Nutrient Sensing Using Chip Scale Electrophoresis and In Situ Soil Solution Extraction

This paper reports an electrophoresis-based microfluidic ion nutrient sensor for the detection of anions in soil solution samples. The sensor is able to analyze the concentration of various anions in extracted soil solutions with high sensitivity as well as high specificity, while it is an approach requiring no labels. The electrophoretic microchip integrates a pair of in-plane conductivity detection microelectrodes. A programmable high voltage power supply unit was designed to achieve precise control over voltage potentials needed for sample and buffer injection and ion separation. An electrical conductivity detector was designed to extract and process the changes in conductivity due to the arrivals of separated anions at the electrodes at various times. An arrival time serves to identify an anionic species, while the peak height indicates the concentration. A soil water extraction device was also designed to extract the soil solution analyte from the bulk soil, by applying vacuum suction. Only a minute amount of solution (on the order of μL) is needed for the electrophoretic measurement. Extracted soil solutions were analyzed for ionic concentrations to demonstrate the feasibility of using this microfluidic sensor, showing a limit of detection of about 7.25 μM.