Surface-Enhanced Raman Scattering-Based Bioimaging - Nanotechnology for Biomedical Imaging and Diagnostics - page 327

band of silver nanoparticles does not partially overlap with interband electronic tran-
sitions observed with gold nanoparticles [45]. This overlap decreases the degree
of coherence motion of free electrons, which produce surface plasmon field. although
silver exhibits significantly stronger localized surface plasmon resonance (LSPR)
and higher SERS enhancement factor (Ef) compared to gold, the superior
biocompatibility of gold makes it a better choice for in vivo and in vitro SERS-based
bioimaging [18, 46].
11.4.2.2 Plasmonic Engineering Plasmonic engineering involves the rational
design of size, shape, and assembly state of the nanostructures to achieve large SERS
enhancement and in turn bright SERS probes. Most of the efforts related to the
development of SERS probes are centered on plasmonic engineering. Various sizes of
shape-controlled gold nanostructures such as spheres, rods, cubes, and stars have been
employed as SERS probes for bioimaging [18, 47, 48]. Shape offers a convenient handle
to tune the optical properties of metal nanostructures. for example, the longitudinal
LSPR of gold nanorods can be tuned from visible to parts of niR by varying the aspect
ratio of nanorods [47]. nanostructures with sharp apexes such as nanostars or
nanopopcorns provide additional enhancement due to concentration of EM field at sharp
apexes (nanoantenna effect) [48-50]. These shape-controlled nanoparticles have been
successfully applied in various in vitro and in vivo imaging applications.
Lightly aggregated or assembled nanoparticles, which host intense EM hot spots
at their interstices, are yet another-attractive class of SERS probes [51-53]. Recently,
we reported a novel class of shape-controlled plasmonic nanostructures, namely,
core-satellite structures, which was achieved through self-assembly using simple
molecular cross-linkers (see fig. 11.5) [16].
Several computational and experimental studies have clearly demonstrated that the
SERS Ef at these EM hot spots is upward of 10 8 , which is nearly four to six orders of
magnitude higher than the corresponding individual nanostructures [16, 54]. Despite
significant efforts toward controlled assembly of plasmonic nanostructures, realizing
scalable nanoparticle assemblies that offer highly uniform, isotropic, and stable SERS
enhancement remains challenging. controlled assembly using colloidal chemistry is a
promising field for high-yield production of metal nanoparticle clusters with small
interparticle gaps. To date, most of the reported methods have used nucleic acid or
other organic molecules as linkers, which lead to long separation distances and thus
weak plasmon coupling [55, 56]. Moreover, only simple clusters, such as dimers and
trimers, have been efficiently synthesized [57, 58]. Recently, the controlled assembly
of gold nanospheres into well-defined nanoparticle clusters has been reported to indi-
cate the outstanding optical performance and therefore ultrasensitive SERS-based
detection-capability [59]. in this work, block copolymers were used to produce highly
symmetric gold nanoparticle clusters with large coordination numbers (up to 7). This
is an area of research that has received significant interest in the last 4-5 years, and we
can expect some important developments in the years to come.
Recently, core-shell plasmonic nanostructures with 1 nm interior gaps, where the
Raman reporter is precisely placed between the two concentric gold layers, have
been demonstrated [60]. The obvious advantage of such SERS probes is the intense