NEW ORLEANS, LA.--With a tip just 25 microns in diameter, a new microelectrode sheds light on the complex natural chemistry of "swamp scum and sea slime"--including the corrosive ocean "biofilms" that damage boats, docks and off-shore platforms, a University of Delaware researcher reported today during the National Association of Corrosion Engineers (NACE) meeting.
Microelectrodes are nothing new, but most existing sensors rely on membranes that don't perform reliably in field environments, Marine Studies Prof. Stephen C. Dexter said. Also, he said, they only characterize a single gaseous compound. "Traditional devices, made of a sensitive membrane placed over an electrode, are limited mainly to measuring things like oxygen and hydrogen sulfide gases," he said, "and you would have to use separate electrodes for each gas."
By contrast, UD's microelectrode, invented by Dexter's colleague, Oceanography Prof. George W. Luther III, is hardy enough to withstand salt marshes, harbors, bays and other swampy marine settings. And, it measures key components of these environments--including dissolved oxygen, iron, manganese, hydrogen sulfide and iodide--simultaneously. Hydrogen sulfide, for example, can be detected at levels as low as one part per billion.
Luther and graduate student Paul Brendel originally developed the probe to learn more about wetland settings, where the delicate balance of nature is constantly changing because of natural chemical events such as the decomposition of organic matter. But, the work could ultimately help researchers develop more effective strategies for preventing or mitigating damage to the environment.
After all, Luther said, "If we want to understand how pollution resulting from human activities might have an impact on fresh water and marine environments, we first need to know exactly what's happening in these systems, on a day-to-day basis. The microelectrode is an extremely useful tool for gathering that information."
With graduate students Brendel and Kunming Xu, Dexter tweaked the technology to analyze thin organic biofilms on metals in seawater. Biofilms can quickly corrode metal structures, and excess metal in seawater also may endanger marine wildlife under some conditions, Dexter said.
NOT YOUR AVERAGE STICK IN THE MUD
"No one was dumb enough to stick these things in the mud before," Luther said jokingly, when asked why his rugged microelectrode wasn't invented sooner. "It's a solid-state device, and we take special steps to prevent it from fouling."
Luther invented the probe by inserting a tiny gold wire, plated with mercury, into the center of a very thin-walled glass tube just 200 microns in diameter and about four centimeters long. Undesirable chemical species were then removed by applying electrical voltages across the surface of the electrode.
After shrinking the sensor's tip to 25 microns--roughly 625 times smaller than a 1/16th-inch segment of a conventional ruler--Dexter and Xu began using Luther's invention to simultaneously measure dissolved oxygen, manganese and iron, as well as pH levels, in seawater biofilms grown on platinum and stainless steel surfaces. The sensor generated accurate measurements at 1.5-micron intervals within the biofilms, he said.
Whenever metals corrode in seawater, Dexter said, they interact with microscopic organisms and metabolic by-products--commonly known as "slime." These interactions trigger an elaborate series of reactions as microorganisms consume oxygen to produce various other chemical species. In this way, microbes may speed the corrosion of metal surfaces.
Researchers won't be able to prevent biofilms from forming on boats, docks and the ocean's surface until they know more about the chemical reactions taking place inside these films, Dexter said. That's easier said than done, because biofilms are "extremely heterogeneous, meaning that their chemistry varies from point to point within the surface," he added.
During preliminary studies of biofilms, Dexter said, "Dissolved manganese species were found in the presence and absence of oxygen, whereas iron species were only detected in anaerobic (oxygen-free) conditions within the biofilms."
A VERSATILE TOOL
Luther field-tested his microelectrode in Hawaii's Kaneohe Bay. "It's very much like a Delaware salt marsh," he said, "in that it cycles iron very rapidly." In the future, he might subject the probe to more extreme tests: in Hawaiian volcanoes and hydrothermal vents, which are loaded with gases such as hydrogen sulfide and methane, as well as iron and other metals. If the probe can withstand Hawaii's low-oxygen hydrothermal vents, he said, "it could probably go just about anywhere." Even, he said, on a deep-sea lander, where it would travel to the ocean floor. Someday, it might be possible to measure chemical species using the microelectrode by remote-control, from on board a ship, Luther said.
Already, Marine Studies Associate Prof. S. Craig Cary is using the probe to measure the chemical components of samples retrieved from deep-sea hydrothermal vents. When oceanic plates move in opposite directions, he noted, the resulting volcanic activity can super-heat water deep in the Earth's crust, ejecting it through the ocean floor at temperatures as hot as 680 degrees Fahrenheit. While hydrothermal vents produce dramatic towers of black smoke or "chimneys," he said, they also generate "diffuse-flow sites," where bacterial communities thrive on a diet of hydrogen sulfide. Cary, who has made nine trips to the ocean floor in a submersible vehicle, brings samples of these organisms back to the laboratory, then measures the chemistry of their dynamic habitat, using the microelectrode probe.
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