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The field of nanotechnology is at the forefront of current scientific research and nanomaterials are finding use in such areas as drug development and medical research, environmental remediation, and electronics. Common nanomaterials include nanotubes, metal rubber, nanometals, quantum dots, and nanopores, nanoparticles and nanomaterials can exist in both nature or be synthesized.
Nanomaterials can be engineered for various applications, but it is critical for researchers to understand their dynamic, time-dependent behavior in a live setting. The properties and behavior of nanomaterials that can affect their reaction and effectiveness include nucleation and growth, phase transformations, and chemical reactions at their surfaces.
As nanomaterials exist at the nanomolecular scale, highly specialized techniques are necessary to image them and elicit information about their chemical and physical properties. Conventional optical microscopy is unable to resolve images of structures at this scale. Techniques utilized include Computed Tomography, Magnetic Resonance Imaging, Positron Emission Tomography, and various Electron Microscopy (EM) methods.
This article will provide a brief overview of EM and how it is used to investigate these “live” nanomaterials.
Electron Microscopy: An Overview
Invented in 1937, the electron microscope was a revolutionary invention that allowed researchers to peer into a world previously invisible to them. Rather than using light, the electron microscope uses a focused electron beam and allowed images of structures that existed beyond the diffraction limit of light to be imaged for the first time. Electron microscopes are powerful enough to see individual atoms.
There are two main types of electron microscopy: Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM.) The main difference between these two techniques is that in TEM, transmitted electrons (which pass through the sample) are used, whereas in SEM electrons which are reflected by the sample are used to create the desired image.
Problems with Using EM to Image Live Samples
As particles in the environment can easily absorb or reflect electrons, the sample under analysis has traditionally needed to be prepared and held in a vacuum, which means that only dead samples could be analyzed and imaged. This prevents the study of the dynamics of the sample in a live environment and does not provide enough information on the dynamic, time-sensitive properties of nanomaterials.
Another problem with using electron microscopy to study live materials is that the electron beam causes damage to the sample. Clearly, EM techniques had to be developed which could study these materials in their live state, whilst retaining the magnification and ability to reveal nanoscale structures that EM provides. Over the past decade, research has been carried out into developing techniques for this purpose.
LCTEM: One EM Technique for Studying Live Nanomaterials
Liquid-phase transmission electron microscopy (LCTEM)is a technique that makes it possible to use a transmission electron microscope to watch chemical reactions unfold in real-time.
The technique works by employing a liquid cell that is made of various materials. One such material is silicon nitride (SiNx); it is used to sandwich a thin (under 500nm) film of liquid, either water or organic solvent containing the target nanomaterial. The SiNx chips include observation windows (50 µm x 200 µm) and are sealed at the tip of a TEM holder. The liquid cell is then inserted into a side-loading microscope.
The electron beam, which is incident on the top observation window, passes through the liquid sample, through the bottom observation window, and is then imaged by the camera. By imaging the sample in this way, it is possible to observe the behavior and thereby elucidate properties of the nanomaterial or nanoparticle under analysis.
However, the extent and type of electron beam damage (especially important for, say, sensitive biological systems and materials) is still crucial as, by merely imaging them, the systems can be greatly perturbed.
One study on nanomaterials using a variant of LCTEM known as variable temperature-liquid phase TEM (VT-LCTEM) was carried out on metal-organic nanotubes (MONTs.) These are tunable porous nanotubular materials that combine metal ions/clusters with organic ligands. Properties of MONTs also include high specific surface area, and superior chemical and thermal stability.
MONTs have potential applications including nanoscaled electronic devices and biomedical sensors. By utilizing VT-LCTEM -the study also used complementary techniques such as single-crystal X-ray diffraction- researchers gained valuable insight into how these nanomaterials grow, form, and evolve, to understand their dynamics in real-time.
Another study published in Nature Communications used LCTEM to study self-assembling peptides, which has the potential for more insightful studies of live organic nanomaterials for such purposes as drug delivery.
Conclusion
Nanotechnology continues to be a cutting-edge field of scientific research. However, understanding the dynamic properties of nanomaterials has proven to be a stumbling block of traditional electron microscopy techniques. With the development of techniques such as LCTEM, scientists are now beginning to understand these factors much more clearly. Research is ongoing to develop this technique and others like it which will likely improve our understanding of these materials in the future and help develop more effective nanotechnological applications.
References and Further Reading
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