Germanium-doped hydroxyapatite: Synthesis and characterization of a new substituted apatite

Abstract

Hydroxyapatite (HAp) is the major component of all boney tissues in mammals. Because of this omnipresence in the living world, HAp possesses an exceptional biocompatibility. The downside of this omnipresence, however, comes in the form of its mild to moderate biological activities. One means of augmenting these activities involves the doping of HAp with foreign ions. Here, the first synthesis and characterization of HAp doped with germanium ions is being reported. Germanium was deliberately integrated into the crystal lattice of HAp in the form of germanate anions. Approximately two thirds of the germanate ions introduced into the hydrothermal solution got incorporated into the HAp lattice, yielding the approximate stoichiometry of Ca_10-x(PO₄)_5.62+y(GeO₃)_0.38(OH)_2-z. Germanates replaced the phosphates of stoichiometric HAp and induced the expansion of the HAp lattice both along the screw axis of the calcium ion hexagons and in the direction parallel to the basal plane. Simultaneously, the larger size and the triple valency of the germanate ion as compared to the smaller and trivalent phosphates prompted the bond distortion and charge compensation through defect formation, which reduced the crystallinity and increased the microstrain of the HAp lattice. Vibrational spectroscopic analyses corroborated these crystallographic effects by demonstrating the enhanced heterogeneity of the environments surrounding the active modes after germanate ions were incorporated into HAp. Conforming to La Châtelier's principle, this reduction of the crystallographic order increased the capacity of the material for integration of adventitious carbonates. However, the inclusion of germanate ions induced a partial shift of these carbonates to the hydroxyl channel sites, thus decreasing the ratio of the B-type carbonation to the A-type carbonation. Introduced into HAp, germanium acted as a superb regulator of the particle size and morphology, enhancing their fineness and uniformity. Inclusion of germanate ions also increased the electrophoretic mobility and hydrodynamic surface charge density of the particles by reducing their size and by inducing a more stochastic distribution of terminal ionic groups due to the bending of the crystal facets. Overall, the doping of HAp with germanate ions facilitated the production of narrowly dispersed nanorods with a moderately enhanced structural disorder and with a pronounced potential for the biomedical niche.