Unveiling a Multiplexed Sensing System for Advanced Biosensing Applications with Thread-Based Electrochemical Sensors
Authors: Sharma Atul , Asci Cihan , Sonkusale Sameer 
Published: August 2024

The miniaturization and portability of electrochemical sensing devices play a crucial role in a wide range of applications, including medical diagnostics, environmental monitoring, and point-of-care testing. Integrating a pseudo-reference electrode addresses challenges linked to traditional counterparts, ensuring enhanced portability and miniaturization without performance compromise. Our research delves into the significance of this innovation, emphasizing its impact on signal stability, reproducibility, and overall sensor reliability. The tailored pseudo-reference for compact electrochemical devices contributes to a highly efficient, cost-effective, and user-friendly design, ideal for point-of-care diagnostics and field applications. This work showcases the design, implementation, and performance evaluation of a portable electrochemical sensing system with a pseudo-reference-integrated readout circuit, improving reliability under diverse operating conditions. In the proposed research, we present the design and implementation of a dual-channel configurable multiplexed sensing platform utilizing thread-based electrochemical sensors for continuous potentiometric and amperometric measurements.

For the pH sensor, the fabrication process initiates with the cleaning (using isopropyl alcohol) and subsequent plasma treatment of polysorb sutures. The cleaned sutures undergo multiple coatings (4 cycles) with conductive carbon ink, followed by baking at 60°C for 30 to 35 minutes to ensure uniform conductivity. Carbon-coated (CC) sutures undergo electrochemical pre-treatment in a solution of 0.50 M H2SO4 and 0.10 M KCl, followed by electrodeposition with polyaniline (PANI) using potentiometric deposition at 0.80 V in the presence of 0.50 M aniline in a solution containing 1.0 M H2SO4. An insulating polymer layer of 5% polyurethane (PU) is applied at the sensor's center, leaving ends exposed for electrical connections and subsequent hydrogel coating for real sample analysis. The O2 sensor fabrication involves cleaning silver-coated threads, coating with dielectric ink for a sensing region, and UV curing. Two identical threads serve as the cathode and anode, with an intentionally longer anode region as a common reference for the pH sensor. This comprehensive approach contributes to developing a versatile and efficient sensing system for multiplexed measurements, showcasing potential applications in diverse environments.

The measurement system includes a signal conditioning circuit (denoted as analog front-end or AFE in Fig. 1) comprising a trans-impedance amplifier (TIA) and an op-amp in the unity-gain configuration: the former is used to convert the current into voltage which originates from the oxygen sensor whereas the latter is used to read out the open-circuit voltage (OCV) for the pH sensor. The outputs of the TIA and the voltage buffer are sequentially selected using a 2:1 multiplexer with a control bit (SEL) before digitizing using an analog-to-digital converter (ADC). There is also a digital-to-analog converter (DAC) generating a bias voltage (VBIAS) of 0.55V for the oxygen sensor as the oxygen electrode requires a certain potential to maximize the dynamic range of the current measurement. The microcontroller handles digital signal processing, which can be connected to a computer for data storage and visualization. The operation of the sensor multiplexing system is validated in standard buffer solutions and gel mediums with different pH and O2 concentrations. The proposed setup demonstrated a sensitivity of ~65 mV/pH for the pH sensor (n=3), and 0.6621 nA/mgL-1 for the O2 sensor (n=3) in a gel medium with common ions and electrolytes. The sensor results were validated against commercial oxygen and pH probes for the same solutions. The developed sensing platform and sensor can precisely monitor and track changes in pH and O2 levels within less than 2 minutes for repeated cycles. This architecture has the potential for extending the low-power multiplexed sensing to multiple biomarkers with an increased number of sensors for ingestible devices, wearable sensing, and cellular agriculture applications.

KEYWORDS: Multiplexed sensing, pseudo-reference, pH and oxygen monitoring, miniaturized circuit, thread-based electrochemical sensor

Figure 1