Discrimination of infrared fingerprints of bulk and surface POH and OH of hydroxyapatites

Design Strategies for Hydroxyapatite-Based Materials to Enhance Their Catalytic Performance and Applicability

Hydroxyapatite (HAP) is a green catalyst that has a wide range of applications in catalysis due to its high flexibility and multifunctionality. These properties allow HAP to accommodate a large number of catalyst modifications that can selectively improve the catalytic performance in target reactions. To date, many studies have been conducted to elucidate the effect of HAP modification on the catalytic activities for various reactions. However, systematic design strategies for HAP catalysts are not established yet due to an incomplete understanding of underlying structure-activity relationships. In this review, tuning methods of HAP for improving the catalytic performance are discussed: 1) ionic composition change, 2) morphology control, 3) incorporation of other metal species, and 4) catalytic support engineering. Detailed mechanisms and effects of structural modulations on the catalytic performances for attaining the design insights of HAP catalysts are investigated. In addition, computational studies to understand catalytic reactions on HAP materials are also introduced. Finally, important areas for future research are highlighted.

Production of Nano Hydroxyapatite and Mg-Whitlockite from Biowaste-Derived products via Continuous Flow Hydrothermal Synthesis: A Step towards Circular Economy

Biowastes from agriculture, sewage, household wastes, and industries comprise promising resources to produce biomaterials while reducing adverse environmental effects. This study focused on utilising waste-derived materials (i.e., eggshells as a calcium source, struvite as a phosphate source, and CH₃COOH as dissolution media) to produce value-added products (i.e., calcium phosphates (CaPs) derived from biomaterials) using a continuous flow hydrothermal synthesis route. The prepared materials were characterised via XRD, FEG-SEM, EDX, FTIR, and TEM analysis. Magnesium whitlockite (Mg-WH) and hydroxyapatite (HA) were produced by single-phase or biphasic CaPs by reacting struvite with either calcium nitrate tetrahydrate or an eggshell solution at 200 °C and 350 °C. Rhombohedral-shaped Mg-WH (23-720 nm) along with tube (50-290 nm diameter, 20-71 nm thickness) and/or ellipsoidal morphologies of HA (273-522 nm width) were observed at 350 °C using HNO₃ or CH₃COOH to prepare the eggshell and struvite solutions, and NH₄OH was used as the pH buffer. The Ca/P (atomic%) ratios obtained ranged between 1.3 and 1.7, indicating the formation of Mg-WH and HA. This study showed that eggshells and struvite usage, along with CH₃COOH, are promising resources as potential sustainable precursors and dissolution media, respectively, to produce CaPs with varying morphologies.

Effective Immobilization of Monomeric Methylene Blue on Hydroxyapatite Nanoparticles by Controlling Inorganic-Organic Interfacial Interactions

We successfully synthesized methylene blue (MB⁺)-immobilized hydroxyapatite (HM) nanoparticles by changing the initial P/Ca molar ratio. The immobilized amount of MB⁺ increased with increasing the initial P/Ca molar ratio from 0.6 to 4.0, and the HM had an elliptical shape (long length, 21-24 nm; short length, 11-13 nm) irrespective of the initial P/Ca molar ratio. Upon increasing the initial P/Ca molar ratio, the number of carbonate ions on the HM surface decreased, which would be owing to the electrostatic repulsion by the surface phosphate ions (i.e., P-O^-), the surface P-OH mainly dissociated to form P-O^-, and the electrostatic interaction of P-O^- with MB⁺ enhanced. The bonding of MB⁺ with surface P-OH and Ca²⁺ sites of hydroxyapatite would be hydrogen-bonding and Lewis acid-base interactions, respectively. The optimum synthesis condition for MB⁺ immobilization at the monomer state was found to be the initial P/Ca molar ratio of 2.0. Here, the existence percentage of the MB⁺ monomer and the molecular occupancy of the surface carbonate ion at the initial P/Ca molar ratio of 2.0 were higher than those at 4.0 with no significant difference in the immobilized amount of MB⁺, indicating that MB⁺ at the initial P/Ca molar ratio of 4.0 is more aggregated than that at 2.0. These results suggested that a part of carbonate ions has a role as a spacer to suppress MB⁺ aggregation. Accordingly, the interfacial interactions between the MB⁺ monomer and the hydroxyapatite surface were clarified, which can effectively be controlled by the initial P/Ca molar ratio. These findings will provide fundamental and useful knowledge for the design of calcium phosphate-organic nanohybrids. We believe that these particles will be the base materials to realize diagnostic and/or therapeutic functions through the molecular state control by optimizing the synthesis conditions.

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

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.

Microbial Volatile Organic Compound (VOC)-Driven Dissolution and Surface Modification of Phosphorus-Containing Soil Minerals for Plant Nutrition: An Indirect Route for VOC-Based Plant-Microbe Communications

We investigated the ability of microbial volatile organic compounds (MVOCs) emitted by Bacillus megaterium (a well-known MVOC producer) to modify the dissolution kinetics and surface of hydroxyapatite, a natural soil mineral. Facilitated phosphate release was induced by the airborne MVOCs in a time-dependent manner. Use of each standard chemical of the MVOCs then revealed that acetic and oxalic acids are crucial for the phenomenon. In addition, the ability of such MVOCs to engineer the apatite surfaces was evidenced by FT-IR spectra showing the COO^- band variation with incubation time and the prolonged acceleration of phosphate release during the negligible acidification of the hydroxyapatite-containing solutions. The formation of calcium oxalate was revealed through SEM-EDS and XRD analyses, suggesting that MVOC oxalic acid interacts with calcium ions, leading to the precipitation of calcium oxalate, thus preventing the recrystallization of calcium phosphates. Gel- and soil-based plant cultivation tests employing Arabidopsis thaliana and solid calcium phosphates (i.e., nano- and microsized hydroxyapatites and calcium phosphate dibasic) demonstrated that these MVOC mechanisms facilitate plant growth by ensuring the prolonged supply of plant-available phosphate. The relationship between the growth enhancement and the particle size of the calcium phosphates also substantiated the MVOC sorption onto soil minerals related to plant growth. Given that most previous studies have assumed that MVOCs are a molecular lexicon directly detected by the dedicated sensing machinery of plants, our approach provides a new mechanistic view of the presence of abiotic mediators in the interaction between plants and microbes via MVOCs.

Lactic Acid Conversion to Acrylic Acid Over Fluoride-Substituted Hydroxyapatites

One of the most interesting intermediates for the chemical industry is acrylic acid, which can be derived from lactic acid by catalytic dehydration in the gas phase. The realization of this reaction is complex due to a strong thermal activation leading to the formation of undesired by-products (acetaldehyde, propanoic acid…) as well as polymerization. We studied this reaction over hydroxyapatites modified by substitution of the hydroxyl groups by fluoride. This notably enabled increasing the selectivity to acrylic acid while reducing the formation of the undesired acetaldehyde. Introduction of fluoride induced a modification of the phosphate (PO 4 3 - ) groups. In the presence of water, fluoride prevented the formation of hydrogenophosphate species (HPO 4 2 - ), which are well-known acid sites responsible for the formation of acetaldehyde by decarboxylation/decarbonylation. Further, we evidenced an important impact of fluoride substitution on crystallinity, specific surface area and on the surface Ca/P ratio. This latter is known to be a key parameter to control the acidity and the basicity of the hydroxyapatites. Using FT-IR spectroscopy with propyne as a probe molecule, we could show that lactic acid was concertedly adsorbed on basic and acid sites, which might be at the origin of the observed superior performances.

Irreversible adsorption of acidic, basic, and water gas molecules on calcium-deficient hydroxyapatite

Hydroxyapatite [Ca10(PO4)6(OH)2, HAP] has P-OH Brønsted acidic sites, Ca2+ Lewis acidic sites, and OH- and O2- basic sites on which acidic and basic gas molecules can be selectively adsorbed, and has no micropore onto which various molecules adsorb regardless of the chemical properties of gas molecules. The interaction between the surface sites and acidic and basic gas and water molecules has been investigated by evaluating the adsorption properties of various molecules on the surfaces of calcium-deficient HAP. The specific adsorption sites were assessed by examining the reversible and irreversible adsorption of NH3, CO2, aldehydes, and water vapor on HAP at the temperature of 298 K, using two HAP samples with different Ca/P ratios, but similar structures and surface areas: Ca-deficient HAP with an extreme lower Ca/P ratio (named P-HAP) and one with a higher Ca/P ratio (named C-HAP). Irreversible adsorption of NH3 on C-HAP is attributed to the adsorption on both Ca2+ Lewis acidic and P-OH Brønsted acidic sites. Irreversible adsorption on P-HAP is attributed to the adsorption on P-OH Brønsted acidic sites only. Irreversible adsorption of CO2 occurred on C-HAP only, and preferentially on OH- basic sites. Acetaldehyde undergoes a catalytic reaction over both OH- basic sites and surface P-OH Brønsted acidic sites at 298 K. Water irreversible adsorption was extensively observed for P-HAP, and water was barely desorbed at low pressures. In situ powder X-ray diffraction showed an asymmetric expansion of the lattice in the [100] direction, indicating that water was incorporated into P-HAP crystals, especially on structural OH- sites. Irreversible adsorption of acidic and basic molecules was therefore less observed on P-HAP than on C-HAP, but P-HAP had considerable irreversible adsorption of water vapor with associated asymmetric lattice expansion. The incorporation of water vapor was first observed and could be useful to improve adsorption or catalytic performance with the mediation of water vapor and/or hydration.

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.

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.