title: Ion-conducting Nanopores in Polymer Membranes for (Bio)Molecular Sensory Applications
Creators: name: Duznovic, Ivana
Publisher: name: UNSPECIFIED
Year: 2021

description: In the recent years, track-etched nanopores became the major impulse in the development of nanofluidic biosensing devices. The fabrication is processed first by bombarding polymeric films with swift heavy ions. Subsequently, damaged zones within the polymer membrane are created, which are transformed into nanopores via chemical etching technique. Here, pore geometry and diameter are controlled by selecting a suitable chemical etchant and track-etching conditions. As-prepared nanopores are cation selective due to the presence of the ionized carboxylic acid moieties under physiological conditions, which are generated as a consequence of the ion-track etching process. The fixed surface charge polarity and concomitant ion-selectivity of nanopores is tuneable by the modification of native carboxylic acid groups. Moreover, these groups are used to attach desired receptors for biorecognition purpose through specific ligand-receptor interactions. Surface modification and biorecognition processes are monitored by measuring the changes in the electric response of the nanopore via current-voltage (IV) experiments. Regarding the design and miniaturization of nanopore-based biosensing devices, this thesis focusses on the three major challenges, which must be taken into account to enable applications in real systems: i) investigating the application of commercially available nanopore membranes and biodegradable membranes as alternative substrates for nanofluidic sensors. ii) Examining innovative receptor-analyte moieties towards their sensitive, selective and reproducible sensing performance. Here, a variety of receptors and analytes are investigated regarding the detection of metal cations, small molecules (histamine) and biomacromolecules (proteins) as well as polyelectrolytes. In case of metal cations, the selective recognition of potassium ion is achieved by immobilizing pseudo-crown ether-moieties on the pore surface. Moreover, ultrasensitive subnanomolar Cu(II)-detection is obtained by decorating nanopores with an amino-terminated copper and nickel (ATCUN) motif. Both metal cations play crucial roles within neuronal systems of living organisms. Hence, monitoring of ion level is beneficial regarding diagnostic applications. Further inflammatory indicators like histamine are also successfully detected by the use of nanopore membranes functionalized with Ni(II)-nitrilotriacetic acid (NTA)-complexes. In Addition, LBL-deposition is achieved inside nanopores through the electrostatic attraction between poly(allylamine hydrochloride) and poly(acrylic acid) with poly(4-vinylpyridine) (PVP) as a hydrogen-bond compound. After the cross-linking of stacked polyelectrolytes, the exposure to basic pH-conditions triggered the PVP-release, leading to the formation of porous networks in the nanopores as observed by changes in the electrical readout and an increased mass transport across the membrane. This represents the proof of concept for the stimulated release of drugs. Moreover, the highly selective and sensitive performance of pore-bounded camellia nanobody-protein is successfully investigated, which are single domain antibodies and, therefore, considered as highly efficient detectors within immune systems. The used nanobodies exhibit high affinity towards fluorescent proteins (GFP and mCherry) as evidenced by IV-changes of modified pore membranes and by CLSM-imaging methods. This study demonstrates novel analyte detection using nanobody as receptors on nanopore surfaces and to date receptor-analyte interactions were performed in macro-sized setup, whose implantation in real system is quite challenging due to their sampling volumes of about 7 mL. Therefore, nanoporous membranes were integrated in miniaturized Lab-on-Chips to investigate the modification and sensing performance. Further, the standard aqueous electrolyte used for IV-measurements is exchanged by human serum to investigate the IV-impact of a more complex medium on receptor-analyte interactions across nanopore membranes.