Date: January 24, 2007
Tom Webster (Thomas_Webster@brown.edu)
Nanoparticles
Nanoparticles
A nanoparticle is a microscopic particle whose size is measured in nanometers (nm).
It is defined as a particle with at least one dimension <100nm. Nanoparticles
made of semiconducting material may also be labeled quantum
dots if they are small enough (typically sub 10nm) that quantization of
electronic energy
levels occurs. They have found their use as bulk and coatings on implants,
drug delivery, and their biggest commercial success as sun screens.
Nanoparticles are of great scientific interest
as they are effectively a bridge between bulk materials
and atomic or molecular structures.
A bulk material should have constant physical properties
regardless of its size, but at the nano-scale this
is often not the case. Size-dependent properties
are observed such as quantum
confinement in semiconductor particles, surface
plasmon resonance in some metal particles
and superparamagnetism in magnetic materials.
Semi-solid and soft nanoparticles have been manufactured.
A prototype nanoparticle of semi-solid nature
is the liposome.
The properties of materials change as their size
approaches the nanoscale. For example, the bending
of bulk copper (wire,
ribbon, etc.) occurs with movement of copper atoms/clusters
at about the 50 nm scale. Copper nanoparticles smaller
than 50 nm are considered super hard materials that
do not exhibit the same malleability and ductility as
bulk copper.
The interesting and sometimes unexpected properties
of nanoparticles are partly due to the aspects of
the surface of the material dominating the properties
in lieu of the bulk properties. The percentage of
atoms at the surface of a material becomes significant
as the size of that material approaches the nanoscale.
For bulk materials larger than one micrometre the
percentage of atoms at the surface is minuscule relative
to the total number of atoms of the material. Suspensions of
nanoparticles are possible because the interaction
of the particle surface with the solvent is
strong enough to overcome differences in density,
which usually result in a material either sinking
or floating in a liquid.
Nanoparticles often have unexpected visible properties
because they are small enough to scatter visible
light rather than absorb it. For example gold nanoparticles
appear deep red to black in solution.
At the small end of the size range, nanoparticles
are often referred to as clusters. Metal, dielectric,
and semiconductor nanoparticles
have been formed, as well as hybrid
structures (e.g., core-shell nanoparticles).
Nanospheres, nanorods,
and nanocups are just a few of the shapes that have
been grown. Semiconductor quantum dots and nanocrystals are
types of nanoparticles. Such nanoscale particles
are used in biomedical applications as drug
carriers or imaging
agents. Various types of liposome nanoparticles
are currently used clinically as delivery systems
for anticancer drugs and vaccines.
Nanoparticle characterization is necessary to establish
understanding and control of nanoparticle synthesis
and applications. Characterization is done by using
a variety of different techniques, mainly drawn from materials
science. Common techniques are electron
microscopy [TEM,SEM], atomic
force microscopy [AFM], dynamic
light scattering [DLS], x-ray
photoelectron spectroscopy [XPS], powder
x-ray diffractometry [XRD], and Fourier
transform infrared spectroscopy [FTIR].
Nanoparticle research is currently an area of intense
scientific research, due to a wide variety of potential
applications in biomedical, optical, and electronic
fields. The National
Nanotechnology Initiative of the United
States government has driven huge amounts
of state funding exclusively for nanoparticle
research.
Read more at: http://en.wikipedia.org/wiki/Nanoparticles |
