Atomic force microscopy of purple membranes

The surface structure of purple membranes was imaged using an atomic force probe mounted in a scanning tunnelling microscope. One of the two different membrane surfaces showed protruding, disc-shaped features forming an hexagonal lattice with about 6 nm centre to centre spacing. These are identified as the cytoplasmic surfaces of trimers of bacteriorhodopsin molecules and are correlated with the structural information on bacteriorhodopsin obtained from numerous earlier electron microscope and diffraction studies.     

Atomic and magnetic force microscopy imaging of thin-film recording heads

Investigations on the use of the scanning probe microscope (SPM) in the atomic force microscopy (AFM) mode for topography imaging and the magnetic force microscopy (MFM) mode for magnetic imaging are presented for a thin-film recording head. Results showed that the SPM is suitable for imaging the surface profile of the recording head, determining the width of the pole gap region, and mapping the magnetic field patterns of the recording head excited under current bias conditions of different polarity. For the cobalt sputter-coated tips used in MFM imaging, it was found that the magnetic field patterns obtained under different polarities of the current bias to the recording head were similar. This can be explained by the nature of the thin-film MFM tip, in which the direction of the tip magnetic moment can follow the stray magnetic field of the sample as the current bias to the recording head reverses in direction.     

Adhesion artefacts in atomic force microscopy imaging

Artefacts that affect contrast and arise from adhesion forces in atomic force microscopy images of aramid fibres (both fresh and plasma-treated) are investigated. It is demonstrated that these stem not only from variations in the chemical composition of the surface but also from certain topographical features (which may appear hidden or enhanced in the images), resulting in changes in the lateral forces that are detected by the cantilever and are comparable to the vertical forces. It is also shown that both types of contribution to the forces can be uncoupled to yield images free from these artefacts, thus allowing more accurate quantitative measurements. These artefactual effects are also generally applicable to many other materials.     

Atomic force microscopy imaging of actin cortical cytoskeleton of Xenopus laevis oocyte

In this study we report an atomic force microscopy (AFM) investigation of the actin cortical cytoskeleton of Xenopus laevis oocytes. Samples consisted of inside‐out orientated plasma membrane patches of X. laevis oocytes with overhanging cytoplasmic material. They were spread on a freshly cleaved mica surface, subsequently treated with Triton X‐100 detergent and chemically fixed. The presence of actin fibres in oocyte patches was proved by fluorescence microscopy imaging. Contact mode AFM imaging was performed in air in constant force conditions. Reproducible high‐resolution AFM images of a filamentous structure were obtained. The filamentous structure was identified as an actin cortical cytoskeleton, investigating its disaggregation induced by cytochalasin D treatment. The thinnest fibres showed a height of 7 nm in accordance with the diameter of a single actin microfilament. The results suggest that AFM imaging can be used for the high‐resolution study of the actin cortical cytoskeleton of the X. laevis oocyte and its modifications mediated by the action of drugs and toxins.     

Atomic force microscopy of a hydrated bacterial surface protein

The protein surface layer of the bacterium Deinococcus radiodurans (HPI layer) was examined with an atomic force microscope (AFM). The measurements on the air-dried, but still hydrated layer were performed in the attractive imaging mode in which the forces between tip and sample are much smaller than in AFM in the repulsive mode or in scanning tunnelling microscopy (STM). The results are compared with STM and transmission electron microscopy (TEM) data.     

Atomic force and scanning electron microscopy of atmospheric particles

Atomic force microscopy (AFM) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) have been used for both morphological and elemental mass analysis study of atmospheric particles. As part of the geometrical particle analysis, and in addition to the traditional height profile measurement of individual particles, AFM was used to measure the volume relative to the projection area for each particle separately, providing a particle shape model. The element identification was done by the EDS analysis, and the element mass content was calculated based on laboratory calibration with particles of known composition. The SEM-EDS mass measurements from two samples collected at 150 and 500 m above the surface of the Mediterranean Sea were found to be similar to mass calculations derived from the AFM volume measurements. The AFM results show that the volume of most of the aerosols that were identified as soluble marine sulfate and nitrate aerosol particles can be better estimated using cylindrical shapes than spherical or conical geometry.     

Atomic force microscopy as a tool for study of human hair

Atomic force microscopy (AFM) is a newly developed microscopic technique that offers high-resolution power, less intrusive measurement, and requires little sample pretreatment for elucidating structures of biological materials in three dimensions and in their natural environment. In this study, AFM has been used not only as an imaging technique for examining human hair structure at high resolution, but also as a tool for quantitative assessment of the effect of treatment in 10 mM phosphate buffered saline of pHs 3.0, 7.0, and 11.0 and heating on human hair structure. It is observed that the hair cuticle is a sensitive indicator of external influences on hair structure, and that its height can be used as a parameter for quantitative assessment. The experimental results obtained show that the swelling of hair caused by the incubation in the buffer decreases with the increase of the pH values and that, depending on the duration of heating, the hair undergoes structural expansion and shrinkage. This study demonstrates that AFM can be used as a valuable alternative to conventional microscopic techniques for hair research.     

Atomic force microscopy observations of acyl chains in phospholipids

The potential use of atomic force microscopy (AFM) to image the mode of assembly and to measure the corresponding lattice parameters of model systems consisting of ordered aggregates of cardiolipin molecules has been investigated. An unprecedented resolution of about 0·2 nm has been achieved on suitably prepared specimens. This enables the orientational order and the positional correlations of the individual molecules in the lattice to be defined, and submolecular details, such as the acyl chains and the polar groups, to be imaged. The structural parameters derived from AFMhave been compared with those obtained by transmission electron diffraction of the same specimen and found to be in excellent agreement. AFM turns out to be a powerful and probably a unique tool to reveal local phase variations in systems, such as biological membranes, that have non-homogeneous composition and organization.     

Atomic force microscopy imaging of fragments from the Martian meteorite ALH84001

A combination of scanning electron microscopy (SEM) and environmental scanning electron microscopy (ESEM) techniques, as well as atomic force microscopy (AFM) methods has been used to study fragments of the Martian meteorite ALH84001. Images of the same areas on the meteorite were obtained prior to and following gold/palladium coating by mapping the surface of the fragment using ESEM coupled with energy-dispersive X-ray analysis. Viewing of the fragments demonstrated the presence of structures, previously described as nanofossils by McKay et al . (Search for past life on Mars — possible relic biogenic activity in martian meteorite ALH84001. Science , 1996, pp. 924–930) of NASA who used SEM imaging of gold-coated meteorite samples. Careful imaging of the fragments revealed that the observed structures were not an artefact introduced by the coating procedure.     

Dynamic behavior of amplitude detection Kelvin force microscopy in ultrahigh vacuum

The acquisition rate of all scanning probe imaging techniques with feedback control is limited by the dynamic response of the control loops. Performance criteria are the control loop bandwidth and the output signal noise power spectral density. Depending on the acceptable noise level, it may be necessary to reduce the sampling frequency below the bandwidth of the control loop. In this work, the frequency response of a vacuum Kelvin force microscope with amplitude detection (AM-KFM) using a digital signal processing (DSP) controller is characterized and optimized. Then, the main noise source and its impact on the output signal is identified. A discussion follows on how the system design can be optimized with respect to output noise. Furthermore, the interaction between Kelvin and distance control loop is studied, confirming the beneficial effect of KFM on topography artefact reduction in the frequency domain. The experimental procedure described here can be generalized to other systems and allows to locate the performance limitations.     

Atomic force microscopy of silica nanoparticles and carbon nanohorns in macrophages and red blood cells

The emerging interest in understanding the interactions of nanomaterial with biological systems necessitates imaging tools that capture the spatial and temporal distributions and attributes of the resulting nano–bio amalgam. Studies targeting organ specific response and/or nanoparticle-specific system toxicity would be profoundly benefited from tools that would allow imaging and tracking of in-vivo or in-vitro processes and particle-fate studies. Recently we demonstrated that mode synthesizing atomic force microscopy (MSAFM) can provide subsurface nanoscale information on the mechanical properties of materials at the nanoscale. However, the underlying mechanism of this imaging methodology is currently subject to theoretical and experimental investigation. In this paper we present further analysis by investigating tip-sample excitation forces associated with nanomechanical image formation. Images and force curves acquired under various operational frequencies and amplitudes are presented. We examine samples of mouse cells, where buried distributions of single-walled carbon nanohorns and silica nanoparticles are visualized.     

C-banding visualized by atomic force microscopy

C-banding is a method used for studying chromosome rearrangements near centromeres and for investigating polymorphisms. In human chromosomes, the C-bands are located at the centromere of all the chromosomes and the distal long arm of the Y chromosome. In this study, we aimed to detect the structural changes in chromosomes during the stages of C-banding by atomic force microscopy. We observed crater-like structures in the chromosomes after 2xSSC (saline sodium citrate) treatment and measured the relative difference between the heights of chromatid and centromere of the chromosomes. Results showed that the relative difference was 3 nm in chromosomes 1, 9, 16, and Y, whereas in the other chromosomes this value was 11.6 nm. After Giemsa staining, the relative difference increased by a factor of 16 in chromosomes 1, 9, 16, and Y. The other chromosomes showed no such increase, which is in accordance with our suggestion that nonhiston proteins associated with DNA in constitutive heterochromatin can make the constitutive heterochromatin resistant to C-banding.     

On the electrical properties of dislocations in ZnS using electric force microscopy

A combination of electric force microscopy (EFM) and noncontact atomic force microscopy (AFM) was used to study microscratching-induced dislocations in sphaleritic ZnS single crystals. Dislocation bands predominantly consisting of either anion-type (S) or cation-type (Zn) dislocations were induced by scratching along either [111] or [111] on a (110) surface. A significant difference of local distortions in electrical potential between the S(g) and Zn(g) dislocation bands was observed from the EFM images. Electric charges of these dislocations were determined quantitatively and the results were compared with theoretical models.     

Scanning electron microscopy studies of protein-functionalized atomic force microscopy cantilever tips

Protein-functionalized atomic force microscopy (AFM) tips have been used to investigate the interaction of individual ligand-receptor complexes. Herein we present results from scanning electron microscopy (SEM) studies of protein-functionalized AFM cantilever tips. The goals of this study were (1) to examine the surface morphology of protein-coated AFM tips and (2) to determine the stability of the coated tips. Based on SEM images, we found that bovine serum albumin (BSA) in solution spontaneously adsorbed onto the surface of silicon nitride cantilevers, forming a uniform protein layer over the surface. Additional protein layers deposited over the initial BSA-coated surface did not significantly alter the surface morphology. However, we found that avidin-functionalized tips were contaminated with debris after a series of force measurements with biotinylated agarose beads. The bound debris presumably originated from the transfer of material from the agarose bead. This observation is consistent with the observed deterioration of functional activity as measured in ligand-receptor binding force experiments.     

Frictional effects in atomic force microscopy of Langmuir-Blodgett films

Frictional effects in atomic force microscopy (AFM) of Langmuir-Blodgett films of 1, 2-dipalmitoyl-snglycero-phosphoglycerol were examined. Height measurements of the Langmuir layers are strongly influenced by the orientation of the cantilevers used in AFM relative to the sample. A simple model is used to describe the frictional effects and to calculate the real height of the monolayers.     

Atomic force microscopy imaging of polycrystalline CuInSe2 thin films

In this study, atomic force microscopy (AFM) imaging has been used to study the structural properties of polycrystalline CuInSe₂ films, which are widely used as absorber materials in thin film solar cell devices. This technique demonstrated an excellent capability for the reproducible imaging of these rough polycrystalline materials. AFM imaging in combination with statistical analysis revealed distinct differences in the structural properties (i.e. grain width and height distributions, root‐mean‐square (RMS) and peak to valley (R_(p–v)) roughness values) as a function of the specific growth technique used and the bulk composition of the films. In the case of Cu‐rich films, prepared by the H₂Se/Ar treatment of Cu/In/Cu alloys, rough surface structures were in general observed. Statistical analysis revealed two distinct distribution of grains in these samples (1.0–2.5 μm and 3–5.5 μm) with large RMS and R_(p–v) roughness values of 380 nm and 2.6 μm, respectively. In‐rich films were characterized by the presence of much smaller, roughly circular clusters with a significant reduction in both the width and height distributions as well as RMS and R_(p–v) roughness values. The most successful growth techniques, in terms of producing homogeneous and dense films, were in the cases of H₂Se/Ar treated metallic InSe/Cu/InSe alloys and the coevaporation of all materials to form CuInSe₂. Both these techniques produced absorber films with very narrow grain width and height distributions as well as small roughness values. It was possible to establish that high efficiency devices are associated with the use of absorber films with narrow width distributions between 0.5 and 2 μm and small RMS (> 300 nm) roughness values. These values are used as a figure of merit in our laboratories to evaluate the structural properties of our CuInSe₂ thin films.     

The mechanism of G-banding detected by atomic force microscopy

The morphologic changes occurring in human chromosomes during G-banding by trypsin treatment on the same metaphase were followed with the aid of an atomic force microscope (AFM). It was found that trypsin treatment alone caused a pattern of collapse in the chromosomes that was clearly dependent on the duration of trypsinization. The progressive pattern of collapse first indicated the loss of internal differentiation between chromatids, then bands, and finally all internal structures, except for edges running around the chromosomes' perimeter. When stained with Giemsa, the collapsed chromosomes partly regained their original form, and transverse ridges appeared that correspond to G-positive band regions. However, the treatment of fixed chromosomes with trypsin for 42 s diminished the chromosomal edges, and the z-dimensions could not be measured even with the subsequent application of Giemsa.     

Improvement of thermally induced bending of cantilevers used for atomic force microscopy

The microfabricated silicon nitride cantilevers that are used for atomic force microscopy (AFM) are, unfortunately, sensitive thermometers. They bend with ambient temperature changes and those due to laser heating. The bend can result in displacements for the silicon nitride cantilevers of an order several hundred nanometers at the tip of the cantilever. If, however, the silicon nitride cantilevers are treated by removing the metallization and annealing at 500°C for 30 min, these displacements can be reduced by one or two orders of magnitude. Silicon cantilevers have approximately a one order of magnitude smaller drift than silicon nitride cantilevers as received from vendors and are improved less by treatment.     

Phase imaging with intermodulation atomic force microscopy

Intermodulation atomic force microscopy (IMAFM) is a dynamic mode of atomic force microscopy (AFM) with two-tone excitation. The oscillating AFM cantilever in close proximity to a surface experiences the nonlinear tip-sample force which mixes the drive tones and generates new frequency components in the cantilever response known as intermodulation products (IMPs). We present a procedure for extracting the phase at each IMP and demonstrate phase images made by recording this phase while scanning. Amplitude and phase images at intermodulation frequencies exhibit enhanced topographic and material contrast.     

Investigation of nanoparticles by atomic and lateral force microscopy

Metallic nanoparticles have been produced on vitreous carbon substrates by means of thermal evaporation. From pictures of the particles, made by a high-resolution scanning electron microscope (HRSEM), a semispherical shape is suggested due to the total mass of deposited material. Atomic force microscopy (AFM) has been applied to this sample in order to get direct topographic information. The AFM has been operated with normal and super tips, the latter having a smaller cone angle and radius, thus following more precisely the contours of an object. Simultaneously lateral-force microscopic (LFM) images have been recorded. Major differences between the contents of HRSEM- and AFM-images are considered, emphasizing the important influence of the tips' geometry. Both the AFM and LFM line scans have been compared with and have qualitatively agreed with those calculated under simplifying assumptions.