 mJ/cm(2), 63.74 mJ/cm(2), respectively.