Biomedical Engineering Reference
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Ca/P ratio, on the one hand, and the crystal shape, on another hand,
a thickness of the DCPD surface layer along the main crystal axis was
estimated to be ~1 nm [205], which is close to dimensions of the
unit-cells (Table 1.3). A similar structure of a crystalline core with the
composition of the stoichiometric HA and a disordered (amorphous)
surface layer was found by other researchers [211]; however, in yet
another study devoted to nanodimensional carbonateapatites [212],
the model of a crystalline core and an outer amorphous layer was
not confirmed. Perhaps, this discrepancy could be explained by the
presence of carbonates. A lack of hydroxide in nanodimensional
apatites was detected; an extreme nanocrystallinity was found to
place an upper bound on OH
possible in apatites [213].
However, it is possible to address the structure of surface
terminations of HA nano-sized particles to be amorphous or
crystalline by properly selecting the preparation parameters and, in
particular, the temperature; thus, nanodimensional HA without the
amorphous layer on the surface has been prepared [214]. The two
types of surfaces (amorphous or crystalline) of nanodimensional HA
appeared to be quite similar in terms of their first hydration layer, as
well as Lewis acid strength of exposed Ca
2+
ions. Both features have a
strong dependence on the local structure of surface sites (well probed
by small molecules, such as H
O and CO) that appeared essentially
unaffected by the organization at a longer range. Interestingly, but
once treated at 573 K, the crystalline surfaces of nanodimensional
HA was found to adsorb multilayers of water in a larger extent than
the amorphous ones [214].
Nevertheless, after summarizing the available data, the following
statements on the structure of nano-sized crystals of apatites have
been made:
(1) they involve non-apatitic anionic and cationic chemical
environments (in another study, the researchers mentioned
on “ordered and disordered HA” [206]),
(2) at least part of these environments are located on the surface
of the nano-sized crystals and are in strong interaction with
hydrated domains,
(3) immature samples show FTIR band fine substructure that
is altered upon drying without leading to long-range order
(LRO) modifications,
(4) this fine substructure shows striking similarities with the
FTIR spectrum of OCP [203].
2
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