Nanoparticles as Contrast Agents for Optoacoustic Imaging - Nanotechnology for Biomedical Imaging and Diagnostics - page 134

(TeOs). The effect of the silica coating is to improve the stability of the nanorods
due to its mechanical rigidity while not influencing the optical properties due to its
transparency at NIR wavelengths. Indeed, studies have shown that the silica-coated
gold nanorods retained their morphologies up to three times higher fluences than
bare nanorods [172].
silica deposition on gold nanorods and HAuNs is usually conducted with a
procedure developed by stöber [179] and later reproduced and further optimized by
several research groups [178, 180, 181]. silica covering involves hydrolysis and suc-
cessive condensation of TeOs [si(c 2 H 5 O) 4 ] in an alcoholic medium in the presence
of ammonium hydroxide (NH 4 OH) as catalyst. The reaction mechanism is shown in
equations 5.1 and 5.2. The first step is hydrolysis in which ethoxy groups are replaced
by OH groups. In the second step, silicon hydroxides undergo polycondensation to
form siO 2 .

( ) + → ( ) +

Si OCH O iOHCHOH
25 4

4 4 (5.1)

2 25

4

( ) →+

4

Si OH SiOHO 2 (5.2)

2 2

The thickness of the layer is dependent upon the concentrations of TeOs and
NH 4 OH, the reaction time, and the number of TeOs additions [172, 174, 180].
Applications with a gold-based contrast agent, such as nanorods or HAuNs,
require two main features: high optical absorbance and effective heat transfer to sur-
rounding media. since silica coating does not change absorbance properties of
nanorods, chen et al . explored silica-covered gold nanorods for 3D OA imaging
system [172]. silica has a significantly higher thermal diffusivity compared to water;
thus, the thickness-dependent thermal stability enhancement can be attributed to a
faster heat distribution over a larger area [172]. changes in the thermal conductivity
could lead to an improved cooling due to a more efficient heat transfer [172].
conversely, low thermal conductivity of the underlying porous layer reduces the
amount of heat escaping from the substrate and contributes to the efficient PA
emission from Au nanoparticle arrays [182].
The amplitude of the PA signal is determined by the grüneisen parameter of the
solvent, which directs heat transfer from the nanoparticles to the liquid [174]. The
effect is also dependent upon the optical absorption of the nanoparticles, laser flu-
ence, and interfacial heat conductivity between the two materials [174]. In
References [172] and [174], the authors suggest that the phenomenon can be
explained by a modified interfacial thermal resistance due to the silica layer. The
authors of this chapter believe that the silica layer prevents overheating of the sur-
rounding water to increase the efficiency of OA response. Our alternative hypo-
thesis is based on slowing the heat transfer through gold/silica/water due to low
thermal conductivity and mismatch of the main energy carriers. It prevents over-
heating of water surrounding the gold nanoparticle and improves heat and optical
energy transfer during the laser pulse as it is shown in Reference [183]. Regardless
of the mechanism, these data suggest that silica-covered gold nanoparticles are
efficient OA amplifiers.