Evaluation of antioxidant properties of particulate matter by DPPH assay

Several acellular methods, defined as oxidative potential (OP) assays, have been developed to assess the particulate matter (PM) oxidative capacity and they are considered as predictors of the ability of dust to generate oxidative stress in living organisms. There is no agreement regarding the most representative assay to measure the OP of PM (Ayres et al., 2008), but methods mostly used on the PM filter extracts are the dithiothreitol (DTT; Cho et al., 2005), the 2′,7′-dichlorofluorescin (DCFH; Hung et al., 2001) and the ascorbic acid (AA; Stoeger et al., 2008) assays. The application of those assays to PM samples collected in field suggests the possible presence of reducing species, naturally present in PM, that compete with oxidizing species, altering OP values. This issue merits to be further investigated. While the use of acellular methods for the assessment of the OP values has been receiving great scientific attention in recent years, there is still a literature gap about acellular assays for identifying reducing species in particulate matter. One of the antioxidant capacity assays, that is currently in use for various matrices, is based on the use of the stable free radical DPPH (2,2-Diphenyl-1-picrylhydrazyl). In this work, this assay was tailored and applied to PM. Both PM2.5/PM10 field samples and dusts produced by specific sources (i.e. brake dust, Saharan dust, road dust, soil dust, coke and certified material NIST1648a) were considered in the work. The effect of the antioxidants on DPPH· radical is estimated according to the procedure described by Chen et al. (2016) with slight modifications. The collected dust and the PM filters are put into 5 ml of DPPH 0.1 mM in ethanol (96%) prepared daily and, then, they are extracted at room temperature for 30 minutes, by shaking the solutions inside glass vials. Then, spectrophotometric measurements are done at 517 nm. The same samples were analysed also for elements, inorganic ions and oxidative potential (AA, DTT and DCFH assays). The comparison between obtained data by OP assays and DPPH assay performed in parallel, allowed us to gain more knowledge on redox properties of PM. The values are interpreted and linked to the chemical composition of the samples. The study of the redox properties of PM may help to deepen the relationship between the composition of PM and its toxicity.