Abstract
In the Raman spectrum of B-type carbonated apatites, the ν1 CO32– mode (at ∼1070 cm–1) overlaps the ν3 PO43– band. The latter is readily observed where the CO32– content is low (up to ∼3 wt%). The CO32– content of bone is considerably higher (∼7–9 wt%). As a result, the ν3 PO43– band becomes completely obscured. The 1000–1100 cm–1 spectral range of carbonated apatite is frequently considered a combined ν3 PO43– and ν1 CO32– region. Here, high-resolution polarized Raman spectroscopy (step size of 0.74 ± 0.04 cm–1) provides new insights into synthetic hydroxyapatite (HAp) obtained as micrometer-sized fibers. Compared to bone mineral (deproteinized bovine bone), spectral features of HAp fibers are highly resolved. In particular, the ν3 PO43– band resolves into nine distinct sub-components: 1028, 1032, 1040, 1043, 1047, 1053, 1055, 1062, and 1076 cm–1. Parameters including full width half-maximum, intensity, area fraction, intensity ratio, and area fraction ratio vary between parallel and perpendicular polarized configurations. It is likely that the ν1 CO32– band of B-type carbonated apatites may contain a small but not insignificant contribution from the 1076 cm–1 sub-component of the ν3 PO43– band. Furthermore, the 1076 cm–1/1047 cm–1 ratio changes between parallel and perpendicular scattering configurations, suggesting that the contribution of the 1076 cm–1 sub-component may vary as a function of local orientation of bone mineral, thus skewing the ν1 CO32– band and compromising accurate estimation of carbonate-to-phosphate ratios in B-type CO32– substituted apatite.
Highlights
Synthetic- and natural-derived calcium phosphates, e.g., hydroxy- or hydroxy(l)apatite (HAp), are by far the most frequently used biomaterials for bone repair.[1]
Raman spectroscopy is frequently employed for nondestructive assessment of bone quality,[4,5] and has been used extensively to study carbonate substitution in bone mineral,[6] which is considered an important marker of bone turnover.[7]
The n2 PO34– band consists of sub-components at 428 cm–1 and 450 cm–1, of which the $428 cm–1 sub-component appears least sensitive to polarization in terms of position, intensity, and shape
Summary
Synthetic- and natural-derived calcium phosphates, e.g., hydroxy- or hydroxy(l)apatite (HAp), are by far the most frequently used biomaterials for bone repair.[1]. In a typical Raman spectrum of B-type carbonated apatites (where CO23– substitutes for PO34–), the symmetric stretching n1 CO23– (b2g) mode overlaps the antisymmetric stretching n3 PO34– (a1g) band. The latter is observed up to $3 wt% CO23– but tends to be completely enveloped by the n1 CO23– peak in bone,[8] where the CO23– content is significantly higher ($7–9 wt%).[9] CO23– substitution for PO34– influences physical properties including crystallite size, solubility, and thermal stability of biological apatites,[10] in effect restricting mineral crystallinity to below that observed for carbonate-free apatites
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