There are numerous studies on the effect of copper (Cu) and copper oxide (CuO) nanoparticles on environmental organisms. Most take into account the effect of soluble copper ions and/or compare the effects of nanoscale Cu and CuOs with effects of coarser Cu and CuO particles. Some studies report a higher toxicity of nanoparticles compared to dissolved ionic copper but in many studies no differences in the effects could be found. Copper is one of the substances known to be ecotoxic, and copper compounds have been used for decades in paints for ship hulls to prevent the growth of algae, mussels and snails.

Copper and copper oxide nanoparticles were tested for their effects on different organisms living in different habitats. CuO nanoparticles are toxic to bacteria . However, copper ions dissolving from the particles are responsible for the effects; they interact with the "protective cover" of the bacteria and induce oxidative stress. In the presence of substances "scavenging" copper ions (chelators), no toxic effects were observed. Surprisingly, however, the toxic effects differed depending on the origin of ion: ions originating from particles act differently than ions from copper salts. Coarser particles released only very few ions and were not toxic.

The protozoan Tetrahymena responded to various copper compounds with a change in the composition of its fat components . At comparable toxicity nano-CuO released less ions compared to copper salts or coarse copper particles, hence an additional effect by the nano-form is assumed.

The effect of Cu nanoparticles on soil-dwelling worms was tested by mixing the particles into the soil. Depending on the worm species, contradictory results have been achieved. Cu nanoparticles had an inhibitory effect on the propagation of worms but administered in the same amounts as copper salt it was more toxic than the particles in one study and less toxic or of equal toxicity in another study. The effect of Cu nanoparticles and copper salts led to very different gene activation response patterns, leading to the assumption that the copper nanoparticles cause a specific effect, which is not comparable to copper ions .

Studies on zebrafish draw the same conclusion additionally showing a damage to the gills. Cu₂O nanoparticles were less toxic for zebrafish than copper salts. Carp showed a growth delay after exposure to nano-CuO, a particle uptake into various organs was also detected .

CuO nanoparticles disrupt the development of Xenopus embryos. The main intake route was the ingestion of particles. Consequently, damage to the gastrointestinal tract was observed, which was due to both the particulate form and the ions. An electron microscopic study showed an uptake of CuO particles (nano and micro) in the intestines of water fleas but for both particle types no uptake from the gut into the body was observed. Water fleas showed a higher sensitivity to nanoparticles compared with coarser particles or salts .

A soil-dwelling water snail ingested more copper from nano copper oxide compared to coarser copper oxide and copper salts. Accordingly nano copper oxide had the greatest effects on growth, feed intake and reproduction of animals ..

Copper oxide particles had a toxic effect on green algae. In order to prevent the leaching of copper ions, CuO particles were coated with a polymer. These particles, however, showed a higher toxicity. The coating probably increases the particle uptake into the cells. Similar results were obtained on blue-green algae that took up more particles in the presence of organic materials. For various aquatic organisms (algae, crustaceans, rotifers) CuO particles were less toxic than copper salts, from the particles used in this study no leaching of ions was observed .

Different plant species (radish, ryegrass and duckweed) were inhibited in their growth in a consistent manner. In addition, DNA damage was observed in radish and ryegrasses. Although the absorption of copper in plants was higher when copper was present than ions, more damage in the genome occurred after exposure to nanoparticles. The duckweed took up more copper when it was present in particulate form. Corn plants showed nanoCuO induced growth retardation but not with Cu particles or coarse salt. A particle uptake by the roots and the distribution in the plant has been demonstrated ^[15]. Significant differences in the sensitivity of individual plant species were observed .

In summary, it can be stated that the known toxic effects of certain doses of copper occur likewise for the nano-form of copper. From the available studies no general statement can be made as to whether the effect is due solely to the ions or particulate form. The observed effects are due in part to the effect of the dissolving copper ions, and copper may be internalised in principle by many organisms. Between the two types of nanoparticles of CuO and Cu, there are no striking differences in the effects on organisms and there is no difference in the solubility.