nano 2007-11-23 10:49
AFM： Resonating probe pursues zeptogram sensitivity
[color=blue]【纳米科技世界快讯】Monitoring the resonance frequency shift of an atomic force microscope (AFM) cantilever is proving to be a very effective way of measuring mass adsorption. Now, scientists in Switzerland have shown that you can improve the sensitivity of the technique by two orders of magnitude by simply increasing the mode of vibration.[/color]
Hans Peter Lang (HPL) based jointly at IBM Research's Zurich laboratory and the University of Basel; and Murali Krishna Ghatkesar (MKG) from the University of Basel put nanotechweb.org in the picture.
[b]How does a cantilever-based mass sensor work and what has been done to improve the resolution of the technique?[/b]
[img]http://img208.imageshack.us/img208/3571/0611061f1be3bis2.jpg[/img]
[color=gray]To test their method, gold layers of different thickness were deposited on the upper side of six out of the eight cantilevers. (Credit: IBM Research, Zurich and the University of Basel)[/color]
[MKG] The cantilever responds to the added mass by decreasing its resonance frequency of vibration. Generally, mass sensitivity is increased by shrinking the inertial mass of the cantilever, but this makes the probe more difficult to handle and expensive to fabricate. Our approach was to operate the cantilevers at higher modes of vibration rather than to reduce their size. We found a linear increase in mass sensitivity with the square of the mode number.
[b]What do you see as the big benefits for such a device?[/b]
[HPL] One of the biggest advantages is the ability to detect masses in the nanogram range when operated in liquids, through to the zeptogram (1 × 10–21 g) range when placed in ultrahigh vacuum. Versatility is also a factor. As well as detecting mass, the probe can sense the structural (conformational) changes of adsorbing molecules in situ and in real-time. This is achieved by the simultaneous measurement of the cantilever's resonance frequency together with its static deflection (to capture any changes in interfacial stress). Finally, the cantilevers are very compact and straightforward to mass-produce using standard silicon microfabrication techniques, which means that the sensor can be integrated seamlessly into other structures.
[b]It's fascinating to see the scanning electron microscope (SEM) images that capture the cantilever's various modes of vibration. How difficult was this to achieve?[/b]
[img]http://img404.imageshack.us/img404/6175/0611062f27f0aqu2.jpg[/img]
[color=gray]SEM images showing the first three modes of vibration. (Credit: IBM Research, Zurich and the University of Basel)[/color]
[color=#808080][/color]
[HPL] Actually this was very easy. To achieve optimal actuation, the cantilever is glued onto the piezocrystal that drives the probe. An external function generator connected to the piezocrystal via the SEM's electrical feed-through supplies the signal. The mass probe will oscillate when the actuation frequency matches the resonance frequency of the cantilever. In an SEM, it always takes a few milliseconds until a picture is acquired. Within that time the cantilever undergoes many oscillations, which means that we will only observe the envelope of the oscillation, and that's what you see on the pictures.
[b]Why did you use a vertical cavity surface emitting laser (VCSEL) light source to illuminate the measurement probe?[/b]
[HPL] This goes back to the AFM, which uses optical beam deflection for position measurement. Since you only have to use one cantilever in an AFM, it is sufficient to use an ordinary laser diode. In our research, we work with an array of (typically eight) closely spaced cantilever sensors and therefore a compact, integrated and powerful light source is required. It turns out that VCSELs, which are actually used in repeaters for fiber-optic telecommunication networks, are perfect for the task.
[b]What are the next steps for you and your team?[/b]
[MKG] In terms of the design, we are now exploring various cantilevers to further increase the mass sensitivity of our sensor probe.
[HPL] Looking at applications, the largest scientific potential lies in the combination of dynamic and static modes of cantilever measurements. Taking biosensing as an example, the cantilever probe could be used to detect the mass of adsorbing proteins as well as the change in conformation when the molecules adhere to a surface. The method could be used in biochemistry to find out more about the processes taking place on lipid and cell membranes.
The researchers presented their work in [url=http://www.iop.org/EJ/abstract/0957-4484/18/44/445502][color=#0000ff][i]Nanotechnology[/i] [b]18[/b] 445502[/color][/url].
april811 2007-11-23 22:55
:victory: :victory: :victory: