Nano-model membrane filters for the well-controlled separation of biomolecules

The separation of proteins into relatively homogeneous groups and sizes is important in biopharmaceuticals. We developed a simple yet general method for engineering cubic mesopore cages inside silica nanotubes (NTs) using an anodic alumina membrane (AAM) as hostage pockets. The three-dimensional mesocage structures with a large unit cell constant of 17.3 nm and uniform pore entrance (∼5 nm) running in the direction perpendicular to the axis of the silica NTs allow the development of size-exclusive nanofilter membranes as a powerful tool for the separation of proteins, such as lysozyme, myoglobin, β-lactoglobulin, and hemoglobin. Cage silica NTs were synthesized within the nanopore of AAM pocket hostages. Quaternary microemulsion liquid crystalline phases of triblock copolymer F127 (PEO100PPO70PEO100) were used in the template-guided synthesis of cubic Im3m mesocage pores inside the NTs. Thus, the development of nanofilter membranes composed of 3D mesocage silica NTs perpendicular to the longitudinal axis of the nanochannels would effectively enhance the size-based separation of proteins. In such nanofilter design, the coating of the pore channels of the AAM with polar silane coupling agents, such as N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride, prior to the addition of surfactant F127/silica composition domains facilitate the production of extremely robust constructed sequences of membranes without the formation of air gaps between silica NTs and AAM walls. Our nanofilter enables the precise modification of the nanoscale pore surfaces of the formed silica NTs with organic moieties, such as trimethylchlorosilane, leading to the efficient filtration of high concentrations of proteins (retentate ≥ 10−4 mol dm−3) without substantial kinetic hindrance.