Environmental Engineering Reference
In-Depth Information
termed as nanoarrays - a miniaturized version of microarrays. They have become
invaluable tools for genomics and proteomics research. Common methods for the
fabrication of nanoarrays are dip-pen nanolithography (Piner et al., 1999), nano-grafting
(Liu et al., 2002), and finely focused ion beam lithography (Bergman et al., 1998).
Nanoarrays provide highly parallel, multiplexed, and ultrasensitive format for diagnostic
applications of biomolecules (Ekins, 1998).
In addition to the above-mentioned two- and three-dimensional nano-structures,
nanoparticles (NPs) are also becoming a critical component in sensors. Nanoparticles are
often presented as a more effective alternative to organic flourophores. NPs are
attractive candidates in sensor applications because of their: i) smaller size (1-100 nm)
and correspondingly high surface to volume ratio, ii) chemically tunable physical
properties related to size and composition, iii) excellent photostability and high
luminescent signal, and iv) possibility of surface modification for biocompatibility. Gold
nanoparticles, quantum dots (QDs), and organic dye doped silica nanoparticles are the
more common examples of NPs used in bioimaging and biosensor applications. NPs
possess very high surface area for the immobilization of analytes. In addition, most of
the nanoparticles are easily synthesized using routine chemical procedures. Gold NPs,
for example, are commonly synthesized by reduction of HAuCl
4
by sodium citrate
solution (Turkevich et al., 1951), two phase reduction method (Brust et al., 1994), and
seeding technique (Murphy and Orendorff, 2005). Extensive information is available on
the synthesis and characterization of QDs (Boatman et al., 2005).
Figure 13.4 b depicts silica nanoparticle doped with dye molecules to enhance
the resulting signal for detection of
E. coli
O157:H7. Several novel organic dye-doped
silica nanoparticles were designed for high dispersibility in water, excellent
photostability, easy surface modification capacity, size uniformity and tunability and
produced high-quality luminescent signals (Santra et al., 2001). A large number of dye
molecules (hundreds to thousands) can be incorporated inside a single silica particle.
Moreover the already developed silica chemistry allows for surface modifications of
dye-incorporated silica NPs. Two methods that are commonly applied for the synthesis
of dye doped silica NPs are the method of Stäber et al. (Shibata et al., 1997) and the
reverse microemulsion method (Yamauchi et al., 1989).
There are many challenges to achieve rapid and accurate detection of small
amounts of target microorganisms present in complex matrices. Thus, concentration of
target and its enrichment or amplification is always required. Genetic targets can be
amplified by polymerase chain reaction, ligase chain reaction, Klenow fragment
amplification, strand displacement amplification or tandem amplification process. As a
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