Samples were analyzed by a LEO (Zeiss) 1530 Scanning Electron Microscope at a magnification of 10,000× as well as 50,000× (inserts).

Four different samples of ZnO particles are indicated as:

• ZnO I (from Nanostructured and Amorphous Materials Inc., with a declared primary particle diameter of 20 nm and a specific surface area (SSA) of 50 m2/g),
• ZnO II (Nanoactive, from Nanoscale Materials Inc., with a declared crystallite size of <10 nm and SSA of 70 m2/g),
• ZnO III (from Sigma-Aldrich, with a declared SSA <10 m2/g), and
• ZnO IV (obtained through the European Commission Joint Research Centre (Ispra, Italy) as reference nanomaterial NM-110, with a diameter of 100 nm and SSA of 14 m2/g).

Electron microscopy of these four samples was performed by Scanning Electron Microscopy using a JOEL 7500F Electron Microscope (JOEL Eching, Germany). Representative pictures are shown in Fig. 1. As can be seen in the figure, there were notable contrasts in morphology of the four samples.