Biomedical Applications of Gold Nanoparticles: Recent Advances and Future Prospects

Introduction: Nanotechnology

“Nanotechnology” broadly defined as the creation of objects and surfaces whose unique functions are governed by the dimensions falling within the nanoscale. The prefix of the term “Nanotechnology” comes from the ancient Greek word να̃νος through the Latin nanus meaning literally dwarf or very small. Within the convention of International System of Units (SI) it indicates the reduction factor of 10^-9 times (Wani, 2014). Thus, nanotechnology manipulates matter at the one billionth of a meter. This one billionth, 10^-9part of a meter represents a nanometer. A nanometer (nm) represents a collection of a few atoms or molecules. “Nanoscale” generally refers to objects 1-100 nm in one or more dimensions. At its lower limit this definition intentionally excludes individual molecules which generally define the lower end of the nanotechnology, i.e. nano derived features are as much a function of larger bulk materials approaching a molecular scale as they are a selective change in molecules’ properties as they aggregate. Properties of the bulk materials of micrometer size or larges are currently well understood and studied by the solid state physicists and material scientists for years. Materials on the nanometer size lying within 1–100 nm scale were not studied by either group in the past. It has been very recently shown that on this size scale the properties of a material become dependent on its size and shape. At the nanoscale new properties develop due to the lack of symmetry at the interface or due to electron confinement that do not scale linearly with size. Thus, the nanometer scale (1-100 nm) incorporates collections of atoms or molecules, whose properties neither represent those of the individual constituents nor those of the bulk (Eustis & El-Sayed, 2006). New properties are observed on this scale due to the interface that is not observed in the bulk or individual atoms. Since the properties depend on the size of the structure, instead on the nature of the material, reliable and continual change can be achieved using a single material, e.g., quantum dots of CdSe of different sizes have differing maximum emissions across the whole visible region, and gold and silver nanostructures have absorption across most of the visible region (Eustis & El-Sayed, 2006). Nanometer scale is also interesting in biological systems (Gill, Dr., And, & Prof., 2005; Nathaniel L. Rosi and Chad A. Mirkin, 2005). Many proteins are nearly 10’s of nm in size. Since structures can be accurately designed on the nanometer scale and these nanostructures can be incorporated into biological systems, due to the similar size scales. The ability to rationally design structures on the same size as biological molecules generates the ability to probe and modify biological systems. Furthermore, biological systems are used to build up nanomaterials of specific shape and function. Nanostructures are being used as drug delivery agents, labeling agents, sensors, and to enhance electromagnetic fields (Eustis & El-Sayed, 2006) as discussed latter.