Topic Name: Scientists Developed Lensless X-ray Technique to View Nanoscale Materials and Biological Specimens
Category: Optical imaging
Research persons: Argonne National Laboratory Scientists
Location: Argonne National Laboratory, U.S. Department of Energy, United States
X-rays have been used for decades to take pictures of broken bones, but
scientists at the U.S. Department of
Energy's (DOE) Argonne National Laboratory
and their collaborators have developed a lensless X-ray technique that can take
images of ultra-small structures buried in nanoparticles and nanomaterials, and
features within whole biological cells such as cellular nuclei.
Argonne scientists along with scientists from the University
of California at Los Angeles, the University
of Melbourne, La Trobe
University and the Australian
Synchrotron developed a way to examine internal and buried structures in
micrometer-sized samples on the scale of nanometers. This is important to the
understanding of how materials behave electrically, magnetically and under
thermal and mechanical stress. Application of this capability to biology and
biomedicine could contribute to our understanding of disease and its
eradication, healing after injury, cancer and cell death.
X-rays are ideally suited for nanoscale imaging because of their ability to
penetrate the interior of the object, but their resolution has traditionally
been limited by lens technology. The new lensless technique being developed at
Argonne avoids this limitation.
“There is no lens involved at all,” said Ian McNulty, the lead Argonne
author on a new publication on this work appearing in the journal Physical
Review Letters. “Instead, a computer uses sophisticated algorithms to
reconstruct the image. We expect this technique will enhance our understanding
of many problems in materials and biological research.” The technique can be
extended beyond the current resolution of about 20 nanometers to image the
internal structure of micrometer-sized samples at finer resolution, reaching
deep into the nanometer scale.
Other types of microscopes, such as electron microscopes, can image
structural details on the nanometer scale, but once the sample reaches sizes of
a few micrometers and larger, the usefulness of these instruments to probe its
internal structure is limited. In many cases, only the surface of the sample can
be studied, or the sample must be sliced to view its interior, which can be
A collaborative team comprising members of the X-ray Microscopy and Imaging
Group at Argonne's Advanced
Photon Source (APS) and a team led by Professor John Miao at the University
of California at Los Angeles developed a powerful new extension of the new
lensless imaging technique that enables high resolution imaging of a specific
element buried inside a sample.
The key is the high intensity X-ray beams created at the APS at Argonne. An
intense, coherent X-ray beam collides with the sample, creating a diffraction
pattern which is recorded by a charge coupled device (CCD) camera. The X-ray
energy is tuned to an atomic resonance of a target element in the sample. Using
sophisticated phase-recovery algorithms, a computer reconstructs an image of the
specimen that highlights the presence of the element. The result is an image of
the internal architecture of the sample at nanometer resolution and without
destructive slicing. By using X-ray energies that coincide with an atomic
absorption edge, the imaging process can distinguish between different elements
in the sample.
If the nucleus or other parts of a cell are labeled with protein specific
tags, it can be imaged within whole cells at a resolution far greater than that
of ordinary microscopes.
Another application of this new method of imaging includes the burgeoning
field of nanoengineering, which endeavors to develop more efficient catalysts
for the petrochemical and energy industries and materials with electrically
programmable mechanical, thermal and other properties.
“There are only a handful of places in the world this can be done and APS
is the only place in the United States at these X-ray energies,” X-ray
Microscopy and Imaging Group Leader Qun Shen said. “We would eventually like
to create a dedicated, permanent laboratory facility at the APS for this imaging
technique that can be used by scientists on a routine basis.”
A dedicated facility would require building an additional beamline at the APS,
which currently has 34 sectors, each containing one or more beamlines.
Note for Nanoparticle
A nanoparticle (which historically has included nanopowder, nanocluster, and nanocrystal) is a small particle with at least one dimension less than 100 nm. This definition can be fleshed out further in order to remove ambiguity from future nano nomemeclature. A nanoparticle is an amorphous or semicrystalline zero dimensional (0D) nano structure with at least one dimension between 10 and 100nm and a relatively large (≥ 15%) size
dispersion. A nanocluster is an amorphous/semicrystalline nanostructure with at least one dimension being between 1-10nm and a narrow size
distribution. This distinction is an extension of the term "cluster" which is used in inorganic/organometallic chemistry to indicate small molecular cages of fixed sizes. A nanopowder is an agglomeration of noncrystalline nanostructural subunits with at least one dimention less than
100nm. A nanocrystal is any nanomaterial with at least one dimension ≤ 100nm and that is
singlecrystalline. Any particle which exhibits regions of crystllinity should be termed nanoparticle or nanocluster based on dimensions. Nanoparticle research is currently an area of intense scientific research, due to a wide variety of potential applications in biomedical, optical, and electronic fields. The National Nanotechnology Initiative of the United States government has driven huge amounts of state funding exclusively for nanoparticle research.
Nanoparticles are of great scientific interest as they are effectively a bridge between bulk materials and atomic or molecular structures. A bulk material should have constant physical properties regardless of its size, but at the nano-scale this is often not the case. Size-dependent properties are observed such as quantum confinement in semiconductor particles, surface plasmon resonance in some metal particles and superparamagnetism in magnetic materials.
The properties of materials change as their size approaches the nanoscale and as the percentage of atoms at the surface of a material becomes significant. For bulk materials larger than one micrometre the percentage of atoms at the surface is minuscule relative to the total number of atoms of the material. The interesting and sometimes unexpected properties of nanoparticles are partly due to the aspects of the surface of the material dominating the properties in lieu of the bulk properties.
Nanoparticles exhibit a number of special properties relative to bulk material. For example, the bending of bulk copper (wire, ribbon, etc.) occurs with movement of copper atoms/clusters at about the 50 nm scale. Copper nanoparticles smaller than 50 nm are considered super hard materials that do not exhibit the same malleability and ductility as bulk copper. The change in properties is not always desirable. Ferroelectric materials smaller than 10 nm can switch their magnetisation direction using room temperature thermal energy, thus making them useless for memory storage. Suspensions of nanoparticles are possible because the interaction of the particle surface with the solvent is strong enough to overcome differences in density, which usually result in a material either sinking or floating in a liquid. Nanoparticles often have unexpected visible properties because they are small enough to confine their electrons and produce quantum effects. For example gold nanoparticles appear deep red to black in solution.
Note for Electron Microscope
An electron microscope is a type of microscope that uses electrons to illuminate a specimen and create an enlarged image. Electron microscopes have much greater resolving power than light microscopes and can obtain much higher magnifications. Some electron microscopes can magnify specimens up to 2 million times, while the best light microscopes are limited to magnifications of 2000 times. Both electron and light microscopes create images with electromagnetic radiation, with their resolving power and magnification limited by the wavelength of the electromagnetic radiation being used to obtain the image. The greater resolution and magnification of the electron microscope is due to the wavelength of an electron being much smaller than that of a light photon.
The electron microscope uses electrostatic and electromagnetic lenses in forming the image by controlling the electron beam to focus it at a specific plane relative to the specimen in a manner similar to how a light microscope uses glass lenses to focus light on or through a specimen to form an image.
Electron microscopes are expensive to buy and maintain. They are dynamic rather than static in their operation: requiring extremely stable high-voltage supplies, extremely stable currents to each electromagnetic coil/lens, continuously-pumped high- or ultra-high-vacuum systems, and a cooling water supply circulation through the lenses and pumps. As they are very sensitive to vibration and external magnetic fields, microscopes aimed at achieving high resolutions must be housed in buildings (sometimes underground) with special services. Some desktop low voltage electron microscopes have TEM capabilities at very low voltages (around 5 kV) without stringent voltage supply, lens coil current, cooling water or vibration isolation requirements and as such are much less expensive to buy and far easier to install and maintain, but do not have the same ultra-high (atomic scale) resolution capabilities as the larger instruments.
The samples largely have to be viewed in vacuum, as the molecules that make up air would scatter the electrons. One exception is the environmental scanning electron microscope, which allows hydrated samples to be viewed in a low-pressure (up to 20 torr), wet environment.
Scanning electron microscopes usually image conductive or semi-conductive materials best. Non-conductive materials can be imaged by an environmental scanning electron microscope. A common preparation technique is to coat the sample with a several-nanometer layer of conductive material, such as gold, from a sputtering machine; however, this process has the potential to disturb delicate samples.
Note for X-ray Microscope
An X-ray microscope uses electromagnetic radiation in the soft X-ray band to produce images of very small objects.
Unlike visible light microscopes, X-rays do not reflect or refract easily, and they are invisible to the human eye. Therefore the basic process of an X-ray microscope is to expose film or use a charge-coupled device (CCD) detector to detect X-rays that pass through the specimen, rather than light which bounces off the specimen. It is a contrast imaging technology using the difference in absorption of soft x-ray in the water window region (wavelength region: 2.3 - 4.4 nm, photon energy region: 0.28 - 0.53 keV) by the carbon atom (main element composing the living cell) and the oxygen atom (main element for water).
Early X-ray microscopes by Kirkpatrick and Baez used grazing-incidence reflective optics to focus the X-rays, which grazed X-rays off parabolic curved mirrors at a very high angle of incidence. An alternative method of focusing X-rays is to use a tiny fresnel zone plate of concentric gold or nickel rings on a silicon dioxide substrate. Sir Lawrence Bragg produced some of the first usable X-ray images with his apparatus in the late 1940's.
The resolution of X-ray microscopy lies between that of the optical microscope and the electron microscope. It has an advantage over conventional electron microscopy in that it can view biological samples in their natural state. Electron microscopy is widely used to obtain images with nanometer level resolution but the relatively thick living cell cannot be observed as the sample has to be sliced thinly and then dried to get the image. However, it should be mentioned that cryo-electron microscopy allows the observation of biological specimens in their hydrated natural state. Until now, resolutions of 30 nanometer are possible using the Fresnel zone plate lens which forms the image using the soft x-rays emitted from a synchrotron. Recently, more researchers have begun to use the soft x-rays emitted from laser-produced plasma rather than synchrotron radiation.
Additionally, X-rays cause fluorescence in most materials, and these emissions can be analyzed to determine the chemical elements of an imaged object. Another use is to generate diffraction patterns, a process used in X-ray crystallography. By analyzing the internal reflections of a diffraction pattern (usually with a computer program), the three-dimensional structure of a crystal can be determined down to the placement of individual atoms within its molecules. X-ray microscopes are sometimes used for these analyses because the samples are too small to be analyzed in any other way.
Note for Charge-Coupled Device
A charge-coupled device (CCD) is an analog shift register, enabling analog signals (electric charges) to be transported through successive stages (capacitors) controlled by a clock signal. Charge coupled devices can be used as a form of memory or for delaying analog, sampled signals. Today, they are most widely used for serializing parallel analog signals, namely in arrays of photoelectric light sensors. This use is so predominant that in common parlance, "CCD" is (erroneously) used as a synonym for a type of image sensor even though, strictly speaking, "CCD" refers solely to the way that the image signal is read out from the chip.
The capacitor perspective is reflective of the history of the development of the CCD and also is indicative of its general mode of operation, with respect to readout, but attempts aimed at optimization of present CCD designs and structures tend towards consideration of the photodiode as the fundamental collecting unit of the CCD. Under the control of an external circuit, each capacitor can transfer its electric charge to one or other of its neighbors. CCDs are used in digital photography and astronomy (particularly in photometry, sensors, medical fluoroscopy, optical and UV spectroscopy and high speed techniques such as lucky imaging).
An image is projected by a lens on the capacitor array, causing each capacitor to accumulate an electric charge proportional to the light intensity at that location. A one-dimensional array, used in line-scan cameras, captures a single slice of the image, while a two-dimensional array, used in video and still cameras, captures the whole image or a rectangular portion of it. Once the array has been exposed to the image, a control circuit causes each capacitor to transfer its contents to its neighbor. The last capacitor in the array dumps its charge into a charge amplifier, which converts the charge into a voltage. By repeating this process, the controlling circuit converts the entire semiconductor contents of the array to a sequence of voltages, which it samples, digitizes and stores in some form of memory. These stored images can then be transferred to a printer, digital storage device or video display.
This research was funded by the Department of Energy's Office of Basic
Energy Sciences as part of its mission to foster and support fundamental
research to expand the scientific foundations for new and improved energy
technologies, and by the National
Argonne National Laboratory brings the world's brightest scientists and
engineers together to find exciting and creative new solutions to pressing
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Department of Energy's Office
In figure, Argonne scientists and collaborators used high energy X-rays from the Advanced Photon Source to create detailed images of nanoscale materials. The scientists are working to develop a dedicated facility for the process at Argonne.
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