Biomedical Engineering Reference
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that provided the largest enhancement should exhibit large Re(
ε
)
and small Im(
ε
) which immediately singled out the noble metals as
best enhancers for excitation with visible light sources. The LSPR of
small Ag and Au particles are extremely sensitive to composition,
morphology, geometry, degree of aggregation, and dielectric
properties of the ambient as demonstrated in the calculations
depicted in Fig. 11.2.
A( )
AB(
A( )
(A)
(B)
)
A
(
)
A
(
)
AB(
)
(C)
(
)
Figure 11.2
Extinction spectra of Ag and Au NPs and their corresponding
dimers. (A) Ag NP in air (green), Ag NP coated with a
monolayer of benzenethiol (blue) and a pair of Ag NP
separated by 1 nm gap (red). (B) Au NP in air (green), Au
coated with a monolayer of benzenethiol (blue), and a
coupled Au NP dimer separated by 1 nm gap (red). See also
Color Insert.
In Fig. 11.2, the LSPR of 20 nm Ag and Au particles are estimated
to lie at 355 nm and 500 nm, respectively, from a Mie scattering
calculation.
12
In the presence of a monolayer of benzenethiol, the
extinction spectra showed a slight red-shifted of the resonances,
whereas a substantial red-shift is observed when particles
aggregated into a dimer (red spectra, Fig. 11.2A,B). Note that
the dimer calculation was carried out using the Discrete Dipole
Approximation (DDA) computer codes of Draine
et al.
13,14
This
sensitivity allows one to design nanostructures and tailor their LSPR
for particular SERS applications. For example, nanorods, nanoshells,
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