| Summary: | The pH behavior in the μm to cm thick diffusion boundary layer (DBL) surrounding many aquatic species is dependent on light-controlled metabolic activities. This DBL microenvironment exhibits different pH behavior to bulk seawater, which can reduce the exposure of calcifying species to ocean acidification conditions. A low-cost time-domain dual-lifetime referencing (<i>t</i>-DLR) interrogation system and an optical fiber fluorescent pH sensor were developed for pH measurements in the DBL interface. The pH sensor utilized dual-layer sol-gel coatings of pH-sensitive iminocoumarin and pH-insensitive Ru(dpp)<sub>3</sub>-PAN. The sensor has a dynamic range of 7.41 (±0.20) to 9.42 ± 0.23 pH units (95% CI, T = 20 °C, <i>S</i> = 35), a response time (<i>t</i><sub>90</sub>) of 29 to 100 s, and minimal salinity dependency. The pH sensor has a precision of approximately 0.02 pH<sub>T</sub> units, which meets the Global Ocean Acidification Observing Network (GOA-ON) “<i>weather</i>” measurement quality guideline. The suitability of the <i>t</i>-DLR optical fiber pH sensor was demonstrated through real-time measurements in the DBL of green seaweed <i>Ulva</i> sp. This research highlights the practicability of optical fiber pH sensors by demonstrating real-time pH measurements of metabolic-induced pH changes.
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