Two-dimensional distribution patterns of ammonium in marine sediments revealed by a novel ammonia planar fluorescence sensor

High-resolution distributions of ammonium are critical to study the rates and patterns of remineralization, nitrification, and denitrification processes in sediments. A novel irreversible ammonia planar optical sensor was developed using o-phthalaldehyde (OPA) as indicator to resolve 2-dimensional (2-D) distributions of ammonia with a limit of detection of 9 nM. The sensor is prepared by immobilizing OPA in a thin layer of polyurethane hydrogel (~10 µm thickness) on a transparent polyester sheet, and covered with a gas permeable PTFE membrane to eliminate interferences from various solutes.  Fluorescence shows maximum emission at 430 nm with excitation at 365 nm, and fluorescence intensity is proportional to ammonia concentration and time of reaction.  No interferences from other major solutes or trace metal ions were observed except hydrogen sulfide when it was higher than 100 µM.  The irreversible sensor has been paired with pH sensors and successfully used to measure 1 and 2-D ammonia – ammonium distributions in subtidal sediments.  Sensing can be extended to 3-D patterns using stacked images.  Images are readily obtained using inexpensive LED excitation and commercial grade digital cameras, with typical pixel resolution of < 50 × 50 mm over areas > 100 cm².  A distinct ammonia consumption zone across the water-sediment interface and complex heterogeneous distribution patterns of ammonia associated with biogenic structures and oxygenation are revealed.  Distributions can be related directly to the corresponding visible or x-radiographic images of the sediment.

Presentation Preference: Poster

Primary Presenter: Qingzhi Zhu, Stony Brook University (qing.zhu@stonybrook.edu)

Authors:

Wen Cong, Stony Brook University/School of Marine and Atmospheric Sciences  (wen.cong@stonybrook.edu)

Qingzhi Zhu, Stony Brook University/School of Marine and Atmospheric Sciences  (qing.zhu@stonybrook.edu)

Robert Aller, Stony Brook University/School of Marine and Atmospheric Sciences  (robert.aller@stonybrook.edu)