Fig. S1 (a) XRD patterns showing that even in the absence of heat treatment the Ca-0Sr-HA sample is indexed as hydroxyapatite (AMCSD 0002281). (b) For heat-treated Ca-XSr-HA particles, the (002) peak shifts towards lower 2θ values as the content of Sr2+ increases. (c) Position of (002) peaks as a function of Sr2+ content in the Ca-XSr-HA particles.

Fig. S2 (a) ν1 and (b) ν4 modes of PO43− group displaces linearly to lower wavenumbers as the content of Sr2+ increases in the Ca-XSr-HA particles.

Fig. S3 SEM images showing the spherical morphology at the nanoscale of Ca-XSr-HA particles containing 10, 30, 70 and 90 mol% Sr2+.

Table S1 Interplanar distance (d-pacing) and peak position corresponding to (002) and (310) planes and lattice parameters (a and c) obtained from the XRD patterns of heat-treated Ca-XSr-HA particles with Sr2+ percentages ranging from 0 to 100 mol%

Table S2 Relative intensity between the absorption bands due to PO43− (1,070 cm−1) and C=O (1730 cm−1) groups before and after immersion into SBF