Environmental Engineering Reference
In-Depth Information
preparing nanomaterials with designed structures possible. As a result there is a
huge wealth of literature on nanomaterials and nanotechnology, but only a fraction
of this has made it as far as commercialisation.
There are two general areas of nanotechnology, those relating to nanomachines
and those relating to more mechanically passive nanoparticles. Nanomachines are
nanoscale devices which incorporate some motion or action as a result of their
structure. Often these machines are single molecules and are therefore sometimes
referred to as molecular machines.
There are several excellent reviews (Liedl et al. , 2007 ; Ozin et al. , 2005 ) on
the subject of molecular machines and they may essentially be divided into two
types, the biologically derived and the non-biologically derived. The biologically
derived examples are often based on protein or DNA molecules or assemblies
which switch between two states depending on the environment in which they are
placed. Typically, switching may be achieved by the addition of certain molecules
to the system. Similarly, the rotary action of ATP has been harnessed to rotate a
nickel bar (150
750 nm). This elegantly captures the ability of nature to function
at this scale, the diameter of the motor itself is about 50 nm (Soong et al. , 2000 ).
However, clearly if machines, rather than the components of them, are to be
prepared in this manner their overall dimensions will exceed the 100 nm limit of
nanotechnology and probably result in a device which is more sensibly measured
in microns.
In order to push the size of these motors to even smaller scales more simple
molecules have been devised. Some are based on shifting a ring along a chain in a
class of molecules called rotaxanes. These molecules are, in fact, composed of a
ring-shaped molecule with a rod-shaped molecule passing through it (Figure 2.3).
In one example (Balzani et al. , 2006) the molecule is comprised of a section which
will harvest light energy. This energy is dissipated within the molecule chain by the
transfer of an electron to a series of acceptors on the chain. As the electron moves
through the chain it affects the interaction of the rod section with the ring section,
resulting in a shifting of the ring along the rod. The concept of a motor is generally
tied to a device for generating rotary motion. This is not an issue in many molecules
as the thermal energy at room temperature is suffi cient to make various groups on
molecules spin on their axis. However, generating motion which is controlled and
unidirectional is more challenging. Others have demonstrated how this may be
achieved and also how it may be put to work (Pijeret et al. , 2005 ; Vicario et al. ,
2006). A series of photochemical excitations and relaxation processes results in the
rotation of two large groups about a formally double bond. The molecules have
been specially designed to resist counter rotation and have a shape which facilitates
rotation in one direction. The authors have been able to show that if similar mol-
ecules are suspended in a liquid crystalline fi lm, excitation of the motor results in
ordered motion of the regular ridges in the fi lm. The magnitude of the process is
suffi ciently large as to allow a glass bar (5
×
m) to be rotated on the surface.
It seems that the mass use of nanotechnology in this form is still some time away.
Interestingly the discussion also highlights the issue of where does nanotechnology
start and chemistry/biology end? Many of the devices discussed here would be
better classed as molecules rather than nanoparticles.
×
3 0
µ
Search WWH ::




Custom Search