Luminescent Silole Nanoparticles as Chemical Sensors for

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Luminescent Silole Nanoparticles as Chemical Sensors for Carcinogenic Chromium(VI) and Arsenic(V) Sarah J . Toal and William C. Trogler* University of California at San Diego, 9500 Gilman Drive, La Jolla CA 92093-0358 *Corresponding author: [email protected]

Introduction The principle objective of the research described herein is to synthesize new materials for the selective sensing of carcinogenic chromium(VI) and arsenic(V) containing species. Both metal ions have been proven to have serious adverse health effects and are regulated by the EPA. As it exists in its predominant environmental form, CrO42- is isostructural with the sulfate ion (SO42-); hence, sulfate transport proteins uptake CrO42-, wherein it damages DNA. Likewise, the predominant environmental form of As(V), AsO43-, is isoelectronic with PO43-, and so it competes in cellular uptake of phosphate. The EPA has set maximum contaminant level goals (MCLG) for chromium and arsenic at 0.10 and 0.05 ppm respectively. The enforcement of these regulations requires the development of sensitive and selective sensors for these carcinogens. Both chromate and arsenate are oxidizing agents, a property which may be exploited to effect their detection. It has recently been shown that silole containing polymers may be used as sensors specific for TNT and other nitroaromatic oxidant species. The method of detection is based on luminescence quenching of the silole polymer by the electron accepting analyte. Polysilole luminescence occurs from a LUMO stabilized by σ*-π* conjugation arising from the interaction between the σ* orbital of the silicon chain and the π* orbital of the butadiene moiety. Recently, it was reported that formation of methyl(phenyl)tetraphenylsilole colloids leads to a significant increase in luminescence. Although poly(tetraphenyl)silole is insensitive to simple inorganic oxidants, functionalization of the silole unit with hydrogen bonding substituents has been performed to incorporate binding regions for anionic i

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© 2005 American Chemical Society

In Nanotechnology and the Environment; Karn, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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oxidants, such as chromate and arsenate. Nanoparticle colloids of poly(tetra -phenyl)silole show not only increased luminescence, but also much greater sensitivity in analyte detection.

Materials and Methods The silole compound used for the detection studies 1A, l-(3-aminopropyl)l-methyl-2,3,4,5-tetraphenylsilole, was designed to have an amino terminated alkyl substituent on the silicon atom, which hydrogen bonds to the oxygens of the chromate and arsenate analytes in solution. It was synthesized from the chloroplatinic acid (H PtCl ) catalyzed hydrosilation of allylamine by 1-methyl2,3,4,5-tetraphenylsilole. The latter silole was prepared by a modified literature procedure/' 2

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Results and Discussion The UV-vis absorption spectrum of silole 1A (Figure 1) exhibits two absorption bands at 248, and 360 nm. The longest wavelength absorption is typical of silole compounds, and is attributed to the π-π* transition of the metallole ring. 5

Wavelength (nm) Silole 1A Figure 1: UV-vis data for 1A in THF Fluorescence studies were performed with an excitation wavelength of 360 nm on a 10 μηι solution of 1A. Silole 1A is a weak luminophore in THF solution with a = 475 nm. However, addition of water to the THF solution causes precipitation as a colloid and increases the luminescence dramatically. Atomic Force Microscopy images were taken on a glass coverslip coated with


171 the nanoparticles, and they reveal the size of the nanoparticles to be on the order of 80-100 nm (Figure 2a). Surface coating of the nanoparticles with Si0 by treatment with Si(OEt) produces particles with sizes on the order of 150 nm, and slows the rate of nanoaggregate settling. 2


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Figure 2: AFM images of silole 1A nanoparticles: 2a, uncoated (1 μηι χ 1 μηι scan), and 2b. silicate coated (5 urn χ 5 urn scan) For uncoated nanoparticles, chromate and arsenate detection studies were made on a 10 μηι solution of 1A in 1:9 THF:H 0. Luminescence quenching is observed at concentrations as low as 0.50 and 5 ppm of Cr(VI) and As(V) respectively. Stern-Volmer plots for the quenching, however, are non-linear, which may reflect nanoparticle surface saturation (Figures 3a-b). 2

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Figure 3: Stern-Volmer plots for luminescence quenching by 3a) Cr(VI) and, 3b) As(V) It has been shown that silole compounds functionalized with anion binding sites show a capability of detecting chromate and arsenate at low concentrations.


172 This research is funded by the U.S. EPA-Science to Achieve Results (STAR) program grant # R829619.

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References i

Lippard, S.J.; Berg, J.M. Principles of Bioinorganic Chemistry; University Science Books; Mill Valley, CA, 1994

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Kaim, W.; Schwederski, B.; Bioinorganic Chemistry: Inorganic Elements in

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the Chemistry ofLife: John Wiley & Sons, Inc., New York, NY, 1994 http://www.epa.gov/safewater/dwh/c-ioc/chromium.html; http://www.epa.gov/safewater/arsenic.html a) Sohn, H.; Calhoun, R. M.; Sailor, M.J.; Trogler, W.C. Angew. Chem. Int. Ed. 2001, 40, 2104; b) Sohn, H; Sailor, M.J.; Magde, D.; Trogler, W.C.; J. Am. Chem. Soc. 2003 Luo, J. et. al., Chem. Commun., 2001,

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Boudjouk, P.; Sooriyakumaran, R.; Han, B. J. Org. Chem., 1986, 51, 2818


Luminescent Silole Nanoparticles as Chemical Sensors for

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