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Cantilevers
Advances in photo- and e-beam lithographic techniques enable the fabrication of more
complex devices at the micrometre and nanometre scale. Microcantilevers (with
nanoscale thickness) can be used to detect biomolecules, micro-organisms and chemicals
by measuring changes in oscillating frequency as a result of binding (see Figure 2.16).
Figure 2.16 (a) SEM image of an array of silicon cantilever (length: 500µm, width; 100µm,
thickness: 500nm) (b) Schematic drawing of a cantilever array with different sensitive layers.
Source: http://www.ewh.ieee.org/tc/sensors/Sensors2003/Lang.pdf
Detection can be performed in both gas and liquid phases, however performances are
deteriorated when detection is operated in liquid environments because the motion of the
cantilever is dampened by the liquid. Recent work at MIT (Burg and Manalis) provides a
solution to this by confining liquid to channels inside the cantilever. This is capable of
weighing single nanoparticles, cells and proteins with a mass resolution of better than
one femtogram.
Functionalizing microcantilevers with target capture DNA, for example, provides a
platform for the formation of a sandwich assay between capture DNA, target DNA, and
DNA modified gold nanoparticle labels. The gold labels provide a site for silver ion
reduction, which increases the mass on the cantilever and results in a detectable
frequency shift that can be correlated with target detection. The detection of viruses and
bacteria is also possible using nanoelectromechanical devices (N.L. Rosi and C.A. Mirkin,
2005).
SERS / Raman detection
Attaching Raman-dye-labelled oligonucleotides to gold nanoparticle probes generates
spectroscopic codes for individual targets, thus permitting multiplexed detection of
analytes (see Figure 2.17). The presence of the target is confirmed by silver deposition
on the gold nanoparticle (as low as 1 fM), and the target identity is confirmed by surfaceenhanced
Raman scattering (SERS) signature. The advantages over fluorophore based
systems are: narrower spectroscopic bandwidths per dye (hence less overlap and noise)
but broader overall spectrum available (potentially allowing greater multiplexing); and
only a single wavelength laser radiation is needed to scan a highly multiplexed array with
numerous target-specific Raman dyes (N.L. Rosi and C.A. Mirkin, 2005).
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