Investigating a droplet microfluidic system for measuring total alkalinity

Autonomous, high-resolution, and long-term monitoring in dynamic marine environments is advantageous to characterize the ocean carbon cycle. Here, we report an autonomous droplet Lab-on-Chip (LoC) sensor enabling high-resolution total alkalinity (TA) measurements with reduced reagent requirements validated in the laboratory. The droplet LoC sensor integrates on-chip droplet absorbance detection to perform multi-point spectrophotometric titrations, where each droplet in a set is an individual titration point. To facilitate droplet stability under varying pH titration conditions, a three-step silica nanoparticle coating method was adapted to render the PMMA channels superhydrophobic while maintaining compatibility with the titration chemistry. The droplet-based approach enables significant reductions in sample and reagent consumption, attaining 15 µL per titration point, which represents a ~67-fold improvement compared to previous designs. Further measurement time was also decreased, with a complete TA measurement taking 190 seconds that is ~6-fold faster than earlier stop-flow analyzers. Multiple certified reference material (CRM) measurements demonstrated a precision of 9.08 µmol kg⁻¹, which is comparable to previous stop-flow microfluidic systems. Once calibrated against CRMs, the device showed a strong correlation between sensor measured TA values and prepared standards across a range of alkalinities from to 1500 to 2500 µmol kg⁻¹, and an accuracy of –3.3 ± 22.9 µmol kg⁻¹. The droplet approach to TA measurements on chip demonstrate the potential of droplet microfluidics for long-term, and high spatiotemporal resolution TA measurements in marine environments.