Separation, immobilization, and biocatalytic utilization of proteins by a supramolecular membrane

Noncovalent Aqua Materials Based on Perylene Diimides

Most robust functional organic materials are currently based on polymers. These materials exhibit high stability, but once formed they are difficult to modify, adapt to their environment, and recycle. Materials based on small molecules that are held together by noncovalent interactions can offer an alternative to conventional polymer materials for applications that require adaptive and stimuli-responsive features. However, it is challenging to engineer macroscopic noncovalent materials that are sufficiently robust for practical applications. This Account summarizes progress made by our group towards the development of noncovalent "aqua materials" based on well-defined organic molecules. These materials are uniquely assembled in aqueous media, where they harness the strength of hydrophobic and π-π interactions between large aromatic groups to achieve robustness. Despite their high stability, these supramolecular systems can dynamically respond to external stimuli. We discuss design principles, fundamental properties, and applications of two classes of aqua materials: (1) supramolecular gels and (2) nanocrystalline arrays. The materials were characterized by a combination of steady-state and time-resolved spectroscopic techniques, electrical measurements, molecular modeling, and high-resolution microscopic imaging, in particular cryogenic transmission electron microscopy (cryo-TEM) and cryogenic scanning electron microscopy (cryo-SEM). All investigated aqua materials are based on one key building block, perylene diimide (PDI). PDI exhibits remarkably stable intermolecular bonds, together with useful chemical and optoelectronic properties. PDI-based amphiphiles carrying poly(ethylene glycol) (PEG) were designed to form linear supramolecular polymers in aqueous media. These one-dimensional arrays of noncovalently linked molecules can entangle and form three-dimensional supramolecular networks, leading to soft gel-like materials. Tuning the strength of interactions between fibers enables dynamic adjustment of viscoelastic properties and functional characteristics. Besides supramolecular gels, we show that simple PDI-based molecules can self-assemble in aqueous medium to form robust organic nanocrystals (ONCs). The mechanical and optoelectronic properties of ONCs are distinctly different from gel-phase materials. ONCs are excellent building blocks for macroscopic free-standing materials that can be used in dry state, unlike hydrogels. Being constructed from small molecules, ONC materials are easy to fabricate and recycle. High thermal robustness, good mechanical properties, and modular design render ONC materials versatile and suitable for a variety of applications. In the future, noncovalent aqua materials can become a sustainable alternative to conventional polymer materials. Examples from our research include stimuli-responsive and recyclable filtration membranes for preparative nanoparticle separation, water purification and catalysis, light-harvesting hydrogels for solar energy conversion, and nanocrystalline films for switchable surface coatings and electronic devices.

Responsive Nanoporous Membranes with Size Selectivity and Charge Rejection from Self-Assembly of Polyelectrolyte "Hairy" Nanoparticles

We report the preparation and characterization of charged nanoporous membranes by self-assembly of "hairy" silica nanoparticles (HNPs) functionalized with polyelectrolyte copolymer brushes. We show that HNP membranes possess high water flux, have well-defined pore sizes, and rejection up to 80% of charged species in solution. The properties of these membranes can be tuned by controlling the length and composition of polymer brushes and the electrolyte concentration in solution. We demonstrate that membrane pore sizes undergo changes of up to 40% in response to changes in the ionic strength of the salt solution. Using MD computer simulations of a coarse-grained model, we link these tunable properties to the conformations of polymer chains in the spaces between randomly packed HNPs. As polymer length increases, the polymers fill the interparticle gaps, and the pore size decreases markedly. On the basis of their straightforward fabrication and tunable properties, HNP membranes may find applications in size- and charge-selective separations, water desalination, and responsive devices.

Template-Guided Assembly of Silk Fibroin on Cellulose Nanofibers for Robust Nanostructures with Ultrafast Water Transport

The construction of multilength scaled hierarchical nanostructures from diverse natural components is critical in the progress toward all-natural nanocomposites with structural robustness and versatile added functionalities. Here, we report a spontaneous formation of peculiar "shish kebab" nanostructures with the periodic arrangement of silk fibroin domains along straight segments of cellulose nanofibers. We suggest that the formation of these shish kebab nanostructures is facilitated by the preferential organization of heterogeneous (β-sheets and amorphous silk) domains along the cellulose nanofiber driven by modulated axial distribution of crystalline planes, hydrogen bonding, and hydrophobic interactions as suggested by all-atom molecular dynamic simulations. Such shish kebab nanostructures enable the ultrathin membrane to possess open, transparent, mechanically robust interlocked networks with high mechanical performance with up to 30 GPa in stiffness and 260 MPa in strength. These nanoporous robust membranes allow for the extremely high water flux, up to 3.5 × 10⁴ L h^-1 m^-2 bar^-1 combined with high rejection rate for various organic molecules, capability of capturing heavy metal ions and their further reduction into metal nanoparticles for added SERS detection capability and catalytic functionalities.

Functional membranes via nanoparticle self-assembly

Nanoporous and ion conductive materials can be prepared by the self-assembly of nanoparticles, providing membranes with size and charge selectivity suitable for separation and possessing proton or lithium transport properties suitable for fuel cells and batteries.