Restricted and Unrestricted Migration Mechanisms of Silica Nanoparticles in Agarose Gels and Their Utilization for the Separation of Binary Mixtures

The migration of SiO₂ nanoparticles (NPs) was investigated during agarose gel electrophoresis with various gel mesh sizes. As a model system, spherical SiO₂ NPs of different sizes were synthesized by Stöber synthesis, and the surface was modified by covalently binding different types of ligands to tailor the ζ-potential as required. In the following step, agarose gels were cast with Tris/borate/EDTA (TBE) buffer solution, thereby controlling the mesh sizes by varying agarose concentrations, with higher concentrations leading to smaller mesh sizes. Additionally, the applied voltage could be varied for different electrophoretic experiments. Generally, large mesh sizes and high applied voltages lead to faster particle migration. The migration of SiO₂ NPs occurs under two regimes: restricted migration, where small mesh size interferes with large particles so that the gel is acting similar to a sieve; and unrestricted migration, where the mesh size does not influence the particles’ migration behavior and they move according to their ζ-potential. Following these regimes, two different mechanisms of NP separation were possible inside agarose gels. Both mechanisms were used to separate binary mixtures into the former individual fractions, as proven by scanning electron microscopy (SEM) or the ninhydrin test.