Biomedical Engineering Reference
In-Depth Information
4.7.1.4 Surface Plasmon Resonance
A surface-sensitive analytical technique that arises from the collective oscilla-
tions of conduction band electrons in resonance with the field of incident radia-
tion induced by molecular adsorption at a noble metal film is called SPR.
112
Aggregation of particles results in the shift in the position of the plasmon reso-
nance peak which is related to the refractive index of the surrounding medium
as predicted by the Mie theory which predicts that SPR occurs when,
Re
(
( )
ω
= -2
M
Є
Є
where Re(є(ω)) is the real component of the metal dielectric constant and є
M
is the medium dielectric constant.
113
Thus, aside from colorimetric sensing
through SPR coupling in the aggregated NPs, targets of interest can also be
detected through changes in the refractive index change that offers: (i) high sen-
sitivity, (ii) longer sensing range through the exponential decay of the evanes-
cent electromagnetic field, (iii) multiple modes of detection such as angle shift,
wavelength shift, and image, (iv) real-time detection at a timescale between 0.1
and 0.001 s for measuring binding kinetics, (v) lateral spatial resolution around
10 µm, enabling multiplexing and miniaturization, and (vi) commercially avail-
able instruments.
114
The SPR resonance frequency is a function of particle size,
shape, dielectric constant/refractive index of the surrounding medium, the mate-
rial composition, pH, and other parameters.
112,115
Noble metals such as Ag and
Au have SPR bands at the visible light region.
116
However, conventional SPR
sometimes fails to measure extremely small changes in refractive index limiting
its application in ultrasensitive detection. This limitation is addressed with the
use of NPs such as AuNP tags to increase the angle shift in SPR reflectivity.
57
This enhanced angle shift results from increased surface area, high dielectric
constant of AuNPs, and the electromagnetic coupling between AuNPs and the
Au film sensor surface. The detection limit of 10 pM for 24-mer ONTs at a
surface density ≤ 8 × 10
8
molecules/cm
2
was close to that of traditional fluores-
cence-based methods for DNA hybridization.
AuNPs attached to DNA was also reported to improve the sensitivity of
transmission SPR spectroscopy.
117
This highly sensitive biosensor was based
on the adsorption of AuNPs onto gold diffraction gratings. The sensor exhibits
enhanced diffraction due to the optical coupling of the planar surface plasmons
in the grating to the localized surface plasmons in the AuNPs that were used and
applied to detect unmodified DNA at a concentration of 10 fM
118
that was used
as a cross-linker for the immobilization of the capture antibodies.
Improvement of nanobiosensor sensitivity using the catalytic growth
of AuNPs has been applied in various analytical methods or integrated with
enzyme reactions for quantitative detection.
119
But, catalytic growth of AuNPs
do not work in some situations, such as SPR and electrochemistry, because
metal matrices used in these techniques, e.g. Au, are susceptible to metal depo-
sition affecting the background seriously. To circumvent these issues, an SiO
2
