| The contradiction between today’s explosive global population growth and limited arable land is intensifying,especially as the COVID-19 pandemic spreads and rages around the world since 2019,further adding to the concerns about food security.As an indispensable means of production for grain security,pesticides play an important role in improving the quality and yield of food.With the green environmental protection concept has gained popularity,China has put forward a plan to achieve zero growth in the use of pesticides in order to meet the social development.However,traditional pesticide formulations have low effective utilization,poor stability and pose a significant threat to the environment.Therefore,there is an urgent need to develop a pesticide formulation with high stability,long duration and environmental friendliness to achieve efficient use of pesticides and guarantee stable and increased food production.The development of an environmentally responsive controlled release platform can effectively solve these problems by intelligently responding to external environmental stimuli to trigger the on-demand release of active ingredients.Based on this background,this study designed nanodiamond/chitosan(DND/CS)-based composite microspheres with natural polymer chitosan(CS)as the substrate,further optimized its structure and constructed a photothermal response controlled release system to systematically study its photothermal,safety and controlled release properties,and finally applied it to plant growth experiments.This work is designed for enhancing the efficiency of pesticide utilization and providing reference solutions to ensure the sustainable development of green agriculture.The main contents of the thesis include the following:(1)The DND/CS-based composite microspheres were designed and prepared by alkali gelation method using natural polymer CS as the substrate.DND/YS-CS microspheres were fabricated by introducing photothermal DND into the yeast-chitosan(YS-CS)matrix.The successful synthesis of the composite microspheres was characterized by scanning electron microscopy(SEM)and infrared spectroscopy(FT-IR),and the effects of DND content on their photothermal,physical,water absorption and swelling properties as well as drug loading properties were also explored.The results indicated that the DND/YS-CS microspheres have excellent light absorption capacity in the full wavelength band(250-2500 nm)and can effectively convert light energy into heat energy.In addition,the mechanical properties of the DND/YS-CS microspheres are proportional to the DND content and the absorption of water is consistent with a pseudo-second-order kinetics model.The plant growth regulator IBA was selected as the model pesticide,and the loading of 3-Indolebutyric Acid(IBA)could reach41.73 μg/mg when the DND content was 1.2 mg/m L.The zebrafish experiment proved that the DND/YS-CS microspheres had good biosafety.Therefore,DND/YS-CS composite microspheres can be an effective carrier for intelligent photothermal response to pesticide release.(2)The DND/CS-based composite microspheres were structurally optimised to obtain DND/CS-based composite microspheres with a semi-interpenetrating network(semi-IPN)structure and their related performance was investigated.CS/PNIPAm/DND microspheres were prepared using CS as the substrate,poly(N-isopropylacrylamide)(PNIPAm)as the temperature sensitive material,and DND as the photothermal material.The successful synthesis of the microsphere was demonstrated using various characterization tools such as FT-IR,SEM and TGA,and the structurally enhanced microspheres were tested for critical dissolution temperature(LCST)and photothermal properties.The results showed that the LCST of CS/PNIPAm was ～33℃.Meanwhile,the CS/PNIPAm/DND composite microspheres retain the excellent light absorption performance and photothermal conversion ability of DND.Furthermore,the temperature changes observed in five light on/off cycles demonstrate the good photothermal stability of CS/PNIPAm/DND.The synergy of DND and PNIPAm can optimize and expand the use of DND/CS-based composite microspheres as pesticide carriers for controlled release applications.(3)IBA controlled release studies were conducted on DND/CS-based composite microspheres and DND/CS-based semi-IPN composite microspheres,and the practical application of DND/CS-based semi-IPN composite microspheres was investigated.The results showed that the cumulative release rate of DND/CS-based composite microspheres was highest for IBA at 1.2 mg/m L of DND,and the release of DND/CS-based composite microspheres with various contents of DND under light was in accordance with the Korsmeyer-Peppas model.The mechanism of IBA release from composite microspheres under light exposure was investigated,and it was revealed that the diffusion rate of drug molecules in the microspheres was accelerated by the heat generated from the absorption of light by DND under light exposure,which resulted in faster drug release.Based on the study of the controlled release behavior of DND/CS-based composite microspheres,the photothermal response release of the DND/CS-based semi-IPN composite microsphere system was explored.IBA release was accelerated when the temperature was higher than LCST of PNIPAm,and the behavior of IBA release at different temperatures was in accordance with the Korsmeyer-Peppas model.Under intermittent light exposure,the drug release rate shows an "on-off" trend with the switch of light.The mechanism of photothermal response to drug release could be that the warming causes PNIPAm to reach its LCST and the PNIPAm chain undergoes a structural transition from irregular coils to spherical shapes,which in turn regulates the release of the drug.Pea growth experiments have shown that the DND/CS-based semi-IPN composite microspheres controlled release system has great potential for practical application. |
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