Infiltration of Macromolecules into Nanoporous Silica Particles

The immobilization of macromolecules within porous materials for applications such as biosensing, biocatalysis, drug delivery, and protein separation requires an understanding of the conditions under which nanopores are accessible to macromolecules. We report the results of a detailed investigation into the infiltration of a polymer probe, poly(acrylic acid) (PAA) of different molecular weights (2000−250 000 g mol-1), in amine-functionalized nanoporous silica particles with a series of pore sizes (4−40 nm). The surface charge of the nanopores and the charge density and conformation of PAA were tuned by changing the PAA solution conditions (e.g., pH and ionic strength) to which the particles were exposed. Thermogravimetric analysis and dynamic light scattering revealed that the extent of PAA infiltration strongly depends upon the relative sizes of the nanopores and the PAA moleculesthe larger the nanopores, the broader the range of PAA molecular weights that can infiltrate the particles. These techniques also revealed that as the pH of the PAA solution increased above 3, the amount of PAA loaded in the particles decreased due to the polymer chains adopting a more extended conformation. In addition, it was found that the ionic strength of the PAA solution played a relatively complex role in PAA infiltration, as electrolytes can screen both the polyelectrolyte charge and the particle surface charge. Loading of PAA in the nanopores was confirmed by transmission electron microscopy of the replicated nanoporous polymer materials, which were prepared by cross-linking the infiltrated polymer and removing the silica template particles. The distribution of PAA in the nanoporous silica particles was examined by confocal laser scanning microscopy after binding fluorescent doxorubicin to the loaded PAA via electrostatic association.