Inhomogeneous DNA bonding to polycrystalline CVD diamond
Introduction
Bioelectronics is a rapidly progressing field at the junction of physics and biochemistry. The basic feature of bio-electronic devices is the immobilization of a biomaterial onto a transducer and the electronic detection of biological functions associated with the biological matrices. The growing use of deoxyribonucleic acid (DNA) micro-arrays and DNA chips in genetics, medicine and drug discovery shifted significant attention towards the realization of miniaturized and fast analytical systems [1], [2], [3], [4]. DNA immobilization techniques have been explored for a variety of substrates like gold, carbon electrodes, and SiOx, to name a few [5], [6], [7]. These substrates show different characteristics with respect to flatness, homogeneity, availability and are also very different in chemical stability, reproducibility and biochemical manipulation. For active electronic bio-applications, the integration of bio-functionalized surfaces with microelectronics and micro-mechanical tools is required, which shifted activities towards chemical and biological modifications of semiconductors [8], [9], [10], [11]. Most of the microelectronic-compatible materials like silicon, SiOx, and gold show, however, degradation of the interfaces which inhibits the development of integrated sensors [11]. Diamond is a promising candidate for bio-electronic devices as it shows good electronic and chemical properties, is considered to be biocompatible and can be grown single-, poly- or nanocrystalline, either by homo-epitaxy or by heteroepitaxy on a variety of substrates. Since it has been shown that the surface of diamond can be chemically modified to bond DNA [11], [12], enzymes [13] and proteins [14], the realization of bio-functionalized diamond electrodes, of bio-field effect transistors (FET), of bio-micro-electro-mechanical systems (MEMS) and of bio-AFM tips [15] for gene surgery are growing fields of research.
Two basic recipes are used to attach bio-molecules to diamond: i) photochemical surface modifications [11], [12] and ii) electrochemical modification [16]. In case of photochemical processing, excitation of electrons from diamond is required which triggers hydrogen cleavage at the diamond surface and which stimulates nucleophilic properties of alkene molecules [17], [18], [19]. Sub-bandgap light illumination of typically 250 nm is used in these experiments. It has been shown that the negative electron affinity of hydrogen terminated diamond gives rise to this transition [17], [18], [19]. The electron affinity is related to the quality of hydrogen termination and can vary from − 1.1 eV to + 1.7 eV [20], dependent on the hydrogen coverage and the crystal orientation.
In case of miniaturized bio-sensors highly uniform interface properties are required down to micro- or sub-micrometer dimensions. Up-to-now, most attachment and characterization experiments have been applied on nano- and polycrystalline diamond layers which are composed out of differently oriented crystallites of varying size and shape as well as of grain boundaries which are decorated with graphite or amorphous carbon. In the case of polycrystalline diamond, smooth surfaces are commercially available which are realized by mechanically polishing thus showing defective electronic surface properties. For mixed composite (graphite/diamond) and/or polished layers, surface Fermi level, negative electron affinity and surface defect densities are not defined and controlled.
In this paper we report about microscopic characterization of polycrystalline diamonds, which have been photochemically modified to covalently bond DNA on the surface. We apply scanning electron microscopy (SEM) to elucidate variations of negative electron affinities on smoothly polished polycrystalline diamond surfaces which have been H-terminated by a standard plasma process. Fluorescence microscopy (FM) is applied to detect variations of color-center labeled double-strand (ds) DNA. Surface morphologies of polished CVD diamond and the properties of DNA films grafted on such films are characterized by atomic force microscopy (AFM). These experiments reveal significant local variations of DNA molecule densities bonded to polycrystalline diamond.
Section snippets
Experimental
Commercially available (Element Six) polished boron-doped polycrystalline diamond films of 5 mm × 5 mm × 0.5 mm size have been used for the photochemical surface modification and DNA attachment experiments. The surface topography of such polished diamond films has been characterized by AFM. Fig. 1 shows a typical result. Please note that these films appear black and smooth using optical microscopy. The surface shows fine structure roughness of about 5 Å and grain induced roughness, which varies
Results and conclusion
To characterize surface electronic properties like electron affinity and surface Fermi level, we applied photo-electron emission spectroscopy, Kelvin force microscopy and scanning electron microscopy (SEM) (for details Refs [22], [23], [24]). We clearly find that SEM bright scale images reveal electron affinity properties of H-terminated and oxidized diamond surfaces qualitatively as shown in Fig. 2. However, a quantitative discussion has not yet been achieved due to the complexity of the
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