Amplitude Modulation Atomic Force Microscopy Research Articles

Fast imaging with alternative signal for dynamic atomic force microscopy

In this paper, a method for imaging in amplitude-modulation atomic force microscopy is developed which enables accurate sample-profile imaging even at high scanning speeds where existing methods that use the actuator input signal fail. The central concept is to use a model of the vertical positioning actuator to compensate for the artifacts introduced due to its compliance in high scanning frequencies. We provide experiments that compare sample-profile estimates from our method with the existing methods and demonstrate significant improvement (by 70%) in the estimation bandwidth. The proposed design allows for specifying a trade-off between the sample-profile estimation error and estimation bandwidth.

Cantilever dynamics in amplitude modulation AFM: continuous and discontinuous transitions

Transitions between the attractive and the repulsive force regimes for amplitude modulation atomic force microscopy (AFM) can be either discontinuous, with a corresponding jump in amplitude and phase, or continuous and smooth. During the transitions, peak repulsive and average forces can be up to an order of magnitude higher when these are discrete. Under certain circumstances, for example, when the tip radius is relatively large (e.g. R > 20–30 nm) and for high cantilever free amplitudes (e.g. A0 > 40–50 nm), the L state can be reached with relatively low set-points only (e.g. Asp/A0 < 0.30). We find that these cases do not generally lead to higher resolution but increase the background noise instead. This is despite the fact that the imaging can be non-contact under these conditions. The appearance of background noise is linked to increasing cantilever mean deflection and tip–surface proximity with increasing free amplitude in the L state.

Linear and nonlinear approaches towards amplitude modulation atomic force microscopy

Frequency response behavior of microcantilever is analytically and experimentally investigated in amplitude modulation Atomic Force Microscopy (AFM). AFM microcantilever probe is modeled as a continuous beam, and tip-sample interaction force is considered to include both attractive and repulsive force regimes. The developed model is compared with the linear lumped-parameters model that has been extensively used in the literature so far. Experimental measurements are also provided for the frequency response of a typical microcantilever-sample system to demonstrate the advantages of the developed model over the linear formulation. The results indicate that the nonlinear continuous model is more accurate, particularly in the estimation of the saturated amplitude value and frequency zone in which the tip-sample contact happens.

Bi-stability of amplitude modulation AFM in air: deterministic and stochastic outcomes forimaging biomolecular systems

The dynamics of the oscillating microcantilever for amplitude modulation atomicforce microscopy (AM AFM) operating in air is well understood theoretically butthe experimental outcomes are still emerging. We use double-stranded DNA onmica as a model biomolecular system for investigating the connection betweentheory and experiment. A demonstration that the switching between the twocantilever oscillation states is stochastic in nature is achieved, and it can be inducedby means of topographical anomalies on the surface. Whether one or the otherattractor basin is accessed depends on the tip–sample separation history used toachieve the imaging conditions, and we show that the behaviour is reproduciblewhen the tip is stable and well characterized. Emergence of background noiseoccurs in certain regions of parameter space regardless of whether two cantileveroscillation states coexist. The low state has been explored in detail and we notethat at low to intermediate values of the free amplitude, noise-free imaging isachieved. The outcomes shown here are general and demonstrate that a thorough andsystematic experimental approach in conjunction with standard modelling givesinsight into the mechanisms behind image contrast formation in AM AFM in air.

Molecular resolution investigation of tetragonal lysozyme (110) face in liquid by frequency-modulation atomic force microscopy

In order to determine the molecular structure by x-ray diffraction analysis, it is very important to grow high quality protein crystals. The molecular resolution imaging of soluble protein crystals such as the tetragonal lysozyme (110) face in saturated solution is demonstrated using frequency-modulation atomic force microscopy (FM-AFM). The surface structure of the (110) face and the crystallographic position of individual molecules were determined from molecular resolution images. For observation of protein crystals, FM-AFM is a favorable technique as an alternative to contact mode or amplitude-modulation AFM.

Influence of thermal noise on measured bond lengths in force measurements using dynamic atomic force microscopy

The use of dynamic methods in atomic force microscopy (AFM) has lead to spectacular advances in force measurements and imaging. There has been a gradual shift to stiffer cantilevers and smaller amplitudes, resulting in higher resolution imaging and three-dimensional force mapping. However, when amplitudes become much smaller than 1 Å, they can approach the same order as the thermal noise of the cantilever. In this article, we explore the effect of thermal noise on force measurements using small-amplitude, dynamic AFM. He restricts himself to off-resonance, amplitude-modulation AFM, as this is easiest to model and analyze. He finds that position and force noise increase roughly with the square root of temperature, as expected from the equipartition theorem; however, a closer look reveals a more complicated behavior due to nonlinearities in the system and the competition of position and force noise in these systems.

Determination and simulation of nanoscale energy dissipation processes in amplitude modulation AFM

We develop a theoretical framework that explains the use of amplitude modulation AFM to measure and identify energy dissipation processes at the nanoscale. The variation of the dissipated energy on a surface by a vibrating tip as a function of its amplitude has a shape that singles out the dissipative process. The method is illustrated by calculating the dynamic-dissipation curves for surface adhesion energy hysteresis, long-range interfacial interactions and viscoelastic processes. We also show that by diving the dissipated energy by its maximum value, the dynamic-dissipation curves become independent of the experimental parameters. In particular, for long-range dissipative processes we have derived an analytical relationship that shows the independence of the normalized dynamic-dissipation curves with respect the free amplitude, cantilever constant or quality factor.

Atomic force microscopy of DNA at high humidity: irreversible conformational switching of supercoiled molecules

Three topologically different double-stranded DNA molecules of the same size (bps) have been imaged in air on mica using amplitude modulation atomic force microscopy (AM AFM) under controlled humidity conditions. At very high relative humidity (>90% RH), localized conformational changes of the DNA were observed, while at lower RH, the molecules remained immobile. The conformational changes occurred irreversibly and were driven principally by superhelical stress stored in the DNA molecules prior to binding to the mica surface. The binding mechanism of the DNA to the mica (surface equilibration versus kinetic trapping) modulated the extent of the conformational changes. In cases where DNA movement was observed, increased kinking of the DNA was seen at high humidity when more surface water was present. Additionally, DNA condensation behavior was also present in localized regions of the molecules. This study illustrates that changes in the tertiary structure of DNA can be induced during AFM imaging at high humidity on mica. We propose that AM AFM in high humidity will be a useful technique for probing DNA topology without some of the drawbacks of imaging under bulk solution.

Controlled ionic condensation at the surface of a native extremophilemembrane

At the nanoscale level biological membranes present a complex interface with the solvent. The functional dynamics and relative flexibility of membrane components together with the presence of specific ionic effects can combine to create exciting new phenomena that challenge traditional theories such as the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory or models interpreting the role of ions in terms of their ability to structure water (structure making/breaking). Here we investigate ionic effects at the surface of a highly charged extremophile membrane composed of a proton pump (bacteriorhodopsin) and archaeal lipids naturally assembled into a 2D crystal. Using amplitude-modulation atomic force microscopy (AM-AFM) in solution, we obtained sub-molecular resolution images of ion-induced surface restructuring of the membrane. We demonstrate the presence of a stiff cationic layer condensed at its extracellular surface. This layer cannot be explained by traditional continuum theories. Dynamic force spectroscopy experiments suggest that it is produced by electrostatic correlation mediated by a Manning-type condensation of ions. In contrast, the cytoplasmic surface is dominated by short-range repulsive hydration forces. These findings are relevant to archaeal bioenergetics and halophilic adaptation. Importantly, they present experimental evidence of a natural system that locally controls its interactions with the surrounding medium and challenges our current understanding of biological interfaces.

Atomically flat SrO-terminated SrTiO3(001) substrate

We show that atomically flat single SrO-terminated SrTiO3(001) substrates can be obtained through simple high-temperature treatment. Amplitude-modulation atomic force microscopy with phase-lag analysis and x-ray photoelectron spectroscopy, have been used to demonstrate that the ratio between the two chemical terminations can be tailored by choosing the annealing time. Moreover, the progressive SrO surface enrichment (up to 100%) is accompanied by a self-assembly process which results in the spatial separation at the nanoscale of both chemical terminations. We further demonstrate that this opens a interesting avenue for selective chemical reaction and growth of oxide nanostructures.

Controlling chaos in dynamic-mode atomic force microscope

We successfully demonstrated the first experimental stabilization of irregular and non-periodic cantilever oscillation in the amplitude modulation atomic force microscopy using the time-delayed feedback control. A perturbation to cantilever excitation force stabilized an unstable periodic orbit associated with nonlinear cantilever dynamics. Instead of the typical piezoelectric excitation, the magnetic excitation was used for directly applying control force to the cantilever. The control force also suppressed the cantilever's occasional bouncing motions that caused artifacts on a surface image.

Quantitative force versus distance measurements in amplitude modulation AFM: a novelforce inversion technique

A new method for extracting quantitative data from amplitude modulation dynamicforce–distance measurements is developed. The method is based on the harmonic oscillatormodel of vibrating atomic force microscope cantilevers, and is capable of extracting boththe conservative and dissipative parts of the tip–sample interaction from a measurement ofoscillation amplitude and phase as a function of distance. Numerical simulations are usedto demonstrate the validity of the method. Further proof of the accuracy of this method isprovided by a measurement of electrostatic forces between an AFM tip and a graphitesample.

DNA Conformation and Biomolecular Motors: New Nanomedicine Research Targets

DNA supercoiling is a feature of almost all DNA molecules. It is a powerful thermodynamic force that drives and directs many DNA associated processes in vivo. The level of supercoiling or DNA spatial conformation is constantly changing due to the activities of proteins and the environmental conditions of the cell. Local and temporal changes in DNA supercoiling affect many cellular processes such as replication, transcription recombination and chromosome organization.DNA biomolecular motors such as DNA topoisomerases and DNA translocases are responsible for maintaining the steady state of supercoiling essential for cell viability. In prokaryotes, DNA supercoiling is expected to play an important role in site-specific recombination, a fundamental process to achieve resolution of dimeric chromosomes, allowing plasmids and chromosome segregation and consequently cell division. During this process, DNA undergoes multiple conformational changes due to the activity of Tyrosine recombinases and a DNA translocase known as FtsK.I use cell biology, biochemical and biophysical techniques to study the role of DNA biomolecular motors and DNA topology in different cellular processes. In vitro, we demonstrate the topology dependence of the different steps in site-specific recombination events using DNA substrates with different superhelical density. By TIRFM, I characterize at the level of single molecule the activity of DNA molecular motors. Using high-resolution amplitude modulation atomic force microscopy (AM-AFM) in physiological buffer we characterize the nature of the forces that drive relevant DNA conformational changes by itself or after protein interaction during site-specific recombination events. Additionally, we observe for the first time the dynamics of DNA and the conformational changes of DNA during site-specific recombination events imaged by high-speed AFM at time resolutions up to 20 ms and sub-nm spatial resolution. Our current research is focus on DNA biomolecular motors as new nanomedice targets.

Direct Mapping Of Surface-bound Liquid With Sub-nanometer Resolution

At the solid-liquid interface molecules of the liquid adopt a particular arrangement which depends primarily on the interfacial energy and geometry. At the nanoscale this molecular arrangement is of central importance in a wide range of fields from molecular biology to surface physics, heterogeneous catalysis and electronics. In biology the role of interfacial liquid is further emphasized by the soft nature of most biomolecules whose conformation and dynamics depends on the surrounding medium. This is the case for protein function and folding (1), self-assembly processes and bio-electronics where a complex interplay between surface-bound liquid molecules and ions strongly affects any motion. Using amplitude modulation atomic force microscopy (AM-AFM) operated in liquid and in a particular regime, it is possible to simultaneously image the topography of the surface-bound liquid while measuring its adhesion energy to the solid investigated. We have used this method to map the binding energy of water to gold nanoparticles coated with mixed ligands, self-assembled in controlled patterns (2, 3). Such functionalized nanoparticles can mimic the typical surface of proteins (hydrophobicity, charge, surface domains) while allowing careful control of the domains’ size and properties (2). Our results show that the average binding energy of water to the surface of the nanoparticles strongly depends on the spatial arrangement of the ligands molecules. The geometry as well as the size of the ligand domains both affect the local adhesion energy of the solvent in a non-linear fashion. Our findings provide experimental and quantitative insight into the inter-play between solvent and surfaces in nanoscale biophysical processes. (1) H. Frauenfelder et al, (2006), Proc. Natl. Acad. Sci. 103, 15469-15472. (2) A. Verma et al, (2008), Nature Mat. 7, 588-595. (3) A. Centrone et al, (2008), Proc. Natl. Acad. Sci. 105, 9886-9891.

Second harmonic atomic force microscopy of living Staphylococcus aureus bacteria

Monitoring higher harmonics of the drive frequency in amplitude-modulation atomic force microscopy can give extra information on local surface properties. The first to fourth harmonics inclusive were monitored on the surface of individual Staphylococcus aureus bacteria and on the polycarbonate filter in which they were trapped. The second harmonic response was sufficient to create the first higher harmonic images of the surfaces of living bacterial cells under aqueous buffer. Mapping the second harmonic signal onto the height (Z piezo-signal) shows that they are largely uncorrelated, suggesting that it measures local surface properties related to mechanics and/or chemical interactions.

Study of a nanoscale water cluster by atomic force microscopy

We present a novel method for investigating a nanometric cluster of water molecules, which includes the formation and manipulation of nanometric water, and the measurement of its mechanical properties. The atomic force microscope based on the quartz tuning-fork sensor is employed to form and manipulate the nanometric water, and the theoretical tool of amplitude-modulation atomic force microscopy is used to obtain the elasticity, viscosity and dissipation energy of it. With high vertical resolution less than approximately 0.1 nm and high force sensitivity of approximately 0.01 N m(-1), this tool facilitates the stable formation and manipulation of a nano-water cluster (approximately 10(4) molecules) in air without 'jump-to-contact' instability, as well as quantitative measurements of its physico-chemical properties. PACS numbers: 47.55.nk, 62.10.+s, 68.37.Ps

A flexible, highly stable electrochemical scanning probe microscope for nanoscale studies at the solid-liquid interface

This work describes the design, fabrication, and application of an ultrastable scanning probe microscope for nanoscale studies at the solid-liquid interface, specifically in electrolyte environments. Quantification of system noise limits in the tunneling mode, mechanical drift rates, and lowest mechanical resonance provided values of < or = 10 pA/Hz(1/2), 2 nm/min (XY) and 0.15 nm/min (Z), and 7.9 kHz, respectively. Measurement of the closed-loop transfer function in the tunneling condition demonstrated linear feedback responses up to 4.2 and 2.5 kHz in ambient and electrochemical conditions. Atomic and molecular resolution imagings have been achieved in ambient, in situ, and electrochemical imaging environments at scan rates up to 80 lines/s. A modular design approach has produced a highly flexible microscope capable of imaging and spectroscopy in tunneling, tapping force [amplitude modulation atomic force microscopy (AFM)], and noncontact force (frequency modulation-AFM) modes.

Effect of Solution Concentration, Surface Bias and Protonation on the Dynamic Response of Amplitude-Modulated Atomic Force Microscopy in Water

The dynamic response of amplitude-modulated atomic force microscopy (AM-AFM) is studied at the solid/water interface with respect to changes in ionic concentration, applied surface potential, and surface protonation. Each affects the electric double layer in the solution, charge on the tip and the sample surface, and thus the forces affecting the dynamic response. A theoretical model is developed to relate the effective stiffness and hydrodynamic damping of the AFM cantilever that is due to the tip/surface interaction with the phase and amplitude signals measured in the AM-AFM experiments. The phase and amplitude of an oscillating cantilever are measured as a function of tip-sample distance in three experiments: mica surface in potassium nitrate solutions with different concentrations, biased gold surface in potassium nitrate solution, and carboxylic acid-terminated self-assembled monolayers (SAMs) on gold in potassium nitrate pH buffers. Results show that, over the range where the higher harmonic modes of the oscillation are negligible, the effective stiffness of the AFM cantilever increases to a maximum as the tip approaches the surface before declining again as a result of the repulsive electrical double layer interaction. For attractive electrical double-layer interactions, the effective stiffness declines monotonically as the tip approaches the surface. Similarly, the hydrodynamic damping of the tip increases and then decreases as the tip approaches the solid/water interface, with the magnitude depending on the species present in the solution.

Bimodal atomic force microscopy imaging of isolated antibodies in air and liquids

We have developed a dynamic atomic force microscopy (AFM) method based on thesimultaneous excitation of the first two flexural modes of the cantilever. Theinstrument, called a bimodal atomic force microscope, allows us to resolve thestructural components of antibodies in both monomer and pentameric forms.The instrument operates in both high and low quality factor environments, i.e.,air and liquids. We show that under the same experimental conditions, bimodalAFM is more sensitive to compositional changes than amplitude modulationAFM. By using theoretical and numerical methods, we study the material contrastsensitivity as well as the forces applied on the sample during bimodal AFM operation.

Adhesion detachment and movement of gold nanoclusters induced by dynamic atomic forcemicroscopy

Via dissipation processes in dynamic atomic force microscopy (AFM), we have investigatedadhesion and frictional properties of deposited nanoclusters, under controlled ambientconditions. Specifically, we have considered gold nanoclusters with nominal diameters of 13and 24 nm physisorbed on silicon substrate. The manipulation experiment has shownunambiguously that the amplitude modulation AFM method and the calibrationprocedure adopted allow us to discern and measure the size dependence of the energydetachment threshold for deposited objects down to the nanometer scale. Moreover,allowing us to switch easily and with high repeatability between imaging andmanipulation during the AFM scans, this operational method proves to be apromising tool for inducing controlled spatial displacements at these length scales.