Nanoparticles for in-vitro and in-vivo biosensing and imaging

28 Metal nanoparticles
Figure 1.12: Simulation of the surface plasmon absorption for gold nanorods of different aspect ratio
by Link and coworkers. [52].
calculated in Figure 1.12b. The dotted line represents a plot of an equation derived by
Link in order to predict the maximum of the longitudinal plasmon band λ max [52]
λ max = (33.34R − 46.31)ɛ m + 472.31 (1.24)
It follows from Equation 1.24 that the maximum of the longitudinal plasmon resonance
also linearly depends on the medium dielectric constant ɛ m . Hence, with increasing
the medium dielectric constant, there is also a red-shift of λ max for a fixed aspect ratio
(Figure 1.12 c with R=3.3).
The fact that gold nanorods exhibit a longitudinal SPR in the near-infrared has
drawn interest of the biophysical community in the use of this particles in in-vivo study
because of larger penetration depth of the laser light: in the region between 650-900 nm,
≥ 300 µm depending on tissue types.
Solid gold nanorods have several advantages over other contrast agents; their synthesis
with various aspect ratios and tunable absorption wavelength in the near infrared region
[58] [59] [8] is quite simple and well-established. The typical size of the nanorods (50-200
nm) is potentially useful in applications such as drug delivery and gene therapy. In addition,
the biosafety of gold is well known and it has been used in vivo since the 1950’s [60]
and recently the low cytotoxicity of gold nanoparticles in human cells has been studied
in detail by Wyatt and coworkers [61].
Moreover, gold nanorods can be used as photo-absorbers in NIR and realize photothermal
therapy. Thus, upon laser irradiation at the surface plasmon absorption band, the
nanoparticles absorb energy and then immediately transfer it into heat. If the nanoparticles
are incorporated or incubated with biomolecules, cells or tissues, heat causes the
sharp increase on the local temperature around the nanoparticles and thus damages to
the surrounding materials. This process is called photothermal destruction and can be