Surface functionalization and biological applications of CVD diamond
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Introduction There is a great deal of current interest in modifying solid substrate surfaces with biomolecules due to their utility as biosensors and for implantable biomedical devices. Physical and chemical properties such as wear resistance, chemical inertness, and biocompatibility are the most important parameters guiding the choice of the substrate. As a wide-bandgap semiconductor, diamond combines ultimate electronic properties with exceptional physicochemical properties and bio-inertness (Table I).1 Applications of diamond ultimately depend on the commercial availability of diamond films. Recent advances made in the controlled deposition of diamond thin films with varying morphologies (Figure 1) have pushed researchers to consider diamond as an alternative carbon-based material.2 Ultranano-, nano-, and microcrystalline diamond layers are dominated by grain boundaries that are decorated with sp2-bonded and amorphous carbon.1 The volume fraction of sp2 carbon and grain boundaries depends on the growth parameters used and varies from layer to layer. While nanocrystalline diamond typically has a surface roughness in the range of 30–50 nm, single-crystalline CVD diamond has been optimized to electronic grade quality with atomically smooth surfaces and defect densities of less than 102 cm–2.
To make diamond thin films valuable for biosensing applications, stable and site-specific immobilization of enzymes, proteins, and peptides is required. Covalent grafting of biological molecules is considered the most viable approach for the formation of extremely robust and stable biointerfaces even at elevated temperatures. The high chemical stability and inertness of diamond for a long time have been considered to be incompatible with surface functionalization strategies developed on other semiconductor surfaces, in particular silicon. Modification of hydrogen-terminated silicon surfaces through silicon-carbon (Si–C) bonds or chemical functionalization of the oxide layer through Si–O–Si bonds using silane chemistry are two of many means for providing new functionalities to the semiconductor surface.3 The chemical stability of the resulting semiconductor/organic interface and its biocompatibility were found to be limited, as surface-active electronic defects are easily formed. The rather rapid formation of an insulating oxide layer on silicon also means that surface modification needs to be performed under well-controlled conditions. These disadvantages have been shown to be overcome using diamond interfaces. In the last 15 years, progress has been made in the development of easy, controllable, and specific surface modification methods for the introduction of different
Sabine Szunerits, University Lille 1, Interdisciplinary Research Institute, Parc de la Haute Borne, France; [email protected] Christoph E. Nebel, Fraunhofer Institute for Applied Solid State Physics, Freiburg, Germany; [email protected] Robert J. Hamers, Department of Chemistry, University of Wisconsin-Madison, USA; [email protected]
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