Characterization of Synthetic Hydroxyapatite Fibers Using High-Resolution, Polarized Raman Spectroscopy

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.