Ions are essential biological regulators, influencing vital processes in all living organisms. Precise and real-time monitoring of ion concentrations and their variations from equilibrium is crucial for understanding health states in various systems, from plant physiology to human health. Abnormal ion levels are often indicative of diseases or organ dysfunction. For example, small deviations (less than 20%) in potassium levels in human serum can have significant clinical consequences. Current ion-sensing technologies, while diverse (including silicon, zinc-oxide, graphene, and OECT-based sensors), often struggle to provide the required multiscale functionality, i.e., detection over wide concentration ranges and simultaneous monitoring of minute fluctuations. Organic electrochemical transistors (OECTs) are promising due to their sensitivity, low voltage operation, biocompatibility, and ease of integration, but their sensitivity is generally limited by the Nernst limit. This paper aims to overcome this limitation and provide a multiscale ion detection method with enhanced performance.

Existing ion detection methods utilize various transistor technologies, such as silicon, zinc-oxide, and graphene ion-sensitive field-effect transistors (ISFETs), porous silicon extended-gate FETs, amorphous indium-gallium-zinc-oxide dual-gate thin-film transistors, organic electrolyte-gated FETs, and OECTs. OECTs are particularly attractive due to their performance in aqueous environments and biocompatibility. However, most OECT-based ion sensors are limited by the Nernst limit (59 mV dec⁻¹), requiring additional amplification circuitry. Recent work has explored improving OECT sensitivity through current-driven architectures, but these methods are not optimal for real-time sensing. The current challenge remains the development of a high-sensitivity approach capable of both wide-range detection and precise tracking of small concentration changes, addressing the need for multiscale ion sensing.

This study utilizes a complementary OECT amplifier configuration, integrating both p-type (PEDOT:PSS) and n-type (BBL) OECTs in a push-pull architecture. This configuration acts as a high-gain amplifier, integrating both ion-to-electron transduction and signal amplification within the same device. The fabrication process involved sputtering gold electrodes onto a glass substrate, depositing parylene-C layers for insulation, and spin-coating PEDOT:PSS and BBL to form the transistor channels. For ion selectivity, ion-selective membranes (ISMs) were incorporated between the gate and channel. Electrical characterization involved measuring the output voltage (V₀) as a function of the input voltage (Vᵢ) and ion concentration (c) using a Keithley 2636A. Real-time measurements were conducted by monitoring V₀ while varying the analyte concentration. The sensitivity was determined by analyzing the shifts in the transfer characteristics and the change in output voltage with respect to the concentration change. The selectivity was assessed by adding other ions to the analyte and observing the output response.

The complementary OECT amplifier exhibits a high sensitivity (up to 1172 mV dec⁻¹ at 0.5 V supply voltage), resulting in a normalized sensitivity exceeding 2300 mV V⁻¹ dec⁻¹, significantly higher than previously reported values and the theoretical limit. This allows for detection over a wide concentration range (10⁻⁵ M – 1 M). The amplifier effectively monitors small concentration variations (two orders of magnitude lower than the detected concentration) with a fast response time (11 s for a 7 × 10⁻⁵ M change). The sensitivity is tunable by adjusting the supply voltage (V_{DD}), enabling adaptation to different applications. Using ion-selective membranes, the system demonstrates selective potassium (K⁺) detection with minimal cross-sensitivity to sodium (Na⁺) and calcium (Ca²⁺). The amplifier's effectiveness was demonstrated in real-time monitoring of potassium in human blood serum, accurately detecting variations smaller than 20% from baseline, which are clinically relevant in detecting hypo- and hyperkalemia.

The results demonstrate a significant advancement in ion-sensing technology. The high sensitivity and wide dynamic range of the complementary OECT amplifier overcome limitations of previous approaches. The ability to tune the sensitivity by adjusting V_{DD} provides flexibility in adapting the sensor to diverse applications. The demonstration of selective K⁺ sensing in blood serum highlights the potential of this technology for biomedical applications, including the real-time monitoring of physiologically relevant ions in complex biological fluids. The low power consumption and ease of fabrication also make it a suitable candidate for future development of portable and wearable sensors.

This work introduces a novel complementary OECT amplifier for multiscale, high-sensitivity ion detection. The device achieves unprecedented sensitivity, enabling real-time monitoring of small ion concentration variations in a wide range. The tunable sensitivity and demonstrated selectivity make this approach highly adaptable and promising for diverse applications, including biomedical sensing. Future work could explore the integration of this technology with other biorecognition elements for broader applications, such as immunosensing or single-molecule detection.

While the study demonstrates excellent performance, there are some limitations. The long-term stability and robustness of the device under continuous operation in physiological fluids require further investigation. The current study focuses on K⁺ detection; more extensive evaluations with a wider range of ions are necessary. Furthermore, the effect of potential fouling from biological samples on the sensor's performance needs to be addressed.