The first syntheses of metallic gold nanoparticles most probably date back to the 5th or 4th century BC where gold specimens were reported in China and Egypt. Since then, they have been exploited for both their curative and aesthetic properties.
The first aspect that made some alchemists see in gold colloidal solutions, the elixir of youth. Their optical properties were exploited for coloration of glass, ceramics, china and pottery. The optical properties of gold nanoparticles filled Faraday’s enthusiasm when he reported in 1857 the synthesis of colloidal solutions of gold exhibiting colors ranging from ruby red to amethyst. He pointed factors impacting the color of those solutions and evidenced that ‘‘the mere variation in the size of particles gave rise to a variety of resultant colors.’’ Many applications of this property have arisen recently, leading to a impressive bibliography. The subject is indeed so fashionable that even the New York Times focused on it. The applications and types of gold nanoparticles have expanded significantly from the bulk aggregation response used in lateral flow assays (e.g. in pregnancy tests). Now their single nanoparticle properties are being used in biomedical, material, optical, and industrial applications. These materials are still being actively researched, and applications utilizing their enhanced various physical and chemical properties are only beginning to be realized.
Colloidal gold, also known as gold nanoparticles, is a suspension (or colloid) of nanometer-sized particles of gold. The most prevalent method for the synthesis of monodisperse spherical gold nanoparticles was pioneered by Turkevich et al. in 1951 and later refined by Frens et al. in 1973. This method uses the chemical reduction of gold salts such as hydrogen tetrachloroaurate (HAuCl4) using citrate as the reducing agent. This method produces monodisperse spherical gold nanoparticles in the range of 10–20 nm in diameter. However, the synthesis of larger gold nanoparticles with diameters between 30 and 100 nm was reported by Brown and Natan via seeding of Au3+ using hydroxylamine. Subsequent research led to the modification of the shape of these gold nanoparticles resulting in rod, triangular, polygonal rods, and spherical particles. These ensuing gold nanoparticles have unique properties, providing a high surface area to volume ratio. Moreover, the gold surface offers a unique opportunity to conjugate ligands such as oligonucleotides, proteins, and antibodies containing functional groups such as thiols, mercaptans, phosphines, and amines, which demonstrates a strong affinity for gold surface. The realization of such gold nanoconjugates coupled with strongly enhanced Plasmon resonance of gold nanoparticles have found applications in simpler but much powerful imaging techniques such as dark-field imaging, and optical imaging for the diagnosis of various disease states. In fact, El Sayed et al. have established the use of gold nanoparticles for cancer imaging by selectively transporting Au nanoparticles into the cancer cell nucleus. In order to selectively transport the Au nanoparticles into the cancer cell nucleus, they conjugated arginine–glycine–aspartic acid peptide and a nuclear localization signal peptide to a 30-nm Au nanoparticles via PEG.
Moreover, the use of gold nanostructures as photothermal agents sets them apart from all nanoprobes. Photothermal therapy is a procedure in which a photosensitizer is excited with specific band light (mainly IR). This activation brings the sensitizer to an excited state where it then releases vibrational energy in the form of heat. The heat is the actual method of therapy that kills the targeted cells. One of the biggest recent successes in photothermal therapy is the use of gold nanoparticles. Spherical gold nanoparticles absorptions have not been optimal for in vivo applications. This is because the peak absorptions have been limited to 520 nm for 10 nm diameter. Moreover, skin, tissues, and hemoglobin have a transmission window from 650 up to 900 nm. This was circumvented by the discovery of gold nanostructures by Murphy and coworkers, who were able to tune the absorption peak of these nanoparticles, from 550 nm up to 1 μm just by altering their morphology and size.
Some Au nanoparticles applications:
Therapeutic Agent Delivery