Stimuli-Triggered Ionic and Molecular Transport through Track-Etched Nanopores

• Main Author: Nasir, Saima
• Format: Others
• Language: English; en
• Published: 2014
• Online Access: https://tuprints.ulb.tu-darmstadt.de/4233/1/Stimuli-Triggered%20Ionic%20and%20Molecular%20Transport%20through%20Track-Etched%20Nanopores.pdf; Nasir, Saima <http://tuprints.ulb.tu-darmstadt.de/view/person/Nasir=3ASaima=3A=3A.html> (2014): Stimuli-Triggered Ionic and Molecular Transport through Track-Etched Nanopores.Darmstadt, Technische Universität, [Ph.D. Thesis]

In nature, ion channels facilitate the selective transport of ions, water and small organic molecules across the cell membrane. Under the influence of external stimuli, biological ion channels change their conformation states in order to enhance/inhibit the ionic transport across the membrane, allowing functions such as communication between cells, nerve conduction and signal transmission. Inspired from the functionality and responsiveness of natural ion channels, an attempt to design artificial nanopore-based stimuli-responsive membranes is demonstrated in this thesis. To achieve this goal, the swift heavy ion irradiation of polymer membranes is performed at the UNILAC linear accelerator (GSI, Darmstadt). The damaged zones (latent ion tracks) in the polymer membrane are selectively removed via asymmetric and symmetric track-etching techniques, leading to the fabrication of conical and cylindrical nanopores, respectively. Due to heavy ion irradiation and the concomitant chemical etching process, chemical moieties on the inner pore walls are produced. The native carboxyl (–COOH) groups on the pore walls are further exploited for the chemical attachment of stimuli-responsive molecules having primary amine in their backbone through carbodiimide coupling chemistry. Thermo-responsive membranes are prepared by the immobilisation of amine-terminated polymer (PNIPAAM–NH2) chains on the inner pore wall via “grafting-to” approach. The effective pore diameter is tuned due to swelling/shrinking of the polymer chains by changing the environmental temperature, leading to decrease/increase in the ionic transport through the modified nanopores. The experimental results exhibit the reversible temperature-dependent variation in the analyte permeation across the multi-pore membranes and ionic conductance of single-pore membrane in response to thermal changes in the electrolyte solution in contact with the nanopores. Light-sensitive nanopores are prepared by decorating the pore walls with monolayers of photolabile molecules. The terminal uncharged photosensitive pyrene moieties are cleaved from the pore surface through UV irradiation, leading to the generation of carboxylate (–COO¯) groups. The photo-triggered permselective ionic transport is evaluated experimentally and theoretically by current–voltage (I–V) and analyte permeation measurements of single-pore and multi-pore membrane, respectively. Moreover, dual-responsive nanopores, i.e., nanopores that respond to both light and pH, are prepared by functionalizing the nanopore surface with photosensitive “caged” lysine chains. The uncharged and hydrophobic photo-labile 4,5-dimethoxy-2-nitrobenzyl (NVOC) groups, protecting the amine and carboxylic acid groups of lysine, are removed by exposing the modified pores to UV light, resulting in the production of hydrophilic amphoteric groups on the inner pore walls. In this experiment, polymer membranes having single and arrays of asymmetric nanopores are used for the light-triggered pH-tunable transport of ionic and molecular analytes through the nanopores. In addition to above mentioned stimuli-responsive systems, the modulation of ionic transport is also achieved through biomolecular conjugation inside the confined geometries. To this end, nanopore surface is modified with a suitable biorecognition element (ligand). Firstly, iron-terPy complex (ligand) is immobilized on the pore walls. The Fe(II) ions incorporated in the iron-terPy complex recognize and bioconjugate with lactoferrin through specific metal ion–protein interactions. The bioconjugation processes inside the nanopore significantly decrease the effective pore diameter available for the transport of ions, resulting in the reduction of ionic flux across the membrane. Secondly, an attempt is made to fabricate nanopore which exhibits reversible biomolecular recognition and conjugations via lectin–carbohydrate interactions. For this purpose, nanopore surface is decorated with mannopyranoside moieties which have the ability to selectively bioconjugate with lectin (ConA) protein. The biomolecular binding (bioconjugation) and unbinding inside the confined geometries gives measurable changes in the conductance of single-pore membrane and permeation rate for the case of multipore membrane. Moreover, the ConA binding/unbinding events inside the confined environment are reversible, allowing several measuring cycles by simply washing the bioconjugated membrane with a mannose solution. Such stimuli-responsive nanoporous systems, described in this PhD research work, have huge potential for biosensing, drug delivery and the design of controlled release platforms, especially when the modulation of nanopore transport properties under biological conditions is required.