Yu. Dekhtyar

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

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).

Computational study of hydroxyapatite structures, properties and defects

Hydroxyapatite (HAp) was studied from a first principle approach using the local density approximation (LDA) method in AIMPRO code, in combination with various quantum mechanical (QM) and molecular mechanical (MM) methods from HypemChem 7.5/8.0. The data obtained were used for studies of HAp structures, the physical properties of HAp (density of electronic states—DOS, bulk modulus etc) and defects in HAp. Computed data confirmed that HAp can co-exist in different phases—hexagonal and monoclinic. Ordered monoclinic structures, which could reveal piezoelectric properties, are of special interest. The data obtained allow us to characterize the properties of the following defects in HAp: O, H and OH vacancies; H and OH interstitials; substitutions of Ca by Mg, Sr, Mn or Se, and P by Si. These properties reveal the appearance of additional energy levels inside the forbidden zone, shifts of the top of the valence band or the bottom of the conduction band, and subsequent changes in the width of the forbidden zone. The data computed are compared with other known data, both calculated and experimental, such as alteration of the electron work functions under different influences of various defects and treatments, obtained by photoelectron emission. The obtained data are very useful, and there is an urgent need for such analysis of modified HAp interactions with living cells and tissues, improvement of implant techniques and development of new nanomedical applications.

A Computational Study of the Properties and Surface Interactions of Hydroxyapatite

Hydroxyapatite (HAP, Ca10(PO4)6(OH)2) was studied from first principles approaches using the local density approximation (LDA) method in combination with various quantum-chemical (QM) and molecular mechanical (MM) methods from HypemChem 7.5/8.0. The data then were used for studies of HAP structures, and the interactions of HAP clusters with ionic species such as citrates. Computed data show that HAP can co-exist in different phases at room temperature, as both hexagonal and monoclinic. Special interest is connected with the ordered monoclinic structure, which could reveal piezoelectric properties. Obtained data on HAP interactions with citrates show the formation of differing HAP nanostructure forms, depending upon the concentration of citrate present.

Sensors for water safety and security

Identification and prioritization of contaminants arising from occasional release or intentional discharge in unsecured water supplies are of paramount importance to assess national and international capacities to be able to respond, sense/detect, isolate, and mitigate safety and security risk vectors in a timely manner. Using advanced and nanoscale materials, a series of novel point and stand-off sensors/detectors for continuous and in-situ monitoring of inorganic, organic, and microbiological pollutants are described. The study is concurrent to our existing efforts to develop remediation strategies for region specific contaminants, defining water quality, modeling of process function water system design, and enhancing water resiliency. New sensing approaches based on interaction of a semiconductor surface with emitting electrons and metal-complexes-based chiroptical switches are also described.

Exoelectron analysis of amorphous silicon

The basic aim of the present article is proposing a new field in the study of structural imperfections in amorphous silicon, particularly in the a‐Si:H system. The method is based on registering photothermostimulated exoelectron emission (PTSE). The PTSE method is brought into effect as a near‐threshold single‐photon photoemission process. Its quantum yield is modulated by change in the density of states created by point defects, such as D centers. It is shown that exoelectron spectroscopy makes it possible to investigate heat‐ and light‐modified changes in the electronic structure of the defects, their annealing, as well as diffusion of atomic particles, such as hydrogen.

Modified Hydroxyapatite Structure and Properties: Modeling and Synchrotron Data Analysis of Modified Hydroxyapatite Structure

First principle modeling and calculations of hydroxyapatite both native and surface modified and having various defects (OH vacancies, H inter-nodes) were performed. Local Density Approximation method used with calculations of Density of States allows us to analyze the experimental obtained work function data. Molecular modeling was confirmed by photo-electron measurements up to 6.5 eV and photoluminescence experimental data from synchrotron DESY up to 30 eV. Brief analysis of the influence of heating, microwave radiation, hydrogenation, and synchrotron radiation on hydroxyapatite surface is presented in this work. New data on the structure of modified hydroxyapatite are obtained.

Size depended electrical properties of Hydroxyapatite nanoparticles

Hydroxyapatite (HAp) nanoparticles is widely in use to fabricate implants. Computer simulation was employed to understand the HAp clusters structural and electrical (dipole momentum and polarization - surface charges) properties. A size (X) of the simulated cluster has an influence on its dipole momentum, polarization and as the result on the electron work function (ϕ). Saturation of hydrogen bonds and the size of the cluster have an influence on its surface polarization. Values of ϕ were experimentally estimated owing to photoelectron emission measurements. The magnitude of ϕ was demonstrated positive correlation on X. The nanoparticles demonstrated a capability to be gathered within conglomerates. This property is confirmed by calculated surface disordering of HAp nanoparticles leading to its higher interaction.

Electrically Functionalized Hydroxyapatite and Calcium Phosphate Surfaces to Enhance Immobilization and Proliferation of Osteoblasts In Vitro and Modulate Osteogenesis In Vivo

Hydroxyapatite (HAP) is used to fabricate implants for bone repair in dentistry and orthopaedics. To functionalise the surface of HAP that is in direct contact with human cells a surface electrical charge deposition has been achieved by means of hydrogenation technology. This technology provides uniform 3 dimensioned processing of the specimen surfaces. The engineered charge was estimated from measurements of the photoelectron emission work function (ϕ). The later was increased to ~ 0.2 eV. The negatively charged HAP surface attached 10 times more osteoblatic cells and increased their proliferation capacity 1.6 times, in contrast to the uncharged one.

Novel Concepts of “Niche-Relief” and Niche-Voltage” for Stem Cells as a Base of Bone and Hematopoietic Tissues Biomimetic Engineering

An affect of relief features and quantitative variables of artificial surfaces on structural-functional status of human lung prenatal stromal cells (HLPSC) and remodeling of Balb/c mice bone/bone marrow system have been studied. Implants with rough (Ra>2 μm) calcium phosphate micro-arc coatings have structural-functional sites (micro-regions) named “niches-relief” which are necessary for maturation and differentiation of HLPSC into secreting osteoblasts in shortterm culture. Maximal remodeling of mouse bone/bone marrow system in 45-day subcutaneous heterotopic test in vivo is also noted under optimal parameter (average index of cellular alkaline phosphatase area to artificial micro-region area is about 43 %) of osteogenic niche in vitro. Probable physical mechanism of osteogenic niche functioning has been determined. It correlates with increasing of electron work function supplied with a negative charge of calcium phosphate nanorelief in the sockets (artificial “niches”). “Niche-voltage” concept for bone tissue biomimetic engineering was proposed.

The Influence of Surface Treatment by Hydrogenation on the Biocompatibility of Different Hydroxyapatite Materials

The influence of hydrogenation on the biocompatibility of different hydroxyapatite (HAP) materials was tested. Materials consisted of pure HAP, HAP substituted with manganese (Mn+2) and with magnesium (Mg+2) – all axially pressed and conventionally sintered for 2 h at 1200 °C; pure HAP isostatic pressed and sintered by a microwave technique for 15 min at temperature of 1200 °C. Biocompatibility was compared by enumeration of the number of osteoblast-like cells to the materials before and after hydrogenation. Obtained results show that the osteoblastic cells demonstrated a higher ability to attach to HAP if its surface was negatively charged. Hydrogenation altered the surface potential; HAP substituted with manganese – HAP(Mn) and with magnesium – HAP(Mg) demonstrated the highest ability to engineer the charge.