New Methods For Colorimetric Determination Of Quinclorac And Dithiocarbamates Pesticides

Quinclorac（QNC）is a quinoline carboxylic acid herbicide used in rice fields widely.Dithiocarbamates（DTCs）pesticides are a kind of broad-spectrum fungicides.The excessive use and improper treatment have led to the presence of those pesticides residues in the environment and foods,resulting in serious environmental pollution and food safety issues.Hence,it is significant to detect those pesticides residues in the environment and foods on site.Here,two colorimetric sensors were developed for the on-site detection of QNC and DTCs.The contents include:（1）Here,a colorimetric aptasensor was constructed for sensitively detecting QNC,a common herbicide.The aptasensor involved a novel amplification strategy and a classical strand displacement strategy.The amplification strategy,termed exonuclease III（Exo III）-assisted cyclic release of phosphorodiamidate morpholino oligomer（PMO）mimic enzyme strategy,was developed based on two new findings on PMO:1)DNA hybridized with PMO could resist Exo III digestion;2)a designed G-rich PMO（named P2）could bind to hemin to form a G-quadruplex PMOzyme with peroxidase-like activity.In this strategy,a designed DNA-PMO duplex（D1-P1）completely hybridized with the other designed DNA（D2）in D2-P2 to trigger D2 degradation by Exo III and cyclic release of P2.After that,the hemin-binding P2 catalyzed colorless3,3′,5,5′-tetramethylbenzidine（TMB）to blue TMB⁺.The cycle process was performed at high Exo III concentrations without strict control and with constant background signals.In this case,the developed strategy was sensitive,efficient,easy to operate,reliable,and ultralow background.Meanwhile,a QNC aptamer was used to develop the strand displacement strategy based on magnetic beads.The colorimetric aptasensor was sensitive and selective for QNC detection with a detection limit of 7.1 ng m L^-1.It was successfully applied to detect QNC in soil and river water with good recovery rates（92–98%）and a relative standard deviation（n=3）<5%.This is the first time that PMO was used for the development of aptasensor.The success of this study could not only develop a highly sensitive aptasensor for the visual detection of QNC but also provide a general reference strategy for developing sensitive aptasensors and other nucleic acid-related sensors.（2）DTCs pesticides are extensively used as fungicides in a variety of crops during their growth,storage,and shipment.The DTCs residue in foods will seriously harm human health.In this study,a novel multicolor colorimetric sensor was developed for visual screening of total DTCs（total of ziram,thiram,and zineb）based on dithiocarboxylic-mediated growth of gold nanobipyramids（Au NBPs）.We demonstrated that DTCs can absorb on Au NBPs seed’s surface via the formation of Au-S bonds and thus impede the 8-hydroxyquinoline（8-HQ）promoted Au NBPs growth,which generates DTCs concentration corresponding color changes.The developed sensor has vivid color changes,short analysis time,higher sensitivity,and excellent specificity.It can be used to detect as low as 50 n M of total DTCs by bare eye observation and 17-18 n M of total DTCs by UV-visible spectrometry.By using the multicolor sensor,we have successfully screened total DTCs in apple and black tea by bare eye observation and detected total DTCs in apple and black tea by UV-visible spectrometry with a recovery of 90%-104%and a relative standard deviation（RSD,n=5）<5%.The results obtained with our method consisted well of those obtained with high-performance liquid chromatography-tandem mass spectrometry（HPLC-MS/MS）,verifying that our method had good accuracy and reliability.Especially,the visual detection limit of our method is much lower than the maximum residue limit of total DTCs in vegetables and fruits.All the above features make our sensor a promising method for rapid on-site screening of total DTCs in vegetables and fruits by only bare eye observation.