Nanosensors - Nanotechnologies for Water Environment Applications - page 431

13.9.1 Cantilevers
Advances in micromachining techniques permits fabrication of micro-electro-
mechanical systems and nano-electro-mechanical systems as cantilever arrays that can
be used to detect biological molecules and whole cells. A cantilever is defined as a
projecting beam or other structure supported only at one end. Cantilever arrays have the
potential to eliminate drawbacks of conventional microarray in terms of increasing
sensitivity, quantification, portability, specificity, and decreasing the time required to
complete an assay (Carrascosa et al., 2006). Fluctuations in bending and oscillation
(before and after a biological entity is captured/hybridized) are measured optically or by
mechanical motion transduction using a piezo-resistor and/or piezoelectric material
(Baselt et al., 1996). Two methods have been described for sensitive transduction of
micromechanical motion: i) the change in frequency due to additional mass or force
constant, in this instance, target hybridization alters the mechanical stress due to changes
in total mass which influences frequency of oscillation, and ii) a change in surface stress
causes the cantilever to bend and deflection is measured. It has been suggested that
measuring changes in frequency seems to be a better choice for biosensing applications
because it can easily quantify mass and allow more stable control (Ziegler, 2004).
Cantilevers have been used extensively for biomolecular recognition to rapidly
obtain high levels of sensitivity and specificity. Specificity, however, is a function of
the antibody used. The most common method to measure the deflection of a cantilever is
with optical techniques. Even single base pair mismatch can be discriminated using
cantilever sensors with a thin film gold attachment (McKendry et al., 2002). A minimum
resolvable mass of 0.37 attograms was calculated using nanoelectromechanical
oscillators functionalized with gold anchors (Ilic et al., 2004). These researchers
detected a single virus, weighing approximately 3 femtograms and could quantitatively
count the number of viruses attached to a single receptor by noting the difference in
eigen frequency due to change in mass (Ilic et al., 2004). Another research group
studied the surface stress due to Watson-Crick base pairing between unlabeled
olionucleotides and measurement was able to discriminate between perfect and signal
base mismatches with 12-mer probes (Fritz et al., 2000). A disadvantage of using optical
techniques is the lack of parallel capacity because of the need to rigorously focus
hundreds of lasers in parallel (Khaled et al., 2003). Therefore, many platforms have been
developed with piezo cantilevers to reduce limitations of optical techniques. One group
used magnetic beads functionalized with streptavidin and a magnetic field to apply an
external force that occurs due to a hybridization event, and obtained a detection limit of
10 -18 M concentrations using a piezoresistor (Baselt et al., 1998). Piezoelectric
cantilevers were used for detecting the abundance of yeast cells in solution (Yi et al.,
2003). Recently, the use of piezoelectric excited cantilevers to detect E. coli O157:H7 at
approximately 10 cells per ml in spiked ground beef samples in 10 min is demonstrated
(Maraldo and Mutharasan, 2007). By further improvement in the piezoelectric excited