Biomedical Engineering Reference
In-Depth Information
single molecule study of chitosan using AFM as well as AFM-based single
molecular force spectroscopy allowed the imaging of a positively charged
single strand of chitosan on negatively charged mica [77].
Scanning electron microscopes (SEM) are commonly used to get a bet-
ter picture of the surface of a nanomaterial. SEM can be used to collect
information on the topography, morphology, crystallographic arrange-
ment and composition of the sample being analysed. h e principle upon
which this microscope works, involves directing a beam of electrons (0.2
- 40 KeV) at the sample which dislodges the sample's electrons, thus gen-
erating signals. h ese signals are used to generate an image of the surface
of the sample. h e sample needs to be electrically conductive to be exam-
ined by SEM. h us, the sample needs to be coated with a thin layer of an
electrically conducting material such as gold or palladium. h is technique
can be used at low vacuum, high vacuum and in wet conditions. An advan-
tage of SEM analysis is that a resolution better than 1 nanometre can be
achieved [75, 78]. Polymer size distribution can easily be visualised with
SEM as well as the surface morphology of nanoi bres and nanoparticles.
Numerous publications display chitosan surface features using SEM, it is
certainly a key tool in polymer surface analysis.
Transmission electron microscopy (TEM) involves
the focusing of a
thin beam of electrons on a sample with a wavelength of < 200 nm. h e
electrons are scattered when they interact with matter and an image of the
interacting electrons is magnii ed and focused onto an imaging device. h e
electrons provide a picture of the sample that is being studied. h e micro-
scope has a resolution of up to 0.2 nm enabling the determination of parti-
cle size, arrangement of atoms in a sample and composition of the sample.
An improved version of this microscope is the scanning-transmission elec-
tron microscope (STEM) which combines attractive features of both TEM
and SEM. Samples are typically analysed at low pressures. h e images gen-
erated can be used to build a three dimensional picture of the sample. A
disadvantage associated with this technique is the higher cost compared to
other characterization techniques [75, 79]. h is technique has previously
been utilized to view amongst others, a chitosan-clay nanocomposite [80].
h e scanning tunnelling microscope (STM) is
intended to image surfaces
at the atomic level. h is technique can also provide information on the elec-
tronic structure of the sample at a specii ed point. h e samples analysed are
required to conduct electricity. During analysis a current hovers over the
surface of the material where changes in surface height and density of states
alter the current. h ese changes are recorded and used to produce images
[81]. h erefore the sample resolution is independent of radiation wave-
length and is based only on the size of the probe which is the size of a single
