Pyrolytic carbon microelectrodes for impedance based cell sensing

Conductive carbon structures can be obtained from a polymer template through a pyrolysis process. These structures can be used as electrodes for bio sensing applications such as electrochemical cell morphology evaluation. This study focuses on the optimization of 2D pyrolysed carbon microelectrodes with the carbon MEMS (C-MEMS) process (1), (2). SU-8 was used as photoresist to define the polymer template on a Si-based carrier substrate, and then pyrolysed. Different electrochemical microchips with carbon working (WE) and counter electrode (CE) were fabricated. The particular focus of this work was on the optimization of the pyrolysis process to decrease the resistivity of the resulting carbon material and improve the performance in cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). A gold pseudo-reference electrode (RE) and gold contact pads were deposited by e-beam evaporation through a shadow mask, and a 5µm thick film of SU-8 was used as passivation layer. To investigate the influence of the different pyrolysis parameters on the obtained carbon, a one-step process was used with a maximum temperature of 900°C and a heating rate of 10°C/min. Then, the samples were kept at the maximum temperature under N 2 for 10 minutes, 1 hour, 2 hours and 10 hours respectively. Focusing on the influence of the atmosphere during pyrolysis, experiments with different gases were performed, using a one-step process with a dwell time of 1 hour. In particular, the experiments were carried out under H 2 and vacuum. The resistivity measurements, performed using a custom made 4-point measurement setup, show that the two hours process in N 2 atmosphere gives the lowest resistivity value of 9,32x10 -3 Ω·cm (Fig.1a). For CV, the ferri-ferrocyanide [Fe(CN) 6 ] 4− / [Fe(CN) 6 ] 3− redox couple was used, after treatment of the electrodes with oxygen plasma. Also in this case the pyrolysis process of two hours under N 2 produced electrodes that displayed the best electrochemical behaviour, with a peak current of 2,88 x10 -4 A and a ΔE p of 240 mV (Fig.1b). To provide an increased sensitivity for impedance based cell sensing, the design of the electrode chip has been modified. The WE is covered by the passivation layer except for a defined number of small circles with a diameter of 250 µm (Fig.1c). The WEs locally increase the current density, and the changes occurring at the multiple WEs dominate the measurement, while the contribution of the CE becomes negligib