The Technology Of Ion Selective Membrane And Its Application In Soil Nitrate Detection

Fertilizer is important for agricultural production, which plays a crucial role in increasing grain yield. In recent years, the overuse of fertilizer especially for nitrogen fertilizer has resulted in serious agricultural non-point source pollution and economic loss. It is significant to develop sensing techniques to detect soil nitrate at low cost for guiding variable fertilizationIon selective electrode (ISE) has been widely used in many fields such as clinical analysis, environmental monitoring, physiological and process control and shows such merits as low cost, simple operation, not affected by solution color and turbidity, etc. In recent years, a new orientation is opened up by the combination of traditional electrochemistry and other discipline such as computer technology, nano materials and so on, which injects vigour into ISE in agriculture application. In this research, the soil nitrate nitrogen is chosen as the sensing object then we studied and developed a stable and low-cost solid state ion selective electrode. Then, a calibration model was built for correcting interference of high content coexisting ions under a wide temperature range. Last, the solid state ion selective electrode and calibration modele were integrated into an automatic detection device being developed by our research team previously to provide an accurate, stable, low-cost means for soil nitrate nitrogen detection. The main contents include the following aspects:1. The fractal characteristics of polypyrrole films doped by nitrate were studied then the relationship between them and sensor performance was explored. By changing polymerization current density, the polypyrrole films doped by nitrate (PPy-NO2) with different morphologies were modified on glassy carbon (GC) substrate. After the fractal analysis, a relationship among the fractal dimension, electrochemical performance and the dynamics characteristic was discussed. The results show that the PPy-NO3- film presents fractal characteristics. Changing preparation conditions (polymerization current density) can significantly alter membrane morphology parameters. The fractal dimension (Df) is more suitable to quantify the complexity of a film surface in the micro scale. In a certain range, there is a positive correlation between D/and sensor performance (linear range, response rate, detection limit) and the lager Df value corresponds to a higher phase diffusion coefficient. Under a polymerization current of 1.2 mA·cm-2, the enhanced performance was showed with the Df=2.58, diffusion coefficient of D=7.48×10-11 cm2·s-1, the linear range of 5.0×10-5-10-1 M, response slope of-53.3 mV·decade-1, response time<60 s and selective coefficient for main interference ions reaching a magnitude of 10-2 order.2. The mechanism of potential drift caused by a water layer was analyzed and a new solid state ion selective electrode was prepared, which based on a composite material of PPy-NO3- and graphene (GR) and could inhibit the potential drift effectively. The new sensor presents good electrochemical analysis capabilities with response slope of 56.2 mV·decade-1, linear range of 5.0×10-5-10-1 M, detection limit of 0.63×10-5 M, response time of< 15 s and logKNO3-, Cl=-2.5. Besides that, the response potential stability was inproved greatly with AE·At-1 of 0.67±0.05 mV·h-1. Compared with standard test results of pratical soil samples, the sensor has high accuracy and could maintain a good stability under direct soil slurry measurement.3. The impact of membrane polarization on the response potential stability was analyzed theoretically and the possible micro structural which could inhibit polarization was inferred. After that, the PPy-NO3-(T) film with nanotube like microstructure was prepared and a corresponding all solid state ion selective electrode (PPy-NO3--(T)/GR/GC) was constructed. Amylose was used as molecular template to induce the oriented growth of pyrrole monomer then the nanotube like microstructure was regulated. The experiment results show that the PPy-NO3-(T) film owns a high charge transfer rate (Rct=95.7 Ω) and large electrochemical capacitor (Cem=0.98 mF、Cd1=4.5×10-6 mF), which could inhibit polarization of solid state ion selective electrode greatly. Besides owning good analytical performance, the corresponding sensor has extremely high stability with AE·At-1 of 80±6μV·h-1.4. An ANN calibration model was constructed for correcting interference of high content coexisting ions and a large temperature change. The model inputs include response electromotive force of a Cl- ion selective electrode, a PPy-NO3--(T)/GR/GC and temperature while the concentration of nitrate nitrogen is as the output. The effect of ANN parameters and their interaction on model performance was analyzed by response surface methodology then related optimum values were calculated. The result shows that optimized model presents good predictive ability with RMSET of 1.84 mg·kg-1 and R2 of 0.97, which runs under optimum parameter of topology 3-8-1, momentum coefficient of 0.73 and learning rate of 0.33. Compared with traditional Nernst equation calibration method, the model shows certain superiority in strong interference situation. The model is suitable for soil NO3--N dection in agiculture application.