Application of Conical Nanopores in SiO2 Membranes for Molecular Separation and Biosensing

Abstract

Solid-state nanopore membranes have emerged as a powerful platform for controlling ionic transport and enabling molecular sensing at the nanoscale. Among the various fabrication techniques, ion-track etching offers a scalable and versatile approach to produce nanopores ranging from single pores to high-density arrays with well-defined geometry and narrow size distribution. In addition, silicon dioxide (SiO2) nanopore membranes provide a well-known and highly tunable surface chemistry, which is critical for regulating ionic transport, selectivity, and interfacial interactions. However, despite these advantages, the application of SiO2 nanopore membranes remains relatively underexplored, particularly in the context of exploiting surface functionality for practical applications. This thesis investigates the application of conical SiO2 nanopore membranes fabricated via ion-track etching for two distinct areas: charge-based molecular separation and ion current rectification (ICR)-based biosensing. The fabrication process enables precise control over pore geometry and density, while surface functionalization using silane chemistry allows systematic tuning of surface charge properties. In the first part of this work, charge-based molecular transport and separation are systematically investigated using model dye molecules with different charge states. The results clearly demonstrate that electrostatic interactions between the nanopore surface and the analyte molecules dominate the transport behavior, enabling highly selective separation of oppositely charged species. Native SiO2 membranes possess a negative surface charge at neutral pH and preferentially transport cationic molecules. Upon aminosilane functionalization, the surface charge is converted to positive, enabling selective transport of anionic species. This charge-dependent tunability results in strong separation performance, with separation factors reaching up to ∼36 and ∼20 for negatively and positively charged membranes, respectively. The influence of electrolyte concentration on transport is further examined, highlighting the crucial role of electric double layer effects in governing ion selectivity and transport rates. In the second part of this work, antibody-functionalized conical SiO2 nanopores are used as a label-free biosensing platform based on ion current rectification for protein detection. The platform is validated using model biomolecules, including bovine serum albumin (BSA) and heart-type fatty acid-binding protein (H-FABP). Ultrasensitive detection is demonstrated, achieving a limit of detection (LOD) as low as ∼0.4 aM for H-FABP with a dynamic detection range spanning from attomolar to nanomolar concentrations. Exceptional selectivity is observed, with clear discrimination of target proteins in the presence of non-target species at concentrations up to six orders of magnitude higher. The sensor further exhibits excellent reproducibility and reusability through effective surface regeneration, enabling repeated sensing cycles. Overall, this work establishes conical SiO2 nanopore membranes as a robust and versatile platform for selective molecular transport and ultrasensitive biomolecular detection. By systematically correlating pore geometry, surface chemistry, and electrostatic interactions with functional performance, this thesis provides fundamental insights and practical design principles for the development of next-generation nanopore-based separation technologies and diagnostic systems. Show more