Capacitance sensitivity study of interdigital capacitive sensor based on graphene for monitoring Nitrates concentrations

In order to increase agricultural productivity, it is necessary to manage the nutrients that the plants receive. In this sense, nitrogen is one of the most important nutrients for the adequate development and growth of plants and whose control contributes to the conservation of the environment. Traditionally, the quantification of nitrates is performed employing methods such as ion chromatography conducted in a laboratory, which leads to problems such as processing time, complexity in the analysis process itself and the need to have people trained to handle such analysis equipment. To alleviate these difficulties, some sensors use electrochemical principles; however, they present several limitations such as manufacturing complexity, cost, and difficulties in conducting real-time measurements. On the other hand, capacitive sensors have also been developed whose operating principle is based on analyzing the changes in the electrical characteristics of these devices produced by variations in the environment surrounding the sensor. For example, solutions with different concentrations of nitrates will produce an alteration of the electric field generated by the electrodes of the capacitor, producing changes in parameters such as its electrical impedance. However, the materials used to manufacture these sensors are metals, being unattractive for agricultural applications due to the corrosion they may suffer. To counteract this problem, it is possible to use other types of materials, among which graphene emerge as it has been shown to have excellent properties such as being a mechanically strong material and presenting good electrical conductivity. For this reason, in this work, the study of the sensitivity of a graphene-based interdigital capacitive sensor applied to the measurements of nitrate concentrations is performed. Different analytical methods were used. In general, the interdigital capacitive sensor shows repeatability in the measurements, especially for concentrations greater than 10 ppm with variability of ∼16%. Also, parameters such as the detection limit of the sensor were calculated, the result of which was 1.71 ppm. Also, measurements were made on agricultural soil samples showing differences in readings with respect to the ion chromatography method. Differences that are attributable to the different methodologies used in the calculation of the nitrate concentration and the fertilization process that was applied to the crop field a few weeks before. Despite the difference between the sensor measurements and the results obtained by ion chromatography, both procedures showed a high amount of nitrate concentration.