Layer-by-layer Assembled Membranes with Immobilized Porins

Abstract

With the synthesis and functionalization of membranes for selective separations, reactivity, and stimuli responsive behavior arises new and advanced opportunities. The integration of bio-based channels is one of these advancements in membrane technologies. By a layer-by-layer (LbL) assembly of polyelectrolytes, outer membrane protein F trimers (OmpF) or "porins" from Escherichia coli with a central pore of ~2 nm diameter at its opening and ~0.7 × 1.1 nm at its constricted region are immobilized within the pores of poly(vinylidene fluoride) microfiltration membranes, as opposed to traditional ruptured lipid bilayer or vesicles processes. These OmpF-membranes demonstrate selective rejections of non-charged organics over ionic solutes, allowing the passage of salts up to 2 times higher than traditional nanofiltration membranes starting with rejections of 84% for 0.4-1.0 kDa organics. The presence of charged groups in OmpF membranes also leads to pH-dependent salt rejection through Donnan exclusion. These OmpF-membranes also show exceptional durability and stability, delivering consistent and constant permeability and recovery for over 160 h of operation. Characterization of solutions containing OmpF, and membranes were conducted during each stage of the process, including detection by fluorescence labelling (FITC), zeta potential, pH responsiveness, flux changes, and rejections of organic-inorganic solutions.

Keywords: Layer-by-layer membranes; OmpF immobilization; functional polymer; selective separation.

Figure 1

Schematic of the extraction process of OmpF from E. coli.

Figure 2

Schematic of layer-by-layer assembly process of polyelectrolytes into a PVDF membrane and subsequent immobilization of OmpF.

Figure 3

SDS-PAGE of OmpF extracted using valeric acid. Lane 1 is the protein marker, and lane 2 is the purified OmpF obtained.

Figure 4

Bare membrane characterization. (a) Top surface PVDF microfiltration membrane (Nanostone PV200); (b) Pore size distribution of PVDF membrane; (c) Top surface PC membrane (50 nm Whatman-Tisch Scientific); (d) Pore size distribution of PC membrane.

Figure 5

Water permeability and pH responsiveness in layer-by-layer functionalization after two layers. Membrane used: PVDF (Nanostone PV200), original pore size of 48 ± 1 nm. Monomer concentration before polymerization = 1.26 M acrylic acid. A_PVDF is the permeability of the bare PVDF membrane to DIUF.

Figure 6

Membrane characterization in each step of layer-by-layer functionalization. (a) Top surface PVDF-PAA membrane; (b) Top surface PVDF-PAA-PAH-OmpF-PSS membrane; (c) Top surface PVDF-PAA-PAH-PSS membrane; (d) Top surface PVDF-PAA-PAH-FITC labelled OmpF-PSS membrane with green tag (FITC) for fluorescence-labeled OmpF and red tag for membrane structure. Membrane used: PVDF (Nanostone PV200), original pore size of 48 ± 1 nm. PAA weight gain ≈ 3.0%. PAH:PAA = 2/1 molar, PSS:PAA = 1/1 molar; OmpF permeated: 1.74 g/m2 of PVDF top surface.

Figure 7

Zeta potential due to pH change in each step of layer-by-layer functionalization. Membrane used: PVDF (Nanostone PV200), original pore size of 48 ± 1 nm. PAA weight gain ≈ 3.0%. PAH:PAA = 2/1 molar, PSS:PAA = 1/1 molar; OmpF permeated: 1.74 g/m² of PVDF top surface. Some error bars are inside the symbols or are negligible.

Figure 8

Permeability and pore size change of functionalized PVDF membrane (Nanostone PV200) per step of layer-by-layer functionalization. PAA weight gain ≈ 3.0%; PAH:PAA = 2/1 molar, PSS:PAA = 1/1 molar. A_PVDF is the permeability of the bare PVDF membrane to DIUF water. d_PVDF is the mean pore size of the bare PVDF membrane to DIUF water.

Figure 9

OmpF concentration and normalized fluorescence (FITC) intensity of feed and permeate streams during layer-by-layer functionalization on PVDF-PAA-PAH membrane. Original PVDF: Nanostone PV200 with pore size of 48 ± 1 nm. FITC labelled OmpF expt: Note the feed concentration for this experiment was 5 μg/ml. Permeate showing very low intensity of FITC Labelled OmpF (red diamond) indicating high retention during each pass.

Figure 10

Rejection of different organic molecular sizes and inorganic salts for each layer of the layer-by-layer functionalization. Ca²⁺ and Na⁺ as CaCl₂ and NaCl, respectively. Membrane used: PVDF (Nanostone PV200). Some error bars are inside the symbols or are negligible.

Figure 11

Rejections of salts at different pH values. Membrane used: PVDF (Nanostone PV200) with OmpF layer (-PAA-PAH-OmpF-PSS) and without OmpF layer (-PAA-PAH-PSS). Membrane used: PVDF (Nanostone PV200) with a layer-by-layer functionalization. Some error bars are inside the symbols or are negligible.

Figure 12

Rejections of solutions with different molecular weights of model organic solutes over the course of three cycles. Insert: First cycle denoting rejections for the smaller molecular weights. For comparison purposes molecule radius is included in the top axis. Membrane used: PVDF (Nanostone PV200) with a layer-by-layer functionalization of PAA-PAH-OmpF-PSS. Some error bars are inside the symbols or are negligible.

Figure 13

Measured and calculated concentrations over time for a solution of sucrose and NaCl. Insert: Filtration selectivity of NaCl from sucrose. Membrane used: PVDF (Nanostone PV200) with layer-by-layer functionalization of PAA-PAH-OmpF-PSS. Sucrose initial concentration, C₀ = 442.05±0.85 mg/L. Initial volume, V₀ = 0.190 L. Some error bars are inside the symbols or are negligible.