> Question: “Why, for example, are titanium dioxide nanoparticles transparent once they have reached a certain size?”
This is due to a physical effect. Any object that is clearly smaller than the wavelength of the visible light is invisible. Visible light is composed of wavelengths in the range of approximately 380 – 790 nanometers.
Particles that measure e.g. 100 nanometers are not visible anymore. This, however, happens only under extraordinary conditions: As soon as several particles are found one in front of or beside the other, they (normally) take on a white color and become visible again due to e.g. diffraction or dispersion. In spite of this, not all of the particles’ chemical and physical properties change at the nanolevel. Absorption properties, for example, persist i.e., the particles do not reflect light anymore, thus are transparent but actually absorb UV radiation.