Biomedical Engineering Reference
In-Depth Information
2.2.1
Nanoparticles
Nanoparticles are nanometer-sized particles that are nanoscale in three dimensions. They include
nanopores, nanotubes, quantum dots, nanoshells, dendrimers, liposomes, nanorods, fullerenes, nano-
spheres, nanowires, nanobelts, nanorings, and nanocapsules. The applications of nanoparticles
include drug delivery systems, cancer targeting, and dentistry
[2]
. Nanoparticles are of great scien-
tific 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 for the
nanoscale this is often not the case. Size-dependent properties are observed such as quantum confine-
ment in semiconductor particles, surface plasmon resonance in some metal particles, and super
paramagnetism in magnetic materials
[16]
. Nanoparticles exhibit a number of special properties rela-
tive to bulk material. Nanoparticles often have unexpected visual properties because they are small
enough to confine their electrons and produce quantum effects. For example, gold nanoparticles
appear deep red to black in solution. The often very high surface-area-to-volume ratio of nanoparti-
cles provides a tremendous driving force for diffusion, especially at elevated temperatures. Sintering
is possible at lower temperatures and over shorter durations than for larger particles. This theoreti-
cally does not affect the density of the final product, though flow difficulties and the tendency of
nanoparticles to agglomerate do complicate matters. The surface effects of nanoparticles also reduce
the incipient melting temperature. Nanoparticles are being applied in various industries, including
medicine, due to various properties such as increased resistance to wear and the killing of bacteria,
but there are worries due to the unknown consequences to the environment and human health
[17]
.
2.2.2
Characterization
The first observations and size measurements of nanoparticles were made during the first decade of
the twentieth century. They are mostly associated with the name of Zsigmondy who made detailed
studies of gold sols and other nanobiomaterials with sizes down to 10 nm and less. Zsigmondy
published a topic in 1914. He used an ultramicroscope that employs a dark field method for seeing
particles with sizes much less than light wavelength. Applications began in the 1980s with the
invention of the scanning tunneling microscope and the discovery of carbon nanotubes and fuller-
enes. In 2000, the US government founded the National Nanotechnology Initiative to direct
nanotechnological development. There are traditional techniques developed during twentieth cen-
tury in Interface and Colloid Science for characterizing nanobiomaterials. These are widely used
for first generation passive nanobiomaterials
[18]
. These methods include several different techni-
ques for characterizing particle size distribution. This characterization is imperative because many
materials that are expected to be nanosized are actually aggregated in solutions. Some of the
methods are based on light scattering. Others apply ultrasound, such as ultrasound attenuation
spectroscopy for testing concentrated nanodispersions and microemulsions. There is also a group of
traditional techniques for characterizing surface charge or zeta potential of nanoparticles in
solutions. This information is required for proper system stabilization, preventing its aggregation or
flocculation. These methods include microelectrophoresis, electrophoretic light scattering, and
electroacoustics. Nanobiomaterials behave differently than other similarly sized particles. It is
therefore necessary to develop specialized approaches to testing and monitoring their effects on
human health and on the environment
[19]
.
