# Safe Functional Modified CuO Nanoparticles?
## 2 Safe(r)-By-Design CuO Nanoparticles
### Surface Modification by Coating or Capping  
The rationale for the surface modification of CuO nanoparticles by coating or capping is that the coating or capping may act as a scavenger for copper ions and reactive oxygen species [46; 47; 48]. It may be noted, though, that coating or capping may also increase nanoparticle hazard mechanisms [23]. An example thereof is the substantial increase in CuO nanoparticle toxicity to the green alga _Chlamydomonas reinhardtii_ and the aquatic macrophyte _Lemma gibba_ by coating CuO nanoparticles with styrene-butylacrylate copolymer [23]. [PERSON] et al. [23] have suggested that this increase in toxicity is linked to changing nanoparticle interactions with cells and toxicity mechanisms. In a safe-by-design study by [PERSON] at al. [47] CuO nanoparticles were coated with citrate, polyvinylpyrrolidone (PVP) and aminomethylphosphonate. Testing was for cytotoxicity in the (human) cell lines BEAS-2B and THP-1 and for inflammation of mouse lung tissue. The reduction in negative impacts by CuO nanoparticles was very limited when coating was with citrate. In the case of coating with PVP, the reduction in negative impact was moderate and in the case of phosphonate coating was relatively large, though not complete. In a safe-by-design study, [PERSON] et al. [45] studied the impact on A549 human lung epithelial cells and _Xenopus laevis_ embryos of capping of CuO with polyethyleneimine (PEI) and polyethylene glycol (PEG). They found that modifying the surface of CuO nanoparticles with PEG reduced hazard, but capping with PEI did not. As to the mechanism underlying reduced hazard, [PERSON] et al. [45] found in the case of exposure to CuO nanoparticles capped with PEG, that the generation of reactive oxygen species in cells was reduced. The presence of Cu ions in lung cells was higher in the case of PEI-capped CuO nanoparticles than in the case of their PEG-coated counterparts. Hazard was not eliminated by capping with PEG. Extrapolation of the findings of [PERSON] et al. [45] to humans and the different ways of intake by humans is subject to uncertainty [49; 50; 51] and the extrapolation to other organisms that can be exposed to CuO nanoparticles is beset by uncertainties [50; 52]. In a safe-by-design study, [PERSON] et al. [46] studied the impact of CuO nanoparticles surface-modified with ascorbate and PEI on short-term pulmonary inflammation in rats. These surface modifications had been tested before as to their cytotoxicity in a mouse macrophage cell line [41]. In the latter study ascorbate-modified CuO particles scored best in reducing cytotoxicity. However, [PERSON] et al. [46] found no significant differences as to toxic effects and toxic potency in the lungs of rats between the two surface modifications. This underlines the uncertainty in extrapolating outcomes of tests in cell lines to organisms. Inhalation hazard to rats was not eliminated by capping. There is a set of studies testing the hazard of CuO nanoparticles coated with polyethylene glycol, carboxylate and methylaminated compounds, if compared with the hazard of -pristine CuO nanoparticles, in a variety of biological settings [43; 48; 53; 54; 55]. These studies will be briefly presented below. Table 1 compares the hazard of CuO nanoparticles coated with polyethylene glycol (PEGylated CuO nanoparticles), with the hazard of pristine CuO nanoparticles. [PERSON] et al. [43] used CuO nanoparticles with polyethylene glycol, carboxylate and methylaminated compounds in 14 days toxicity tests with earthworms. In fresh soil, CuO nanoparticles with a carboxylate and methylaminated coating were more toxic than pristine CuO nanoparticles, whereas PEGylated CuO nanoparticles had the lowest toxicity. In aged soil, Cu nanoparticles that had a methylaminated organic coating were more toxic than pristine CuO nanoparticles, whereas carboxylated and PEGylated CuO nanoparticles had (similar) lower toxicities than pristine CuO nanoparticles. Using CuO nanoparticles with the same coatings as used by [PERSON] et al. [43], [PERSON] et al.