Graphene-based field-effect transistors integrated with microfluidic chip for real-time pH monitoring of seawater

Seawater pH is an important parameter in marine and environmental researches, and it demands sensitive, portable, rapid and real-time sensing pH sensors. Here, we propose a graphene field-effect transistor (Gr-FET)-based pH sensor on flexible polyimide (PI) substrate integrated with microfluidic chip for real-time seawater pH detection. The monolayer graphene was grown by chemical vapor deposition, and transferred onto PI substrate to form transistor. The formed Gr-FET integrated with microfluidic channel forming the pH sensing chip, which is 2 × 3 cm² in size, and 2 mm in thickness. Gr-FET-based pH sensor on PI substrate presents sensitivity of 23.98 mV/pH and 8.07 mV/pH in 1 × PBS and seawater solutions, respectively, and realizes pH detection in 1 min. The different ion type and concentration in the seawater solution could be contributed to the sensitivity reduction of the sensors in seawater. Real-time pH detection results of local fresh seawater show the fluctuation within 3% comparison with commercial pH sensor. The proposed Gr-FET-based pH sensor is economic, portable, fast and promising to realize real-time pH detection.