A recent study by Wilkinson
et al. addressed the use of solution-engineered palladium (Pd) nanoparticles as a model for health effect studies of automotive particulate pollution. The authors state that over 60% of platinum group metals (PGMs) (Ru, Os, Rh, Ir, Pd, and Pt) are used for the production of automobile catalytic converters [
11,
12]. These converters are constructed by deposition of PGMs on a honeycomb-cordierite substrate covered by a washcoat of cordierite and γ-Al
2O
3[
11]. There may be health effects due to particles which are emitted from converters, but it is extremely difficult to generate PGM-NPs. Therefore, the authors sought an alternative for the production of these NPs by the use of solution synthesis [
11] with the production of dispersions of hydrophilic spherical Pd nanoparticles (Pd-NPs) of uniform shape and size (10.4 ± 2.7 nm) in one step by Bradley's reaction (solvothermal decomposition in an alcohol or ketone solvent). A similar approach also provided mixtures of Pd-NPs and nanoparticles of non-redox-active metal oxides, such as Al
2O
3. The authors furthermore studied particle aggregation in applied media by DLS and NP tracking analysis.
Three years earlier, a group from Munich reported the preparation and characterization of Pd/Al
2O
3 and Pd nanoparticles as standardized test material for chemical and biochemical studies of traffic related emissions [
13]. Specifically, two series of Pd particles were prepared: Pd NPs with 2–4 nm dispersed on aluminium oxide particles of a diameter range between 0.1 to 30 μm and "Pd-only" NPs of 5–10 nm in diameter [
13]. The Pd/alpha-Al
2O
3 particles were reported to be very similar to particles emitted from catalytic converters by mechanical abrasion. The Pd-only particles were suggested to be useful
e.g. for exposure studies in which the presence of aluminium could lead to interferences when studying biological and biochemical effects [
13]. In contrast to the study by Leopold
et al. who characterized the NPs using transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), selective area diffraction (SAD), laser granulometry and graphite furnace atomic absorption spectrometry (GFAAS) for the measurement of Pd concentrations [
13], the group of Wilkinson
et al. also performed toxicological experiments in order to assess putative health effects of the produced Pd NPs and nanocomposite mixtures. For this purpose, they used human primary bronchial epithelial cells (PBEC) and human alveolar carcinoma cell line (A549) as model system for NP-exposure [
11]. They reported that a cellular uptake of Pd nanoparticles was only visible in PBEC, as determined by TEM. However, they found pronounced and dose-dependent effects on cellular secretion of soluble biomarkers both in PBECs and A549 cells and a decreased responsiveness of human epithelial cells to the pro-inflammatory cytokine TNF-α [
11]. Interestingly, when cells were incubated with higher doses of the Pd nanoparticles, induction of apoptosis and caspase activation were observed present in PBEC but not in A549 cells [
11]. In summary, the authors concluded that this mode of Pd NP generation is applicable to study the effects of Pd NPs. It is important to realize that this study cannot be used for an exact toxicological assessment for traffic-related health effects of Pd NPs since it is too preliminary.