Characterization of a reduced graphene oxide based biosensor via infrared spectroscopy
Date of Issue2018-02-06
School of Materials Science and Engineering
The fabrication of a small and user-friendly sensor, which has a high dynamic range in which it is sensitive and specific for certain analytes is a major topic in research in the last decades. The first fabrication of a graphene monolayer in 2004 caused a boom in material research because its unique electrical properties make it a potential candidate to replace materials like copper (a conductor) and silicon (a semiconductor). However, many preparation steps have to be optimized. Furthermore, diverse factors which could potentially effect the functionality of the device have to be determined. Graphene is often used in biosensors which utilise biomolecules like DNA, aptamers or proteins. Those can specifically interact with the analyte which is further detected by the device. A grapheneba.sed field-effect transistor (GFET) was developed in our group which uses a protein as a sensing molecule. The protein is the odorant binding protein 14 from the honey bee (Apis mellifera). It binds specific odorants and can consequently be used for an artificial nose. The first part of the thesis analyses the fabrication of the FET step by step via surface sensitive methods. Beside static measurements, dynamic measurements were done for critical steps. This data was used for optimization of the setup. The graphene product of Freicht et al. showed the best result for optimal surface coverage. Hydroiodic acid showed the best reduction properties. Measurements, done via infrared spectroscopy, showed that OBP14 adsorbs stable via the PBSE linker on the surface. Further experiments were done via polarized attenuated total reflection (ATR) infrared spectroscopy to determine the orientation of certain vibrations and the surface concentration of the protein and linker. The protein concentration was determined to be 9.68 ± 3.03pM I cm2 or 157 ± 50ng I cm2 respectively. This is equivalent to a surface concentration of approximately 60%. The last part deals with the effect of pH and ligand binding on the sensor protein. Therefore, the protein was immobilized via His-tag on the surface of an ATR-crystal and the environment changed periodically. The decrease in pH showed a significant spectral change which is attributed to the weakening of H-bridges in the protein and rearrangement of helix 7, which causes opening of the binding pocket. Additionally, a theoretical mechanism for ligand uptake and release was proposed. The ligand modulation showed only a very weak signal. Due to the low intensities and poor signal-to-noise ratio it was not possible to obtain clear information on structural changes or the kinetics of ligand binding.