Detection Of Organophosphorus Pesticides By Liquid Crystal Sensor Based On Metal Ion Probe

Organophosphorus pesticides are currently the most widely used pesticides.However,their residues also cause serious food safety problems.The development of real-time rapid detection of organophosphorus pesticide residues is,therefore,of great significance.For the detection of organophosphorus pesticides,the commonly used methods include chromatographic analysis,spectral analysis,enzyme inhibition,etc.Although they are effective,most of them require well-trained professionals and expensive instruments and equipment,which are not suitable for on-site detection.Therefore,this thesis aims to develop a label-free liquid crystal sensor for real-time and rapid detection of organophosphorus pesticides.In this thesis,metal ions are used as probes to construct a liquid crystal sensor based on the mechanism of the competitive coordination reaction between P=O or P=S of organic phosphorus and cyano group（-CN）of liquid crystal 5CB with metal ions.The addition of organophosphorus pesticides will cause the orientation transition of liquid crystal mesogen,which leads to the optical structure change of liquid crystals from the"black"to the"bright"under the polarizing microscope.This change can be easily observed and used for reporting the presence of the organophosphorus pesticides.The liquid crystal sensor is constructed by a bottom slide fixed by metal perchlorate,an upper slide modified by N-dimethyl-N-octadecyl-3-aminopropyl-trimethylsilane（DMOAP）,and a liquid crystal layer in between.The detailed study about this liquid crystal sensor for organophosphorus pesticides detection are as the follows:First,the preparation for the metal ion probe layer was studied.Metal ion salts are fixed on the carboxyl group-modified glass surface by spin coating.After the parameters for spin coating were determined,the detection of four metal perchlorates of copper perchlorate,iron perchlorate,zinc perchlorate and aluminum perchlorate for organophosphorus pesticides are investigated respectively.It was found that the aluminum perchlorate provided the sensitive detection.When the concentration of aluminum perchlorate is 8 mM,the detection limit of the liquid crystal sensor was 1μg/mL for glyphosate（P=O）and 0.1μg/mL for malathion（P=S）.Moreover,the liquid crystal sensor based on Al³⁺probe can avoid the interference of other commonly used pesticides such as spirotetramat and thiamethoxam,providing a good specificity for the examination of organophosphorus pesticides.In the following,the influence of anions on the detection of organophosphorus by metal cations is explored.When metal nitrate was used instead of metal nitrate,it was found that no change was observed after introducing organophosphorus pesticides into the liquid crystal sensor.This proved that perchlorate played an important role on the identification of organophosphorus pesticides by metal ion probe.This was further confirmed by the infrared spectrum,that is,as a weakly coordinating anion,perchlorate would not affect the coordination of aluminum ions with cyano groups in liquid crystals,while nitrate,as a strong coordinating anion,would inhibit the combination of aluminum ions and cyano groups.On this basis,the detection by the mixing of the these two aluminum salts on organophosphorus pesticides was studied.It was found that the mixed aluminum salt has improved performance compared with pure aluminum perchlorate.When the probe concentration was 10 mM and Al（ClO₄）₃:Al（NO₃）₃is 7:3,the sensor based on Al³⁺can also detect 1μg/mL of glyphosate and 0.1μg/mL of malathion.Finally,liquid crystal sensor built on microchannels for real-time detection of organophosphorus pesticides was explored.The substrate was functionalized by metal ions.After the PDMS with microchannels was bound to the functionalized glass slide,the organophosphorus pesticides,water,and liquid crystals are sequentially injected into the microchannels through the syringe to examine the organophosphorus pesticides.It showed the liquid crystal sensor built on microchannels can identify 20μg/mL of glyphosate and 1μg/mL of malathion in a real-time way.In this thesis,a liquid crystal sensor is developed by using metal ions as the probes,which can specifically identify glyphosate（P=O）and malathion（P=S）,two representative pesticides for organophosphorus pesticides.When Al³⁺was used as a metal ion probe,the detection limits of glyphosate and malathion were 1μg/mL and 0.1μg/mL,respectively,and were not disturbed by adding other types of pesticides.