Ultrahigh Ionic Exclusion through Carbon Nanomembranes.

The collective "single-file" motion of water molecules through natural and artificial nanoconduits inspires the development of high-performance membranes for water separation. However, a material that contains a large number of pores combining rapid water flow with superior ion rejection is still highly desirable. Here, a 1.2 nm thick carbon nanomembrane (CNM) made from cross-linking of terphenylthiol (TPT) self-assembled monolayers is reported to possess these properties. Utilizing their extremely high pore density of 1 sub-nm channel nm-2 , TPT CNMs let water molecules rapidly pass, while the translocation of ions, including protons, is efficiently hindered. Their membrane resistance reaches ≈104 Ω cm2 in 1 m Cl- solutions, comparable to lipid bilayers of a cell membrane. Consequently, a single CNM channel yields an ≈108 higher resistance than pores in lipid membrane channels and carbon nanotubes. The ultrahigh ionic exclusion by CNMs is likely dominated by a steric hindrance mechanism, coupled with electrostatic repulsion and entrance effects. The operation of TPT CNM membrane composites in forward osmosis is also demonstrated. These observations highlight the potential of utilizing CNMs for water purification and opens up a simple avenue to creating 2D membranes through molecular self-assembly for highly selective and fast separations.

Biphasic Formation of 2D Nanomembranes by Photopolymerization of Diacetylene Lipids as Revealed by Infrared Difference Spectroscopy.

Two-dimensional nanomembranes are promising materials for filtration or separation by providing the basis for controlled and rapid transport between two compartments. The polymerization by UV light of diacetylene-containing lipids at an interface produces free-standing 2D nanomembranes. Here, we analyzed in situ the nanomembrane formation of 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DiynePC) and 1-palmitoyl-2-(10,12-tricosadiynoyl)-sn-glycero-3-phosphoethanolamine (PTPE) on germanium using light-induced infrared difference spectroscopy with attenuated total reflection to obtain insights into the kinetics and mechanism of the polymerization process. Our interpretation is supported by atomic force microscopy and density functional theory. Formation of the polymer network is evidenced by changes in the frequency of C═O stretches acting as infrared probes. However, spectral and kinetic analysis revealed a biphasic process in the monolayer. In both phases, losses in signal of CH2 stretches are observed which are not in agreement with the accepted mechanism of chain propagation for diacetylene polymerization. These signals are dominant in the second phase and are assigned to termination reactions with some contributions from intramolecular consecutive reactions. This finding now provides a spectroscopic measure for the identity and integrity of the nanomembrane complementary to microscopic analysis. We deduce that limited 2D mobility on the solid support promotes intramolecular termination, leading to smaller domains.

Characterization of Robust and Free-Standing 2D-Nanomembranes of UV-Polymerized Diacetylene Lipids.

Free-standing lipid membranes are promising as artificial functional membrane systems for application in separation, filtration, and nanopore sensing. To improve the mechanical properties of lipid membranes, UV-polymerized lipids have been introduced. We investigated free-standing as well as substrate-supported monolayers of 1-palmitoyl-2-(10,12-tricosadiynoyl)- sn-glycero-3-phosphoethanolamine (PTPE) and 1,2-bis(10,12-tricosadiynoyl)- sn-glycero-3-phosphocholine (DiynePC) and characterized them with respect to their structure, morphology, and stability. Using helium ion microscopy (HIM), we were able to visualize the integrity of the lipid 2D-nanomembranes spanning micrometer-sized voids under high-vacuum conditions. Atomic force microscopy (AFM) investigations under ambient conditions revealed formation of intact and robust pore-spanning 2D-nanomembranes up to 8 × 2 µm2 in size. Analysis by attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) verified a distinct reduction of signal at 2143 cm-1 from diacetylene groups in the 2D-nanomembranes after UV-polymerization. Further high-resolution AFM investigations of unpolymerized lipid monolayers revealed a well-ordered two-dimensional network, when deposited on highly oriented pyrolytic graphite (HOPG). These structures were inhibited for polymerized adlayers. Structural models for the molecular arrangement of the adlayers are proposed and discussed.