A Low-Cost Sensor for Assessment of Aqueous Solutions Properties

Monitoring of aqueous solution parameters such as pH and solute concentration is of utmost importance in various industries such as oceanic and agriculture. Herein, we report an innovative, low-cost interdigitated electrode (IDE) sensor built on a printed circuit board (PCB) for assessment of aqueous solutions properties through electrochemical impedance spectroscopy (EIS). We designed and fabricated two variants of the proposed sensor with two different electrode widths. The sensors were characterized in “dry” mode (with no electrolyte) using an equivalent electric circuit, and the associated model parameters were derived. In the “wet” mode, we used saline solutions with a concentration varying from 0.03% to 0.9%. Three different equivalent electrical circuit models were fit to the EIS data acquired using the sensors. Our observations showed that the series resistance of the electrochemical cell decreases as the solution concentration increases, which was used to calibrate the sensors. The results demonstrated that the developed sensors could reliably distinguish between samples with slight differences (0.03%) in their concentration. We also studied the effects of pH within the range of 4–7.6. From the models fit to the acquired EIS data, we extracted two appropriate circuit parameters to calibrate the sensor for pH measurement: one exhibits an almost linear response with an absolute sensitivity of $91.802~\Omega $ /pH over the entire pH measurement range (relative sensitivity ranging from 56.35% at low pH to 70.36% at high pH), and the other demonstrates a nonlinear response with a relative sensitivity ranging from 248.4% at low pH to 471.96% at high pH.