Atmospheric particulate matter (PM) is known to induce oxidative stress by generating reactive oxygen species (ROS) when inhaled. To understand the sources of ROS production by PM, evaluating its individual components is essential. Previous studies have reported that metals and oxidized derivatives of polycyclic aromatic hydrocarbons (PAHs) exhibit high ROS generation capacity. However, the diversification of energy sources has raised concerns about the emergence of new harmful air pollutants, such as nitrogen-containing organic compounds, including nitrogen heterocyclic compounds (PANHs). PANHs can act as ligands, forming coordination complexes with metals such as iron, but the ROS production capability of such complexes remains unclear.

This study evaluated the oxidative potential (OP) of coordination complexes as an indicator of ROS production using the Dithiothreitol (DTT) assay. Test samples included 1,10-Phenanthroline iron(II) perchlorate (Fe-phen), Tris(1,10-Phenanthroline) cobalt(II) bis(hexafluorophosphate) (Co-phen), and Tris(2,2'-Bipyridine) cobalt(II) bis(hexafluorophosphate) (Co-bpy). For comparison, divalent iron [Fe(II)], cobalt [Co(II)], copper [Cu(II)], and manganese [Mn(II)] were also tested. DTT consumption rates were measured spectrophotometrically at 415 nm after reaction with the test samples for 20 minutes.

The formation of coordination complexes generally increased OP compared to individual metal ions or ligands. Notably, the Fe-phen complex exhibited a DTT consumption rate 50 times higher than Fe(II) alone, while the Co-bpy complex showed a 10-fold increase compared to Co(II). Furthermore, the DTT consumption rates of all complexes exceeded those of Cu(II), which has previously been reported to exhibit relatively high OP. To thoroughly evaluate the contribution of these complexes to the OP of atmospheric particles, it is essential to obtain atmospheric concentration data for each complex. Therefore, future studies should focus on developing sample preparation methods as well as separation and analytical techniques for detecting and quantifying these complexes in real atmospheric particles.