Graphene quantum dot soil moisture sensor

A soil sensor to measure moisture content in soil is fabricated and described. The sensor is based on graphene quantum dots (GQDs) and is highly sensitive. The simplicity of the process and use of an inexpensive material GQDs make it an affordable low cost sensing unit in comparison to existing soil moisture sensing units. It is a resistive micro-sensor with interdigitated electrodes (IDE) where MEMS fabrication process is used and GQDs serve as the channel material between IDEs. The sensor is tested in two different soil environments viz. white clay (passive soil) and bentonite clay (an active soil). The conductance of IDE structure with GQDs changes from 0.06 × 10−6 1/Ω to 0.68 × 10−6 1/Ω in white clay as the gravimetric moisture content changes from 4 to 45%. For bentonite soil, the conductance of the sensor changes from 0.06 × 10−6 1/Ω to 0.48 × 10−6 1/Ω across the gravimetric moisture range of 11 to 90%. The sensor shows higher sensitivity at higher moisture content which can be attributed to Grotthuss chain reaction and ionic conductivity. Response time of the fabricated micro-sensor is found to be 2-3 min which is the lowest response time reported for a resistive based soil moisture sensor. The sensitivity of sensor towards gravimetric moisture contents is 0.014 × 10−6 1/Ω per percent increase in moisture content (1/Ω/1%) for the white clay and 0.005 × 10−6 1/Ω/1% for the bentonite soil.