Droplet in the Ocean: Development and Deployment of Droplet Microfluidic Total Alkalinity Sensors in Wide-Range Environments

Total alkalinity (TA) is the balance of proton acceptors and donors in the ocean. With increasing ocean acidification, TA measurements are more important than ever for monitoring the ocean's buffering system. In situ TA sensors allow us to measure rapidly changing alkalinity in environments like estuaries or coral reefs, or to evaluate marine Carbon Dioxide Removal (mCDR) applications. Currently, commercially available and research prototype in situ TA sensors have measurement range and sample frequency limitations. To address this, a cost-effective droplet microfluidic sensor has been developed, utilizing lab-on-chip (LOC) technology to create discrete aqueous droplets suspended in hydrophobic oil. In each droplet, a single-point closed-cell titration occurs, sampling ~0.75 µL every 6 seconds. The small volume, high throughput nature of droplet flow improves on the limitations of current sensors, increasing the effective measurement ranges with alternating titrant acid concentrations. This improves the analytical range and speed of analysis for in situ, high-frequency TA measurements.  Here I will present results from an initial closed-cell lab-based prototype and from field deployments on the underway system of a tall ship and in a local estuary. Measurements are in close agreement with those made by equivalent benchtop methods. This technology could offer a new and important tool in the analysis of the marine carbonate system in fast-changing environments.