The detection of biological samples with nanocantilevers is not as simple as was previously thought
Micro- and nanocantilevers have been used to detect bacteria, viruses and DNA molecules, often at the level of single objects, and have shown great potential for a variety of sensing applications in pharmaceutical and biological research. In general, the detection process relies on the extra mass of the sample shifting the resonance frequency of the cantilever to lower values. However, new research by Javier Tamayo of the IMM-CNM laboratory in Madrid and co-workers suggests that other effects could be equally important1.
Tamayo and colleagues dipped silicon cantilevers in a suspension of E. coli, dried them and then measured the resonance frequency. They found that, on average, the resonance frequency decreased by a small amount (1%) as expected. However, in 24% of their measurements the frequency actually increased. The Spanish team argue that this is caused by the stiffness of the bacteria making the cantilever more rigid, and therefore shifting the resonance to higher frequencies. This view is supported by theoretical results reported in a second paper2.
It is possible to disentangle the two effects by controlling the distribution of the bacteria on the cantilever because the mass effect is greatest when the bacteria are concentrated at the free end of the cantilever, whereas the rigidity effect is larger when they are at the other end.
Ramos, D., Tamayo, J., Mertens, J., Calleja, M. & Zaballos, A. Origin of the response of nanomechanical resonators to bacteria adsorption. J. Appl. Phys. 100, 10.1063/1.2370507 106105 (2006).
Tamayo, J., Ramos, D., Mertens, J. & Calleja, M. Effect of the adsorbate stiffness on the resonance response of microcantilever sensors. Appl. Phys. Lett. 89, 10.1063/1.2388925 224104 (2006).
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Rodgers, P. Ups and downs for cantilevers. Nature Nanotech (2006). https://doi.org/10.1038/nnano.2006.197