Bioinspired Citrate-Apatite Nanocrystals Doped with Divalent Transition Metal Ions

Bioinspired citrate-carbonate-apatite (cAp) nanocrystals doped with divalent transition metal (M) ions, i.e., Mn2+, Co2+, and Ni2+, were prepared by batch thermal decomplexing of Ca2+/M/citrate/phosphate/carbonate solutions at 80 and 37°C, with initial Ca2+/M molar ratios of 9:1 and 5:5, and at different crystallization times ranging from 1 to 96 h. A thorough chemical, crystallographic, and morphological characterization was carried out on the doped nanocrystals revealing that (i) using similar crystallization conditions the amount of incorporated M normally followed the order Mn2+ > Co2+ > Ni2+; (ii) the growth of nanocrystals was clearly enhanced at 80°C and when a lower amount of M was incorporated in the crystal structure; (iii) the increase of the M content increased the aspect ratio (length/width) of the M-doped cAp nanocrystals compared to undoped ones, (iv) the incorporation of the 6.5 wt % of M is a threshold for the long-range order of the nanocrystal; in fact, with a higher M content, amorphous materials were mainly produced. Additionally, we found that citrate controlled the activity of hydrated M ions in solution, the extension of the doping process, and protected the M ions against oxidation in solution as well as in the outermost layer of doped nanocrystals. Preliminary in vitro cytotoxicity studies on the murine MS1 endothelial cell line showed that the produced Mn2+- and Co2+-doped nanocrystals were highly biocompatible at doses comparable to those of undoped ones, with the exception of the nanocrystals substituted with the highest Co2+ content at the higher doses.