An atomic force microscope that can sense forces equivalent to the weight of an individual virus has been developed by a team of physicists from the Australian National University (ANU). The technique used to obtain such a high level of sensitivity uses a laser to cool the nanowire probes in the microscope to -256°C, which minimises vibrations. The research has been described in a paper published on 14 August in Nature Communications.
Atomic force microscopes achieve high sensitivity measurements of microscopic features by scanning a wire probe over a surface. However, the probes, around 500 times finer than a human hair, are prone to vibration. ‘At room temperature the probe vibrates, just because it is warm, and this can make your measurements noisy,’ said Dr Ben Buchler, a co-author of the research.
The new method developed by at the Quantum Optics Group of the Research School of Physics and Engineering at ANU, however, uses a laser to prevent the probes from vibrating. ‘We can stop this motion by shining lasers at the probe,’ said Giovanni Guccione, a PhD student at ANU.
In this way, the resolution of these atomic-force microscopes, which are a primary tool for measuring nanoscopic structures and the tiny forces between molecules, is improved considerably. ‘The level of sensitivity achieved after cooling is accurate enough for us to sense the weight of a large virus that is 100 billion times lighter than a mosquito,’ said Professor Ping Koy Lam, the leader of the Quantum Optics Group.
The ANU team first used a 200nm-wide silver gallium nanowire coated with gold as force sensor. The laser was then used to control the vibration of the probe. ‘The laser makes the probe warp and move due to heat. But we have learned to control this warping effect and were able to use the effect to counter the thermal vibration of the probe,’ explained Guccione.
However, the probe cannot be used while the laser is on as the laser effect overwhelms the sensitive probe. So, the laser is turned off and the measurements are obtained within a few milliseconds before the probe heats up. By taking measurements over a number of cycles of heating and cooling, an accurate value can be found.
‘We now understand this cooling effect really well,’ said PhD student Harry Slatyer. ‘With clever data processing we might be able to improve the sensitivity, and even eliminate the need for a cooling laser.’