Parametric resonance based mass sensing under ambient and liquid conditions

The parametric excitation of a cantilever for AFM applications using a closed-loop electronic feedback has been previously discussed [1, 2]. The parametrically excited cantilever enables the detection of small frequency shifts due to its sharp, controllable and non-Lorentzian resonance peak, making it a suitable detector for mass sensing applications. By attaching a small particle of hygroscopic material at the free end of a cantilever, the mass can be controllably changed as the humidity is varied. The increase in mass due to the adsorbed water causes a small downshift in the resonance frequency of the cantilever that can easily be detected because of the narrow resonance peak under parametric excitation. Using a commercial cantilever, the smallest mass change observed is of the order 1 x 10$^{-12}$ grams under ambient conditions. Efforts to extend this work to improve dynamic sensing under liquids will also be described. \\[4pt] [1] M. Moreno-Moreno \textit{et al.}, Appl. Phys. Lett. \textbf{88}, 193108 (2006). \\[0pt] [2] G. Prakash \textit{et al.}, Phys. Rev. B, \textbf{79}, 094304 (2009).