Counterion selection for ion controls of CuO-nanoparticles is important for assessing soil biological responses

Environmental evaluations of metal nanoparticles (NP) rely on seperating the effects of the metal NP from its dissolution products. However, the coordinating or counter anion used in experimental controls may potentially influence biotic indicators used in ecotoxicology and soil health monitoring, thereby compromising the ability to detect real nanoparticle effects and potentially confounding interpretation of metal-NP impacts. Using the example of copper oxide (CuO) NP, we demonstrate for the first time that depending on the anion used in the metal ion control (CuCl₂ versus CuSO₄), different and even opposite conclusions may be drawn for CuO-NP effects. This include a key biological indicator such as enzyme activity in soil samples. Moreover, this effect was specific to environmental conditions and indicator type, raising important methodological and interpretive implications to assess the CuO-NP impacts on soils. Our findings imply that assessments of soil health impacts of metal-NP should consider multiple coordinating anion controls for a given metal, especially when the counterion is known to impact the biological indicator including nutrient ions.