Diffusion of Proteins across Silica Colloidal Crystals.

We studied the diffusion of three model proteins, lysozyme (Lz), bovine hemoglobin (BHb), and bovine serum albumin (BSA), normal to the (111) plane of sintered silica colloidal crystals with three different pore "radii" (7.5, 19, and 27 nm). We demonstrated that these colloidal crystals exhibit size selectivity when the nanopores are sufficiently small (7.5 and 19 nm). Because these nanopores are still larger than the diffusing proteins, the observed size selectivity can be attributed to the tortuosity of the colloidal nanopores. Larger (27 nm) nanopores led to higher transport rates but at the cost of selectivity. In addition to the size selectivity, we also demonstrated that 19 nm nanopores possess shape selectivity for the proteins of comparable molecular weights. We showed that the high temperature sintering required for the preparation of sintered colloidal crystals reduces the extent of interactions between the proteins and the nanopore surface, which appear to play a minor role in the diffusion, and that transport selectivity is decided solely by protein size and shape. Taken together, our observations suggest that sintered silica colloidal crystals constitute promising nanoporous membranes for protein separations, with easily controllable pore size, size and shape selectivity, and minimal surface fouling.