C. J. Roberts

Characterization of protein-resistant dextran monolayers

A range of synthetic thiolated dextrans of varying molecular weights and degrees of thiol substitution have been investigated as well-defined monolayer coatings for the reduction of nonspecific protein adsorption. Atomic force microscopy and surface plasmon resonance (SPR) analysis revealed that the surface coverage of the dextran monolayers increased with an increasing degree of thiol substitution, but conversely decreased with increasing molecular weight. SPR was then employed to monitor bovine serum albumin protein adsorption to thiolated dextran monolayers from a flowing buffered solution. Whilst a significant reduction of protein adsorption to a thiolated dextran layer coated surface compared to an uncoated surface was observed, the degree of conversion of hydroxyls to thiol groups and molecular weight was shown to affect the protein-resistant performance of the dextran layer.

A scanning tunneling microscopy comparison of passive antibody adsorption and biotinylated antibody linkage to streptavidin on microtiter wells

An antiferritin antibody was either, (a) passively adsorbed to microwells or (b) biotinylated and immobilised to streptavidin coated microwells. Scanning tunnelling microscope (STM) imaging of these well surfaces coated with a platinum (95%) carbon (5%) coating (Pt/C) conductive layer showed a randomly oriented array of antibodies for passive adsorption whereas for biotin-streptavidin immobilisation there was a more uniform and even distribution of antibodies on the well surface. On further incubation with ferritin STM imaging showed that for passive adsorption approximately 5% of the surface was functional, while for the biotinylated antibody it was greater than 60%. The images presented in this paper show graphically the loss of functionality that occurs using passive adsorption and, conversely, the preservation of antibody functionality using the biotin-streptavidin linkage for antibody immobilisation. These results correlate well with the work of others in the field.

Scanning probe microscopy of biomedical interfaces

The development of the scanning probe microscopes over the past decade has provided a number of exciting new surface analytical techniques making a significant progress in the characterisation of biomedical interfaces. In this review, several examples are presented to illustrate that SPM is a powerful and promising tool for surface investigations including biomolecules, cell membranes, polymers and even living cells. The ability of the SPM instrument to monitor adhesion phenomena and provide quantitative information about intermolecular interactions is also described. Moreover, the huge potential of the scanning probe microscopes to study dynamic processes at interfaces under nearly physiological conditions is highlighted. Novel applications in the field of biochemistry, microbiology, biomaterial engineering, drug delivery and even medicine are discussed.

Direct atomic force microscopy observations of monovalent ion induced binding of DNA to mica.

Multivalent ions in solution are known to mediate attraction between two like‐charged molecules. Such attraction has proved useful in atomic force microscopy (AFM) where DNA may be immobilized to a mica surface facilitating direct imaging in liquid. Theories of DNA immobilization suggest that either ‘salt bridging’ or fluctuation in the positions of counter ions about both the mica surface and DNA backbone secure DNA to the mica substrate. Whilst both theoretical and experimental evidence suggest that immobilization is possible in the presence of divalent ions, very few studies identify that such immobilization is possible with monovalent ions. Here we present direct AFM evidence of DNA immobilized to mica in the presence of only monovalent ions. Our data depict E. coli plasmid pBR322 adsorbed onto the negatively charged mica both after short (10 min) and long (24 h) incubation periods. These data suggest the need to re‐explore current theories of like‐charge attraction to include the possibility of monovalent interactions. We suggest that this DNA immobilization strategy may offer the potential to image natural processes with limited immobilization forces and hence enable maximum conformational freedom of the immobilized biomolecule.

Surface damage of sputtered gold films at the high and low gap resistance settings of a scanning tunnelling microscope

Abstract The use of a scanning tunnelling microscope to alter surface morphology is now established for a number of materials from the atomic scale to the nanometer. Presented here is a novel technique utilizing the field emission mode of the microscope at high bias voltages (i.e., high gap resistances) for the creation of stable features on gold films after the removal of surface material. Further investigations reveal that damage to the gold films may also occur during normal scanning at low, but not unusual, gap resistances. This value was dependent upon the grain size of the film and varied between approximately 100 and 10 M ohm for grain diameters between 3 and 12 nm. This second and separate surface damage phenomena is ascribed to a sweeping effect of the tip as demonstrated by the controlled removal of a gold island at a specific gap resistance.

Observation of a super‐periodic feature on gold with a scanning tunneling microscope

In this letter we present the first reported images of a super‐periodic hexagonal lattice observed on gold in air by scanning tunneling microscopy. The surface features presented extend over large areas (up to 1.0×105 nm2) with an average periodicity of 8 nm, and a corrugation of 1 nm. The impact of comparable results on highly oriented pyrolytic graphite suggests the need for some caution when using gold in biomolecular scanning tunneling microscopy studies.

Controlled nanometre-scale line and symbol formation on graphite in air using a scanning tunnelling microscope

The authors demonstrate the first reported production of etched lines on a highly oriented pyrolytic graphite surface using a scanning tunnelling microscope. This has been achieved by moving the imaging tip, biased at a minimum of 3.5 V of either polarity, in a controlled fashion over the substrate at a velocity of 10 nm s-1 or greater. The potential of the technique to control the width and depth of the lines is investigated. The resultant etched structures may be used to evaluate the performance of the piezo-crystal tube and tip. The potential of this technique in the study of bio-molecules is noted.

Controlled surface damage using a scanning tunnelling microscope on platinum and platinum-carbon films and the in-situ correction of tip defects

Abstract The use of a scanning tunnelling microscope to alter surface morphology from the nanometer to the atomic scale has been described for a number of materials. Here a technique for the creation of stable features on platinum and platinum-carbon films is demonstrated. This is achieved by applying tunnelling biases exceeding ±2.9-3.1 V and ±3.2-3.4 V for the platinum and platinum-carbon films, respectively. Moving a tip which is highly biased generally results in trenches being created in the surface, although, in the case of the platinum-carbon films, material may also be deposited. These effects are ascribed to an electrostatic interaction between the tip and the surface. We also identify the presence of positive and negative contrast artefacts on a platinum film and discuss the possible application of this work to studies of biomolecules. In addition, we demonstrate the correction of a tip defect in-situ through the application of high-voltage biases.

Visualization of the Surface Degradation of Biomedical Polymers in Situ with an Atomic Force Microscope

The ability of the atomic force microscope (AFM) to observe dynamic polymer/liquid interfaces has been utilized to visualize in situ morphological changes occurring during the biodegradation of polymer surfaces. Using this technique we demonstrate the differential rate of degradation of amorphous and crystalline material and study the kinetics of erosion of an immiscible polymer blend. The use of atomic force microscopy in this applied area of research is furthering our knowledge of the importance of the relationship between surface morphology and degradation kinetics and promises to become an invaluable tool in the evaluation of novel biodegradable materials.

Scanning Tunneling Microscopy Studies on Xanthan Gum

The application of the scanning tunneling microscope (STM) to imaging biological molecules has generated much interest over recent years. Research has mainly concentrated on obtaining images of DNA, proteins and small organic molecules. However, the work we present concerns polysaccharides, specifically xanthan gum, commonly used in the food and oil industries. We have employed high-resolution metallic shadowing to overcome some of the problems associated with imaging biological molecules by STM, such as tip-sample interaction effects and lack of conduction. The images displayed demonstrate the ability of the STM to image replicas of discrete polysaccharide molecules. We directly correlate and compare these results with electron micrographs of similarly prepared molecules. In addition, the effect of different sample deposition techniques and substrates on the resultant images of xanthan gum is demonstrated. We discuss the implications for sample preparation procedures for future STM studies on biological molecules.