Study On Stability And Antibacterial Performance Of Photosensitizer-coated Nanoemulsions

As we all know,people often use high temperature,high pressure,chemical drugs and other treatment methods to prevent the growth and infection of microorganisms in food,especially bacteria.However,these traditional sterilization methods can lead to drug resistance,as well as the destruction of food appearance and loss of nutrients.In recent years,photodynamic sterilization has been widely used to kill pathogenic bacteria,especially food-borne bacteria,due to its high efficiency and low side effects.To address this shortcoming,this thesis has significantly improved the performance of photosensitizers by coating them with nanoemulsions loading.The stability,rheology and photosensitizer encapsulation effect of the prepared photosensitizer coated nanoemulsions were characterized and analyzed and the photodynamic bactericidal performance of the prepared nanoemulsions was evaluated by plate counting method.The feasibility of combining photodynamic bactericidal technology with nanoemulsions technology to stabilize the bactericidal activity was verified,followed by an attempt to enhance the photodynamic bactericidal performance and to explain the mechanisms involved in the antimicrobial activity of the coated photosensitizer nanoemulsions in relation to their fluorescence spectral properties,single-linear oxygen yield and post-sterilization SEM image analysis.The specific aspects of this study will be developed in the following ways:Firstly,natural bamboo yellow was selected as the raw material from which Hypocrellin A(HA)was extracted to be used as a photosensitizer and nanoemulsions coated with HA were prepared using lecithin as an emulsifier.The particle size and Zeta potential of the nanoemulsions on the first and seventh day were tested at different oilwater ratios and emulsifier contents.The results showed that the best stability and the smallest change in particle size of the nanoemulsions of bamboo red mycotoxin were obtained at an oil-water ratio of 1:9 and an emulsifier content of 4.0 wt.%,which was the best preparation process for the screening.Stability tests,rheological tests,absorbance tests and laser confocal microscopy of the emulsions under different conditions showed that the prepared nanoemulsions had good stability,rheological properties and were able to effectively encapsulate the HA.In vitro simulated digestion of the nanoemulsions showed that the nanoemulsions were stable for digestion in humans.In addition,photodynamic bactericidal tests with LED light at 630 nm showed that the nanoemulsions had a good photodynamic bactericidal effect on S.aureus,with a bactericidal rate of over 90%.Therefore,the feasibility of applying the nanoemulsions in the form of encapsulated photosensitizers for the photodynamic bactericidal treatment of food-borne bacteria can be demonstrated.Secondly,curcumin(CUR)and riboflavin tetra butyrate(RTB)were selected as photosensitizers and lecithin was used as an emulsifier to prepare nanoemulsions coated with the CUR/RTB composite photosensitizer.The fluorescence spectra of the nanoemulsions were tested using fluorescence spectroscopy and the results showed that the composite photosensitizer nanoemulsions had higher peak widths as well as higher peak intensities.Stability,rheology and laser confocal microscopy of the nanoemulsions under different environments showed that the composite photosensitizer nanoemulsions were more stable than the other single photosensitizer nanoemulsions and had better rheology and better encapsulation of the two photosensitizers.In vitro simulated digestion tests of the nanoemulsions showed that the prepared nanoemulsions could be stably digested in humans with excellent digestive stability and little change in bio accessibility.In addition,post-light assays of the nanoemulsions for singlet oxygen showed that the composite photosensitizer nanoemulsions could produce singlet oxygen more efficiently.Finally,photodynamic bactericidal tests under LED light at 455 nm and scanning electron microscopy observations of bacteria after sterilization showed that the prepared composite photosensitizer nanoemulsions were more effective against S.aureus and were found to have a greater degree of cell membrane breakage after sterilization.The bactericidal mechanism of the composite photosensitizer nanoemulsions can be summarized as follows: the bi-directional synergistic photosensitizer in the composite photosensitizer nanoemulsions can receive a wider range of wavelengths of light than a single photosensitizer,allowing the photosensitizer to be excited more and therefore produce more singly linear oxygen,which allows for stronger damage to the cell membrane of the target bacteria during the photodynamic bactericidal process,resulting in a stronger bactericidal performance.This allows for greater damage to the cell membrane of the target bacteria during photodynamic sterilization,resulting in greater bactericidal performance.This further demonstrates the feasibility of encapsulating composite photosensitizer nanoemulsions in the photodynamic sterilization of food-borne bacteria and provides an idea to enhance the photodynamic sterilization performance of food-borne bacteria.