| Soil/cultivation substrates are the supporting medium for plant crop, and also store the nutrient and water which are essential for plant growing. It can provide a stable and suitable growing environment for plant, and the plant roots obtain nutrient and water according to the need. pH is one of the most important fundamental properties for soil/cultivation substrates, also is a significant influence factor for fertility and nutrient effectiveness. Hence, measuring and controlling the pH of the soil/cultivation substrates timely promotes a better controlling the water and fertilization condition for soil/cultivation substrates, and has an important significance in agriculture production.Aiming at the in situ pH measurement for soil/cultivation substrates, considering the finite using condition of commercial pH sensor, after the analysis of the research status of the pH sensor and the special physical and chemical properties, an all-solid-state pH sensor based on modification membrane was presented. Furthermore, the studies of the sensing mechanism and in situ pH measurement for soil/cultivation substrate were conducted. The important conclusions and innovative achievements of this work are as following.(1) Study on the sensing mechanism of antimony pH electrode based on Nafion modification. Firstly, relevant theories of the antimony pH electrode based on Nafion modification were presented and analyzed. Secondly, some basic theories and methods of molecular dynamics simulation were introduced. Thirdly, with the molecular dynamics simulation software Materials Studio, the interaction energy of Nafion-antimony interface was calculated through molecular dynamics simulation, which is -471.04kcal/mol. The results showed that the Nafion membrane and the antimony surface attracted each other and the attraction was very tight. Lastly, the diffusion of hydronium ion in simulation cells with different hydration degree Nafion membranes was studied, with the help of the simulation software Materials Studio. The simulation results indicated that the diffusion of hydronium ion improved with the increase of the hydration degree of the Nafion membrane. In other words, the more the Nafion membrane wet, the faster the hydronium ion diffuse.(2) Study on the antimony pH electrode based on Nafion modification. Firstly, the Nernst response mechanism of the antimony pH electrode was analyzed, and the ideal Nernst response slope of the antimony pH electrode was determined as 59.16mV/pH at 25?. Secondly, the antimony pH electrode based on Nafion modification was fabricated combined the printed circuit board PCB technology, magnetron sputtering deposition technology and spin coating technology.Thirdly, surface morphology and electron energy spectrum of the antimony pH electrode with different antimony film thickness were performance using ultra high resolution field emission scanning electron microscopy (FESEM) with energy dispersive spectrophotometry (EDS). The performance results showed that pores were observed at different degrees in all surfaces,which is quite obvious when the thickness is ?50 nm, and with the increasing of the thickness,the pore decrease, the compactness increase, and the copper content decrease. Lastly, to investigate the effect of the thickness of the Sb sensing thin film on the performance of antimony pH electrodes,electrochemical experiments were conducted in Sb thin film thickness of ?50,?150, ?240, and?330 nm. Experimental results demonstrated excellent features of the pH electrodes with ?240 nm thickness Sb thin film, including high sensitivity, short response time, high resolution,stability, reversibility, and long-term stability.(3) Study on the Ag/AgCl reference electrode based on chitosan-graphene modification.Firstly, the response mechanism of the Ag/AgCl reference electrode was analyzed. Secondly, the Ag/AgCl reference electrode based on chitosan-graphene modification was fabricated combined the printed circuit board PCB technology, magnetron sputtering deposition technology and spin coating technology. Thirdly, surface morphology and electron energy spectrum of the Ag/AgCl electrode were performance using ultra high resolution field emission scanning electron microscopy (FESEM) with energy dispersive spectrophotometry (EDS). The qualitative and semi-quantitative analysis results showed that the best thickness of silver film was about 4?m,the sputtering time was determined as 45min, and the best chlorination time was determined as 0.5 min. To obtain a stable and efficient modification solution, the electrochemical performance of a bared Ag/AgCl electrode, an Ag/AgCl electrode modified by chitosan, an Ag/AgCl electrode modified by graphene, and an Ag/AgCl electrode modified by chitosan-graphene was investigated. The performance results demonstrated excellent features of the Ag/AgCl quasi-reference electrode modified by chitosan-graphene in all parameters, including durability,reversibility, and cycle voltammetry behavior.(4) Study on the performance of the all-solid-state pH sensor based modification membrane.Firstly, a new all-solid-state pH sensor based modification membrane was proposed, which consist of antimony pH electrode based on Nafion modification and Ag/AgCl reference electrode based on chitosan-graphene modification. Secondly, the response properties of the developed pH sensor were determined in phosphate buffer solution (PBS). The results indicated that the sensitivity of the developed pH sensor was 52.3mV/pH, which was close to the ideal Nernst response slope, the response time was 20s, and the measurement deviation in short term was less than 2mV, and the deviation of the response slope was less than 2mV/pH in long term measurement. Lastly, the developed pH sensor was applied to actual pH measurement, including the solution samples and extract solution samples of soil/cultivation substrate. The test results show that the measuring error in solution sample was less than that in solid cultivation substrates extract solution, and the measuring error in acid condition was larger than in other conditions obviously. Comparing with the commercial pH meter, the absolute error of the developed pH sensor was less than 0.6pH, and the relative error was less than 10% in all pH measurement.(5) Study on the double compensation method of the all-solid-state pH sensor based on modification membrane. Firstly, combined with Nernst response mechanism and the special request of soil/cultivation substrate in situ pH measurement, the double compensation method,which was temperature and humility two-step compensation method,was presented for soil/cultivation substrate in situ pH measurement. Secondly,the experimental study of the double compensation method was conducted using the peat and vermiculite, and the mathematic models of the double compensation method were obtained through analysis. Lastly, the pH detector based on double compensation method for soil/cultivation substrates was developed, and relative experiment was conducted. The experiment results showed that the pH measuring absolute error was positive value, and range from 0.12 to 0.27, relative error was ranged from 1.87% to 4.95%after double compensation. The measuring error was relative low, and close to the request of soil pH measurement of national standard (NY/T 1377-2007).The development of the pH sensor based on modification membrane and the study of the temperature-humility double compensation method for in situ pH measurement of soil/cultivation substrates, provide a new method and a new direction for in situ pH measurement of complex measurement system,such as soil/cultivation substrates. This work also can improve the measuring speed and the measuring accuracy of the soil/cultivation substrates information measurement in cultivation process, as well as the production efficiency of facility horticulture.Furthermore, this work also will be considered as a reliable technical support and theoretical basis in future development of measuring instruments and apparatus in situ for soil/cultivation substrates. |
PDF Full Text Request