Rapid Detection Of Organophosphorus Pesticide And Nitrate By Electrochemical Methods

Pesticides and fertilizer have been playing important roles in agriculture. However, their excessive use brings about severe contaminations of food and environment, posing threats to human. Therefore, the development of rapid detection method is of great importance. Among all the available methods, electrochemical methods are characterized with easy operation, high sensitivity and low-cost. The obstacle for rapid analysis by electrochemical method is the complicated treatment of the electrode. The working electrode plays an essential role for sensing, which requires different fabrication/modification procedures to meet the requirements of various analytes. Besides, the regeneration of the used electrode is a must. So, the easy-to-use electrode would facilitate the analysis and meet the requirement of on-site and rapid detection. For this purpose, carbon paste electrode, screen-printed electrode and all-solid-state electrode are fabricated to facilitate the measurements of pesticide and nitrate. In addition, the sample pretreatment is an indispensable and time-consuming procedure, which is disadvantageous for rapid analysis. To overcome this obstacle, ink-based electrodes are developed so that electrodes can be written on solid samples for direct analysis without sample pretreatment. This written electrode provides a common strategy for the in situ detection without sample pretreatment. So, it can be a promising candidate in the development of rapid detection method. Also, this written electrode can be used as a sensor to detect the activities of plant in real time, providing a new window to look into the world of plant.The research contents, results and conclusions are summarized as follows:(1) Combining acetylcholinesterase inhibition method with electrochemical analysis for the rapid detection of organophosphorus pesticide.Silver electrode is capable for the rapid detection of organophosphorus pesticide. Cyclic voltammetry and amperometry are used to assess the characteristics of thiocholine, the catalytic product of acetylcholinesterase. The silver electrode shows an advantage in detection of thiocholine at a low working potential, which allows it to detect the activity of enzyme in real time. The silver electrode shows an liner response to thiocholine from 5.2×10-7 to 2.5×10-5mol/L with a LOD of 6.2 ppb for paraoxon. Recovery rates for Chinese cabbage and Fuji apple are 92.05% and 106.11%, respectively. So, silver electrode is capable for the detection of pesticide. After that, disposable silver electrode is fabricated by screen-printing method and organophosphorus pesticide of P=S type is detected. This type of organophosphorus pesticide dose not inhibit the activity of acetylcholinesterase until P=S is converted into P=O. The chemicals used for the conversion are interferences for the detection of thiocholine. However, the screen-printed silver electrode is capable to detect thiocholine at 0 V, and it shows no response to the interfering chemicals. The LOD for chlorpyrifos is 2.5 ppb. Recovery rates of cucumber and Fuji apple are 101.81% and 107.42%. Due to the specificity to thiocholine, silver electrode can be used without modification. The screen-printing method is capable for the fabrication of disposable and low-cost silver electrode. These merits make silver electrode an ideal candidate for the rapid detection of organophosphorus pesticide.(2) Electrochemical detection of organophosphorus pesticide An electrode made by n-octylpyridinium hexafluorophosphate and carbon nanotube is developed for the extraction and detection of organophosphorus pesticide. The electrode can accumulate pesticide from the solution and increase the peak current. The ability of extraction results from the π-π stacking effect between the pesticide and n-octylpyridinium hexafluorophosphate, and the accumulation is linear from 0 to 14 min. With the preconcentration of 3 min, the electrode shows a linear response to methyl parathion in the range 0.1 to 2.5μg/mL. A self-correcting algorithm is used to improve the sensitivity and the limit of detection reaches 0.008μg/mL. Recovery rates of pear and soil samples are in the range 91.15% to 101.70%. These results shows that this electrode provides a simple approach for the detection of organophosphorus pesticide(3) Rapid detection of nitrate Graphene made from graphite is used as the solid-contact layer for the fabrication of an all-solid-state nitrate-selective electrode. Electrochemical impedance spectroscope shows the feasibility of graphene as the ion-to-electron transducer, and graphene can prevent the formation of water layer which would lead to instability of the signal. The fabricated electrode shows a linear response from 10-4.3 to 10-1 mol/L with a Nernstian slope of 57.9 mV/decade of nitrate concentration. The limit of detection is 3×10-5 mol/L. the electrode is used to detect the nitrate in drinking water, and the results are compatible with those obtained by a national standard method. So, this electrode provides a handy alternative for the rapid detection of nitrate.(4) Fabrication of electrodes on agricultural products for direct analysis without sample pretreatmentA novel strategy for direct analysis of agricultural products is realized. A facile way to fabricate electrodes on solid samples is developed, which allows in situ sensing of electroactive analytes without sample pretreatment. Two inks are developed and electrodes can be obtained by writing inks on the sample. After curing at room temperature for 5-8 min, the written electrode can be used for in situ detection. This method is used to detect methyl parathion, and the recovery rates of Fuji apple, Chinese cabbage and Chinese chives are between 85.5% and 97.0%. Elimination of sample pretreatment makes this method simple and suitable for the on site and rapid detection. In addition, this method is capable for in situ detection of nitrite. So, it can be a common strategy for the detection of electroactive analytes. This method is of great importance for the development of rapid detection method. In addition to chemical sensing, this written sensor can be used as a flexible and stretchable strain sensor. It can be written on the object and is capable for wearable detection such as tiny angle change, body motion and plant activities.