Fluorosensor Reveals Seafloor Habitats
Robert C. Aller likes to muck about when he works. He studies worms, clams and other animals that live in aquatic sediments. These creatures create complex, three-dimensional chemical microenvironmental and transport path networks that Aller, a marine sciences professor at Stony Brook University in New York state, likens to tree roots or ant tunnels. By measuring solute distributions, such as pH, researchers can trace the networks and estimate material movement across the seafloor.
Traditionally, these measurements have been performed with microelectrodes, which sample only a point. Now, by making use of a fluorosensor and other photonic devices, Aller can perform planar pH measurements in marine sediment and water and resolve temporal as well as spatial variations. In tests of the pH sensor, which he and his fellow researchers Qingzhi Zhu and Yanzhen Fan reported in the Nov. 15 issue of Environmental Science & Technology, it quantified complex biogeochemical patterns over areas larger than 100 cm2 with a 50-µm resolution.
A false-color image reveals a vertical section of the pH distribution associated with the burrow of the "clam worm" Nereis diversicolor in Flax Pond, a salt marsh on the north shore of Long Island in New York. Low-pH regions below the sediment/water interface and around the burrow represent sites of remineralization and oxidation. Courtesy of Robert C. Aller.
Key to the approach is a planar fluorosensor foil made by covalently linking the fluorescent dye 8-hydroxy-1,3,6-pyrenetrisulfonic acid trisodium salt to a transparent 10-µm-thick polyvinyl alcohol membrane backed by a 125-µm-thick polyester sheet. The dye responds to pH changes over the range of interest by a shift in fluorescence emission. For their sensor, the scientists selected a polymer that was both rugged and transparent, enabling the imaging of visual structure and pH fluorescence while the sensor was deployed.
To use the sensor, they placed the foil in contact with water and sediment in a small container. They illuminated it with a xenon arc lamp filtered by a computer-controlled monochromator from Newport Corp.’s Oriel Instruments of Stratford, Conn., or with two LEDs with wavelengths of 420 and 505 nm. In either case, the angle of incidence was less than 30°.
They collected the resulting fluorescence emission using a Canon 2048 × 3072-pixel commercial digital camera equipped with lenses and a 540-nm emission filter. The camera was located at a right angle to the fluorosensor plate, so light reflected from the foil did not interfere with the emission measurement. For pH measurements, they captured the fluorescence intensity at 540 nm with excitation at 506 and then 428 nm.
In their tests, the investigators calibrated the intensity ratio to pH measurements by using electrodes in solutions with known pH values. The results show that the ratio correlated with pH changes from acidic at 5.5 to basic at 8.6 and that the sensor compared well with the traditional method.
“The results obtained for pH are analytically indistinguishable from those obtained from pH electrode sensors,” Aller said. He added that the sensor is not affected by oxygen or sulfides, two reactive solutes often found in marine environments, and that it is relatively insensitive to temperature.
Beyond such research in the marine sciences, the sensor may have potential applications in environmental monitoring for regulatory purposes. Also, the group is developing planar fluorosensors to track other solutes, such as CO2.
Finally, the sensor could have instructional uses. Today, students read a number from an electrode and try to make sense of it. Tomorrow, the procedure may be more colorful and of greater educational value.
“Imagine being able to take a high-resolution picture of pH or other solutes in marine sediment or soil in real time to demonstrate biogeochemical and physical processes to students,” Aller said.
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