Satoshi Horikoshi

Chemical oxidation and DNA damage catalysed by inorganic sunscreen ingredients

Titanium dioxide (TiO2) has been noted (US Federal Register, 43FR38206, 25 August 1978) to be a safe physical sunscreen because it reflects and scatters UVB and UVA in sunlight. However, TiO2 absorbs about 70% of incident UV, and in aqueous environments this leads to the generation of hydroxyl radicals which can initiate oxidations. Using chemical methods, we show that all sunscreen TiO2 samples tested catalyse the photo‐oxidation of a representative organic substrate (phenol). We also show that sunlight‐illuminated TiO2 catalyses DNA damage both in vitro and in human cells. These results may be relevant to the overall effects of sunscreens.

Photooxidation mechanism of dye alizarin red in TiO2 dispersions under visible illumination: an experimental and theoretical examination

Visible light-induced photocatalytic oxidation of the dye alizarin red (AR) has been examined in TiO2 aqueous dispersions. The ESR spin-trapping technique was used to detect active oxygen radicals formed during in situ visible irradiation of AR/TiO2 dispersions. Evidence for the production of superoxide (O2⋅− or HOO⋅) (formed in the reduction of O2) and hydroxyl radicals (⋅OH) (formed by a multistep reduction) in the initial photoexcitation stage is presented. Meanwhile, the pathway of the photooxidation of AR is theoretically predicted on the basis of molecular orbital (MO) calculations by frontier electron densities and point charges on all the individual atoms of the dye. The relatively high negative point charges on the sulfonic oxygens lead to a strong adsorption of the dye onto the TiO2 particle surface through the sulfonate function. The position of the dye molecule attacked by the active oxygen species (e.g., O2⋅− or HOO⋅ radicals) and/or O2 is correlated with frontier electron densities, there is a perfect agreement between MO calculations and the results of experiments. A plausible mechanism of photooxidation under visible irradiation is discussed.

Photocatalyzed N-demethylation and degradation of methylene blue in titania dispersions exposed to concentrated sunlight

Abstract S imulated wastewaters that con tain methylene blue (MB) were bleached in a photocatalytic aqueous TiO 2 dispersion illuminated by concentrated sunlight using a parabolic round concentrator reactor (PRCR). The kinetic analysis was carried out well when the temporal concentration variation was a function of the concentrated light energy irradiated. The p hotocatalyzed N-demethylation of MB takes place concurrently with photocatalytic decomposition of MB by pseudo- first-order kinetics. The dependence of the photo- decomposition kinetics on the initial concentration of MB is consistent with the Langmuir– Hinshelwood model. Elimination of TOC (total organic carbon) also occurs by pseudo- first-order kinetics prior to full bleaching of the aqueous TiO 2 dispersion, after which the TOC level decreases only slightly. Also, compared with the open to air, the photodegradation of MB is not influenced by molecular oxygen bubbl ing continuously through the reactant suspensions during illumination . TiO 2 loadings and flow rates markedly a ffect the degradation o f MB. Under concentrated sunlight, t he relative photonic efficiency of MB photodegradation i s ζ rel =0.49 (relative to phenol) . T he efficiency ζ rel for the degradation of MB is independent of photoreactor geometry (cylindrical bottle reactor versus round-bot tomed flask), of light sources (solar light concentrator versus a Hg lamp) and of the operating mode used (flow versus batch operation).

Environmental remediation by an integrated microwave/UV-illumination method II.: Characteristics of a novel UV–VIS–microwave integrated irradiation device in photodegradation processes

Abstract In this brief article we examined the characteristics of a novel UV–VIS–microwave integrated irradiation device and explore its usefulness using the now well-established photo-degradation of a cationic dye (rhodamine-B; RhB) as a test substrate. The device consisted of a double quartz cylindrical plasma photoreactor (DQCPP) equipped with a microwave (MW) powered electrodeless Hg/Ne irradiation source. The latter is activated only by the MW radiation. The integrated MW–UV–VIS radiations so produced and incident on the DQCPP reactor significantly enhanced the decomposition of the RhB dye through simultaneous photocatalytic and photosensitization processes.

In vitro photochemical damage to DNA, RNA and their bases by an inorganic sunscreen agent on exposure to UVA and UVB radiation

Abstract The fate of DNA, RNA and their pyrimidine and purine bases was examined on exposure to UVA and UVB radiation in the presence of a physical sunscreen agent (TiO2 anatase/rutile particles) to assess the potential damage that such an agent may cause on contact with such substrates. DNA and RNA were partially decomposed and the bases were converted to carbon dioxide (nitrogen atoms to ammonia and nitrate ions) in a Pyrex reactor under conditions simulating UVA and UVB sunlight. The physical and chemical damage inflicted on DNA and RNA was also confirmed by scanning electron microscopy and gel permeation chromatography.

An in vitro systematic spectroscopic examination of the photostabilities of a random set of commercial sunscreen lotions and their chemical UVB/UVA active agents

The photostabilities of a random set of commercially available sunscreen lotions and their active ingredients are examined spectroscopically subsequent to simulated sunlight UV exposure. Loss of filtering efficacy can occur because of possible photochemical modifications of the sunscreen active agents. Changes in absorption of UVA/ UVB sunlight by agents in sunscreen lotions also leads to a reduction of the expected photoprotection of human skin and DNA against the harmful UV radiation. The active ingredients were investigated in aqueous media and in organic solvents of various polarities (methanol, acetonitrile, and n-hexane) under aerobic and anaerobic conditions The UV absorption features are affected by the nature of the solvents with properties closely related to oil-in-water (o/w) or water-in-oil (w/o) emulsions actually used in sunscreen formulations, and by the presence of molecular oxygen. The photostabilities of two combined chemical ingredients (oxybenzone and octyl methoxycinnamate) and the combination oxybenzone/titanium dioxide were also explored. In the latter case, oxybenzone undergoes significant photodegradation in the presence of the physical filter TiO2.

Environmental remediation by an integrated microwave/UV-illumination technique: IV. Non-thermal effects in the microwave-assisted degradation of 2,4-dichlorophenoxyacetic acid in UV-irradiated TiO2/H2O dispersions

Abstract After establishing proof of concept in the first three articles of this series, in which we used the rhodamine-B dye to test the methodology, we herein report a study on the photocatalyzed degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) in aqueous TiO2 dispersions that were subjected to simultaneous irradiation by UV light and microwave (MW) radiation. The characteristics of the process were monitored for the effect of microwave radiation by examining the fate of each substituent group, viz. the carboxylic acid group, the chlorine substituents and the benzene ring of the 2,4-D structure. The degradation dynamics of the microwave-assisted process were examined by decrease of UV absorption, loss of total organic carbon (TOC), formation of Cl− ions (dechlorination), and identification of intermediates by electrospray mass spectral techniques. The thermal effect of the microwave radiation to the system was considered by comparing results from MW-generated heat versus externally applied heat. The effect of dissolved oxygen or absence of oxygen on the degradative process was also investigated. The greater efficiency of the MW-assisted process is ascribed to a non-thermal effect of microwave radiation on the break-up of the aromatic ring of 2,4-D (oxidation) but evidently not on the dechlorination process (reduction) for which MW radiation seems to have a negligible influence, if any. Factors involved in this non-thermal component and a mechanism are proposed for the initial stages of the degradation.

Hydroxyl radicals in microwave photocatalysis. Enhanced formation of OH radicals probed by ESR techniques in microwave-assisted photocatalysis in aqueous TiO2 dispersions

Formation of OH radicals during TiO2 photocatalyzed oxidations driven by simultaneous UV and microwave radiation was probed by the electron spin resonance technique employing a novel setup in which the ESR sample (contained the DMPO spin-trap agent and P-25 TiO2 or pure anatase or pure rutile in aqueous media) could be irradiated by both UV light and microwave radiation; the microwave generator was coupled through an N connector to the ESR sample cavity and to a photoreactor in which phenol was photooxidized by simultaneous irradiation with UV light and microwaves to assess whether the photooxidative process correlated with the concentration of OH radicals formed.

Fate of amino acids upon exposure to aqueous titania irradiated with UV-A and UV-B radiation Photocatalyzed formation of NH3, NO3−, and CO2

Abstract The fate of nitrogen in various amino acids was examined following their photooxidative (and/or reductive) transformation catalyzed by UV-A and UV-B illuminated aqueous TiO 2 dispersions. The nitrogens in the amino acids are photoconverted predominantly into NH 3 (analyzed as NH 4 + ) and to a lesser extent into NO 3 − ions; NH 4 + /NO 3 − ratios span the range 3–12 after ca. 8 h irradiation. Extensive evolution of CO 2 is also observed; in some cases it is quantitative. Variations in the NH 4 + /NO 3 − ratio in the transformation of amino acids are dependent on the substrates molecular structure. Some of the steps in an otherwise complex mechanism of the heterogeneous photocatalyzedmineralization are described.

Photocatalytic degradation mechanism for heterocyclic derivatives of triazolidine and triazole

In an attempt to improve the understanding of the basic mechanisms on the degradation of pollutants in water by TiO2 photocatalysis, we discussed the primary degradation mechanism of three triazolidine derivatives, such as 1,2,4-triazolidine-3,5-dione (TRIANE), 4-hydroxy-1,2,4-triazolidine-3,5-dione (OH-TRIANE) and 4-phenyl-1,2,4-triazolidine-3,5-dione (Ph-TRIANE) and one triazole derivative of the 4-phenyl-1,2,4-triazole-2,5-dione (Ph-TRIOLE), on the basis of the experimental results together with molecular orbital (MO) calculation of frontier electron density and partial charge. The above four heterocycles were selected as molecular probe, principally because the highest frontier electron density was situated at different places of the molecule, while their structures were very similar, two major pathways have been revealed by LC/MS analysis for each heterocyclic compound. The pathway (a) corresponded to the hydroxylation of the atom bearing the highest electron density, via oxidation on nitrogen atom of azo group with respect to the photodegradation of TRIANE, OH-TRIANE and Ph-TRIOLE and opening of aromatic ring when Ph-TRIANE was used. This initial attack occurred with the preferential electrophilic attack of OH radicals. The pathway (b) was caused by the attack of active species on to carbon atom of carbonyl group. Considering the calculation of the relative electrophilic density of this carbon atom for the four heterocyclics and the evolution of large amount of N2 gas at the initial degradation step, it has been suggested that this step (b) was favored by adsorption of carbonyl group on TiO2 surface as theoretically determined by partial charge and confirmed by IR analysis. The participation of hole (h+) to form R–CO+ was envisaged in this step. The presence of 1,2,4,5-tetrazixane-3,6-dione also agrees with the participation of TiO2 surface. The nitrogen inorganic analysis (N2, NH4+ and NO3−) determined by gas chromatography and ion liquid chromatography, show that the hydrazo group were photoconverted mainly into N2 gas and partially to NH4+ ions. No nitrate ions were observed at the beginning of the degradation even when a OH substituent was present on one nitrogen atom (case of OH-TRIANE). However, the presence of –OH group increased the NO3−/NH4+ ratio observed after a few hours of irradiation. The presence of OH also improved the total mineralization of carbon atom into CO2, which was attributed to mesomeric effect of –OH group.