Page 295 - Mechanical design of microresonators _ modeling and applications

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Microcantilever and Microbridge Systems for Mass Detection

294 Chapter Six
Generally, biomolecular adsorption of target analytes to function-
alized regions of a cantilever-based sensor can alter mechanical stress
within the oscillator and its total mass and thus influence both the
bending and the natural frequency of the cantilever, respectively.
Signal transduction is generally achieved by employing an optical
deflection (or interferometric) system to measure the mechanical
bending or the frequency spectra resulting from additional loading by
the adsorbed mass. Within such a configuration, a collimated laser
beam is focused onto the free end of the cantilever and is reflected onto
a split photodiode. The dc offset of the difference signal between the two
cells of the photodiode quantifies the cantilever bending while the
resonant peak of the ac signal extracted by a spectrum analyzer
corresponds to the natural frequency of the cantilever. In the case of
interferometric detection, an ac change in the intensity of the reflected
light corresponds to the cantilever natural frequency.
Most of the current work has been devoted to the immobilization of
target species onto the surface of the resonating structure. In such a
scenario, pathogen binding events are not confined to a particular por-
tion of the device and can occur anywhere on the surface. Since both
the resonant frequency shift and deflection are highly dependent on
the position of the adsorbed material, it is difficult to determine the
exact amount of additional mass present without any visual inspec-
tion. To circumvent these limitations, one can construct arrays of
surface micromachined oscillators with precisely positioned catalyzing
anchors. The incorporation of prefabricated adsorption sites allows
adequate control of chemical surface functionality for the detection of
analytes of interest. For example, by using electron beam lithogra-
phy, stress-free polycrystalline silicon and low-stress silicon nitride
micromachined resonators with evaporated gold contact pads can be
fabricated. Alkanethiol molecules can be subsequently adsorbed from
solution onto the Au anchors, creating a dense thiol monolayer with the
tail end group pointing outward from the surface. A common feature of
the alkanethiol self-assembled monolayer (SAM) systems is the strong
interaction between the functional group and the gold substrate. The
van der Waals interactions among the molecules permit dense packing of
the monolayer into a supermolecular hierarchical organizations of inter-
locking components. Typically, the total amount of material in a well-
í2
packed alkanethiol SAM on gold is approximately 8.3 × 10 í10 mol·cm .
Alkanethiolates offer unique opportunities for precise tailoring of the
length of the alkane chain and chemical properties of both the head
and tail groups, thus making them excellent systems for further
engineering of the chemical surface functionality following the
assembly of the SAM. Due to their extreme flexibility, SAMs allow the

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