An internally referenced leaky waveguide sensor for monitoring of pH

Wensley, Thomas (2023). An internally referenced leaky waveguide sensor for monitoring of pH. University of Birmingham. Ph.D.

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Abstract

Leaky waveguide (LW) sensors have the potential to offer label-free, low-cost and long-term continuous monitoring of pH, particularly in situations in which contaminants or additives may hinder sensor functionality or where maintenance is difficult. The pH of water supply and wastewater treatment affects a range of industrial, household and environmental factors, and one of its requirements is compatibility with the presence of free chlorine. In this project, a polyion hydrogel sensor for near-neutral pH detection is developed, additionally incorporating a compatible, chemically inert waveguide for common-path, automatic internal referencing and correction for matrix effects.

To meet this need, leaky waveguides (LW) were developed from porous polyacrylamide hydrogels. Fabrication of the sensor hydrogel from linear polymer or dissolved monomer were shown to provide suitable LWs of mesoporous structure and refractive index sensitivities in the 130-150 degrees per refractive index unit (RIU) range. Fresnel diffraction was also observed around the leaky mode. Addition of ionisable copolymers to the gel were shown to provide pH-dependent swelling detectable by monitoring of the leaky mode, and a linear range of up to 5 pH units was
demonstrated.

Additionally, a polysaccharide-based sensor design was constructed using agarose and chitosan as respective pH-inert and -sensitive polymers. Multi-layered waveguides capable of sustaining independent leaky modes were produced by sequential deposition of the polysaccharides. A combined, common-path LW sensor, one of the first of its kind, was produced and a sensitivity of 0.280 degrees/pH unit and a typical linear range of pH 4-8 demonstrated. By substituting agarose for linear polyacrylamide, a sensitivity of 0.273 degrees/pHU was achieved.

A fully synthetic waveguide was developed through incorporation of N,N-dimethylamino moieties into the hydrogel using an acrylate and acrylamide comonomer as the linking group. The former provided superior initial sensitivity at 0.132 degrees/pHU and a neutral pH range suitable for environmental monitoring but suffered from leaching and degradation while in prolonged use. The acrylamide comonomer provided a stable pH-sensitive waveguide of sensitivity 0.091 degrees/pHU and a typical linear range of pH 4-8. Easier incorporation of the latter comonomer indicates the acrylate monomer forms crosslinked microparticles during polymer formation.

For the first time, multi-layered synthetic waveguides for pH sensing were then produced by adapting the method for polysaccharide waveguides, with deposition of linear polyacrylamide onto a cast waveguide of higher density. The internally referenced sensor retains a pH sensitivity of 0.091 degrees/pHU, and a tested range of pH 4-8. Sensitivity to selected interferants was also assessed for both single-layer and internally referenced dimethylamino-bearing leaky waveguides. In both states the sensor was shown to be susceptible to dye staining and interference from monovalent salt solutions but effectively eliminated nonspecific interactions.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):