Biomedical Engineering Reference
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physico-chemical techniques and chemical analysis methods [169,
198-210] with a special attention to the “nano” effect (i.e
, an enhanced
contribution of the surface against the volume). Unfortunately, no
publications on the structure of other nanodimensional and/or
nanocrystalline calcium orthophosphates were found in the available
literature.
Due to a nanocrystalline nature, various diffraction techniques
have not yet given much information on the fine structural details
related to apatite nano-sized crystals (assemblies of nano-sized
particles give only broad diffraction patterns, similar to ones from
an amorphous material — see Chapter 2) [198, 199]. Nevertheless,
the diffraction studies with electron microprobes of 35 ± 10 nm
in diameter clearly indicated a crystalline character of the nano-
sized particles in these assemblies. Furthermore, high-resolution
transmission electron microscopy results revealed that nano-sized
particles of HA behaved a fine monocrystalline grain structure [169,
198].
`Therefore, a recent progress on the structure of nanodimensional
and nanocrystalline apatites has relied mainly on diverse
spectroscopic methods, which are sensitive to disturbances of
the closest environments of various ions. Namely, the structure
analysis revealed an existence of structural disorder at the particle
surface, which was explained by chemical interactions between the
orthophosphate groups and either adsorbed water molecules or
hydroxyl groups located at the surface of nano-sized apatites [200].
More to the point, infrared (FTIR) spectra of nanocrystalline apatites,
in the
.
domain, revealed the existence of additional bands of
orthophosphate ions which could not be assigned to an apatitic
environment and which were not present in well-crystallized apatites
(Fig. 3.1). These bands were assigned to non-apatitic environments
of PO
ν
PO
4
4
4
3−
4
2−
ions of the nano-sized crystals. Thus, FTIR
spectra can be used to provide a sufficiently accurate evaluation of
the amounts of such environments. Furthermore, the non-apatitic
environments were found to correspond to hydrated domains of the
nano-sized crystals, which were distinct from the apatite domains
[202]. Hence, precipitated crystals of nano-sized apatite appeared to
have a hydrated surface layer containing labile ionic species, which
easily and rapidly could be exchanged by ions and/or macromolecules
from the surrounding fluids [201, 202, 209]. For the as-precipitated
apatites, such a layer appears to constitute mainly by water molecules
and HPO
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