Self-assembly of nano-hydroxyapatite on multi-walled carbon nanotubes

Inspired by self-assembly of nano-hydroxyapatite (nHA) on collagen associated with the 67 nm periodic microstructure of collagen, we used multi-walled carbon nanotubes (MWCNTs) with approximately 40 nm bamboo periodic microstructure as a template for nHA deposition to form a nHA–MWCNT composite. The assembled apatite was analyzed by transmission electron microscopy and scanning electron microscopy. Defects that were analogous to edge dislocations along the carbon nanotubes’ multi-walled surfaces were the nucleation sites for nHA after these defects had been functionalized principally into carboxylic groups. Spindle-shaped units consisting of an assembly of near parallel, fibril-like nHA polycrystals were formed and oriented at a certain angle to the long axis of the carbon nanotubes, unlike nHA–collagen in which the nHA is oriented along the longitudinal axis of the collagen molecule. One possible explanation for this difference is that there are more bonds for calcium chelation (–COOH, >CO) on the collagen fibril surface than on the surface of MWCNTs. Spindle-shaped units that are detached from the MWCNT template are able to maintain the ordered parallel structure of the nHA polycrystal fibril. We have thus created a self-assembled hydroxyapatite on MWCNTs.