Study On Microfluidic System For Rapid Detection Of Soil Nutrients

It is of importance to ensure high yield and efficiency in agricultural production via accurately and quickly acquiring the soil nutrition content at a low cost.Furthermore,it is effective to avoid soil deterioration and environmental pollution caused by excessive fertilization,which has important social and economic value.In the traditional detection methods of soil nutrients,large-scale analytical instruments and numerous chemical reagents are adopted by professional inspectors.It exists plenty of problems,such as poor time efficiency,high cost and excessive reagent consumption.In this thesis,microfluidic techniques for rapid detection of soil nutrients including nitrogen,phosphorus and potassium are investigated by optimizing the capacitively coupled contactless conductivity detection(C4D),which combined with two ion separation methods-capillary electrophoresis(CE)and microchip electrophoresis(MCE),respectively.Supported by the National Key Research and Development Program of Research on Portable Soil Available State Nutrient Real-time Analysis and Decision Technology(2017YFD0700501-2),CE-C4D and MCE-C4D systems for rapid detection of soil nutrients are built and applied in the field with capillary and microfluidic chip as the carrier.The main contents and innovation points are as follows:(1)For the problem of poor accuracy of soil nutrient detection due to scant sensitivity and high noise level of traditional C4D systems,this paper proposes an excitation circuit design method with high-frequency transformers by an in-depth study of mathematical model and frequency response features of C4D technology.Meanwhile,the method matches the optimization of circuit parameters and electrode size,as well as the application of a fuzzy PID temperature control algorithm.It causes the detection limits of K+,NH4+,NO3-and H2PO4-in the constructed CE-C4D system to be 0.7,0.6,0.9 and 0.4μg/L,which are substantially better than related reports.In the design of the MCE-C4D system,a chip design scheme with an integrated reference channel was proposed,combined with the application of a baseline compensation circuit and a lockin amplifier circuit.It achieves the elimination of background currents and the suppression of system noise,resulting in an increase of C4D signal peak height by more than 5 times and a reduction of the noise level of the system to about 0.06 mV.The experiments on standard solutions and soil samples provide evidence of the feasibility and superior sensitivity of the C4D method for soil nutrient detection.(2)For the problems of the conventional soil nutrient detection of slow speed and complex operation process,a separated electrode design scheme was proposed.The electrode detection substrate composed of a four-electrode structure was fabricated through a PCB process.The detection chip only contains the channel structure,which has cheap materials cost and can be quick replacement.It has no risk of crosscontamination owing to no cleaning process,leading to a great improvement of detection efficiency and significantly simplified operation process,which enables the detection of a single nutrient indicator within 90 seconds.(3)For the problem that it is difficult to promote and apply the rapid soil nutrient detection system in vast rural areas,the detection device is made with a high level of integration and portability through the multiplexing technology of high-voltage systems and the miniaturization design of detection circuits.Compared with the rapid detection methods using optical and ion-selective electrodes,the device,with no complex synthetic electrode materials and precision optical devices,has relatively low manufacturing cost and better transportation stability.It can as a useful supplement to traditional soil nutrient detection methods,and obtain information on nutrient distribution with better timeliness and resolution,providing more efficient data guidance for agricultural production.