Page 296 - Mechanical design of microresonators _ modeling and applications

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Microcantilever and Microbridge Systems for Mass Detection

Microcantilever and Microbridge Systems for Mass Detection 295
creation of ordered supermolecular structures, thereby making them
attractive building blocks for superlattices and molecularly tailored
surface properties.
Mass detection can be performed in the nano- and microdomain by
means of relatively simple devices such as cantilever- or bridge-based
systems whose experimentally monitored static deflection or resonant
frequency shift offers quantitative assessment of the mass that attaches
to such a device. Static deflection methods are structured around the
fact that adsorbed matter induces a stress gradient into the structure,
which produces deformations (deflections) in beam structures. On the
other hand, mass that attaches in either a pointlike or a layerlike
manner can be considered as a gravity force acting on a beam and
generating deflections. Resonant detection methods rely on the change
of the sensing system’s mass (through attachment of extraneous
substances) or on the combined alteration of mass and stiffness (as is
the case with layerlike deposition) which produces a shift in the
relevant resonant frequency (usually bending or torsional).
This chapter includes the study of mass detection by means of static
deflection interrogation methods which analyze the gravity effects of
point or layer forces in conjunction with the stiffness change (in the case
of layer forces), but is mainly dedicated to studying evaluation methods
of the resonant frequency shift.
The drive toward ever-smaller mechanical resonators is simple:
minute amounts of deposited mass in the realm of femtograms and even
attograms (10 í18 g) have already been detected, and the promise of
downscaling 3 orders of magnitude, which is equivalent to molecule-
level detection, can only be achieved by very small-dimension devices
which enable detection of significant resonant shifts.
Nano- and micromass detectors are therefore implemented as sensors
in a variety of applications such as chemical, biological, or clinical anal-
ysis; environmental control; and monitoring of industrial applications
by study of variations in temperature, viscosity, mass, stress, or
1
electric/magnetic fields (Raiteri et al. ). Nanocantilevers, nanobridges,
or systems based on these components are generally the structural
implementations of these detectors. The original structure is coated
with a layer that will capture the substance of interest by generally
chemical reactions; this coating process, known as functionalizing, is
followed by mass addition. Comparison between the significant amount
values (deflection in static methods and frequency shift in resonant
methods) measured before and after mass immobilization constitutes
the metric for evaluating the quantity of attached mass. The static
deflection method can be used, among other numerous applications, to
detect hydrogen or mercury vapors by employing functionalized

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