Colloidal particles can exhibit interesting examples of quantum size effects. Colloids are a heterogeneous mixture of small particles suspended in a solvent.
Each container in the image below contains a nanoparticle solution, and each container contains particles of a different size to the others. The particle material is the same in each container, only the size is changing
Despite the fact that the particles are all composed of the same material, the colour of each solution is different. This is because the differences in particle size affect the electronic structure of the particles and therefore their optical absorption properties.