Functionalized Responsive Membranes for Environmental Applications (invited paper)

The functionalization of microfiltration membrane pores with macromolecules undergoing conformational changes provides nano-scale interactions in a confined domain.  The development of these types of biomimetic , tunable , nanocomposite membranes with stimuli-responsive properties provide opportunities in flux tunability and separation selectivity control.  Depending on the types of functionalized groups (such as , chain length , charge of groups , biomolecule , etc.) , these membranes could be used in applications ranging from metal (or oxyanions) separation to enzymatic catalysis. The use of microfiltration membrane-based sorbents containing polypeptides which provides nano-scale interaction is a novel technique to achieve very high (1 – 1.5 g/g sorbent) metal sorption and selective separations under convective flow conditions.  In addition to toxic metal separations , one can reduce the sorbed metals directly to nanoparticles for environmental catalysis. These membranes (with functionalized polypeptides or other polyelectrolytes) are also used for obtaining nanofiltration type separations at pressure < 3 bar.  For example , we have demonstrated 90 % separations of arsenic oxyanions with high water flux at pressure of about 1 bar. By taking advantage of helix-coil transitions one can get tunable separations by altering pH. In addition , electrostatic self assembly in pores (layer-by-layer , LBL) can also be achieved through alternate adsorption of cationic and anionic poly-aminoacids or polyelectrolytes under convective flow conditions. Non-stoichiometric immobilization of charged multilayers within a confined pore geometry leads to an enhanced volume density of ionizable groups in the membrane phase leading highly tunable separations. Multilayer assemblies of polyelectrolytes provide excellent platform for protein/enzyme immobilization by providing reusability and high reactivity. Our results with Glucose oxidase enzyme with glucose as substrate showed very high (70 to 80% of homogeneous activity) activity , and continuous synthesis of hydrogen peroxide and gluconic acid. The presentation will include the role of nano-domain interactions for selective separations , full-scale synthesis aspects , stimuli responsive permeability , polypeptide assembly in membrane pores , and integrated enzyme-nanoparticle systems for environmental applications. This research has been supported by NIEHS and NSF-IGERT program.