X-ray microscopy of biological objects with carbon kappa and with synchrotron radiation

Correlative Cryo-imaging Using Soft X-Ray Tomography for the Study of Virus Biology in Cells and Tissues

Viruses are obligate intracellular pathogens that depend on their host cell machinery and metabolism for their replicative life cycle. Virus entry, replication, and assembly are dynamic processes that lead to the reorganisation of host cell components. Therefore, a complete understanding of the viral processes requires their study in the cellular context where advanced imaging has been proven valuable in providing the necessary information. Among the available imaging techniques, soft X-ray tomography (SXT) at cryogenic temperatures can provide three-dimensional mapping to 25 nm resolution and is ideally suited to visualise the internal organisation of virus-infected cells. In this chapter, the principles and practices of synchrotron-based cryo-soft X-ray tomography (cryo-SXT) in virus research are presented. The potential of the cryo-SXT in correlative microscopy platforms is also demonstrated through working examples of reovirus and hepatitis research at Beamline B24 (Diamond Light Source Synchrotron, UK) and BL09-Mistral beamline (ALBA Synchrotron, Spain), respectively.

Nanometer-Resolution Imaging of Living Cells Using Soft X-ray Contact Microscopy

Soft X-ray microscopy is a powerful technique for imaging cells with nanometer resolution in their native state without chemical fixation, staining, or sectioning. The studies performed in several laboratories have demonstrated the potential of applying this technique for imaging the internal structures of intact cells. However, it is currently used mainly on synchrotrons with restricted access. Moreover, the operation of these instruments and the associated sample-preparation protocols require interdisciplinary and highly specialized personnel, limiting their wide application in practice. This is why soft X-ray microscopy is not commonly used in biological laboratories as an imaging tool. Thus, a laboratory-based and user-friendly soft X-ray contact microscope would facilitate the work of biologists. A compact, desk-top laboratory setup for soft X-ray contact microscopy (SXCM) based on a laser-plasma soft X-ray source, which can be used in any biological laboratory, together with several applications for biological imaging, are described. Moreover, the perspectives of the correlation of SXCM with other super-resolution imaging techniques based on the current literature are discussed.

A guide into the world of high-resolution 3D imaging: the case of soft X-ray tomography for the life sciences

In the world of bioimaging, every choice made determines the quality and content of the data collected. The choice of imaging techniques for a study could showcase or dampen expected outcomes. Synchrotron radiation is indispensable for biomedical research, driven by the need to see into biological materials and capture intricate biochemical and biophysical details at controlled environments. The same need drives correlative approaches that enable the capture of heterologous but complementary information when studying any one single target subject. Recently, the applicability of one such synchrotron technique in bioimaging, soft X-ray tomography (SXT), facilitates exploratory and basic research and is actively progressing towards filling medical and industrial needs for the rapid screening of biomaterials, reagents and processes of immediate medical significance. Soft X-ray tomography at cryogenic temperatures (cryoSXT) fills the imaging resolution gap between fluorescence microscopy (in the hundreds of nanometers but relatively accessible) and electron microscopy (few nanometers but requires extensive effort and can be difficult to access). CryoSXT currently is accessible, fully documented, can deliver 3D imaging to 25 nm resolution in a high throughput fashion, does not require laborious sample preparation procedures and can be correlated with other imaging techniques. Here, we present the current state of SXT and outline its place within the bioimaging world alongside a guided matrix that aids decision making with regards to the applicability of any given imaging technique to a particular project. Case studies where cryoSXT has facilitated a better understanding of biological processes are highlighted and future directions are discussed.

Fabrication of 200 nanometer period centimeter area hard x-ray absorption gratings by multilayer deposition

We describe the design and fabrication trials of x-ray absorption gratings of 200 nm period and up to 100:1 depth-to-period ratios for full-field hard x-ray imaging applications. Hard x-ray phase-contrast imaging relies on gratings of ultra-small periods and sufficient depth to achieve high sensitivity. Current grating designs utilize lithographic processes to produce periodic vertical structures, where grating periods below 2.0 μm are difficult due to the extreme aspect ratios of the structures. In our design, multiple bilayers of x-ray transparent and opaque materials are deposited on a staircase substrate, and mostly on the floor surfaces of the steps only. When illuminated by an x-ray beam horizontally, the multilayer stack on each step functions as a micro-grating whose grating period is the thickness of a bilayer. The array of micro-gratings over the length of the staircase works as a single grating over a large area when continuity conditions are met. Since the layers can be nanometers thick and many microns wide, this design allows sub-micron grating periods and sufficient grating depth to modulate hard x-rays. We present the details of the fabrication process and diffraction profiles and contact radiography images showing successful intensity modulation of a 25 keV x-ray beam.

A numerical study of resolution and contrast in soft X-ray contact microscopy

We consider the case of soft X-ray contact microscopy using a laser-produced plasma. We model the effects of sample and resist absorption and diffraction as well as the process of isotropic development of the photoresist. Our results indicate that the micrograph resolution depends heavily on the exposure and the sample-to-resist distance. In addition, the contrast of small features depends crucially on the development procedure to the point where information on such features may be destroyed by excessive development. These issues must be kept in mind when interpreting contact microradiographs of high resolution, low contrast objects such as biological structures.

Soft X-ray microscopes and their biological applications

In this review we propose to address the question: for the life-science researcher, what does X-ray microscopy have to offer that is not otherwise easily available?

We will see that the answer depends on a combination of resolution, penetrating power, analytical sensitivity, compatibility with wet specimens, and the ease of image interpretation.

Microscopic imaging of cells

The microworld was revealed to investigators through a glass bead or a hanging water droplet long before optics was understood. The cellular structure of plants was well resolved by such simple magnifying glasses, van Leeuwenhoek, the Dutch merchant and amateur microscopist, was the first to report to the English Royal Society his observations of bacteria with his single-lens microscope in 1665.

Contact microscopy with synchrotron radiation

Soft X-ray contact microscopy with synchrotron radiation offers the biologist, and especially the microscopist, a way to morphologically study specimens that could not be imaged by conventional TEM, STEM, or SEM methods (i.e., hydrated samples, samples easily damaged by an electron beam, electron-dense samples, thick specimens, unstained, low-contrast specimens) at spatial resolutions approaching those of the TEM, with the additional possibility to obtain compositional (elemental) information about the sample as well. Although flash X-ray sources offer faster exposure times, synchrotron radiation provides a highly collimated, intense radiation that can be tuned to select specific discrete ranges of X-ray wavelengths or specific individual wavelengths that optimize imaging or microanalysis of a specific sample. This paper presents an overview of the applications of X-ray contact microscopy to biological research and some current research results using monochromatic synchrotron radiation to image biological samples.

Contact x-ray microscopy. A new technique for imaging cellular fine structure

Contact x-ray microscopy potentially allows living, wet cells to be visualized at a resolution of up to 100 A. Furthermore, differential absorption by specific elements permits the study of the distribution of those elements in biological specimens. In contact x-ray microscopy, soft x-rays (10 A to 100 A) pass through a biological sample and expose an underlying x-ray sensitive polymer (resist), producing an image that reflects the photon absorbance within the specimen. The high penetrating power of soft x-ray enables images to be obtained from specimens up to several microns thick. In this paper, the technique is described, some of the areas currently under study are considered, and biological examples of the use of contact x-ray microscopy are given.

Direct imaging of live human platelets by flash x-ray microscopy

A 100-nanosecond pulse of long-wavelength x-rays was used to produce high-resolution stop-motion images of living human platelets. Although some aspects of the structure conform to those seen in dehydrated specimens, novel features are apparent. The technique should permit detailed stop-motion examination of the interaction of platelets with their surrounding medium as well as exploration of the phagocytic and secretory activities of a wide variety of other cells.

Improved detail in biological soft X-ray microscopy: study of blood platelets

Improved image quality in soft x-ray contact microscopy can be obtained by examining the resist with transmission rather than scanning electron microscopy. Application of the new technique to air-dried preparations of human blood platelets reveals structures not visible in the same cells with transmission electron microscopy or when the resist is examined by scanning electron microscopy. As seen by the new technique, platelet pseudopods contain a central structure connected to a network in the platelet and dense bodies exhibit a lamellar structure.

Soft x-ray lithographic studies of interphase chromosomes

Soft x-ray absorption lithograph patterns of purified interphase human nuclei and chromosome arrays, imaged on PMMA resists, were examined by scanning EM. The patterns obtained were compared to those utilizing more conventional sources, including transmission EM, scanning EM, high voltage EM, and various light microscopic techniques. The x-ray resist images revealed orderly arrays of absorption profiles in the 3-dimensional specimen with both mild and more extensive developments of the resist. Dense chromatin at the edge of interphase nuclei revealed aligned periodic peaks on the order of 2200 A diameter, with substructure. The periodicity and alignment of interphase chromosomes were entirely consistent with birefringent data on nuclei indicating a high degree of 3-dimensional order. This degree of 3-dimensional order was observed in nuclei containing essentially DNA and histones with only very few other minor (probably structural) proteins. Sonication and nuclease treatment to disperse interphase chromosomes revealed similar absorption periodicities in individual chromosome fibers. Analysis of x-ray absorption profiles thus appears to offer significant new insights into the ordered structure of these defined biological specimens.

Flash X-ray Microscopy

Soft x-ray contact microscopy, utilizing single-shot exposures of approximately 60 nanoseconds duration in polymethyl methacrylate, has been realized with a resolution of 300 angstroms. The radiation spectrum is intense in the "window" between 23 and 44 angstroms where water is transparent compared to biological materials, and therefore permits viewing of wet samples.

High resolution microchemical analysis using soft X-ray lithographic techniques

High resolution x-ray lithographic studies of cells from chick embryo hearts dried by the CO2 critical point method have been made with soft x-ray radiation of different wavelengths. A marked difference in the relief replica in polymethyl methacrylate (PMMA) resulting from the differential absorption by the dried cells of carbon K alpha radiation at 4.48 nm and broad band synchrotron radiation (SR) with lambda is greater than 1.5 nm demonstrates the potential usefulness of the technique in making high resolution (approximately or equal to 10 nm) chemical identification of the constitutents which make up the various parts of the cell.

Seeing with a New Light: Synchrotron Radiation

The special properties of synchrotron light are leading to a rapid increase in its utilization for both research and technology. At wavelength in the ultraviolet region of the broad spectrum in these beams a number of atomic, molecular, and solid-state spectroscopies are being pursued; soft x-rays are being used for spectroscopy, lithography, microscopy, and topography; at still shorter wavelengths, advantage is taken of scattering properties to probe the structure of matter. Characteristics of synchrotron radiation and of the sources which produce it are described, and some typical investigations and applications are presented to suggest the versatility of these sources.

Transmission microscropy of unmodified biological materials: comparative radiation dosages with electrons and ultrasoft x-ray photons

The minimum radiation dosage in a specimen consistent with transmission microscopy at resolution d and specimen thickness t is calculated for model specimens resembling biological materials in their natural state. The calculations cover 10(4)-10(7) eV electrons and 1.3-90 A photons in a number of microscopy modes. The results indicate that over a considerable part of the (t,d)-plane transmission microscopy on such specimens can be carried out at lower dosage with photons than with electrons. Estimates of the maximum resolutions obtainable with electrons and photons, consistent with structural survival of the specimen, are obtained, as are data on optimal operating conditions for microscopy with the two particles.

High-resolution soft x-ray microscopy

X-ray micrographs of biological materials have been obtained with a resolution better than 100 angstroms by using x-ray resist as the recording medium. A high-resolution scanning electron microscope with a short-focal-length final lens, operating in the "low-loss" mode, is used to make the smallest features in the x-ray replica visible.

Potential operating region for ultrasoft x-ray microscopy of biological materials

Calculations are presented which indicate an extensive suboptical region in the microscopy of biological materials in their natural state which is accessible to ultrasoft x-ray transmission microscopy. Throughout most of the region, radiation dosage levels to the specimen are lower than in electron microscopy.

Reflective multilayer coatings for the far uv region

Progress in the field of reflective multilayer coatings for the wavelength region between 50 A and 2000 A is reviewed. All the coatings contain absorbing materials, absorption losses are minimized by positioning strongly absorbing materials into the nodes of the standing wave inside the coating. Above lambda = 1200 A, ideal coatings with a reflectivity approaching 100% are theoretically possible; the theoretical predictions have been confirmed for coatings up to six layers at wavelengths around 2000 A. Below lambda = 1000 A, no absorption-free material is available that can be used as a spacer layer to cover the antinodes of the standing wave field. This limits the theoretically obtainable reflectivity. However, even at the shortest wavelength a reflectivity of 30% is still possible. Experimental results have been obtained for wavelengths between 100 A and 200 A for coatings up to nine layers. Discrepancies between experiment and theory can be explained as due to insufficient knowledge of the optical constants of the films used. Extensive future work on the optical constants of materials and their dependence on film thickness and deposition conditions is required for further improvement.