Next Generation of Selective and Biocompatible Neural Interface—Cleanroom-Free Carbonized Parylene C Electrodes for Neurotransmitter Detection

Laser engraving is progressively becoming popular for creating porous graphitic carbon structures known as Laser-Induced Graphene (LIG), used in various sensing applications. LIGs are produced by directing a laser beam onto carbon-rich polymers, causing localized heat reactions that transform the $SP_{3}$ carbon atom hybridization into a three-dimensional porous graphitic structure. This method is maskless, scalable, reproducible, cost-effective, and rapid, producing graphite layers with high electrical conductivity and excellent electrocatalytic properties. These features make LIGs superior to graphene created by traditional methods such as chemical vapor deposition (CVD) and wet chemistry.<br/><br/>Traditionally, perylene has been the primary material for developing neural interfaces and probes, and it has been validated in the bioelectronic community. We chose perylene C due to its pinhole-free surface, high flexibility, mechanical strength, and biocompatibility, making it ideal for constructing implantable neural interfaces. However, fabricating electrodes from perylene can be time-consuming and typically requires cleanroom facilities. In this work, we use laser power to carbonize the perylene C substrate, simplifying the complex process of neural electrode microfabrication.<br/><br/>We selected acetylcholine and dopamine as representative neurotransmitters essential to brain activity and requiring distinct detection mechanisms to highlight the versatility of our platform. Our proof-of-concept platform comprises two modules: (1) an acetylcholine (ACh) potentiometric sensor and (2) a voltammetric dopamine sensor. The ACh sensor includes a working electrode with an ACh-specific sensing membrane and a reference electrode with a reference membrane. Similarly, the dopamine sensor features carbonized parylene C working and counter electrodes and an Ag/AgCl reference electrode.<br/><br/>The parylene C substrate is carbonized through laser ablation, producing electrode arrays with individual working areas that have a 500–600 µm radius. This one-step fabrication process eliminates the complexities of the multilayer photolithography approach. In the acetylcholine (ACh) sensing module, the electrical potential (emf) between the ACh sensor and the reference electrode corresponds to the activity of ACh in the biofluid, following the Nernst Equation: emf=E⁰+(RT/zF) log (a_{&lt;span style="font-size:10.8333px"&gt;ACh&lt;/span&gt;}). A theoretical slope of 61.5 mV/decade is expected at body temperature, known as the Nernstian slope. The ACh-sensing membrane will be applied to the working electrode area. Due to the electroactive nature of dopamine, we utilized square wave voltammetry (SWV) techniques to measure physiologically relevant concentrations for dopamine detection.<br/><br/>The ACh sensing module showed a Nernstian slope of 54.9 ± 0.8 mV/decade and a 42 nM detection limit. Additionally, our sensor successfully differentiated dopamine from other common interfering substances in the brain, including serotonin, ascorbic acid, and uric acid, within physiologically relevant concentrations.<br/><br/>Advances in neural interface technology require innovative materials and fabrication techniques to improve sensing performance and in-vivo applicability. In this work, we present a transformative approach to neural probe fabrication and neurotransmitter sensing, which promises enhanced performance in neural interfaces. Future research will focus on optimizing sensor integration to in-vivo applications and expanding the range of detectable neurotransmitters.