The work on DNA follows two major research lines: i) the development of covalent coupling techniques to establish hybrid systems based on semiconductors, especially carbon-based materials, and nucleic acids; ii) the development of fast and label-free methods to localize and to identify point mutations (single nucleotide polymorphisms, SNPs) in DNA fragments. SNP-type DNA defects are known to be responsible – or to be co-factors – in more than 400 hereditary diseases.
Concerning the coupling of DNA to sensor platforms, we have developed the ‘fatty acid & EDC route’, comprising the photochemical attachment of ω-unsaturated fatty acids, followed by a zero-length crosslinking step to biochemical probe molecules. These probe molecules can be single-stranded DNA and aptamers, but also immunoglobulin-type antibodies. The fatty acid & EDC approach allows immobilizing probe molecules at a predefined distance from the sensor surface and, moreover, it was proven to work on CVD diamond, carbon nanowalls, silicon, and cubic boron nitrite. In the case of diamond, this coupling route allows for no less than 35 denaturation-hybrididization cycles without loss of DNA-binding capacity. Extensive studies of the molecular arrangement of the probe DNA on sensor surfaces have been performed by quantitative fluorescence-intensity microscopy and by spectroscopic ultraviolet ellipsometry in cooperation with the BESSY II synchrotron facility.
The label-free sensors for SNP detection are based on electrochemical impedance spectroscopy. Sensor electrodes with covalently attached probe DNA allow for the real-time detection of hybridization events, as well as for the detection of thermally- or chemically induced denaturation. In contrast to microarrays, the electronic readout takes only a few minutes and the sensor electrodes can be used repetitively without regeneration steps. There is strong evidence that the denaturation-time constants correlate with the melting temperature of complementary- and SNP-containing DNA duplexes. In this sense, the electronic readout has the potential to become a fast equivalent to the established but tedious SNP identification by biochemical melting techniques with optical monitoring.