Development of optical fibre pH sensors for marine microenvironments

The oceans absorb approximately one-third of the CO2 emission into the atmosphere causing a decline in seawater pH, a process known as ocean acidification (OA). This decline in pH reflects changes to the seawater carbonate system and is expected to have an impact on the marine environment and ecosystems. pH changes along coastal ecosystems are highly variable and knowledge of these marine environments can be used to enhance our understanding of OA impacts on marine organisms.

The micro-to-centimetre thick layer directly surrounding many aquatic organisms is known as the diffusion boundary layer (DBL). The DBL creates a region which reduces the exposure of calcifying species to OA conditions. The pH within the DBL is dependent on light-controlled metabolic activities and exhibits different pH behaviour to bulk seawater. The challenge of detecting in situ pH variations and attributing OA effects highlights a need for a fresh research approach and innovative analyses.

The objective of this research is to develop optical fibre pH sensors capable of continuous pH measurement, and suitable for measuring pH variation in marine microenvironments. The development, characterisation, and applications of optical fibre pH sensors are described. The pH sensing components consist of pH-sensitive indicators immobilised in an optimised sol-gel matrix, minimising indicator leaching without the need for a covalent bond. This research explores two approaches, absorbance-based and fluorescence-based pH sensors.

The absorbance-based sensor applied meta-cresol purple (mCP) as the pH-sensitive indicator. The pH sensor has a usable lifetime of 7 days and a dynamic range of pH 7.4 to 9.7. This self-referencing pH sensor was utilised for real-time pH measurements within the DBL of the seaweed Ulva sp., and successfully used to monitor metabolic activity for 100 hours, achieving a precision of 0.02 pH units. This sensor conformed to the GOA-ON Weather quality guideline and demonstrated its capability to identify short-term variation in biological and environmental studies.

The fluorescence pH sensor utilises a time-domain dual-lifetime referencing scheme (t-DLR). The fluorescence pH sensing materials required the synthesis of pH-sensitive iminocoumarin and the encapsulation of pH-inert reference Ru(dpp)3 in polyacrylonitrile (PAN). The pH is determined from the ratio of the combined excitation intensity to the emission intensity of the reference indicator. This approach allows the signal to be referenced internally, independent of fluorescence dye concentration and variations in excitation light intensity. The t-DLR instrumentation used commercial electronic and optical components, integrated with custom-made electronic circuits. The pH sensor has a dynamic range of pH 7.8 to 9.3 and a precision of 0.02 pH units. The pH sensor was insensitive to changes in salinity and had negligible dye leaching and minimal photobleaching.

This work accomplished the development of mCP-based and dual-layer t-DLR fluorescent-based optical fibre pH sensors. This highlights the versatility of optical fibre pH sensors and the potential for a wider range of applications.

Chen W. H., 2023. Development of optical fibre pH sensors for marine microenvironments. PhD thesis, University of Otago, 279 p. Thesis.

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