Surface-Enhanced Raman Scattering Based on Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 255

and nanoprisms can all be tuned by controlling aspect ratio, 15-18
shell-radius ratio, 19,20 and edge-sharpness, 21 respectively.
Another outcome of the electromagnetic enhancement theory is
the predicted distance dependence built into the SERS enhancement
factor, G , which is proportional to ( a + d ) -12 . This inherently restricts
the SERS effect to adsorbate molecules that are situated either
directly on the surface or in the immediate vicinity of the NP. This
has also had an impact on the design of SERS-based sensors. 22 The
EM theory also predicted that additional signiicant enhancements
can be produced through tight coupling of NPs as a result of coupling
of surface plasmon dipoles from the individual NP. Consider the
simplest coupled-NP systems as a set of two NPs (dimer) separated
by a small inter-particle distance. Theoretical calculation 23,24 has
demonstrated that a simple dimer excited with light polarized parallel
to the inter-particle axis showed enormous optical ield strength in
the inter-particle gap and the expected SERS enhancement exceeded
10 11 . Figure 11.2 shows ield amplitude images calculated by DDA
for a single NP (Fig. 11.2C), and a coupled NP dimer separated by
a distance of 1 nm (Fig. 11.2D). The differential ield enhancement
(normalized to the incident ield) for a coupled dimer to a single
spherical NP was ~6 orders of magnitude in this case, with the
highest ield amplitude located at the center of the gap between
particles. Hence, a molecule situated in the inter-particle gap of
dimer particles excited with a light polarized parallel to the inter-
particle axis will beneit from the intense ield enhancement, thus
enabling detection of very few to even a single molecule by SERS. 25-28
The overall enhancement of dimer particles, however, depends
strongly on the separation distances as illustrated in References 23
and 27. The enhancement by dimer particles also exhibited strong
dependence on the polarization of the incident radiation with the
strongest enhancement being observed for light polarized parallel
to the inter-particle axis. Conversely, dimers excited with light
polarized perpendicular to the inter-particle axis, had a region of
highest localized optical ields at the two separate polar regions of
the NP while the ield in the inter-particle gap was depleted, forming
a “cold spot.”
A inal result of the EM theory is an imposed size-limitation of
the nanostructure for the overall enhancement of the SERS active
system. SERS-active systems must ideally possess structures in the
5-100 nm range. The upper limit is dictated by the dipole plasmon