A nanoparticle is an ultra fine particle with at least one dimension between 1-100 nanometers (nm). One nanometer is equal to one billionth of a meter. The lower size limit helps to distinguish a particle from random clusters of atoms. The upper limit is the largest at which size related property differences normally manifest themselves.
This definition is widely accepted, though it is a bit arbitrary. There are published references to nanoparticles at sizes outside the 1-100 nm range. What makes such particles of interest to scientists are the unique material properties that sometimes result from their size. When particles manifest such properties, they will likely be considered nanoparticles even if they do not fit precisely within the defined size range.
It is not necessarily the case that a nanoparticle will display property differences from larger instances of the same material. When it does occur, the property differences may be due to quantum effects. It is also true that at the nanoscale, particles of a material have a relatively larger surface area compared to their volume. The proportionally larger exposed surface can make nanoparticles much more chemically active. This may be another cause of their unexpected properties.
A quantum dot is a semiconductor nanoparticle about 1-20 nm in diameter. Its structure is essentially the same as larger semiconductors. The electronic properties it displays can be very different, however. These properties are the result of the quantum size effect. When physical size approaches the wavelength of an electron, the relationship between voltage and conductance can be different than at larger scales.
Gold and silver are relatively inert in bulk amounts. On the nanoscale, however, they demonstrate unique catalytic properties. For example, silver nanoparticles are an effective antibiotic. Nanoparticles of gold have proven to be efficient at removing volatile organic compounds from the atmosphere, even at room temperature.
Nanotechnology is concerned with making use of the unique properties of these ultra fine particles to engineer systems that function on the molecular or atomic levels. The special properties of the particles are seen to have potential in computer technology, medicine and environmental engineering. They may also form the building blocks for complex devices designed to operate on the microscopic level.
Concerns have been expressed about human exposure to nanoparticles. Animal research has demonstrated that some types of nanoparticles can reach the brain and other organs when inhaled. Inflammation and fibrosis in the lungs has also been reported. Explosion and fire in the workplace have proven to be the principle hazards of these particles, however.