AAWSAP DIRD, Biosensors and BioMEMS: A Survey of the Present Field, March 2010

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Microcantilever based sensors fall into
two categories: static sensors and
vibrational sensors. Static sensing
microcantilevers are usually covered
with a gold film, which is then coated
with a substance having a specific
affinity. Upon binding the specific
selective chemical layer
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analyte (such as a protein), stress is
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generated resulting in bending of the
microcantilever.
The deflection is often measured using a
narrow light beam from a small laser.
♦ reacting chemical compound
Light reflected off the bottom of the
cantilever falls onto a surface, where a
position sensitive detector (PSD) can
Figure 34. Principle of a Microcantilever That Bends
determine how much the beam bends.
When It is Loaded With an Adherent Mass
Typically the bend is exceedingly slight,
but the sensitivity of the optical readout system is high enough that small amounts
(nanograms to picograms) of adsorbed material can be detected. These devices are
mostly used for measuring things in the air and not well suited for immersion in fluids.
In vibrational sensing, an external motor is used to vibrate the microcantilever through
a certain range of speeds, and a sensing mechanism then reads out the amplitude of
the vibration. By seeing at what speed the cantilever vibrates best, the resonant
frequency is obtained. As bioparticles bind to the microcantilever, the resonant
frequency will be slower due to the increased mass.
The mass of attached analyte can be determined from the frequency of the
microcantilever. The microcantilever has its own natural frequency w. When a
biomolecule binds to the microcantilever, it changes the mass of the microcantilever.
This in turn affects the resonant frequency. We can use that change to determine the
mass that has been attached to the microcantilever.
In some sense it is like a tuning fork that changes its pitch when touched. The surface
has an affinity for adhesion of a certain material and nothing else. When these
materials are present, they adhere and increase the mass and so the pitch moves
lower.
The sensitivity of these systems to the loaded mass increases as the mass of the
cantilever beam decreases and with it the resonant frequency increases. Decreasing the
overall dimensions of the beam results in an corresponding increase in their sensitivity.
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