Techniques for Molecular Detection: From Mid-Infrared Spectrometers to Nanopore Devices

Citation

Choi, Changsoon (2025) Techniques for Molecular Detection: From Mid-Infrared Spectrometers to Nanopore Devices. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/82w6-ka57. https://resolver.caltech.edu/CaltechTHESIS:06012025-202413304

Abstract

This thesis presents the development and characterization of three distinct molecular detection platforms aimed at improving accessibility and scalability. These systems utilize optical and electrical sensing techniques to analyze solid, liquid, and gas-phase samples, addressing challenges in portability, cost-effectiveness, and throughput.

First, a compact mid-infrared spectrometer was developed using a continuously variable filter composed of Ge/BaF2 distributed Bragg reflectors and a 2-D microbolometer array. This design enables high signal-to-noise ratio measurements across the long-wave infrared region (7.5 - 10.5 µm) without requiring moving parts. The system was calibrated using polymer samples and validated through spectral measurements of gases and liquids using gas cells and attenuated total reflection configurations. The results demonstrate good agreement with commercial Fourier transform infrared (FTIR) spectrometers while maintaining a significantly smaller form factor and lower cost.

Next, a tunable diode laser absorption spectroscopy system was implemented for methane detection. The system uses wavelength-modulated distributed feedback lasers and lock-in detection to achieve high sensitivity. Both free-space and fiber-optic configurations were developed, with detection limits down to 0.5 ppm-m for the free-space system. Real-time gas monitoring was demonstrated using retroreflectors and beam-steering optics, enabling multi-point detection with a single source-detector pair.

Finally, a design for a multiplexed nanopore detector was proposed to improve throughput. Electric properties of solid-state nanopores were investigated through analytical modeling, numerical simulations, and experimental conductance measurements. A new sensing architecture incorporating embedded electrodes was introduced. Simulations confirmed that this design enables independent detection of translocation events at each pore. Prototype devices were fabricated to validate the concept.

Together, these platforms offer scalable and accessible alternatives to conventional molecular detection systems, with potential applications in environmental monitoring, industrial gas sensing, and real-time molecular diagnostics.

Thesis Files