A nanoshell is a spherical particle engineered at the nanoscale, typically between 10 and 200 nanometers in diameter. They are composite materials, meaning they are made of at least two different components combined to produce characteristics that are distinct from the individual materials.
The Structure of a Nanoshell
A nanoshell possesses a two-part construction, often described as a core-shell architecture. This design consists of a central, spherical core made of one material that is completely encapsulated by an outer metallic layer, or shell. The structure is analogous to a microscopic piece of candy with a distinct center and a thin outer coating.
The core is commonly made of silica, a type of silicon dioxide, because it provides a stable and inert scaffold whose surface can be readily modified. Gold is chosen for the shell for its biocompatibility, non-toxic nature, and its distinct interactions with light. The combination of a non-conducting (dielectric) core with a conducting metal shell gives the nanoshell its specialized optical and physical properties.
The dimensions of these layers are controlled with high precision. The silica core can have a diameter ranging from approximately 40 to 180 nm. The thickness of the gold shell is also meticulously managed, measuring only a few nanometers.
How Nanoshells Interact with Light
The interaction between nanoshells and light is governed by a phenomenon known as localized surface plasmon resonance. When light of a specific wavelength strikes the metallic surface of the nanoshell, it causes the free electrons in the gold to oscillate collectively. This resonant oscillation results in the particle either strongly absorbing or scattering the light. The energy from the absorbed light is then rapidly converted into heat.
Scientists can precisely control which wavelengths of light the nanoshell will interact with by adjusting the ratio of the core’s diameter to the shell’s thickness. Decreasing the shell’s thickness relative to the core’s diameter shifts the particle’s light absorption peak to longer wavelengths.
This tunability allows nanoshells to be engineered to interact with near-infrared (NIR) light. The NIR spectrum, from 700 to 900 nanometers, is known as the “biological window” because light in this range can penetrate deeply into human tissue with minimal absorption by water and hemoglobin. By designing nanoshells to absorb light in this window, an external light source can be used to activate them deep within the body.
Medical Applications in Treatment and Diagnosis
Nanoshells have led to the development of several medical applications, particularly in oncology. Their ability to be tuned to the near-infrared spectrum makes them effective for photothermal therapy, a non-invasive treatment method. In this approach, nanoshells are administered and accumulate in tumors due to the leaky blood vessels that characterize many cancers, a phenomenon known as the enhanced permeability and retention (EPR) effect.
Once the nanoshells have gathered at the tumor site, an external near-infrared laser is shone on the tissue. The light passes harmlessly through healthy tissue but is strongly absorbed by the nanoshells concentrated in the tumor. This absorption causes the nanoshells to heat up, raising the local temperature to levels above 42°C, which is sufficient to destroy cancerous cells through a process called photothermal ablation. This method is precise, as cell death is confined to the illuminated area containing the nanoshells.
Beyond therapy, nanoshells also serve as contrast agents in medical imaging. Their capacity to strongly scatter light makes them highly visible in techniques like optical coherence tomography (OCT). When nanoshells accumulate in a tumor, they enhance the contrast between diseased and healthy tissue, allowing for more accurate diagnosis and monitoring.
Another application is in targeted drug delivery. Therapeutic agents can be attached to the surface of gold nanoshells. These drug-loaded particles can then be delivered directly to a target site, such as a tumor. Some systems are designed so the drug is released when the nanoshell is heated by a laser, providing a controlled release that can increase treatment efficacy and reduce systemic side effects.
Fabrication and Engineering
The creation of nanoshells is a multi-step process requiring precise chemical synthesis. The process begins with the synthesis of uniform, spherical silica nanoparticles to serve as the cores. These nanoparticles are created using the Stöber method, which allows for control over the size of the silica spheres.
Following the formation of the silica cores, their surfaces must be prepared for the gold shell. This involves functionalizing the silica surface by attaching very small gold colloid particles, just one or two nanometers in diameter, to the surface of the silica cores. These tiny gold particles act as seeds for the growth of the complete shell.
The final stage involves growing the gold shell around the seeded cores. The silica particles, now decorated with gold seeds, are placed in a chemical solution containing a gold salt. A reducing agent is then added to the solution, causing the gold ions to deposit onto the gold seeds. This process continues until the individual seeds grow and merge, forming a continuous, thin metallic shell over the entire silica core.