New insights into the application of pair distribution function studies to biogenic and synthetic hydroxyapatites

Emily L Arnold,Charlene Greenwood,Keith D Rogers,Dean S Keeble

doi:10.1038/s41598-020-73200-2

Emily L Arnold, Charlene Greenwood + Show 2 more

Open Access

https://doi.org/10.1038/s41598-020-73200-2

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Abstract

Biogenic and synthetic hydroxyapatites are confounding materials whose properties remain uncertain, even after years of study. Pair distribution function (PDF) analysis was applied to hydroxyapatites in the 1970’s and 1980’s, but this area of research has not taken full advantage of the relatively recent advances in synchrotron facilities. Here, synchrotron X-ray PDF analysis is compared to techniques commonly used to characterise hydroxyapatite (such as wide angle X-ray scattering, Fourier-transform infrared spectroscopy and thermogravimetric analysis) for a range of biogenic and synthetic hydroxyapatites with a wide range of carbonate substitution. Contributions to the pair distribution function from collagen, carbonate and finite crystallite size were examined through principal component analysis and comparison of PDFs. Noticeable contributions from collagen were observed in biogenic PDFs when compared to synthetic PDFs (namely r < 15 Å), consistent with simulated PDFs of collagen structures. Additionally, changes in local structure were observed for PDFs of synthetic hydroxyapatites with differing carbonate content, notably in features near 4 Å, 8 Å and 19 Å. Regression models were generated to predict carbonate substitution from peak position within the PDFs.

Highlights

Biogenic and synthetic hydroxyapatites are confounding materials whose properties remain uncertain, even after years of study
This was in an effort to explain a noticeable reduction in diffractogram intensity when biogenic HA was compared to synthetic apatites with a comparable 002 coherence length (CL), supposedly due to the presence of 45 wt% stable amorphous calcium phosphate (ACP), a theory proposed by Harper and Posner in 196618
Results were plotted according to the sample group, and clusters can be seen, most prominently Group 4, made up of the majority of biogenic samples

Summary

Introduction

Biogenic and synthetic hydroxyapatites are confounding materials whose properties remain uncertain, even after years of study. Contributions to the pair distribution function from collagen, carbonate and finite crystallite size were examined through principal component analysis and comparison of PDFs. Noticeable contributions from collagen were observed in biogenic PDFs when compared to synthetic PDFs (namely r < 15 Å), consistent with simulated PDFs of collagen structures. The PDF is a real-space analysis technique which allows local structures to be analysed (atom–atom pair distances from 1 to 50 Å for some experimental designs), making it ideally suitable for nano-crystalline materials, such as biogenic hydroxyapatite (HA). Research continued focusing mainly on differentiating ACP from H A19,20, determining the effect of carbonate substitution and different preparation mediums[21] From these it was concluded that a maximum of 10 wt% ACP was present in biological bone mineral. Calcium deficient HA was analysed and compared to biogenic H A24

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