Suzanne P. Jarvis

Accurate formulas for interaction force and energy in frequency modulation force spectroscopy

Frequency modulation atomic force microscopy utilizes the change in resonant frequency of a cantilever to detect variations in the interaction force between cantilever tip and sample. While a simple relation exists enabling the frequency shift to be determined for a given force law, the required complementary inverse relation does not exist for arbitrary oscillation amplitudes of the cantilever. In this letter we address this problem and present simple yet accurate formulas that enable the interaction force and energy to be determined directly from the measured frequency shift. These formulas are valid for any oscillation amplitude and interaction force, and are therefore of widespread applicability in frequency modulation dynamic force spectroscopy.

Noninvasive determination of optical lever sensitivity in atomic force microscopy

Atomic force microscopes typically require knowledge of the cantilever spring constant and optical lever sensitivity in order to accurately determine the force from the cantilever deflection. In this study, we investigate a technique to calibrate the optical lever sensitivity of rectangular cantilevers that does not require contact to be made with a surface. This noncontact approach utilizes the method of Sader et al. [Rev. Sci. Instrum. 70, 3967 (1999)] to calibrate the spring constant of the cantilever in combination with the equipartition theorem [J. L. Hutter and J. Bechhoefer, Rev. Sci. Instrum. 64, 1868 (1993)] to determine the optical lever sensitivity. A comparison is presented between sensitivity values obtained from conventional static mode force curves and those derived using this noncontact approach for a range of different cantilevers in air and liquid. These measurements indicate that the method offers a quick, alternative approach for the calibration of the optical lever sensitivity.

Development of liquid-environment frequency modulation atomic force microscope with low noise deflection sensor for cantilevers of various dimensions

We have developed a liquid-environment frequency modulation atomic force microscope (FM-AFM) with a low noise deflection sensor for a wide range of cantilevers with different dimensions. A simple yet accurate equation describing the theoretical limit of the optical beam deflection method in air and liquid is presented. Based on the equation, we have designed a low noise deflection sensor. Replaceable microscope objective lenses are utilized for providing a high magnification optical view (resolution: <3μm) as well as for focusing a laser beam (laser spot size: ∼10μm). Even for a broad range of cantilevers with lengths from 35to125μm, the sensor provides deflection noise densities of less than 11fm∕Hz in air and 16fm∕Hz in water. In particular, a cantilever with a length of 50μm gives the minimum deflection noise density of 5.7fm∕Hz in air and 7.3fm∕Hz in water. True atomic resolution of the developed FM-AFM is demonstrated by imaging mica in water.

Nanoscale Mechanical Characterisation of Amyloid Fibrils Discovered in a Natural Adhesive

Using the atomic force microscope, we have investigated the nanoscale mechanical response of the attachment adhesive of the terrestrial alga Prasiola linearis (Prasiolales, Chlorophyta). We were able to locate and extend highly ordered mechanical structures directly from the natural adhesive matrix of the living plant. The in vivo mechanical response of the structured biopolymer often displayed the repetitive sawtooth force-extension characteristics of a material exhibiting high mechanical strength at the molecular level. Mechanical and histological evidence leads us to propose a mechanism for mechanical strength in our sample based on amyloid fibrils. These proteinaceous, pleated β-sheet complexes are usually associated with neurodegenerative diseases. However, we now conclude that the amyloid protein quaternary structures detected in our material should be considered as a possible generic mechanism for mechanical strength in natural adhesives.

Stretching the α-helix: a direct measure of the hydrogen-bond energy of a single-peptide molecule

Abstract Atomic force microscopy was used to measure the force required to stretch individual molecules of the peptide cysteine 3 –lysine 30 –cysteine from the α-helical state into a linear chain (approximately 200 pN). The measured force versus peptide elongation was used to calculate the work done in breaking the hydrogen bonds which give rise to the helical structure. The average experimental value of the hydrogen-bond energy (20.2 kJ/mol) is in good agreement with reported theoretical calculations. In addition, the stiffness of individual peptides was measured directly using a force modulation technique and found to vary from approximately 0.005–0.012 N/m during elongation.

Directly probing the effects of ions on hydration forces at interfaces.

Understanding the influence of water layers adjacent to interfaces is fundamental in order to fully comprehend the interactions of both biological and nonbiological materials in aqueous environments. In this study, we have investigated hydration forces at the mica-electrolyte interface as a function of ion valency and concentration using subnanometer oscillation amplitude frequency modulation atomic force microscopy (FM-AFM). Our results reveal new insights into the nature of hydration forces at interfaces due to our ability to measure high force gradients without instability and in the absence of lateral confinement due to the use of an atomically sharp tip. We demonstrate the influence of electrolytes on the properties of both primary and structural hydration forces and reveal new insights into the interplay between these phenomena in determining the interaction forces experienced by a nanoscale object approaching an interface. We also highlight the difficulty in directly comparing hydration force data from different measurement techniques where the nature of the perturbation induced by differing interaction geometries is likely to dramatically affect the results.

Visualization of Ion Distribution at the Mica−Electrolyte Interface

Local ionic environments within nanometer proximity of a surface play a major role in the interactions which occur there and can be of critical importance in, for example, colloid suspensions, as well as biological function. Such environments often vary significantly from bulk properties, as we show here by the direct imaging of a range of monovalent (Li(+), Na(+)) and divalent (Ca(2+), Mg(2+)) cations distributed at the liquid-solid interface of mica. We image local charge distributions relative to the atomic lattice of mica and adjacent structured water and explain how their location is influenced by the electrostatic characteristics of the underlying lattice.

Quantitative measurement of solvation shells using frequency modulated atomic force microscopy

The nanoscale specificity of interaction measurements and additional imaging capability of the atomic force microscope make it an ideal technique for measuring solvation shells in a variety of liquids next to a range of materials. Unfortunately, the widespread use of atomic force microscopy for the measurement of solvation shells has been limited by uncertainties over the dimensions, composition and durability of the tip during the measurements, and problems associated with quantitative force calibration of the most sensitive dynamic measurement techniques. We address both these issues by the combined use of carbon nanotube high aspect ratio probes and quantifying the highly sensitive frequency modulation (FM) detection technique using a recently developed analytical method. Due to the excellent reproducibility of the measurement technique, additional information regarding solvation shell size as a function of proximity to the surface has been obtained for two very different liquids. Further, it has been possible to identify differences between chemical and geometrical effects in the chosen systems.

Phase modulation atomic force microscope with true atomic resolution

We have developed a dynamic force microscope (DFM) working in a novel operation mode which is referred to as phase modulation atomic force microscopy (PM-AFM). PM-AFM utilizes a fixed-frequency excitation signal to drive a cantilever, which ensures stable imaging even with occasional tip crash and adhesion to the surface. The tip-sample interaction force is detected as a change of the phase difference between the cantilever deflection and excitation signals and hence the time response is not influenced by the Q factor of the cantilever. These features make PM-AFM more suitable for high-speed imaging than existing DFM techniques such as amplitude modulation and frequency modulation atomic force microscopies. Here we present the basic principle of PM-AFM and the theoretical limit of its performance. The design of the developed PM-AFM is described and its theoretically limited noise performance is demonstrated. Finally, we demonstrate the true atomic resolution imaging capability of the developed PM-AFM by i...

Beneficial characteristics of mechanically functional amyloid fibrils evolutionarily preserved in natural adhesives

While biological systems are notorious for their complexity, nature sometimes displays mechanisms that are elegant in their simplicity. We have recently identified such a mechanism at work to enhance the mechanical properties of certain natural adhesives. The mechanism is simple because it utilizes a non-specific protein folding and subsequent aggregation process, now thought to be generic for any polypeptide under appropriate conditions. This non-specific folding forms proteinaceous crossed β-sheet amyloid fibrils, which are usually associated with neurodegenerative diseases. Here we show evidence for the beneficial mechanical characteristics of these fibrils discovered in natural adhesives. We suggest that amyloid protein quaternary structures should be considered as a possible generic mechanism for mechanical strength in a range of natural adhesives and other natural materials due to their many beneficial mechanical features and apparent ease of self-assembly.