Industrialisation has led to unprecedented levels of outdoor air pollution, posing a significant health risk to human beings. Consequently, there is an urgent need to replace fossil fuels with sustainable energy sources, thereby mitigating these risks and providing a safer outdoor and indoor environment. Titanium dioxide is a versatile transition metal oxide with applications ranging from energy conversion to environmental remediation. However, it faces limitations, particularly in its absorption spectrum and charge separation efficiency, and enhancing these properties remains a significant challenge. In this research work, we have decorated the surface of TiO2 hybridising it with noble-metal and/or noble-metal oxides (Ag and/or CuxO) to improve the photocatalytic performances (monitoring the removal of nitrogen oxides and benzene, and hydrogen generation from water splitting) under simulated solar-light irradiation. Our results showed that titania modified with an Ag:Cu molar ratio equal to 1:1, not only exhibited the most promising performance in terms of nitrogen oxides and benzene removal, it was the optimum amount for the light-induced generation of hydrogen from water splitting.

Outdoor and indoor air pollution is a global environmental concern in modern society. Although many policies and regulations on air quality have been promulgated worldwide over the past decades, airborne pollution still negatively affects health and therefore the life-style of human beings. One of the strategies to challenge this problem might be reducing the amount of airborne pollutant by mineralising them via photoinduced reactions. Photocatalytic oxidation of gaseous pollutants via titanium dioxide is one of the most investigated solar photochemical reactions. In this research work, by means of a green sol-gel procedure, we have coupled titania to graphene (0.5 and 1.0 wt%) aiming to increase the solar photocatalytic activity of the produced hybrid materials. The photocatalytic reactions were assessed by monitoring the removal of nitrogen oxides and two different volatile organic compounds (benzene and isopropanol). Photocatalytic mechanism was investigated by means of EPR spin trapping experiments.Our results highlight the exceptional characteristics of the TiO2/graphene hybrid material (1.0 wt% graphene), and its suitability for multi-purpose applications in the field of environmental remediation. Compared to unmodified titania, the hybrid material with 1.0 wt% graphene shows a clear enhancement in the photocatalytic removal of those hazardous pollutants – corresponding to more than twice the photocatalytic degradation rate. In addition, the same material is highly stable and shows fully recyclability over repeated tests. Hybrid titania-graphene materials could thus be exploited to grant a safer outdoor and indoor environments, having a beneficial impact on public health and thus on the quality of our lives.

The present work describes the preparations of active carbon (AC) - titania composites with 2 different AC/TiO2 ratios, their characterization using SEM, BET, determination of particle size, and evaluation of adsorption ability and photocatalytic activity using benzoic acid (BA) in the aqueous phase and toluene in the gaseous phase. Composite materials based on AC/TiO2 were prepared from two types of active carbon (having a BET surface area of 1737 m(2) g(-1) and 890 m(2) g(-1), respectively) and TiO2 (P25, BET surface area 45 m(2) g(-1)) by mixing. The main studied parameter was milling of AC. Immobilized photocatalyst films were prepared by drop-casting of aqueous composite suspensions on glass substrates and drying. The composite films prepared from milled AC have much better adhesion to substrate in aqueous media than those from non-milled AC, which is due to a decrease of particle size by about 1 order during milling. The ability of composites prepared from milled AC to remove BA was more than 2.5 times higher than that of composites from non-milled AC. In comparison with TiO2, prepared composites showed similar or higher photocatalytic activity and the irradiation of AC/TiO2 composites in aqueous suspensions resulted in the generation of a comparable concentration of the hydroxyl radical spin-adduct ((DMPO)-D-center dot-OH) as with TiO2.

Nanostructured systems showing reversible colour switching are envisaged to play a significant role in photo-switches, photo-optical sensors, smart windows, displays, optical storage memories. Most of the materials exhibiting reversible colour switching are organic compounds. However, their UV-light activation, low thermal and chemical stability, as well as harmful synthesis methods, are of limit for their extensive use. In this research, we have created an inorganic switchable photochromic material exploiting: (i) TiO2 ability of creating an exciton upon excitation, (ii) copper as the chromophore, and (iii) graphene’s extraordinarily high electron mobility. Our material showed itself to be able to work under visible-light, its photochromic property being three times faster than conventional titania based photochromic materials, reaching a stable change in colouration after only 30 mins of visible-light irradiation (versus > 120 min in conventional Cu-TiO2). With the addition of just 1 wt% graphene, the material exhibited a staggeringly stable photochromic switching over repeated cycles. These results relate to the best previously reported values for any form of TiO2-based photochromic material. This is therefore an excellent candidate for smart self-cleaning windows, and other chromic devices and applications.

In order to find ways to characterize oxygen-saturated aqueous TiO 2 suspensions, the formation of photo-induced free radicals was followed by EPR spectroscopy, using as indicators N-oxide and nitrone spin trapping agents, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), 3,3,5,5-tetramethyl-1-pyrroline N-oxide (TMPO), α-(4-pyridyl-1-oxide)-N- tert -butylnitrone (POB N), 4-(N-methylpyridyl)-N- tert -butylnitrone (MePyBN), as well as semi-stable free radicals, 4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxyl (TEMPOL), cation radical of 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid), diammonium salt (ABTS) and 1,1-diphenyl-2-picrylhydrazyl (DPPH). DMPO and TMPO are efficiently oxidized to the EPR-silent products via radical in termediates. Conversely, the nitrone spin traps (POBN and MePyBN) showed selective formation of hydroxyl radical spin adducts upon continuous irradiation of oxygenated TiO 2 suspensions. Their concentrations increased proportionally with the amount of photocatalyst and irradiation time. The EPR spectrum of the semi-stable free radicals TEMPOL, ABTS ·+ or DPPH is gradually eliminated during irradiation, and this system represents a simple technique for the evaluation of TiO 2 activity.

Commercial sunscreen products containing titanium dioxide were irradiated with λ > 300 nm and the formation of oxygen- (OH, O2-/OOH) and carbon-centered radicals was monitored by EPR spectroscopy and spin trapping technique using 5,5-dimethyl-1-pyrroline N-oxide, α-phenyl-N-tert-butylnitrone (PBN), α-(4-pyridyl-1-oxide)-N-tert-butylnitrone as spin traps, and free nitroxide radical 4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxyl. The photoinduced production of singlet oxygen was shown by 4-hydroxy-2,2,6,6-piperidine. The generation of reactive oxygen radical species upon irradiation of sunscreens significantly depends on their composition, as the additives present (antioxidants, radical-scavengers, solvents) can transform the reactive radicals formed to less harmful products. The continuous in situ irradiation of titanium dioxide powder, recommended for cosmetic application, investigated in different solvents (water, dimethyl sulfoxide, isopropyl myristate) resulted in the generation of oxygen-centered reactive radical species (superoxide anion radical, hydroxyl and alkoxyl radicals).

Protoberberinium salts, i.e. berberine (I), palmatine (II) and jatrorrhizine (III) prepared from Mahonia aquifolium (Pursh) Nutt. belong to isoquinoline alkaloids possessing interesting biological activity (e.g. antibacterial, antimalarial, antitumor). The characteristic UV/Vis absorption band maxima of I–III iodide salts were found in regions 350 and 425 nm in dimethylsulfoxide (DMSO) and ethanol solvents, and were only negligibly influenced by substitution changes on the C‐2 and C‐3 positions. The fluorescence intensity of protoberberinium salts monitored in ethanol solutions was significantly lowered by iodide counter‐ions, and decreased in the order berberine > palmatine > jatrorrhizine. EPR spectroscopy supplied evidence of the formation of super‐oxide anion radicals and singlet oxygen upon irradiation of berberine in oxygenated DMSO solvent. The photochemical generation of O2·− and 1O2 in DMSO solutions of palmatine and jatrorrhizine was substantially lower, and probably reflected the replacement of a photolabile methylenedioxy group at C‐2 and C‐3 positions in the berberine molecule by two methoxy groups in palmatine, and methoxyl (C‐2) and hydroxyl (C‐3) substitution in jatrorrhizine. Additionally, the powder EPR spectra of protoberberinium iodides I–III measured at 290 K revealed the presence of single‐line EPR signals (geff = 2.0044), which were attributed to hydroperoxidic structures produced by the autoxidation process. The photochemical reactions of protoberbenium salts producing reactive oxygen species after UVA excitation should be integrated in biological activity investigations, as well as in their applications in skin disorder treatment. Copyright © 2004 John Wiley & Sons, Ltd.

The production of reactive radical species upon irradiation (λ > 300 nm) of six triarylmethane dyes, i.e. C.I. Basic Red 9 (42500; Basic Fuchsin), C.I. Basic Violet 3 (42555; Crystal Violet), C.I. Basic Green 4 (42000; Malachite Green Oxalate), C.I. Acid Green 16 (44025; Acid Green V), C.I. Basic Blue 11 (44040; Victoria Blue R), C.I. Acid Blue 93 (42780; Aniline Blue), as well as a phenothiazine dye C.I. Basic Blue 9 (52015; Methylene Blue) and a xanthene dye C.I. Acid Red 87 (45380; Eosin Y) in different solvents (water, ethanol, dimethylsulphoxide), and subsequently on paper substrate was investigated by means of EPR spectroscopy. The EPR experiments using spin trapping agents (5,5-dimethyl-1-pyrroline-N-oxide, DMPO and 2,3,5,6-tetramethyl nitrosobenzene, ND) showed evidence of the free radical formation in both deoxygenated and aerated systems during photoexcitation. The photooxidation processes in presence of oxygen resulted in the electron transfer generating super-oxide anion radicals (Type I process), simultaneously with energy transfer producing singlet oxygen (Type II process) confirmed by 2,2,6,6-tetramethyl-4-piperidinol (TMP).

Camptothecin (CPT) is an anticancer drug that inhibits topoisomerase I (Topo 1), an enzyme closely linked to cell division, by forming a ternary DNA-CPT-Topo I complex. However, it has been shown that UVA-irradiated CPT in the absence of Topo I produces DNA damage. It has been proposed that free radicals are the key species responsible for the DNA cleavage. It has also been shown that the presence of metal ions enhances the activity of several anticancer drugs. Therefore, we attempt here to explore and identify free radicals generated in these processes. We describe a detailed spectroscopic study of UVA-irradiated CPT and the Cu(II)-CPT complex. From the low-temperature EPR spectrum of the Cu(II)-CPT complex, a proximity between the Cu(II) ion and the 20-hydroxy group of the lactone E ring of CPT is proposed. Upon irradiation (lambda = 365 nm) of the Cu(II)-CPT complex in deoxygenated dimethyl sulfoxide (DMSO), the EPR signal of Cu(II) measured in situ at room temperature shows formal first-order exponential decay with a formal half-life of 11 min. By the use of a specific Cu(I) chelating agent, neocuproine, it was shown that, during this process, Cu(II) is reduced to Cu(I). When the photochemical experiments are repeated in oxygen-saturated DMSO solutions, analogous phenomena are observed, characterized by a formal half-life of 16 min for Cu(II), except that there is an induction period of similar to3 min. Application of the spin-trap agent 5,5-dimethyl-l-pyrroline N-oxide (DMPO) shows that during this induction period the only radical formed is the superoxide radical, trapped as the (DMPO)-D-.-O-2(-) adduct. The loss in EPR signal intensity of the Cu(II)-CPT complex upon irradiation is accompanied by the appearance of a new EPR signal at g approximate to 2.0022. Application of the spin traps nitrosodurene (ND) and DMPO revealed that the main radical product formed upon continuous irradiation of CPT in DMSO solutions is the hydroxyl radical (trapped in DMSO as the (CH3)-C-. adduct). Application of 2,2,6,6-tetramethyl-4-piperidinol has revealed that irradiation of CPT in aerated DMSO solution also leads to the formation of singlet oxygen (O-1(2)). In the Cu(II)-CPT system, the formation of methyl radicals is suppressed, and the generation of two new radical adducts originating from camptothecin ring cleavage is identified. A mechanism of photochemically generated radicals that include the superoxide radical and the radical cation of the {Cu(II)...CPT.+) complex followed by the reduction of Cu(II) to Cu(I) is proposed. The EPR experiments on irradiated CPT in the absence of copper (II) support the importance of the 20-hydroxy group of the lactone E ring in the antitumor activity of the drug mediated through the initially generated hydroxyl radical. In the presence of Cu(H), there is blockage of the 20-hydroxy group of CPT, and the generation of hydroxyl radicals is strongly suppressed. In this case, there is a mixture of radicals of various origin generated as a result of irradiation, which are capable of causing DNA damage. We propose that the superoxide radical, hydroxyl radical, singlet oxygen, and carbon-centered radicals generated from CPT in close proximity to DNA can cause considerable damage to DNA. These findings would help to explain the experiments indicating that the photoactivated camptothecin interacts specifically with guanines, consistent with preferential stimulation of topoisomerase I cleavage at sites that bear a guanine at their 5'-DNA terminus.Taken together, our spectroscopic experiments indicate that CPT is a promising photosensitizer and that radicals and singlet oxygen generated upon illumination play a central role in DNA cleavage and in the induction of apoptosis in cancer cells.

The radical intermediates produced upon UV irradiation of deoxygenated alcoholic titanium dioxide suspensions of nitrosobenzene, nitrobenzene, 2-nitrosotoluene, 2,3,5,6-tetramethylnitrosobenzene, 3,5-di-bromo-4-nitrosobenzenesulfonate (sodium salt), 2,4,6-tri-t-butyl-nitroso-benzene, and 2-methyl-2-nitrosopropane were investigated using in situ EPR technique. Nitrosobenzene is efficiently photoreduced in TiO2 suspensions (toluene/alcohol, 1:1 (v/v)) forming exclusively one stable radical intermediate corresponding to C6H5NOH species. The formation of this radical species is consistent with the proposed photocatalytic reduction mechanism, occurring from the primary generated nitrosobenzene mono-anion by the hydrogen abstraction from surroundings. The origin of hydrogen added to the nitroso group was demonstrated by the photocatalytic experiments using deuterated methanol, where the production of C6H5NOD was established. Additionally, an identical radical C6H5NOH was detected, when nitrobenzene was reduced under analogous experimental conditions. The photoinduced electron transfer from TiO2 to nitroso compounds is accompanied by alcohol oxidation via the photogenerated titanium dioxide valance band holes forming alkoxy and hydroxyalkyl radicals. Production of hydroxyalkyl radicals (CH2OH, CH(OH)CH3, C(OH)(CH3)2) with redox potentials suitable for a direct electron transfer to nitroso compounds represents an alternative reaction pathway for their reduction. On the other hand, the investigated nitroso derivatives are efficient spin-trapping agents, therefore, formation of nitroxyl radical spin adducts was observed in the photocatalytic experiments. The EPR spectra monitored upon irradiation of substituted nitrosobenzene derivatives in alcoholic TiO2 suspensions reveal the correlation between nitrosobenzene derivative first step reduction potentials and yield of radical species produced.

Chromium-, manganese- and cobalt-doped titanium dioxide photocatalysts containing 0.2, 0.5 or 1at.% of metal-dopant were investigated by UV–VIS, FT-IR, near-IR and electron paramagnetic resonance (EPR) spectroscopic techniques. The presence of the doping ions in the titania structure caused significant absorption shift to the visible region compared to pure TiO2 powder (P25 Degussa). The EPR spectra of TiO2 powders containing chromium showed the superposition of three types of individual paramagnetic species (β-, γ- and δ-signals), whose relative EPR intensity is significantly dependent on the dopant concentration, as well as on the photocatalysts preparation and treatment. The EPR spectra of the chromium-doped photocatalysts heated in H2/N2 atmosphere corresponded to the Cr(III) ions occupying vacated cation sites in the rutile or anatase crystal lattice. The characteristic feature of the EPR spectra of the Mn/TiO2 samples is a sharp six-line Mn(II) component centered on geff=1.99, flanked by shoulders with a weak feature, which appeared on geff=2.66 and 4.32. The photocatalytic activity of the various metal-doped TiO2 samples was tested in aqueous or dimethylsulfoxide (DMSO) suspensions using EPR spin trapping technique with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trap. The ability of the irradiated photocatalysts to generate reactive oxygen species, namely hydroxyl radicals and super-oxide anion radicals, which were trapped as the corresponding DMPO-adducts was investigated.

Electron paramagnetic resonance (EPR) in situ spectroscopy was applied in the study of photoinduced electron transfer between 3,3',5,5'-tetramethylbenzidine (TMB) and C70 in different solvent systems. The changes found in UV-vis spectra pointed at ground state charge transfer complex formation [C70-TMB] in benzonitrile. Upon selective excitation of C70 using steady-state monochromatic irradiation with a wavelength of 546 nm, two EPR singlets were observed, which were assigned to C70 mono- and di-anion. In the photochemical and cathodic in situ reductions, identical EPR spectra of anion radicals were obtained. C70 mono-anion was investigated also in frozen 1,2-dichlorobenzene solutions within the temperature range from 110 to 210 K, and at 110 K the anisotropic EPR spectrum of C70 mono-anion was simulated assuming an axially symmetric g-matrix with gparallel - gperpendicular = 0.00165.

The photoinduced electron transfer to water-soluble mono-adducts and various bis-functionalized C60 derivatives I–VI (see Scheme 1 in the text) were investigated by EPR spectroscopy in heterogeneous water/2-propanol TiO2 system, and also in aqueous solutions containing l-ascorbic acid as an electron donating substrate. Irradiated TiO2 suspensions of C60C(COONa)2/γ-CD and C60(C4H10N+)/γ-CD yielded an EPR signal A, characterized by gA=2.0000 and peak-to-peak width, ppA=0.095 mT, assigned to the fullerenes' mono-anion. The latter transforms with continuing irradiation into radicals B and C (gB=2.0005, gC=2.0009). In contrast, the three water-soluble anionic bis-adducts (IV–VI) gave rise to only a single narrow mono-anion EPR line M (ppM<0.02 mT) positioned at gM=2.0007.