A new ratiometric, planar fluorosensor for measuring high resolution, two-dimensional pCO2 distributions in marine sediments

CO2 distributions in sediments reflect a broad set of C cycling reactions associated with the decomposition or synthesis of organic matter and with mineral dissolution or precipitation. In order to examine transport-reaction processes controlling CO2 at the seafloor, a new ratiometric optode fluorosensor was developed which allows high resolution two-dimensional imaging of pCO2 (partial pressure of CO2) distributions in sediments and overlying water. The sensor film consists of a fluorescent pH indicator dye 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt (HPTS, PTS−), tetraoctylammonium cation (TOA+), and tetraoctylammonium hydroxide (TOAOH) immobilized within an ethyl cellulose membrane, backed by a polyester sheet, and coated with a gas permeable silicone membrane. The ratio of fluorescence intensity at 515 nm following dual excitation at 475 and 405 nm of the sensor optode correlates directly with pCO2 from 0 to 20 matm. The sensor is sensitive, stable, and precise, with fast response time (∼2.5 min) and good reversibility. It is insensitive to oxygen, total ammonia, and background reflectance but is subject to interference from dissolved hydrogen sulfide and is temperature dependent. The optode film can withstand repeated insertion into deposits, and retains its properties after continuous exposure to marine sediment for weeks. Depending on the camera used, the fluorosensor readily measures pCO2 patterns at pixel sizes 55 × 55 μm to 10 × 10 μm over areas exceeding 150 cm2. In addition to resolving the well-known increases of CO2 that typically occur with depth in deposits, the sensor reveals complex heterogeneous distributions and previously undocumented time-dependent reaction phenomena associated with both inhabited and abandoned biogenic structures.