https://nanohybrids.net/pages/plasmonics
The unique optical properties of plasmonic nanoparticles have been
observed for thousands of years. Since ancient times artists have used
colloidal nanoparticles of gold, silver, and copper to give color to
pottery and stained glass. The beautiful range of colors results from
adjustable optical properties in certain plasmonic nanoparticles. The
phenomena that provides tunable control of nanoparticle light absorption
and scattering is known as surface plasmon resonance (SPR).
Understanding of SPR has led to the development of many new
plasmonic nanoparticle structures with utility in many applications
including photocacoustic imaging, photothermal cancer therapy, biosensor and immunoassay development, and surface enhanced Raman spectroscopy (SERS).
Plasmon Defined
Electromagnetic radiation has photons and mechanical vibration has phonons. Similarly, plasma oscillation has plasmons. A plasmon is a quasi-particle defined as a quantum of plasma oscillation, commonly observed in metals.
Metallic bonding consists of a sea of negative electrons
surrounding islands of positive nuclei. This sea of electrons flows like
the tide, oscillating about their atomic islands. This collective
oscillation of the free electron gas with respective to the fixed
positive nuclei is a plasmon. Plasmons play a large role in the optical
characteristics of metals.
Surface Plasmon Resonance (SPR)
SPR occurs in plasmonic metal nanoparticles when the free surface
electrons collectively oscillate, induced by light of specific
wavelength. Figure 1 illustrates surface plasmon
resonance (SPR) for a metallic sphere. When an incoming electromagnetic
wave matches the frequency of the oscillation of the electron cloud, SPR
resonance occurs and the light is absorbed.
Figure 1: Schematic showing surface plasmon resonance (SPR) for a metallic sphere
This effect depends upon the polarizability of a particular
nanoparticle. The polarizability is dependent upon numerous factors,
including the size, shape, material composition, surface coating, and
medium. Each of these factors can be tuned to change the resonance
wavelength, though some have a larger effect than others.
For spherical plasmonic nanoparticles, the resonant wavelength
depends on the particle’s radius, material composition, and the
refractive index of the medium. Increasing the radius or the medium's
refractive index will cause a red shift of plasmon resonance (increases
the wavelength at which plasmon resonance occurs).
According to Gans theory, polarizability, and therefore plasmon
resonance wavelength, is highly dependent on both size and shape. When
symmetry is broken, a particle gains additional modes of plasmon
resonance. In the case of gold nanorods, this means that they have two
SPR wavelengths: transverse and longitudinal. Figure 2 illustrates the two plasmon resonances of gold nanorods.
Figure 2: Schematic showing the two SPRs of gold nanorods
The plasmonic gold nanorod is more easily polarized
longitudinally, meaning the SPR occurs at a lower energy, and thus
higher wavelength. As the aspect ratio (ratio of length to width) of a
nanorod is increased for a fixed diameter, the longitudinal and
transverse plasmon resonances are both affected; however, the
longitudinal axis is more polarizable and more sensitive to aspect ratio
changes. In gold nanorods, the longitudinal surface plasmon resonance
(LSPR) wavelength can be tuned from 550 nm to over 2000 nm by adjusting
to longer aspect ratios, while the transverse surface plasmon resonance
(TSPR) remains relatively constant at ~510 – 520 nm. As a convention,
the peak LSPR (as opposed to TSPR) wavelength is often quoted to define
gold nanorods with absorbance spanning the visible to near-infrared
region (NIR).
Surface Coating Effects
Since the SPR wavelength is dependent upon
interfacial properties, the medium surrounding plasmonic nanoparticles
is also an important factor. As the refractive index of the surrounding
medium is increased, the SPR red-shifts to longer wavelengths. This
effect allows plasmonic nanoparticles to be used as efficient molecular
sensors. When molecules adsorb to or desorb from the particle surface,
the local refractive index changes, resulting in an SPR wavelength
shift. This effect is also why gold nanoparticles exhibit different SPR
wavelengths dependent upon surface coating.
NanoHybrids Gold NanoRods
are designed at a specific aspect ratio to achieve ultimate peak
absorbance of 780 nm, 808 nm, or 850 nm after coating. Our CTAB
(Cetrimonium bromide), PEG (Polyethylene glycol), silica, and PEG-silica
coated nanorods are all synthesized to have the same size distributions
and aspect ratios independent of coating; any variations in LSPR are
then due to the different surface coatings.
Plasmonics in Photoacoustic Imaging (Optoacoustic Imaging)
Plasmonic properties of contrast agents play a crucial role in optical imaging techniques like photoacoustic imaging.
When a plasmonic nanoparticle is irradiated with light corresponding to
its SPR wavelength, the plasmon, or collective motion of electrons on
the surface of the nanoparticle, generates heat. In photoacoustic
imaging, laser light is delivered in a pulsed manner, causing the
nanoparticles to warm in a transient fashion and transfer heat to local
surroundings. This principle serves as the foundation for using gold
nanoparticles in Plasmonic PhotoThermal Therapy (PPTT) to treat various
cancers.
Since the coefficient of thermal expansion is much greater for
water than for gold, the water surrounding plasmonic nanoparticles
thermoelastically expands in response to heat deposition. This
expansion, generates a pressure wave that can be read as sound by a
transducer and is the source of photoacoustic signal in photoacoustic
imaging techniques.
Special Properties of gold nanorods and silica-coated gold nanorods
As previously described, the SPR wavelength of gold nanorods can be
tuned from the visible to NIR to match the desired incident wavelength
or laser source. Since organic tissue absorbs light minimally in the
near-IR region, properly tuned gold nanorods make for excellent contrast
agents in vivo.
In photoacoustic imaging, the laser source creates rapid heating
of the rods, which can cause melting, leading to degradation in optical
properties, and the loss of repeatable imaging. With silica-coated nanorods,
the heat quickly dissipates into the silica and the risk of melting is
significantly minimized. This efficient heat transfer provides a large
photoacoustic signal enhancement (from 3 to 10 fold) while also
stabilizing the nanorods for repeated use.
