Plasmonics & Surface Plasmon Resonance
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).
--- Invisibly small particles of gold can manipulate properties of light. (Gold Nanoparticle Photonics and Plasmonics - Nature, Vol 495, 2013) ---
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.
Contact us for more information about the plasmonic properties of gold nanoparticles. We would be glad to help you figure out if our particles are a good fit for your research.
Related links:
Photoacoustic Imaging: 'The sound of light'