Grantee Research Project Results
Final Report: Nanostructured Porous Silicon and Luminescent Polysiloles as Chemical Sensors for Carcinogenic Chromium(VI) and Arsenic(V)
EPA Grant Number: R829619Title: Nanostructured Porous Silicon and Luminescent Polysiloles as Chemical Sensors for Carcinogenic Chromium(VI) and Arsenic(V)
Investigators: Trogler, William C. , Sailor, Michael J.
Institution: University of California - San Diego
EPA Project Officer: Hahn, Intaek
Project Period: January 1, 2002 through December 31, 2004
Project Amount: $400,000
RFA: Exploratory Research: Nanotechnology (2001) RFA Text | Recipients Lists
Research Category: Nanotechnology , Safer Chemicals
Objective:
The objective of this research project is to develop new, selective, solid-state sensors for chromium(VI) and arsenic(V) based on redox quenching of the luminescence from nanostructured porous silicon and polysiloles. There are several research components to the problem. Luminescent porous silicon (PSi), surface functionalization and polymer coatings will be tested as ways to enhance binding of the chromate and arsenate anions. Highly luminescent polysiloles that are chain-terminated with anion binding regions also will be explored as chromate and arsenate sensors. Chemical modification to vary the redox potential of the polysilole excited state will be used as a way to impart chemical selectivity. Both approaches will be combined by encapsulating the polysilole in a nanotextured microcavity between two Bragg stacks constructed from porous silicon. Such 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. 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.
Summary/Accomplishments (Outputs/Outcomes):
The hydrophobic nature of polysiloles precludes direct detection of ionic oxidants, such as chromate, which have little affinity for these hydrophobic materials. Luminescent polysiloles that are chain-terminated with anion binding regions have been shown to be an effective chromate sensor down to 10 ppm sensitivity. Colloidal nanoparticulate (~120 nm by atomic force microscopy) suspensions of these polymeric materials display increased sensitivity toward chromate; however, nitrate and perchlorate show only a weak quenching ability. Colloidal suspensions of 3-aminopropyl-methyl(tetraphenyl)silole nanoparticles can be used as selective chemosensors for carcinogenic chromium(VI) analyte. Methylhydrosilole is functionalized by hydrosilation of allylamine and the colloid is prepared by the rapid addition of water to a THF solution of the aminopropylsilole. The method of detection is through electron-transfer quenching of the fluorescence of the silole colloid ( λem = 485 nm at 360 nm excitation) by the analytes with ppb detection limits. Stern Volmer plots are linear up to 10 ppm in the case of chromium, but exhibit saturation behavior near 5-10 ppm for arsenic. Dynamic light scattering experiments and atomic force microscopy measurements show the particle sizes to be around 100 nm in diameter and dependent on solvent composition, with a particle size dispersity of ± 25 percent. The fluorescence lifetimes of the silole in solution and colloid are approximately 31 ps and approximately 4.3 ns, respectively, while the silole has a lifetime of 6 ns in the bulk solid. A minimum volume fraction of 80 percent water is necessary to precipitate the colloid from THF, and the luminescence continues to rise with higher water fractions. Colloids in a pH 7 phosphate buffered suspension show both higher sensitivity and greater selectivity (> 100-fold ) for CrO42- detection than for other oxoanion interferents, NO3-, NO2-, SO42-, and ClOO4-. Arsenate ion also has been detected by this method down to low ppm levels.
Other work has focused on surface functionalization as a means to enhance porous Si sensors, and to stabilize the material against oxidation in air and water for use in environmental sensing. We showed that the chemical stability of porous Si can be increased by replacing residual Si-H species on the surface with methyl groups. The advantage of this microporous material is that it will readily admit small inorganic analytes, such as chromate and arsenate, and exclude biomolecules that might confound the measurements. Developed sensors need to be compatible with silicon microcircuit fabrication technology. Subtasks accomplished included the development of an etching tool for automated preparation of multilayer porous Si structures (Bragg stacks), development of the chemistry to stabilize PSi films in harsh environments, and construction of a prototype device for pollutant detection. Several experiments designed to quantify the stability of the modified porous Si samples were performed involving the use of chemical oxidants and solutions that mimic those used in bioassay applications or that might be encountered in environmental sensor applications.
(1)
The hydride-terminated surface of p-type or p++ -type porous silicon was stabilized by electrochemical reduction of organohalides in acetonitrile solutions as shown in equation (1). We found that reduction of 6-iodo-ethylhexanoate, 1-iodo-6-(trifluoroacetylamino)hexane, iodomethane, 1-bromohexane, or ethyl 4-bromobutyrate at a porous Si cathode results in removal of the halogen and attachment of the organic fragment to the porous Si surface via a Si-C bond. A two-step procedure involving the attachment of the functional group of interest followed by attachment of methyl groups (by reduction of iodomethane) to residual, more sterically inaccessible sites on the porous Si surface is found to yield a more stable material. Three tests of the chemical stability of the modified surfaces were performed: 1) treatment with dimethylsulfoxide (a chemical oxidant for porous Si); 2) treatment with aqueous Cu2+; and 3) exposure to 10 percent ethanol in a solution of phosphate-buffered (pH = 7.4) aqueous saline. The reactions are characterized by atomic force microscopy, Fourier transform infrared, and optical reflectivity spectroscopies. The data indicate that electrochemical alkylation greatly improves the stability of porous Si against oxidation and corrosion, and that the methyl capping procedure provides the most stable material yet reported. A prototype detector, Figure 1, has been constructed.
Figure 1. Prototype Chemical Detector (top). Internal workings (bottom) show the porous Si chip (far left), the optical components, and the radio transmitter for data transfer. This system has been modified to sample liquids for the present work.
Journal Articles on this Report : 6 Displayed | Download in RIS Format
Other project views: | All 25 publications | 8 publications in selected types | All 6 journal articles |
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Cunin F, Schmedake TA, Link JR, Li YY, Koh J, Bhatia SN, Sailor MJ. Biomolecular screening with encoded porous-silicon photonic crystals. Nature Materials 2002;1(1):39-41. |
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Lin H, Mock J, Smith D, Gao T, Sailor MJ. Surface-enhanced Raman scattering from silver-plated porous silicon. Journal of Physical Chemistry B 2004;108(31):11654-11659. |
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Lin H, Gao T, Fantini J, Sailor MJ. A porous silicon−palladium composite film for optical interferometric sensing of hydrogen. Langmuir 2004;20(12):5104-5108. |
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Sailor MJ, Link JR. "Smart dust'': nanostructured devices in a grain of sand. Chemical Communications 2005;(11):1375-1383. |
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Toal SJ, Jones KA, Magde D, Trogler WC. Luminescent silole nanoparticies as chemoselective sensors for Cr(VI). Journal of the American Chemical Society 2005;127(33):11661-11665. |
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Toal SJ, Sohn H, Zakarov LN, Kassel WS, Golen JA, Rheingold AL, Trogler WC. Syntheses of oligometalloles by catalytic dehydrocoupling. Organometallics 2005;24(13):3081-3087. |
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Supplemental Keywords:
heavy metals, effluent, discharge, ecosystem, indicators, aquatic, nanotechnology, innovative technology, chemistry, optics, monitoring, analytical, measurement methods, electroplating industry, mining, groundwater,, RFA, Scientific Discipline, Toxics, Water, Ecosystem Protection/Environmental Exposure & Risk, POLLUTANTS/TOXICS, Sustainable Industry/Business, National Recommended Water Quality, Sustainable Environment, Physics, Environmental Chemistry, Chemistry, Arsenic, Technology for Sustainable Environment, Analytical Chemistry, Monitoring/Modeling, Biochemistry, New/Innovative technologies, Chemistry and Materials Science, Water Pollutants, Engineering, Environmental Engineering, 33/50, biosensing, nanosensors, environmental monitoring, chemical sensors, chromium & chromium compounds, nanotechnology, environmental sustainability, polysiloles, environmentally applicable nanoparticles, chemical sensor, nanostructured porous silicon, carcinogens, sustainability, water quality, innovative technologiesRelevant Websites:
http://www-chem.ucsd.edu/Faculty/bios/trogler.html Exit
http://www-chem.ucsd.edu/Faculty/bios/sailor.html Exit
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.