Nanoshell

Part of a series of articles on Nanomedicine

Nanotoxicology Nanosensor Nanoshell Nanorobotics

See also Nanotechnology

A nanoshell, or rather a nanoshell plasmon, is a type of spherical nanoparticle consisting of a dielectric core which is covered by a thin metallic shell (usually gold). These nanoshells involve a quasiparticle called plasmon which is a collective excitation or quantum plasma oscillation where the electrons simultaneously oscillate with respect to all the ions.

The simultaneous oscillation can be called plasmon hybridization where the tunability of the oscillation is associated with mixture of the inner and outer shell where they hybridize to give a lower energy or higher energy. This lower energy couples strongly to incident light whereas, the higher energy is an anti-bonding and weakly combines to incident light. The hybridization interaction is stronger for thinner shell layers, hence, the thickness of the shell and overall particle radius determines which wavelength of light it couples with. Nanoshells can be varied across a broad range of the light spectrum that spans the visible and near infrared regions. The interaction of light and nanoparticles affects the placements of charges which affects the coupling strength. Incident light polarized parallel to the substrate gives a s-polarization (Figure 1b), hence the charges are further from the substrate surface which gives a stronger interaction between the shell and core. Otherwise, a p-polarization is formed which gives a more strongly shifted plasmon energy causing a weaker interaction and coupling.

Theory

These plasmonic nanoshells offer a spectral tune-ability that is directly related to their size. By increasing or decreasing the inner and outer dimensions of the nanoshell, different wavelengths of light must be used to incur scattering or absorption. However, scattering and absorption are also dependent upon the current charge of the plasmonic shell. As mentioned above in the introduction, the higher energy modes (strong coupling between the inner and outer diameters) react poorly to incident light, while lower energy modes (weak coupling between the inner and out diameters) react better with incident light. Additionally, as opposed to current dyes used in MRI scanning or dual modality imagery (an imaging strategy that uses x-rays and radionuclide imaging), these plasmonic nanoshells allow for multiple different sizes to be used at once, with multiple different functions. Because of the specificity of the shell size related to the light required to meet resonance, only one size of plasmonic nanoshells will be activated by any tuned activation light, so long as the spectral range of the activation light is small enough. This means that the individual functions of the nanoshells can be activated in any order that is desired.

The plasmonic shells can be described as having multiple attainable energy levels. These energy levels can be considered, and are actually quite similar to atomic energy levels (the main difference being scale). Thus based off of the ratio inheirent to the thinkness of the nanoshell, a total of four different energy levels can be reached by charging/activating the plasmon with the correct activation light. This dependence on the inner and outer dimensions of the plasmonic shell can be described through electromagnetic theory, it has also recently been verified using ab initio quantum mechanical electronic structure methods.

Synthesis

These nanoshell plasmons are created through a chemical process where by a silica dielectric core which has its outer most parts functionalized by oxide nanoparticles. This is accomplished by using aminopropyltiethoxy- or –methoxysilane. Once this is accomplished, the dielectric core is exposed to small (1-2 nm in diameter) gold nanoparticles. These particles then begin to cover the surface to a uniform depth that is dependent upon the amount of time they were exposed to these gold nanoparticles (this uniformity is governed by the laws of entropy). A nanoshell is synthesized in a multistep process :

1. Obtain gold nanoparticles in a solution (usually tetrachloroauric acid and a reducing agent)

   This solution phase synthesis of the gold nanoparticles uses a reduction using tetrachloroauric acid by a reducing agent. There are several different reducing agents used and all can greatly affect the uniformity of the nanoparticle

2. Attach a very small seed colloid onto the dielectric nanoparticles (such as: zinc selenide, sapphire, and glass) giving a discontinuous shell
3. Grow a continuous shell by using a chemical reduction of the metal attached to the dielectric nanoparticles (the most common reduction agents being formaldehyde and carbonmonoxide)

If a uniform shell is not obtained then it can greatly affect the optical properties of the nanoshell. A good example of this is a nanoegg, which is a metallic nanoshell that has a nonuniform thickness. This characteristic nonuniformity causes additional hybridized plasmon resonances in the spectrum making the coupling not as effective.

Applications

Since nanoshells possess highly favorable optical and chemical properties it is often used for biomedical imaging, therapeutic applications, fluorescence enhancement of weak molecular emitters, surface enhanced Raman spectroscopy and surface enhanced infrared absorption spectroscopy.

Active Plasmonics

Nanoshell plasmonics can be coated in nanoparticles to modify or drive a reaction near a metallic surface when properly excited. Additionally, the scattering of light from these plasmonics can be controlled and even directed based on the surface particles, geometry, and size.

Energy Applications

Nanoshell plasmonics can be used in harvesting solar radiation for energy applications. This is accomplished by redirecting incident light into the waveguide and evanescent surface modes of thin film photovoltaic devices.

Using multiple layered plasmons to purify water is also being investigated.

For more information on the research behind energy applications, and the collaborations behind this research, please visit the Halas group website listed below in the external links.

Cancer Treatment

Gold shelled nanoparticles, which are spherical nanoparticles with silica cores and gold shells, are used in cancer therapy and bio imaging enhancement. Theranostic probes – capable of detection and treatment of cancer in a single treatment - are nanoparticles that have binding sites on their shell that allow them to attach to a desired location (typically cancerous cells) then can be imaged through dual modality imagery (an imaging strategy that uses x-rays and radionuclide imaging) and through near-infrared fluorescence. The reason gold nanoparticles are used is due to their vivid optical properties which are controlled by their size, geometry, and their surface plasmons. Gold nanoparticles (such as AuNPs) have the benefit of being biocompatible and the flexibility to have multiple different molecules, and fundamental materials, attached to their shell (almost anything that can normally be attached to gold can be attached to the gold nano-shell, which can be used in helping identifying and treating cancer). The treatment of cancer is possible only because of the scattering and absorption that occurs for plasmonics. Under scattering, the gold plated nanoparticles become visible to imaging processes that are tuned to the correct wavelength which is dependent upon the size and geometry of the particles. Under absorption, photothermal ablation occurs, which heats the nanoparticles and their immediate surroundings to temperatures capable of killing the surrounding cells. This is accomplished with minimal damage to cells in the body due to the utilization of the "water window" (the spectral range between 800-1300nm). As the human body is comprised mostly of water, this optimizes the light used versus the effects rendered.

These gold nanoshells are shuttled into tumors by the use of phagocytosis where phagocytes engulf the nanoshells through the cell membrane to form an internal phagosome, or macrophage. After this it is shuttled into a cell and enzymes are usually used to metabolize it and shuttle it back out of the cell. These nanoshells are not metabolized so for them to be effective they just need to be within the tumor cells and photoinduced cell death (as described above) is used to terminate the tumor cells. This scheme is shown in Figure 2.

Nanoparticle-based therapeutics have been successfully delivered into tumors by exploiting the enhanced permeability and retention effect, a property that permits nanoscale structures to be taken up passively into tumors without the assistance of antibodies.[4] Delivery of nanoshells into the important regions of tumors can be very difficult. This is where most nanoshells try to exploit the tumor’s natural recruitment of monocytes for delivery as seen in the above figure. This delivery system is called a "Trojan Horse".

This process works so well since tumors are about ¾ macrophages and once monocytes are brought into the tumor, it differentiates into macrophages which would also be need to maintain the cargo nanoparticles. Once the nanoshells are at the necrotic center, near-infrared illumination is used to destroy the tumor associated macrophages.

Additionally, these nanoparticles can be made to release antisense DNA oligonucleotides when under photo-activation. These oligonucleotides are used in conjunction with the photo-thermal ablation treatments to perform gene-therapy. This is accomplished because nanoparticle complexes are delivered inside of cells then undergo light induced release of DNA from their surface. This will allow for the internal manipulation of a cell and provide a means for monitoring a group cells return to equilibrium.

Another example of nanoshell plasmonics in cancer treatment involves placing drugs inside of the nanoparticle and using it as a vehicle to deliver toxic drugs to cancerous sites only. This is accomplished by coating the outside of a nanoparticle with iron oxide (allowing for easy tracking with an MRI machine) then once the area of the tumor is coated with the drug filled nanoparticles, the nanoparticles can be activated using resonant light waves to release the drug.

External links

• halas.rice.edu