Application of DPPH assay for assessment of particulate matter reducing properties

Different acellular assays were developed to measure particulate matter’s (PM) oxidative potential (OP) to predict the PM ability to generate oxidative stress in living cells and organisms. However, there are still fundamental unresolved issues regarding the influence of assay’s design and the tests’ operative condition on OP obtained results. An additional important point is related to the stability of the species: short-life oxidant species can react and redox equilibria among PM native species could occur during both the sample storage and the extraction phase. In this regard, it is possible that, within the conditions that influence OP results, there is also the reaction and/or competition between oxidant and reducing species naturally occurring in PM. Some studies proved the presence of species with likely reducing characteristics (Menetrex et al, 2009; Nemmar et al, 2013). However, due to the heterogeneous PM composition, is very difficult to identify the exact chemical constituents and, thus, the nature of reducing species is still unknown. This study was focused on the application of the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay to PM, in order to evaluate the presence of reducing species. The DPPH assay was adapted to PM and showed good analytical performances in terms of sensitivity and good repeatability of measurements. This assay is commonly performed to biological matrices and, to our knowledge, was never applied to PM before. DPPH assay was performed on seven types of PM, representing possible PM contributes (certified urban dust NIST1648a; brake dust; Saharan dust; coke dust; calcitic soil dust; incinerator dust; and diesel particulate matter certified material NIST1650b) and to PM10 and PM2.5 field samples, in parallel with OP assays (OPAA, OPDTT, OPDCFH). Results revealed that DPPH assay gave a linear response and that detectable amounts of reducing species were present in PM (Frezzini et al, 2019). Moreover, the combined application of DPPH and OPs assays to PM field filters treated with different extraction and conservation systems, permitted to identify the presence of competitive equilibria among PM during extraction and conservation steps. PM filters were also chemically analysed by using inductively coupled plasma mass spectrometry (ICP-MS) to determine micro and trace element concentration, with the aim of trying the evaluation of the impact of different emission sources on PM redox properties. More detailed studies about PM native species competitive equilibria will permit to gain more knowledge about OP measurements representativeness.