Ultrananocrystalline Diamond Surfaces Research Articles

Nanostructured modified ultrananocrystalline diamond surfaces as immobilization support for lipases

Ultrananocrystalline diamond (UNCD) is a promising and - so far - relatively unexploited material for biocatalytic applications. In this study, UNCD films, deposited on silicon wafers by microwave plasma-assisted chemical vapor deposition, were investigated as supports for immobilization of the enzyme lipase B from Candida antarctica (CalB). The motivation for this work was the potential of utilization of the resulting biomaterials for the fabrication of flow bioreactors, harnessing both the benefits of the UNCD and the biocatalysts. Prior to immobilization, the UNCD surfaces were structured by reactive ion etching with a randomly distributed gold nanodroplet mask that was applied on top of the UNCD films, in order to increase the area for interaction with the enzyme. Furthermore, structured UNCD films were subjected to modification by UV/ozone treatment or trifluoromethane plasma which rendered the surfaces oxygen-terminated and hydrophilic or fluorine-terminated and hydrophobic, respectively. Two established coupling strategies, based on carbodiimide and oxalyl chloride chemistry, were investigated for covalent immobilization of CalB on structured oxygen-terminated surfaces. However, non-covalent immobilization on fluorine- and as-grown hydrogen-terminated surfaces, mediated by physical adsorption of the enzyme, appeared to be more efficient. Moreover, the data strongly suggested that the chemical nature of the termination and the hydrophobicity of the surface influence the substrate selectivity of the enzyme. Non-covalently immobilized CalB on strongly hydrophobic fluorine-terminated surfaces showed a ~5.5-fold higher preference for the short-chain substrate p-nitrophenyl butyrate over the medium-chain ester p-nitrophenyl laurate, whereas on hydrophilic O-terminated surfaces this selectivity was only ~1.9-fold. Moderately hydrophobic H-terminated surfaces conferred a selectivity ratio of ~3.5, which is in between the two former values and therefore in line with the hypothesized dependence of the substrate selectivity on the hydrophobicity of the surface material. These primary results can help on the optimization of the design of immobilized biocatalysts on UNCD surfaces to produce columns for flow bioreactors. Prime noveltyNanostructured modified ultrananocrystalline diamond (UNCD) surfaces were studied as immobilization support for lipases. The physical adsorption of lipase CalB was more efficient on hydrophobic F- or H-terminated surfaces than the two covalent coupling approaches on hydrophilic O-terminated UNCD. Furthermore, the chemical nature of the surface termination had an impact on the substrate selectivity by catalyzed hydrolysis.

Combining TEM, AFM, and Profilometry for Quantitative Topography Characterization Across All Scales.

Surface roughness affects the functional properties of surfaces, including adhesion, friction, hydrophobicity, biological response, and electrical and thermal transport properties. However, experimental investigations to quantify these links are often inconclusive because surfaces are fractal-like, and the values of measured roughness parameters depend on measurement size. Here, we demonstrate the characterization of topography of an ultrananocrystalline diamond (UNCD) surface at the angstrom scale using transmission electron microscopy (TEM), as well as its combination with conventional techniques to achieve a comprehensive surface description spanning 8 orders of magnitude in size. We performed more than 100 individual measurements of the nanodiamond film using both TEM and conventional techniques (stylus profilometry and atomic force microscopy). While individual measurements of root-mean-square (RMS) height, RMS slope, and RMS curvature vary by orders of magnitude, we combine the various techniques using the power spectral density and use this to compute scale-independent parameters. This analysis reveals that "smooth" UNCD surfaces have an RMS slope greater than 1, even larger than the slope of the Austrian Alps when measured on the scale of a human step. This approach of comprehensive multiscale roughness characterization, measured with angstrom-scale detail, will enable the systematic evaluation and optimization of other technologically relevant surfaces, as well as systematic testing of the many analytical and numerical models for the behavior of rough surfaces.

Patterning of the surface termination of ultrananocrystalline diamond films for guided cell attachment and growth

The surface patterning of a biomaterial, i.e. placing areas with different surface chemistries next to each other is an important step for various biomedical and biotechnological applications, e.g. in biosensor technology, drug screening or tissue engineering. In the current work we investigated the patterning of the surface termination of ultrananocrystalline diamond (UNCD) films prepared by microwave plasma chemical vapor deposition. In order to achieve a high wettability contrast in the neighboring areas O- and F-terminations were selected. The sequence of the modifications and the feasibility of different masking techniques were tested in advance. Based on the achieved results the following process of surface patterning was applied. Initially the whole UNCD surface was modified by UV/O3 treatment rendering the surface O-terminated and strongly hydrophilic (contact angle against water of 11°). The partial masking of this surface before the second modification was achieved with a gold hard mask photolithographically structured and wet etched in form of a grid. After the second modification with CHF3 plasma which provided F-termination and hydrophobic character of the surface (contact angle>110°), the gold layer was removed. The results of the patterning of the surface termination were visualized by scanning electron microscopy revealing different contrasts of the areas with different terminations. Finally, human SH-SY5Y neuroblastoma cells were plated on the patterned UNCD surfaces and they exhibited preferential attachment and growth on the hydrophilic grid.

Biological evaluation of ultrananocrystalline and nanocrystalline diamond coatings.

Nanostructured biomaterials have been investigated for achieving desirable tissue-material interactions in medical implants. Ultrananocrystalline diamond (UNCD) and nanocrystalline diamond (NCD) coatings are the two most studied classes of synthetic diamond coatings; these materials are grown using chemical vapor deposition and are classified based on their nanostructure, grain size, and sp3 content. UNCD and NCD are mechanically robust, chemically inert, biocompatible, and wear resistant, making them ideal implant coatings. UNCD and NCD have been recently investigated for ophthalmic, cardiovascular, dental, and orthopaedic device applications. The aim of this study was (a) to evaluate the in vitro biocompatibility of UNCD and NCD coatings and (b) to determine if variations in surface topography and sp3 content affect cellular response. Diamond coatings with various nanoscale topographies (grain sizes 5-400 nm) were deposited on silicon substrates using microwave plasma chemical vapor deposition. Scanning electron microscopy and atomic force microscopy revealed uniform coatings with different scales of surface topography; Raman spectroscopy confirmed the presence of carbon bonding typical of diamond coatings. Cell viability, proliferation, and morphology responses of human bone marrow-derived mesenchymal stem cells (hBMSCs) to UNCD and NCD surfaces were evaluated. The hBMSCs on UNCD and NCD coatings exhibited similar cell viability, proliferation, and morphology as those on the control material, tissue culture polystyrene. No significant differences in cellular response were observed on UNCD and NCD coatings with different nanoscale topographies. Our data shows that both UNCD and NCD coatings demonstrate in vitro biocompatibility irrespective of surface topography.

Strong attachment of circadian pacemaker neurons on modified ultrananocrystalline diamond surfaces

Diamond is a promising material for a number of bio-applications, including the fabrication of platforms for attachment and investigation of neurons and of neuroprostheses, such as retinal implants. In the current work ultrananocrystalline diamond (UNCD) films were deposited by microwave plasma chemical vapor deposition, modified by UV/O3 treatment or NH3 plasma, and comprehensively characterized with respect to their bulk and surface properties, such as crystallinity, topography, composition and chemical bonding nature. The interactions of insect circadian pacemaker neurons with UNCD surfaces with H–, O– and NH2-terminations were investigated with respect to cell density and viability. The fast and strong attachment achieved without application of adhesion proteins allowed for advantageous modification of dispersion protocols for the preparation of primary cell cultures. Centrifugation steps, which are employed for pelletizing dispersed cells to separate them from dispersing enzymes, easily damage neurons. Now centrifugation can be avoided since dispersed neurons quickly and strongly attach to the UNCD surfaces. Enzyme solutions can be easily washed off without losing many of the dispersed cells. No adverse effects on the cell viability and physiological responses were observed as revealed by calcium imaging. Furthermore, the enhanced attachment of the neurons, especially on the modified UNCD surfaces, was especially advantageous for the immunocytochemical procedures with the cell cultures. The cell losses during washing steps were significantly reduced by one order of magnitude in comparison to controls. In addition, the integration of a titanium grid structure under the UNCD films allowed for individual assignment of physiologically characterized neurons to immunocytochemically stained cells. Thus, employing UNCD surfaces free of foreign proteins improves cell culture protocols and immunocytochemistry with cultured cells. The fast and strong attachment of neurons was attributed to a favorable combination of topography, surface chemistry and wettability.

Cell growth on different types of ultrananocrystalline diamond thin films.

Unique functional materials provide a platform as scaffolds for cell/tissue regeneration. Investigation of cell-materials’ chemical and biological interactions will enable the application of more functional materials in the area of bioengineering, which provides a pathway to the novel treatment for patients who suffer from tissue/organ damage and face the limitation of donation sources. Many studies have been made into tissue/organ regeneration. Development of new substrate materials as platforms for cell/tissue regeneration is a key research area. Studies discussed in this paper focus on the investigation of novel ultrananocrystalline diamond (UNCD) films as substrate/scaffold materials for developmental biology. Specially designed quartz dishes have been coated with different types of UNCD films and cells were subsequently seeded on those films. Results showed the cells’ growth on UNCD-coated culture dishes are similar to cell culture dishes with little retardation, indicating that UNCD films have no or little inhibition on cell proliferation and are potentially appealing as substrate/scaffold materials. The mechanisms of cell adhesion on UNCD surfaces are proposed based on the experimental results. The comparisons of cell cultures on diamond-powder-seeded culture dishes and on UNCD-coated dishes with matrix-assisted laser desorption/ionization—time-of-flight mass spectroscopy (MALDI-TOF MS) and X-ray photoelectron spectroscopy (XPS) analyses provided valuable data to support the mechanisms proposed to explain the adhesion and proliferation of cells on the surface of the UNCD platform.

Influence of the surface termination of ultrananocrystalline diamond/amorphous carbon composite films on their interaction with neurons

Ultrananocrystalline diamond (UNCD) films have been deposited by microwave plasma chemical vapor deposition from 17% CH4/N2 mixtures. In order to change the original hydrogen termination of the UNCD surfaces, the films have subsequently been subjected either to the so-called UV/O3 treatment which leads to OH-terminated surfaces, or to NH3/N2 plasmas which introduces NH2 groups but also a certain amount of OH groups. These three types of surfaces have been characterized by X-ray photoelectron spectroscopy, contact angle and ζ-potential measurements. The contact angle measurements have shown that as-grown UNCD surfaces are highly hydrophobic but became highly hydrophilic after both treatments. The ζ-potential measurements revealed that the isoelectric point of the H-terminated as-grown surface is distinctively higher than that of either UV/O3 or NH3/N2 plasma treated surfaces. Finally, the interactions of these surfaces with neurons of the cockroach Leucophaea maderae have been investigated. These studies have shown that especially the two treated surfaces allow for a fast, strong attachment of these cells without compromising their viability and without changing their normal physiological responses. These results will be discussed in terms of those obtained with the different surface characterization techniques.

Development of ultrananocrystalline diamond (UNCD) coatings for multipurpose mechanical pump seals

The reliability and performance of silicon carbide (SiC) shaft seals on multipurpose mechanical pumps are improved by applying a protective coating of ultrananocrystalline diamond (UNCD). UNCD exhibits extreme hardness (97 GPa), low friction (0.1 in air) and outstanding chemical resistance. Consequently, the application of UNCD coatings to multipurpose mechanical pump seals can reduce frictional energy losses and eliminate the downtime and hazardous emissions from seal failure and leakage. In this study, UNCD films were prepared by microwave plasma chemical vapor deposition utilizing an argon/methane gas mixture. Prior to coating, the SiC seals were subjected to mechanical polishing using different grades of micron-sized diamond powder to produce different starting surfaces with well-controlled surface roughnesses. Following this roughening process, the seals were seeded by mechanical abrasion with diamond nanopowder, and subsequently coated with UNCD. The coated seals were subjected to dynamic wear testing performed at 3600 RPM and 100 psi for up to 10 days during which the seals were periodically removed and inspected. The UNCD-coated seals were examined using Raman microanalysis, scanning electron microscopy, optical profilometry, and adhesion testing before and after the wear testing. These analyses revealed that delamination of the UNCD films was prevented when the initial SiC seal surface had an initial roughness >0.1 μm. In addition, the UNCD surfaces showed no measurable wear as compared to approximately 0.2 μm of wear for the untreated SiC surfaces.

Controlling Surface Functionality through Generation of Thiol Groups in a Self-Assembled Monolayer

A lithographic method to generate reactive thiol groups on functionalized synthetic diamond for biosensor and molecular electronic applications is developed. We demonstrate that ultrananocrystalline diamond (UNCD) thin films covalently functionalized with surface-generated thiol groups allow controlled thiol-disulfide exchange surface hybridization processes. The generation of the thiol functional head groups was obtained by irradiating phenylsulfonic acid (PSA) monolayers on UNCD surfaces. The conversion of the functional headgroup of the self-assembled monolayer was verified by using X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and fluorescence microscopy. Our findings indicate the selective generation of reactive thiol surface groups. Furthermore, we demonstrate the grafting of yeast cytochrome c to the thiol-modified diamond surface and the electron transfer between protein and electrode.

Method for Characterizing Nanoscale Wear of Atomic Force Microscope Tips

Atomic force microscopy (AFM) is a powerful tool for studying tribology (adhesion, friction, and lubrication) at the nanoscale and is emerging as a critical tool for nanomanufacturing. However, nanoscale wear is a key limitation of conventional AFM probes that are made of silicon and silicon nitride (SiNx). Here we present a method for systematically quantifying tip wear, which consists of sequential contact-mode AFM scans on ultrananocrystalline diamond surfaces with intermittent measurements of the tip properties using blind reconstruction, adhesion force measurements, and transmission electron microscopy (TEM). We demonstrate direct measurement of volume loss over the wear test and agreement between blind reconstruction and TEM imaging. The geometries of various types of tips were monitored over a scanning distance of approximately 100 mm. The results show multiple failure mechanisms for different materials, including nanoscale fracture of a monolithic Si tip upon initial engagement with the surface, film failure of a SiNx-coated Si tip, and gradual, progressive wear of monolithic SiNx tips consistent with atom-by-atom attrition. Overall, the method provides a quantitative and systematic process for examining tip degradation and nanoscale wear, and the experimental results illustrate the multiple mechanisms that may lead to tip failure.

Surface properties of differently prepared ultrananocrystalline diamond surfaces

Ultrananocrystalline diamond/amorphous carbon composite films have been deposited by microwave plasma chemical vapour deposition from 17% CH 4/N 2 mixtures at 600 °C. Thereafter the films were subjected to various treatments (plasma processes, UV/O 3 exposure) to obtain hydrogen, oxygen, and fluorine terminated surfaces, which then have been characterized with respect to their composition, roughness, wettability, and other properties. Among others, it will be shown that H- and F-terminated surfaces are very stable even if exposed for long time to air, while O-terminated ones are prone to contaminations. H- and O-termination can be patterned by applying the UV/O 3 treatment through a mask. Finally, it will be shown that a non-fouling poly(ethylene glycol) layer can be grafted directly on oxygen terminated surfaces by an atom transfer radical polymerization process using α-bromoisobutyryl bromide as an initiator.

Origin of Ultralow Friction and Wear in Ultrananocrystalline Diamond

The impressively low friction and wear of diamond in humid environments is debated to originate from either the stability of the passivated diamond surface or sliding-induced graphitization/rehybridization of carbon. We find ultralow friction and wear for ultrananocrystalline diamond surfaces even in dry environments, and observe negligible rehybridization except for a modest, submonolayer amount under the most severe conditions (high load, low humidity). This supports the passivation hypothesis, and establishes a new regime of exceptionally low friction and wear for diamond.

Surface chemistry and bonding configuration of ultrananocrystalline diamond surfaces and their effects on nanotribological properties

We present a comprehensive study of surface composition and nanotribology for ultrananocrystalline diamond (UNCD) surfaces, including the influence of film nucleation on these properties. We describe a methodology to characterize the underside of the films as revealed by sacrificial etching of the underlying substrate. This enables the study of the morphology and composition resulting from the nucleation and initial growth of the films, as well as the characterization of nanotribological properties which are relevant for applications including micro-/nanoelectromechanical systems. We study the surface chemistry, bonding configuration, and nanotribological properties of both the topside and the underside of the film with synchrotron-based x-ray absorption near-edge structure spectroscopy to identify the bonding state of the carbon atoms, x-ray photoelectron spectroscopy to determine the surface chemical composition, Auger electron spectroscopy to further verify the composition and bonding configuration, and quantitative atomic force microscopy to study the nanoscale topography and nanotribological properties. The films were grown on $\mathrm{Si}{\mathrm{O}}_{2}$ after mechanically polishing the surface with detonation synthesized nanodiamond powder, followed by ultrasonication in a methanol solution containing additional nanodiamond powder. The $s{p}^{2}$ fraction, morphology, and chemistry of the as-etched underside are distinct from the topside, exhibiting a higher $s{p}^{2}$ fraction, some oxidized carbon, and a smoother morphology. The nanoscale single-asperity work of adhesion between a diamond nanotip and the as-etched UNCD underside is far lower than for a silicon-silicon interface ($59.2\ifmmode\pm\else\textpm\fi{}2$ vs $826\ifmmode\pm\else\textpm\fi{}186\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}∕{\mathrm{m}}^{2}$, respectively). Exposure to atomic hydrogen dramatically reduces nanoscale adhesion to $10.2\ifmmode\pm\else\textpm\fi{}0.4\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}∕{\mathrm{m}}^{2}$, at the level of van der Waals' interactions and consistent with recent ab initio calculations. Friction is substantially reduced as well, demonstrating a direct link between the surface chemistry and nanoscale friction. The proposed mechanism, supported by the detailed surface spectroscopic analysis, is the elimination of reactive (e.g., ${\mathrm{C}}^{*}$), polar (e.g., $\mathrm{C}\mathrm{O}$), and $\ensuremath{\pi}$-bonded $(\mathrm{C}\mathrm{C})$ surface groups, which are replaced by fully saturated, hydrogen-terminated surface bonds to produce an inert surface that interacts minimally with the contacting counterface.

Structured Polymer Grafts on Diamond

In this work, a facile method for the preparation of structured and functional polymer grafts on diamond surfaces is described. Uniform poly(styrene) (PS) grafts with a thickness of approximately 110 nm were created directly onto oxidized ultrananocrystalline diamond (UNCD) surfaces by the self-initiated photografting and photopolymerization of bulk styrene with UV irradiation. The stable covalent bonding of the PS grafts allows polymer analogue reactions with drastic reaction conditions without noticeable detachment of the polymer coating. Thus, various functionalities, such as nitro, sulfonic, and aminomethyl groups have been successfully incorporated to the polymer grafts. Furthermore, the reactivity contrast between hydrogenated and oxidized UNCD surfaces allows for the preparation of structured polymer grafts. Finally, we have demonstrated the good reactivity and accessibility of the incorporated pendant functional groups.