FULL PAPER 1800318 (1 of 7) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmattechnol.de Transparent Graphene/PEDOT:PSS Microelectrodes for Electro- and Optophysiology Pranoti Kshirsagar, Simon Dickreuter, Michael Mierzejewski, Claus J. Burkhardt, Thomas Chassé, Monika Fleischer, and Peter D. Jones* DOI: 10.1002/admt.201800318 activity, in both in vitro [1] and in vivo [2] research as well as clinical applications. [3] In recent years, the use of calcium- or voltage-sensitive imaging and the rise of optogenetics have advanced the sym- biosis of electro- and optophysiology. [4,5] The combination of these complemen- tary methods suffers from complica- tions such as light-induced artifacts in electrical recordings [4] or obscured fields of view due to opaque electrode arrays. Optical artifact-free transparent graphene electrocorticography (ECoG) electrodes of 100 × 100 μm 2 have been reported [6] but smaller dimensions are required for single-unit recording. Extracellular action potentials are recorded from cells in the immediate vicinity (≈50 μm) of the microelectrode (Figure 1) which are, however, shadowed by the opaque electrode. The development of transparent conductive materials suggests a solution for such chal- lenges, and materials including indium tin oxide (ITO) have been used for the electrically conductive paths connecting microelectrodes to their amplifiers. However, the micro- electrode material itself—which directly contacts cells and biological fluids (including proteins and enzymes)—must additionally be nontoxic, biostable, and exhibit low electro- chemical impedance. Here carbon nanomaterials can offer a solution. Carbon allotropes used for transparent micro- electrodes include boron-doped diamond [7] (2.1 Ω cm 2 , 50% transparent), carbon nanostructures [8] (<0.7 Ω cm 2 , 40% transparent), and graphene [9–11] (30–150 Ω cm 2 , >90% transparent). High transparency consistently comes at a cost to impedance. While the high temperatures (500–1000 °C) required to deposit various carbon allotropes limit process and material compatibility, graphene is routinely transferred to target substrates to circumvent such restrictions. However, its high specific impedance prevents miniaturization of gra- phene microelectrodes to dimensions (≈30 μm) for recording single-unit activity. Active devices such as electrolyte-gated graphene transis- tors amplify signals to avoid the problems of high imped- ance, but until now require additional opaque materials and more complex electronics. [12] Low recording noise of passive microelectrodes requires reducing the electrode–electro- lyte interfacial impedance. [13] Impedance may be decreased 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. P. Kshirsagar, M. Mierzejewski, Dr. C. J. Burkhardt, Dr. P. D. Jones NMI Natural and Medical Sciences Institute at the University of Tübingen Reutlingen 72770, Germany E-mail: peter.jones@nmi.de P. Kshirsagar, S. Dickreuter, Prof. T. Chassé, Prof. M. Fleischer Center for Light-Matter-Interaction, Sensors and Analytics (LISA + ) Eberhard Karls University Tübingen Tübingen 72076, Germany S. Dickreuter, Prof. M. Fleischer Institute for Applied Physics Eberhard Karls University Tübingen Tübingen 72076, Germany Prof. T. Chassé Institute of Physical and Theoretical Chemistry Eberhard Karls University Tübingen Tübingen 72076, Germany The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admt.201800318. Microelectrode Array Technology 1. Introduction Electrophysiology has classically been supported by micros- copy to target specific biological structures and identify puta- tive cell types. Microelectrode arrays (MEAs) have become standard tools for recording and stimulation of bioelectrical Adv. Mater. Technol. 2018, 1800318