Nanosensors - Nanotechnologies for Water Environment Applications - page 428

resolve than reflective fluorescent signals. This leads to improvement in assay
specificity. SERS technology employing the sandwich hybridization recognition of
target DNA with the gold nanoparticle labeling was demonstrated (Cao et al., 2002). The
Raman signal can be further enhanced by silver reduction. Silver formed on the
roughened surface of gold nanoparticle is found to significantly promote not only the
grey-level signal (Taton et al., 2000) but also the Raman signals around the roughened
gold nanoparticle surfaces (Cao et al., 2002). Compared to the grey-scale scanometric
detection (Taton et al., 2000), this strategy provides multiplexing capabilities and better
sensitivity (1.20 × 10 7 copies of target gene/mL). This scheme made an improvement in
sensitivity of two orders of magnitude to that of conventional Cy3 labeling based DNA
array with a detection limit of 3.01×10 9 copies of synthetic target gene/mL (Taton et al.,
2000). As demonstrated by a research group, thousands of identification codes written
onto the silver nanoparticles could be expressed in various fluorescent intensities by
adjusting the concentration of each dye conjugate (Docherty et al., 2004). The silver
nanoparticles can then be identified individually by their unique Raman signals
providing high throughput with a simpler chemistry.
13.7.3 Bio-Bar-Code Based Signaling
Bio-bar-code assay employs two particle probes (iron-oxide and gold)
immobilized with oligonucleotide sequences specific to the target sequence. The iron-
oxide particle probes are immobilized with oligonucleotides complementary to the target
sequence. Similarly, gold particle probes have oligonucleotides complementary to the
target sequence and also unique bar-code identifier sequences. In this design, the signal
from target DNA is amplified through two steps. First, the signal of target DNA strand is
switched to a greatly amplified signal of a massive amount of bar-code DNA. This is
carried out by a novel gold nanoparticle with two-component conjugate DNA that
hybridizes with the target DNA at one end and a massive amount of bar-code DNA
strands at the other end. Then the bar-code DNA is detected by a glass slide based
sandwich hybridization between a gold nanoparticle probe and a capture DNA. The
signal is further amplified by silver enhancement and detected by scanometric method
(Taton et al., 2000). The assay sensitivity of this approach is comparable to that of PCR-
based techniques (Nam et al., 2004). This scheme succeeded to detect the target anthrax
lethal factor associated gene at 500 zM, i.e., ~300 copies of functional gene/mL sample.
The entire assay can be carried out in 3-4 h, regardless of the target concentration.
13.8 Magnetic Particle Based Immunoassays for the Detection of
Pathogens: Commercial Devices