The surface reactivity of iron oxide nanoparticles as a potential hazard for aquatic environments: A study on Daphnia magna adults and embryos

Massimiliano Magro,Marco De Liguoro,Eleonora Franzago,Fabio Vianello,Davide Baratella

doi:10.1038/s41598-018-31483-6

Abstract

Nano-ecotoxicology is extensively debated and nanomaterial surface reactivity is an emerging topic. Iron oxide nanoparticles are widely applied, with organic or inorganic coatings for stabilizing their suspensions. Surface active maghemite nanoparticles (SAMNs) are the unique example of naked iron oxide displaying high colloidal and structural stability in water and chemical reactivity. The colloidal behavior of SAMNs was studied as a function of the medium salinity and protocols of acute and chronic toxicity on Daphnia magna were consequently adapted. SAMN distribution into the crustacean, intake/depletion rates and swimming performances were evaluated. No sign of toxicity was detected in two model organisms from the first trophic level (P. subcapitata and L. minor). In D. magna, acute EC50 values of SAMN was assessed, while no sub-lethal effects were observed and the accumulation of SAMNs in the gut appeared as the sole cause of mortality. Fast depuration and absence of delayed effects indicated no retention of SAMNs within the organism. In spite of negligible toxicity on D. magna adults, SAMN surface reactivity was responsible of membrane bursting and lethality on embryos. The present study offers a contribution to the nascent knowledge concerning the impact of nanoparticle surface reactivity on biological interfaces.

Highlights

As nanotechnology starts to move into large scale production, inevitably nanoscale products and by-products will enter into the aquatic environment
As the preparation of stable colloidal suspensions remains a significant challenge for preparative nanotechnology[37], the employment of stabilizing coatings involves the screening of nanoparticle surface by substitution with the properties of the coating shell
The structural integrity of Surface active maghemite nanoparticles (SAMNs) was the object of several studies[45], thanks to the robustness of stoichiometric pure maghemite nanocrystals, their dissolution can likely be excluded as a factor determining toxicity

Summary

Introduction

As nanotechnology starts to move into large scale production, inevitably nanoscale products and by-products will enter into the aquatic environment. Ecotoxicity effects may be ruled by the physical form of nanomaterials and/or the presence of dissolved species[20,21], which may contribute to toxicity through different modes of action This can introduce confounding factors for the assessment of the eco-toxicology of nanomaterials, which possibly shadow the specific effects of nanoparticles. It is recommended that already-established models for the speciation of trace metals should be used to account for the effects of the dissolved metal fraction[22,23,24,25], facilitating the interpretation of a possible nanomaterial effect On these bases, novel findings should be evaluated as to whether nanoparticles can physically interact with test organisms, undergo dissolution in the aqueous media, and internalize/discretely localize in/on the test organisms. The evaluation of the toxicity of unmodified iron oxide nanoparticles in the aquatic environment could generate a paradox as their poor colloidal stability can compromise the reliability of common tests, which assume the solubility as an implicit requisite[38]

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Conclusion