Probing electronic structure of biomineralized hydroxyapatite inside nanoclay galleries

Hydroxyapatite, the most abundant mineral in the human body, is also an important component in design of biomaterials for bone tissue regeneration. Synthetic hydroxyapatite mineralized in the laboratory often does not exhibit the same biological and morphological properties of biogenic hydroxyapatite in human bone. A biomimetic hydroxyapatite structure is synthesized using biomineralization routes inside the clay galleries of montmorillonite clay. Amino acids are used to modify the clay galleries. These amino acids are used to mineralize hydroxyapatite. The molecular interactions between nanoclay, modifiers inside nanoclay (amino acids) and biomineralized hydroxyapatite result in unique morphology, structure and stoichiometry of the biomineralized hydroxyapatite. Prior studies have indicated that this biomineralized hydroxyapatite inside nanoclay galleries is an effective component of tissue engineering bone scaffolds that elicits an optimal biological response from human mesenchymal stem cells. Here, a detailed electron energy-loss spectroscopy (EELS) study is reported that elucidates the differences in hydroxyapatite, biomineralized hydroxyapatite and β-tricalcium phosphate (β-TCP). Comparison of EELS low-loss transitions and energy loss near-edge structure (ELNES) of P-L_2,3 edges for these three compounds is done to determine if there are differences in their electronic structures. These changes observed experimentally are compared with prior predictions and simulations using molecular dynamics studies. The simulations predict attractive and repulsive interactions between phosphate, modified MMT clay and aminovaleric acid (amino acid) molecules. Kramers-Kronig analysis is performed on the loss spectra obtained to yield the real and imaginary parts of the dielectric function of the apatites (ε₁ and ε ₂). We have also used the ε₂ spectra obtained to calculate the AC conductivity spectra for the apatites. This study represents a unique experimental probe into molecular interactions in complex biomineralized hydroxyapatite structures. The small changes observed in the energy loss spectra appear to play important biological roles in biomineralized hydroxyapatite such as the ability to differentiate human mesenchymal stem cells into osteoblasts without growth media.