Transparent Graphene/PEDOT:PSS Microelectrodes for Electro‐ and Optophysiology

Abstract

Conventional opaque electrodes in microelectrode array (MEA) technology obstruct the view of cells in their immediate vicinity (e.g., ≈50 µm) from which the strongest extracellular action potentials are recorded. This limitation has been overcome by transparent graphene electrodes which allow for optical access essential for novel applications such as optogenetics and calcium imaging. Downscaling, necessary for high resolution single‐unit electrophysiological recordings, has been a significant challenge due to inferior electrochemical impedance and correspondingly lower signal‐to‐noise ratio. Here, the combination of graphene with the conductive polymer poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as a transparent microelectrode material for in vitro MEAs is presented and their application with optical imaging and electrophysiology is demonstrated. Optimal graphene/PEDOT:PSS microelectrodes display transparencies of 84% over the visible spectrum and impedance magnitude of (166 ± 13) kΩ at 1 kHz. The balance of transparency and 1 kHz impedance can be tuned from ≈90% and 700 kΩ to 50% and 42 kΩ.

Low impedance and highly transparent microelectrode arrays (MEA) for in vitro neuron electrical activity probing

“…A similar dependence of surface impedance to the thickness of the electrode material was already shown by Kshirsagar et al [3] who demonstrated that increasing the PEDOT:PSS layer thickness (electrodeposited by different time durations) led to a decrease of the impedance value. As a comparison, the impedance of a PEDOT:PSS layer electrodeposited during 0.2 sec is around 700 kΩ whilst those of PEDOT:PSS layers electrodeposited during 10 sec is around 50 kΩ-the thickness of the PEDOT:PSS layer is proportional to the deposition time [3].…”

“…As a comparison, a transparency of 84% was obtained for graphene/PEDOT:PSS-based MEAbut with higher surface impedance value of 117.3 ± 9.2 MΩ µm 2 at 1 kHz (30 μm diameter) [3] while an 80% transparency was obtained using ITO-based MEAs [26] with a surface impedance value of 1250 MΩ µm 2 (80 μm diameter) [9]. Table 1 and Fig.…”

“…Microelectrode arrays (MEA) are recognized as standard tools allowing recording in vitro biological activity of neuronal cells [1,2], cardiac cells [3,4] and from brain slices [5,6].…”

“…PEDOT:PSS has been already used to produce low impedance microelectrodes by coating such materials as gold [21,1,5,4,22] or platinum [17]-as well as to reduce the impedance of graphene microelectrodes [3]. Whereas in previous publications, PEDOT:PSS was used to improve the recording performance of the opaque as well as transparent microelectrodes, in this work, PEDOT:PSS will be used as the sole conductive element of the electrodes.…”

Advanced Electrical and Optical Microsystems for Biointerfacing

“…In addition to gold nanomesh, PEDOT:PSS coating can effectively improve the electrochemical performance of other transparent MEAs such as graphene. [ 261 ] Recently we demonstrated that covering the open spaces between interconnected metal grid transparent microelectrodes with ITO could significantly increase the effective ESA without changing GSA. [ 262 ] The resulting microelectrodes exhibit improved electrochemical impedance (5.4–18.4 Ω cm 2 ) than both metal grid and ITO references, high optical transparency (59–81%), and superior mechanical flexibility with no changes in electrical performance after 5000 bendings against a small radius at 5 mm.…”