Novel applications of colourimetric gas sensors

Abstract

This thesis is focused on developing new applications of existing and novel colourimetric indicators, primarily for the detection of carbon dioxide (CO₂).

The Introduction chapter gives a short history of CO₂ indicators from aqueous solutions, to ‘dry’ ink-based indicators, and the onset of plastic films. Some existing applications of colourimetric CO₂ indicators are discussed.

The Experimental chapter outlines the most important instrumentation and methods used throughout this PhD, to include UV-Vis spectroscopy, photography and digital colour analysis (DCA), lifetime O₂ monitoring, microbiological techniques, general plastic film CO₂ indicator preparation, and gas mixing.

In Chapter 3, two commercial colourimetric CO₂ indicators are replicated in the lab- one for monitoring the level of dissolved CO₂ in an aquarium, and another for monitoring the level of CO₂ in breath. Photography and DCA are used to convert the colour data from the indicators into a quantitative measure of CO₂.

In Chapter 4 the preparation of a plastic film indicator for the measurement of CO₂ in aquaria is described, based on extrusion of low-density polyethylene (LDPE) mixed with a small amount of a CO₂ sensitive pigment. The pigment comprises a mixture of a pH-indicator dye, xylenol blue (XB), with a base, coated onto silica. Digital photography and DCA quantifies the response of the XB-LDPE indicator in an aquarium.

In Chapter 5, a new form of micro-respirometry, %CO2-μR, is used to measure total viable count (TVC) of bacteria. In %CO₂-μR, the apparent absorbance of a small, XB-LDPE 3D-printed CO₂ indicator is measured at any t, as it is related to the %CO₂ dissolved in inoculated growth medium. The potential of %CO₂-μR for measuring the bacterial load of CO₂-generating aerobes and anaerobes is studied and discussed briefly.

Finally, a colourimetric indicator for the detection of microbial hydrogen (H₂) evolution is described in Chapter 6. A H₂-sensitive indicator was fabricated based on the dye methylene blue (MB), and was used to determine the TVC of a range of H2-producing anaerobes.

Date of Award	Jul 2026
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Department for the Economy
Supervisor	Amilra De Silva (Supervisor), Peter Nockemann (Supervisor) & Andrew Mills (Supervisor)