A rapid response, planar fluorosensor for measuring two‐dimensional pCO2 distributions and dynamics in marine sediments

Abstract

CO2 distributions in marine sediments are closely coupled to early diagenetic reactions and transport processes. An existing planar optode designed to measure pCO2 in sediments and overlying water was modified to substantially lower response times and to allow for enhanced resolution of CO2 dynamics. Sensing foils are fabricated by entrapping fluorescent indicator 8‐hydroxy‐1,3,6‐pyrenetrisulfonic acid‐tetraoctylammonium ion‐pair (TOA+PTS4−, molar ratio 1:1) and tetraoctylammonium hydroxide (TOAOH) in a hydrophobic ethyl cellulose polymer matrix on an inert transparent polyester support, and coated with gas permeable silicone membrane. The response time of the optimized sensor is 18 s (0 → 5 matm), allowing measurement of transient pCO2 distributions associated with microenvironments such as burrows. As typically configured, the ratiometric sensor has a working range of 0 to ~40 matm pCO2 and a detection limit of 0.193 matm (3σ), and shows fluorescence emission at 515 nm with dual excitation at 470 and 407 nm, and a well‐defined isosbestic point. The sensor is sensitive, precise, stable, and reversible, and retains its properties after continuous exposure in marine sediments for approximately 2 weeks at 15°C, with a longer or shorter lifetime at lower or higher temperatures respectively. This sensor has been successfully used to measure two‐dimensional pCO2 distributions and dynamics in marine sediments and overlying water with a typical pixel resolution of ~ 50×50 µm over an area of ~150 cm2.