MicroRaman spectral study of the PO4 and CO3 vibrational modes in synthetic and biological apatites.

The carbonate and phosphate vibrational modes of different synthetic and biological carbonated apatites were investigated by Raman microspectroscopy, and compared with those of hydroxyapatite. The ν1 phosphate band at 960 cm−1 shifts slightly due to carbonate substitution in both A and B sites. The spectrum of type A carbonated apatite exhibits two ν1 PO4 3− bands at 947 and 957 cm−1. No significant change was observed in the ν2 and ν4 phosphate mode regions in any carbonated samples. The ν3 PO4 3− region seems to be more affected by carbonation: two main bands were observed, as in the hydroxyapatite spectrum, but at lower wave numbers. The phosphate spectra of all biominerals apatite were consistent with type AB carbonated apatite. In the enamel spectrum, bands were observed at 3513 and at 3573 cm−1 presumably due to two different hydroxyl environments. Two different bands due to the carbonate ν1 mode were identified depending on the carbonate substitution site A or B, at 1107 and 1070 cm−1, respectively. Our results, compared with the infrared data already reported, suggest that even low levels of carbonate substitution induce modifications of the hydroxyapatite spectrum. Increasing substitution ratios, however, do not bring about any further alteration. The spectra of dentine and bone showed a strong similarity at a micrometric level. This study demonstrates the existence of acidic phosphate, observable by Raman microspectrometry, in mature biominerals. The HPO4 2− and CO3 2− contents increase from enamel to dentine and bone, however, these two phenomena do not seem to be correlated.