Biomedical Engineering Reference
In-Depth Information
exhibit a strong extinction band that is not present in the spectrum
of the bulk metal. The extinction band results when the incident
photon frequency is resonant with the collective oscillation of the
conduction electrons and is known as PPR, also known as localized
surface plasmon resonance (LSPR). The extinction cross-section
and peak wavelength of the PPR band is highly dependent on the
local environment of the nanoparticle (i.e., refractive index of the
surrounding medium),
12-16
and furthermore, the binding events
to those functionalized nanoparticles.
17-21
However, the signal-
to-noise ratio for slide-based PPR sensors obtained by either the
transmission mode with a single pass of light or relection mode
with a double pass of light through a NMNP layer is not so high, as
a result of the low absorbance of the NMNP layer. The absorbance
of such a NMNP layer can be enhanced by optical waveguide (OW)
sensing
22
which is based on the absorption of the evanescent ield
via multiple total internal relections. By this approach, we have
developed a novel iber optic particle plasmon resonance (FO-PPR)
biosensing platform that exploits PPR of self-assembled NMNPs on
the unclad portion of an optical iber for monitoring the refractive
index (RI) of solution bulk and for label-free detection of molecular
or biomolecular binding at the surface of NMNPs.
23-25
Other OW-
based PPR biosening platforms are also possible and are described
later in this chapter. For such OW-PPR biosensing platforms, the
nanoparticles act as chromophores, while their interactions with the
analytes will result in colorimetric changes. Hence, with a suitable
receptor immobilized on the surface of the NMNPs, the resulting OW-
PPR sensor can detect the corresponding analyte even if the analyte
is spectroscopically silent in the UV-vis region. The FO-PPR sensing
platform has been demonstrated to be sensitive to biomolecular
binding events at the pico-molar level.
23,24
In comparison to the SPR sensor, the OW-PPR sensor retains
many of the desirable features of the SPR sensor and has at least
nine additional advantages. First, without the need of employing a
momentum matching scheme by using either a prism or a grating to
excite the surface plasmons in SPR sensors,
26
OW-PPR sensors have
a simple optical coniguration and, hence, a high potential being
miniaturized to a portable format. Second, OW-PPR measurement
is based on extinction spectroscopy and, hence, the normalization
by the relative change of transmission response alleviates the
demand on precise optical alignment. Third, sensitivity of SPR
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