Ions dissolved in aqueous media play a fundamental role in plants, animals, and humans. Therefore, the in situ quantification of the ion concentration in aqueous media is gathering relevant interest in several fields including biomedical diagnostics, environmental monitoring, healthcare products, water and food test and control, agriculture industry and security. The fundamental limitation of the state-of-art transistor-based approaches is the intrinsic trade-off between sensitivity, ion concentration range and operating voltage. Here we show a current-driven configuration based on organic electrochemical transistors that overcomes this fundamental limit. The measured ion sensitivity exceeds by one order of magnitude the Nernst limit at an operating voltage of few hundred millivolts. The ion sensitivity normalized to the supply voltage is larger than 1200 mV V−1 dec−1, which is the largest value ever reported for ion-sensitive transistors. The proposed approach is general and can be extended to any transistor technology, thus opening opportunities for high-performance bioelectronics.

The conductivity of poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) can be strongly enhanced by treatment with high boiling solvents as dimethyl sulfoxide (DMSO). The effect of various DMSO solvent treatment methods on the performance of organic electrochemical transistors (OECTs) based on PEDOT:PSS is studied. The treatments include mixing PEDOT:PSS with DMSO before film deposition, exposing a deposited PEDOT:PSS film to a saturated DMSO vapor, and dipping a PEDOT:PSS film in a DMSO bath. Compared to dry PEDOT:PSS, operating in the OECT configuration causes a significant reduction of its conductivity for all treatments, due to the swelling of PEDOT:PSS by the direct contact of the conductive channel with the electrolyte. The dipping method gives rise to the highest OECT performance, reflected in the highest on/off ratio and transconductance. The improved conductivity and device performance after dipping arise from an enhanced charge carrier mobility due to enhanced structural order.

The integrity of CaCo‐2 cell barriers is investigated by organic electrochemical transistors (OECTs) in a current‐driven configuration. Ion transport through cellular barriers via the paracellular pathway is modulated by tight junctions between adjacent cells. Rupturing its integrity by H2O2 is monitored by the change of the output voltage in the transfer characteristics. It is demonstrated that by operating the OECT in a current‐driven configuration, the sensitive and temporal resolution for monitoring the cell barrier integrity is strongly enhanced as compared to the OECT transient response measurement. As a result, current‐driven OECTs are useful tools to assess dynamic and critical changes in tight junctions, relevant for clinical applications as drug targeting and screening.

Electrodes coated with poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) have been employed to measure the integrity of cellular barriers. However, a systematic experimental study of the correlation between tissue integrity and impedance of the sensing device has not yet been conducted. Using impedance spectroscopy, how the impedance ratio of the biological tissue to the recording device affects the recording ability of the latter is investigated. PEDOT:PSS‐coated electrodes of various dimensions are employed and the effect of their size to their sensing efficiency is examined. The biotic/abiotic ensemble is modeled with a simple equivalent circuit and an analytical expression of the total impedance as a function of frequency is extracted. The results reveal a critical impedance ratio of the biological tissue to the sensor which allows for efficient sensing of the tissue integrity. This work provides the ground rules for improved impedance‐based biosensors with optimized sensitivity.