Nanosensors - Nanotechnologies for Water Environment Applications - page 427

When a similar dye-doped silica nanoparticle was employed for labeling at the
DNA level (Lian et al., 2004), the detection limit was 2.35 × 10 5 copies of genomic
DNA/mL sample. Clearly, the enhanced signal intensity did not eliminate the need to
carry out pre-amplification before hybridization to a microarray. Similarly,
tetramethylrhodamine doped silica nanoparticle signaling probes could achieve a
detection limit of 4.82 × 10 7 copies/mL (Zhao et al., 2003). This could be explained by
the very high number of dye or NP that could be attached to the cell compared to a DNA
target. When using microarays, use of fluorophore based labeling is commonly applied
to bioassays for pathogen detection (Zhao et al., 2003). Use of alternative dyes (e.g.,
Alexa Fluor and Tyramide Signal Amplification) was reported to yield a higher signal
and more photostable (Panchuk-Voloshina et al., 1999). However, the enhancement in
signal sensitivity is generally insufficient to eliminate the need to carry out PCR before
the hybridization step.
13.7 Signaling Methods
13.7.1 Quantum Dot Signaling
Quantum dots (QDs) are semiconductor nanoparticles and compared to
conventional fluorophores, have a narrow, tunable, symmetric emission spectrum and
are photochemically stable (Bruchez et al., 1998). Another remarkable advantage of
QDs is that a single wavelength can be shared by different-sized QDs for simultaneous
excitation (Alivisatos, 1996); (Nirmal and Brus, 1999). QD labels that are 20 times
brighter and 100 times more stable against photobleaching, and one-third as wide in
spectral width as traditional rhodamine labeling array are already available (Chan and
Nie, 1998). For multiplex identification, the microbeads carrying semiconductor QDs
can be exploited (Han et al., 2001). QDs can also be embedded into polymeric
microbeads in precisely controlled ratios to vary the signal intensity of microbeads. A
combination of intensity (determined by numbers of embedded quantum dots) and colors
(determined by QD size and emission wavelength) can be used to create a variety of
label coding. For example, the use of 10 intensity levels and 6 colors could theoretically
code one million nucleic acid or protein sequences. These highly luminescent QDs are
thus considered ideal for multiplex microorganism detection (Han et al., 2001).
13.7.2 Surface-Enhanced Raman Scattering (SERS) Signaling
Surface-enhanced Raman scattering or surface-enhanced Raman spectroscopy
(SERS) is a surface sensitive technique, which amplifies Raman scattering signals of
adsorbed molecules on rough metal surfaces. SERS is known to enhance Raman signals
by up to 100,000-fold (Kneipp et al., 1999) leading to the detection of a single molecule.
Apart from the enhanced intensity, SERS signal is much sharper and thus easier to