Disinfection of Legionella pneumophila by photocatalytic oxidation

Photocatalytic Bacterial Destruction and Mineralization by TiO2-Based Photocatalysts: A Mini Review

This work presents an overview of the reports on the bacterial cell photocatalytic destruction and mineralization process in the presence of TiO2-based photocatalysts. The presented research included experiments conducted in air and water. Numerous works confirmed that a photocatalytic process with TiO2 led to bacteria and their organic residues' mineralization. Additionally, based on the obtained results, a possible two-stage mechanism of photocatalytic mineralization in the presence of TiO2-based materials was proposed. To help future studies, challenges of photocatalytic microorganism mineralization are also proposed. There are some aspects that need to be addressed, such as the lack of standardization of conducted research or relatively small amount of research on photocatalytic microorganism mineralization. According to our best knowledge, in the available literature, no work regarding a summary of previous research on photocatalytic bacterial mineralization process was found.

Assessing impacts of municipal wastewater treatment plant upgrades on bacterial hazard contributions to the receiving urban river using SourceTracker

Upgrading municipal wastewater treatment plants (MWTPs) has been implemented in many megacities of China to reduce the discharges of nutrients and other pollutants and improve water quality of highly urbanized rivers. However, the contribution of MWTP discharge to bacterial hazards in the receiving rivers after upgrades has been largely unknown. In this study, high-throughput sequencing and shotgun metagenomics were applied to investigate the changes in the abundance, composition, potential risks, and contributions of bacteria and antibiotic resistance genes (ARGs) from effluent to receiving river after upgrading the third-largest MWTP in China with denitrification biofilters, ultrafiltration, ozonation, and disinfection processes. The annual loadings of total nitrogen and 27 types of pharmaceuticals were reduced by 42.4% ± 13.2% and 46.2% ± 15.4%, respectively. Bacterial biomass decreased from (3.58 ± 0.49) to (1.23 ± 0.27) × 107 16S rRNA gene copies/mL, and identified biomarkers in effluent and downstream shifted due to the adopted processes. Opportunistic pathogen bacteria downstream were also reduced. Although the relative abundance of total ARGs in MWTP effluent increased from 1.10 ± 0.02 to 2.19 ± 0.03 copies/16S rRNA gene after upgrades, that of total and high-risk ARGs downstream showed no significant difference. More importantly, the Bayesian-based SourceTracker method provided valuable insight by revealing that the contributions of MWTP discharge to downstream bacteria (from 44.2% ± 1.5%-31.4% ± 0.9%) and ARGs (from 61.2% ± 5.3%-47.6% ± 4.1%) were significantly reduced following the upgrades, indicating upgrading MWTP showed integrated benefits to the bacterial hazards in the receiving river. This study provides useful information for better control of bacterial hazard risks and operational strategy for the improvement of the urban aquatic ecosystem.

Biosafe Bi2O2Se ultrathin nanosheet for water disinfection via solar-induced photothermal synergistic effect

Solar-induced sterilization via photothermal synergy has attracted enormous attention due to its zero-energy consumption and the elimination of hazardous chemical disinfectant. Herein, we successfully synthesized a super biosafety Bi₂O₂Se with crossed nanosheet structure (Bi₂O₂Se-CN) for the sterilization of Escherichia coli (E. coli) via solar-induced photothermal synergistic effect. In comparison to bulk Bi₂O₂Se, the lower light reflection and more efficient photogenerated charge carrier separation under visible-infrared light irradiation resulted in the excellent sterilization effect of Bi₂O₂Se-CN, with a sterilization efficiency of 99.9% under the synergistic effect of light and heat. The crossed ultrathin nanosheet structure and suitable band gap width of Bi₂O₂Se-CN are fundamental reasons for its enhanced light absorption and charge carrier separation efficiency. Mechanistic studies showed that Bi₂O₂Se-CN can completely inactivate bacteria via generating a large amount of reactive oxygen species (•O₂^-, •OH, and ¹O₂) to attack the cell membrane, which further resulted in the reduced activity of intracellular enzymes and the leakage of intracellular contents. The biosafety property of Bi₂O₂Se-CN was confirmed by in vivo toxicological evaluation on the mice model. This work provided new ideas for the design of more efficient, energy-saving, biocompatible and environmental friendly solar water purification projects.

A Review of the Use of Semiconductors as Catalysts in the Photocatalytic Inactivation of Microorganisms

Obtaining clean and high-quality water free of pathogenic microorganisms is a worldwide challenge. Various techniques have been investigated for achieving an effective removal or inactivation of these pathogenic microorganisms. One of those promising techniques is photocatalysis. In recent years, photocatalytic processes used semiconductors as photocatalysts. They were widely studied as a green and safe technology for water disinfection due to their high efficiency, being non-toxic and inexpensive, and their ability to disinfect a wide range of microorganisms under UV or visible light. In this review, we summarized the inactivation mechanisms of different waterborne pathogenic microorganisms by semiconductor photocatalysts. However, the photocatalytic efficiency of semiconductors photocatalysts, especially titanium dioxide, under visible light is limited and hence needs further improvements. Several strategies have been studied to improve their efficiencies which are briefly discussed in this review. With the developing of nanotechnology, doping with nanomaterials can increase and promote the semiconductor’s photocatalytic efficiency, which can enhance the deactivation or damage of a large number of waterborne pathogenic microorganisms. Here, we present an overview of antimicrobial effects for a wide range of nano-photocatalysts, including titanium dioxide-based, other metal-containing, and metal-free photocatalysts. Promising future directions and challenges for materials research in photocatalytic water disinfection are also concluded in this review.

Portable point-of-use photoelectrocatalytic device provides rapid water disinfection

Portable water purification devices are needed to provide safe drinking water in rural communities, developing communities with low quality centralized water distribution, and military or recreational applications. Filtration, ultraviolet light, or chemical additives provide a spectrum of alternatives to remove pathogens from water. For the first time, we design, fabricate and demonstrate the performance of a small portable photoelectric point-of-use device, and document its performance on pathogen inactivation. The device utilizes a commercial teacup from which TiO₂ nanotube photoanodes were produced in-situ and, with a small rechargeable battery powered 365 nm light emitting diode, was able to achieve 5-log inactivation of Escherichia coli in 10 s and 2.6-log of Legionella in 60 s of treatment in model water samples. Treatment of natural water achieved a 1-log bacteria inactivation after 30 s due to matrix effects. The electro-photocatalytic disinfection reactor in a kup (e-DRINK) can provide a feasible and affordable solution to ensure access to clean water. More broadly, this work demonstrates the potential for illumination to improve the efficiency of electrocatalytic surfaces.

Modified graphene supported Ag-Cu NPs with enhanced bimetallic synergistic effect in oxidation and Chan-Lam coupling reactions

Herein, well dispersed Ag–Cu NPs supported on modified graphene have been synthesized via a facile and rapid approach using sodium borohydride as a reducing agent under ambient conditions. Dicyandiamide is selected as an effective nitrogen source with TiO₂ as an inorganic material to form two kinds of supports, labelled as TiO₂–NGO and NTiO₂–GO. Initially, the surface area analysis of these two support materials was carried out which indicated that N-doping of GO followed by anchoring with TiO₂ has produced support material of larger surface area. Using both types of supports, ten nano-metal catalysts based on Ag and Cu were synthesized. Benefiting from the bimetallic synergistic effect and larger specific surface area of TiO₂–NGO, Cu@Ag–TiO₂–NGO is found to be a highly active and reusable catalyst out of other synthesized catalysts. It exhibits excellent catalytic activity for oxidation of alcohols and hydrocarbons as well as Chan–Lam coupling reactions. The nanocatalyst is intensively characterized by BET, SEM, HR-TEM, ICP-AES, EDX, CHN, FT-IR, TGA, XRD and XPS.

Cu@Ag–TiO₂–NGO prepared from modified graphene by simple methodology exhibits enhanced catalytic activity towards oxidation and Chan–Lam coupling due to the synergistic effect between Ag and Cu NPs.

Biomedical Waste Management by Using Nanophotocatalysts: The Need for New Options

Biomedical waste management is getting significant consideration among treatment technologies, since insufficient management can cause danger to medicinal service specialists, patients, and their environmental conditions. The improvement of waste administration protocols, plans, and policies are surveyed, despite setting up training programs on legitimate waste administration for all healthcare service staff. Most biomedical waste substances do not degrade in the environment, and may also not be thoroughly removed through treatment processes. Therefore, the long-lasting persistence of biomedical waste can effectively have adverse impact on wildlife and human beings, as well. Hence, photocatalysis is gaining increasing attention for eradication of pollutants and for improving the safety and clearness of the environment due to its great potential as a green and eco-friendly process. In this regard, nanostructured photocatalysts, in contrast to their regular counterparts, exhibit significant attributes such as non-toxicity, low cost and higher absorption efficiency in a wider range of the solar spectrum, making them the best candidate to employ for photodegradation. Due to these unique properties of nanophotocatalysts for biomedical waste management, we aim to critically evaluate various aspects of these materials in the present review and highlight their importance in healthcare service settings.

Prediction of a Stable Organic Metal-Free Porous Material as a Catalyst for Water-Splitting

It is of practical significance to find organic metal-free catalyst materials. We propose a new graphene-like carbon nitride structure, which was able to meet these requirements well. Its primitive cell consists of eight carbon atoms and six nitrogen atoms. Hence, we called this structure g–C8N6. The stability of the structure was verified by phonon spectroscopy and molecular dynamics simulations. Then its electronic structure was calculated, and its band edge position was compared with the redox potential of water. We analyzed its optical properties and electron–hole recombination rate. After the above analysis, it is predicted that it is a suitable photocatalyst material. To improve its photocatalytic performance, two methods were proposed: applied tensile force and multilayer stacking. Our research is instructive for the photocatalytic application of this kind of materials.

A review on ZnO nanostructured materials: energy, environmental and biological applications

Zinc oxide (ZnO) is an adaptable material that has distinctive properties, such as high-sensitivity, large specific area, non-toxicity, good compatibility and a high isoelectric point, which favours it to be considered with a few exceptions. It is the most desirable group of nanostructure as far as both structure and properties. The unique and tuneable properties of nanostructured ZnO shows excellent stability in chemically as well as thermally stable n-type semiconducting material with wide applications such as in luminescent material, supercapacitors, battery, solar cells, photocatalysis, biosensors, biomedical and biological applications in the form of bulk crystal, thin film and pellets. The nanosized materials exhibit higher dissolution rates as well as higher solubility when compared to the bulk materials. This review significantly focused on the current improvement in ZnO-based nanomaterials/composites/doped materials for the application in the field of energy storage and conversion devices and biological applications. Special deliberation has been paid on supercapacitors, Li-ion batteries, dye-sensitized solar cells, photocatalysis, biosensors, biomedical and biological applications. Finally, the benefits of ZnO-based materials for the utilizations in the field of energy and biological sciences are moreover consistently analysed.

Superior disinfection effect of Escherichia coli by hydrothermal synthesized TiO2-based composite photocatalyst under LED irradiation: Influence of environmental factors and disinfection mechanism

The photocatalytic inactivation of Escherichia coli (E. coli) under light-emitting diode (LED) light irradiation was performed with P/Ag/Ag₂O/Ag₃PO₄/TiO₂ photocatalyst to investigate the photocatalytic bactericidal activity. Our work showed that this composite photocatalyst possessed remarkable bacterial disinfection ability and could completely inactivate 10⁸ cfu/mL of E. coli within just 40 min under the optimum catalyst loading of 0.5 g/L. The effects of different environmental factors, including light wavelength, light intensity, temperature, solution pH and inorganic ions, on the inactivation efficiency were evaluated. The results showed that bacteria inactivation by P/Ag/Ag₂O/Ag₃PO₄/TiO₂ was more favorable with blue colored LED irradiation, light intensity at 750 W/m², temperature in the range of 30-37 °C and pH values at natural or slightly alkaline condition. The existence of different inorganic ions under normal environmental level had no significant impact on the bactericidal performance. In addition, during the inactivation process, the morphology changes of E. coli cells were directly observed by scanning electron microscope (SEM) and further proved by the measurement of K⁺ leakage from the inactivated E. coli. The results demonstrated that the photocatalytic inactivation caused drastic damage on bacterial cells membrane. Furthermore, the mechanisms of photocatalytic bacterial inactivation were also systemically studied and the results confirmed that the excellent disinfection activity of P/Ag/Ag₂O/Ag₃PO₄/TiO₂ resulted from the major reactive species: h⁺ and ·O₂^- from photocatalytic process instead of the leakage of Ag⁺ (≤0.085 ± 0.005 mg/L) from photocatalyst. These results indicate that P/Ag/Ag₂O/Ag₃PO₄/TiO₂ photocatalyst has promising potential for real water sterilization application.

Photocatalytic oxidation of nitrogen oxides over {001}TiO2: the influence of F- ions

A series of anatase TiO₂ nanosheets with different percentage of {001} facets ({001}TiO₂) were synthesized through a hydrothermal route using tetrabutyltitanate as a titanium precursor and HF as a shape controlling agent. The amount of HF exhibits an obvious influence on the structures and activities of TiO₂ samples. The adsorbed surface F^- ions on the {001} facets of the anatase TiO₂ were removed by washing them with NaOH solution. The as-prepared catalysts were characterized by X-ray diffraction, Brunner-Emmet-Teller measurements, ultraviolet-visible diffuse reflectance spectroscopy, scanning electron microscopy, transmission electron microscopy techniques, and X-ray photoelectron spectroscopy analysis. The results indicated that all the as-prepared catalysts showed an anatase crystalline and nanosheet structure, as well as a strong ultraviolet light absorbance. With the increase of HF content, the crystallite size and the percentage of {001} facets increased first and later decreased, state the opposite change observed in BET. When the content of HF was 4.4 mL, the percentage of {001} facets reached the maximum up to 61.62%. After all samples were treated with 0.1 M NaOH solution, the percentage of {001} facets increased to a maximum of 64.46%. All the samples washed by NaOH solution exhibited much higher photocatalytic activity for NO_x oxidation under UV light irradiation than P25, {101}TiO₂, and {001}TiO₂ without NaOH washing, suggesting that the surface F^- ions inhibited the photocatalytic NO_x oxidation. Moreover, the results showed that the NaOH-washed {001}TiO₂ has a high humidity tolerance.

Introduction

The last 50 years have witnessed a growing awareness of the fragile state of most of the planets’ drinking water resources. Access to freshwater will become even more important in the near future, as the world’s population rises from 7 billion today to 9 billion by 2050. The World Health Organization (WHO) has estimated that 80 % of illnesses in the developing world are water related, resulting from poor water quality and lack of sanitation [1]. There are 3.3 million deaths each year from diarrheal diseases caused by bacteria such as Escherichia coli, Salmonella sp. and Cholera sp., parasites and viral pathogens. In the 1990s, the number of children who died of diarrhoea was greater than the sum of people killed in conflicts since World War II [2]. It is also estimated that around 4 billion people worldwide experience to have no or little access to clean and sanitized water supply, and millions of people died of severe waterborne diseases annually [3, 4].

Forming heterojunction: an effective strategy to enhance the photocatalytic efficiency of a new metal-free organic photocatalyst for water splitting

Photocatalytic water splitting is a new technology for the conversion and utilization of solar energy and has a potential prospect. One important aspect of enhancing the photocatalytic efficiency is how to improve the electron-hole separation. Up to now, there is still no ideal strategy to improve the electron-hole separation. In this article, for metal-free organic photocatalysts, we propose a good strategy- forming heterojunction, which can effectively improve the electron-hole separation. We provide a metal-free organic photocatalyst g-C₁₂N₇H₃ for water splitting. The stability of g-C₁₂N₇H₃ has been investigated, the X-ray diffraction spectra has been simulated. Using first-principles calculations, we have systematically studied the electronic structure, band edge alignment, and optical properties for the g-C₁₂N₇H₃. The results demonstrated that g-C₁₂N₇H₃ is a new organocatalyst material for water splitting. In order to enhance the photocatalytic efficiency, we provided four strategies, i.e., multilayer stacking, raising N atoms, forming g-C₉N₁₀/g-C₁₂N₇H₃ heterojunction, and forming graphene/g-C₁₂N₇H₃ heterojunction. Our research is expected to stimulate experimentalists to further study novel 2D metal-free organic materials as visible light photocatalysts. Our strategies, especially forming heterojunction, will substantially help to enhance the photocatalytic efficiency of metal-free organic photocatalyst.

TiO2/CdS porous hollow microspheres rapidly synthesized by salt-assistant aerosol decomposition method for excellent photocatalytic hydrogen evolution performance

TiO₂/CdS porous hollow microspheres have been one-pot rapidly synthesized and exhibit an excellent photocatalytic activity of 996 μmol h⁻¹ under visible light.

Comparison of Infectious Agents Susceptibility to Photocatalytic Effects of Nanosized Titanium and Zinc Oxides: A Practical Approach

Nanotechnology contributes towards a more effective eradication of pathogens that have emerged in hospitals, veterinary clinics, and food processing plants and that are resistant to traditional drugs or disinfectants. Since new methods of pathogens eradication must be invented and implemented, nanotechnology seems to have become the response to that acute need. A remarkable achievement in this field of science was the creation of self-disinfecting surfaces that base on advanced oxidation processes (AOPs). Thus, the phenomenon of photocatalysis was practically applied. Among the AOPs that have been most studied in respect of their ability to eradicate viruses, prions, bacteria, yeasts, and molds, there are the processes of TiO2/UV and ZnO/UV. Titanium dioxide (TiO2) and zinc oxide (ZnO) act as photocatalysts, after they have been powdered to nanoparticles. Ultraviolet (UV) radiation is an agent that determines their excitation. Methods using photocatalytic properties of nanosized TiO2 and ZnO prove to be highly efficient in inactivation of infectious agents. Therefore, they are being applied on a growing scale. AOP-based disinfection is regarded as a very promising tool that might help overcome problems in food hygiene and public health protection. The susceptibility of infectious agents to photocatalylic processes can be generally arranged in the following order: viruses > prions > Gram-negative bacteria > Gram-positive bacteria > yeasts > molds.

TiO2 hybrid photocatalytic systems: impact of adsorption and photocatalytic performance

For the past 40 years, titanium dioxide (TiO2) nanomaterials have attracted immense attention because of their potential applications in the photodegradation of organic pollutants, photocatalytic water splitting for H2 generation, and dye-sensitized solar cells. Despite the fact that the potential applications of TiO2 nanoparticles are ubiquitous, they are not problem free, examples include a large interfacial area that causes slow charge carrier transport, a wide optical band gap that leads to limited applications using solar light, and single-phase and nanoscales features that induce fast recombination of photo-reduced carriers. Therefore, this review highlights the development associated with the adsorption photocatalysis hybrid system for treating wastewater. The immobilization of TiO2 photocatalysts in metal oxide, carbon, and ceramic materials to form TiO2 hybrid systems could prevent the problem of particle recovery, adsorption capacity, and the separation process. Such hybrid systems require significant effort of optimizing the specific surface area-to-volume ratio of the supported photocatalysts with its photocatalytic activities.

Advances in photocatalytic disinfection of bacteria: Development of photocatalysts and mechanisms

Photocatalysis has attracted worldwide attention due to its potential in solar energy conversion. As a "green" advanced oxidation technology, it has been extensively used for water disinfection and wastewater treatment. This article provides a review of the recent progress in solar energy-induced photocatalytic disinfection of bacteria, focusing on the development of highly efficient photocatalysts and their underlying mechanisms in bacterial inactivation. The photocatalysts are classified into TiO2-based and non-TiO2-based systems, as TiO2 is the most investigated photocatalyst. The synthesis methods, modification strategies, bacterial disinfection activities and mechanisms of different types of photocatalysts are reviewed in detail. Emphasis is given to the modified TiO2, including noble metal deposition, non-metal doping, dye sensitization and composite TiO2, along with typical non-TiO2-based photocatalysts for bacterial disinfection, including metal oxides, sulfides, bismuth metallates, graphene-based photocatalysts, carbon nitride-based photocatalysts and natural photocatalysts. A simple and versatile methodology by using a partition system combined with scavenging study is introduced to study the photocatalytic disinfection mechanisms in different photocatalytic systems. This review summarizes the current state of the work on photocatalytic disinfection of bacteria, and is expected to offer useful insights for the future development in the field.

Comparing methods of determining Legionella spp. in complex water matrices

BACKGROUND

Legionella testing conducted at environmental laboratories plays an essential role in assessing the risk of disease transmission associated with water systems. However, drawbacks of culture-based methodology used for Legionella enumeration can have great impact on the results and interpretation which together can lead to underestimation of the actual risk. Up to 20% of the samples analysed by these laboratories produced inconclusive results, making effective risk management impossible. Overgrowth of competing microbiota was reported as an important factor for culture failure. For quantitative polymerase chain reaction (qPCR), the interpretation of the results from the environmental samples still remains a challenge. Inhibitors may cause up to 10% of inconclusive results. This study compared a quantitative method based on immunomagnetic separation (IMS method) with culture and qPCR, as a new approach to routine monitoring of Legionella.

RESULTS

First, pilot studies evaluated the recovery and detectability of Legionella spp using an IMS method, in the presence of microbiota and biocides. The IMS method results were not affected by microbiota while culture counts were significantly reduced (1.4 log) or negative in the same samples. Damage by biocides of viable Legionella was detected by the IMS method. Secondly, a total of 65 water samples were assayed by all three techniques (culture, qPCR and the IMS method). Of these, 27 (41.5%) were recorded as positive by at least one test. Legionella spp was detected by culture in 7 (25.9%) of the 27 samples. Eighteen (66.7%) of the 27 samples were positive by the IMS method, thirteen of them reporting counts below 10(3) colony forming units per liter (CFU l(-1)), six presented interfering microbiota and three presented PCR inhibition. Of the 65 water samples, 24 presented interfering microbiota by culture and 8 presented partial or complete inhibition of the PCR reaction. So the rate of inconclusive results of culture and PCR was 36.9 and 12.3%, respectively, without any inconclusive results reported for the IMS method.

CONCLUSION

The IMS method generally improved the recovery and detectability of Legionella in environmental matrices, suggesting the possibility to use IMS method as valuable indicator of risk. Thus, this method may significantly improve our knowledge about the exposure risk to these bacteria, allowing us to implement evidence-based monitoring and disinfection strategies.

Chances and limitations of nanosized titanium dioxide practical application in view of its physicochemical properties

Nanotechnology is a field of science that is nowadays developing in a dynamic way. It seems to offer almost endless opportunities of contribution to many areas of economy and human activity, in general. Thanks to nanotechnology, the so-called nanomaterials can be designed. They present structurally altered materials, with their physical, chemical and biological properties entirely differing from properties of the same materials manufactured in microtechnology. Nanotechnology creates a unique opportunity to modify the matter at the level of atoms and particles. Therefore, it has become possible to obtain items displaying new, useful properties, i.e. self-disinfecting and self-cleaning surfaces. Those surfaces are usually covered by a thin layer of a photocatalyst. The role of the photocatalyst is most of the time performed by the nanosized titanium dioxide (nano-TiO2). Excitation of nano-TiO2 by ultraviolet radiation initiates advanced oxidation processes and reactions leading to the creation of oxygen vacancies that bind water particles. As a result, photocatalytic surfaces are given new properties. Those properties can then be applied in a variety of disciplines, such as medicine, food hygiene, environmental protection or building industry. Practically, the applications include inactivation of microorganisms, degradation of toxins, removing pollutants from buildings and manufacturing of fog-free windows or mirrors.

Photocatalytic antibacterial activity of nano-TiO2 (anatase)-based thin films: effects on Escherichia coli cells and fatty acids

Titanium dioxide is a photocatalyst with well-known ability to oxidise a wide range of organic contaminants as well as to destroy microbial cells. In the present work TiO2 nanoparticles with high specific surface area (150m(2)/g) were used to prepare nanostructured films. The TiO2 nanoparticle-based film in combination with UV-A illumination with intensity (22W/m(2)) comparable to that of the sunlight in the UV-A region was used to demonstrate light-induced antibacterial effects. Fast and effective inactivation of Escherichia coli cells on the prepared thin films was observed. Visualization of bacterial cells under scanning electron microscopy (SEM) showed enlargement of the cells, distortion of cellular membrane and possible leakage of cytoplasm after 10min of exposure to photoactivated TiO2. According to the plate counts there were no viable cells as early as after 20min of exposure to UV-A activated TiO2. In parallel to effects on bacterial cell viability and morphology, changes in saturated and unsaturated fatty acids - important components of bacterial cell membrane-were studied. Fast decomposition of saturated fatty acids and changes in chemical structure of unsaturated fatty acids were detected. Thus, we suggest that peroxidation and decomposition of membrane fatty acids could be one of the factors contributing to the morphological changes of bacteria observed under SEM, and ultimately, cell death.

Systematic approach to in-depth understanding of photoelectrocatalytic bacterial inactivation mechanisms by tracking the decomposed building blocks

A systematic approach was developed to understand, in-depth, the mechanisms involved during the inactivation of bacterial cells using photoelectrocatalytic (PEC) processes with Escherichia coli K-12 as the model microorganism. The bacterial cells were found to be inactivated and decomposed primarily due to attack from photogenerated H2O2. Extracellular reactive oxygen species (ROSs), such as H2O2, may penetrate into the bacterial cell and cause dramatically elevated intracellular ROSs levels, which would overwhelm the antioxidative capacity of bacterial protective enzymes such as superoxide dismutase and catalase. The activities of these two enzymes were found to decrease due to the ROSs attacks during PEC inactivation. Bacterial cell wall damage was then observed, including loss of cell membrane integrity and increased permeability, followed by the decomposition of cell envelope (demonstrated by scanning electronic microscope images). One of the bacterial building blocks, protein, was found to be oxidatively damaged due to the ROSs attacks, as well. Leakage of cytoplasm and biomolecules (bacterial building blocks such as proteins and nucleic acids) were evident during prolonged PEC inactivation process. The leaked cytoplasmic substances and cell debris could be further degraded and, ultimately, mineralized with prolonged PEC treatment.

Disinfection of Escherichia coli Gram negative bacteria using surface modified TiO2: optimization of Ag metallization and depiction of charge transfer mechanism

The antibacterial activity of silver deposited TiO2 (Ag-TiO2 ) against Gram negative Escherichia coli bacteria was investigated by varying the Ag metal content from 0.10 to 0.50% on the surface of TiO2 . Ag depositions by the photoreduction method were found to be stable. Surface silver metallization was confirmed by EDAX and XPS studies. Photoluminescence studies show that the charge carrier recombination is less for 0.1% Ag-TiO2 and this catalyst shows superior bactericidal activity under solar light irradiation compared to Sol gel TiO2 (SG-TiO2 ) due to the surface plasmon effect. The energy levels of deposited Ag are dependent on the Ag content and it varies from -4.64 eV to -1.30 eV with respect to the vacuum energy level based on atomic silver to bulk silver deposits. The ability of electron transfer from Ag deposit to O2 depends on the position of the energy levels. The 0.25% and 0.50% Ag depositions showed detrimental effect on bactericidal activity due to the mismatch of energy levels. The effect of the EROS (External generation of the Reactive Oxygen Species by 0.1% Ag-TiO2 ) and IROS (Interior generation of Reactive Oxygen Species within the bacteria) on the bactericidal inactivation is discussed in detail.

Photocatalysts for Solar-Induced Water Disinfection: New Developments and Opportunities

Recent years have seen a surge of interest in the application of solar energy for water disinfection by using nanostructured photocatalysts elaborately designed and fabricated. Photocatalysis has its unique advantage for utilizing sunlight to drive the disinfection process. The highly reactive oxygen species (ROS) serve as the main oxidants and are capable of inactivating microorganisms, including viruses, bacteria, spores and protozoa. This chapter presents an overview of current research activities that center on the preparation, characterization and application of highly efficient photocatalysts for water disinfection under both UV and visible light irradiation. It is organized into two major parts. One is the development of TiO2-based photocatalysts including surface noble metal modified, ion doped, dye-sensitized, and composite TiO2. The other part is the introduction of new types of photocatalysts and advanced technologies that have recently fascinated the scientific community. Particular attention is given to the pioneering fields such as graphene-based photocatalysts, plasmonic-metal nanostructures and naturally occurring photocatalysts. Finally, we conclude with a discussion of what major advancements are needed to move the field of photocatalytic water disinfection forward.

Genetic studies of the role of fatty acid and coenzyme A in photocatalytic inactivation of Escherichia coli

The roles of bacterial cellular components, namely, fatty acid profile and coenzyme A, in photocatalytic inactivation of bacteria were investigated. Escherichia coli BW25113, as a "parental strain", and its isogenic single-gene deletion mutants E. coli JW1081 (fabF(-) mutant) and E. coli JW3942 (coaA(-) mutant) showed different susceptibilities towards photocatalytic inactivation by titanium dioxide (TiO(2), irradiated by UVA lamps (λ = 365 nm)). Regulating the fatty acid composition through pre-incubation temperature adjustment demonstrated the crucial role of cell membrane fatty acid profile in bacterial susceptibility towards photocatalytic inactivation, while the lower coenzyme A level in coaA(-) mutant correlated well with its lower susceptibility towards photocatalytic inactivation. Furthermore, transmission electron microscopic study demonstrated the photocatalytic destruction process of bacterial cells. This is the first study using single-gene deletion mutants to explore better understanding of the photocatalytic inactivation mechanism of E. coli.

Monitoring and control of UV and UV-TiO2 disinfections for municipal wastewater reclamation using artificial neural networks

The use of ultraviolet (UV) irradiation as a physical wastewater disinfection has increased in recent years, especially for wastewater reuse. The UV-TiO(2) can generate OH radicals, which is highly effective to inactivate microorganisms in wastewater disinfection. However, both UV and UV-TiO(2) disinfections create multiple physical, chemical, and bio-chemical phenomena that affect their germicidal efficiency. It is difficult to build a precise control model using existing mathematic models. This study applies artificial neural network (ANN) models to control UV and UV-TiO(2) disinfections. Experimental results indicate that the ANN models, which precisely generate relationships among multiple monitored parameters, total coliform counts in influent and effluent, and UV doses, can be used as control models for UV and UV-TiO(2) disinfections. A novel ANN control strategy is applied to control UV and UV-TiO(2) disinfection processes to meet three total coliform count limits for three wastewater reuse purposes. The proposed controlled strategy effectively controls UV and UV-TiO(2) disinfection, resulting in acceptable total coliform counts in effluent for the three wastewater reuse purposes. The required UV doses for UV-TiO(2) disinfection were lower than those for UV disinfection, resulting in energy saving and capacity reduction of 13.2-15.7%.

Enhanced photocatalytic H₂-production activity of graphene-modified titania nanosheets

Graphene-modified TiO(2) nanosheets with exposed (001) facets (graphene/TiO(2)) were prepared by microwave-hydrothermal treatment of graphene oxide (GO) and hydrothermally synthesized TiO(2) nanosheets with exposed (001) facets in an ethanol-water solvent. These nanocomposite samples showed high photocatalytic H(2)-production activity in aqueous solutions containing methanol, as sacrificial reagent, even without Pt co-catalyst. The optimal graphene content was found to be ∼1.0 wt%, giving a H(2)-production rate of 736 μmol h(-1) g(-1) with a quantum efficiency (QE) of 3.1%, which exceeded the rate observed on pure TiO(2) nanosheets by more than 41 times. This high photocatalytic H(2)-production activity is due to the deposition of TiO(2) nanosheets on graphene sheets, which act as an electron acceptor to efficiently separate the photogenerated charge carriers. The observed enhancement in the photocatalytic activity is due to the lower absolute potential of graphene/graphene˙(-) (-0.08 V vs. SHE, pH = 0) in comparison to the conduction band (-0.24 V) of anatase TiO(2), meanwhile the aforementioned absolute value is higher than the reduction potential of H(+) (0 V), which favors the electron transfer from the conduction band (CB) of TiO(2) to graphene sheets and the reduction of H(+), thus enhancing photocatalytic H(2)-production activity. The proposed mechanism for the observed photocatalytic performance of TiO(2) nanosheets, modified with a small amount of graphene, was further confirmed by photoluminescence spectroscopy and transient photocurrent response. This work not only shows a possibility for the utilization of low cost graphene sheets as a substitute for noble metals (such as Pt) in the photocatalytic H(2)-production but also for the first time shows a significant enhancement in the H(2)-production activity by using metal-free carbon material as an effective co-catalyst.

Photocatalytic disinfection using titanium dioxide: spectrum and mechanism of antimicrobial activity

The photocatalytic properties of titanium dioxide are well known and have many applications including the removal of organic contaminants and production of self-cleaning glass. There is an increasing interest in the application of the photocatalytic properties of TiO(2) for disinfection of surfaces, air and water. Reviews of the applications of photocatalysis in disinfection (Gamage and Zhang 2010; Chong et al., Wat Res 44(10):2997-3027, 2010) and of modelling of TiO(2) action have recently been published (Dalrymple et al. , Appl Catal B 98(1-2):27-38, 2010). In this review, we give an overview of the effects of photoactivated TiO(2) on microorganisms. The activity has been shown to be capable of killing a wide range of Gram-negative and Gram-positive bacteria, filamentous and unicellular fungi, algae, protozoa, mammalian viruses and bacteriophage. Resting stages, particularly bacterial endospores, fungal spores and protozoan cysts, are generally more resistant than the vegetative forms, possibly due to the increased cell wall thickness. The killing mechanism involves degradation of the cell wall and cytoplasmic membrane due to the production of reactive oxygen species such as hydroxyl radicals and hydrogen peroxide. This initially leads to leakage of cellular contents then cell lysis and may be followed by complete mineralisation of the organism. Killing is most efficient when there is close contact between the organisms and the TiO(2) catalyst. The killing activity is enhanced by the presence of other antimicrobial agents such as Cu and Ag.

Preparation and enhanced visible-light photocatalytic H2-production activity of CdS-sensitized Pt/TiO2 nanosheets with exposed (001) facets

CdS-sensitized Pt/TiO(2) nanosheets with exposed (001) facets were prepared by hydrothermal treatment of a Ti(OC(4)H(9))(4)-HF-H(2)O mixed solution followed by photochemical reduction deposition of Pt nanoparticles (NPs) on TiO(2) nanosheets (TiO(2) NSs) and chemical bath deposition of CdS NPs on Pt/TiO(2) NSs, successively. The UV and visible-light driven photocatalytic activity of the as-prepared samples was evaluated by photocatalytic H(2) production from lactic acid aqueous solution under UV and visible-light (λ ≥ 420 nm) irradiation. It was shown that no photocatalytic H(2)-production activity was observed on the pure TiO(2) NSs under UV and/or visible-light irradiation. Deposition of CdS NPs on Pt/TiO(2) NSs caused significant enhancement of the UV and visible-light photocatalytic H(2)-production rates. The morphology of TiO(2) particles had also significant influence on the visible-light H(2)-production activity. Among TiO(2) NSs, P25 and the NPs studied, the CdS-sensitized Pt/TiO(2) NSs show the highest photocatalytic activity (13.9% apparent quantum efficiency obtained at 420 nm), exceeding that of CdS-sensitized Pt/P25 by 10.3% and that of Pt/NPs by 1.21%, which can be attributed to the combined effect of several factors including the presence of exposed (001) facets, surface fluorination and high specific surface area. After many replication experiments of the photocatalytic hydrogen production in the presence of lactic acid, the CdS-sensitized Pt/TiO(2) NSs did not show great loss in the photocatalytic activity, confirming that the CdS/Pt/TiO(2) NSs system is stable and not photocorroded.