Page 136 - Bio Engineering Approaches to Cancer Diagnosis and Treatment

134 CHAPTER 6 Laser-assisted cancer treatment

E local = E inc i , + ∑ E j = E inc i , + ∑ A P , (6.17)
ij
j
= j 1
= j 1
Elocal=Einc, i+∑j=1i≠jEj=Einc, i+ ≠ ij ≠ ij
∑j=1i≠jAijPj,
Aij where A is the interaction matrix between all the dipoles in the particles. The scat-
ij
tering efficiency is:
k
Q sca = 2 | E ( r) ˆ | 2 (6.18)
′ ⋅rdS.
sca
2
2
Qsca=kπr Ei ∫dS|Esca(r)′⋅rˆ| dS. πrE i ∫ dS
2
2
Fourier transform and DDSCAT are the applicable methods to solve Eq. (6.18).
6.2 Photothermal therapy
Photothermal therapy (PTT) refers to using photothermal agents to selectively destroy
cancer cells by converting optical energy into heat. When photothermal agents expose
to light, the electrons transfer from the ground state to the excited state. Then, the
excited electron suddenly relaxes through nonradiative decay channels which leads
to increasing the kinetic energy and subsequently raising the temperature around the
targeted tissue. This heat is applied to destroy cancer cells. Several kinds of the pho-
toabsorbing agent have been used in PTT including natural [5] or synthetic chromo-
phores [6], noble metals [7], and carbon-based nanostructures [8]. In comparison to
synthesize chromophores such as naphthalocyanines and indocyanine green, natural
chromophores suffer from very low absorption cross-section. However, synthetic dye
molecules exhibit photobleaching during laser irradiation. There has been a tremen-
dous effort to develop nanoparticles for PTT.
The noble metals are considered a great deal of attention due to their four to
five times higher cross-section absorption compare to the synthetic chromophores.
Recently, gold nanoshells (GNSs), gold nanocages (GNCs), gold nanorods (GNRs),
gold nanostars (GNSTs), carbon nanotubes, and graphene oxide (GO) have been
considered as versatile nanoparticles for PTT on account of their strong absorption in
near-infrared (NIR) region, high stability, and acceptable biocompatibility. Fig. 6.1 is
shown these nanoparticles which is further described in the following section.

6.2.1 Gold nanoshells
GNSs compose of a dielectric core surrounded by a thin layer of gold. The surface
plasmon resonance (SPR) of GNSs is related to the plasmon difference between the
inner and outer shells. By changing the ratio of core radius to shell thickness, SPR
wavelength is tuned from visible to NIR [9]. The typical method to fabricate GNSs
is seed-mediated growth which composed of two steps—gold nanoparticle is first
attached to the dielectric core and then growth under the specific condition to form a
shell. The main studies of using GNSs for PTT are summarized in Table 6.1. GNSs,
with the trade name of AuroShell is currently being examined for clinical study [10].
When AuroShell injects intravenously in a patient, will accumulate in tumors due