Nanoplastic paradox: unraveling the complex toxicity of nano-sized polyethylene†

The increasing production of polyethylene has inevitably led to its accumulation in the environment, which gradually created a global environmental problem. Unfortunately, despite the huge attention paid to microplastics and the increasing research interest in nanoplastics in the environment, we are still far from fully understanding their true toxicity. The aim of the present work was to mimic the real oxidation process that forms nano-sized polyethylene (nPE) particles from larger waste, and to characterize the obtained nPE in terms of their chemical properties and cytotoxicity. Environmentally relevant nanoplastic was used to avoid the biased interpretations of its impact on cell viability in vitro and on the physiology of the American cockroach (Periplaneta americana) as an in vivo model. The results obtained might be at least surprising, as the toxicity of nPE in both in vitro and in vivo tests increased with the dilution of the solution. This apparent contradiction is explained by the unique physicochemical properties of the obtained nanomaterial and the increasing contribution of the polar layer of nanoparticles formed during oxidation. Thus, the presence of Lewis acidic sites and the polarity of the surface underlie the observed phenomena: (i) the ability of nanoplastic particles to agglomerate and aggregate, (ii) their influence on cell viability, (iii) the decrease in the membrane potential of neurons, and (iv) the changes in the behavior of highly resistant organisms such as Periplaneta americana.