Computational and experimental studies of size and shape related physical properties of hydroxyapatite nanoparticles

Abstract

In this work, the properties of hydroxyapatite (HAP) nanoparticles (NPs) have been studied both theoretically and experimentally focusing on computational analysis. HAP is widely used to fabricate implants, for drug delivery, etc. The physical properties of the nanosized HAP particles play an important role in the interaction with cells in the human body and are of great interest. Computer simulation was employed to understand the properties of HAP clusters (Ca(5)(PO(4))(3)OH) including formation energies, dipole moments and polarization (surface charges) by molecular mechanics (MM + , OPLS) and mostly by quantum semi-empirical Hartree-Fock (PM3) methods. The size of the simulated cluster is found to affect its dipole moment, polarization, and, finally, the electron work function- ϕ. These parameters depend on the concentration of hydrogen atoms H (or protons) at the surface. Values of ϕ were experimentally estimated via photoelectron emission measurements. The magnitude of ϕ was demonstrated to have a positive correlation on sizes. The NPs demonstrated a capability to be gathered within conglomerates. This property is confirmed by the calculated data for various sizes. Their sizes have a positive correlation on ϕ by the native particles. The main results show that the distributions of dipole moments have very different space orientations (along the OX, OY and OZ axes, the OZ axis is oriented along the OH column) and change with the addition of hydrogen atoms, which saturate the broken hydrogen bonds. This electrical property of NP leads to different behaviors and motions with consequent aggregation: (1) for the case of NPs having dipole moment oriented preferably perpendicular to the OZ axis (with more hydrogen bonds saturated by added H)-the HAP NP aggregates with hexagonal orientation and forms a wider and more spherical shape (sphere-like or bundle-like); (2) for the case of NPs having dipole moment oriented along the OZ axis (as is the case in the absence of added protons or non-saturated hydrogen bonds)-the NPs firstly rotated and oriented along this axis to form the most elongated cylindrical shape (rod-like).

Компьютерные Исследования Наноструктур Гидроксиапатита, Их Особенности И Свойства

“…Naturally, HAP is a mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but it is often written as Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two molecular units. The specific feature of HAP is that the OH − ions form inner channels along the c axis with various random OH-dipoles orientations under different external conditions, which allow the transfer of protons along this axis under special conditions (such as temperature heating and applied electric field with necessary critical values) [28][29][30][31][32]35]. Usually HAP has 2 main different forms of nano-crystalline structures (in hexagonal P63/m and monoclinic P21/b space group structures) and amorphous structure.…”

“…The authors of [30] picked out another cluster (HAP 3-411), which is shown in Fig 5,a and Fig. 6,d, and undertook the study of various sizes and shapes of HAP NPs formation under different conditions, particularly with variation of protons or pH of the medium.…”

“…Right figures present the interaction of two HAP NP clusters: a) with clusters orientation along the OZ-axis and b) with clusters orientation parallel to the OZ-axis; c) changes of energies with distances between HAP NPs. The green color in the above figures shows the selection option computed by the HyperChem molecular structure in the self-consistent field of another structure (printed with permission from IOP Publishing, LTD Copyright Clearance Center [30] The result of such modeling is shown in Fig. 7,c (the modeled and interacting molecular structures are presented in Fig.…”

“…It is also useful for creating of new HAP nanostructures having wide biomedical applications [13][14][15][16][17][18][19]. The results of these studies [7,9,11,[19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] allow us to clarify possible mechanisms of structural changes and electrical properties of HAP in the bulk crystal and on the crystal surface (electrical potential, charges and polarization). And this, in turn, will allow one to improve the efficiency of HAP interactions with the living bone cells (osteoblasts, osteoclasts, etc.)…”

“…Beyond HAP crystal bulk model, a special HAP model of the layered slab supercells units, which includes vacuum spaces between the layered slabs forming HAP surface, are considered and analysed too [5]. More computational details are described in [22,24,26,27,[30][31][32][33][34][35]67] and in sections below.…”