Subscriber access provided by NEW YORK MED COLL
Article
A Step Toward Control of the Surface Structure of Biomimetic Hydroxyapatite Nanoparticles: Effect of Carboxylates on the {010} P-Rich/Ca-Rich Facets Ratio Yuriy Sakhno, Pavlo Ivanchenko, Michele Iafisco, Anna Tampieri, and Gianmario Martra J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp510492m • Publication Date (Web): 25 Feb 2015 Downloaded from http://pubs.acs.org on March 1, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
A Step Toward Control of the Surface Structure of Biomimetic Hydroxyapatite Nanoparticles: Effect of Carboxylates on the {010} P-rich/Ca-rich Facets Ratio
Yuriy Sakhno,a* Pavlo Ivanchenko,a Michele Iafisco,b Anna Tampieri,b Gianmario Martra a* a
Department of Chemistry and Interdepartmental Centre “Nanostructured Interfaces and Surfaces-
NIS”, University of Torino, Via P. Giuria 7, 10125 Torino, Italy b
Institute of Science and Technology for Ceramics (ISTEC), National Research Council (CNR),
Via Granarolo 64, 48018 Faenza (RA), Italy
ABSTRACT
Two types of biomimetic hydroxyapatite (HA) nanoparticles were prepared by acid-base neutralization reactions, using Ca(OH)2 or Ca(CH3COO)2 as s calcium source, to evaluate the effect of acetate anions on particle formation. High-resolution transmission electron microscopy observations provided evidence that in both cases, nanoparticles are elongated along the c-axis, but to a more limited extent when prepared in the presence of acetates, and are mainly limited by {010} facets. IR spectra of nanoparticles containing adsorbed CO revealed that the actual termination of 1 ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 31
these are both of the {010}_Ca−rich and {010}_P−rich type, the latter being significantly more abundant for HA nanoparticles grown in the medium containing CH3COO- species. Moreover, these nanoparticles appeared to be more sensitive toward aggregative stacking by thermal treatment, resulting in a significant decrease in specific surface area, while retaining the size of primary particles.
KEYWORDS: Hydroxyapatite nanoparticles, acetate anions, HR-TEM, FTIR, adsorbed CO. 1. INTRODUCTION Hydroxyapatite (HA), [Ca10(PO4)6(OH)2], the calcium phosphate phase thermodynamically stable in physiological environment, is nowadays considered an important bioactive and bioresorbable biomaterial for applications in orthopaedy and dentistry.1-3 In particular, since the group of Webster reported about the beneficial effect of hydroxyapatite nanosizing in improving the response elicited in osteoblasts4 several research works have been devoted to the disclosure of preparation routes for the production of nano-hydroxyapatites. The surface of biomaterials plays a significant functional role, further enhanced when they are in a nanosize form, because it is the place where the interaction with host biological media occurs, as recognized since the definition of the concept of biological/biomedical surface science.5-7 Moreover, nano-HA have raised an increasing interest as heterogeneous catalysts,8-9 another functionality mainly ruled by surface features. Focusing on nano-HA morphology, needle-like10-13 and plate-like,14-16 nanoparticles can be prepared, while in bone tissue only the second type seems to be present.17-19 For both morphologies the prevailing surface terminations are of the {010} type, which in the case of needle-like nano-HA are the lateral facets of hexagonal nanoparticles elongated along the crystallographic c-axis, whilst for plate-like nanoparticles they are the basal facets of nanoparticles preferentially grown along both the c- and a- (or b-) axis,15-16,
20
breaking the crystal symmetry through a mechanism still
matter of investigation.14, 20 2 ACS Paragon Plus Environment
Page 3 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
The role of different surface crystal planes when considering the interaction of HA with biomolecules has been initially recognized by Kandori et al., who demonstrated that basic proteins are mainly adsorbed on HA {001} faces, whereas the acidic ones preferentially interacts with HA {010} surfaces.21 This latter type of surfaces can be thought as resulting from the interruption of a sequence of layers of the ...A−B−A−A−B−A−A−B−A−... type, where A and B correspond to Ca3(PO4)2 and Ca4(PO4)2(OH)2 chemical compositions, respectively. Hence, a {010} surface termination of the ... −A−B−A type give origin to stoichiometric HA{010} surfaces, whilst ... −A−A−B, or ... −B−A−A types result in two nonstoichiometric HA{010} facets, labelled as (010)_Ca-rich and (010)_P-rich, respectively. The coexistence of these non-stoichiometric surfaces on HA particles was reported also by Sato et al.22 and by Ospina et al..23 Moreover, theoretical calculations indicated that the HA {010} stoichiometric surface reacts spontaneously with water,2425
generating P-OH and new Ca-OH surface hydroxyl groups, resulting in {010} facets labelled as
{010}R (R=“reacted”). A pictorial overview of all these types of {010} terminations is displayed in Scheme 1.
Scheme 1. Upper part: side view of the sequence of A) Ca3(PO4)2 and B) Ca4(PO4)2(OH)2 layers in the bulk structure of HA along the cross section perpendicular to the (010) surface. Bottom part: top view of the three possible (010) terminations (see text for details). The upper part is a courtesy by Dr. F. Chiatti, PhD Thesis, Torino, 2014; the bottom part is adapted from ref.26
3 ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 31
Despite experimental evidence of the exposure of these terminations can be provided by HRTEM,22-23 the occurrence of proper orientation and stability of HA nanoparticles under the electron beam in order to observe so subtle differences is not an obvious task. Conversely, the use of probe molecules does not usually suffer of relevant limitations, and IR spectroscopy of adsorbed CO has been already used to investigate the Lewis acidity of Ca2+ ions exposed at the surface of HA nanoparticles,15-16, 27 as well as the influence of Mg2+ substitution on the basic properties of HA28 and features of surface hydroxy groups.29 In this respect, Chiatti et al. reported that the spectra of adsorbed CO can be exploited to evaluate the relative contribution of (001) and three types of (010) terminations to the surface of HA nanoparticles.26 This was the basis of the present study, aimed to the evaluation of the effects of the presence of carboxylate anions in the synthesis medium on textural and surface features of HA nanoparticles. In particular, acetates were considered, as calcium acetate is among the possible source of Ca2+ ions in HA synthesis.30-32 The interest towards the action of carboxylate groups stems from the increasing awareness of their role in addressing chemical, structural and textural properties of hydroxyapatite.33-37 Indeed, this is a bio-inspired approach, which aim to mimic the apatite formation mechanisms that occurred in nature, since it is well-known that nucleation and/or growth of bone crystals is regulated by acidic functional macromolecules, such as bone sialoprotein38-39 and osteopontin40 as well as small acidic molecules, such as citrate, which, in particular, was found to stabilize size and morphology of bone apatite41-42 Turning to synthetic apatites, citrate were found to inhibit apatite crystal growth by strongly interacting with the surface,43 and very recently were proposed to play an important role in driving the bio-inspired formation of platy apatite nanocrystals.20 Furthermore, it was reported that amino acids can bind preferentially to specific HA crystal face and inhibit growth in the direction perpendicular to that face.44-47 Thus, on one hand they can be viewed as shape controller of HA nanoparticles, but, on the other hand, amino acids amino acids were also found to increase the bioactivity of synthetic HA.48-50 At the best of our knowledge, in all these cases the effects of carboxylates bearing molecules on the structure on HA surfaces formed in the 4 ACS Paragon Plus Environment
Page 5 of 31
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
presence of these agents was not investigated, including the research works on HA prepared by using Ca(CH3COO)2, where, for instance, the attention was focused on the relative importance of stoichiometry-non-stoichiometry vs structural order–disorder in determining structural feature of nano-HA.31 Hence, this paper is intended as a contribution to the knowledge of factor which can affect the structure of the most abundantly exposed surface termination of HA nanoparticles. 2. EXPERIMENTAL 2.1. Materials Two types of HA nanoparticles were prepared using a similar procedure (i.e. similar Ca/P ratio of reactants, pH of reaction medium and temperature), but changing the source of calcium ions, namely Ca(OH)2 and Ca(CH3COO)2, resulting in materials hereafter referred to as HA−1 and HA−2, respectively. For the preparation of HA−1, an H3PO4 solution (1.26 M, 0.6 L) was dropped into a Ca(OH)2 suspension (1.35 M, 1 L) and kept at 310 K, to accomplish the following theoretical reaction: 5Ca(OH)2 + 3H3PO4 → Ca5(PO4)3OH + 9H2O HA−2 was prepared by dropping the H3PO4 solution (0.21 M, 0.8 L) into a Ca(CH3COO)2 solution (0.35 M, 0.8 L), kept at 310 K as in the previous case, and pH of approximately 10 by addition of NH4OH solution, resulting in the occurrence of the following theoretical reaction: 5Ca(CH3COO)2 + 3H3PO4 + 10NH4OH → Ca5(PO4)3OH + 10CH3COOH + 9H2O + 10NH3 In both cases reaction mixtures were stirred overnight. The stirring was then stopped and mixtures were left standing for 2 hours to allow for deposition of the inorganic phase. HA powders were then isolated by centrifugation of the mother liquor, repeatedly washed with water and dried at 213 K under outgassing (residual pressure: 3 mbar) overnight. HA granular fractions having dimensions
