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PubMed2005

Chain structures of surface hydroxyl groups formed via line oxygen vacancies on TiO2(110) surfaces studied using noncontact atomic force microscopy.

Structures of surface hydroxyl groups arranged on a reduced TiO2(110) surface that had line oxygen vacancies were studied using noncontact atomic force microscopy (NC-AFM). NC-AFM results revealed that by increasing the density of oxygen vacancies on the TiO2(110) surface, line oxygen vacancies were formed by removal of oxygen atoms in a bridge oxygen row on the TiO2(110) surface. After the TiO2(110) surface with the line oxygen vacancies was exposed to water, the surface showed hydroxyl chain structures that were composed of hydroxyl groups linearly arranged in a form of two rows on the line oxygen vacancies and on a neighboring bridge oxygen row. In-situ NC-AFM measurements of these surfaces exposed to water at room temperature revealed that hydroxyl chain structures were formed at the line oxygen vacancy. Annealing above 500 K was sufficient to remove the hydroxyl chain structures on the TiO2(110) surface and allowed line oxygen vacancies to reappear on the surface. The line oxygen vacancies are active sites for water dissociation. In conclusion, the formation of the hydroxyl chain structure suggests that the surface hydroxyl groups on a TiO2(110) surface can be controlled by preparing oxygen vacancy structures on the surface.

Journal Article↗
PubMed1998

Analysis of various sequence-specific triplexes by electron and atomic force microscopies.

Sequence-specific interactions of 20-mer G,A-containing triple helix-forming oligonucleotides (TFOs) and bis-PNAs (peptide nucleic acids) with double-stranded DNA was visualized by electron (EM) and atomic force (AFM) microscopies. Triplexes formed by biotinylated TFOs are easily detected by both EM and AFM in which streptavidin is a marker. AFM images of the unlabeled triplex within a long plasmid DNA show a approximately 0.4-nm height increment of the double helix within the target site position. TFOs conjugated to a 74-nt-long oligonucleotide forming a 33-bp-long hairpin form extremely stable triplexes with the target site that are readily imaged by both EM and AFM as protruding DNA. The short duplex protrudes in a perpendicular direction relative to the double helix axis, either in the plane of the support or out of it. In the latter case, the apparent height of the protrusion is approximately 1.5 nm, when that of the triplex site is increased by 0.3-0.4 nm. Triplex formation by bis-PNA, in which two decamers of PNA are connected via a flexible linker, causes deformations of the double helix at the target site, which is readily detected as kinks by both EM and AFM. Moreover, AFM shows that these kinks are often accompanied by an increase in the DNA apparent height of approximately 35%. This work shows the first direct visualization of sequence-specific interaction of TFOs and PNAs, with their target sequences within long plasmid DNAs, through the measurements of the apparent height of the DNA double helix by AFM.

Amino Acid Sequence↗
PubMed2005

Observation of the mica surface by atomic force microscopy.

Freshly cleaved mica and a mica surface treated with pure water and dilute-salt solution have been investigated by Atomic Force Microscopy (AFM). On the bare mica surface (after repeated scanning), small dots and islands were observed. The disappearance of these dots and islands has also been captured by AFM. We believe these structures to be condensed water. The water meniscus between AFM tip and mica surface is considered as the source of this water structure. On the mica surface treated with pure water and dilute-salt solution, network structures are frequently observed by AFM.

Journal Article↗
PubMed2003

Theoretical analysis of the atomic force microscopy characterization of columnar thin films.

In this paper, the theoretical analysis of the influence of finite linear dimensions of an atomic force microscope tip on profiles of the upper boundaries of columnar thin films and their statistical quantities is performed. This analysis is based on a numerical evaluation of the main statistical quantities, i.e. the standard deviations of the heights and slopes, one-dimensional distributions of the probability density of heights and slopes and power spectral density function, corresponding to a simulated columnar structure of the thin films. It is shown that the strongest misrepresentation of the measured profiles of the upper boundaries of the columnar films originates in the cases when the linear dimensions of the columns are smaller or comparable with the linear dimensions of the tip. Further, it is shown that using a surface reconstruction procedure one can correct (improve) the boundary profiles and their statistical quantities partially. The results of this analysis enable us to perform rough estimation of the errors achieved within atomic force microscopy studies of the real columnar thin films. Moreover, these results allow to estimate the corrections of the statistical quantities mentioned above to be obtained using the surface reconstruction.

Journal Article↗
PubMed2004

Monitoring the transformation of colloidal crystals by styrene vapor using atomic force microscopy.

The stages of transformation of a colloidal crystalline film of latex spheres to a new periodic structure were imaged by atomic force microscopy. Colloidal crystalline films were prepared with 320 nm diameter poly(styrene-co-2-hydroxyethyl methacrylate) (PSt/HEMA) spheres. The hexagonally ordered surfaces of the colloidal crystalline films were transformed with styrene vapor at room temperature to a new morphology having holes in the surface and the same periodicity as the original films. The surfaces of colloidal crystals and the transformed films have a raspberry-like texture superposed on the 320 nm hexagonal periodicity. Both height images and phase images reveal that the latex spheres shrink and the transformation proceeds by an order-disorder-order sequence. The final structure is an interconnected colloidal array with smaller polystyrene particles dispersed in a continuous PSt/HEMA matrix.

Journal Article↗
PubMed1992

Atomic force microscopy imaging of double stranded DNA and RNA.

A procedure for imaging long DNA and double stranded RNA (dsRNA) molecules using Atomic Force Microscopy (AFM) is described. Stable binding of double stranded DNA molecules to the flat mica surface is achieved by chemical modification of freshly cleaved mica under mild conditions with 3-aminopropyltriethoxy silane. We have obtained striking images of intact lambda DNA, Hind III restriction fragments of lambda DNA and dsRNA from reovirus. These images are stable under repeated scanning and measured contour lengths are accurate to within a few percent. This procedure leads to strong DNA attachment, allowing imaging under water. The widths of the DNA images lie in the range of 20 to 80nm for data obtained in air with commercially available probes. The work demonstrates that AFM is now a routine tool for simple measurements such as a length distribution. Improvement of substrate and sample preparation methods are needed to achieve yet higher resolution.

Aluminum Silicates↗
PubMed1996

Membrane knobs of unfixed Plasmodium falciparum infected erythrocytes: new findings as revealed by atomic force microscopy and surface potential spectroscopy.

Cerebral malaria, a severe complication of malaria, is caused by the obstruction of cerebral microvessels by Plasmodium falciparum-infected erythrocytes. Such cells adhere to endothelial cells by means of "knobs" induced on the red cell membrane by the parasites. When atomic force microscopy was used to investigate the structure of the knobs of unfixed infected red cells, each knob was found to consist of two distinct subunits, knob components that have never been seen in chemically fixed knobs examined by conventional transmission electron microscopy. Surface potential spectroscopy revealed that the knobs have a positive charge (+20 mV), whereas the remainder of the red cell plasma membrane is negatively charged. Since endothelial plasma membranes have a negative charge, the charge difference between knobs and endothelium may play a significant role in cytoadherence between the two cell types. The subunit structure of the knobs may be a steric necessity to align adherence molecules so that they can exert their effect. This study shows that the atomic force microscope has great potential for examination of cells in their native state; in combination with surface potential spectroscopy, it may uncover fundamental processes and mechanisms in cell function.

Animals↗
PubMed2000

Atomic force microscopy of the cell nucleus.

In mammals and plants, the cell nucleus is organized in dynamic macromolecular domains involved in DNA and RNA metabolism. These domains can be visualized by light and electron microscopy and their composition analyzed by using several cytochemical approaches. They are composed of chromatin or ribonucleoprotein structures as interchromatin and perichromatin fibers and granules, coiled bodies, and nuclear bodies. In plants, DNA arrangement defines chromocentric and reticulated nuclei. We used atomic force microscopy to study the in situ structure of the plant cell nucleus. Samples of the plants Lacandonia schismatica and Ginkgo biloba were prepared as for electron microscopy and unstained semithin sections were mounted on glass slides. For comparison, we also examined entire normal rat kidney cells using the same approach. Samples were scanned with an atomic force microscope working in contact mode. Recognizable images of the nuclear envelope, pores, chromatin, and nucleolus were observed. Reticulated chromatin was observed in L. schismatica. Different textures in the nucleolus of G. biloba were also observed, suggesting the presence of nucleolar subcompartments. The observation of nuclear structure in situ with the atomic force microscope offers a new approach for the analysis of this organelle at high resolution.

Animals↗
PubMed1997

High-Resolution Atomic Force Microscopy of Native Valonia Cellulose I Microcrystals

We have studied the surface of native Valonia cellulose I microcrystals under propanol and waterby atomic force microscopy (AFM). Ultra-high-resolution images of the surface are presented, as well as lower resolution morphological observations of whole crystals. The pitch of 0.52 nm along the molecule due to the asymmetrical glucose unit and the intermolecular spacing of approximately 0.6 nm are clearly resolved in both imaging environments. The relationship between the crystalline bulk and the surface are discussed, with particular attention being paid to previous crystallographic studies. We also show that the glucose units along the cellulose chains are not topographically equivalent due to the twofold screw symmetry and accordingly present strong evidence of triclinic character by direct surface imaging, rather than by taking average measurements in reciprocal space. The crystallographic distinction between monoclinic and triclinic structure is a displacement of the cellulose chains by a quarter of the c axis period, resulting in either a stagger or a diagonal shifting, respectively, of the cellobiose unit along the chain axis by 0.26 nm. This structural identification (in real space) represents, as far as we are aware, the highest resolution AFM imaging of a biological specimen to date. This study opens up the future possibility of identifying the localized triclinic or monoclinic nature of the Valonia cellulose surface with AFM.

Journal Article↗
PubMed2005

Nanoscale electrical characterization of semiconducting polymer blends by conductive atomic force microscopy (C-AFM).

For the first time local electrical characteristics of a blend of two semiconducting polymers were studied with conductive atomic force microscopy (C-AFM). The investigated mixture is potentially interesting as the active layer in plastic photovoltaic devices. Besides conventional topography analysis of morphology and phase separation, the internal structure of the active layer was investigated by observing the current distribution with nanoscale spatial resolution. Similar to force spectroscopy, current imaging spectroscopy was performed during scanning the sample surface. Different types of current-voltage (I-V) characteristics were extracted from the array of spectroscopic data obtained from each point of the scans, and local heterogeneities of the electric characteristic were determined and discussed.

Journal Article↗
PubMed2003

Measuring molecular weight by atomic force microscopy.

Absolute-molecular-weight distribution of cylindrical brush molecules were determined using a combination of the Langmuir Blodget (LB) technique and Atomic Force Microscopy (AFM). The LB technique gives mass density of a monolayer, i.e., mass per unit area, whereas visualization of individual molecules by AFM enables accurate measurements of the molecular density, i.e., number of molecules per unit area. From the ratio of the mass density to the molecular density, one can determine the absolute value for the number average molecular weight. Assuming that the structure of brush molecules is uniform along the backbone, the length distribution should be virtually identical to the molecular weight distribution. Although we used only brush molecules for demonstration purpose, this approach can be applied for a large variety of molecular and colloidal species that can be visualized by a microscopic technique.

Journal Article↗
PubMed2001

Adhesion Forces between Lewis(X) Determinant Antigens as Measured by Atomic Force Microscopy.

The adhesion forces between individual molecules of Lewis(X) trisaccharide antigen (Le(X) ) have been measured in water and in calcium solution by using atomic force microscopy (AFM, see graph). These results demonstrate the self-recognition capability of this antigen, and reinforce the hypothesis that carbohydrate-carbohydrate interaction could be considered as the first step in the cell-adhesion process in nature.

antigens↗
PubMed2005

Comparative study of erythritol and lactose monohydrate as carriers for inhalation: atomic force microscopy and in vitro correlation.

The adhesion of micronised salbutamol sulphate to two carrier excipients, lactose monohydrate and erythritol, was investigated using the atomic force microscope (AFM) colloid probe technique and correlated with their respective physico-mechanical properties and aerosolisation performance. The particle size, morphology and moisture sorption properties of the carriers were similar thereby allowing direct comparison of functionality. AFM force measurements (n = 1024 force curves) were obtained between salbutamol sulphate drug probes (n = 4) and the excipients, as 63-90 microm sieve fractions and atomically smooth crystals. In general, significant differences in drug adhesion to lactose monohydrate and erythritol were observed (ANOVA, p<0.05), with erythritol exhibiting relatively greater adhesiveness. A linear relationship between drug probe adhesion to lactose monohydrate and drug probe adhesion to erythritol was established with salbutamol sulphate-lactose monohydrate adhesion being 60-70% of that of the erythritol system. In vitro analysis suggested good correlation with the adhesion measurements. The aerosolisation of salbutamol sulphate from erythritol carrier particles was significantly less (ANOVA, p<0.05) than from lactose monohydrate, with a fine particle dose (<6.4 microm) of 41.9 +/- 7.4 microg and 24.9 +/- 3.1 microg for the lactose monohydrate and erythritol carriers, respectively (n = 3).

Administration, Inhalation↗
PubMed2001

Structural Analysis of Collapsed, and Twisted and Collapsed, Multiwalled Carbon Nanotubes by Atomic Force Microscopy.

A fully collapsed multiwalled carbon nanotube (MWCNT1) section and a different twisted and fully collapsed MWCNT were observed with tapping-mode atomic force microscopy. The collapsed section of MWCNT1 was significantly more flexible than the uncollapsed sections, and advanced 120 nm within 1 month. The collapse of MWCNT1 was most likely initiated by its interaction with the surface, and possibly a water meniscus. The ability of carbon nanotubes to radially deform under the influence of surface interactions is in striking contrast with their extremely high axial rigidity.

Journal Article↗
PubMed2004

In situ annealing and thickening of single crystals of C294H590 observed by atomic force microscopy.

The annealing behavior of twice-folded crystals of the long-chain alkane C294H590 is examined in situ, in real time, by atomic force microscopy (AFM). AFM is capable of following processes in real time provided that the time scale is sufficiently long for several images to be collected during the process. In this paper, we focus on the temperature dependence and the thickened morphology. We are able to investigate where the thickening starts and how this depends on temperature and how melting is influenced by morphology. By following the motion of holes within the crystal, a lower limit for the rate of diffusion of crystalline polyethylene is estimated. We also focus on the substrate effect on the crystal morphology and thickening, using mica, glass, and graphite.

Journal Article↗
PubMed2002

Visualization of thermally fluctuating surface structure in noncontact atomic-force microscopy and tip effects on fluctuation: theoretical study of Si(111)-(square root[3] x square root[3])-Ag surface.

We investigated noncontact atomic-force microscopy (NC-AFM) images of a thermally fluctuating surface structure together with tip effects based on the first-principles electronic state calculation. As an example the Si(111)-(square root[3] x square root[3])-Ag (square root[3]-Ag) surface is studied. We have succeeded in theoretically visualizing the thermal fluctuation of the square root[3]-Ag surface at room temperature, and in reproducing the observed NC-AFM image for the first time. Further, the pinning effect of the thermal fluctuation of the square root[3]-Ag surface by the tip is clarified, which shows a novel ability of NC-AFM to modify the surface structure.

Journal Article↗
PubMed2004

Flexibility and size heterogeneity of the LH1 light harvesting complex revealed by atomic force microscopy: functional significance for bacterial photosynthesis.

Previous electron microscopic studies of bacterial RCLH1 complexes demonstrated both circular and elliptical conformations of the LH1 ring, and this implied flexibility has been suggested to allow passage of quinol from the Q(B) site of the RC to the quinone pool prior to reduction of the cytochrome bc(1) complex. We have used atomic force microscopy to demonstrate that these are just two of many conformations for the LH1 ring, which displays large molecule-to-molecule variations, in terms of both shape and size. This atomic force microscope study has used a mutant lacking the reaction center complex, which normally sits within the LH1 ring providing a barrier to substantial changes in shape. This approach has revealed the inherent flexibility and lack of structural coherence of this complex in a reconstituted lipid bilayer at room temperature. Circular, elliptical, and even polygonal ring shapes as well as arcs and open rings have been observed for LH1; in contrast, no such variations in structure were observed for the LH2 complex under the same conditions. The basis for these differences between LH1 and LH2 is suggested to be the H-bonding patterns that stabilize binding of the bacteriochlorophylls to the LH polypeptides. The existence of open rings and arcs provides a direct visualization of the consequences of the relatively weak associations that govern the aggregation of the protomers (alpha(1)beta(1)Bchl(2)) comprising the LH1 complex. The demonstration that the linkage between adjacent protomer units is flexible and can even be uncoupled at room temperature in a detergent-free membrane bilayer provides a rationale for the dynamic separation of individual protomers, and we may now envisage experiments that seek to prove this active opening process.

Bacterial Proteins↗
PubMed2002

Direct detection by atomic force microscopy of single bond forces associated with the rupture of discrete charge-transfer complexes.

Atomic force microscopy (AFM) was used to measure the chemical binding force of discrete electron donor-acceptor complexes formed at the interface between proximal self-assembled monolayers (SAMs). Derivatives of the well-known electron donor N,N,N',N'-tetramethylphenylenediamine (TMPD) and the electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) were immobilized on Au-coated AFM tips and substrates by formation of SAMs of N,N,N'-trimethyl-N'-(10-thiodecyl)-1,4-phenylenediamine (I) and bis(10-(2-((2,5-cyclohexadiene-1,4-diylidene)dimalonitrile))decyl) disulfide (II), respectively. Pull-off forces between modified tips and substrates were measured under CHCl(3) solvent. The mean pull-off forces associated with TMPD/TCNQ microcontacts were more than an order of magnitude larger than the pull-off forces for TMPD/TMPD and TCNQ/TCNQ microcontacts, consistent with the presence of specific charge-transfer interactions between proximal TMPD donors and TCNQ acceptors. Furthermore, histograms of pull-off forces for TMPD/TCNQ contacts displayed 70 +/- 15 pN periodicity, assigned to the rupture of individual TMPD-TCNQ donor-acceptor (charge-transfer) complexes. Both the mean pull-off force and the 70 pN force quantum compare favorably with a contact mechanics model that incorporates the effects of discrete chemical bonds, solvent surface tensions, and random contact area variations in consecutive pull-offs. From the 70 pN force quantum, we estimate the single bond energy to be approximately 4-5 kJ/mol, in reasonable agreement with thermodynamic data. These experiments establish that binding forces due to discrete chemical bonds can be detected directly in AFM pull-off measurements employing SAM modified probes and substrates. Because SAMs can be prepared with a wide range of exposed functional groups, pull-off measurements between SAM-coated tips and substrates may provide a general strategy for directly measuring binding forces associated with a variety of simple, discrete chemical bonds, e.g., single hydrogen bonds.

Journal Article↗