Figure 4. a, b) The lattice
cell structure of hydroxyapatite. c) Types of Hydroxyapatite
Vibrational Modes.
Carbonate
The biomineralization process of calcium carbonate is similar to that of
hydroxyapatite, both occurring by interaction with biological
substrates. Mostly as amorphous calcium carbonate (ACC), aragonite, and
calcite occur in the shells of crustaceans and the teeth of sea
urchins.[60,61,168], a portion of carbonate is
also present in vertebrate enamel and bone. During the maturation of
hydroxyapatite, carbonate ions replace the hydroxyl or phosphate in it,
thus changing the stability and solubility of the mineral phase. The
embedding of carbonate in the lattice causes a significant increase in
crystal solubility and deterioration of
crystallinity.[169–171] The result shown in the
Raman spectrum is an increase in the ratio of ν1CO32-/ν1PO43-, indicating a decrease in
crystallinity (Table 2 ). A-type carbonate (OH substituted by
CO32-) and B-type carbonate
(PO43- substituted by
CO32-) are both present in enamel and
bone, but the exact spatial distribution is
unknown.[172,173] It is worth affirming that the
content of B-type carbonate dominates and is associated with increased
B-type carbonate substitution (the ratio of B-type/A-type increased.)
and deterioration in the mechanical properties of biomineralized
tissues.[174–176] The increase in B-type
substitution also leads to an increase in the half-peak width of
ν1 PO43-, indicating
an increase in hydroxyapatite crystal defects.
Table 1. Raman
spectroscopic band assignments for biomineralization.