Carbon-based transparent microelectrodes for optical investigation and electrophysiology

Abstract

The goal of this work was to develop carbon based transparent electrodes for advancement of microelectrode array (MEA) technology by allowing the possibility of combining optical methods with classical electrophysiology. Recent years have seen a surge of interest in novel methods such as optogenetics and calcium imaging with the focus on understanding the complex neuronal networks. The conventional microelectrode materials obstruct the optical access, which is from the substrate side with an inverted microscope, and this limitation is overcome by using carbon materials. This work was focused on three main materials - carbon nanostructures, graphene and graphene/PEDOT:PSS (poly(3,4- ethylenedioxythiophene)). The transparency often comes at the cost of high electrochemical impedance. This challenge was tackled by using a novel combination of chemical vapour deposited (CVD) graphene and PEDOT:PSS.

Carbon nanostructures were grown at 550 °C by CVD with acetylene as the carbon source. The morphology was studied by scanning electron microscope (SEM) and the presence of nanostructures mixed with amorphous carbon confirmed by Raman spectroscopy. The semitransparent nature was revealed by UV-Vis measurements. The electrochemical impedance was in the acceptable range for electrophysiological recordings. The functionality of the carbon nanostructure microelectrodes was confirmed by recording electrogenic signals from cardiomyocytes where the optical inspection of the cells through the semitransparent microelectrodes was possible. The mechanical robustness and biocompatibility was revealed by studying the electrode-cell ultrastructure.

Graphene was grown by CVD with methane as the carbon source and integrated in the MEA fabrication process. The largely single layer graphene was investigated with SEM and Raman spectroscopy. The excellent transparency over the entire microelectrode was revealed by optical transmittance measurements. The graphene microelectrodes displayed high electrochemical impedance which led to high noise during electrophysiology. The functionality of the transparent graphene mircoelectrodes was checked with cardiomyocytes where high amplitude signals were detected similar to recording with standard electrodes, however, the smaller amplitude signals went unrecorded owing to the high noise.

Graphene/PEDOT:PSS microelectrodes were fabricated by electrodeposition of the conducting polymer PEDOT:PSS on graphene microelectrodes. Optical microscopy revealed that PEDOT:PSS followed the graphene surface and the continuous coverage of the latter by the former reduced to sparse coverage with decreasing amount of PEDOT:PSS. Raman spectroscopy, especially in the case of lower PEDOT:PSS amounts, revealed the presence of PEDOT:PSS on regions which appeared transparent optically. This information was crucial in understanding the electrodeposition mechanism. The electrochemical impedance was found to be comparable with the commercially available TiN microelectrodes and the applicability was tested with cardiomyocytes. Optical imaging was possible through the transparent microelectrodes. An optimum balance between the optical transparency and electrochemical impedance was obtained which allows flexibility in producing microelectrodes for a wide range of applications.

This work presents a comprehensive view on carbon based transparent microelectrodes for novel applications employing combinations of electro- and opto-physiology. The electrodes fabricated in this work are expected to go a long way in assistance with decoding the complex biological systems and provide insights on the single cell level.