Ion-association complexes unite classicaland non-classical theories for the biomimeticnucleation of calcium phosphate

摘要

Despite its importance in many industrial, geological and biological processes, the mechanismof crystallization from supersaturated solutions remains a matter of debate. Recentdiscoveries show that in many solution systems nanometre-sized structural units are alreadypresent before nucleation. Still little is known about the structure and role of these so-calledpre-nucleation clusters. Here we present a combination of in situ investigations, which showthat for the crystallization of calcium phosphate these nanometre-sized units are in factcalcium triphosphate complexes. Under conditions in which apatite forms from an amorphouscalcium phosphate precursor, these complexes aggregate and take up an extra calcium ion toform amorphous calcium phosphate, which is a fractal of Ca2(HPO4)3 lusters. The calciumtriphosphate complex also forms the basis of the crystal structure of octacalcium phosphateand apatite. Finally, we demonstrate how the existence of these complexes lowers the energybarrier to nucleation and unites classical and non-classical nucleation theories.