Configurable Organic Electrochemical Transistors with Printed Functional Materials for Multiplexed Sweat Analysis

Monitoring human sweat with wearable biochemical sensors could give insights into the hydration, fatigue, and health status, tracking the variations of biomarkers non-invasively and in real-time. In this field, organic electrochemical transistors (OECTs) are gaining significant importance thanks to their high signal amplification, simple architecture, and mechanical flexibility of the organic layer. Despite these properties, their use to analyze complex biological fluids, such as sweat, is restricted due to a lack of cross-sensitivity, stability, and sensing mechanism studies; their integration level into wearables is also limited. This thesis investigates OECT sensors with different gate materials and bio-recognition layers at the electrode interfaces to achieve multiplexed sensing. Some insights into their sensing mechanisms are derived by combining electrical modeling with electrochemical impedance spectroscopy (EIS). Devices with different architectures can be combined into multi-OECT platforms to detect multiple analytes: ions, pH, metabolites, and antigens. By using additive manufacturing techniques, integrating the sensors array into microfluidics is facilitated, leading to digitally-configurable wearable platforms. The first study explores OECTs for real-time multi-ion sensing. The OECT array is made by inkjet printing poly(3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) channels and silver nanoparticles gates and contacts. To improve their specificity, different ion-selective membranes for K+, Na+, and H+ sensing are cast onto the PEDOT:PSS channels. The OECTs integrated into a flexible fluidic channel show successful discrimination of different ions in real-time at the sweat concentration ranges. In the second study, hybrid PEDOT:PSS and polyaniline (PANI) active channels were developed and investigated to optimize the pH sensing performances compared to OECTs with H+-selective membranes. With the active layer made of a combination of the inkjet-printed PEDOT:PSS and the electro-polymerized PANI, high pH sensitivity and linearity are achieved in the 4 to 10 pH range. EIS analysis relates the sensor response to a substantial increase of capacitance on the PANI/PEDOT:PSS interface at acidic pH.In the third study, inkjet-printed OECT devices for enzymatic sensing of glucose and lactate, exploiting a full-graphene gate electrode, are investigated. The OECT sensors with graphene gates, in the presence of ferrocene, show higher sensitivity, linearity, and repeatability than OECTs made of standard inkjet-printed silver nanoparticle-based gate electrodes and previously reported printed devices. EIS reveals a decrease in charge-transfer resistance at the graphene gate interface during the enzymatic detection. The fourth study shows OECTs with antibodies immobilized on gold gates to detect low cortisol concentrations. High signal amplification of the binding events at the gate interface was achieved by optimizing the PEDOT:PSS channel design, measuring cortisol in the nM-range in PBS and real sweat samples. Experimentally, the sensor response was attributed to an increase in capacitance of the gate interface with the binding, which analytical electrical models confirmed.The outcome of these studies enables the development of novel wearable and configurable organic electrochemical transistor arrays for multiplexed sensing. The integrated multi-OECT platform is finally demonstrated to analyze ions in sweat.