Size effects are observed with the reduction in size of structural elements, particles, crystallites and grains below a certain threshold. Such effects occur at the average size of crystal grains of less than 100 nm, and is more clearly observed at the grain size below 10 nm. Quantum size effects result in the electronic properties of a substance or material and are associated with a reduction of the dimensions of the electron gas, which leads to a change in the energy spectrum (see, e.g., a blue shift
The effect of particle size on the physicochemical properties of a substance can be explained by the presence of surface pressure acting on the substance. This additional pressure, which is inversely proportional to the particle size, leads to an increase in Gibbs energy and, consequently, an increase in saturated vapour pressure above the nanoparticles
, reduction of the boiling temperature of the liquid phase and melting of the solid (Figure). Changes are also observed in other thermodynamic characteristics, the equilibrium constants and standard electrode potentials. For example, as the size of silver nanoparticles is reduced, the standard potential of Ag+/Ag couple may become negative, and silver will dissolve in dilute acids with evolution of hydrogen.
The size effect is highly relevant for heterogeneous catalysis
. In many cases, nanoparticles exhibit catalytic activity where larger particles are not active. For example, gold nanoclusters catalyse the selective oxidation of styrene in air to benzaldehyde:
whereas gold particles of larger size have no effect on this reaction.
Size effects in biology are quite different. Biomolecules, polymers and intracellular structures are nanoscale, but their properties (functions) are determined primarily by their structure rather than dimensions.
However, the interaction of artificial structures with biological structures depends not only on the structure, but the dimensions as well. For instance, the permeability of the skin and blood vessels to liposomes
depends on the size of the latter. As a consequence, packaging drugs in liposomes results in a change in the important pharmacological properties of the latter, such as blood circulation time and distribution in the organs. Creating nanoscale surface
relief on synthetic materials facilitates cells adhesion
compared to micro-relief, and is used in tissue engineering
. The size and topography of nanoparticles affect the mechanism and efficiency of their endocytosis
, as well as intracellular localisisation. The toxicity of the particles can also be determined by their dimensions. For example, 1.4 nm gold nanoparticles have the highest toxicity compared with other sizes, since they insert into the major groove of DNA
and induce cell