A rapid response, planar fluorosensor for measuring twodimensional pCO₂ distributions and dynamics in marine sediments

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⁺PTS^4-, 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^oC, 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.