Characterization of nanoscale adhesion between a fluoroalkyl silane monolayer and a silicon AFM tip. Complex character of the interaction potential

Recent advances in sensing the inter-biomolecular interactions at the nanoscale - A comprehensive review of AFM-based force spectroscopy

Biomolecular interactions underpin most processes inside the cell. Hence, a precise and quantitative understanding of molecular association and dissociation events is crucial, not only from a fundamental perspective, but also for the rational design of biomolecular platforms for state-of-the-art biomedical and industrial applications. In this context, atomic force microscopy (AFM) appears as an invaluable experimental technique, allowing the measurement of the mechanical strength of biomolecular complexes to provide a quantitative characterization of their interaction properties from a single molecule perspective. In the present review, the most recent methodological advances in this field are presented with special focus on bioconjugation, immobilization and AFM tip functionalization, dynamic force spectroscopy measurements, molecular recognition imaging and theoretical modeling. We expect this work to significantly aid in grasping the principles of AFM-based force spectroscopy (AFM-FS) technique and provide the necessary tools to acquaint the type of data that can be achieved from this type of experiments. Furthermore, a critical assessment is done with other nanotechnology techniques to better visualize the future prospects of AFM-FS.

Comparison of the Physicochemical Properties of Carboxylic and Phosphonic Acid Self-Assembled Monolayers Created on a Ti-6Al-4V Substrate

This study compared the tribological properties in nano- and millinewton load ranges of Ti‑6Al-4V surfaces that were modified using self-assembled monolayers (SAMs) of carboxylic and phosphonic acids. The effectiveness of the creation of SAMs with the use of the liquid phase deposition (LPD) technique was monitored by the contact angle measurement, the surface free energy (SFE) calculation, X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR) measurements. The obtained results indicated that more stable and well-ordered layers, which were characterized by the lowest values of the coefficient of friction, adhesion, and wear rate, were obtained using phosphonic acid as a surface modifier. Based on the obtained results, it was found that the Ti-6Al-4V alloy modified by phosphonic acid would be the most advantageous for practical applications, especially in micro- and nanoelectromechanical systems (MEMS/NEMS).

Effect of the Chain Length and Temperature on the Adhesive Properties of Alkanethiol Self-Assembled Monolayers

Stable and hydrophobic self-assembled monolayers of alkanethiols are promising materials for use as lubricants in microdevices and nanodevices. We applied high-rate dynamic force spectroscopy measurements to study in detail the influence of the chain length and temperature on the adhesion between methyl-terminated thiol monolayers and a silicon nitride tip. We used the Johnson-Kendall-Roberts model to calculate the number of molecules in adhesive contact and then the Dudko-Hummer-Szabo model to extract the information about the position and the height of the activation barrier per single molecule. Both parameters were determined and analyzed in the temperature range from 25 to 65 °C for three thiols: 1-decanethiol (measured previously), 1-tetradecanethiol, and 1-hexadecanethiol. We associate the increase of the activation barrier parameters versus the chain length with lower stiffness of longer molecules and higher effectiveness of adhesive bond formation. However, we relate the thermal changes of the parameters rather to rearrangements of molecules than to the direct influence of temperature on the adhesive bonds.

Influence of Temperature on the Nanoadhesion of a Methyl-Terminated Thiol Monolayer: A New Insight with High-Rate Dynamic Force Spectroscopy

Alkanethiols form stable, homogeneous, and well-organized self-assembled monolayers and can be used as lubricants in micro- and nanodevices when sufficiently hydrophobic and resistant to sudden temperature changes. In this paper, we demonstrate a new analysis method which provides a deep physical insight into adhesive interactions and their temperature dependencies at the single molecule level. We have focused on the adhesion between a silicon nitride tip and a 1-decanethiol self-assembled monolayer in the temperature range from 25 to 85 °C. We performed dynamic force spectroscopy measurements and applied theoretical models of adhesive-mechanical interactions and thermally activated unbinding to obtain detailed information on the adhesive interactions. The parameters of the interaction potential describing a single adhesive bond were calculated, and their temperature dependence was discussed. Although the changes of the adhesion force versus temperature are significant and nonmonotonic, the energy of the activation barrier of a single adhesive bond appears temperature independent. We attribute observed changes in the position of the activation barrier to the interplay between the rupture and rebinding of adhesive bonds, as well as to thermal reorganization, in particular the change of the tilt angle of thiol molecules in the self-assembled monolayer.

Repetitive dissociation from crocidolite asbestos acts as persistent signal for epidermal growth factor receptor

Mesothelioma is an incurable form of cancer located most commonly in the pleural lining of the lungs and is associated almost exclusively with the inhalation of asbestos. The binding of asbestos to epidermal growth factor receptor (EGFR), a transmembrane signal protein, has been proposed as a trigger for downstream signaling of kinases and expression of genes involved in cell proliferation and inhibition of apoptosis. Here, we investigate the molecular binding of EGFR to crocidolite (blue asbestos; Na2(Fe(2+),Mg)3Fe2(3+)Si8O22(OH)2) in buffer solution. Atomic force microscopy measurements revealed an attractive force of interaction (i.e., bond) as EGFR was pulled from contact with long fibers of crocidolite. The rupture force of this bond increased with loading rate. According to the Bell model, the off-rate of bond dissociation (k(off)) for EGFR was 22 s(-1). Similar experiments with riebeckite crystals, the nonasbestiform variety of crocidolite, yielded a k(off) of 8 s(-1). These k(off) values on crocidolite and riebeckite are very rapid compared to published values for natural agonists of EGFR like transforming growth factor and epidermal growth factor. This suggests binding of EGFR to the surfaces of these minerals could elicit a response that is more potent than biological hormone or cytokine ligands. Signal transduction may cease for endogenous ligands due to endocytosis and subsequent degradation, and even riebeckite particles can be cleared from the lungs due to their short, equant habit. However, the fibrous habit of crocidolite leads to lifelong persistence in the lungs where aberrant, repetitious binding with EGFR may continually trigger the activation switch leading to chronic expression of genes involved in oncogenesis.

Nanoadhesion on rigid methyl-terminated biphenyl thiol monolayers: a high-rate dynamic force spectroscopy study

Nanoadhesion on a self-assembled monolayer of 4-methyl-4'-mercaptobiphenyl is measured using a modified atomic force microscope. The dependence of the adhesion force on the loading rate is analyzed with the Dudko-Hummer-Szabo model, and the kinetic and interaction potential parameters for a single terminal group are extracted. The energy and location of the activation barrier suggest that the adhesion is dominated by van der Waals dispersion forces. The humidity effect on the nanoadhesion is also studied. The results are compared with previously measured values for methyl-terminated alkane thiols and the influence of the thiol rigidity on the adhesion force is discussed.

Interpreting the widespread nonlinear force spectra of intermolecular bonds

Single molecule force spectroscopy probes the strength, lifetime, and energetic details of intermolecular interactions in a simple experiment. A growing number of these studies have reported distinctly nonlinear trends in rupture force with loading rate that are typically explained in conventional models by invoking complex escape pathways. Recent analyses suggested that these trends should be expected even for simple barriers based on the basic assumptions of bond rupture dynamics and thus may represent the norm rather than the exception. Here we explore how these nonlinear trends reflect the two fundamental regimes of bond rupture: (i) a near-equilibrium regime, produced either by bond reforming in the case of a single bond or by asynchronized rupture of multiple individual bonds, and (ii) a kinetic regime produced by fast, non-equilibrium bond rupture. We analyze both single- and multi-bonded cases, describe the full evolution of the system as it transitions between near- and far-from-equilibrium loading regimes, and show that both interpretations produce essentially identical force spectra. Data from 10 different molecular systems show that this model provides a comprehensive description of force spectra for a diverse suite of bonds over experimentally relevant loading rates, removes the inconsistencies of previous interpretations of transition state distances, and gives ready access to both kinetic and thermodynamic information about the interaction. These results imply that single-molecule binding free energies for a vast number of bonds have already been measured.