Thesis

Preparation of a fibre optic sensor based on AgNp/silica composite for pH measurement

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Awarding institution
  • University of Strathclyde
Date of award
  • 2024
Thesis identifier
  • T16928
Person Identifier (Local)
  • 201890949
Qualification Level
Qualification Name
Department, School or Faculty
Abstract
  • The measurement of hydrogen ion concentration in a solution, commonly referred to as pH, holds significant importance in various industries. However, measuring pH under high-temperature and high-pressure (HTHP) conditions has posed a formidable challenge due to the lack of reliable sensors and instrumentation. Recent research has demonstrated the potential of utilising the localised surface plasmon resonance (LSPR) phenomenon seen by gold nanoparticles deposited on optical fibres for pH detection under varying temperature and pressure conditions. Usingthis reported stability of optical fibres in HTHP conditions, this project aimed to create an optical fibre-based pH sensor using silver nanoparticles (AgNp) embedded within a silica matrix, which is suitable for use under HTHP conditions. In this work, a AgNp/silica coating is applied on a glass optical fibre to create an optical pH sensor. The initial stage of the project involved the preparation of colloidal silver nanoparticles and silica gel as separate entities, with a focus on stabilising the colloidal solution and optimising silica gel production. Subsequently, setbacks were overcome in creating the AgNp/silica coating, refining the sol-gel method, and enhancing adhesion through careful adjustments of gelation temperature and heat treatment procedures. The project optimised the gelation process to achieve goodadhesion on the glass while maintaining sufficient silver nanoparticles for pH sensitivity. Characterisation through UV-Vis spectroscopy indicated an extinction peak at 400 nm, which confirms the presence of silver nanoparticles. Afterwards, the AgNp/silica coating was applied to the optical fibres. In the preparation of the optical fibre, the fibre was etched using 7 M sodium hydroxide (NaOH). This marked a successful milestone in utilising NaOH as an etchant for optical fibres. The coated optical fibre reached an ideal core diameter of 104 ± 1 microns, which was required for pH sensitivity. Extensive experimentation led to fine-tuning the sensor setup, ensuring optimal sensitivity while emphasising the delicate balance between light source voltage and detector integration time. Durability testing revealed susceptibility to acidic environments, underscoring theneed for coating improvement. The project explored pH measurement under ambient conditions, demonstrating the sensor's accuracy and stability in various chemical conditions. A calibration curve was constructed, and the coating's reproducibility and repeatability were thoroughly analysed, further validating its reliability as a pH sensing tool. The calibration curve provided insights into pH measurement using theoptical method. Additionally, the performance and sensitivity of the sensor were investigated and compared between sensors prepared in the same batch and those from batch-to-batch production, offering insights into the sensor's characteristics. Furthermore, an evaluation was conducted to assess the accuracy of the sensor in comparison to the potentiometric pH measurement technique. The project's most challenging phase involved testing the sensor in HTHP conditions, revealing issues related to the stability of the AgNp/silica coating at higher temperatures and pressures. It was observed that the pH decreased as the temperature of the solution increased. The experimental apparatus was constructed, which allowed experiments up to a gauge pressure of 5 bar, equivalent to 156 °Ctemperature, where a higher dissolution of the coating was observed. Nonetheless, the sensor was stable at lower gauge pressures, such as 2 bar, even during multiple heating and cooling cycles. Despite these challenges, the inclusion of silver nanoparticles showed promise in improving the sensor's consistency and stability. In conclusion, this project represents a significant effort in developing a robust opticalpH sensor with potential applications in industries such as oil and gas, chemicals, and environmental monitoring. The work underscores the ongoing challenge of maintaining sensor integrity under HTHP conditions, showing the need for further research to enhance coating resilience and a more robust calibration technique. Despite these challenges, this project pushes the boundaries of pH measurement technology and contributes substantially to the field of optical pH sensing. It is an exciting step towards overcoming the limitations of pH measurement in extremeenvironments, opening new avenues for precise monitoring and control in critical industries.
Advisor / supervisor
  • Roy, Sudipta
  • Chen, Yi-Chieh
Resource Type
DOI
Date Created
  • 2023

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