Environ. Sci. Technol. 2004, 38, 4409-4413
Colored Thin Films for Specific Metal Ion Detection CAROLINE L. SCHAUER,† MU-SAN CHEN, RONALD R. PRICE, PAUL E. SCHOEN, AND FRANCES S. LIGLER* Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, Washington, D.C. 20375-5348
This paper describes the investigation of chitosan and poly(allylamine) (PAH) for the creation of a multi-film, colorbased dipstick for the detection of metal ions in solution. Thin, colored films of chitosan and PAH cross-linked with hexamethylene 1,6-di(aminocarboxysulfonate) (HDACS) are created where color is due to film thickness and optical interference effects. The films are investigated for their ability to selectively detect aqueous metal ions via changes in thickness and/or color. Chitosan-HDACS films were selective for Cr(VI) over all other metal ions tested including Cr(acac)3 and Cr(NO3)3‚9H2O, and PAH-HDACS films were selective for Cu(II) and Cu(I) salts over all other metal ions tested. The irreversible, selective changes due to metal ion solutions were not caused by varying the pH. Potomac River water was also tested using the two films, with results indicating the presence of Cu(II) in the aqueous sample.
Introduction Heavy metal detection continues to be a priority as high levels of metals have been isolated from soil and drinking water in residential areas. As heavy metals have been demonstrated to have carcinogenic properties at high concentrations, they continue to be an important analyte for careful monitoring of the environment. There are many laboratory-based metal ion detection methods in use today. Liquid (1) or gas (2) chromatography (3), atomic absorption (4), flow-injection systems (5), electrochemistry (6), fluorescent sensors (7, 8), inhibition-based enzymatic assays (9), solid-phase extraction (10, 11), and immunoassay (12) are being used to measure metal ions in solutions. An ideal metal ion sensor for the residential analysis of water would be an inexpensive, rapid, color-based, dipstick test for heavy metal salts in waste and drinking water. The dipstick would have different panels of generic and sensitive films that change colors, ideally visible to the naked eye, in response to the presence of heavy metal ions in water. Such a basic test would not supplant current laboratory-based techniques, which provide more detailed and rigorous information. However, it would supply the public with a powerful, albeit basic, tool and a means of determining whether further, more sophisticated, testing would be warranted. Previous work with colored thin films of cross-linked chitosan (Figure 1A), the soluble form of chitin, and poly* Corresponding author phone: (202)404-6002; fax: (202)404-8897; e-mail:
[email protected]. † Current address: Department of Materials Science and Engineering, Drexel University, 3141 Chestnut St., Philadelphia, PA 19104. 10.1021/es035047+ Not subject to U.S. Copyright. Publ. 2004 Am. Chem. Soc. Published on Web 07/10/2004
(allylamine) hydrochloride (PAH) (Figure 1B) demonstrated that these films change their thickness and color in response to all metal ion solutions tested, therefore creating a generic sensor platform (13). The response of the generic sensing films of cross-linked chitosan and PAH were found not to be dose-dependent, with all metal ion solutions tested generating similar thickness and color responses. The creation of a specific sensing film was investigated with a focus on metal ion selectivity, reliability, and readout. Two specific, metal ion sensitive films are presented here. They utilize a new cross-linker, hexamethylene 1,6-di(aminocarboxysulfonate) (HDACS) (Figure 1C) (14), which was used to prepare both chitosan-HDACS and PAH-HDACS colored thin films from colorless starting materials. ChitosanHDACS was found to be specific for Cr(VI) over other metal ions tested, and PAH-HDACS was specific for Cu(II) and Cu(I). The films were tested against a range of environmentally relevant pHs to eliminate the possibility of effects due to pH and were used to test Potomac River water.
Experimental Procedures Chemicals and Materials. Chitosan, of low molecular weight (approximately 250 kDa) and greater than 70% deacetylation, sodium nitrate (NaNO3), and sodium sulfate (Na2SO4) were supplied by Sigma (St. Louis, MO). Poly(allylamine) (PAH) 50% solution in water, cobalt(II) chloride hydrate (CoCl2‚ H2O), chromium(VI) oxide (CrO3), zinc(II) chloride (ZnCl2), manganese(II) chloride tetrahydrate (MnCl2‚4H2O), copper(II) nitrate hemipentahydrate (Cu(NO3)2‚2.5H2O), copper(II) chloride (CuCl2), copper(II) sulfate pentahydrate (CuSO4‚ 5H2O), copper(I) chloride (CuCl), chromium(III) acetylacetonate (Cr(acac)3), chromium(III) nitrate nonahydrate (Cr(NO3)3‚9H2O), and iron(III) nitrate nonahydrate (Fe(NO3)3‚ 9H2O) were purchased from Aldrich (Milwaukee, WI). Glacial acetic acid, ethylenediamine tetraacetic acid (EDTA), nickel(II) sulfate hexahydrate (NiSO4‚6H2O), cadmium(II) chloride anhydrous (CdCl2), and mercury(II) nitrate monohydrate (Hg(NO3)2‚H2O) were obtained from Fisher Scientific (Brightwaters, NY). All water was deionized and double distilled, and all compounds were used as received. Synthesis of HDACS is described elsewhere (14). A Potomac River sample was taken on April 10, 2003, at high water levels, to avoid added particulates, in the C&O Canal National Park approximately six miles upstream from Washington, DC. The sample was stored in the refrigerator in a new 50 mL plastic Falcon tube for a week to allow the settling of any particulate, and only the clear solution was used for testing. The sample was then sent to Exygen Research (State College, PA) and was analyzed using a PE Sciex inductively coupled plasma mass spectrometer (ICP/MS). Polished silicon wafers (10 cm diameter, phosphorus doped, crystal face 100, n-type) were purchased from MEMC Electronic Materials, Inc. (St. Peters, MO). Gold-coated silica slides, used for reflectance FT-IR measurements, were prepared by electrolytically depositing a 5 nm chromium layer on a piranha-cleaned (1:4 30% H2O2 in concentrated H2SO4) silica substrate followed by a 55 nm gold layer. Preparation of Films. The silicon chips (approximately 4 cm2) were cleaned using a modified RCA (Radio Corporation of America) protocol (15). First, the chips were immersed in a solution of NH4OH/H2O2/H2O (1:1:5) at 80 °C for 5 min. The chips were then rinsed repeatedly with water and immersed in a solution of HCl/H2O2/H2O (1:1:5) at 80 °C for 5 min. After additional rinsing with water, the chips were dried under a N2 stream. VOL. 38, NO. 16, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Chitosan-HDACS Filmsa
FIGURE 1. Structure A is chitin when X . Y and chitosan when Y . X; structure B is PAH, and structure C is HDACS. To create the cross-linked chitosan-HDACS films, 45 mg of HDACS was added to the chitosan solution, 225 mg of chitosan/30 mL of 2.5% acetic acid. To create the crosslinked PAH-HDACS films, 60 mg of HDACS was added to 2.5 mL of PAH (50% solution in water) in 27.5 mL of water. For spin coating, ∼0.4 mL of the above solution was placed on the chip covering the majority of the surface. The chips were spun at 2000 rpm for 30 s using a spin-coater from Specialty Coating Systems, Inc. (Indianapolis, IN). The substrates were placed on an 80 °C hot plate for 30 min to dry, then into a 120 °C oven for 2 h to cross-link and cure. Ellipsometry. The ellipsometry measurements were taken on a J. A. Woollam Co., Inc multiwavelength ellipsometer (Lincoln, NE) with an angle of incidence of 70°. Films were dried with a stream of N2 prior to measurement. The assumption was made for ellipsometry measurements that the manually entered refractive index of the thin films remained unchanged following the metal ion interaction. The refractive indices of the cross-linked polymers were first measured as a thick cast film on an Abbe prism refractometer. Reflectance Spectroscopy. The reflectance of the films was measured using a USB2000 miniature fiber optic spectrometer (Ocean Optics Inc., Dunedin, FL). Films were placed perpendicular to the light source, and only the color back reflected at 180° to the film was measured. ChitosanHDACS and PAH-HDACS produced clear, optical interference-colored films, with a highly reflective peak characteristic of the film’s thickness. All films were dried before reflectance measurement. Reflectance FT-IR. Reflectance FT-IR was taken using a liquid N2 cooled Nicolet Magna IR 750 spectrophotometer, series II (Madison, WI). Spectra were recorded for 128 scans at a resolution of 8 cm-1 and were referenced to the spectrum of an unmodified gold substrate. Samples for FT-IR measurements were dip coated onto a 1 × 1 in. gold substrate. Solvent and Metal Ion Tests. Each of the dried films was first measured using ellipsometry and reflectance spectroscopy. The film was placed into 5 mL of 50 parts per million (ppm) aqueous metal ion solution or solvent for 5 min at room temperature. The film was removed, dried under a stream of N2, and measured with ellipsometry and reflectance spectroscopy. The changes in thickness and reflectance maximum were used to identify the metal ion present.
Results and Discussion Chitosan Films. Chitosan is a nontoxic, biodegradable, and functional biopolymer consisting primarily of β(1f4) linked 4410
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chitosan-HDACS
thickness change (nm)b
reflectance change (nm)c
CrO3 Cr(acac)3 NaNO3 H2O CuSO4‚5H2O Fe(NO3)3‚9H2O NiSO4‚6H2O Na2SO4 MnCl2‚4H2O CoCl2‚H2O CdCl2 Hg(NO3)3‚H2O ZnCl2 Cr(NO3)3‚9H2O CH2Cl2 MeOH
8.7 ( 0.3 -2.5 ( 0.2 -5.1 ( 4.1 -6.3 ( 0.7 -6.9 ( 0.9 -8.9 ( 3.8 -9.7 ( 0.9 -10.3 ( 0.8 -10.8 ( 1.8 -11.5 ( 3.2 -12.4 ( 0.9 -13.8 ( 0.4 -14.7 ( 1.2 -18.8 ( 3 -22.2 ( 5.9 -30.1 ( 19.8
14 ( 1 -30 ( 1 -42 ( 6 24 ( 3 -39 ( 3 -42 ( 2 -42 ( 6 -47 ( 3 -49 ( 3 -45 ( 16 -60 ( 4 -53 ( 7 -61 ( 3 -14 ( 3 -45 ( 0 -29 ( 2
a Mean plus standard deviation of three samples after dipping into a 50 ppm solution, except for the solvents, which were used pure. b Thickness determined by ellipsometry and ordered according to increasingly negative changes from a 124 ( 1 nm initial thickness reading. c Reflectance maximum wavelength shift from a 454 ( 3 nm initial reading.
2-amino-2-deoxy-β-D-glucopyranose units. Many factors determine chitosan’s capacity for absorption of metal ions including pH, concentration, temperature, percent deacetylation, interaction time, and chain length. For example, if the pH is below the pKa (between 6.2 and 6.8) of chitosan, then the amines become approximately 90% protonated, resulting in a positively charged chitosan, which is a good adsorbent for anions. Alternatively, if chitosan is above the pKa, then the amines are deprotonated resulting in a good adsorbent for cations. The creation of thin films of chitosan-Resimene/tetraethylene glycol (TEG) has been previously documented (13, 16). Studies on the effects of cross-linking chitosan-HDACS in the bulk powder have also been undertaken (14). ChitosanHDACS colored, thin films were investigated here for their ability to distinguish metal ions using a change in optical properties and thickness. The films are measured using reflectance and ellipsometry before and after dipping into a 50 ppm aqueous metal salt solution. The chitosan-HDACS films used in the metal ion study were blue in color with a thickness of 124 ( 1 nm and a reflectance maximum wavelength at 454 ( 3 nm. Chitosan-HDACS Film Response. The chitosan-HDACS films’ response to various 50 ppm metal ion solutions is given in Table 1 as decreasing thickness. The chitosan-HDACS films were found to have a 7% increase in thickness upon interaction with the CrO3 solution (in bold), but a thickness decrease was observed with all other samples including Cr(acac)3 (2% decrease) and Cr(NO3)3‚9H2O (15% decrease) (in bold). The reflectance maximum peak after dipping into the CrO3 solution increased in wavelength or red shifted 3%. The reflectance maximum after dipping into the Cr(acac)3 decreased in wavelength or blue shifted 7%, while Cr(NO3)3‚ 9H2O caused a blue shift of 3%. The entry for H2O is italicized because, while the thickness decreased 5%, the reflectance increased 5%. The percent change in thickness was investigated using a two-tailed test, or t-test, to determine the statistical probability that the thickness changes were real. Comparing a blank chitosan-HDACS film that was measured twice to determine the error of the ellipsometer to all thickness changes observed, the P ) 1.8 E-09. Comparing the thickness changes of Cr(VI) to all of the metal ions and solvents tested, a P ) 7.8 E22 was observed. Therefore, the Cr(VI) response
TABLE 2. PAH-HDACS Filmsa
FIGURE 2. Dose-response curve for chitosan-HDACS and CrO3. Thickness measured using an ellipsometer before and after dipping into a CrO3 solution. Each point indicates the mean ( standard deviation of three samples. has a low probability of being similar to a solvent or other metal ion. Chitosan-HDACS Cr(VI) Specificity. Chromium is present in waste solutions in two forms, Cr(III) and Cr(VI), and the inherent properties of these two species are considerably different. Cr(VI) is toxic due to the ease with which it penetrates biological membranes and its highly oxidizing effect; it is thus considered a carcinogenic agent. Cr(III), however, is essential for maintaining the metabolism of lipids, glucose, and proteins (17). Chitosan membranes have been investigated for their ability to remove chromium(III and VI) from aqueous solutions (18). Chitosan flakes (19), microemulsions (20), and particles (21) have been used to study the absorption of Cr(III). Chitosan membranes, cross-linked and uncross-linked (18, 22), chitosan coated ceramic alumina (23), and hollow fiber modules made of chitosan (24) have been used for the adsorption of Cr(VI). Acidic solutions (below pH 4) of chitosan have been determined to partially reduce Cr(VI) to Cr(III) by about 60% (25). According to some published literature, Cr(III) (21) binds to amines on chitosan, and Cr(VI) (26) prefers chitin over chitosan; therefore, the binding may occur through the acetylamine or primary or secondary alcohol groups. The protonated amines in chitosan can also undergo electrostatic attraction to the Cr(VI) anion, which would make chitosan a better sorbant for Cr(VI) over chitin. A dose-response curve (Figure 2) was run to identify the effect of different concentrations of Cr(VI) on the thickness of chitosan-HDACS films. The curve from 8 ppm to 0 ppb (parts per billion) indicates that the films can detect as low as 8 ppb CrO3, but they are unable to quantify the amount of CrO3 present. A curve of the higher concentrations of CrO3 is shown in the Supporting Information. The EPA allowable upper limit maximum contaminant level (MCL) for Cr(VI) in drinking water is 0.1 mg/L or 100 ppb (27). So, the chitosanHDACS films are responsive in the relevant range at the beginning of the linear part of the concentration curve. PAH Films. Previous work with thin films of cross-linked PAH-disuccinimidyl suberate-gluteraldehyde/TEG demonstrated the films to be a generic sensor for metal ions (13). The films increased in thickness responding to CuSO4‚5H2O, CdCl2, CrO3, CoCl2‚H2O, MnCl2‚4H2O, and ZnCl2. Previous work by Rivas and Seguel, using uncross-linked PAH in solution, indicates that PAH selectively binds divalent metal ions, such as Cu(II), Co(II), and Ni(II) (28). Therefore, thin films of PAH cross-linked with HDACS were investigated for their potential sensitivity to metal ions. Rivas and Seguel noted a change in IR spectra of uncrosslinked PAH upon interaction with either Cu, Co, or Ni (28) where the PAH solution bound the metal ions in a 1:4 ratio and shifted the NH2 and C-N pellet IR peaks by as much as
PAH-HDACS
thickness change (nm)b
reflectance change (nm)c
CuSO4‚5H2O CuCl2 Cu(NO3)2‚2.5H2O CuCl Hg(NO3)3‚H2O NiSO4‚6H2O ZnCl2 NaNO3 Na2SO4 MnCl2‚4H2O Cr(NO3)3‚9H2O CoCl2‚H2O CH2Cl2 H2O CdCl2 Fe(NO3)3‚9H2O MeOH CrO3
116 ( 77 42 ( 6 37 ( 6 15 ( 9 -1 ( 0.2 -3 ( 0.4 -4 ( 1 -5 ( 0 -7 ( 4 -8 ( 7 -12 ( 2 -13 ( 4 -60 ( 9 -67 ( 12 -116 ( 6 -127 ( 12 -138 ( 4 -153 ( 55
23 ( 5 41 ( 19 22 ( 9 27 ( 1 -20 ( 1 -8 ( 4 -3 ( 0.1 -15 ( 6 -16 ( 7 -11 ( 3 -35 ( 8 -3 ( 1 0(0 -15 ( 6 -11 ( 7 -15 ( 4 29 ( 7 23 ( 21
a Mean plus standard deviation of three samples after dipping into a 50 ppm solution, except for the solvents, which were used pure. b Change in thickness from initial measured using ellipsometry and ordered as increasingly negative thickness changes from a 388 ( 41 nm initial reading. c Reflectance maximum peak wavelength shift from a 740 ( 20 nm initial reading.
40 cm-1. We investigated a thin film of PAH-HDACS to identify changes to the IR spectra upon interaction with CuSO4‚5H2O. The sample underwent the following series of experimental procedures for IR measurements: the film was spun onto a gold coated substrate, dried under a stream of N2, measured, heated at 120 °C for 2 h to further cross-link, measured, dipped into 50 ppm CuSO4‚5H2O solution, measured, allowed to sit into a N2 atmosphere overnight, and measured. However, we were unable to identify any large IR peak shifts resulting from the film’s interaction with the CuSO4‚5H2O solution (see Supporting Information). PAH-HDACS Film Response. The thickness and optical characteristics of PAH-HDACS thin films were measured using ellipsometry and reflectance. Cross-linking of PAH to HDACS occurs quickly in solution above pH 7. PAH films, with controlled thickness, were made by varying the amount of HDACS. The PAH-HDACS films’ response to the metal ion solution is ordered in Table 2 according to decreasing thickness. The films were found to increase in thickness upon interaction with 50 ppm CuSO4‚5H2O solutions 32% increase, CuCl2 11% increase, and Cu(NO3)2‚2.5H2O 10% increase (in bold), with a thickness decrease for all other metal ions. Interestingly, the Cu(I) solution also caused an increase in thickness and reflectance similar to the Cu(II) solutions. Both methanol and CrO3, indicated by the italicized entry, had a decrease in thickness by ellipsometry, 34 and 37%, respectively, but a 3-4% red shift, or increase, in the reflectance maximum. PAH-HDACS Cu(II) Specificity. The percent change in thickness was investigated using a t-test to determine the statistical probability that the Cu(II) thickness changes were significantly different than those generated by the solvents. Comparing a blank PAH-HDACS film that was measured twice to determine the error of the ellipsometer to all thickness changes observed, the P ) 0.031. Comparing the thickness changes induced by the three Cu(II) solutions to those generated by all of the metal ions, except Cu(I), and solvents tested generated a P ) 0.001. Therefore, the Cu(II) response has a low probability of being similar to a solvent or other metal ion. A series of concentrations (Figure 3) were tested to identify the effect of different concentrations of CuSO4‚5H2O on the VOL. 38, NO. 16, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Effect of pH on Film Thicknessa pH
chitosan-HDACS
PAH-HDACS
4 5 6 7
-12 ( 1 -8 ( 4 -10 ( 2 -14 ( 4
-17 ( 1 -16 ( 2 -16 ( 2 -16 ( 1
a Change in thickness (nm) from ellipsometry. Mean plus the standard deviation of three samples.
FIGURE 3. Dose-response curve for PAH-HDACS and CuSO4‚5H2O. Thickness measured using an ellipsometer before and after dipping into a CuSO4‚5H2O solution. Each point indicates the mean ( standard deviation of three samples.
FIGURE 4. Correlation between thickness and index of refraction demonstrating the phase change of reflected light off the surface of a thin film. PAH-HDACS films. While the films could detect the presence of Cu(II) at concentrations as low as parts per trillion (ppt), as seen in the initial positive change in thickness, a dosedependent response was not observed. However, according to the EPA the MCL is 1.3 mg/L or 1.3 ppm (27), which places the beginning of the more responsive range of the sensor in the relevant range. To determine if the measured thickness increase was in fact an actual increase and not due to a change in the index of refraction, an artificial step was scratched into a film. The thickness change of the step was measured before and after dipping into a 50 ppm CuSO4‚5H2O solution using a profilometer (Tencore alpha stepper). The thickness of the artificially created step was 209 ( 10 nm. After dipping into a 50 ppm CuSO4‚5H2O solution for 5 min and allowing the sample to dry under a stream of N2, the thickness increased to 232 ( 16 nm. This indicated that there was a 23 nm or 11% thickness increase separate from any change in the index of refraction. Using FILM WIZARD software (Scientific Computing International), the changes in the thickness observed from profilometry were modeled. The initial thickness and index of refraction data were entered into the program, and a reflectance curve was plotted. Then the final thickness after the dip and the initial index of refraction were entered into the program, and a reflectance curve was plotted. The modeled reflectance wavelength maximum changes from 545 nm, before a 50 ppm CuSO4‚5H2O dip, to 619 nm, after 50 ppm CuSO4‚5H2O dip, which equals a 74 nm red shift or 14% change. The difference between the 11% thickness change and the 14% reflectance maximum change can be explained by a change in the index of refraction. In Figure 4, using a modified Fresnel’s equation, Φ, or phase change, multiplied by wavelength, λ, equals the index of refraction, n, times the optical distance, d. So 3% of the reflectance 4412
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change correlates to a change in the index of refraction of 0.03 or a change from 1.38 to 1.41. PAH-HDACS Reversibility. Reversibility of the PAHHDACS films was also investigated using a similar technique for Fe(NO3)3‚9H2O with chitosan-Resimene/ TEG films (13). Samples were dipped into the 4 parts per thousand (ppt) CuSO4‚5H2O solution, dried under a stream of N2, and measured using ellipsometry and reflectance. The films became cloudy and grainy from the interaction with the CuSO4‚5H2O solution. Then, the samples were dipped into 10.7 ppth EDTA, a metal ion chelator, in water, dried, and measured. The films did not revert back to the original values for thickness or reflectance indicating that the binding to the films was not reversible under these conditions. Environmental Conditions. The chitosan-HDACS and PAH-HDACS films were tested to determine any effects due to pH. The films were dipped into environmentally relevant pH solutions ranging from pH 4-7; the resulting data are displayed in Table 3. Both PAH-HDACS and chitosan-HDACS films were found to have similar changes in thickness after dipping, independent of pH. Since the pH solutions, monitored using a pH meter, were made from both NaOH and HCl, the final solution would contain NaCl. The pH solutions are unbuffered, to prevent additional changes in thickness and reflectance from the buffering components. The 1% change in thickness for both films was comparable to changes generated by the Na2SO4 or NaNO3 solutions (Tables 1 and 2), indicating that the film was probably responding to the sodium salt rather than to a pH effect. The films changed thickness from the pH test in the opposite direction than the case of the Cr(VI) for chitosan-HDACS or the Cu(II) for PAHHDACS. The films were tested under real world, environmental conditions. Potomac River water was spiked to a concentration of 800 ppm CuSO4‚5H2O and compared to an unspiked sample. In the unspiked sample, the PAH-HDACS films gave a 5.4 ( 1.0 nm increase in thickness and 15 ( 2 nm red shift of the reflectance maximum, and the chitosan-HDACS responded with a 6.8 ( 2.5 nm loss in thickness and a 31 ( 1 blue shift, in the reflectance maximum. This coincides with the presence of copper in the Potomac River water. In the 800 ppm CuSO4‚5H2O spiked river water, the PAH-HDACS thickness increased by 8.8 ( 2.0 nm, and the reflectance red shifted 21 ( 3 nm. Interestingly, after the river water was spiked with CuSO4‚5H2O, a light blue precipitation was observed indicating that the additional copper (II) salt was in solution at a concentration different than that expected from the amount added. According to the Army Corps of Engineers, there was 8-12 times the allowable 1.3 ppm MCL (10-16 ppm) of copper, as well as 9-664 times the MCL for aluminum, 5-8 times the MCL for lead, 3-4 times the MCL for mercury, 2-3 times the MCL for selenium, and 3-252 times the MCL for zinc in the Potomac River from grab samples at the C&O National Park in March and May 2002 (29). Preliminary analysis of sludge being dumped into the Potomac River at the C&O National Historic Park, among other places, indicates the presence of high levels of arsenic, lead, mercury, chromium, copper, zinc, nickel, and selenium (29).
The Potomac River water sample (40 mL) was analyzed for copper, lead, selenium, zinc, aluminum, and mercury using ICP/MS by Exygen Research. Their findings indicate a 0.347 mg/L (347 ppb) of aluminum with a limit of quantification of 0.0005 mg/L; 0.005 mg/L (5 ppb) of copper with a limit of quantification of 0.001 mg/L; 0.002 mg/L (2 ppb) lead with a limit of quantification of 0.0003 mg/L; and 0.009 mg/L zinc (9 ppb) with a limit of quantification of 0.0005 mg/L. The presence of selenium or mercury was at or below the limits of quantification. Therefore, the thin colored PAH-HDACS film was able to distinguish copper salts in a complex solution. By changing chitosan’s cross-linker from Resimene to HDACS, the response of the films to the metal ion solution changes from generic to selective for Cr(VI). Metal binding to the chitosan or PAH films may depend on the ligand potential of either polymer (30). Possible binding sites for binding of the metal ions in chitosan-HDACS include ureas, amines, amides (80% deacetylated) and alcohols and in PAHHDACS, amines and ureas. However, the type and arrangement of the ligands may not be solely responsible for the color change in the film upon metal binding. For instance, the wavelength of reflective light may change as a function of swelling or shrinking. In addition, aggregation of the metals on the film surface might change the refractive index of the film, which was assumed to be constant after the metal ion interaction for all ellipsometry measurements. The metal salts have varying indexes of refraction from 1.46 to 1.65, which would affect the ellipsometry reading if they deposited on top of or intercalated into the film in large quantities. While the films do not provide quantitative information on the concentration of metal ions present, using the response of both films provides the capability of rapidly discriminating copper ions in the presence of other metal ions. The combination of these and additional films may offer a simple dipstick method for detecting environmental pollution by heavy metal ions.
Acknowledgments The authors thank Eric R. Welsh and Chris Rowe Taitt for helpful discussions. This work was supported by the Office of Naval Research. This work was performed while C.L.S. held a National Research Council Research Associateship Award at the NRL. The work and views expressed here are those of the authors and do not reflect the policy or opinion of the U.S. Navy or the Department of Defense.
Supporting Information Available Information on pellet FT-IR spectrum of HDACS, PAHHDACS film, and corresponding graph. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review September 22, 2003. Revised manuscript received May 11, 2004. Accepted May 12, 2004. ES035047+
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