Obtaining accurate, high-frequency and long-term seawater pH data by using coupled lab-on-chip and optode sensing technologies

The marine science community requires accurate, cost-effective, and reliable pH sensors capable of long-term, stable operations in-situ from coastal to deep-sea environments. Spectrophotometric pH sensors, based on lab-on-chip (LOC) technology, offer measurement frequencies of every 10 min and provide good performance (pH difference 0.02 RMSE) relative to validation samples with long-term use. However, for applications where higher-frequency measurements are important, this maximum sample rate may be limiting, in addition to the power requirements needed to operate the sensor.

In contrast, commercially available pH optodes (PyroScience GmbH) are relatively inexpensive, consume little power and are contained within a comparatively small form-factor package, but with intense use the pH sensitive membrane can photo-oxidise, causing signal drift. The combination of LOC and optode technologies, however, can be used to provide long-term, high-frequency and high-stability in-situ pH data, but protocols to correct for sensor drift need to be developed and evaluated.

To examine sensor drift and develop protocols to account for it, we suspended two LOC pH sensors with two pH optodes at 0.5 m depth from a floating pontoon within a harbour in Southampton, UK for six months (June–December 2023). This is a highly dynamic tidal environment with substantial biofouling. The optode (AquapHOx-L-pH, PyroScience GmbH) and an independent pH sensor (Deep SeapHOx V2, Sea-Bird Scientific) measured at a high frequency (e.g., ≤ 5 min) alongside a LOC pH sensor measuring at a lower frequency (e.g., ≤ 2 h). Triplicate lab validated co-samples were collected each week, in addition to dedicated sensors monitoring the temperature, salinity, dissolved oxygen and tidal height. We find good agreement, i.e., mean ΔpH =  between the SeapHOx and LOC sensors (3182 data points in common), in addition to individual performances of 0.02 RMSE relative to validation samples. As expected, we found significant signal drift (e.g., generally ≤ 0.012 pH d−1) and pH offsets (e.g., 0.1–0.2) with the optodes after intensive use in a high biofouling environment. However, by coupling LOC pH data to high frequency optode data, we corrected the optode signal drift/offset and achieved a similar field performance (∼ 0.02 RMSE relative to validation samples) as the SeapHOx sensor even when using ultra-low LOC pH sensor measurement frequencies (e.g., several days to weeks per LOC measurement). Overall, this work provides the oceanographic community with guidelines on how to achieve accurate, rapid, and long-term pH measurements, while also balancing power requirements, by combining two complementary pH sensing technologies.