The term “nanoparticle” or “nanomaterial” is somewhat difficult to define rigorously. A nanoparticle has dimensions somewhere in the nanometer regime, that is, a diameter of 1 to 100 nm. Thus nanoparticles span the range from clusters of atoms in solution at the small end to colloidal particles at the large end. Nanoparticles may be amorphous or consist of only a few unit cells of crystalline material. A very large fraction of their atoms are near the surface, see Figure 11. Nanoparticles may be surrounded by vacuum, a gaseous atmosphere, water, or other fluid. In the natural environment, nanoparticles...