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Posted: Feb 18, 2009
Cutting edge nanotechnology
(Nanowerk Spotlight) Back in 2006, researchers introduced the concept of a carbon nanotube (CNT) knife that, in theory, would work like a tight-wire cheese slicer. In the meantime, other research groups have developed similar approaches, for instance for cutting and sharpening carbon nanotubes (see for instance: Nanotechnology grinders). Now, the group that introduced the CNT nanoknife in 2006 has refined their design and demonstrated the feasibility of fabricating a nanoknife (compression-cutting tool at the nanoscale) based on an individual CNT.
"We stretched an individual nanotube between two tungsten needles in a manner that allowed us to test the mechanical strength of assembled device" Gurpreet Singh tells Nanowerk. "A force test on the prototype nanoknife indicated that failure was at the weld while the CNT was unaffected by the force we applied. In situ load tests on the nanoknife indicated maximum breaking force to be in micro Newton range."
Singh explains that, in biotechnology and medical studies, the three-dimensional cryo-electron microscopy of frozen–hydrated samples is important for structural and functional studies of cells.
"An intrinsic problem to cutting sections of frozen–hydrated samples with conventional diamond or glass knives is that the angle included at the knife’s cutting edge bends the sections sharply away from the block face, inducing compressive stresses on the upper surface of the section relative to the bottom, which leads to cracking of the sample surface when it is laid flat," he says. "A possible solution to this problem is to use a multi- walled carbon nanotube in place of a conventional diamond knife. This device would reduce the angle by which the sample is bent during cutting, due to the small diameter of a carbon nanotube."
In their recent work, Singh and collaborators, Professor J Richard McIntosh and Paul Rice from the University of Colorado at Boulder and Professor Roop L Mahajan from the Virginia Polytechnic Institute and State University describe initial steps towards the development of a CNT-based knife.
SEM micrograph showing a mechanical bending test of the nanotube being pushed by the AFM tip. (Image: Dr. Singh, Virginia Tech)
The team has demonstrated pick and place of individual nanotubes, followed by mechanical bending tests performed in a conventional electron microscope using an atomic force microscope tip. They found that the maximum force taken by the device before failure is in close range with the actual cutting forces observed in a microtome and is currently limited only by the poor strength of the amorphous carbon weld.
Singh points out that most electron microscopy (EM) of cells is carried out on samples that have been treated in ways that sometimes may cast doubt on the reliability of the information obtained: cells are treated with chemical cross-linkers (fixatives), suffused with organic solvents to replace cellular water, and then with a plastic resin, so they can be cut with a diamond knife to a suitable thickness (<100 nm) and withstand the vacuum of the EM.
"These treatments, plus the heavy metal stains that are used to give contrast, introduce concern about the validity of the structures seen."
In contrast, a tightly strung metal wire is an effective cutting tool, so long as the wire and its support are strong and the material is not too hard (think cheese). "One can imagine that CNTs could cut vitreous water, vitrified cells, and other comparatively soft materials in the same way" says Singh. "Maybe, one day, similar devices might even be used for nanosurgery."
Since Epon resin is commonly used to embed cell biological samples that have been fixed and dehydrated in preparation for microtomy and then transmission electron microscopy, Singh's team performed cutting experiments with their nanoknife on a sharp, gold-coated Epon block.
Although the block clearly showed indentation mark due to the nanotube, characterization of the cutting process has been limited by the lack of high resolution imaging of the polymeric specimen in the SEM, which made it difficult to locate a small cut or mark.
Cutting process being carried out by the nanoknife. (Image: Dr. Singh, Virginia Tech)
According to Singh, the surface roughness of specimen, which is of the same order as nanotube diameter, also contributes to the problem.
Another problem is that the current method for fabricating nanoknives is time consuming and expensive, due to the uncertainties involved in nanotube separation, welding and attachment to the needles.
"We will address these problems in future experiments, in which we plan to replace the glass support with a variable gap silicon microelectromechanical system (MEMS) type structure to prepare tightly strung nanotubes," explains Singh. "We plan to use a high-resolution focused ion beam (FIB) SEM with an in-built micromanipulator to pick and place the nanotubes. Welding of nanotubes to MEMS will be performed by injecting metal organic precursors in a controllable manner; strong welds can thus be formed by depositing material of choice, such as platinum."