Aerosol Jet Printed Ion Selective Organic Electrochemical Transistors for Non-Invasive Detection of Neural Health

Monitoring nerve function is important to understand and treat many prevalent neural diseases. An effective and non-invasive way of doing so is by monitoring ion concentration in bio-fluids such as sweat. It is believed that there are normal and subnormal thresholds for ion concentrations in sweat that can be directly correlated with the health of the nervous system and can be used for early detection of diseases such as neuropathy. By monitoring sodium concentrations, which are the most abundant ion in sweat, correlations will be made for predictive analysis of diabetic neuropathy. Traditionally, potentiometric sensors are used for this application. However, it is often difficult to fabricate and miniaturize the necessary reference electrode and such sensors face issues of high output impedance. The proposed system does not encounter these challenges and uses organic electrochemical transistors (OECTs) with an ion selective membrane (ISM) functionalized channel for real-time measuring, up to 30 minutes at a time.<br/>Initial characterizations of the ISM were based on redox potentials determined by cyclic voltammetry (CV). This process involved recording voltammograms for interfering and primary analyte ions with a Na⁺ selective membrane to ensure selectivity for primary ions in a repeatable and reliable way. Gold electrodes were printed using an Optomec Aerosol Jet 5X printer were used for the CV testing. These devices were coated with a drop-casted layer of poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS), acting as the ion-to-electron transducer, and a layer of ISM solution. Given that the average redox peak for PEDOT:PSS was found to be 1.15 ± 0.03V, the measured values showed an obvious and repeatable selectivity for Na⁺ when compared to K⁺ of the same concentration. The average redox peak potential for 0.1M NaCl was found to be 1.24 ± 0.03V and for 0.1M KCl, it was found to be 1.16 ± 0.02V. This difference in redox peak potential allows for clear selectivity towards Na⁺ ions. An OECT will be used to increase the signal to noise ratio, thus reducing the need for signal amplification and thereby improving sensitivity. This device is fabricated on a flexible Kapton substrate, using aerosol jet printed gold and PEDOT:PSS as the electrode material and the device channel respectively. The ISM solution is drop-casted on the channel as well. Kapton is used to ensure that the form-factor is biocompatible both in terms of wearability and material composition. The proposed sensor is designed to not only distinguish between varying concentrations of Na⁺ but also different ions. So far, the devices have been tested against K⁺, Ca²⁺ and Mg²⁺ ions and has shown reliable selectivity such that minimal current responses are produced when these ions are introduced. The chosen concentrations for the Na⁺ selective device range from 0.005M to 0.5M which is adequate to measure ion concentrations in human sweat which are between 0.08M and 0.128M [1,2,3]. Detailed CV and OECT results from the ion sensor will be presented.<br/><br/><u>References</u><br/>[1] Baker, L.B. Sweating rate and sweat sodium concentration in athletes: A review of methodology and intra/interindividual variability. Sports Med. <b>2017</b>, 47, 111–128.<br/>[2] Pirovano, P.; Dorrian, M.; Shinde, A.; Donohoe, A.; Brady, A.J.; Moyna, N.M.; Wallace, G.; Diamond, D.; McCaul, M. A wearable sensor for the detection of sodium and potassium in human sweat during exercise. Talanta 2020, 219, 121145.<br/>[3] Yang, M.; Sun, N.; Lai, X.; Li, Y.; Zhao, X.;Wu, J.; Zhou,W. Screen-Printed Wearable Sweat Sensor for Cost-Effective Assessment of Human Hydration Status through Potassium and Sodium Ion Detection. Micromachines 2023, 14, 1497.