An Autonomous pH Sensor for Real-Time High-Frequency Monitoring of Ocean Acidification in Estuarine and Coastal Areas

<i>In situ</i> pH sensing is crucial for the real-time monitoring of ocean acidification and investigations into the marine carbon cycle. Although ion sensitive field-effect transistor (ISFET) has been proven suitable for marine pH monitoring, its supply and implementation remain challenging. An underwater pH sensor for environmental analysis (uSEA-pH) based on ISFET was developed herein, incorporating a modified commercial laboratory pH probe through engineering design. Laboratory characterization demonstrated that uSEA-pH exhibited a Nernstian response (slope -57.60 ± 1.05 mV/pH, R² > 0.999), rapid response time (∼7 s), and low measurement uncertainty (<0.01 pH). The sensor supports a sampling frequency of 1 Hz with an average power consumption of only 0.72 W. Its compact design (self-contained with battery: Φ15 × 45 cm; miniaturized version: Φ6.4 × 21 cm) facilitates deployment on various observational platforms. During high-frequency underway monitoring in the Pearl River Estuary and Dongshan Bay, uSEA-pH successfully detected subtle pH variations (<0.05 pH). In extended <i>in situ</i> deployments, buoy-mounted uSEA-pH reliably recorded tidal-driven pH fluctuations in Dapeng Bay (27 days) and Xiamen Bay (7 days), generating over 2.3 million field measurements. This study presents a viable, robust, and high-resolution approach for continuous pH monitoring in estuarine and coastal areas.