Nanoparticles as Contrast Agents for Optoacoustic Imaging - Nanotechnology for Biomedical Imaging and Diagnostics - page 126

since then, OA imaging experiments have been conducted using gold nanorods in
a variety of phantoms, cells , and in vivo models of disease. Agarwal et al . [126]
imaged gel beads dispersed with gold nanorods implanted in the hind leg of a mouse
using a bimodal ultrasound—a PA system where a clinical ultrasonography system
was synchronized to a laser. The groups of Li and Wang, who were active in many
aspects of synthesis and applications of gold nanorods, published one of the first clear
demonstrations of molecular imaging using antibody-coupled nanoparticles [127].
In the following publications, song et al . used gold nanorods as tracers in OA
mapping of lymph drainage to identify sLNs in rat models [128]. The system used
was a deeply penetrating PA microscopy system. cui et al . [129] also applied PA
microscopy to visualize gold nanorods accumulated at a tumor site in a mouse model,
to guide high-intensity focused ultrasound (HIfu) for therapy. Taruttis et al . [130]
showed real-time imaging of circulating gold nanorods in blood vessels in the neck
region of a mouse injected with the particles. gold nanorods were also shown to be
capable of targeting multiple molecules [96, 121, 131].
A number of studies show that antibody-conjugated gold nanorods as a molecu-
lar-specific contrast agent can enhance the sensitivity of OAT for imaging of tumors
[106, 107]. In the studies [127, 132], two corresponding monoclonal antibodies were
conjugated to nanorods with different aspect ratios to target HeR2 and cXcR4 mol-
ecules. By simply switching the wavelength of the excitation laser, multiple molec-
ular signatures could be obtained both in vitro and in vivo . This approach allowed
monitoring of the expression levels of different oncogenes within cancer cells simul-
taneously. The feasibility of OA imaging of inflammatory responses using bioconju-
gated nanorods was demonstrated ex vivo [133]. gold nanorods were conjugated to
anti-intercellular adhesion molecule-1, and stimulated endothelial cells labeled with
bioconjugated nanorods were scanned using a high-frequency transducer. The results
demonstrated that OA images could differentiate between inflamed cells, which were
targeted with OA contrast agents, and control cells.
gold nanorods are commonly synthesized using a toxic surfactant cTAB. An
important approach to ensure biocompatibility of nanorods is to replace cTAB with
Peg [30, 134, 135]. With Pegylated gold nanorods, the group of Oraevsky [15, 134,
136, 137] successfully demonstrated OA imaging as a noninvasive technique in small
animal models. figure 5.13 shows 3D OAT of a mouse treated with an intravenous
injection of Pegylated gold nanorods solution.

5.4.4 gold Nanocages

With a more complicated shape, gold nanocages represent a class of optically tunable
nanoparticles with cuboidal symmetry and “hollow” shape [102, 138, 139]. Hollow gold
nanoparticles are made via galvanic replacement chemistry from the template of silver
sols [140-142]. silver redox potential (Ag + /Ag 0.8V vs. standard hydrogen electrode
(sHe)) has a lower redox potential than gold (Aucl 4 − /Au 0.99V vs. sHe) [139, 143,
144], leading to the following replacement reaction between Ag nanocubes and HAucl 4 :

0 − 0 + −

3 Ag + AuCl = Au + 3 Ag + 4 Cl

() () () () () s 4 aq s aq aq