Photocatalytic Bacterial Inactivation by the Carbon-based Materials: Bactericidal Performances and Mechanisms

The ever-increasing global challenges towards water security and shortage emphasize the importance of developing efficient and low-cost disinfection technologies. Currently, photocatalysis has been extensively studied for water disinfection, presenting as a “green”, cost-effective, and promising strategy. This technique only requires light-responsive semiconductors and suitable light source such as visible light or even sunlight, during which bacteria species are killed by the photoinduced reactive species. Unlike traditional water disinfection techniques, such as ozonation, chlorination and ultraviolet irradiation, there are less disinfection byproducts formation and bacterial resistance evolution during the photocatalytic inactivation process. Therefore, extensive efforts have been devoted to developing efficient photocatalyst for water disinfection. However, the high cost and low efficiency of photocatalysts restricted the real-world widespread application.In this regard, this study investigated visible-light-driven, low-cost, and efficient carbon-based photocatalysts for different purpose of water disinfection. Firstly, to address drinking water safety, the treated rape pollen-MoS2 (PM) biohybrid was fabricated through a facile one-pot hydrothermal treatment method, by in situ anchoring of defective MoS2 nanosheets onto the treated rape pollen skeleton. The four common waterborne bacterial species (i.e., Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, and Bacillus pumilus) were selected to evaluate the photocatalytic bacterial inactivation performances of the PM biohybrids. Results showed that the PM biohybrid showed excellent dispersibility, high stability, and efficient charge-carrier separation and migration ability, resulting in the highly enhanced photocatalytic inactivation performance towards various waterborne bacteria under different light sources. The inactivation mechanisms were systematically investigated. Reactive species (RSs), including electrons, holes, and reactive oxygen species (•O2- and •OH), played major roles in inactivating bacteria. The antioxidant system of bacteria exhibited a self-protection capacity by eliminating the photo-generated RSs from PM biohybrid at the early stage of inactivation. With the accumulation of RSs, the cell membrane and membrane-associated functions were destroyed, as suggested by the collapse of cell envelope and subsequent loss of cell respiration and ATP synthesis capacity. The microscopic images further confirmed the destruction of the bacterial membrane. After losing the membrane barrier, the oxidation of cytoplasmic proteins and lipids caused by invaded RSs occurred readily. Finally, the leakage of DNA and RNA announced the irreversible death of bacteria. These results indicated that the bacterial inactivation began with the membrane rupture, followed by the oxidation and leakage of intracellular substances.Secondly, to address irrigation water safety towards agricultural application, graphitic carbon nitride (g-C3N4), a typical carbon-based material with good physic-chemical stability and biocompatibility as well as visible light response, was introduced. Besides, the plant-pathogen, Rhizobium radiobacter (also name as Agrobacterium radiobacter or Agrobacterium tumefaciens), which is a ubiquitous plant pathogen to cause the crown gall diseases, was used as model bacterium species. the results showed that the g-C3N4 could act as an effective antagonist to the ubiquitous plant-pathogen R. radiobacter with good photostability and reusability under visible light. The applying of g-C3N4 efficiently improved the survivability of tobacco seedlings under the stress of R. radiobacter and showed no adverse effect on tobacco growth compared with the common metal-based TiO2 and CdS photocatalysts. The systematic mechanism studies revealed that the photoinduced reactive species (RSs) were strongly involved in the photocatalytic bacterial inactivation process, with an effectiveness of h+ >·O2- >·OH > H2O2 > e– > 1O2. The direct contact between g-C3N4 and bacterial cells was also essential for the effective bacterial inactivation. Although bacterial self-protection system (SOD and CAT enzymes) functioned in the initial period, the accumulated RSs damaged cell membrane structure as well as membrane-associated respiration and ATP synthesis ability, finally leading to the leakage of cellular building blocks (K+, TOC, DNA and RNA) and irreversible cell death. This study illustrated that the g-C3N4 could be used as an ideal photocatalytic bactericide towards agriculture application to improve the plant resistance to pathogens with good biocompatibility and low-cost.In summary, the project provides two promising light-driven carbon-based biocompatible photocatalysts for water treatment from the perspective of different practical applications. Besides, it also provides a new insight into the combination of semiconductors with earth-abundant carbon-based biomaterials for fabricating high-performance photocatalysts. The mechanistic study contributes to better understand the photocatalytic bacterial inactivation process from macro to micro dimension.日益嚴峻的全球水安全和水資源短缺問題強調了發展高效和低成本消毒技術的重要性。目前，光催化技術已被廣泛應用於水消毒領域，是一種綠色、經濟、有發展前途的技術。這項技術只需要合適的光響應半導體和光源，如陽光。在此過程中，半導體吸收光子產生電子和空穴以及次生活性氧自由基，用以殺滅細菌。與傳統的臭氧、氯化、紫外線等水消毒技術不同，光催化滅活過程中無消毒副產物的形成且可減少細菌耐藥性，從而實現有效水體除菌。因此，人們致力於開發高效的水消毒光催化劑。然而，光催化劑的高成本和低效率限制了其在現實世界中的廣泛應用。基於此，本研究探討了可見光驅動，低成本，高效率的碳基光催化劑對不同水體的滅菌，包括飲用水和灌溉水。首先，為了解決飲用水安全問題，通過一種簡便的水熱處理方法，將有缺陷的MoS2納米片原位錨定在處理過的油菜花粉碳骨架上，成功制備了處理過的油菜花粉-MoS2復合催化劑。四種常見的水性細菌（大腸桿菌、銅綠假單胞菌、金黃色葡萄球菌和短小芽孢桿菌）被選來用於對復合催化劑的光催化殺菌性能評價。結果表明，復合催化劑具有良好的分散性、高穩定性、高效的電荷載體分離和遷移能力，在不同光源下對各種水性細菌的光催化滅活性能都有很大的提高。隨後，其失活機理進行了系統性的研究。活性自由基，包括電子、空穴和活性，在細菌滅活過程中中起到了主要作用。細菌的抗氧化系統在滅活初期通過在一定程度上消除復合催化劑產生的活性自由基而表現出一定的自我保護能力。隨著活性自由基的積累，細胞膜及膜相關功能被破壞，表現為細胞膜崩塌，細胞呼吸和ATP合成能力喪失。顯微圖像進一步證實了細菌膜的破壞。在失去膜屏障後，入侵的活性自由基容易引起細胞質蛋白質和脂質的氧化。最後，DNA和RNA的水解泄漏宣告了細菌不可逆轉的死亡。這些結果表明，細菌的失活是從細胞膜破裂開始的，隨後是細胞內物質的氧化和滲漏。其次，為了解決灌溉用水安全問題，本研究介紹了一種具有良好理化穩定性和生物相容性以及可見光響應的典型碳基材料石墨碳氮化物（g-C3N4）。此外，以廣泛存在可引起冠癭病的植物病原菌&根瘤菌（又稱農桿菌或農桿菌腫瘤學）為模型菌作為驗證對象。結果表明，g-C3N4對植物病原菌具有良好的光殺菌效果， 並表現了良好的穩定性和可重復利用性，可作為一種有效的植物病原菌拮抗劑。與常見的金屬基光催化劑TiO2和CdS相比，g-C3N4的應用有效地提高了煙草幼苗在農桿菌脅迫下的存活能力，且對煙草生長無不良影響。系統機理研究表明，光誘導活性自由基參與了光催化細菌的滅活過程，其作用效果表現為空穴大於超氧自由基大於羥基自由基大於過氧化氫大於電子大於單線態氧。同時，g-C3N4與細菌細胞的直接接觸對有效滅活也至關重要。雖然細菌自我保護系統（SOD和CAT酶）在初期起作用，但積累的活性自由基可導致細胞膜結構、膜相關呼吸系統和ATP合成能力受到損傷，最終導致胞內基礎物質（K+、TOC、DNA、RNA）的滲漏和不可逆細胞死亡。本研究表明，g-C3N4可作為一種理想的光催化殺菌劑應用於農業應用，以提高植物對病原菌的抗性，具有良好的生物相容性和低成本。綜上所述，本項目從不同的實際應用角度，為水處理提供了兩種前景廣闊的光驅動碳基生物相容性光催化劑。同時，它還為為制備高性能光催化劑，比如半導體與地球豐富的碳基生物的結合，提供了新的見解。本研究有助於更好地了解光催化細菌從宏觀到微觀的滅活過程。XIAO, Kemeng.Ph.D. Chinese University of Hong Kong 2021.Includes bibliographical references (leaves )Abstracts in English and ChineseTitle from PDF title page (viewed on …