Maros Gregor

Charge specific protein placement at submicrometer and nanometer scale by direct modification of surface potential by electron beam.

The understanding and the precise control of protein adsorption is extremely important for the development and optimization of biomaterials. The challenge resides in controlling the different surface properties, such as surface chemistry, roughness, wettability, or surface charge, independently, as modification of one property generally affects the other. We demonstrate the creation of electrically modified patterns on hydroxyapatite by using scanning electron beam to tailor the spatial regulation of protein adsorption via electrostatic interactions without affecting other surface properties of the material. We show that domains, presenting modulated surface potential, can be created to precisely promote or reduce protein adsorption.

Electrical properties of hydroxyapatite

Abstract Despite being one of the mostly studied biomaterials for orthopedic, dental, protein purification and stem cell applications, electrical properties of hydroxyapatite has received only limited attention. Since the prediction in 2005 of the possibility of piezo and pyroelectricity in hydroxyapatite several theoretical and experimental works in this field may lead to new understandings of electrical behaviors of calcified tissues in vertebrates. Also, the ability of creating discrete electrostatic domains on nanocrystalline films of hydroxyapatite will open the possibility of understanding how surface charge influences biological interactions. The outlook for future endeavours in this field will be discussed.

Direct creation of microdomains with positive and negative surface potential on hydroxyapatite coatings

A method for the direct patterning of electrostatic potential at the surface of hydroxyapatite is presented here. Microdomains of surface potential have been created on hydroxyapatite coatings by a 20 keV focused electron beam with minimal alterations of surface chemistry. The success of such approach has been confirmed by Kelvin probe force microscopy measurements, which show that this method is capable of creating micron sized positive and negative local electrostatic potential. The shape and potential difference of these domains were found to depend on the dose of total injected charge from the electron beam as well as the speed with which such charge is injected.

Directly created electrostatic micro-domains on hydroxyapatite: probing with a Kelvin Force probe and a protein

Micro-domains of modified surface potential (SP) were created on hydroxyapatite films by direct patterning by mid-energy focused electron beam, typically available as a microprobe of Scanning Electron Microscopes. The SP distribution of these patterns has been studied on sub-micrometer scale by the Kelvin Probe Force Microscopy method as well as lysozyme adsorption. Since the lysozyme is positively charged at physiological pH, it allows us to track positively and negatively charged areas of the SP patterns. Distribution of the adsorbed proteins over the domains was in good agreement with the observed SP patterns.

Experimental and computational studies on toughness enhancement in Ti-Al-Ta-N quaternaries

Design of hard ceramic material coatings with enhanced toughness, which prevents crack formation/propagation leading to brittle failure during application, is a primary industrial requirement. In this work, experimental methods supported by ab initio density functional theory (DFT) calculations and electronic structure analyses are used to investigate the mechanical behavior of magnetron sputtered Ti-Al-Ta-N hard coatings. The as-deposited Ti1-x-yAlxTayN (y = 0–0.60) films exhibit a single phase cubic sodium chloride (B1) structure identified as TiAl(Ta)N solid solutions. While the hardness H of Ti0.46Al0.54N (32.5 ± 2 GPa) is not significantly affected by alloying with TaN (H of the quaternary nitrides varies between 26 ± 2 and 35 ± 4 GPa), the elastic stiffness monotonically decreases from 442 to 354 GPa with increasing Ta contents, which indicates improved toughness in TiAlTaN. Consistent with the experimental findings, the DFT results show that Ta substitutions in TiAlN reduce the shear resistance due...

Surface Charge and Carbon Contamination on an Electron-Beam-Irradiated Hydroxyapatite Thin Film Investigated by Photoluminescence and Phase Imaging in Atomic Force Microscopy

The surface properties of hydroxyapatite, including electric charge, can influence the biological response, tissue compatibility, and adhesion of biological cells and biomolecules. Results reported here help in understanding this influence by creating charged domains on hydroxyapatite thin films deposited on silicon using electron beam irradiation and investigating their shape, properties, and carbon contamination for different doses of incident injected charge by two methods. Photoluminescence laser scanning microscopy was used to image electrostatic charge trapped at pre-existing and irradiation-induced defects within these domains, while phase imaging in atomic force microscopy was used to image the carbon contamination. Scanning Auger electron spectroscopy and Kelvin probe force microscopy were used as a reference for the atomic force microscopy phase contrast and photoluminescence laser scanning microscopy measurements. Our experiment shows that by combining the two imaging techniques the effects of trapped charge and carbon contamination can be separated. Such separation yields new possibilities for advancing the current understanding of how surface charge influences mediation of cellular and protein interactions in biomaterials.

Elimination of surface band bending on N-polar InN with thin GaN capping

0.5–1 μm thick InN {0001} films grown by molecular-beam epitaxy with N- or In-polarity are investigated for the presence of native oxide, surface energy band bending, and effects introduced by 2 to 4 monolayers of GaN capping. Ex situ angle-resolved x-ray photo-electron spectroscopy is used to construct near-surface (GaN)/InN energy profiles, which is combined with deconvolution of In3d signal to trace the presence of InN native oxide for different types of polarity and capping. Downwards surface energy band bending was observed on bare samples with native oxide, regardless of the polarity. It was found that the In-polar InN surface is most readily oxidized, however, with only slightly less band bending if compared with the N-polar sample. On the other hand, InN surface oxidation was effectively mitigated by GaN capping. Still, as confirmed by ultra-violet photo-electron spectroscopy and by energy band diagram calculations, thin GaN cap layer may provide negative piezoelectric polarization charge at the G...

Studies of YBa2Cu3O6+x degradation and surface conductivity properties by Scanning Spreading Resistance Microscopy

Local surface conductivity properties and surface degradation of c-axis oriented YBa2Cu3O6+x (YBCO) thin films were studied by Scanning Spreading Resistance Microscopy (SSRM). For the surface degradation studies, the YBCO surface was cleaned by ion beam etching and the SSRM surface conductivity map has been subsequently repeatedly measured over several hours in air and pure nitrogen. Average surface conductivity of the scanned area was gradually decreasing over time in both cases, faster in air. This was explained by oxygen out-diffusion in both cases and chemical reactions with water vapor in air. The obtained surface conductivity images also revealed its high inhomogenity on micrometer and nanometer scale with numerous regions of highly enhanced conductivity compared to the surroundings. Furthermore, it has been shown that the size of these conductive regions considerably depends on the applied voltage. We propose that such inhomogeneous surface conductivity is most likely caused by varying thickness of degraded YBCO surface layer as well as varying oxygen concentration (x parameter) within this layer, what was confirmed by scanning Auger electron microscopy (SAM). In our opinion the presented findings might be important for analysis of current–voltage and differential characteristics measured on classical planar junctions on YBCO as well as other perovskites.

MgB2 radio-frequency superconducting quantum interference device prepared by atomic force microscope lithography

A new method of preparation of radio-frequency superconducting quantum interference devices on MgB2 thin films is presented. The variable-thickness bridge was prepared by a combination of optical lithography and of the scratching by an atomic force microscope. The critical current of the nanobridge was 0.35μA at 4.2K. Noncontact measurements of the current-phase characteristics and of the critical current versus temperature have been investigated on the authors’ structures.

Structure of Hydrogen Gas Sensing TiO2 Thin Films Prepared by Sol-Gel Method and their Comparison with Magnetron Sputtered Films

TiO2 thin films with a thickness of about 150 nm were deposited by spin coating method on sapphire substrate from a sol-gel system. The hydrogen sensing properties of TiO2 films annealed at various temperatures were studied and correlated with their structure, optical and electrical properties. The annealing temperatures in the range of 600 800 °C lead to anatase films with a roughness in the range of 0.6 0.9 nm. Their sensitivity towards hydrogen is low. The thin films annealed at temperatures in the range 900 1000 °C consist of rutile phase and their roughness increased to 11.7 13.5 nm. They showed good hydrogen sensitivity with optimal operating temperature 200 250 °C. The structure and sensing properties of the prepared films are compared with those synthesized with magnetron sputtering. The maximum of sensitivity was measured on the thin films with diameter of the grains about 100 nm in both cases, i.e. on thin films prepared by sol-gel method as well as on thin films prepared by magnetron sputtering. The maximum sensitivity correlates with the diameter of the grains and dont depend on the allotropy of the titanium dioxide anatase or rutile.