Noninvasive Assessment of Microbial Activity by Realtime Monitoring Degradation of Cellulose Acetate via Electrochemical Impedance Measurement

Abstract Bacteria play essential roles in maintaining soil health, including decomposition of organic and inorganic materials as well as processing nutrients and fertilizers. Careful monitoring of microbial activity has become of great interest in agricultural processes as high levels of activity and diversity can be correlated to soil quality, nutrient availability, as well as provide insight on future crop yields. Although many methods and devices have been developed to enable a better understanding and quantification of the microbial activity levels, most of them are based on laboratory culture assays or are difficult to implement in fields. Here, for the first time, we demonstrate feasibility of a non-destructive, real-time monitoring technique that allows measuring the degradation kinetics of a cellulose acetate (CA) membrane as a direct indicator of the level of cellulase activity of the microorganism within the targeted ecosystem. The sensors are made by depositing a 50-micron thick layer of CA onto circular shaped interdigitated electrodes (IDEs) on a FR-4 glass epoxy substrate with an active sensing area of 12 mm2. The kinetic degradation of the CA membrane was determined by measuring the changes in differential impedance across the IDEs at an operating frequency of 10 kHz. As a proof of concept, the sensors were tested in liquid culture media to monitor the cellulase activity of Pseudomonas aeruginosa, as commonly found bacteria in soil and water. The biodegradation of the CA membrane was successfully measured as a distinct decrease in the impedance of the sensor as the number of Colony Forming Units (CFU/mL) increased in the culture environment with an average linear sensitivity of -139.6Ω/log(CFU/mL) and R2 = 0.98. Sensors were also tested on samples taken from agricultural fields used for crop production. Results of sensors performance in soil provides evidence for using this technique for in-field measurements. Unlike conventional culture-based and DNA extraction techniques, the proposed method provides a real-time assessment of the microorganisms’ activity within their ecosystem. Furthermore, the proposed simple electrical sensing approach paves the road towards developing a new class of biosensors able to easily interface with common wireless data acquisition modules for precision agricultural applications.