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

In the Raman spectrum of B-type carbonated apatites, the ν(1) CO(3)(2–) mode (at ∼1070 cm(–1)) overlaps the ν(3) PO(4)(3–) band. The latter is readily observed where the CO(3)(2–) content is low (up to ∼3 wt%). The CO(3)(2–) content of bone is considerably higher (∼7–9 wt%). As a result, the ν(3) PO(4)(3–) band becomes completely obscured. The 1000–1100 cm(–1) spectral range of carbonated apatite is frequently considered a combined ν(3) PO(4)(3–) and ν(1) CO(3)(2–) 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 ν(3) PO(4)(3–) 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 ν(1) CO(3)(2–) band of B-type carbonated apatites may contain a small but not insignificant contribution from the 1076 cm(–1) sub-component of the ν(3) PO(4)(3–) 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 ν(1) CO(3)(2–) band and compromising accurate estimation of carbonate-to-phosphate ratios in B-type CO(3)(2–) substituted apatite.