EPA Nanotechnology and the Environment ... - Mechatronics

Nanotechnology and the Environment: Applications and Implications STAR Progress Review Workshop
Nanostructured Porous Silicon and Luminescent Polysiloles as
Chemical Sensors for Carcinogenic Chromium(VI) and Arsenic(V)
William C. Trogler
University of California, San Diego, CA
Environmental Benefits
The chief goal of this research project is to develop
new selective solid state sensors for carcinogenic and toxic
chromium(VI) and arsenic(V) based on redox quenching
of the luminescence from nano-structured porous silicon
and polysiloles. Nano-structured porous silicon as well as
polysilole nanowire coatings will be chemically modified
to enhance the binding of the chromate and arsenate anions.
Chemical modification to vary the redox potential of
the polysilole excited state also will be used as a way to
impart chemical selectivity. Both approaches will be combined
by encapsulating the polysilole in a nanotex-tured
microcavity between two Bragg stacks constructed from
porous silicon. Such optical devices have been shown to
provide significant detection sensitivity enhancements. The
nanoporous material will readily admit small inorganic
analytes, such as chromate and arsenate, and exclude
biomolecules that might confound the measurements.
Sensors based on silicon wafer and polymer technologies
also are readily adaptable to fabrication. The fluorescence
quenching detection modality also is manufacturable. The
essential electronics require a blue or ultraviolet LED as
the excitation source and an inexpensive photodiode detector.
Potential applications of such real-time detection
devices include remote sensing and industrial process control.
The focus on chromium(VI) and arsenic(V) detection
is dictated by the redox quenching mechanism that is
being used, as well as by the importance of chromium(VI)
and arsenic(V) as regulated chemicals under the Safe
Drinking Water Act.
Chromium(VI) detectors will be developed that can
sense the analyte at concentrations at least as low as the
0.1 ppm action level mandated by the Safe Drinking Water
Act with at least a 10 percent accuracy. For arsenic(V),
the target range is 10-50 ppb at the same level of analytical
accuracy. The deployment of remote sensors for
natural waters and for industrial wastewater that could
be used to signal alerts in real time would become possible.
An electronic sensor method would prove more
beneficial than the grab-sampling procedure currently
used for detecting these problematic water contaminants.
A second goal of the project is the development of polymer
coated “litmus paper” for qualitative detection of
chromium(VI) and arsenic(V). Given the low expense
of polymer-coated paper, this could prove to be a useful
adjunct to current qualitative methods.
Those that benefit from remote sensing applications
would include research scientists trying to understand
the variations in chromium and arsenic pollutants in
natural waters, municipal source water monitoring, as
well as federal regulatory monitoring. Because electronic
sensors can be engineered either wired or wireless, the
range of possible applications is truly immense. Industries
involved in metal working and electroplating also
would benefit from the ability to monitor waste streams
for continuous compliance (in real time) with pollution
regulations. Electronic sensors also could be employed
to monitor mine runoff for the presence of toxic soluble
chromium and arsenic that could enter ground or natural
water systems. Chromium is widely used as a corrosion
inhibitor in closed water boilers and chillers. Electronic
sensors could be used to monitor leaks into nearby groundwater
as well as into heat exchange systems.
The simple “litmus paper” application of the thin film
polymer indicators could prove useful for field studies
and monitoring. Such test strips also would find application
in science education (both K-12 and university level)
for informing the public about metals in water supplies.
Qualitative test strips would be most economically feasible
for monitoring water supplies for compliance at the
tap, which is the ultimate measure of safety.
The Office of Research and Development’s National Center for Environmental Research 31