One question is whether there might be systematic trends in EEO2 vs. NP mass. If reactivity is proportional to the number of reactive sites on the NP surface, then it would depend on the density of reactive surface sites and the NP surface area. The surface area is approximately taken into account in calculating EEO2, assuming spherical NP shape (surface area ∝ M 2/3 ), and M 2/3 scaling would still hold for non-spherical NPs if the shapes of different size NPs are similar. In either case, EEO2 would tend to be massindependent, assuming the reactive site density is also sizeindependent. As noted above, however, many of the larger NPs are almost certainly aggregates of smaller primary particles, and in that scenario, the actual surface areas and the EEO2 values should increase with mass.