TETRAHEDRAL AMORPHOUS CARBON – GRAPHENE HYBRID ELECTRODE FOR DETECTION OF DOPAMINE Master’s thesis for the degree of Master of Science in Technology

The real time in vivo detection of dopamine and other neurotransmitters in awake behaving animals is a long-standing goal. Carbon nanomaterials have emerged as promising candidates for electrochemical detection of dopamine. Diamond-like carbon films have wide water window, low capacitive background current and high chemical stability. By combining ultrathin tetrahedral amorphous carbon (ta-C) with Ti under layer the electron transfer properties can be enhanced without deterioration in the desired properties of ta-C. Such a bilayer thin films can also be modified with other carbon allotropes, such as graphene. To the best knowledge of the author this reports for the first time a ta-C electrode modified with reduced graphene oxide (rGO) for electrochemical detection of dopamine. Ti/ta-C bilayer electrodes with varying taC top layer thickness were fabricated and optimized in terms of electron transfer properties. Both types of electrodes were subjected to cyclic voltammetry experiments, Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy. The ta-C thickness was found to affect the electron transfer kinetics, while the dopamine detection limit of 5 μM remained unchanged. The electron transfer properties were found to improve with decreasing ta-C thickness and best performance was observed with 7 nm ta-C thickness. At a thickness below 7 nm the electron transfer properties start deteriorating due to excessive oxidation of the Ti/ta-C interface. The ta-C electrode showed poor selectivity towards dopamine. An order of magnitude improvement in sensitivity and a significant increase in selectivity towards dopamine was achieved. The rGO modified electrodes were able to detect 500 nM DA without any data treatment. Modification with rGO also resulted in significant improvement in electron transfer kinetics of dopamine. The amount of added rGO and the stacking of the graphene sheets were found to affect electron transfer in both inner and outer sphere systems. Finally oxidative treatments of the rGO resulted in increased current response and selectivity towards dopamine of all rGO electrodes, highlighting the role of oxygen containing functional groups in the electro-oxidation of both L-ascorbic acid and dopamine. By combining the stability, good electron transfer properties, and the reasonably low capacitive currents of the Ti/ta-C bilayer electrode with the more electrochemically active rGO, an electrode with low detection limit and improved selectivity towards dopamine was achieved. This electrode also exhibited wide enough water window and sufficiently low capacitive background currents for electrochemical detection of dopamine.