Silver nanoparticles are made wholly or partly from metallic silver, exist in various shapes, and range 1–100 nm in diameter. Their small size and ability to induce cell death through multiple mechanisms makes them fantastic candidates for anti-cancer therapies.
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Silver nanoparticles as chemotherapeutic agents
Silver nanoparticles are potent bactericides and chemotherapeutic agents. They can induce cell apoptosis through double-strand DNA breaks, oxidative stress, and chromosomal instability.
Although larger particles (~100 nm) tend to induce these effects more strongly than smaller particles (~10 nm), smaller particles are more cytotoxic, as they can enter the cell and localize to the nucleus easily.
Silver nanoparticles can also modulate p-glycoprotein activity, which is responsible for the transport of drugs across the cell membrane. Thus, over or under activity of this protein promotes multi-drug resistance by removing drugs from the cell interior or preventing their entry to the cell.
Drug delivery using silver nanoparticles
Nanoparticles make excellent drug delivery platforms as they can release a single or multi-type payload of small molecule chemotherapeutics in the vicinity or interior of a cancer cell.
Silver nanoparticles can also be coated with other materials to reduce their toxicity, improve biological retention time, or allow specific targeting of a tissue or cancer cell.
Antibodies, proteins, or other biomolecules are frequently used as target ligands, which ensures the accumulation of nanoparticles within the tumor due to the enhanced permeability and retention effect in tumors.
Silver nanoparticles as diagnostic agents
Silver nanoparticles are plasmonic, meaning that conduction electrons belonging to the particle can be made to collectively oscillate in the electromagnetic field of incident light.
Frequencies of light in phase with these oscillations are intensely scattered or absorbed by the nanoparticle, creating a phenomenon known as surface plasmon resonance.
Shaping the silver nanoparticle allows for the surface plasmon resonance wavelength to be fine-tuned to the tissue transparency window, the wavelength of light which is most penetrating through biological tissue. Thus, they have applications as image contrast agents or biosensors to improve the resolution of existing imaging techniques or replace fluorescent molecules.
Radiation dose enhancement
Atoms of high atomic weight are useful radiation dose enhancers that can amplify and focus a beam of radiation in a small area around them.
This is done through a series of photoelectric, compton scattering, and other similar processes, where a shower of low energy electrons from the nanoparticle generate ions which leads to the destruction of a nearby cell.
Using this method, lower doses of radiation can be delivered while maintaining efficacy against the target tumor leading to fewer negative side effects.
Combining the properties of silver nanoparticles as drug delivery vehicles, diagnostic agents, radiation dose enhancers, and cyototoxic materials presents an interesting novel approach to treat cancer.
The creation of nanocomposite materials including silver, such as silver-gold nanoparticles, allows optimizing specific properties of interest, with improved results compared with nanoparticles made wholly from gold or silver alone.
This applies to photothermal therapy, where the heating of a particle struck by incident light is enhanced by the creation of this nanocomposite.