Integration of AC bias-stabilized SiNRFETs with microscale Ag/AgCl pseudo-reference electrodes for lab-on-chip biosensing application

Ion-sensitive field-effect transistors (ISFETs) are highly promising for lab-on-chip integration due to their excellent scalability, high sensitivity, and rapid response times. However, on-chip integration of reliable microscale reference electrodes, essential for ISFET operation in microfluidic systems, remains a significant challenge. This study presents a microfluidic pH meter that integrates silicon nanoribbon field-effect transistor (SiNRFET) sensors with on-chip microscale AgCl/Ag pseudo-reference electrodes (p-REs). The microscale p-REs, fabricated using CMOS-compatible processes and protected with a photoresist layer, demonstrated rapid stabilization and long-term stability. Moreover, the short distance between the integrated p-REs and the paired SiNRFET sensors within the microfluidic environment minimized streaming potential, ensuring measurement accuracy regardless of flow rate variations. In addition, applying an AC pulse gate bias effectively reduced ISFET signal drift caused by ion penetration into the pH-sensitive layer, enhancing the system's reliability for long-term monitoring. To showcase the platform's biosensing capabilities, the microfluidic pH meter was finally used to monitor bacterial metabolism on-chip via measuring pH changes induced by metabolic activity with microliter-level samples. This work establishes a robust lab-on-chip biosensing platform, enabling real-time and long-term biosensing in microscale environments.