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Sporicidal performance induced by photocatalytic production of organic peroxide under visible light irradiation

View Article: PubMed Central - PubMed

ABSTRACT

Bacteria that cause serious food poisoning are known to sporulate under conditions of nutrient and water shortage. The resulting spores have much greater resistance to common sterilization methods, such as heating at 100 °C and exposure to various chemical agents. Because such bacteria cannot be inactivated with typical alcohol disinfectants, peroxyacetic acid (PAA) often is used, but PAA is a harmful agent that can seriously damage human health. Furthermore, concentrated hydrogen peroxide, which is also dangerous, must be used to prepare PAA. Thus, the development of a facile and safe sporicidal disinfectant is strongly required. In this study, we have developed an innovative sporicidal disinfection method that employs the combination of an aqueous ethanol solution, visible light irradiation, and a photocatalyst. We successfully produced a sporicidal disinfectant one hundred times as effective as commercially available PAA, while also resolving the hazards and odor problems associated with PAA. The method presented here can potentially be used as a replacement for the general disinfectants employed in the food and health industries.

No MeSH data available.


Relationship between the potential of oxygen reduction and the band structures of TiO2 and WO3.
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f5: Relationship between the potential of oxygen reduction and the band structures of TiO2 and WO3.

Mentions: The presence of hydrogen peroxide is critical to the generation of organic peroxide. The conduction level of a photocatalyst is closely related to generation of hydrogen peroxide. To demonstrate the importance of the conduction level, we evaluated the survival rate of B. subtilis spores in a system containing TiO2, which is (globally) the most widely used and applied photocatalyst. It is well known that although TiO2 exhibits high photocatalytic activity, its conduction band level is much more negative than the potential of the multi-electron reduction of oxygen. As a result, the single-electron reduction of oxygen (O2 + e− = O2−, −0.284 V; O2 + H + + e− = HO2, −0.046 V) preferentially proceeds over TiO2 instead464748. Also, it was reported that O2− and HO2 can transfer to H2O2 through some subsequent reactions484950. However, TiO2 should rapidly consume hydrogen peroxide producing during irradiation as shown in Supplementary Figure 5, which suppressed increasing the concentration of hydrogen peroxide. Hence, it seems that organic peroxide cannot be generated via TiO2. The band edge of the TiO2 used in this work was observed at ca. 400 nm, as shown in Supplementary Figure 1, indicating an estimated band gap of 3.1 eV. The crystal structure of the TiO2 was a mixture of anatase and rutile phases (Supplementary Figure 2B), and the particle size was estimated to be ca. 20 nm (Supplementary Figure 3B). The surface area of the TiO2 was found to be 54 m2 g−1, larger than that of the WO3. Supplementary Figure 6 shows that the B. subtilis spores were not inactivated in aqueous ethanol solution upon illumination with either UV or visible (λ > 420 nm) light, indicating that TiO2 is not suitable for the photocatalytic inactivation of bacterial spores. Next, the quantitative amount of organic peroxide, hydrogen peroxide, acetic acid, and formic acid produced in the presence of TiO2 in 8:2 (v/v) ethanol/water under UV light irradiation was investigated. As shown in Fig. 2, no MTSO peak detected, indicating that the TiO2 did not produce organic peroxide. As shown in Supplementary Figure 7, the formation of acetic acid and formic acid was confirmed, indicating that the photocatalytic oxidative reaction proceeded. In contrast, the production of hydrogen peroxide was not observed, in accordance with the difficulty of the multi-electron reduction of oxygen over TiO2. These results revealed that WO3 is a more suitable visible-light-driven photocatalyst for the inactivation of bacterial spores than TiO2, and suggests that the presence of hydrogen peroxide is key to the production of organic peroxide. As shown in Fig. 5, the valence band levels of TiO2 and WO3 have similar positions because band levels consist of O2p orbitals, which enable the photocatalytic oxidative decomposition of ethanol to proceed. However, the conduction band level of TiO2 is much more negative than that of WO3, which causes the single-electron reduction of oxygen to preferentially occur. This difference explains why illuminated TiO2 did not produce hydrogen peroxide, precluding the formation of organic peroxide in the presence of this photocatalyst.


Sporicidal performance induced by photocatalytic production of organic peroxide under visible light irradiation
Relationship between the potential of oxygen reduction and the band structures of TiO2 and WO3.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5036025&req=5

f5: Relationship between the potential of oxygen reduction and the band structures of TiO2 and WO3.
Mentions: The presence of hydrogen peroxide is critical to the generation of organic peroxide. The conduction level of a photocatalyst is closely related to generation of hydrogen peroxide. To demonstrate the importance of the conduction level, we evaluated the survival rate of B. subtilis spores in a system containing TiO2, which is (globally) the most widely used and applied photocatalyst. It is well known that although TiO2 exhibits high photocatalytic activity, its conduction band level is much more negative than the potential of the multi-electron reduction of oxygen. As a result, the single-electron reduction of oxygen (O2 + e− = O2−, −0.284 V; O2 + H + + e− = HO2, −0.046 V) preferentially proceeds over TiO2 instead464748. Also, it was reported that O2− and HO2 can transfer to H2O2 through some subsequent reactions484950. However, TiO2 should rapidly consume hydrogen peroxide producing during irradiation as shown in Supplementary Figure 5, which suppressed increasing the concentration of hydrogen peroxide. Hence, it seems that organic peroxide cannot be generated via TiO2. The band edge of the TiO2 used in this work was observed at ca. 400 nm, as shown in Supplementary Figure 1, indicating an estimated band gap of 3.1 eV. The crystal structure of the TiO2 was a mixture of anatase and rutile phases (Supplementary Figure 2B), and the particle size was estimated to be ca. 20 nm (Supplementary Figure 3B). The surface area of the TiO2 was found to be 54 m2 g−1, larger than that of the WO3. Supplementary Figure 6 shows that the B. subtilis spores were not inactivated in aqueous ethanol solution upon illumination with either UV or visible (λ > 420 nm) light, indicating that TiO2 is not suitable for the photocatalytic inactivation of bacterial spores. Next, the quantitative amount of organic peroxide, hydrogen peroxide, acetic acid, and formic acid produced in the presence of TiO2 in 8:2 (v/v) ethanol/water under UV light irradiation was investigated. As shown in Fig. 2, no MTSO peak detected, indicating that the TiO2 did not produce organic peroxide. As shown in Supplementary Figure 7, the formation of acetic acid and formic acid was confirmed, indicating that the photocatalytic oxidative reaction proceeded. In contrast, the production of hydrogen peroxide was not observed, in accordance with the difficulty of the multi-electron reduction of oxygen over TiO2. These results revealed that WO3 is a more suitable visible-light-driven photocatalyst for the inactivation of bacterial spores than TiO2, and suggests that the presence of hydrogen peroxide is key to the production of organic peroxide. As shown in Fig. 5, the valence band levels of TiO2 and WO3 have similar positions because band levels consist of O2p orbitals, which enable the photocatalytic oxidative decomposition of ethanol to proceed. However, the conduction band level of TiO2 is much more negative than that of WO3, which causes the single-electron reduction of oxygen to preferentially occur. This difference explains why illuminated TiO2 did not produce hydrogen peroxide, precluding the formation of organic peroxide in the presence of this photocatalyst.

View Article: PubMed Central - PubMed

ABSTRACT

Bacteria that cause serious food poisoning are known to sporulate under conditions of nutrient and water shortage. The resulting spores have much greater resistance to common sterilization methods, such as heating at 100 °C and exposure to various chemical agents. Because such bacteria cannot be inactivated with typical alcohol disinfectants, peroxyacetic acid (PAA) often is used, but PAA is a harmful agent that can seriously damage human health. Furthermore, concentrated hydrogen peroxide, which is also dangerous, must be used to prepare PAA. Thus, the development of a facile and safe sporicidal disinfectant is strongly required. In this study, we have developed an innovative sporicidal disinfection method that employs the combination of an aqueous ethanol solution, visible light irradiation, and a photocatalyst. We successfully produced a sporicidal disinfectant one hundred times as effective as commercially available PAA, while also resolving the hazards and odor problems associated with PAA. The method presented here can potentially be used as a replacement for the general disinfectants employed in the food and health industries.

No MeSH data available.