shows the darkening of the lymph nodes after injection of cold Ge-SPION@PEG. It
is possible to affirm, thus, that radiolabeled IONPs have emerged as imaging systems with a clear capacity of allowing non-invasive quantitative imaging results
with the great advantage of supporting simultaneously multi-modality imaging.
2.3 Gold Nanoparticles
Gold NPs (AuNPs), or colloidal gold, can be defined as suspension gold metal
particles, in the nanometer scale suspended in a fluid, usually aqueous medium that
can be synthesized to have between 3 and 200 nm in diameter. In the molecular
imaging field, AuNPs have been extensively applied due to their unique optical and
electronic properties, as well as its high stability and biological compatibility.
AuNPs can also be easily surface-modified, synthesized with a controllable morphology and are considered bio-inert and nontoxic [53]. Many subtypes of gold
NPs, classified according to their size, shape, and physical properties, including
gold nanorods, nanospheres, nanoshells, nanocages as well as gold
surface-enhanced Raman scattering (SERS) NPs have been developed and preclinically investigated for molecular imaging [54].
The most commonly used method of AuNPs synthesis, the cluster beam method,
was developed more than 50 years ago and involves citrate reduction of HAuCl 4 , as
suggested by Turkevich et al. [55]. The dissolution of Au(III) salt or Au(I) complex
to Au(0) by a reducing agent, in the presence of Lewis base ligands, usually results
in monodispersed particles with controlled average diameters of 10–60 nm. For
smaller AuNPs (*5 nm), other types of reducing agents can be used [56] while for
larger particles, a method described by Goia and Matijevic [57] used. Other
methods of synthesis and optimization protocols, including electrochemical
oxidation/reduction procedure and two-step seed-mediated surfactant-assisted protocol have been proposed [58].
It is important to highlight the significance of the optical properties of AuNPs,
considering that their absorption and emission are within the visible range of light.
Because the light scattered from AuNPs is in the visible portion of the electromagnetic spectrum in accordance with their plasmon bands, an effect known as
“localized surface plasmon resonance” (LSPR) that happens when an electromagnetic wave passes through, it is possible to optically track the position of individual
NPs, paving the way for imaging applications [59] and, thus, AuNPs have shown
potential as contrast agents for optoacoustic cancer imaging [60]. Furthermore,
since SPR peaks of gold nanostructures can be easily tuned between visible and
NIR windows by simply changing the shape and size of AuNPs, they have been
variously harnessed for fluorescence (FL), photoacoustic (PA) and Raman imaging,
as well as for photothermal therapy (PTT) [61–63]. The biggest advantage of
AuNPs in imaging lies in their multiplexing ability. The inherent optical properties
and high X-ray absorption coefficient allow their use as multimodal contrast agents,
with widespread applications in optical, MR, CT and radionuclide imaging [27].
20
C. A. Ferreira et al.
is possible to affirm, thus, that radiolabeled IONPs have emerged as imaging systems with a clear capacity of allowing non-invasive quantitative imaging results
with the great advantage of supporting simultaneously multi-modality imaging.
2.3 Gold Nanoparticles
Gold NPs (AuNPs), or colloidal gold, can be defined as suspension gold metal
particles, in the nanometer scale suspended in a fluid, usually aqueous medium that
can be synthesized to have between 3 and 200 nm in diameter. In the molecular
imaging field, AuNPs have been extensively applied due to their unique optical and
electronic properties, as well as its high stability and biological compatibility.
AuNPs can also be easily surface-modified, synthesized with a controllable morphology and are considered bio-inert and nontoxic [53]. Many subtypes of gold
NPs, classified according to their size, shape, and physical properties, including
gold nanorods, nanospheres, nanoshells, nanocages as well as gold
surface-enhanced Raman scattering (SERS) NPs have been developed and preclinically investigated for molecular imaging [54].
The most commonly used method of AuNPs synthesis, the cluster beam method,
was developed more than 50 years ago and involves citrate reduction of HAuCl 4 , as
suggested by Turkevich et al. [55]. The dissolution of Au(III) salt or Au(I) complex
to Au(0) by a reducing agent, in the presence of Lewis base ligands, usually results
in monodispersed particles with controlled average diameters of 10–60 nm. For
smaller AuNPs (*5 nm), other types of reducing agents can be used [56] while for
larger particles, a method described by Goia and Matijevic [57] used. Other
methods of synthesis and optimization protocols, including electrochemical
oxidation/reduction procedure and two-step seed-mediated surfactant-assisted protocol have been proposed [58].
It is important to highlight the significance of the optical properties of AuNPs,
considering that their absorption and emission are within the visible range of light.
Because the light scattered from AuNPs is in the visible portion of the electromagnetic spectrum in accordance with their plasmon bands, an effect known as
“localized surface plasmon resonance” (LSPR) that happens when an electromagnetic wave passes through, it is possible to optically track the position of individual
NPs, paving the way for imaging applications [59] and, thus, AuNPs have shown
potential as contrast agents for optoacoustic cancer imaging [60]. Furthermore,
since SPR peaks of gold nanostructures can be easily tuned between visible and
NIR windows by simply changing the shape and size of AuNPs, they have been
variously harnessed for fluorescence (FL), photoacoustic (PA) and Raman imaging,
as well as for photothermal therapy (PTT) [61–63]. The biggest advantage of
AuNPs in imaging lies in their multiplexing ability. The inherent optical properties
and high X-ray absorption coefficient allow their use as multimodal contrast agents,
with widespread applications in optical, MR, CT and radionuclide imaging [27].
20
C. A. Ferreira et al.