Preparation Of Insecticides-Loaded Mesoporous Silica Nanoparticles And Their Biological Activity On Plutella Xylostella

Plutella xylostella L.is a cosmopolitan lepidopteron insect pest of numerous vegetables and crops.The extensive use of insecticides has resulted in the emergence of resistance in P.xylostella.Indoxacarb belongs to a class of insecticides known as oxadiazines,is the first commercialized pyrazoline kind voltage-dependent sodium channel blocker.A moderate level of resistance to indoxacarb has evolved in field populations of P.xylostella in China.Moreover,this insect has developed resistance against methoxyfenozide,belongs to a group of biorational insecticides known as insect growth regulators for controlling lepidopteran insect pests.Thus,development of innovative strategies to overcome the insecticide resistance and effective control of P.xylostella is highly desirable.Inspired by the concept and breakthrough of nanomedical strategies to treat multidrug resistance,nanotechnology may find potential application in overcoming or delaying insecticide resistance.Carbon dots-embedded fluorescent mesoporous silica nanoparticles（FL-SiO₂NPs）with different sizes were successfully developed using classical sol-gel method and calcination.Indoxacarb-loaded fluorescent silica nanoparticles（IN@FL-SiO₂NPs）with the particle size of 187 nm and loading content of 24%were facilely prepared using solution adsorption method,while two sizes of methoxyfenozide-loaded fluorescent silica nanoparticles（Me@FL-SiO₂NPs-70 nm and Me@FL-SiO₂NPs-150 nm）were also successfully synthesized with loading content of 15%and 16%,respectively.The nanoparticles were fully characterized by scanning and transmission electron microscopy,Fourier transform infrared spectra,surface areas,pore size and thermogravimetric analysis.Indoxacarb from IN@FL-SiO₂NPs was released at different p H values（4.8,7.2 and 8.9）.The release of indoxacarb from IN@FL-SiO₂NPs was p H sensitive,and indoxacarb released faster in basic solutions compared to acidic and neutral media.This p H responsive release pattern can find potential application in sustainable plant protection.In order to investigate the effect of different particle sizes on the release behaviors of insecticide-loaded NPs,Me@FL-SiO₂NPs-70 nm and Me@FL-SiO₂NPs-150nm were released at p H 7.5.Methoxyfenozide from Me@FL-SiO₂NPs-70 nm released a little faster compared to that from Me@FL-SiO₂NPs-150 nm.IN@FL-SiO₂NPs,Me@FL-SiO₂NPs-70 nm and Me@FL-SiO₂NPs-150 nm exhibited better insecticidal activity against P.xylostella than indoxacarb TC and methoxyfenozide TC under the same doses of active ingredient applied.IN@FL-SiO₂NPs showed a mortality of 90%,while indoxacarb TC displayed 76%at the highest concentration of 25 mg/L.Similarly,at all concentrations the mortality（%）was higher in IN@FL-SiO₂NPs treated insects than those treated with indoxacarb TC.Moreover,the lethal concentration such as LC₅₀value was 1.6 mg/L for IN@FL-SiO₂NPs,while 2.4 mg/L for indoxacarb TC after 48 h exposure.In case of methoxyfenozide,more than 90%mortality was observed for both sizes of Me@FL-SiO₂NPs,while methoxyfenozide TC exhibited 81%at the maximum concentration of 200 mg/L after 72h.The LC₅₀value was 24 mg/L for methoxyfenozide TC,14 mg/L for Me@FL-SiO₂ NPs-70 nm and 15mg/L for Me@FL-SiO₂ NPs-150 nm after 72 h exposure,respectively.Moreover,the activities of detoxification enzymes including GST,Car E,ACh E and P450of P.xylostella were suppressed by treatment with insecticide-loaded NPs,showed that NPs could also involve in reduction of enzymes.Furthermore,the entering of FL-SiO₂NPs into the midgut of P.xylostella was confirmed by confocal laser scanning microscope（CLSM）.For comparison,P.xylostella under treatment with water as control was also observed under CLSM.The control exhibited no fluorescent signal,while the larvae treated with FL-SiO₂NPs showed strong fluorescence under a laser excitation wavelength of 448 nm.In this thesis,the development,mechanism of nano formulation of insecticides in P.xylostella was studied.The remarkable results of higher mortality against P.xylostella along with decreased enzymes activities predicts that nano-based delivery systems of insecticides could be possibly used to delay or overcome the insecticide resistance against many insects.In the future,we will conduct the bioactive and biochemical studies of insecticide loaded FL-SiO₂NPs on insecticide resistant strains of P.xylostella.