The radical intermediates formed upon UVA irradiation of titanium dioxide suspensions in aqueous and non-aqueous environments were investigated applying the EPR spin trapping technique. The results showed that the generation of reactive species and their consecutive reactions are influenced by the solvent properties (e.g., polarity, solubility of molecular oxygen, rate constant for the reaction of hydroxyl radicals with the solvent). The formation of hydroxyl radicals, evidenced as the corresponding spin-adducts, dominated in the irradiated TiO₂ aqueous suspensions. The addition of ¹⁷O-enriched water caused changes in the EPR spectra reflecting the interaction of an unpaired electron with the ¹⁷O nucleus. The photoexcitation of TiO₂ in non-aqueous solvents (dimethylsulfoxide, acetonitrile, methanol and ethanol) in the presence of 5,5-dimethyl-1-pyrroline N-oxide spin trap displayed a stabilization of the superoxide radical anions generated via electron transfer reaction to molecular oxygen, and various oxygen- and carbon-centered radicals from the solvents were generated. The character and origin of the carbon-centered spin-adducts was confirmed using nitroso spin trapping agents.

In order to find ways to characterize oxygen-saturated aqueous TiO2 suspensions, the formation of photo-induced free radicals was followed by EPR spectrosc

Thermal decomposition of potassium persulphate (K₂S₂O₈) was studied in detail by the EPR spin trapping technique in dimethylsulphoxide (DMSO), employing 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), 5-ethoxycarbonyl-5-methyl-1-pyrroline-N-oxide (EMPO) and 5-(diisopropoxyphosphoryl)-5-methyl-1-pyrroline-N-oxide (DIPPMPO) as spin traps. DMPO and/or its derivatives exclusively trapped the primary formed SO₄^{• −} radical anions producing stable spin adducts with half-lives exceeding 2 h at room temperature. High-resolution EPR spectra of these adducts showed unusually rich hyperfine structure due to the interaction of the unpaired electron with all magnetically active nuclei of the spin trap moiety. In contrast to aprotic DMSO solvent, ^•DMPO-OH adducts dominated in mixed DMSO/water solutions with water content higher than 50%. The thermal decomposition of K₂S₂O₈ in DMSO represents an effective source of free radicals for the radical scavenging capacity (RSC) determination assay, applicable to hydrophilic as well as hydrophobic antioxidants. Efficiency of the assay is demonstrated with a series of cereal samples.

The g-C₃N₄/TiO₂ nanopowders prepared by the annealing of melamine and TiO₂ P25 at 550 °C were investigated under dark and upon UV or visible-light photoactivation using X- and Q-band electron paramagnetic resonance (EPR) spectroscopy. The EPR spectra of powders monitored at room temperature and 100 K showed the impact of the initial loading ratio of melamine/TiO₂ on the character of paramagnetic centers observed. For the photocatalysts synthesized using a lower titania content, the paramagnetic signals characteristic for the g-C₃N₄/TiO₂ nanocomposites were already found before exposure. The samples annealed using the higher TiO₂ loading revealed the photoinduced generation of paramagnetic nitrogen bulk centers (g-tensor components g₁ = 2.005, g₂ = 2.004, g₃ = 2.003 and hyperfine couplings from the nitrogen A₁ = 0.23 mT, A₂ = 0.44 mT, A₃ = 3.23 mT) typical for N-doped TiO₂. The ability of photocatalysts to generate reactive oxygen species (ROS) upon in situ UV or visible-light photoexcitation was tested in water or dimethyl sulfoxide by EPR spin trapping using 5,5-dimethyl 1-pyrroline N-oxide. The results obtained reflect the differences in photocatalyst nanostructures caused by the differing initial ratio of melamine/TiO₂; the photocatalyst prepared by the high-temperature treatment of melamine/TiO₂ wt. ratio of 1:3 revealed an adequate photoactivity in both spectral regions.