Experimental evidence and structural modeling of nonstoichiometric (010) surfaces coexisting in hydroxyapatite nano-crystals

First-principles based theoretical calculations of atomic structures of hydroxyapatite surfaces and their charge states in contact with aqueous solutions

Surface charge states of biomaterials are often important for the adsorption of cells, proteins, and foreign ions on their surfaces, which should be clarified at the atomic and electronic levels. First-principles calculations were performed to reveal thermodynamically stable surface atomic structures and their charge states in hydroxyapatite (HAp). Effects of aqueous environments on the surface stability were considered using an implicit solvation model. It was found that in an air atmosphere, stoichiometric {0001} and P-rich {101̄0} surfaces are energetically favorable, whereas in an aqueous solution, a Ca-rich {101̄0} surface is the most stable. This difference suggests that preferential surface structures strongly depend on chemical environments with and without aqueous solutions. Their surface potentials at zero charge were calculated to obtain the isoelectric points (pH_PZC). pH_PZC values for the {0001} surface and the Ca-rich {101̄0} surface were obtained to be 4.8 and 8.7, respectively. This indicates that in an aqueous solution at neutral pH, the {0001} and Ca-rich {101̄0} surfaces are negatively and positively charged, respectively. This trend agrees with experimental data from chromatography and zeta potential measurements. Our methodology based on first-principles calculations enables determining macroscopic charge states of HAp surfaces from atomic and electronic levels.

Internalization of Fluoride in Hydroxyapatite Nanoparticles

Hydroxyapatite (HAP) is a cost-effective material to remove excess levels of fluoride from water. Historically, HAP has been considered a fluoride adsorbent in the environmental engineering community. This paper substantiates an uptake paradigm that has recently gained disparate support: assimilation of fluoride to bulk apatite lattice sites in addition to surface lattice sites. Pellets of HAP nanoparticles (NPs) were packed into a fixed-bed media filter to treat solutions containing 30 mg-F/L (1.58 mM) at pH 8, yielding an uptake of 15.97 ± 0.03 mg-F/g-HAP after 864 h. Solid-state ¹⁹F and ¹³C magic-angle spinning nuclear magnetic resonance spectroscopy demonstrated that all removed fluoride is apatitic. A transmission electron microscopy analysis of particle size distribution, morphology, and crystal habit resulted in the development of a model to quantify adsorption and total fluoride capacity. Low- and high-estimate median adsorption capacities were 2.40 and 6.90 mg-F/g-HAP, respectively. Discrepancies between experimental uptake and adsorption capacity indicate the range of F^- that internalizes to satisfy conservation of mass. The model was developed to demonstrate that F^- internalization in HAP NPs occurs under environmentally relevant conditions and as a tool to understand the extent of F^- internalization in HAP NPs of any kind.

Towards the control of the biological identity of nanobiomaterials: Impact of the structure of 011¯0 surface terminations of nanohydroxyapatite on the conformation of adsorbed proteins

High-resolution transmission electron microscopy, ζ-potential and in-situ IR spectroscopy of adsorbed CO were combined for elucidating the ratio between {011¯0}_ Ca-rich: {011¯0}_ P-rich terminations of {011¯0} facets, i.e. the surfaces with the highest morphological importance, in two nanohydroxyapatite samples. Bovine serum albumin was found to form at least a monolayer on the surface left accessible to protein molecules by the agglomeration of nanoparticles when suspended in the buffered incubation medium. Noticeably, the conformation of adsorbed proteins appeared sensitive to the ratio between the two types of {011¯0} terminations, also resulting in a difference in the surface exposed toward the exterior by the adsorbed protein layer(s).

Hydroxyapatite nanowires rich in [Ca-O-P] sites for ethanol direct coupling showing high C6-12 alcohol yield

Herein, we have shown that the [Ca-O-P] sites exposed on hydroxyapatite are clearly responsible for C-C formation in ethanol direct-coupling, and their high density accelerates the C-C coupling rate and boosts C6-12 alcohol production. Notably, nanowire-like hydroxyapatite exhibited 30.4% selectivity to n-butanol and 63.9% selectivity to C6-12OH at a conversion of 45.7% at 325 °C, and thereby close to 30% yield of C6-12OH, which is greatly higher than that using the state-of-the-art catalysts (6%).

HRTEM study of individual bone apatite nanocrystals reveals symmetry reduction with respect to P63/m apatite

In this study we present the first crystal structure model for bone apatite based on the analysis of individual nanocrystals by high resolution transmission electron microscopy (HRTEM). Crystallographic image processing of the obtained HRTEM images from different projections indicates symmetry reduction with respect to P6₃/m stoichiometric apatites and the presence of threefold symmetry along the c axis. Based on HRTEM observations and the measured Ca/P = 2 ratio we propose a structural model with phosphate-to-carbonate substitution and O vacancies localized along c axis, which explains the observed loss of 6₃ screw axis parallel, and the shift of mirror plane perpendicular to the c axis. Also, the presence of non-equivalent (010) surfaces has been proven. These results on the atomic structure of bone apatite nanocrystals contribute to the understanding of their biochemically controlled nucleation processes.

Probing the surface structure of hydroxyapatite through its interaction with hydroxyl: a first-principles study

Understanding the interaction of the hydroxyapatite (HAp) surface with hydroxyl originating from either the alkalescent physiological environment or HAp itself is crucial for the development of HAp-based biomaterials. Periodical density functional theory calculations were carried out in this study to explore the interaction of the HAp (100), (010) and (001) facets with hydroxyl. Based on a comparison study of Ca-rich, PO4-rich and Ca-PO4-OH mixed surfaces, the interaction pattern, interaction energy and effect of an additional water molecule on the Ca-OH interaction were comprehensively studied. The formation of CaOH on the Ca-rich surface was energetically favored on (100) and (001), while Ca(OH)2 was energetically favored on (010). The Ca-water interaction was competitive, but had lower interaction energy than Ca-OH. Furthermore, Ca-O bonding and its influence on the OH stretching vibration were analyzed. Our calculations suggest that the hydroxyl-coated surface structure is more appropriate than the commonly used Ca-terminated surface model for studying HAp surface activity in its service environments.

About the Genetic Mechanisms of Apatites: A Survey on the Methodological Approaches

Apatites are properly considered as a strategic material owing to the broad range of their practical uses, primarily biomedical but chemical, pharmaceutical, environmental and geological as well. The apatite group of minerals has been the subject of a huge number of papers, mainly devoted to the mass crystallization of nanosized hydroxyapatite (or carboapatite) as a scaffold for osteoinduction purposes. Many wet and dry methods of synthesis have been proposed. The products have been characterized using various techniques, from the transmission electron microscopy to many spectroscopic methods like IR and Raman. The experimental approach usually found in literature allows getting tailor made micro- and nano- crystals ready to be used in a wide variety of fields. Despite the wide interest in synthesis and characterization, little attention has been paid to the relationships between bulk structure and corresponding surfaces and to the role plaid by surfaces on the mechanisms involved during the early stages of growth of apatites. In order to improve the understanding of their structure and chemical variability, close attention will be focused on the structural complexity of hydroxyapatite (HAp), on the richness of its surfaces and their role in the interaction with the precursor phases, and in growth kinetics and morphology.

Control of calcium accessibility over hydroxyapatite by post-precipitation steps: influence on the catalytic reactivity toward alcohols

Hydroxyapatites exhibit Ca²⁺ and POH surface acid sites, which relative accessibilities can be varied by synthesis to tune the catalytic reactivity toward alcohols.

The effects of strontium on bone mineral: A review on current knowledge and microanalytical approaches

The interest in effects of strontium (Sr) on bone has greatly increased in the last decade due to the development of the promising drug strontium ranelate. This drug is used for treating osteoporosis, a major bone disease affecting hundreds of millions of people worldwide, especially postmenopausal women. The novelty of strontium ranelate compared to other treatments for osteoporosis is its unique effect on bone: it simultaneously promotes bone formation by osteoblasts and inhibits bone resorption by osteoclasts. Besides affecting bone cells, treatment with strontium ranelate also has a direct effect on the mineralized bone matrix. Due to the chemical similarities between Sr and Ca, a topic that has long been of particular interest is the incorporation of Sr into bones replacing Ca from the mineral phase, which is composed by carbonated hydroxyapatite nanocrystals. Several groups have analyzed the mineral produced during treatment; however, most analysis were done with relatively large samples containing numerous nanocrystals, resulting thus on data that represents an average of many crystalline domains. The nanoscale analysis of the bone apatite crystals containing Sr has only been described in a few studies. In this study, we review the current knowledge on the effects of Sr on bone mineral and discuss the methodological approaches that have been used in the field. In particular, we focus on the great potential that advanced microscopy and microanalytical techniques may have on the detailed analysis of the nanostructure and composition of bone apatite nanocrystals produced during treatment with strontium ranelate.