ABSTRACT
The ability to regulate transport across cellular boundaries is essential to the cell’s existence as an open system.1 There is a steady trafc of ions, molecules, polymers, and other species across the plasma membrane. Consider the chemical exchanges between a human muscle cell and the extracellular uid that surrounds it. Sugars, amino acids, and other
nutrients enter the cell; waste products of metabolism leave. The cell takes in oxygen for cellular respiration and expels carbon dioxide. It also regulates its concentrations of inorganic ions, such as Na+, K+, Ca2+, and Cl-, by shuttling them one way or the other across the plasma membrane. Mother Nature has created natural channels that are highly selective, that is, they allow certain molecules and ions to pass
20.1 Introduction ..................................................................................................................................................................... 539 20.2 Nomenclature of Nanoparticles ....................................................................................................................................... 540 20.3 Synthesis and Characterization of Cylindrical-/Conical-/Diamond-Shaped Pores and Membranes .............................. 540
20.3.1 Template Synthesis of Nanotubes and Some Recent Advances .......................................................................... 540 20.3.2 Conical Nanotubes ............................................................................................................................................... 541
20.3.2.1 Conical Nanopores in Tracked-Etched Polymer Films ........................................................................ 541 20.3.2.2 Conical Nanotube Membranes with Uniform Nanopores .................................................................... 541 20.3.2.3 Nonchemical Etching of Porous Membranes ....................................................................................... 542 20.3.2.4 Gold Particle Etching ............................................................................................................................ 543 20.3.2.5 Forming Nanopores by Gas Generation through Salt Decomposition with Polymer Films ................ 543 20.3.2.6 Nanoporous Films Based on Block Copolymers .................................................................................. 544 20.3.2.7 Fabrication and Characterization of Nanotubes Using Layer-by-Layer Self-Assembly ....................... 544 20.3.2.8 Diamond-Shaped CNTs Using Template Synthesis.............................................................................. 545
20.4 Functionalized Nanotubes ............................................................................................................................................... 545 20.4.1 Attaching Different Functional Groups to the Inside versus Outside Surfaces .................................................. 546 20.4.2 Nanotubes for Chemical and Bioextraction and Biocatalysis: Demonstration of Potential Drug
Detoxication Using Nanotubes .......................................................................................................................... 546 20.4.3 Carbon Nanotube Filter Membranes and Modulation of Enzyme Activity through Mechanical
Compression of the Filter .................................................................................................................................... 548 20.4.4 Two-Dimensional Pores in Track-Etched Glass Films ........................................................................................ 548
20.5 Biosensing Using Nanoporous Membranes ..................................................................................................................... 549 20.5.1 Real-Time Biosensor ............................................................................................................................................ 549
20.5.1.1 Fabrication of Reproducible Conical Nanopores in Polymer Membranes ........................................... 549 20.6 Nanotube Membranes for Bioseparations........................................................................................................................ 552
20.6.1 Antibody-Functionalized Nanotube Membranes for Selective Enantiomeric Separations ................................. 552 20.6.2 Functionalized Nanotube Membranes with Hairpin-DNA Transporter with Single-Base Mismatch Selectivity ....553 20.6.3 Enantioseparations of Amino Acids Using Apoenzymes Immobilized in a Porous Polymeric Membrane ....... 556
20.7 Other Interesting Applications of Nanoporous Membranes ............................................................................................ 557 20.7.1 Mimicking Articial Ion Channel Using DNA-Functionalized Conical Nanotubes .......................................... 557 20.7.2 Gated Single Nanopore for Controlling Ion Transport ........................................................................................ 559 20.7.3 Nanoparticles Synthesizer and Sieve ................................................................................................................... 559 20.7.4 Lithography Using Conical Photopen .................................................................................................................. 560
20.8 Conclusions ...................................................................................................................................................................... 562 Acknowledgments ..................................................................................................................................................................... 563 References ................................................................................................................................................................................. 563
more easily than others (or they reject them). For example, there are highly specialized potassium protein channels that allow potassium cations to pass through with high selectivity than other ions. Other examples include water, Na+, Ca2+, Cl-, glucose protein channels, and nuclear pore complexes. The highly selective transportation of molecules and ions is controlled by molecular recognition between transporting species and protein channels present in the cell membranes.
