17-03-2014, 09:25 PM
Abstract—Medical applications of metal nanoparticles are
the subject of intense research due to their unique
properties which make them suitable for both diagnostic
and therapeutic use. One such property is the Surface
Plasmon Resonance (SPR) which results in strong
enhancement of the absorption and scattering of
electromagnetic radiation. The combination of metal type,
size, and shape characteristics provides unique tunability
of a nanostructure’s optical properties. Several types of
nanoparticles have been explored for medical and
biological applications. Here we present a theoretical
investigation of a novel metal nanostructure which has the
unique property of distinct absorption and scattering
plasmon bands. This could be beneficial for combined
diagnostic and therapeutic applications since the
diagnostic and therapeutic laser wavelengths can be
decoupled for increased efficacy and safety. For this
purpose, it is desirable to have the most intense scattering,
with minimal absorption, in the near-infrared for imaging
and the opposite in the red, for therapy. The efficiency
factor for various metals, shapes and sizes was first
calculated using the Discrete Dipole Approximation (DDA)
method. From the results, nanostructures consisting of
combinations of cubes and small spheres were considered
to have the desired property and were thoroughly
investigated. The number (diameter) and material (silver
or gold) of the nanospheres were varied in order to obtain
the optimum nanostructure with distinct absorption and
scattering plasmon band. Given its properties, this
nanostructure have the potential to be used for
enhancement of various imaging and therapeutic methods.
Keywords—nanocube; nanosphere; silver; gold; absorption;
scattering; imaging; therapy
I. INTRODUCTION
Metal nanoparticles can have unique optical properties due
to the existence of Surface Plasmon Resonance (SPR). By
varying the size, shape, metal type, external medium, and the
coupling and interactions between adjacent nanoparticles, the
SPR frequency can range from visible to near-infrared (NIR)
wavelengths [1, 2]. Most of the research effort focuses on
noble metals, such as silver and gold, since they exhibit strong
absorption and scattering enhancements which can be exploited
in imaging, sensing, and therapeutic applications [3, 4]. Gold
nanostructures have been extensively studied as potential
theranostic agents [5-7]. Recent reports discuss the synthesis,
preparation, and surface modifications necessary and suggest
potential applications in sensing, e.g. with Surface Enhanced
Raman Spectroscopy (SERS) [8], imaging, e.g. with Optical
Coherence Tomography (OCT) [9] and therapy, e.g. with
photothermal therapy [10].
Optical medical imaging and spectroscopic applications can
significantly benefit from using NIR light, which minimizes
absorption and scattering and therefore allows deeper
penetration of the incident radiation into tissue. Photothermal
applications, on the other hand, could benefit from
nanostructures having strong absorption with limited
scattering, for better efficiency. Here we present a theoretical
investigation of a novel metal nanostructure which has the
property of distinctly separated absorption and scattering
plasmon bands. This could be beneficial for combined
diagnostic and therapeutic applications since the diagnostic and
therapeutic laser wavelengths can be decoupled for increased
efficacy and safety. For this purpose, it is desirable to have the
most intense scattering, with minimal absorption, in the nearinfrared
for imaging and the opposite in the red, for therapy.
Such a structure would allow independent control of imaging
and therapy for combined theranostics.
Several metal nanostructures, gold and silver, nanospheres,
nanoshells, nanorods, nanocubes and tetrahedral, have been
investigated so far. Their optical properties have been
extensively studied using electrodynamic methods. Extinction
spectra were calculated for various metals, sizes and shapes
and compared with experimental results [1, 2, 11-13]. Small
nanospheres, exhibit mostly absorption plasmon bands in
visible range [2, 14]. As the size increases, scattering becomes
more intense, overlapping with absorption which is not optimal
for theranostic applications. Silver is mostly a scattering
material but its SPR is in the visible wavelength range [15].
Gold SPR is shifted to longer wavelengths, but with, again,
overlapping absorption and scattering spectra. Similar is the
case for most nanostructures investigated so far [13-16]. A
nanostructure having distinct absorption (at visible
wavelengths) and scattering (at NIR wavelengths) plasmon
bands, suitable for combined theranostic applications, has not
yet been described.