An irreversible planar optical sensor for multi-dimensional measurements of sedimentary H2S

Hydrogen sulfide (H2S) is produced by sulfate reducing bacteria during anaerobic metabolism in sediment. Knowledge of the detailed distributions of H2S permits quantification of the rates and patterns of SO42 − reduction and sulfidic diagenetic processes. A novel H2S sensor based on a diphenylcarbazone (DPCO) indicator was developed to readily and rapidly resolve a wide range of H2S distributions in natural sediments in multi-dimensions. 2-D sensing membranes were prepared by immobilizing DPCO-Zn2 + complex in polyurethane hydrogel (D4) on transparent polyester sheets. A gas permeable silicone layer was applied as an outermost layer to eliminate or minimize possible interfering reactions. The immobilized DPCO-Zn2 + complex has a maximum absorbance at 530 nm, but DPCO shows no absorption at this wavelength. The working principle of the sensor is competition (ligand replacement) between DPCO and sulfide ion with Zn2 + to form the more stable ZnS. After exposure to H2S, the absorbance of the sensor is inversely correlated with H2S concentration and the time of reaction. By immobilizing different amounts of DPCO-Zn2 + complex in the membrane, a series of sensors can be made to measure H2S in different depositional environments. The DPCO-Zn2 + based sensor is only sensitive to H2S gas, and its performance is independent of other dissolved gases such as O2, CO2, NH3, and N2. Because the reaction of DPCO-Zn2 + with H2S has stable products, the reacted sensor can be optically scanned using a simple flatbed or pen scanner immediately or at a later time for quantification. By simple manipulations of exposure times (1-20 min) and DPCO-Zn2 + loading, high resolution (50 × 50 μm), 2-D or 1-D H2S distribution profiles are readily obtained in natural sediment over a very wide range of H2S concentrations (~ 5-4000 μM). Serial spatial deployment of sensor membranes also allows reconstruction of complex 3-D patterns of H2S, for example, in bioturbated marine sediments or plant rhizospheres.