Mauro Murgia

Organic Electronic platforms for biosensing are being demonstrated at a fast pace, especially in healthcare applications where the use of organic (semi‐)conductive materials leads to devices that efficiently interface living matter. Nevertheless, interesting properties of organic (semi‐)conductors are usually neglected in the development of (bio‐)sensors. Among these, the non‐linear response when operated under dynamic biasing conditions (i.e., with pulsed driving voltages), thus mimicking synaptic plasticity phenomena, offers promising and largely unexplored possibilities for bio‐sensing. The artificial synaptic response's figures of merit reflect the composition of the surrounding environment and, ultimately, the ion concentration and dynamics at the organic (semi‐)conductor/electrolyte interface. Therefore, new sensing strategies that rely on the effect of target analytes on the short‐term plasticity response of Organic Neuromorphic Devices are being demonstrated. This work presents the development of a label‐free Single Electrode Neuromorphic Device (SEND) specifically designed for in vivo real‐time mapping of dopamine concentration. The device response is investigated as a function of the driving frequency, resulting in the determination of the optimal operational configuration for minimally invasive neuromorphic devices. It exhibits stable multi‐parametric response in complex fluids, in brain's mechanical models and in vivo, enabling monitoring of local variations of dopamine concentration in the rat brain.

Successful translation of organic transistors as sensors and transducers to clinical settings is hampered by safety and stability issues. The operation of such devices demands driving voltages across the biotic/abiotic interface, which may result in undesired electrochemical reactions that may harm both the patient and the device. In this study, a novel operational mode is presented for electrolyte‐gated organic transistors that avoid these drawbacks: the common‐drain/grounded‐source configuration. This approach reverts the standard common‐source/common‐ground configuration and achieves maximum signal amplification while applying null net bias across the electrolyte, with no parasitic currents. The viability of the proposed configuration is demonstrated by recording in vivo the somatosensory evoked activity from the barrel cortex of rats. The main inherent advantage of transistors with respect to passive electrodes is preserved in the proposed scheme: a superior signal‐to‐noise ratio is achieved which enables the detection of evoked activity at the single‐trial level. Then, common‐drain/grounded‐source organic transistors are proposed as ideal candidate devices for a harmless translational recording platform.

The photochemical core of every photosynthetic apparatus is the reaction center, a transmembrane enzyme that converts photons into charge‐separated states across the biological membrane with an almost unitary quantum yield. A light‐responsive organic transistor architecture, which converts light into electrical current by exploiting the efficiency of this biological machinery, is presented. Proper surface tailoring enables the integration of the bacterial reaction center as photoactive element in organic transistors, allowing the transduction of its photogenerated voltage into photomodulation of the output current up to two orders of magnitude. This device architecture, termed light‐responsive electrolyte‐gated organic transistor, is the prototype of a new generation of low‐power hybrid bio‐optoelectronic organic devices.

Microelectrode arrays are used for recording and stimulation in neurosciences both in vitro and in vivo. The electrodeposition of conductive polymers, such as poly(3,4‐ethylene dioxythiophene) (PEDOT), is widely adopted to improve both the in vivo recording and the charge injection limit of metallic microelectrodes. The workhorse of conductive polymers in the neurosciences is PEDOT:PSS, where PSS represents polystyrene‐sulfonate. In this paper, the counterion is the fluorinated polymer Nafion, so the composite PEDOT:Nafion is deposited onto a flexible neural microelectrode array. PEDOT:Nafion coated electrodes exhibit comparable in vivo recording capability to the reference PEDOT:PSS, providing a large signal‐to‐noise ratio in a murine animal model. Importantly, PEDOT:Nafion exhibits a minimized polarization during electrical stimulation, thereby resulting in an improved charge injection limit equal to 4.4 mC cm⁻², almost 80% larger than the 2.5 mC cm⁻² that is observed for PEDOT:PSS.

Monitoring of bioelectric signals in peripheral sympathetic nerves of small animal models is crucial to gain understanding of how the autonomic nervous system controls specific body functions related to disease states. Advances in minimally-invasive electrodes for such recordings in chronic conditions rely on electrode materials that show low-impedance ionic/electronic interfaces and elastic mechanical properties compliant with the soft and fragile nerve strands. Here we report a highly stretchable low-impedance electrode realized by microcracked gold films as metallic conductors covered with stretchable conducting polymer composite to facilitate ion-to-electron exchange. The conducting polymer composite based on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) obtains its adhesive, low-impedance properties by controlling thickness, plasticizer content and deposition conditions. Atomic Force Microscopy measurements under strain show that the optimized conducting polymer coating is compliant with the micro-crack mechanics of the underlying Au-layer, necessary to absorb the tensile deformation when the electrodes are stretched. We demonstrate functionality of the stretchable electrodes by performing high quality recordings of renal sympathetic nerve activity under chronic conditions in rats.

Electrolyte-gated organic field-effect transistors (EGOFETs), based on ultrathin pentacene films on quartz, were operated with electrolyte solutions whose pH was systematically changed. Transistor parameters exhibit nonmonotonic variation versus pH, which cannot be accounted for by capacitive coupling through the Debye–Helmholtz layer. The data were fitted with an analytical model of the accumulated charge in the EGOFET, where Langmuir adsorption was introduced to describe the pH-dependent charge buildup at the quartz surface. The model provides an excellent fit to the threshold voltage and transfer characteristics as a function of the pH, which demonstrates that quartz acts as a second gate controlled by pH and is mostly effective from neutral to alkaline pH. The effective capacitance of the device is always greater than the capacitance of the electrolyte, thus highlighting the role of the substrate as an important active element for amplification of the transistor response.

Water-gated organic transistors have been successfully exploited as potentiometric transducers in a variety of sensing applications. The device response does not depend exclusively on the intrinsic properties of the active materials, as the substrate and the device interfaces play a central role. It is therefore important to fine-tune the choice of materials and layout in order to optimize the final device performance. Here, polyethylene naphthalate (PEN) has been chosen as the reference substrate to fabricate and test flexible transistors as bioelectronic transducers in liquid. PEN is a biocompatible substrate that fulfills the requirements for both bio-applications and micro-fabrication technology. Three different semiconducting or conducting polymer thin films employing pentacene, poly(3-hexylthiophene) or poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) were compared in terms of transconductance, potentiometric sensitivity and response time. The different results allow us to identify material properties crucial for the optimization of organic transistor-based transducers operating in water.