Extensive libraries of nanoparticles, composed of an assortment of different sizes, shapes, and materials, and with various chemical and surface properties, have already been constructed. The field of nanotechnology is under constant and rapid growth and new additions continue to suppliment these libraries. The classes of nanoparticles listed below are all very general and multi-functional, however, some of their basic properties and current known uses in biotechnology, and particularly nanomedicine, are described here.
- Fullerenes: Buckyballs and Carbon tubes
Both members of the fullerene structural class, buckyballs and carbon tubes are carbon based, lattice-like, potentially porous molecules. Buckyballs are spherical in shape while carbon tubes are cylindrical. The diameter of a carbon tube can be several nm but the length can be much greater, up to several mm, depending on its intended use.Carbon tubes have many applications in materials science due to their strength and unique electrical properties. However, they have also found use in the field of biomedicine as carriers for vaccines, drugs and other molecules. A single wall carbon tube is a one-atom-thick sheet of graphite, resembling chicken wire, rolled seamlessly into a tube. There are also multi-walled and other types of tubes depending on the shape, diameter, density (hollow versus solid) and other properties.
- Liposomes
Liposomes are lipid-based nanoparticles used extensively in the pharmaceutical and cosmetic industries because of their capacity for breaking down inside cells, once their delivery function has been met. Liposomes were the first engineered nanoparticles used for drug delivery but problems such as their propensity to fuse together in aqueous environments and release their payload, have lead to replacement, or stabilization using newer alternative nanoparticles.
- Nanoshells
Also referred to as core-shells, nanoshells are spherical cores of a particular compound surrounded by a shell or outer coating of another, which is a few nanometers thick. One application in biomedicine is to create nanoshells that absorb at biologically useful wavelengths, depending on the shell thickness.One common formula for the construction of nanoshells is to use silica for the core and another sticky compound to adhere gold particles to the outside surface, creating the shell. Nanoshells such as these have been used to kill cancer cells in mice. Once injected into a tumor, radiation is applied and the nanoshells heat up enough to kill the tumor cells.
- Dendrimers
Dendrimers are highly branched structures gaining wide use in nanomedicine because of the multiple molecular "hooks" on their surfaces that can be used to attach cell-identification tags, fluorescent dyes, enzymes and other molecules. The first dendritic molecules were produced around 1980, but interest in them has blossomed more recently as biotechnological uses are discovered.Nanomedical applications for dendrimers are many and include nanoscale catalysts and reaction vessels, micelle mimics, imaging agents and chemical sensors, and agents for delivering drugs or genes into cells. There are two basic structural types. One is the globular structure with a central core from which branches radiate. The second type has no central core and consists simply of a series of highly branched polymers.
- Quantum dots
Also known as nanocrystals, quantum dots are nanosized semiconductors that, depending on their size, can emit light in all colours of the rainbow. These nanostructures confine conduction band electrons, valence band holes, or excitons in all three spacial directions. Examples of quantum dots are semiconductor nanocrystals and core-shell nanocrystals, where there is an interface between different semiconductor materials. They have been applied in biotechnology for cell labelling and imaging, particularly in cancer imaging studies.
- Superparamagnetic nanoparticles
Superparamagnetic molecules are those that are attracted to a magnetic field but do not retain residual magnetism after the field is removed. Nanoparticles of iron oxide with diameters in the 5-100 nm range, have been used for selective magnetic bioseparations. Typical techniques involve coating the particles with antibodies to cell-specific antigens, for separation from the surrounding matrix.Used in membrane transport studies, superparamagenetic iron oxide nanoparticles (SPION) are applied for drug delivery and gene transfection. Targeted delivery of drugs, bioactive molecules or DNA vectors is dependent on the application of an external magnetic force that accelerates and directs their progress towards the target tissue. They are also useful as MRI contrast agents.
- Nanorods
Typically 1-100 nm in length, nanorods are most often made from semiconducting materials and used in nanomedicine as imaging and contrast agents. Nanorods can be made by generating small cylinders of silicon, gold or inorganic phosphate, among other materials.
Current concerns over the safety of nanoparticles have lead to the development of many new facets of research. As a result, our collection of knowledge about nanoparticle interactions within cells is still rapidly growing. As resarch progresses in this exciting new area of biotechnology, new nanoparticles are continuously being discovered and new applications to nanomedicine will be found.
