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

In the Raman spectrum of B-type carbonated apatites, the ν₁ CO₃^2- mode (at ∼1070 cm^-1) overlaps the ν₃ PO₄^3- band. The latter is readily observed where the CO₃^2- content is low (up to ∼3 wt%). The CO₃^2- content of bone is considerably higher (∼7-9 wt%). As a result, the ν₃ PO₄^3- band becomes completely obscured. The 1000-1100 cm^-1 spectral range of carbonated apatite is frequently considered a combined ν₃ PO₄^3- and ν₁ CO₃^2- 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 ν₃ PO₄^3- 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 ν₁ CO₃^2- band of B-type carbonated apatites may contain a small but not insignificant contribution from the 1076 cm^-1 sub-component of the ν₃ PO₄^3- 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 ν₁ CO₃^2- band and compromising accurate estimation of carbonate-to-phosphate ratios in B-type CO₃^2- substituted apatite.