University of Delaware
Office of Public Relations
UpDate - Vol. 16, No. 23, March 13



              Environmental probe calculates marine life
     
     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,
Stephen C. Dexter, marine studies professor and researcher,
reported at 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, Dexter said, and added, 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," Dexter said,
"and you would have to use separate electrodes for each
gas."
     By contrast, UD's microelectrode, invented by Dexter's
marine studies colleague, Oceanography Prof. George W.
Luther III, is hardy enough to withstand salt marshes,
harbors, bays and other swampy marine settings. Plus, 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 such natural chemical events 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.
     No 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.
     Some day, it might be possible to measure chemical
species using the microelectrode by remote-control, from on
board a ship, Luther said. Already, S. Craig Cary, marine
studies, is using the probe to measure the chemical
components of samples retrieved from deep-sea hydrothermal
vents.
                        -Ginger Pinholster and Tracey Bryant