Surface-modified silica colloidal crystals: nanoporous films and membranes with controlled ionic and molecular transport.

Nanoporous membranes are important for the study of the transport of small molecules and macromolecules through confined spaces and in applications ranging from separation of biomacromolecules and pharmaceuticals to sensing and controlled release of drugs. For many of these applications, chemists need to gate the ionic and molecular flux through the nanopores, which in turn depends on the ability to control the nanopore geometry and surface chemistry. Most commonly used nanoporous membrane materials are based on polymers. However, the nanostructure of polymeric membranes is not well-defined, and their surface is hard to modify. Inorganic nanoporous materials are attractive alternatives for polymers in the preparation of nanoporous membranes. In this Account, we describe the preparation and surface modification of inorganic nanoporous films and membranes self-assembled from silica colloidal spheres. These spheres form colloidal crystals with close-packed face centered cubic lattices upon vertical deposition from colloidal solutions. Silica colloidal crystals contain ordered arrays of interconnected three dimensional voids, which function as nanopores. We can prepare silica colloidal crystals as supported thin films on various flat solid surfaces or obtain free-standing silica colloidal membranes by sintering the colloidal crystals above 1000 °C. Unmodified silica colloidal membranes are capable of size-selective separation of macromolecules, and we can surface-modify them in a well-defined and controlled manner with small molecules and polymers. For the surface modification with small molecules, we use silanol chemistry. We grow polymer brushes with narrow molecular weight distribution and controlled length on the colloidal nanopore surface using atom transfer radical polymerization or ring-opening polymerization. We can control the flux in the resulting surface-modified nanoporous films and membranes by pH and ionic strength, temperature, light, and small molecule binding. When we modify the surface of the colloidal nanopores with ionizable moieties, they can generate an electric field inside the nanopores, which repels ions of the same charge and attracts ions of the opposite charge. This allows us to electrostatically gate the ionic flux through colloidal nanopores, controlled by pH and ionic strength of the solution when surface amines or sulfonic acids are present or by irradiation with light in the case of surface spiropyran moieties. When we modify the surface of the colloidal nanopores with chiral moieties capable of stereoselective binding of enantiomers, we generate colloidal films with chiral permselectivity. By filling the colloidal nanopores with polymer brushes attached to the pore surface, we can control the ionic flux through the corresponding films and membranes electrostatically using reversibly ionizable polymer brushes. By filling the colloidal nanopores with polymer brushes whose conformation reversibly changes in response to pH, ionic strength, temperature, or small molecule binding, we can control the molecular flux sterically. There are various potential applications for surface-modified silica colloidal films and membranes. Due to their ordered nanoporous structure and mechanical durability, they are beneficial in nanofluidics, nanofiltration, separations, and fuel cells and as catalyst supports. Reversible gating of flux by external stimuli may be useful in drug release, in size-, charge-, and structure-selective separations, and in microfluidic and sensing devices.