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Cancer Therapies Utilizing Gold Nanoparticles
The use of gold nanoparticles in recent developments of cancer therapy treatments is being explored as drug delivery systems and target imaging for cancerous cells. Gold nanoparticles—known for their chemical inertness and high hydrophilicity—perform well in the biological system and offer very low toxicity compared to previously developed methods.

Targeted Drug Delivery
Current methods of cancer treatment necessitate large amounts of platinum-containing drugs that target all rapidly dividing cells. This method of treatment can leave a wake of destruction in non-cancerous cells and can be toxic in such large quantities. Targeted drug delivery using gold nanoparticles relieves these issues while still utilizing the platinum anticancer drugs. These gold surfaces must be functionalized to form stable, hydrophobic monolayers with the nanoparticles. They are tightly bound with a cyclic disulfide to prevent displacement by strong nucleophiles in the body. Gold-based delivery of the anticancer drug was proven to have a higher uptake than the unaccompanied drug. The drug by itself was shown to take up to twenty-four hours to fully accumulate in the cancerous site, submitting the patient to longer exposure of toxicity. Gold delivery allows the hydrophobic drug to navigate hydrophilic areas of the body, resist protein absorption, and inhibit colloidal aggregation for a more efficient delivery. One specific drug, silicon phthalocyanine 4, attached to a gold nanoparticle will take less than two hours to maximally accumulate at the tumor and can then be irradiated with light to activate treatment. PEG ligands on the nanoparticles stabilize silicon phthalocyanine 4 while also forming a cage to protect the drug from the aqueous phase.

Imaging
Gold nanoparticles, which have many available techniques for synthesis and a high conjugation to coordinate with many ligands, are also easily excited. The ability for electrons to be excited is a favorable optical quality that can be utilized in cancer cell imaging. Iodine, currently used in X-ray imaging, is cleared out of the body quickly by the kidneys because of its relatively small size. Gold, a larger molecule, strongly attenuates x-rays and remains in the body longer. Though many other molecules can improve x-ray contrast, gold has been proven the most biocompatible because it is highly inert and stable at physiological temperature and pH.

Phototherapy
Gold nanoparticles also respond favorably to certain wavelengths of light by producing standing oscillations of the surface electrons known as plasmons. When the wavelength of light is in resonance with the surface oscillations, the Plasmon Phenomenon occurs. These standing oscillations created are easily identified and highly specific. The dielectric constant of the surrounding medium determines how much electron density can be accommodated at the surface of the gold nanoparticle, thus affecting the plasmon frequency. Such information can be obtained about the medium through this method. A capping material is also necessary on the surface of the nanoparticle to prevent aggregation and precipitation out of the solution, which can affect the plasmon resonance. In particular, gold nanorods are useful due to their anisotropism creating transverse and longitudinal resonance. Surface Plasmon based imaging is essential in multiple techniques useful in tumor imaging. One of these techniques is Surface Enhanced Raman-Scattering. The strong electromagnetic field on the surface of the nanoparticle and the ability to tune to the near-infrared window of light minimizes interference to produce a high-resolution image useful for flow cytometry and contrast imaging.

Photoacoustic
Another technique utilizing the Plasmon phenomenon is photoacoustic imaging. Adding light to a sample will result in an ultrasonic sound wave, which is then detected on an ultrasound detector and displays the properties of the internal tissue. Different tissues absorb different wavelengths of light, distinguishing between cancerous and non-cancerous materials. This method is non-ionizing and highly sensitive, making it an ideal imaging method.