Two Photon Laser Techniques for 3D Miniaturized Structures: Resistive-pulse Sensing and Potential Bio-uses

Abstract

Reliable analysis of individual nanoparticles is becoming increasingly critical to a broad range of application scenarios. Such measurements provide a gateway to understand the physical, chemical, and biological processes in complex systems. For various nanoparticle analysis purposes, one need to not only detect the size of nanoparticles, but track the dynamics of individual analyte in selected space. Fluorescence microscopy techniques and dynamic light scattering (DLS) serve as the most-commonly explored methods in nanoparticle studies. However, these measurements have been limited by the observation time and the measurement rate.Rapid advances in label-free strategies are transforming the way sensing systems are studied, especially with a view on developing novel devices for single nanoparticle analysis. Due to their inherently simple operating principle, which is based on recording the changes in the ionic current through a miniaturized pore that is separated by two electrolyte-filled reservoirs, resistive-pulse sensing (RPS) pore sensors have been gaining prominence for a wide range of analysis: from performing the sequence of DNA/RNA to unravelling the underlying mechanisms of bio-systems. In particular, considerable effects have been devoted to the RPS investigation of nanoparticles, providing profound implications for uses in analysing the size, structure and surface feature of nanoparticles.For a typical RPS system, micro-/nano-pore plays a central role in nanoparticle analysis, through which characteristic resistive pulses are generated due to their different ion transport properties. To fully understand the information contained within the signal, it is critical to prepare accurate pores with full control over pore’s three-dimensional (3D) geometrical features.\xa0 However, the current state-of-the-art of RPS pore fabrication technologies are still not able to achieve reliable control over the 3D internal geometry of the produced pores. Hence, novel techniques enabling the generation of geometrically well-defined micro-/nano-pores are currently of immense interest.Recently, two-photon polymerization (TPP) based techniques have become a powerful tool for rapid and accurate 3D prototyping of microscopic structures. In TPP fabrication, a femtosecond pulsed laser beam is tightly focused on a photosensitive resist material consisting of a mixture of monomers and photo-initiators. This laser-activated reaction leads to localized crosslinking of photosensitive resist material, and by moving the focal point in through the liquid material, arbitrary 3D structures can be formed. By taking advantage of precision optics, tailored 3D structures with up to 100 nm resolution can be realized, suitable for the fabrication of micro-/nano-pores with configurable internal geometries.This PhD project aims to develop robust TPP-based fabrication techniques for accurate RPS pores for the first time. Firstly, a brand-new 3D ablation process based on two-photon femtosecond laser is introduced to generate accurate micro-hole structures in plastic substrates (Chapter 3). This strategy can create arbitrary shaped micro-well structures in plastics through a rapid ablation process without any masks, providing suitable substrate that is required to load the sensing pore construct. Next, this pre-treated plastic substrate is utilized for loading the miniaturized hollow 3D constructs, which is to be realized through the TPP fabrication system (Chapter 4). Systematic investigations are performed to achieve optimized fabrication conditions, including laser power level, structural design, and post-processing. As a proof-of-concept, classical 3D hollow micro-devices, including micro-pore, micro-needle, micro-pump and micro-electrode, are demonstrated and characterized.As TPP platform has been experimentally evaluated for the preparation of tailored 3D micro-constructs, accurate miniaturized RPS pores are then formulated for robust nanoparticle analysis (Chapter 5). TPP based nanolithography is introduced for reliable preparation of customizable RPS pores. For the first time, accurate micro- and nano-pores with different cone angles have been successfully prepared for experimental studies. Subsequently, accurate 3D pores were studied for selected RPS analysis: cis- and trans-conical pores for the investigation of pore’s preferential transport capability; symmetrical pores for the electrical tracking of nanoparticle position; and cylindrical pores for the surface charge analysis of chemically distinct nanoparticles of the same size.Lastly, a promising new strategy based on vertically arranged double-pore construct was introduced to analyse and control single molecular transportation (Chapter 6). TPP nanolithography affords to generate accurately controlled dual-pore systems that have enhanced sensing and controlling capabilities in nanoparticle analysis. By modifying the geometric features of dual-pore systems, the translocating nanoparticles can be analysed both collectively and individually. Moreover, this vertically stacked dual-pore system can be deployed as a modulator to control each nanoparticle event, and even trap/release single nanoparticle within a confined space.In summary, the proposed two-photon laser based fabrication platform serves as robust toolkit for the preparation of accurate RPS pore constructs. The knowledge and methodologies developed in this Thesis can serve as a solid foundation for the further developments of novel/enhanced types of RPS analysis platforms, thus will contribute the development of whole single-molecular analysis systems and help to inspire the next stages in this exciting field of research.