Gold Nanoparticles

Gold nanoparticles have been extensively studied over the last decade in numerous therapeutic applications. By definition gold nanoparticles are composed of nanosized colloidal gold particle suspended in water or other liquids that exhibit optical, electronic, and molecular recognition properties [2]. These unique properties of gold nanoparticles can also be fine-tuned to obtain desired effect by adjusting the size, shape, and surface chemistry [9]. These nanoparticles also come in various shapes such as nanorods, nanostars, nanocubes, nanotriangles, nanoclusters, and nanoshells, each with a unique set ofproperties and applications. Nanoparticles of noble metals like gold display surface plasmon resonance, a property that is responsible for strong electromagnetic fields with high optical absorption and emission properties [10—12]. Contrary to quantum dots are the smaller gold nanoparticles that absorb light in the lower wavelength region reflecting red light. As their size increases these dots emit more clear or translucent color [12].

Gold nanoparticles can be prepared by chemical, physical, and biological methods. Most commonly the process involves reduction of dissolved chloroauric acid (H [AuCl4]) to generate Au⁺ ions from Au³⁺ ions, followed by disproportionation reaction, three Au⁺ ions generate one Au³⁺ and two Au⁰ ions. The Au⁰ ions act as nucleation centers around which Au⁺ ions are reduced. The resulting unstable surface charge on particles is then stabilized with surfactants to prevent aggregation resulting in dispersion within colloidal suspension [10].

Photodynamic therapy (PDT) is a treatment with a combination of drug and photosensitizer agents, which are exposed to certain wavelength of visible light to trigger photochemical reaction. This generates singlet oxygen molecules that cause tumor cell death. Several confounding factors are involved in this type of therapy with limited application. Gold nanoparticles are biocompatible with low cytotoxicity rendering them one of the safest candidates for biomedical applications like imaging (sensory probes), therapeutic drug delivery, and catalysis [3]. Larger gold nanoparticles (>40 nm) are applied for imaging because of high scattering properties. Smaller particles

(<20 nm), applied for photothermal therapy, are capable of absorbing most of the incident light energy creating sufficient heat to denature proteins leading to tumor cell death [11,12].