Conjugation-regulating Synthesis Of Porphyrin-based Covalent Organic Frameworks And Its Photodynamic Inactivation Of Bacteria

As the consumption of fruits and vegetables increases,the outbreaks of foodborne diseases caused by the contamination of pathogens in fruits and vegetables are becoming more and more serious.Therefore,effective sterilization methods to inactivate foodborne pathogens in fruits and vegetables are of extreme importance to economy and human health.As a new sterilization method,photodynamic inactivation is attracting a lot of attention due to its efficiency,broad-spectrum and no drug resistance.Photosensitizer plays a key role in photodynamic inactivation.However,traditional photosensitizers mainly suffer from self-aggregation in solution,which highly diminishes their photosensitizing activity.A competent approach to overcome the self-aggregation is the synthesis of porphyrin-based covalent organic frameworks（Por-COFs）with ordered porous structure to immobilize porphyrin with a forced and well-defined arrangement.Moreover,the ordered structure with high porosity provides photosensitizers a good access to O₂and a convenient release of reactive oxygen species.In this thesis,a conjugation-regulating strategy for the synthesis of Por-COFs with high photosensitizing activity was developed for photodynamic inactivation of bacteria.The main results are as follows:Firstly,a conjugation-regulating strategy was developed to design and synthesize Por-COFs with high photosensitizing activity.Terephthalaldehyde（Da）,2,5-Dihydroxyterephthalaldehyde（Dha）and 2,5-Diethoxyterephthalaldehyde（Deta）were selected to condense with 5,10,15,20-Tetrakis（4-aminophenyl）porphyrin（Tph）to synthesize COF-366,Dha Tph and JNU-2,respectively.The effects of synthetic conditions such as solvent,reaction temperature and reaction time on the synthesis of Por-COFs were studied in detail.The prepared Por-COFs were characterized by Fourier transform infrared spectroscopy,powder X-ray diffraction,nitrogen adsorption experiments and so on.All the prepared Por-COFs had good stability,crystallinity and pore properties,and provided promising candidates for subsequent application in photodynamic sterilization.Secondly,the mechanism of conjugation effect on photosensitizing activity was proposed and the potential of Por-COFs for photodynamic inactivation of bacteria was explored.The singlet oxygen quantum yield of JNU-2 was 0.53,significantly higher than that of COF-366（0.23）and Dha Tph（0.35）,indicating the highest photosensitive activity of JNU-2.Compared to COF-366 without any additional conjugation regulation,the hydroxyl groups in Dha and the ethoxy groups in Deta further expanded the conjugation of Dha Tph and JNU-2 via the formation of intralayer extendedπ-cloud delocalization and p-πconjunction,respectively.The extension of conjugation for Por-COFs resulted in the increase of intersystem crossing process and significantly improved their photosensitizing activity.Furthermore,the prepared Por-COFs were applied for photodynamic inactivation of bacteria in fruits and vegetables.After irradiation with a white LED light(150 m W cm^-2)for 20 min,0.1 mg m L^-1JNU-2 exhibited the highest photosensitizing activity and superior antibacterial effects toward Staphylococcus aureus（99.1%）and Escherichia coli（96.8%）.In contrast,COF-366 and Dha Tph under the same conditions only caused 65.0%and 83.7%death of Staphylococcus aureus,and 55.8%and 71.5%death of Escherichia coli,respectively.Moreover,no dark toxicity of 0.5 mg m L^-1JNU-2 was observed.Treatment with 0.1 mg m L^-1 JNU-2 eliminated more than 99%aerobic mesophilic germs from strawberries,lettuce and cherry tomato after white LED irradiation,preliminarily showing the effectiveness of the application potential in photodynamic inactivation of bacteria in fruits and vegetables.