Entrapment of Individual DNA Molecules and Nanoparticles in Porous Alumina Membranes

Depth-resolved fluorescence imaging allows the motion of single DNA molecules and single nanoparticles at the liquid/solid interface to be recorded in real time. Porous alumina membranes were employed as model chromatographic packing material. Using a suitable pH and ionic strength, adsorptive interactions are suppressed. The effects of 3-dimensional topography, specifically the presence of nanopores, on DNA and nanoparticle migration across the surface are, thus, revealed. The residence times and the number of immobilized DNA molecules or particles increased as the pores size increased. Yet, we found that the pore diameter must be significantly larger than the particle diameter or the DNA short radius before entrapment can occur. Furthermore, the depth distribution of particles does not conform to one-dimensional diffusion in the pores, probably because of collisions with the walls. These observations provide new insights into conventional liquid chromatography as well as size-exclusion chromatography and membrane separations.