Attogram Sensitivity Using
Nanoelectromechanical Resonant Mass Detectors
B. Ilic1,
H. G. Craighead1, S. Krylov3, W. Senaratne2,
C. Ober2, P. Neuzil4
1School of Applied Physics, Cornell Nanoscale Facility and Nanobiotechnology Center, 2Department of Materials Science, Cornell University, Ithaca, NY, 3Department of Solid Mechanics Materials and Systems, Tel Aviv University, ISRAEL, 4Institute of Bioengineering and Nanotechnology, Singapore
Interest in sensors and actuators and the availability of new fabrication approaches is leading to growing interest in micro and nano-electromechanical systems (MEMS and NEMS), oscillators and resonant systems. One of the possible applications of NEMS resonant devices is sensitive detection of bound mass. With selective binding surfaces, the devices could be exquisitely sensitive to binding of selected chemical or biological species. The fabrication capability could allow for the creation of large arrays of detection devices.
We have fabricated NEMS oscillators with integrated circular Au contacts with sub-attogram mass detection sensitivity (Fig. 1). Surface micromachined, poly-crystalline silicon and silicon nitride nanomechanical oscillators were used to detect the presence of well-defined mass loading. In order to highlight the mass loading effects of selectively immobilized dinitrophenyl poly(ethylene glycol) undecanthiol based molecules (DNP-PEG4- C11thiol) to prefabricated Au contacts on the surface of the NEMS resonator, the size of the circular Au element varied from d=50nm-400nm. The observed shift in the natural frequency of the resonator was correlated to our analytical models (Fig. 2). The theoretical frequency shifts calculated by using Euler-Bernoulli are approximately in accordance with the experimental data. As verified by finite element analysis, when rotational inertia corrections are applied the approach allows for a more accurate determination of the eigenfrequency. Discrepancies based on the simplified model are ascribed to subtle variations in the complex morphology of the Au element which lead to modifications of the nucleation sites for the thiolate self assembled monolayer. Control experiments utilizing oscillators without Au contacts, did not show a shift in the natural frequency signifying selectivity of the thiolate binding (Fig 3c-d). We estimate 0.39 X 10-18grams as the smallest resolvable mass for an oscillator with dimensions of l=4mm, w=500nm, t=160nm, with a 1mm X 1mm paddle. Based on our mass sensitivity calculations, this technique presents an opportunity for detection of a single large biomolecule adsorbed on the surface of the NEMS oscillator. Utilizing vacuum encapsulation we demonstrate sensing capability in the attogram regime of adsorbed self assembled monolayer. Our studies suggest that further tailoring of device dimensions and mechanical properties could additionally extend the mass sensitivity to the zeptogram regime.
Publication Reference:
B. Ilic, H. G. Craighead, S. Krylov, W. Senaratne, C. Ober, P. Neuzil, "Attogram Detection Using Nanoelectromechanical Oscillators", Journal of Applied Physics, 95, 3694-3703 (2004).
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