Author Identifiers

Wade Lonsdale
ORCID: 0000-0001-5558-5403

Date of Award

2018

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Science

First Advisor

Professor Kamal Alameh

Second Advisor

Dr Magdalena Wajrak

Field of Research Code

0301, 0399, 0906, 0205, 0912

Abstract

The measurement of pH is undertaken frequently in numerous settings for many applications. The common glass pH probe is almost ideal for measuring pH, and as such, it is used almost ubiquitously. However, glass is not ideal for all applications due to its relatively large size, fragility, need for recalibration and wet-storage. Therefore, much research has been undertaken on the use of metal oxides as an alternative for the measurement of pH.

Here, a solid-state potentiometric pH sensor is developed using ruthenium metal oxide (RuO2). Initially, pH sensitive RuO2 electrodes were prepared by deposition with radio frequency magnetron sputtering (RFMS) in a reactive oxygen plasma, onto screen-printed carbon based electrical contacts (substrates). These electrodes performed well, between pH 4 and 10, exhibiting Nernstian pH sensitivity, low hysteresis and low drift rate. However, these electrode were found to exhibit less than ideal properties outside this range (pH 2-12), though this could be overcome using a pH 12 conditioning protocol. Later, improved RuO2 pH sensitive electrodes were developed and characterised. Elimination of the carbon substrate material resulted in electrodes that displayed excellent performance from pH 2 to 12, even without pH 12 conditioning.

Whilst this RuO2 electrode displayed excellent pH sensing performance, RuO2 along with all other metal oxide based pH sensors suffer from interference caused by strong oxidising and reducing agents. To reduce this interference, Ta2O5 and Nafion protective layers were studied. Using a combination of sputter deposited Ta2O5 (80 nm) and thermally cured drop-cast Nafion, an electrode was manufactured, which was immune to interference from dissolved oxygen, and resistant to stronger redox species. This electrode was found to outperform an unprotected RuO2 electrode and was suitable for application in several common beverage samples.

In order to construct a potentiometric pH sensor a reference electrode is also required. Here, a pH insensitive reference electrode was developed by modification of the pH sensitive RuO2 electrode with a porous polymer junction containing SiO2. The reference electrode showed very low sensitivity to pH and KCl. The reference electrode provided a suitably stable potential over short periods of time, allowing accurate pH measurements to be made. The potential of the reference electrode was found to drift over longer time periods, however, this could be accounted for by recalibration.

The developed working and reference electrodes were then used to construct a pH sensor. The sensor displayed excellent performance between pH 2 and 6; close to Nernstian sensitivity (-55.3 mV/pH), linear response (R2=1.0000) and excellent reproducibility (hysteresismV). The sensor was applied to several beverage samples, where it was shown to perform accurately, results within ±0.08 pH of a commercial glass pH sensor. The sensor develop here would be suitable for development into handheld and in-situ type pH sensor devices.

Access Note

Due to copyright restrictions, Chapters 2, 3 and 6 are currently not available in this version of the thesis

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