A Low-voltage electron microscope (LVEM) is an

electron microscope An electron microscope is a microscope that uses a beam of electrons as a source of illumination. It uses electron optics that are analogous to the glass lenses of an optical light microscope to control the electron beam, for instance focusing it ...

which operates at accelerating voltages of a few kilo

electronvolt In physics, an electronvolt (symbol eV), also written electron-volt and electron volt, is the measure of an amount of kinetic energy gained by a single electron accelerating through an Voltage, electric potential difference of one volt in vacuum ...

s (keV) or less. Traditional electron microscopes use accelerating voltages in the range of 10-1000 keV. Low voltage imaging in transmitted electrons is possible in many new scanning electron detectors. A low cost alternative is a dedicated tabletop low voltage

transmission electron microscope Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a gr ...

. While its architecture is very similar to a conventional transmission electron microscope, it has a few key differences that enable it to take advantage of a 5 keV electron source, but trading off many advantages of higher voltage operations, including higher resolution, the possibility of X-ray microanalysis and

EELS Eels are ray-finned fish belonging to the order (biology), order Anguilliformes (), which consists of eight suborders, 20 Family (biology), families, 164 genus, genera, and about 1000 species. Eels undergo considerable development from the earl ...

, etc. Recently a new low voltage transmission electron microscope has been introduced that operates at variable voltage ranges between 6–25 kV.

Advantages

Higher contrast

A substantial decrease in electron energy allows for a significant improvement of contrast of light elements. The comparison images below show that decreasing the acceleration voltage from 80 kV to 5 kV significantly enhances the contrast of test samples. The improved contrast is a direct result of increased electron scattering associated with a reduced accelerating voltage. LVEM brings an enhancement of imaging contrast nearly twenty times higher than for 100 kV. This is very promising for biological specimens which are composed of light elements and do not exhibit sufficient contrast in classical TEMs. Further, a relatively low mean free path (15 nm) for organic samples at 5 kV means that for samples with constant thickness, high contrast will be obtained from small variations in density. For example, for 5% contrast in the LVEM bright field image, we only need a difference in density between the phases of 0.07 g/cm³. This means that the usual need to stain polymers for enhanced contrast in the TEM (typically done with

osmium Osmium () is a chemical element; it has Symbol (chemistry), symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a Abundance of elements in Earth's crust, trace element in a ...

or ruthenium tetroxide) may not be necessary with the low voltage electron microscopy technique.

Stain not required

The improved contrast allows for the significant reduction, or elimination, of the heavy metal

negative stain In microscopy, negative staining is an established method, often used in diagnostic microscopy, for contrasting a thin specimen with an optically opaque fluid. In this technique, the background is stained, leaving the actual specimen untouched, ...

ing step for TEM imaging of light elements (H, C, N, O, S, P). While staining is beneficial for experiments aimed at high resolution structure determination, it is highly undesirable in certain protein sample preparations, because it could destabilize the protein sample due to its acid pH and relatively high heavy metal concentration. The addition of stain to sectioned samples such as biological materials or polymers can also introduce imaging artifacts. LVEM experiments carried out on an extracted membrane protein sample that was analyzed with and without the staining procedure show a marked improvement in the appearance of the sample when standard staining is omitted. Results show that LVEM could be even more useful than conventional EM for this particular application because it avoids the potentially disrupting staining step, thus providing an undisturbed image of the protein's aggregation state. Additionally, The ability to eliminate the staining step could aid to improve safety in the lab, as common heavy metal stains, such as

uranyl acetate Uranyl acetate is the acetate salt of uranium oxide, a toxic yellow-green powder useful in certain laboratory tests. Structurally, it is a coordination polymer with formula UO2(CH3CO2)2(H2O)·H2O. Structure left, 260px, Structure (from X-ray ...

do have associated health risks.

Resolution

The first low-voltage electron microscopes were capable of spatial resolutions of about 2.5 nm in TEM, 2.0 nm in STEM, and 3.0 nm in SEM modes. The SEM resolution has been improved to ~1.2 nm at 800 eV by 2010, while a 0.14 nm TEM resolution at 15 keV has been reported in 2016.

Limitations

Currently available low voltage microscopes are only able to obtain resolutions of 1–3 nanometers (nm). While this is well beyond resolutions possible from optical (light) microscopes, they are not yet able to compete with the atomic resolution obtainable from conventional (higher voltage) electron microscopes. Low voltage limits the maximum thickness of samples which can be studied in the TEM or STEM mode. Whereas it is about 50–90 nm in conventional TEM, it decreases to around 20–65 nm for LVEM at 5 kV. However, thicknesses of the order of 20 nm or less are required to attain the maximal resolution in the TEM and STEM modes 5 kV. These thickness are sometimes achievable with the use of an ultramicrotome. in 2015 these limitations were overcome with a 25 kV low voltage electron microscope that can produce high quality results with thin sectioned samples up to around 100 nm+.

Application areas

LVEM is especially efficient for the following applications. *

Antibodies An antibody (Ab) or immunoglobulin (Ig) is a large, Y-shaped protein belonging to the immunoglobulin superfamily which is used by the immune system to identify and neutralize antigens such as bacteria and viruses, including those that caus ...

Cell biology Cell biology (also cellular biology or cytology) is a branch of biology that studies the structure, function, and behavior of cells. All living organisms are made of cells. A cell is the basic unit of life that is responsible for the living an ...

Drug discovery In the fields of medicine, biotechnology, and pharmacology, drug discovery is the process by which new candidate medications are discovered. Historically, drugs were discovered by identifying the active ingredient from traditional remedies or ...

Education Education is the transmission of knowledge and skills and the development of character traits. Formal education occurs within a structured institutional framework, such as public schools, following a curriculum. Non-formal education als ...

Histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology that studies the microscopic anatomy of biological tissue (biology), tissues. Histology is the microscopic counterpart to gross anatomy, which looks at large ...

Materials science Materials science is an interdisciplinary field of researching and discovering materials. Materials engineering is an engineering field of finding uses for materials in other fields and industries. The intellectual origins of materials sci ...

Nanomedicine Nanomedicine is the medical application of nanotechnology, translating historic nanoscience insights and inventions into practical application. Nanomedicine ranges from the medical applications of nanomaterials and biological devices, to n ...

Nanoparticles A nanoparticle or ultrafine particle is a particle of matter 1 to 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 100 nm in only two directions. At ...

* Nanotubes *

Pathology Pathology is the study of disease. The word ''pathology'' also refers to the study of disease in general, incorporating a wide range of biology research fields and medical practices. However, when used in the context of modern medical treatme ...

Polymers A polymer () is a substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeating subunits derived from one or more species of monomers. Due to their broad spectrum of properties, b ...

Proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, re ...

*Tissue samples *

Toxicology Toxicology is a scientific discipline, overlapping with biology, chemistry, pharmacology, and medicine, that involves the study of the adverse effects of chemical substances on living organisms and the practice of diagnosing and treating ex ...

Viruses A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are found in almo ...

References

External links

WENDMANs VIEWS on NANOTECH BlogLVEM5 low voltage electron microscope from Delong America
{{DEFAULTSORT:Low Voltage Electron Microscope Electron microscopy Scientific techniques