Amir Khabibullin

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.

Grafting PMMA Brushes from α-Alumina Nanoparticles via SI-ATRP

Alumina nanoparticles are widely used as nanofillers for polymer nanocomposites. Among several different polymorphs of alumina, α-alumina has the most desirable combination of physical properties. Hence, the attachment of polymer chains to α-alumina to enhance compatibility in polymeric matrixes is an important goal. However, the chemical inertness and low concentration of surface hydroxyl groups have rendered polymer modification of α-alumina a long-standing challenge. Herein, we report that activation of α-alumina in concentrated or molten NaOH as well as in molten K2S2O7 increased polymer graft density up to 50%, thereby facilitating the synthesis of α-alumina brush particles with uniform grafting density of 0.05 nm(-2) that are readily miscible or dispersible in organic solvents or in chemically compatible polymeric hosts.

Nanoporous membranes with tunable pore size by pressing/sintering silica colloidal spheres.

We prepared robust nanoporous membranes with controlled area and uniform thickness by pressing silica colloidal spheres into disks followed by sintering. Three different diameters of silica particles, 390, 220, and 70 nm, were used to prepare the membranes with different pore size. In order to evaluate their size-selectivity, we measured the diffusion of polystyrene particles through these membranes. Although pressed silica colloidal membranes do not possess visible order or uniform pore size, they showed size-selective transport. We also demonstrated that pressed silica colloidal membranes can be functionalized via pore-filling. Sulfonated polymer brushes were grown inside the pores via surface-initiated atom transfer radical polymerization, which resulted in a material with high proton conductivity suitable for fuel cell applications.

Thermal analysis of charge-transfer complex formed by nitrogen dioxide and substituted calix[4]arene

Simultaneous thermal analysis with evolved gas analysis (STA-EGA) was used to study the ability of 1,3-alternate conformer of tert-butylcalix[4]arene with four n-propoxy substituents (1) to be applied for detection of nitrogen dioxide in reversible sensors. Solid calixarene 1 forms an intensively colored charge-transfer complex (CTC) with gaseous NO2/N2O4. Using the STA-EGA method, the nature and conditions of CTC bleaching were characterized, including the conditions of its reversible change of color from white to dark blue and back at CTC formation and decomposition. For this, the thermal stability of CTC and its regeneration products were studied. This, together with the ion thermograms for evolved gases, gives the information on the oxidation of 1 by nitrogen dioxide if present.

The effect of sulfonic acid group content in pore-filled silica colloidal membranes on their proton conductivity and direct methanol fuel cell performance

We prepared mesoporous silica colloidal membranes pore-filled with polymer brushes with different degrees of sulfonation. In these membranes, the assembly of silica colloidal spheres serves as a rigid matrix containing a continuous network of interconnected mesopores and providing mechanical and thermal stability, non-swelling and water retaining properties, while sulfonic acid group-containing polymer brushes grown on the surface of silica provide proton conductivity. We studied the proton conductivity of these membranes as well as open circuit voltage and linear polarization of the fuel cells prepared using these membranes as a function of sulfonic acid group content in the pore-filling polymer brushes. We found a sigmoidal dependence of the proton conductivity on the amount of sulfonic acid groups. We showed that the proton conductivity of the membrane does not increase significantly after reaching ca. 75% sulfonic acid group content. The fuel cell performance, on the other hand, decreased after reaching ca. 65–70% sulfonic acid group content, which was attributed to the increased methanol permeability of the membranes at higher sulfonic acid group content.