由於潔淨用水日漸短缺，科學家著力研究各種水淨化方法，其中以光催化技術作水淨化處理為可行的方法之一。光催化是以半導體光催化劑在光照射下所產生的活性物種（reactive oxidative species）進行消毒，其中的失活原理、各活性物種的作用和活性物種對細菌的攻擊方位，雖然已有廣範的研究，但當中仍有不清之處，比如說過氧化氫(H₂O₂)在光催化失活的作用便是其中之一，在光催化系統中所產生的H₂O₂濃度一般較低，因此其對細菌失活的效能仍然存有爭議。 / 本研究設計一種新的反應器去研究H₂O₂在連續供應模式中的失活動力學。在 8 mM 的H₂O₂下，10⁵的大腸桿菌(Escherichia coli)在8小時內完全失活。而在 2 mM 的H₂O₂ 下，並無出現顯著失活，由於該濃度遠遠高於一般光催化系統所產生的濃度（<50 μM），因此可以推斷，即使一般光催化系統所產生的H₂O₂是連續供應，也不會使細菌失活。然而在光照的情況下，其失活動力學大為不同，在強光照射(200 mW cm⁻²)下，H₂O₂的失活效率顯著增強，證明光照和過氧化氫之間存有協同效應。這現象亦出現於光預處理過(light pretreated)的大腸桿菌，進一步證實了光照改變細菌的生理機能，從而使其易於被H₂O₂失活。 / 其後我們使用RNA測序(RNA sequencing)去檢測的大腸桿菌的基因表達水平在光照下的變化，以便研究光照和H₂O₂之間的協同作用的機理。大多數涉及抵抗氧化的基因，包括過氧化氫酶(catalase, CAT)和超氧化物歧化酶(superoxide dismutase,SOD)的表達、DNA修復及細菌內的鐵含調控等等，其mRNA 水平沒有顯著的增加或減少，只有dps、fes和sodB有明顯的變化。此外，還有幾種調控細胞內的銅合量（cutA和cueR）和細胞膜組成（ompA、ompC、resx和gnsB）的基因在光照下產生顯著變化。 經RNA測序後，我們選定了10個目標基因，並選擇相對的大腸桿菌變異體(mutants)，對比他們和母體(E. coli BW25113)經過光預處理後被H₂O₂的失活效能。雖然這次研究並未找到相關基因，但研究結果表示，光照和H₂O₂的協同效應，應該是光照增加細胞膜的通透性和提高細菌內Fenton劑含量，使細菌內的羥基自由基(·OH)的濃度增加，因此加強對細菌DNA的損傷。 / 最後，我們亦比較了AgBr/Ag/Bi₂WO₆在不同的光源的照射下的對大腸桿菌的光催化失活效率。雖然發光二極管(light emitting diode)和熒光管都常用於室內照明，但AgBr/Ag/Bi₂WO₆的細菌失活效率在兩者的光照下表現出顯著的差異，而不同的發射波長下的細菌失活效率和AgBr/Ag/Bi₂WO₆光學吸收表現出良好的相關性。此外，相對其他顏色的發光二極管，綠色發光二極管照射下在犧牲劑研究(scavenger study)的結果大為不同，進一步表明了光照的發射波長(emissionwavelength)對光催化失活機制的影響。 / 本研究揭示了H₂O₂和光照在光催化失活中的重要性，並演示了H₂O₂和光照射之間的協同作用，也闡明了光照的屬性如何影響光催化下各活性物種的產生。本研究不僅提供了一個新的角度去探討的光照、H₂O₂和細菌的生理狀態在光催化失活中的重要性，也提供了新的方向和方法去研究光催化失活機制的。 / Due to the increasing concern for the need of clean drinking water, different methods for water purification have been developed. Photocatalysis, which makes use of semiconductor photocatalyst for the generation of reactive charged and oxidative species (ROSs) under light irradiation, is one of the most promising methods for water disinfection. The mechanisms of the photocatalytic inactivation have been extensively investigated. Different factors, including the roles of ROSs and the ROSs target site(s) of bacterial cell, were elaborated by different studies. However, there are still controversial issues on the role of H₂O₂ in photocatalytic inactivation. The effectiveness of the low concentration of H₂O₂ in the bacterial inactivation process is still under question. / This study designs a new reactor to study the kinetic of H₂O₂ inactivation in continuous supply mode. Complete inactivation of 5-log Escherichia coli within 8 h is achieved when 8 mM of H₂O₂ is applied. No significant inactivation was observed when 2 mM H₂O₂ is applied, this concentration of H₂O₂ is much higher than that detected in common photocatalytic system (< 50 μM). The results show that H₂O₂ produced by common photocatalytic system is not harmful to bacterial cell, even they are produced continuously. However, when light irradiation of 200 mW cm⁻² , using Xenon lamp as lighting source, was applied to the system, the inactivation efficiency of H₂O₂ was significantly enhanced, which demonstrate the synergistic effect between the light irradiation and H₂O₂. The enhancement of inactivation by H₂O₂ can also be observed with light pretreated E. coli K-12, further confirms that light irradiation alter the physiology of the bacterial cell which increases their sensitivity to H₂O₂. / In order to find out the mechanism(s) of the synergism between the light irradiation and H₂O₂, RNA sequencing (RNA-Seq) was used to reveal the change of gene expression level of the E. coli under light irradiation. The mRNA level of most of the genes involve in catalase (CAT) and superoxide dismutase (SOD) expression, DNA repairing and intracellular iron regulation did not have significant increase or decrease. Only dps, fes and sodB showed significantly changes. Moreover, some genes that related to regulation of intracellular copper (cutA and cueR) and membrane composition (ompA, ompC, resX and gnsB) also showed significantly changes under light irradiation. After the RNA-Seq, ten genes were chosen as the possible target genes that related to the mechanism(s). Then the inactivation of E. coli BW25113 (parental strain) and the isogenic deleted mutants by H₂O₂ with light pretreatment were conducted and compared. Although the gene(s) that directly involved in the mechanisms of the synergy between H₂O₂ and light irradiation are not identified in the study, the results show that genes that are important to bacterial defense of oxidative damages, such those responsible for CAT and SOD expression and DNA repairing, are not involved in the mechanism(s). Increase of cell permeability and intracellular Fenton’s reagent content should be the main causes for the enhancement of H₂O₂ under light irradiation. / Finally, the inactivation efficiency of E. coli K-12 using AgBr/Ag/Bi₂WO₆ under different lighting sources is compared. The results show that inactivation efficiency under different emission wavelength are highly correlated with the optical absorption of the AgBr/Ag/Bi₂WO₆. Photocatalytic inactivation under two indoor lighting sources, LED lamps and Fluorescence tubes, also showed significant difference. The result of scavenger study under green LED lamps is completely different from those under other colour of LED lamps, indicates that emission wavelength also has great influence in photocatalytic inactivation mechanisms. / This study reveals the roles of H₂O₂ and light irradiation in photocatalytic inactivation and demonstrates the synergism between the H₂O₂ and light irradiation. The influence of the properties of light irradiation, including the light intensity and major emission wavelength, on the ROSs production by photocatalyst is also reported as well. This study not only provides a new perspective to the importance of light irradiation, H₂O₂ and the physiology of bacteria in photocatalytic inactivation, but a new approach in the investigation of photocatalytic inactivation mechanisms as well. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Ng Tsz Wai. / Thesis (Ph.D.) Chinese University of Hong Kong, 2014. / Includes bibliographical references (leaves 111-131). / Abstracts also in Chinese.
| Identifer | oai:union.ndltd.org:cuhk.edu.hk/oai:cuhk-dr:cuhk_1202851 |
| Date | January 2014 |
| Contributors | Ng Tsz Wai (author.), Wong, Po-Keung (thesis advisor.), Chinese University of Hong Kong Graduate School. Division of Life Sciences. (degree granting institution.) |
| Source Sets | The Chinese University of Hong Kong |
| Language | English, Chinese |
| Detected Language | English |
| Type | Text, bibliography, text |
| Format | electronic resource, electronic resource, remote, 1 online resource (xix, 131 leaves) : illustrations (chiefly color), computer, online resource |
| Rights | Use of this resource is governed by the terms and conditions of the Creative Commons “Attribution-NonCommercial-NoDerivatives 4.0 International” License (http://creativecommons.org/licenses/by-nc-nd/4.0/) |
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