Nanosensors - Nanotechnologies for Water Environment Applications - page 432

cantilevers these researchers could detect a single E. coli O157:H7 cell (Campbell and
Mutharasan, 2007). Cantilevers do have a lower limit on sensitivity due to noise of the
displacement transducer, which is based on fundamental phase noise processes and
readout circuitry, and will also change when the cantilever is introduced in air or placed
in a hybridization solution. Ekinci et al. (2004) reported a detection limit of 2.53 x 10 -18
g (change in frequency equals to 3000 kHz bandwidth) for a resonating bridge oscillator
placed in a vacuum.
13.9.2 Surface Plasmon Resonance (SPR)
Surface plasmon resonance (SPR) tools coated with a nano-thin layer of gold
(with or without further immobilization of gold nanoparticles), exploit waveguides and
resonant mirrors to recognize changes in analyte structure, thus permitting the detection
of label free molecular interactions (Karlsson, 2004). In traditional SPR, a nano-thin
layer of gold (conducting film) is used to capture certain angles of incidence from a light
emitting diode. The polarized energy from captured angles of incidence light will
couple into a surface plasmon (packets of electrons) wave traveling along the interface
between the gold layer and the analyte. The loss of this energy is observed as a sharp
reduction of reflectivity. The angle at which this occurs varies with the surface
refractive index (RI) of the analyte; therefore the surface RI is measured by the
reflectivity across a range. Thus, a reflectivity versus angle spectrum is obtained using a
detector array. In summary, a change in the surface RI of the analyte will change the
angles in which incidence light is lost from the LED, increasing loss of incidence light,
and causing a change in reflectivity that is observed by a diode array (Chinowsky et al.,
2003). An example of a portable sensor utilizing SPR phenomenon is SPREETA TM
developed by Texas Instruments. SPREETA TM eliminates labeling of the target and the
need for external readers. The highly integrated small size permits fast hybridization of
DNA (often less than 5 minutes), and high potential for point-of-use assays of microbial
water quality.
Incorporating gold nanoparticles with SPR leads to a level of sensitivity
equivalent with traditional fluorescence-based methods for hybridization of target
oligonucleotides (He et al., 2000). For this technique, a nano-layer thin sheet of gold is
attached to the surface of the SPR. Typical immobilization chemistries to the thin gold
layer include strong bonds (30-40 kcal/mol) between thiol and gold resulting in a stable
monolayer connection that is extremely robust (Csaki et al., 2001). Van der Waals
interactions among the thiol molecules generate dense packing into monolayer
organization of interlocking components, establishing an ideal density of immobilized
capture probes (Ilic et al., 2004). For probe immobilization to the SPR, a flow through
mechanism and a robotic spotter have been used. The highest level of sensitivity for
SPR was observed by using flow through immobilization (Lee et al., 2001).
Microfluidic channels fabricated from polydimethylsiloxane were incorporated with