Incorporating nanocarbons as neural electrode coatings can enhance their surface area. However, microfabricating the electrodes requires the use of harmful chemicals and extreme heating. In an article recently accepted by the journal iScience, researchers adopted a scalable, facile and safe reduction technique to obtain reduced graphene oxide (rGO) films by using Vitamin C (VC) for the reduction reaction.
Study: Vitamin C-reduced graphene oxide improves the performance and stability of multimodal neural microelectrodes. Image Credit: Africa Studio/Shutterstock.com
The VC-rGO coatings showed conductivity of approximately 44 siemens per centimeter. The rGO/gold (Au) microelectrodes showed an approximate eight-fold lower impedance and 400 times higher capacitance than pure Au, thus enhancing the injection capacity and charge storage. rGO/Au arrays allowed the voltammetric detection of dopamine (DA) in vitro and enabled high-resolution microscale recording in vivo.
Coating Materials for Neural Electrodes
The circuitry, underlying brain function, and disease depend on the ability to modulate and record neural activities with highly reliable electrodes. These electrodes are composed of doped inorganic materials with high conductivity, biocompatibility, electrochemical stability, and facile patterning ability into a different structure via standard lithographic techniques.
Moreover, simulating the cellular-level activity requires resolving the corresponding temporal and spatial scales by miniaturizing the electrode contacts. However, the inorganic material-based microscale electrodes suffer from high impedance and exhibit modest charge injection capacity, resulting in degradation of the signal-to-noise ratio (SNR) of recordings and restricting safe neuromodulation.
Nanoscale roughening and surface coating of the metallic or silicon (Si) electrodes are common strategies to overcome such limitations. These strategies improve the analytical detection by providing additional adsorption sites and high effective surface area. Moreover, these modifications lead to enhanced capacity for safe storage and charge delivery.
Titanium nitride (TiN), carbon nanotubes (CNTs), nanodiamonds, conductive polymers (CPs), and hybrid materials are a few common materials employed to enhance the electrode surface. However, these materials have limitations in their practical applicability.
Reduced graphene oxide (rGO) has predominant capacitive nature, ease of processing, electrochemical stability, lower impedance, tunability, and high charge delivery, which aid in its application as a neural electrode coating material.
VC-rGO Films in Multimodal Neural Microelectrodes
In the present work, the researchers presented a novel method to produce rGO coatings for application in neural microelectrode arrays. These coatings were safe and compatible with commonly used electrode materials, with facile integration into the conventional microelectrode process.
This method leveraged the advantage of VCs’ slow reaction kinetics to employ them as reducing agents at ambient temperature. The researchers demonstrated a facile construction method to obtain neural microelectrode arrays coated with rGO, wherein GO and VC was spray-casted onto bare Au micropatterned Si wafers.
The electrochemical and electronic properties of the coating were maximized by optimizing the heating time and VC concentration. Additionally, a one-step coating process in fabricating parylene-C encapsulated rGO/Au microelectrode arrays for cortical stimulation, microelectrocorticography (μECoG) recordings, and neurochemical sensing were also demonstrated.
The rGO/Au microelectrode’s electrochemical properties were characterized in vitro. The results indicated that rGO coatings showed better stability and enhanced the electrode surface area, resulting in reduced impedance and increased charge storage/charge injection capacity, compared to previously reported Au electrodes or other coating materials.
In addition to the enhanced surface area, the rGO coating also increased the number of adsorption sites, which enabled in vitro dopamine DA detection with low detection limit and high sensitivity.
The rGO/Au μECoG array's feasibility was demonstrated to monitor neural circuits in the microscale range and at high resolution by showing a high-density mapping of induced cortical responses in the somatosensory cortex of a rat from stimulating its whisker.
The present study proposed a novel strategy to enhance neural microelectrode’s stimulation, recording, and biochemical detection properties using rGO coatings. The rGO films leveraged the kinetics of VC and completed the reduction in a biocompatible, safe, highly scalable, and non-destructive manner.
The reduction of VC is optimized to attain compatibility with polymeric substrates in terms of their comfort and softness for their applications in implantable medical devices. Additionally, the processing of rGO and the designed film deposition method allowed their facile integration into the microfabrication process to produce neural microelectrodes.
The demonstrated VC-rGO coatings were adequately conductive, stable, and significantly enhanced the electrochemical properties compared to metallic electrodes. The rGO-coated electrode’s impedance under uninterrupted charge injection in combination with the VC-rGO DC’s conductivity in electrolytic and atmospheric environments indicated the VC-rGO film’s potential stability and versatility for long-term application.
Additionally, the excellent charge transfer and charge storage properties of rGO-coated electrodes demonstrated their ability to serve as promising candidates for in vivo stimulation studies and chronic recording.
In future work, the researchers anticipate completing concurrent in vivo recording and stimulation experiments by employing rGO/Au arrays for recording electrophysiology and stimulation of neural tissue with a direct passage of electric charge.
Brendan B. Murphy, Nicholas V. Apollo, Placid Unegbu, Tessa Posey, Nancy Rodriguez-Perez (2022). Vitamin C-Reduced Graphene Oxide Coatings Improve the Performance and Stability of Multimodal Microelectrodes for Neural Recording, Stimulation, and Dopamine Sensing. iScience. https://www.sciencedirect.com/science/article/pii/S2589004222009245