Document Type

Thesis - Open Access

Award Date

2026

Degree Name

Master of Science (MS)

Department / School

Electrical Engineering and Computer Science

First Advisor

Xiaojun Xian

Abstract

Continuous monitoring of gases is a critical challenge for wearable health monitoring systems and portable environmental gas detection. The technology used for gas detection, including electrochemical sensors, metal oxide semiconductors, and non-dispersive infrared systems, is based on bulky optical components, which limit their practicality as an ideal tool for wearable and field-deployable stages. This thesis investigates if the passive colorimetric chemistry read-out provided by a low-power CMOS imager-based architecture can provide a non-invasive alternative across two different application domains: medical transcutaneous CO₂ monitoring systems and toxic gas detection from environmental. Two sensing platforms were developed and evaluated. In the first project, we hypothesized that incorporating a cobalt-based metal-organic framework (MOF-71) into a pH-sensitive colorimetric sensing matrix would enhance sensitivity to CO₂ relative to indicator-only controls, and that a 2×3 sensor array imaged by a lensless CMOS sensor would enable reliable quantification without active skin heating. Experimental results confirmed that MOF-71 incorporation produced consistently larger absorbance changes under identical CO₂ exposure conditions. The integrated platform achieved a detection limit of approximately 26 ppm under high-humidity conditions, a dynamic range spanning 0 to 20,000 ppm, reversible response over multiple exposure cycles with response and recovery times of approximately 50 seconds, and strong selectivity against representative interferents including acetone, ammonia, ethanol, and oxygen. Feasibility for transcutaneous monitoring was demonstrated using skin-gas samples collected from four healthy adult subjects, with the sensor distinguishing rest versus light physical activity conditions and correlating with a commercially available sensor as a reference instrument. The second project is a proof of concept, which is studied to extend the imaging-based framework to a reflectance-based phenomenon, using an opaque thin-layer chromatography silica gel plate as substrate, and was used to detect ammonia and formaldehyde, two major toxic chemicals of significant environmental and occupational health concern. Our results confirmed: ammonia sensing exhibits reversible behavior suitable for continuous, reusable monitoring, but formaldehyde sensing shows an irreversible reaction, establishing it as a single-use platform. Calibration curves with R² value exceeding 0.97 were achieved for ammonia across all RGB channels but for the formaldehyde channel B showed 0.974 R².

Publisher

South Dakota State University

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Rights Statement

In Copyright