Ho Yin Yip

Visible-Light-Driven Photocatalytic Inactivation of E. coli K-12 by Bismuth Vanadate Nanotubes: Bactericidal Performance and Mechanism

Bismuth vanadate nanotube (BV-NT), synthesized by a template-free solvothermal method, was used as an effective visible-light-driven (VLD) photocatalyst for inactivation of Escherichia coli K-12. The mechanism of photocatalytic bacterial inactivation was investigated by employing multiple scavengers combined with a simple partition system. The VLD photocatalytic bacterial inactivation by BV-NT did not allow any bacterial regrowth. The photogenerated h(+) and reactive oxidative species derived from h(+), such as OH(ads), H(2)O(2) and HO(2)/O(2)(-), were the major reactive species for bacterial inactivation. The inactivation by h(+) and OH(ads) required close contact between the BV-NT and bacterial cells, and only a limited amount of H(2)O(2) could diffuse into the solution to inactivate bacterial cells. The direct oxidation effect of h(+) to bacterial cells was confirmed by adopting F(-) surface modification and anaerobic experiments. The bacterial cells could trap e(-) in order to minimize e(-)-h(+) recombination, especially under anaerobic condition. Transmission electron microscopic study indicated the destruction process of bacterial cell began from the cell wall to other cellular components. The OH(ads) was postulated to be more important than OH(bulk) and was not supposed to be released very easily in the BV-NT bacterial inactivation system.

Naturally Occurring Sphalerite As a Novel Cost-Effective Photocatalyst for Bacterial Disinfection under Visible Light

The photocatalytic disinfection capability of the natural semiconducting mineral sphalerite is studied here for the first time. Natural sphalerite can completely inactivate 1.5 × 10(7) cfu/mL E. coli K-12 within 6 h under visible light irradiation. The photocatalytic disinfection mechanism of natural sphalerite is investigated using multiple scavengers. The critical role that electrons play in bactericidal actions is experimentally demonstrated. The involvement of H(2)O(2) in photocatalytic disinfection is also confirmed using a partition system combined with different scavengers. Moreover, the photocatalytic destruction of bacterial cells is observed through transmission electron microscopic analysis. A catalase activity study reveals that antioxidative enzyme activity is high in the initial stage of photocatalytic disinfection but decreases with time due to damage to enzymatic functioning. Natural sphalerite is abundant and easy to obtain and possesses excellent visible-light photocatalytic activity. These superior properties make it a promising solar-driven photocatalyst for large-scale cost-effective wastewater treatment.

Visible-light-driven BiOBr nanosheets for highly facet-dependent photocatalytic inactivation of Escherichia coli

Bismuth oxybromide (BiOBr) nanosheets with fully exposed {001} and {010} facets are synthesized via a facile hydrothermal method. Significant differences in photocatalytic inactivation towards Escherichia coli K-12 under visible light irradiation are found to be highly dependent on the dominantly exposed facets. In comparison with BiOBr with dominant {010}-facet (B010) nanosheets, BiOBr with dominant {001}-facet (B001) nanosheets exhibit remarkably higher photocatalytic activity in bacterial inactivation. This superior activity is ascribed to the more favorable separation and transfer of photogenerated electron–hole pairs as well as more oxygen vacancies of B001 nanosheets. Due to the faster production and further accumulation of ˙O2− and h+ within a short time, the VLD photocatalyst of B001 nanosheets can completely inactivate 107 colony forming unit (CFU) mL−1 (i.e. 7-log reduction) bacterial cells within 2 h; while only 1- and 6.5-log reductions of bacterial cells can be achieved within 2 and 6 h, respectively, by B010 nanosheets due to limited amounts of h+ and ˙O2− generated.

A Recyclable Mineral Catalyst for Visible-Light-Driven Photocatalytic Inactivation of Bacteria: Natural Magnetic Sphalerite

Motivated by recent studies that well-documented mineral photocatalyst for bacterial inactivation, a novel natural magnetic sphalerite (NMS) in lead-zinc deposit was first discovered and evaluated for its visible-light-driven (VLD) photocatalytic bactericidal properties. Superior to the reference natural sphalerite (NS), vibrating sampling magnetometeric (VSM) analysis revealed the ferromagnetic property of NMS, indicating its potential for easy separation after use. Under the irradiation of fluorescence tubes, NMS could inactivate 7 log10 Gram-negative Escherichia coli K-12 without any regrowth and metal ions leached out from NMS show no toxicity to cells. The cell destruction process starting from cell wall to intracellular components was verified by TEM. Some products from damaged cells such as aldehydes, ketones and carboxylic acids were identified by FTIR with a decrease of cell wall functional groups. The relative amounts of potassium ion leakage from damaged cells gradually increased from initial 0 to approximately constant concentration of 1000 ppb with increasing reaction time. Superoxide radical (•O2(-)) rather than hydroxyl radical (•OH) was proposed to be the primary reactive oxidative species (ROSs) responsible for E. coli inactivation by use of probes and electron spin resonance (ESR). H2O2 determined by fluorescence method is greatly involved in bacterial inactivation in both nonpartition and partition system. Multiple cycle runs revealed excellent stability of recycled NMS without any significant loss of activity. This study provides a promising natural magnetic photocatalyst for large-scale bacterial inactivation, as NMS is abundant, easily recycled and possessed an excellent VLD bacterial inactivation ability.

Porous Fluorinated SnO2 Hollow Nanospheres: Transformative Self-assembly and Photocatalytic Inactivation of Bacteria

Highly porous surface fluorinated SnO2 hollow nanospheres (SnO2(F) HNS) were produced in high yield by a hydrothermal treatment of stannous fluoride in the presence of hydrogen peroxide. Two important processes in terms of oriented self-assembly and in situ self-transformation were highlighted for the formation of as-prepared SnO2(F) HNS, which were largely relying on the directing effects of selected specific chemical species in the present synthesis system. Significantly, these SnO2(F) HNS showed considerable activity in photocatalytic inactivation of a surface negatively charged bacterium, Escherichia coli K-12, in aqueous saline solution. The dominant reactive species involved in the inactivation process were also identified.

Organic-free synthesis of {001} facet dominated BiOBr nanosheets for selective photoreduction of CO2 to CO

{001} facet dominated BiOBr nanosheets are fabricated via a facile hydrothermal method in the presence of nitric acid without any organic additive and applied for CO2 photoreduction. The concentration of nitric acid easily regulates the thickness of the obtained BiOBr nanosheets. When employing concentrations of 0, 0.1, 0.5, 1 and 4 M nitric acid, the corresponding surface area and percentage of exposed {001} facets of BiOBr-0, BiOBr-0.1, BiOBr-0.5, BiOBr-1 and BiOBr-4 nanosheets significantly increase in sequence. BiOBr-0 nanosheets are incapable of converting CO2 into CO. However, BiOBr-0.1, BiOBr-0.5, BiOBr-1 and BiOBr-4 nanosheets show successively enhanced CO2 photoreduction performance. Surprisingly, they exhibit high selectivity for converting CO2 into CO with negligible generation of CH4. In particular, BiOBr-4 shows the highest CO production rate of 4.45 μmol g−1 h−1 under simulated sunlight irradiation. The electronic structure analysis demonstrates that the conduction band minimum is significantly raised to endow BiOBr-4 with reduction power for CO2/CO conversion, in comparison with the incapability of BiOBr-0. The breakthrough in CO2 reduction of BiOBr-4 nanosheets is ascribed to the larger active surface area, higher electron transfer, more effective charge carrier separation and significantly raised reduction ability.

Visible-light-driven photocatalytic bacterial inactivation and the mechanism of zinc oxysulfide under LED light irradiation

Zinc oxysulfide (ZnO0.6S0.4) nanoparticles, prepared via a coprecipitation–calcination method, were used as an effective visible-light-driven (VLD) photocatalyst for the inactivation of a typical Gram-negative bacterium, Escherichia coli K-12 for the first time. An energy-saving white light emitting diode (LED) lamp was employed as the visible light (VL) source. Compared to the only UV-responsive pure ZnO and ZnS, the light active region of ZnO0.6S0.4 was expanded as far as 550 nm in the VL region. Significantly, the obtained ZnO0.6S0.4 nanoparticles showed considerable VLD photocatalytic bacterial inactivation activity under white LED irradiation. The mechanism of inactivation was investigated in-depth. Photogenerated holes (h+) and hydrogen peroxide (H2O2) were predominantly responsible for the bacterial inactivation. Moreover, H2O2 was evidenced to be derived only from electrons in the conduction band of ZnO0.6S0.4 in the present photocatalytic system. The integrated damage from the direct oxidation effect of the h+ and continuous accumulation of H2O2 resulted in a high bacterial inactivation efficiency of ZnO0.6S0.4 nanoparticles under visible white LED lamp irradiation. The destruction process of bacterial cells by the ZnO0.6S0.4 photocatalyst was also monitored. This was shown to begin with an attack of the cell membrane and then end in the release of intracellular components.

Role of in Situ Resultant H2O2 in the Visible-Light-Driven Photocatalytic Inactivation of E. coli Using Natural Sphalerite: A Genetic Study

This study investigated how a natural sphalerite (NS) photocatalyst, under visible light irradiation, supports photocatalytic bacterial inactivation. This was done by comparing parent E. coli BW25113, and its two isogenic single-gene knock-out mutants, E. coli JW0797-1 (dps(-) mutant) and JW1721-1 (katE(-) mutant), where both dps and KatE genes are likely related to H2O2 production. NS could inactivate approximately 5-, 7- and 7-log of E. coli BW25113, JW0797-1, and JW1721-1 within 6 h irradiation, respectively. The two isogenic mutants were more susceptible to photocatalysis than the parental strain because of their lack of a defense system against H2O2 oxidative stress. The ability of in situ resultant H2O2 to serve as a defense against photocatalytic inactivation was also confirmed using scavenging experiments and partition system experiments. Studying catalase activity further revealed that in situ H2O2 played an important role in these inactivation processes. The destruction of bacterial cells from the cell envelope to the intracellular components was also observed using field emission-scanning electron microscopy. Moreover, FT-IR was used to monitor bacterial cell decomposition, key functional group evolution, and bacterial cell structures. This is the first study to investigate the photocatalytic inactivation mechanism of E. coli using single-gene deletion mutants under visible light irradiation.

Dual Roles of Capsular Extracellular Polymeric Substances in Photocatalytic Inactivation of Escherichia coli: Comparison of E. coli BW25113 and Isogenic Mutants

ABSTRACT The dual roles of capsular extracellular polymeric substances (EPS) in the photocatalytic inactivation of bacteria were demonstrated in a TiO2-UVA system, by comparing wild-type Escherichia coli strain BW25113 and isogenic mutants with upregulated and downregulated production of capsular EPS. In a partition system in which direct contact between bacterial cells and TiO2 particles was inhibited, an increase in the amount of EPS was associated with increased bacterial resistance to photocatalytic inactivation. In contrast, when bacterial cells were in direct contact with TiO2 particles, an increase in the amount of capsular EPS decreased cell viability during photocatalytic treatment. Taken together, these results suggest that although capsular EPS can protect bacterial cells by consuming photogenerated reactive species, it also facilitates photocatalytic inactivation of bacteria by promoting the adhesion of TiO2 particles to the cell surface. Fluorescence microscopy and scanning electron microscopy analyses further confirmed that high capsular EPS density led to more TiO2 particles attaching to cells and forming bacterium-TiO2 aggregates. Calculations of interaction energy, represented by extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) potential, suggested that the presence of capsular EPS enhances the attachment of TiO2 particles to bacterial cells via acid-base interactions. Consideration of these mechanisms is critical for understanding bacterium-nanoparticle interactions and the photocatalytic inactivation of bacteria.

Noble-metal loading reverses temperature dependent photocatalytic hydrogen generation in methanol–water solutions

Co-catalysts and sacrificing reagents are important components in artificial photocatalytic processes. Here we demonstrate that noble-metal loading reverses the temperature dependent photocatalytic activity trends of photocatalytic hydrogen (H2) generation with methanol as a sacrificing reagent. This finding suggested that visible and infrared light can enhance photocatalytic H2 generation via a heat effect over noble-metal/photocatalysts.