Biomedical Engineering Reference
In-Depth Information
2.1 Introduction
The nanoscale world with structures smaller than 250 nm remained unob-
served for a long time owing to limitations in the resolution of classic optical
microscopy as described by Abbe in 1873* [1]. However, already at that time,
synthetic nanomaterials and nanoparticles were manufactured. An example
is colloidal gold nanoparticles. If synthesized in dispersion, they form a ruby
fluid, which was studied by Faraday in the 1850s. He concluded that it was
composed of gold in very fine metallic form not visible under any microscope
available [2-3]. With the use of electrons as probe particles and the invention
of transmission electron microscopy (TEM) by Knoll and Ruska in 1932, it
became possible to overcome optical resolution limits and to observe and
study nanoscaled structures [4]; early samples were of natural origin: viruses
[5], the endoplasmic reticulum [6], and exfoliated graphites [7]. Soon, a new
quality of material research became possible by the control and manipulation
of structures at the nanometric level. This new quality of control is the cen-
tral idea behind what is called “nanotechnology” today. Its prospects were
outlined in 1959 in Feynman's famous lecture, “There's plenty of room at the
bottom.”
†
Structure optimization at the nanometer scale turned out to be an
astonishingly successful strategy. Nowadays, new nanoscaled materials are
being designed at a steadily increasing rate. The speed of nanotechnologi-
cal innovation can safely be predicted to increase even further. It is stimu-
lated by nanostructured materials showing a broad range of surprising or
even revolutionary new properties. Prominent examples in this context are
carbon nanotubes. Their extreme material properties stem from a number
of size- and structure-related phenomena emerging solely at the nanoscale.
Already simple geometrical considerations predict an increase in surface-
to-volume ratio with decreasing particle size. This is linked to an increasing
fraction of surface or grain-boundary atoms of the nanoparticle that exhibit
modified and generally enhanced reactivity. Other astonishing material
properties can be predicted from more complicated arguments of quantum
theory, which are commonly applied to small atom clusters or atom lattices:
size-constrained effects allow for tuning absorption and emission properties
of nanoscaled pigments and semiconducting quantum dot dyes. Band-gap
tailored nanoscaled solid-state structures revolutionize microelectronics and
optics. In composites, nanoparticle additives can dramatically enhance sur-
face hardness of paints by modifications of the particle-matrix interphase.
During the last decades, a close relation between nanotechnological inno-
vation speed and analytical capabilities became apparent. However, the
*
For visible light, this law limits the resolution to
d
= λ/(
n
sin α), where α is the half open-
ing angle of the microscope objective,
n
is the diffraction index of medium, and λ is the
wavelength.
†
http://calteches.library.caltech.edu/47/2/1960Bottom.pdf.
