Three-Dimensional Conductometric Network Based on Reduced Graphene Oxide for Soil pH Sensors

In this work, we have synthesized l-arginine-functionalized 3D macroporous graphene, formed through the assembling of reduced graphene oxide nanosheets. The soil pH microsensor is fabricated on the patterned Si/SiO2 substrates with Au-interdigitated electrodes employing 3D graphene/l-arginine structure as a sensing platform. The sensor exhibits an increase in conductance toward an increase in pH of the buffer solutions (pH = 3–10) with a sensitivity of 97 μS/pH and response of 650%. The room-temperature pH sensing measurements are performed to detect the pH variations in different types of soil samples. The conductance of the sensor is correlated to the change in soil pH with increase of gravimetric moisture contents displaying a sensitivity of 105 μS/pH and 98 μS/pH for black soil and red soil, respectively. The sensor effectively detects soil pH changes against varying concentrations of CaCl2 solution (1–20 mM) with a sensitivity of 121 μS/pH (black soil) and 101 μS/pH (red soil). In comparison to the commercial pH meter, the 3D graphene/l-arginine-based microsensor demonstrates a relative standard deviation of less than 4% in soil pH sensing measurements. The designed pH sensor displays an excellent response with good reversibility, satisfactory linearity, low hysteresis (3%), a response time of 30–35 s, and stability of around 2 months.