Ultralow Impedance Graphene Microelectrodes with High Optical Transparency for Simultaneous Deep Two‐Photon Imaging in Transgenic Mice

Abstract

The last decades have witnessed substantial progress in optical technologies revolutionizing our ability to record and manipulate neural activity in genetically modified animal models. Meanwhile, human studies mostly rely on electrophysiological recordings of cortical potentials, which cannot be inferred from optical recordings, leading to a gap between our understanding of dynamics of microscale populations and brain-scale neural activity. By enabling concurrent integration of electrical and optical modalities, transparent graphene microelectrodes can close this gap. However, the high impedance of graphene constitutes a big challenge toward the widespread use of this technology. Here, it is experimentally demonstrated that this high impedance of graphene microelectrodes is fundamentally limited by quantum capacitance. This quantum capacitance limit is overcome by creating a parallel conduction path using platinum nanoparticles. A 100 times reduction in graphene electrode impedance is achieved, while maintaining the high optical transparency crucial for deep two-photon microscopy. Using a transgenic mouse model, simultaneous electrical recording of cortical activity with high fidelity is demonstrated while imaging calcium signals at various cortical depths right beneath the transparent microelectrodes. Multimodal analysis of Ca 2+ spikes and cortical surface potentials offers unique opportunities to bridge our understanding of cellular dynamics and brain-scale neural activity.

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

“…A good balance was found with 1 s PEDOT:PSS deposition, which achieved (166 ± 13) kΩ at 1 kHz while maintaining (84 ± 4)% transmittance. This compares favourably to graphene/Pt electrodes which were only ≈50% transparent with 20% higher specific impedance at 1 kHz. Recording of beating chicken cardiomyocytes and fluorescence imaging of transgenic mouse retina ganglion cells showed the excellent functionality of the transparent graphene/PEDOT:PSS MEA.…”

“…(Impedances lower than the empirical expectation can result from roughness and defects in graphene and shunt capacitances). Recently, reduction of impedance to 1.4 Ω cm 2 at 1 kHz was achieved by electrodeposition of Pt, but also reduced transparency to ≈50% …”

Multifunctional Flexible Biointerfaces for Simultaneous Colocalized Optophysiology and Electrophysiology

“…Figure 3d presents a quantitative performance comparison of the Au nanogrid electrodes with other state‐of‐the‐art transparent electrodes reported in the literature for electrophysiology, including flexible Au‐graphene mesh, [ 42 ] Au/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) mesh, [ 26 ] undoped graphene, [ 24 ] nitrogen‐doped graphene (N‐graphene), [ 43 ] platinum nanoparticle‐deposited graphene (Pt‐graphene), [ 44 ] CNT, [ 25 ] and rigid ITO. [ 22 ] The electrode impedance is normalized to electrode size to eliminate the influence of electrode dimension on impedance.…”

Advanced Electrical and Optical Microsystems for Biointerfacing

“…In addition to chemical doping using nitric acid, Lu et al reported that the impedance of graphene microelectrodes could be reduced by 100 times by electrodepositing platinum nanoparticles on CVD graphene. [ 258 ] However, the improved electrochemical performance comes at a cost of reduced transparency as platinum nanoparticles are opaque. In addition to calcium imaging, optogenetic modulation is integrated with graphene MEAs.…”