Spatial distribution of Auger electrons emitted from internalised radionuclides in cancer cells: the photoresist autoradiography (PAR) method

Microdosimetric evaluation of Auger electron-emitting radionuclides involves a detailed evaluation of energy deposition at a nanometre scale. To perform Monte Carlo modelling of such energy deposition, accurate information regarding the spatial distribution of the radionuclide is required. A recent addition to the methods for determining the spatial distribution of cellular internalised radionuclides is based on detection in a polymer photoresist (e.g. polymethyl methacralate), followed by atomic force microscopy analysis of the resultant 3D pattern. In comparison with present practice, the method offers greater spatial resolution and improved quantification. The volume of the pattern is proportional to the total dose, thereby permitting assessment of variability of accumulated activity, while the variation in depth across the pattern reflects the lateral spatial distribution in the local fluence per unit area. An added advantage is the similarity in response to ionising radiation of an organic polymer compared to that of biological material. A pattern in the resist from radiation emitted by a radionuclide treated cell gives additional spatial information about the energy deposited in the resist.

Alpha and recoil track detection in poly(methyl methacrylate) (PMMA)--towards a method for in vitro assessment of radiopharmaceuticals internalized in cancer cells

A method for detection and characterization of single MeV α-particle and recoil tracks in PMMA photoresist by atomic force microscopy (AFM) analysis has been demonstrated. The energy deposition along the track is shown to lead to a latent pattern in the resist due to contrast reversal. It has been shown that the pattern, consisting of conical spikes, can be developed by conventional processing as a result of the dissolution rate of poly(methyl methacrylate) (PMMA) being greater than that for the modified material in the cylindrical volume of the track core. The spikes can be imaged and counted by routine AFM analysis. Investigations by angular-resolved near-grazing incidence reveal additional tracks that correspond to recoil tracks. The observations have been correlated with modelling, and shown to be in qualitative agreement with prevailing descriptions of collision cascades. The results may be relevant to technologies that are based on detection and characterization of single energetic ions. In particular, the direct visualization of the collision cascade may allow more accurate estimates of the actual interaction volume, which in turn will permit more precise assessment of dose distribution of α-emitting radionuclides used for targeted radiotherapy. The results could also be relevant to other diagnostic or process technologies based on interaction of energetic ions with matter.

Dissolution Mechanisms of CaTiO3 - Solution Analysis, Surface Analysis and Electron Microscope Studies Implications for Synroc

The perovskite CaTiO3 is one of the major phases of the Synroc titanate mineral assemblage. Its chemical durability in an aqueous environment, which is relevant to the Synroc concept, has been investigated by solution analysis, surface analysis and electron microscopy. In general it has been found that dissolution due to base catalyzed hydrolysis is the most significant mechanism of attack; the results suggest that an ion exchange mechanism is confined to the first monolayer. Below 90°C the extent of attack, and release of Ca into solution, is limited by the formation of a titanaceous amorphous layer (” 100A thickness) which imposes a reaction constraint at the film-solid interface. Dissolution may also be constrained by the pH and Ca2+concentration in the bulk liquid. Above 90°C dissolution is relatively less constrained due to instability of the amorphous layer which is replaced by nucleation and epitaxial growth of TiO2 on the dissolving substrate. Thermodynamic stability of CaTiO3 for T< 90°C can easily be engineered into the waste repository, while hydrothermal stability (T > 90°C) is more difficult to achieve.

Two-dimensional stick-slip on a soft elastic polymer: pattern generation using atomic force microscopy

It has been demonstrated that it is possible to create laterally differentiated frictional patterning 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 PDMS surface exhibiting higher friction. With higher loading forces ([Formula: see text] 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 are manifested in the form of a series of shallow channels spaced evenly apart on the polymer surface. Detailed friction loop analysis acquired during the manipulation process showed that the lateral force changed according to the strength of trapping of the tip with the polymer surface exhibiting significant in-plane deformation due to lateral forces being imposed. An incremental increase in the initial loading force resulted in an increase in in-plane displacement and a greater spacing between the stick lines/channels in the slow-scan direction. A decrease in channel length in the fast-scan direction is 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.

Hierarchy of DNA immobilization and hybridization on poly-L-lysine using an atomic force microscopy study

The atomic force microscopy has been used to analyze the immobilization of single stranded DNA on poly-L-lysine-coated glass and subsequent hybridization with complimentary DNA with the Z-threshold parameter and fractal analysis methods. The poly-L-lysine layer, which has a thickness of approximately 7 nm, presents nano-defects that could be critical for DNA immobilization by acting as a nucleation sites for ssDNA and subsequently for dsDNA aggregates. The Z-threshold for the dsDNA aggregates is much larger than for ssDNA, but the statistical fractal dimension is very similar, suggesting a conformal increase of the dimensions of the dsDNA aggregates mainly in the Z-direction, due to an effective ssDNA-ccDNA molecular recognition. This study demonstrates the use of fractal analysis in conjunction with the distribution of heights to evaluate the efficiency of DNA-DNA molecular recognition on surfaces and the impact of nanodefects.

Interactions of poly(amino acids) in aqueous solution with charged model surfaces—analysis by colloidal probe

Biomolecules in a confined solution environment may be subject to electrostatic forces with a range up to 100 nm, while van der Waals interaction will account for shorter-range forces. The response of two model poly(amino acids)--poly-L-lysine and poly-L-glutamic acid--has been investigated for a silica/Si-oxide surface at pH 6. The model amino acids were adsorbed, or covalently coupled, to colloidal probes consisting of a microsphere attached to a force-sensing lever. The methodology was based on sensing interaction between the probe and a flat surface through carrying out force versus distance analysis with a scanning force microscope. The results were analyzed within the framework of the conventional DLVO theory. The outcomes illustrate both repulsive and attractive long-range interactions that will hinder, or promote, colloidal biospecies in solution entering the region of attractive short-range interactions at the physical interface. Large 'snap-on' distances were observed for some systems and have been ascribed to compression of the 'soft' functionalized layers. Those observations and measurements of adhesion provided insight into conformation of the adsorbed species and strength of attachment. The results have implications for the efficacy of methods and devices that seek to exploit the properties of micro/nano-fluidic systems.

Atomic force microscopy analysis of wool fibre surfaces in air and under water

Wool fibre surfaces have been treated by solvent cleaning which leaves the native covalently bound surface lipid layer intact, and by alcoholic alkali which removes the lipid layer. The resultant surfaces have been analysed by atomic force microscopy (AFM), with particular emphasis on force-distance (F-d) methods. Methodologies were developed for investigation in situ in water of both the surface topography and the characteristics of the lipid layer. Longitudinal surface texturing was resolved in images of wool fibre surfaces in air; the texturing remained prominent after exposure to water. High resolution F-d curves revealed features associated with the lipid layer. A simple formalism was used to show that the layer had a thickness of a few nm, and an effective stiffness of some 0. 12+/-0.01N/m. Strong adhesive interactions, equivalent to a pressure of 0.1MPa, acted on the tip at the tip-to-substrate interface. The methodology and formalism are likely to be relevant in the broad field of thin-film analysis and for fibre technology.

Imaging and force-distance analysis of human fibroblasts in vitro by atomic force microscopy

The structure of human fibroblasts have been characterised in vitro by atomic force microscopy (AFM) operated in the imaging or in the force versus distance (F-d) modes. The choice of cell substrate is important to ensure good adhesion. Of greater significance in the context of AFM analysis, is the observation that the substrate affects the imaging conditions for in vitro analysis of live cells. For instance, very rarely will glass coverslips lead to acceptable outcomes (i.e., resolved cytoskeletal structure). Activated tissue culture dishes, on the other hand, promote conditions that routinely result in good quality images. Those conditions are then unaffected by adoption of relatively high force loadings (more than 10 nN), large fields of view (100 x 100 microm2) and high scan speeds (up to ca. 200 microm/sec), all of which exceed values recommended in the literature. Plasma membranes are fragile in the context of AFM analysis (F-d analysis gives an equivalent Young's Modulus of ca. 5 kPa). However, the present work suggests that fragility per se need not be a problem, rather it is the adhesive interactions with the tip, which under some circumstances may exceed 20 nN, that are the source of poor imaging conditions. The present results, being supported by a qualitative model, suggest that the activated substrate acts as a preferential scavenger of cellular debris thus preventing the tip from biofouling, and will therefore promote low adhesion between tip and membrane. Good imaging conditions provide non-destructive in vitro information about cytoskeletal structure and dynamics, as shown in two examples concerned with cytochalasin treatment and with the MTT assay.

Interchromosomal DNA-containing fibres in human cells

Interchromosomal fibres were observed by light microscopy in metaphases of HeLa cells and human lymphocytes prepared by spreading on a water surface. Autoradiography after labelling with tritiated thymidine showed that the fibres contained DNA. They were resistant to pronase treatment.