Feb 12, 2009
CNT cheese wire cuts bio-samples to size
Many applications in biology and engineering demand very thinly sliced material. For example, biologists engaged in studying life at the cellular level often need samples that are 5–100 nm thick. These very thin samples are required to visualize nanometer-sized protein complexes in a transmission electron microscope (TEM). Most living cells are too thick for investigation directly in a TEM, so they must be sliced thin enough to be electron transparent.
For many years this problem has been solved by treating cells with chemicals, replacing cellular water with a liquid resin, hardening the resin and then using a glass or diamond knife to cut slices 20–500 nm thick. More recently, biologists have been developing ways to freeze their samples so that ice crystals do not form. These "vitrified" samples provide a more reliable preservation of the living state. Sections have been cut from such samples with a diamond knife, but the frozen cell is subjected to considerable pressure as the knife passes through; the resulting section bends sharply at the knife's edge and suffers from both compression and fissures at the section surface.
To get around these issues, researchers at CU-Boulder and the Institute for Critical Technology and Applied Science, US, are instead using an individual carbon nanotube (CNT) stretched between two sharpened tungsten needles as a prototype nanoknife that could ultimately cut frozen cells without distortions. The idea is a nanoscale equivalent of a tight-wire cheese slicer or a compression cutting tool – the thin diameter and high mechanical strength of a CNT should allow it to press through a frozen cell without major displacement of either the vitrified water or the components of the cell.
The scientists used individual multi-walled CNTs to allow visualization and easy manipulation of the fibers in a conventional scanning electron microscope. The mechanical strength and cutting abilities of the nanoknife were studied using a micromanipulation system fitted in the SEM.
Looking at the prototype in detail, the principal weakness was found to be the weld between the CNT and the tungsten wires. Nonetheless, the resulting device could withstand ~1 µN (the force required to cut small, but useful slices of cells) and the result points towards a new generation of cutting devices constructed from CNTs. Such nanoknives should have a variety of uses in nanofabrication across biological and engineering fields.
About the author
Gurpreet Singh received his PhD in mechanical engineering from the University of Colorado at Boulder (CU-Boulder) and the work presented here is part of his doctoral studies. He is currently working as a postdoctoral associate at the Institute for Critical Technology and Applied Science (ICTAS) at the Virginia Polytechnic Institute and State University while he is looking for faculty positions at other universities. Senior author Prof. J R McIntosh is a university distinguished professor (Emeritus) in the MCD-Biology department at CU-Boulder. Co-author, Prof. Roop L Mahajan is the director of ICTAS and Tucker chair professor of engineering at the Virginia Polytechnic Institute and State University. Paul Rice is the director of microscopy lab. at NCF, CU-Boulder.