A pH plate fluorosensor (optode) for early diagenetic studies of marine sediments

Pore water pH distributions are closely coupled to early diagenetic reactions and transport processes in surficial sediments. In this study, an optical plate fluorosensor for rapid two-dimensional detection of H+ concentration patterns in sediment pore waters was developed. The dual excitation (405/450 mm) single emission (520 nm) pH fluorophore HPTS (8-hydroxypyrene 1, 3, 6, trisulfonic acid trisodium salt) was immobilized onto transparent, supporting sensor foils. Excitation/emission spectra of immobilized HPTS exhibited pH dependent shifts in seawater standards, and fluorescence excitation ratios were used for two-wavelength, ratiometric detection of pH distributions in sediment and bottom water close to the sediment-water interface (pH approximate to 6.5 to 8.5). Sensor foils were fixed to the inner sides of plastic box corers or glass plate aquaria containing sediment and overlying seawater. Equilibration with pore water occurred in seconds, and fluorescence response (excitation/emission) was scanned nondestructively using a monochrome, integrating CCD camera. The two-dimensional sensor system has a vertical and horizontal resolution of 56 X 54 mum (5 X 5 pixels) applied over an image of 34 X 26 mm. Reproducibility and accuracy were best when foils were prepared from solutions of similar ionic strength to samples. Optical sensor pH distributions correlated well with high-resolution, vertical distributions measured simultaneously with a minicombination pH electrode, and two-dimensional pH patterns demonstrated directly the transport-reaction heterogeneity associated with microtopography and biogenic structures in surface deposits and overlying waters close to the sediment-water interface. Sensor foils retained calibration characteristics for at least 50 d.