Importance of defect nanocrystalline calcium hydroxyapatite characteristics for developing the dental biomimetic composites

Abstract

Applying a set of structural-spectroscopic methods of analysis we have studied the relationships between the structure, morphology and sorption characteristics of the samples of nanocrystalline carbonate-substituted hydroxyapatite (C-HAP) synthesized with the use of biogenic calcium source (poultry eggshell) obtained by the chemical deposition from a solution. With the use of X-ray diffractometry and X-ray microanalysis it was found that the appearance of impurities in the structure of synthesized material was due to their presence in calcium carbonate of the poultry eggshell and this could result in the changes of parameters in the elementary crystal cell of hydroxyapatite without formation of the additional phosphate phases. Optical and ESR spectroscopy allowed us to find that in the presence of the inherited set of impurities inclusion of carbonate ions into the structure of our synthesized hydroxyapatite takes place with the formation of C-HAP of B-type. An increase in the content of phosphate groups PO 4 in the process of the materials synthesis in the atmosphere results in the reduction of that of one of structurally bound CO 3 groups. A decrease in the pH value in the solution during the synthesis connected with the increase in the PO 4 3− anions content leads to the change of morphology of the C-HAP samples due to the increase in the size of intracrystalline defects – nanopores at the surface of C-HAP nanocrystals. Such a change in the morphology of the materials resulted in the changes of the sorption characteristics of the samples that were measured by means of thermal desorption of nitrogen technique. The specific surface area of C-HAP powders determined by means of the BET technique was ∼55.5 ± 0.9 m 2 /g, exceeding by several times that of the known analogs. Meanwhile, despite the developed surface the C-HAP samples remained rather stable in the atmosphere of the saturated water vapour and the main losses under polarization are related to Maxwell-Wagner losses. Analysis of characteristics for the C-HAP samples obtained from a biogenic source of calcium demonstrated their potential significance for obtaining of biomimetic materials imitating structure, morphology and anisotropy of the native hard tissues of the human tooth. © 2019 The Authors