Composites Containing Nanohydroxyapatites and a Stable TEMPO Radical: Preparation and Characterization Using Spectrophotometry, EPR and 1H MAS NMR

Natalia Byra,Sylwester Krukowski,Jaroslaw Sadlo,Waclaw Kolodziejski

doi:10.3390/ma15062043

Abstract

Hydroxyapatite is the main constituent of mammalian hard tissues. Basic applications of synthetic hydroxyapatites include bone and dental implantology and drug delivery systems. The study of hydroxyapatite surface properties could give greater insight into the processes of bone mineralization and degradation. Nitroxide radicals are stable radicals that exhibit anticancer and antioxidative properties and are often used as spin probes to study the dynamics of complex biological systems. In this work, we attempted to adsorb the stable 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) on two hydroxyapatites (HAs) differing in specific surface area and the degree of hydration. The adsorption was carried out from cyclohexane, 1-chlorobutane and water. The solutions after adsorption were studied spectrophotometrically, while the obtained composites were characterized via NMR and EPR spectroscopy. The results show that it is possible to reproducibly obtain fairly stable composites, where the main factors influencing the adsorbed amount of the radical are solvent polarity and specific surface area of hydroxyapatite. The Langmuir isotherm was determined to be the most suitable adsorption model. The analysis of EPR and NMR spectra allowed us to determine the distribution of the TEMPO molecules on the hydroxyapatite surface, as well as a probable adsorption mechanism. The HA/TEMPO composites could potentially be used to study certain properties of hydroxyapatite surfaces with EPR spectroscopy. They could also be used as fillers after hard tissue surgery, as well as metal-free MRI contrasts.

Highlights

Due to the high occurrence rate of bone and teeth diseases, bone substitute biomaterials are extensively researched nowadays
We found three reasons which could explain the loss of paramagnetic properties: the formation of the so called “electron paramagnetic resonance (EPR)-inactive” complexes, described by Katter et al [49], the dimerization of tetramethylpiperidine-1-oxyl radical (TEMPO) [63] and the disproportionation of the radical molecules represented by component 2 in the EPR spectra
It is possible to reproducibly adsorb the TEMPO radical on hydroxyapatite, even if it contains a monolayer of surface water;

Summary

Introduction

Due to the high occurrence rate of bone and teeth diseases, bone substitute biomaterials are extensively researched nowadays. Carbonated calcium-deficient hydroxyapatite, which belongs to the group of phosphate minerals called apatites, is the main constituent of mammalian hard tissues. Synthetic apatites can be used to obtain dental and bone implants as well as to fill bone defects. Biological apatites have a high specific surface area (SSA), are nanocrystalline and nonstoichiometric—some of the calcium ions are replaced by sodium, magnesium, potassium or strontium cations. The locations of phosphate ions and hydroxyl ions can be occupied by carbonate ions named B and A, respectively. The reference compound in this group is stoichiometric hydroxyapatite, HA (Ca10(PO4)6(OH)). The reference compound in this group is stoichiometric hydroxyapatite, HA (Ca10(PO4)6(OH)2) It is a suitable material for preliminary research on hydroxyapatites [7]

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