Particle size is one of the most important parameters not only for description of fundamental properties of materials, but also within the biological systems as it can affect a number of key features and processes, such as drug targeting, delivery or distribution. Regarding nanosized particles, ISO standard ISO/TS 27687:2008 (ISO, 2008) provides their definition as an object with a size between 1 and 100 nm. In the EU, the respective Commission Recommendation (EC, 2011) defines a “nanomaterial” to be a “natural, incidental or manufactured material containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for 50% or more of the particles in the number size distribution, one or more external dimensions is in the size range 1–100 nm”.