Selective and Biocompatible Neural Probes with a Fluorous Sensing Phase

Laser engraving is becoming a widely popular method for producing porous graphitic carbon structures, known as Laser-Induced Graphene (LIG), for sensing applications. LIGs are produced by ablating carbon-rich polymers(mostly polyimide) using a laser beam, which creates local photothermal reactions converting SP₃ carbon atom hybridization into 3D porous graphitic structure. This method is a maskless, scalable, reproducible, cost-effective, and fast approach to producing graphene layers with high electrical conductivity and electrocatalytic nature, which makes LIGs superior over graphene created using conventional methods such as chemical vapor deposition (CVD) and wet chemistry. <br/><br/>Here, we developed the first use of a fluorous-phase potentiometric sensor for the measurement of acetylcholine (ACh), a vital neurotransmitter in brain activity. Fluorous compounds (molecules with high content of fluorine atoms) are extremely non-polar and non-polarizable to the extent that they are not miscible with both aliphatic compounds such as hexanes, and with water and other hydrophilic compounds. Therefore, fluorinated compounds are both hydrophobic and lipophobic. Since, the living systems are made of water and lipophilic compounds, makes fluorocarbons bio-orthogonal and nontoxic, meaning that they do not interfere with biology. Biocompatibility of fluorocarbons has resulted in many biomedical applications such as artificial blood, bio-orthogonal imaging contrast agents, and delivery and imaging vehicles. Fluorocarbons are used as coatings in biomedical devices to lower sensor biofouling and cell adhesion to the device. Exceptional selectivities that far outperform a conventional lipophilic sensing membrane were also reported for ionophore-doped fluorous sensing membranes. Such gain in selectivity is attributed to non-coordinating and poorly solvating properties of the fluorous-phase. The high selectivity, biocompatibility, and potentially exceptional resilience to biofouling make fluorous-phase sensors ideal for neural applications. Neurotransmitters present in nanomolar concentrations are a complex matrix (requiring high selectivity). Moreover, studying the dynamics of change and the correlation of such changes to behavior and stimulation is desired (requiring long-term in-vivo studies through implantable sensors).<br/><br/>We showed that selectivity gain in the fluorous phase resulted in more than an order of magnitude improvement in the limit of detection (LOD). Moreover, to move towards in-vivo application of the fluorous-phase ACh sensor (where probe size and flexibility are critical), we have developed the first compact and flexible solid-contact fluorous-phase potentiometric sensor. We fabricated the electrodes using laser-induced carbonization of flexible polyimide films.<br/><br/>Utilizing the laser engraving to fabricate neuroprostheses could enbale more efficient fabrication approach toward neural probe by eliminating the intricate and expensive microfabrication approach. We are utilizing polyimide due to providing a pinhole-free, high flexibility, high mechanical strength, and biocompatible platform to construct implantable ACh-detecting sensors consisting of a working electrode (containing an acetylcholine organic sensing membrane) and a reference electrode (containing a reference membrane). The electrical potential (emf or electromotive force) between the ACh sensor and reference electrode correlates to the activity of ACh in the biofluid, according to the Nernst Equation (E=E°+(RT/nF) Log (a_ACh)), where E° shows the standard potential, R the universal gas constant, T temperature, F the Faraday constant, n the charge of the ion, and a_ACh the activity of ACh. A theoretical slope of 59.2 mV/decade is expected at room temperature and is defined as the Nernstian slope. The fluorous-phase solid-contact LIG ion exchanger electrode showed a near Nernstian slope of 54.9 ± 0.8 and a 42 nM limit of detection.