Contact Mode Imaging Research Articles

Two-dimensional stick–slip on a soft elastic polymer: pattern generation using atomic forcemicroscopy

It has been demonstrated that it is possible to create laterally differentiated frictionalpatterning and three-dimensional structures using an atomic force microscope(AFM) probe on the surface of a soft elastic polymer, poly(dimethylsiloxane)(PDMS). The resulting effect of contact mode imaging at low loading forces(<100 nN), observed in the lateral force mode, revealed a homogeneous pattern on the PDMSsurface exhibiting higher friction. With higher loading forces ( nN) the effect is non-uniform, resulting in structures with depths on the nanometre scale.The topographic and frictional data revealed stick–slip responses in both the fast(orthogonal to the long axis of the lever) and slow (parallel to the long axis of the lever)directions of probe travel from scanning in a raster pattern. The stick–slip events aremanifested in the form of a series of shallow channels spaced evenly apart on the polymersurface. Detailed friction loop analysis acquired during the manipulation process showedthat the lateral force changed according to the strength of trapping of the tip with thepolymer surface exhibiting significant in-plane deformation due to lateral forces beingimposed. An incremental increase in the initial loading force resulted in an increase inin-plane displacement and a greater spacing between the stick lines/channels inthe slow-scan direction. A decrease in channel length in the fast-scan directionis also observed as a result of an increase in static friction with normal force,resulting in greater surface deformation and shorter track length for sliding friction.

An experimental study of the contact mode AFM scanning capability of polyimide cantilever probes

This paper presents a preliminary exploration of high-speed contact mode performed with a polyimide probe. The probe is batch micromachined by a lithographic manufacturing process. It offers a spring constant of <0.1 N/m, a resonance frequency of about 50 kHz, and a tip diameter of 50–100 nm. The probe is particularly suitable for scanning soft specimens such as biological and polymeric samples. Topographical contact mode imaging at high scanning rates of 48 Hz (1.47 mm/s) has been demonstrated, detecting features <100 nm over a 15 μm scan, yielding >7 bit resolution at 48 Hz. Scanning rates of 16 Hz (0.5 mm/s) have been demonstrated for lateral force imaging with spatial resolution of 100 nm over a 15 μm scan, which translates into >7 bit resolution at 16 Hz. These results suggest that the probe can be used in high throughput applications.

Nanoindentation and contact-mode imaging at high temperatures

Technical issues surrounding the use of nanoindentation at elevated temperatures are discussed, including heat management, thermal equilibration, instrumental drift, and temperature-induced changes to the shape and properties of the indenter tip. After characterizing and managing these complexities, quantitative mechanical property measurements are performed on a specimen of standard fused silica at temperatures up to 405 °C. The extracted values of hardness and Young's modulus are validated against independent experimental data from conventional mechanical tests, and accuracy comparable to that obtained in standard room-temperature nanoindentation is demonstrated. In situ contact-mode images of the surface at temperature are also presented.

Comparative study of the conditions required to image live human epithelial and fibroblast cells using atomic force microscopy

Successful imaging of living human cells using atomic force microscopy (AFM) is influenced by many variables including cell culture conditions, cell morphology, surface topography, scan parameters, and cantilever choice. In this study, these variables were investigated while imaging two morphologically distinct human cell lines, namely LL24 (fibroblasts) and NCI H727 (epithelial) cells. The cell types used in this study were found to require different parameter settings to produce images showing the greatest detail. In contact mode, optimal loading forces ranged between 2-2.8 x 10(-9) and 0.1-0.7 x 10(-9) (N) for LL24 and NCI H727 cells respectively. In tapping (AC) mode, images of LL24 cells were obtained using cantilevers with a spring constant of at least 0.32 N/m, while NCI H727 cells required a greater spring constant of at least 0.58 N/m. To obtain tapping mode images, cantilevers needed to be tuned to resonate at higher frequencies than their resonance frequencies to obtain images. For NCI H727 cells, contact mode imaging produced the clearest images. For LL24 cells, contact and tapping mode AFM produced images of comparable quality. Overall, this study shows that cells with different morphologies and surface topography require different scanning approaches and optimal conditions must be determined empirically to achieve images of high quality.

Atomic Force Microscopy Study of Stainless-Steel and Nickel-Titanium Files

Abstract Atomic force microscopy (AFM) is a well established and documented tool for materials investigation. The aim of this study was to evaluate the topography of conventional stainless-steel files and both hand and rotary nickel-titanium (NiTi) files by using AFM. One endodontic file of each of the following was selected: stainless-steel K-file Dentsply, stainless-steel K-file Moyco, hand NiTi K-file Nitiflex, hand NiTi Greater Taper, rotary NiTi Greater Taper, and rotary NiTi Quantec. The analyses were performed on twenty different points located along a 3-mm section starting at the tip of each file. Root mean square (RMS) parameters for contact mode imaging microscopy variations were measured. The differences between RMS values were tested by ANOVA with Fisher's protected LSD test for multiple comparisons (p < 0.05). RMS of depth profile data was used to determine any statistically significant difference in vertical amplitude. According to results, all instruments showed topographic irregularities distributed on surface. Endodontic files manufactured by the same method and alloy demonstrated significant differences, whereas no significant differences were found for instruments produced by different alloys and methods (p < 0.05). The hand NiTi Greater Taper, rotary NiTi Greater Taper, and rotary NiTi Quantec showed greater values of vertical amplitude topography compared to K-Dentsply and Nitiflex files (p < 0.05). The AFM technique proved to be a valuable research tool in the investigation of endodontic files topography.

Structural Effects of Sodium Hypochlorite Solutions on Gutta-Percha Cones: Atomic Force Microscopy Study

Atomic force microscope (AFM) is a well-established methodology for structural characterization of materials. The purpose of this study was to investigate the effects of sodium hypochlorite (NaOCl) solutions on gutta-percha cone structure using AFM. Three standardized gutta-percha cones were cut 3 mm from the tip, attached to a glass base and immersed in 0.5, 2.5, or 5.25% NaOCl solutions. After 1 and 5 min the samples were positioned in the atomic force microscope. The analyses were performed on twelve different points ( n = 12) located between 1 and 2 mm from the tip after each period of immersion in NaOCl. Gutta-percha cone without any NaOCl treatment were used as control. Root mean square (RMS) parameters for contact mode imaging and force modulation microscopy variations were measured. The differences between RMS values were tested by ANOVA with Fisher’s protected LSD test for multiple comparisons (p < 0.05). Aggressive deteriorative effects on gutta-percha cone elasticity were observed for 5.25% NaOCl at 1 min when compared to the control (p < 0.05). In addition, 2.5% and 5.25% NaOCl solutions caused topographic changes after 5 min when compared to the control (p < 0.05). Conversely, 0.5% NaOCl solution did not cause any alteration on topography or elasticity of gutta-percha cone structure when compared to the control (p < 0.05). Thus, 0.5% NaOCl solution is a safe alternative for rapid decontamination of gutta-percha cones.

Effects of 2% chlorhexidine and 5.25% sodium hypochlorite on gutta‐percha cones studied by atomic force microscopy

To compare the effects of 2% chlorhexidine (CHX) and 5.25% sodium hypochlorite (NaOCl) on gutta-percha (GP) cone structure using atomic force microscopy (AFM). Two standardized GP cones were sectioned 3 mm from the tip, attached to a glass base and immersed in 2% CHX or 5.25% NaOCl for 1, 5, 10, 20 and 30 min. Untreated GP cones were used as control. Topography and elasticity analyses were performed on 12 different regions located between 1 and 2 mm from the tip. Root mean square (RMS) parameters for contact mode imaging and force modulation microscopy variations were measured. The differences between RMS values were tested by anova with Fisher's protected LSD test for multiple comparisons. There was no deterioration in the topography and physical properties studied when 2% CHX was used in comparison with the control (P < 0.05). The RMS parameter for topography increased after 10 min of 5.25% NaOCl exposure in comparison with the control (P < 0.05). In addition, 5.25% NaOCl increased the elasticity of the GP cone after an immersion time of 1 min in comparison with the control (P < 0.05). Two per cent CHX did not change GP cone structure following up to 30 min exposure. Conversely, 5.25% NaOCl caused elastic changes after 1 min exposure.

NANOBUBBLES AT THE INTERFACE OF HOPG AND ETHANOL SOLUTION

Recently, it has been put forward that bubbles of nanometer scale (nanobubbles) can exist on solid surfaces immersed in aqueous solution, especially on hydrophobic substrates. This interfacial phenomenon has invited extensive interests in both scientific and commercial fields. Experimental evidence including the study of atomic force microscopy (AFM) has been provided to support the existence of nanobubbles. But, there are still many debates in this field. In this paper, a degassing process is combined with direct AFM imaging to prove that the features in the AFM images are nanobubbles. It is found that the morphology of nanobubbles on the highly oriented pyrolytic graphite (HOPG) surface was noticeably deformed during contact mode imaging and their size is extremely sensitive to the imaging force of AFM. By observing nanobubbles in ethanol solution of different concentrations, we found that ethanol solution more concentrated than 20% (V/V) may have a destructive effect on nanobubbles.

A comparative study of contact resonance imaging using atomic force microscopy

We present here a comparative study of atomic force microscope (AFM) imaging in contact mode when either the cantilever carrying the probing tip or the sample is excited at ultrasonic frequencies. The cantilever or the sample can be excited by piezoelectric transducers attached to them. When the AFM tip is in contact with the sample surface the contact resonance curve depends on the local tip–sample contact stiffness. By measuring the contact resonance as a function of position one can image the local stiffness of the sample surface. We will report here imaging carried out on piezoelectric material and thin metal film using both the modes. The comparison shows that both give similar results.

Mapping of enzyme activity by detection of enzymatic products during AFM imaging with integrated SECM–AFM probes

With the integration of submicro- and nanoelectrodes into atomic force microscopy (AFM) probes using microfabrication techniques, an elegant approach combining scanning electrochemical microscopy (SECM) with AFM has recently been introduced. Simultaneous contact mode imaging of a micropatterned sample with immobilized enzyme spots and imaging of enzyme activity is shown. In contrast to force spectroscopy the conversion of an enzymatic byproduct is directly detected during AFM imaging and correlated to the activity of the enzyme.

PRECISION MAPPING OF ERYTHROCYTE TRAUMA BY ATOMIC FORCE MICROSCOPY

Over the past years experimental advances have made it possible to elucidate the components of the cell membrane and its structure, thus rendering the measurement of micro-mechanical properties much more efficient than previously. This study investigates the use of Atomic Force Microscopy (AFM) to discern subtle alterations to the red cell due to exposure within artificial organs. Red cells were stressed both biochemically and by controlled exposure to supra-physiological shear. AFM imaging was performed using a Multimode AFM (Nanoscope IIIA, Digital Instruments), using tapping mode, contact mode, and force-volume imaging. Force maps obtained using force-volume imaging were analysed using Igor Pro software (Wavemetrics), and macros developed in our laboratory (by N. Almqvist). Preliminary force maps have been obtained on specimens of human erythrocytes: both in a stress-free condition and in a tensed or echinocytic condition. Ongoing finite-element simulation models are being developed to compliment these experimental studies, with the ultimate goal of inverting nano-scale elastic properties of the blood cells. This preliminary success encourages the use of AFM force mapping as a sensitive tool to evaluate sub-lytic trauma to red blood cells. Future studies aim to investigate the elasticity and adhesion other relevant cells including leukocytes and platelets.

Surface physical studies of barium titanate ceramics

The electric conductivity of barium titanate ceramics and its temperature dependence are strongly related to grain structure and dopant concentration. First results of a systematic study using surface physical techniques are presented starting with undoped ceramic samples. The samples were sintered and treated according to our experiences with single crystalline barium titanate. Scanning electron microscopy and atomic force microscopy (AFM) contact mode images of these samples revealed the grain structure and the surface of the grains. A heavily faceted surface was found on the polished and unpolished sample. The surface structure of the polished sample indicates the orientation of the grains. Additional current images taken in AFM contact mode reveal differences in conductivity of individual grains as well as particular insulating segregations on some parts of grain surfaces.

Cross talk between friction and height signals in atomic force microscopy

Atomic force and lateral force microscopes use a four quadrant p-i-n detector to measure the motion of a laser beam reflected from the top of a cantilever. If the detector is rotated slightly in the plane of the p-i-n diode, this will cause frictional forces to be detected as a false height signal. In this article, we will show how this coupling between friction and height signals can adversely affect the measurement of topology at height scales below 10 nm. This will be demonstrated with contact mode images of single-walled carbon nanotubes. We will also show how to detect this effect and possible ways to correct for it.

Characterization of the Adhesive Mucilages Secreted by Live Diatom Cells using Atomic Force Microscopy

Atomic Force Microscopy (AFM) resolved the topography and mechanical properties of two distinct adhesive mucilages secreted by the marine, fouling diatom Craspedostauros australis. Tapping mode images of live cells revealed a soft and cohesive outer mucilage layer that encased most of the diatom's siliceous wall, and force curves revealed an adhesive force of 3.58 nN. High loading force, contact mode imaging resulted in cantilever 'cleaned' cell walls, which enabled the first direct observation of the active secretion of soft mucilage via pore openings. A second adhesive mucilage consisted of strands secreted at the raphe, a distinct slit in the silica wall involved in cell-substratum attachment and motility. Force measurements revealed a raphe adhesive strand(s) resistant to breaking forces up to 60 nN, and these strands could only be detached from the AFM cantilever probe using the manual stepper motor.

Atomic Force Microscope Images of Nanobubbles on a Hydrophobic Surface and Corresponding Force−Separation Data

Domains, apparently nanobubbles, have been observed on a hydrophobic glass surface in water using an atomic force microscope operated in tapping mode. Phase images show the domains to be softer than the underlying substrate. Complementary force curves between a silica colloid probe and the glass surface display features characteristic of the hydrophobic interaction, including a jump-in distance that is comparable to the height of the imaged domains. Images and force curves have been acquired over a range of pH conditions to probe the nature of the overall interaction and gain some insight into the conformation of nanobubbles on the sample surface. The bubbles appear to regrow following their removal by the application of high loads through either contact mode imaging or tapping mode imaging at high drive amplitudes. They are not present in a solvaphilic fluid (ethanol) but regrow following the subsequent reintroduction of H2O. The correlation between image and force data, supported by existing results in the literature, provides strong evidence to favor nanobubbles as the origin of the hydrophobic force.

Observation and Modification of the Surface of Some Organic Crystals by Means of Atomic Force Microscopy

Crystals of hexamethylenetetramine (HMT) with aliphatic dicarboxylic acids in 1:1 molar ratio exhibit different behaviour depending on the number n of carbons in the aliphatic chain. For n=8 and 10, incommensurate phases have been observed, while for n=9, a commensurate disordered phase is found. The structure consists of alternating layers of roughly spherical HMT and acid chains. Atomic force microscope (AFM) has been used in order to characterise these layers in several different crystals. Images have been collected in contact and tapping modes. Contact mode images reveal that the tip interaction removed systematically an integer number of layers of the crystal. Morphological changes of the surface are observed in the images collected in tapping mode.

Tapping and contact mode imaging of native chromosomes and extraction of genomic DNA using AFM tips

It is very important both in medicine and biology to clarify the chromosomal structure to understand its functions. In a standard cytogenetic procedure, chromosomes are often fixed in a mixture of acetic acid and methanol. This process most likely changes the mechanical property of chromosomes. We adopted a method to prepare native and unfixed chromosomes from mouse 3T3 cells and used tapping and contact mode atomic force microscopy (AFM) to image and manipulate them. Modified AFM tips were used to image chromosomes in contact mode in air, and then the chromosome samples were immobilized on a substrate and placed in a buffer solution to pull out DNA–histone complexes from them after they were optimally treated with trypsin. From the AFM images, we could see several bands and granular structures on chromosomes. We obtained force curves indicating long fiber extensions from native chromosomes both with low (in high concentration of NaCl) and high forces (physiological conditions). The result suggested that the degree of chromosome condensation decreased in high concentration of salt. It agrees with the known fact of histone H1 dissociation in a high concentration of salt. We intend to pull out DNA–histone complexes from chromosomes for later molecular operations on them using an AFM.

Hybrid optical fiber-apertured cantilever near-field probe

In this letter, we propose a hybrid optical fiber-apertured cantilever probe for optical near-field applications. A thermal SiO2 cantilever beam with a SiO2 pyramidal tip was formed by Si micromachining technique and bonded with an optical fiber using a polyimide adhesive layer. A subwavelength aperture at the apex of the SiO2 tip was formed by etching the SiO2 in a buffered-HF solution. Optical near-field imaging in contact mode was observed with the fabricated probe. The probe could work in contact mode because the cantilever at the end of the fiber can flexibly move on the sample surface. By detecting the far-field light which is reflected-back by the tip of the cantilever, the vibration of the cantilever was observed using the probe itself. With the proposed structure, a hybrid fiber bundle-apertured cantilever array is feasible for application in parallel near-field processing or data storage.

Atomic Force Microscopy and Scanning Electron Microscopy Reveal Genome–dependent Ultrastructure of Seed Surface

AbstractBoth scanning electron microscopy (SEM) and contact mode imaging via atomic force microscopy (AFM) have been utilized to elucidate the ultrastructure of mung bean seed surfaces. The results indicate: 1) that AFM is useful in the examination of seed surface ultrastructure ex-vaccuo without the need for additional complex preparative procedures; and 2) that both the cotyledon and seed coat of different strains of mung beans bear specific ultrastructural details unique to each strain. To our knowledge, these are the first AFM images of seed surfaces.

Models for quantitative charge imaging by atomic force microscopy

Two models are presented for quantitative charge imaging with an atomic-force microscope. The first is appropriate for noncontact mode and the second for intermittent contact (tapping) mode imaging. Different forms for the contact force are used to demonstrate that quantitative charge imaging is possible without precise knowledge of the contact interaction. From the models, estimates of the best charge sensitivity of an unbiased standard atomic-force microscope cantilever are found to be on the order of a few electrons.