High-definition images of minute objects are standard these days including the imaging of bacteria and viruses, and even molecules and individual atoms in extremely fine details. Atomic force microscopy, which involves bringing a vibrating tip into contact with or close to the sample surface, is often used for this purpose. Until now, however, the choice was between fast imaging techniques, which run the risk of destroying sensitive samples, and gentle imaging techniques that take more time.
Now, researchers from the Faculty of Electrical Engineering and Information Technology at TU Wien have succeeded in finding a way around this dilemma: instead of the previous standard method, which just involves directly vibrating a tiny arm with a fine tip, this method involves vibrating a plate with a smaller customised arm with an ultra-fine tip attached to it. By skilfully connecting the two components, the possible measuring speed can be increased to such an extent that even videos of sensitive objects should be possible, such as living cells that are in the process of reacting to a medication. TU Wien has already applied for a patent for the microstructure consisting of a plate and an arm that is intended to form the core of advanced future atomic force microscopes.
Feeling an image point by point
“In an atomic force microscope, we use a tiny arm, known as a cantilever, which is just a few micrometres in size. When the cantilever is vibrated at its resonant frequency, it vibrates extremely quickly, typically a few hundred thousand times per second,” says Prof. Ulrich Schmid from the Institute of Sensor and Actuator Systems. The cantilever has an ultra-fine tip attached to it. When it comes very close to the surface of the sample, a force acts between the sample and the vibrating tip at the atomic level. This changes the vibrational movement of the cantilever; it vibrates in a slightly different way and this change is measured.
This measurement must be carried out point by point; a new measurement result is obtained for each part of the sample and all of these numerical values must then be combined into an image on the computer. In practice, the crucial factor is how long it takes to create such an image. If you want to image the object in a short amount of time, you need to use a cantilever that vibrates at high speed. “You can achieve this by making the cantilever more and more mechanically stiff,” explains Ulrich Schmid. “However, the problem with this is that the stiffer and less flexible the vibrating part of the measuring apparatus is, the more likely it is to destroy the sample. For this reason, many biological samples could only previously be imaged using particularly gentle techniques, which take more time accordingly.” It was therefore not previously possible to observe samples like this over a short time scale and visualise rapid changes.
Vibrating plate, gentle measurement
Researchers have now found a way to circumvent this problem using a physical trick, namely by attaching the cantilever to a small plate. Here, the plate vibrates in its resonant frequency instead of the cantilever. Certain resonant frequencies of the plate can be stimulated in a very specific way, causing the cantilever to passively move with it in the process. This means that the cantilever itself can be much softer than before, even at higher vibrational frequencies.
“We have produced a connected vibrating system consisting of cantilever and vibrating plate, which is far less stiff than a cantilever on its own even at high frequencies when vibrating at its own frequency,” explains Jonas Hafner, “and less stiff means less destructive for sensitive samples.”
The new measuring sensor works in liquids, which is particularly important for biological probes. A patent is already pending for the microstructure – this was applied for with the assistance of Research and Transfer Support at TU Wien, and the first important results have also been presented in two scientific publications.
The new sensing element can display its strengths anywhere where high measuring speeds are to be achieved for soft materials or surfaces. In future, it is intended to be used for biological samples or biochemical processes, but it could also play an important role in material research, such as for fragile polymer structures for example.
These research activities were supported by the project SPM2.0 (Scanning probe microscopies for nanoscale fast, tomographic and composition imaging) as part of a Marie Curie Skłodowska European Training Network (https://spm20.eu/).
Prof. Ulrich Schmid
Institut für Sensor- und Aktuatorsysteme
Technische Universität Wien
Gußhausstraße 27-29, 1040 Wien