ABSTRACT
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 (t-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)3-PAN. The sensor has a dynamic range of 7.41 (±0.20) to 9.42 ± 0.23 pH units (95% CI, T = 20 °C, S = 35), a response time (t90) of 29 to 100 s, and minimal salinity dependency. The pH sensor has a precision of approximately 0.02 pHT units, which meets the Global Ocean Acidification Observing Network (GOA-ON) "weather" measurement quality guideline. The suitability of the t-DLR optical fiber pH sensor was demonstrated through real-time measurements in the DBL of green seaweed Ulva sp. This research highlights the practicability of optical fiber pH sensors by demonstrating real-time pH measurements of metabolic-induced pH changes.
ABSTRACT
This study presents the development of an optical fiber pH sensor based on evanescent wave absorbance for continuous pH measurements in marine microenvironments. The sensing layer consists of an optimized sol-gel matrix of tetraethoxysilane and dimethyldiethoxysilane, which substantially improves the entrapment efficiency of the pH indicator meta-cresol purple, leading to a long useable lifetime. The optical fiber pH sensor conforms to the Global Ocean Acidification Observing Network "weather" measurement quality guideline with precision of approximately 0.02 pH units, has a dynamic pHT range of 7.4-9.7 in seawater, a response time of 2.5-6.5 min and a useable lifetime of 7 days. The optical fiber pH sensor has additional advantages of being self-referencing, without the need of an external sensor reference, having a simple fabrication method and basic spectrometer instrumentation. The suitability of the optical fiber pH sensor was demonstrated in real-time measurements of the ecologically significant green seaweed Ulva sp. The optical fiber pH sensor monitored pH variations due to metabolic activity over 7 days within the seaweed canopy and 4 days within the diffusion boundary layer interface, demonstrating the suitability for measurements in marine microenvironments.
