In-filed Monitoring of Plant Stress with a Low-cost Electrochemical Sensor

Introduction:: Monitoring plant stress is crucial for maintaining healthy and productive crops. Plants are constantly exposed to various environmental stresses, such as drought, extreme temperatures, pests, and diseases, which can negatively impact their growth, development, and yield. Stress monitoring allows farmers and researchers to identify and diagnose plant stress early on, providing an opportunity to implement timely interventions to mitigate the damage and maintain crop productivity. Salicylic Acid (SA) is a phytohormone responsible for plant stress signaling. It also has a crucial function in plants' defense mechanism against biotrophic and hemibiotrophic pathogens. As a result, SA detection helps understand plant metabolism under stress conditions.

Point-of-care diagnostic technologies allow decentralized, low-cost, and more accessible analysis and monitoring. They provide a rapid first line of diagnosis and prevent timely detection measures, thus, increasing the capability to predict potential problems.

Here we present a low-cost electrochemical device for monitoring plants' stress through Salicylic Acid (SA) detection. The device is integrated with a wireless reader to send the data to a smartphone through Bluetooth Low Energy (BLE) protocol.

Materials and Methods:: Electrochemical techniques refer to a set of analytical and investigative methods that typically link the transfer of electrons between electrodes and the chemical species in the solution. In Differential Pulse Voltammetry (DPV), a voltage is applied to the working electrode, and the resulting current, generated by a redox reaction, is measured. By increasing the voltage through consecutive pulses, the capacitive/background current is eliminated, lowering the detection limit of the system.

Laser-induced graphene (LiG) is a 3D porous carbon material that is created through the use of a laser to pattern and transform a polymer material into graphene. LiG can be fabricated using low-cost, scalable techniques and has promising characteristics for sensor application. We have used Mini 14 (Epilog Laser Machines) to fabricate LiG Working Electrodes (as shown in Figure 1a) on a PI film after optimizing the laser power strength and speed.

The characterization of LiG electrodes was done in 1 mM Ferricyanide and 1 mM Ferrocyanide solution in PBS buffer through running cyclic voltammetry. To determine the SA concentration in the leaves of Epipremnum pinnatum, a buffer solution containing 50 mMKClO₄ is utilized, and the leaves are mixed with the buffer solution using a blender. Then, this solution is spiked with different concentrations of SA.

A portable and cost-effective potentiostat is designed for the purpose of continuous monitoring of salicylic acid (SA) concentration in plants, whereby the collected data is transmitted to a nearby smartphone through the utilization of Bluetooth Low Energy technology.

Results, Conclusions, and Discussions:: Results
The scalable fabrication process of LiG electrodes enables the cost-efficient production of these systems. The characterization of these electrodes reveals the formation of a graphene layer on the PI film. Moreover, the laser machine parameters have been meticulously optimized to produce electrodes that are not only reproducible but also display exceptional electrochemical performance. The cyclic voltammetry test for surface characterization of LiG electrode is shown in Figure 1b.

Differential pulse voltammetry measurements have been done in the prepared solution using a glassy carbon electrode as the working electrode, Standard Ag/AgCl as the reference electrode, and Platinum wire as the counter electrode. The peak current value shows a linear correlation to the concentration of SA (Figure 1c), with an R-squared value of 0.988, between 10-100 uM.


Conclusions
A low-cost LiG-based device to monitor plant stress, integrated with a portable potentiostat, is developed. The detection of salicylic acid, a phytohormone responsible for plant stress signaling, using a glassy carbon electrode has been demonstrated.


Discussion
By transferring the measurements from a conventional glassy carbon electrode to the newly constructed LiG electrodes, the sensor for monitoring plant stress can be effectively utilized.

Acknowledgements (Optional): : The authors would like to acknowledge the Provost Strategic Direction Research (SDRA) award, USC Center for Sustainable Solutions, 3M, Powell Foundation, and USC WiSE.

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