Razi University

 Continuous monitoring of chemical species in water resources, particularly ammonia and nitrate, is of great importance due to their health and environmental consequences. Although conventional instrumental methods offer high accuracy, they require specialized equipment and high costs. In this study, two simple and low-cost sensing systems based on cellulosic substrates were designed, fabricated, and evaluated for the measurement of ammonia and nitrate. In the first part, a paper-based sensor based on polyaniline (PANI) was developed for ammonia detection. Polyaniline was synthesized via chemical oxidative polymerization of aniline in an acidic medium and then applied onto a paper substrate. The chemical structure of the polymer was characterized by FTIR, and the surface morphology was examined by SEM. The resistive response of the sensor in NaOH medium at a concentration of 0.01 M and a contact time of 180 s was selected as the optimal condition. Using the response surface methodology, the optimal amount of polyaniline was determined to be 0.070505 g and the optimal NaOH concentration was 0.085583 M. The calibration curve was obtained in two distinct linear ranges: 0–90 µM (equation: R = -0.8778C + 2556.6, R² = 0.9881) and 200–1000 µM (equation: R = -0.351C + 2494.4, R² = 0.991). The limit of detection (LOD) and limit of quantification (LOQ) were calculated as 0.33 and 1.01 µM, respectively. The repeatability of the sensor response was evaluated with an RSD of 0.46% in standard solution and an average RSD of 0.20% in real samples. Furthermore, the capability of the sensor's kinetic response for discriminating bottled water samples was investigated. Eight commercial brands (Kimia, Meyva, Saeo, Azmarg, Aquafina, Damavand, Dasani, and Bisheh) were tested with five independent replicates. Principal component analysis (PCA) showed that the first two components explained 99.31% of the total variance, and the samples were well separated in the principal component space. The confusion matrix, for the evaluated dataset, showed no classification error, and the performance metrics for the examined classes were 1.00. This result is limited to the same eight samples and experimental conditions. In the second part, a method based on nanocellulose and digital imaging was presented for nitrate measurement. Nanocellulose was prepared from tissue paper via acid hydrolysis and used as the substrate for the colorimetric reaction. The colorimetric reaction was carried out using zinc powder, 3-nitroaniline, and 1-naphthylamine in an acidic medium, and the optimal amounts of these components were determined to be 0.04, 0.005, and 0.003 g, respectively. The RGB calibration curve was obtained in the range of 0–70 mg/L with the equation R = -0.4133C + 147.71 and R² = 0.9732. The LOD and LOQ were calculated as 3.74and 11.35 µM, respectively. Comparison of the RGB method with the spectrophotometric reference method in real samples (well water, agricultural water, tap water, flowing water, wastewater, and watermelon) showed that the relative error between the two methods ranged from 4.54% to -9.88%. The results of this study indicate that cellulosic substrates can serve as suitable platforms for the fabrication of simple, low-cost, and portable sensors for water quality monitoring. However, the obtained results are limited to the laboratory conditions and the examined samples, and their generalization to other conditions and aqueous matrices should be made with caution. Keywords: Polyaniline, Paper-based sensor, Ammonia, Nanocellulose, Digital imaging, Nitrate, Colorimetry