Sensitive Surface-Enhanced Raman Scattering (SERS) Detection of Nitroaromatic Pollutants in Water Menghan Wang,a Benedetto De Vivo,a Wanjun Lu,b Maurizio Muniz-Mirandac,* a b c
Universita` Federico II, Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse (DiSTAR), 80134 Napoli, Italy China University of Geosciences (CUGW), Department of Marine Science and Engineering, Wuhan 430074, China Universita` di Firenze, Dipartimento di Chimica ‘‘Ugo Schiff’’, Via della Lastruccia 3, 50019 Sesto Fiorentino, Italy
The increasing and urgent demand for clean water requires new approaches for identifying possible contaminants. In the present study, polymer substrates with embedded silver nanoparticles are employed to reveal the presence of traces of nitroaromatic compounds in water on the basis of the surface-enhanced Raman scattering (SERS) effect. These platforms provide an easy and sensitive method of detecting of low concentrations of these organic pollutants in contaminated water. Index Headings: Surface-enhanced Raman scattering; SERS; Silver nanoparticles; Pollutant; Aromatic nitro-derivatives; Water.
INTRODUCTION Clean water, free from toxic chemicals, is essential to both human health and economic development; this is clear when we consider its many uses, such as for drinking water, in agriculture, for zoo-technical uses, and in industrial systems. Moreover, the demand for clean water is continuously rising due to global industrialization and the socioeconomic growth of emerging countries. Unfortunately, the aquifers that are the sources of drinking water can become polluted due to atmospheric contamination, the discharge of liquid contaminants, and percolation from contaminated soils. Moreover, water pollution can cause soil contamination from the accumulation of both heavy metals and toxic or carcinogenic compounds in marine and fluvial sediments. In marine waters, as well as in flow waters and aquifers, chemical compounds (usually herbicides, pesticides, and antibacterial agents used in agriculture and zoo prophylaxis) can be present. Many of these compounds have molecular structures similar to natural products and are degraded naturally or through their own chemical or photochemical instability. But when these compounds are stable, they can alter the ecosystem, accumulating in living organisms or diffusing through the environment by volatilization, dissolution, or percolation from contaminated soils. As a consequence, the effects of the pollution can manifest themselves very far from the original contamination site. Nitrophenols are a class of pollutants that are now arousing environmental alarm as being responsible for severe damage to vegetation.1,2 These compounds can derive from combustion or, to a larger extent, from reactions in atmosphere by nitration or oxidation. 4-Nitrophenol (PNP) is one of the most Received 10 December 2013; accepted 4 February 2014. * Author to whom correspondence should be sent. E-mail: muniz@unifi. it. DOI: 10.1366/13-07428
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studied pollutants because it readily is absorbed by and accumulates in soils, dramatically modifying humus pH. Therefore, this pollutant does not degrade and contaminates flow waters, aquifers, and marine waters.3,4 Nitroanilines are aromatic compounds that also may entail severe risks to the environment and, therefore, to the human health. 4-Nitroaniline (PNA), in particular, is commonly used in the synthesis of dyes, antioxidants, and pharmaceuticals; in gum inhibitors; in poultry medicines; and as a corrosion inhibitor. The compound is toxic, is particularly harmful to all aquatic organisms, and can cause long-term damage to the environment.5 For this reason, it has been categorized as a priority pollutant by the environmental protection agencies of many countries. Among the most recent techniques that have been proposed to detect pollutants, Raman spectroscopy allows for the identification of different molecules on the basis of their vibrational bands, providing an unambiguous molecular fingerprint. However, the low sensitivity of Raman scattering, along with possible spectral interference due to fluorescence emission, impairs the use of this technique for the recognition of molecular traces. In these cases, researchers can use the surface-enhanced Raman scattering (SERS) effect,6 which enhances the Raman intensity of molecules adsorbed onto metal substrates by many orders of magnitude and promotes a drastic quenching of fluorescence. Huge magnifications of the Raman signal are observed when a molecule adheres to the nanostructure surfaces of metals with high optical reflectivity, such as Ag, Au, and Cu. The SERS enhancement factors are generally up to 107 with respect to the Raman intensities of non-adsorbed molecules. By means of experimental procedures that combine microscopy and spectral observation beyond the light diffraction limit, researchers have achieved enhancement factors of 1014–1015, thus ensuring the detection of single molecules. Thanks to its peculiar properties, SERS spectroscopy, since its discovery at the beginning of the 1970s, has achieved a leading role in the analytical investigation of very low concentrations of contaminants, allowing for spectroscopic detection at the subpicogram level. As demonstrated in the SERS spectroscopy literature, 7–11 researchers can identify aromatic nitroderivatives adsorbed onto Ag nanoparticles by observing the strong enhancements of their Raman bands. In this study, we propose using SERS platforms of Ag nanoparticles embedded into a polymer matrix to identify trace amounts of PNP and PNA in water.
0003-7028/14/6807-0784/0 Q 2014 Society for Applied Spectroscopy
APPLIED SPECTROSCOPY
FIG. 1. The UV-Vis extinction of Ag-PDMS samples with different metal densities.
EXPERIMENTAL Polydimethylsiloxane (PDMS) elastomer kits (Sylgard 184, Dow Corning) and silver benzoate (Sigma-Aldrich) were used as received. The kit contains the elastomer (PDMS) and the curing agent, which is composed of dimethyl, methylhydrogen siloxane, dimethyl siloxane, dimethylvinylated, and trimethylated silica, tetramethyltetravinyl cyclotetrasiloxane, and ethyl benzene. The samples of PNP and PNA (Sigma-Aldrich) were analytical grade, and the solutions were prepared with highpurity water from a Milli-QPLUS (Millipore) ultrapure water system. The preparation of PDMS with embedded Ag nanoparticles (Ag-PDMS) was performed following the procedure used by Goyal et al.12 The elastomer (8 g) was mixed thoroughly with the curing agent in a weight ratio of 10 : 1 and then degassed under vacuum to remove trapped air bubbles. Silver benzoate (3 mL of a 5 3 103 M solution in hexane) was added to the polymer and sonicated for 15 min to obtain a homogeneous mixture. Subsequently, the color of the mixture changed to brown. It was then cast on glass slides and cured under vacuum at room temperature. The ultraviolet visible (UV-Vis) absorption spectra of Ag-PDMS were obtained in the 200–800 nm region using a Cary 5 spectrophotometer (Varian). Macro-Raman spectra were obtained using a HG-2S monochromator (Jobin-Yvon), a cooled RCA-C31034A photomultiplier (RCA), and the 514.5 nm exciting line supplied by an Ar ion laser (coherent) emitting at 50 mW. A defocused laser beam and a rotating device were adopted to impair thermal effects. The micro-Raman spectra were measured using a RM2000 instrument (Renishaw) equipped with an Arþ laser emitting at 514.5 nm. Sample irradiation was accomplished using the 503 objective lens of a Leica microscope DMLM (Leica Microsystems). The backscattered Raman signal was fed into the monochromator through 40 lm slits and detected by an air-cooled charge-coupled detector (CCD) (2.5 cm1 per
FIG. 2.
Raman spectra of PDMS and Ag-PDMS. Excitation: 514.5 nm.
pixel) filtered by a double holographic notch filtering system (Renishaw). The spectra were calibrated with respect to a silicon wafer at 520 cm1.
SPECTROSCOPIC CHARACTERIZATION OF THE SILVER–POLYDIMETHYLSILOXANE SUBSTRATE The Ag-PDMS substrate was characterized using UVVis absorption and Raman spectroscopy (Figs. 1 and 2, respectively). The occurrence of a surface plasmon resonance band around 400 nm indicates the formation of Ag nanoparticles embedded in the polymer matrix. When we increased the content of silver benzoate and, consequently, the density of the silver nanoparticles, the absorbance center moved from 410 to 428 nm (Fig. 1). Both PDMS and silver benzoate have several strong Raman bands. The first step in SERS applications using Ag-PDMS is to recognize and eliminate the Raman bands of both the polymer and benzoate. As shown in Fig. 2, strong and sharp bands occur around 1000 and 1600 cm1, due to the ring vibrations of the benzoate and intensified by the SERS effect promoted by the Ag nanoparticles.
SURFACE-ENHANCED RAMAN SCATTERING SPECTRA OF 4-NITROPHENOL AND 4NITROANILINE Both PNP and PNA are nitroaromatic compounds that exhibit sizable solubilities in water (at 25 8C, PNP: 3.2 g/L at pH 5–6 and 5.0 g/L at pH 7; PNA: 1.9 g/L at pH 5–7). Thus, they can be considered possible pollutants of the aqueous environment (both wastewaters and aquifers) and, consequently, of soil that has come into contact with contaminated water. Both PNP and PNA have polar groups (a hydroxyl and an amino group, respectively) that can strongly interact with PDMS (as shown in Fig. 3), leaving the nitrogroups free to interact with the Ag nanoparticles embedded in the polymer matrix. Hence, both compounds can be adsorbed onto the Ag-PDMS
APPLIED SPECTROSCOPY
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FIG. 3. Interaction scheme of PNP and PNA with PDMS.
substrate, and researchers can easily detect them using the SERS effect by simply depositing drops of aqueous solution containing the compounds onto the polymer surface and drying the samples at room temperature. In addition, both compounds show a very strong Raman band in the 1300–1350 cm1 region (see Fig. 4), due to the symmetric stretching mode of the nitrogroup, where no band of benzoate and polysiloxane (constituents of the polymer matrix) occurs (Fig. 2). Consequently, the presence of these aromatic derivatives can be clearly evidenced by the occurrence of the marker band of the nitrogroup, as shown in Fig. 5. To obtain the SERS spectra in Fig. 5, it was sufficient to deposit drops of a much diluted solution of PNP or PNA onto the polymer surface. The ligand molecules that resulted were strongly bound to the Ag-PDMS matrix because we could observe the SERS spectra without loss of intensity even after washing the sample for a long time in running solvent. Finally, we subtracted the Raman signal of Ag-PDMS from the observed SERS spectra, obtaining the difference spectra shown in Figs. 6 and 7 for adsorbed PNP and PNA, respectively. These SERS spectra closely correspond to those reported in the literature for both compounds adsorbed onto Ag colloidal particles11,13,14 when the nitrogroups interact with the metal surface. By employing a rotating device for the sample and a defocused laser beam, we can obtain
SERS spectra without the occurrence of spurious bands due to thermal effects. As shown in Figs. 6 and 7, the limit of detection (LOD) for these compounds can reach concentrations of 107 M or less; consequently, this procedure can be considered a sensitive method for detecting these pollutants in water or in soil that has been in contact with contaminated water. Not only can this procedure be used to reveal the presence of nitroderivatives in water, but also to identify the specific nitrophenols and nitroanilines by the assignment of the observed SERS bands. Similar results can be obtained by using a microRaman apparatus and the same laser exciting line (514.5 nm). The laser beam is focused onto Ag aggregates under a micro-Raman spectroscope at different depths of the polymer after immersing the polymer in a PNP solution (106 M concentration). After washing the sample for a long time in running water, we achieved effective spectra up to 200 lm below the polymer surface, demonstrating that the PNP molecules are extracted from the aqueous solution and enter into the polymer matrix. However, the optimum spectrum for adsorbed PNP was acquired at the surface of the substrate (Fig. 8). When the focus was set on deeper Ag aggregates instead, the inevitable photoreaction of benzoate occurred. As shown in Fig. 8, the background
FIG. 4. Raman spectra of PNP and PNA in aqueous solutions. Excitation: 514.5 nm.
FIG. 5. The SERS spectra of Ag-PDMS with PNP and PNA. Excitation: 514.5 nm.
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FIG. 6. The SERS spectra of PNP after subtracting Ag-PDMS. Excitation: 514.5 nm.
signal of the fluorescence emission increases with the polymer depth while, in contrast, the marker band of benzoate (1000 cm1) strongly decreases. Moreover, large and broad bands appear in the high-frequency region (at 1380 and 1580 cm1) due to the Raman bands of amorphous carbon produced during the laser irradiation.
CONCLUSION We used SERS platforms composed of Ag nanoparticles embedded in a polymer matrix to reveal the presence of amounts of nitroaromatic compounds, which are dangerous pollutants in flow waters, aquifers, and fluvial and marine waters. Using SERS to detect these molecules has these advantages: (1) Very high sensitivity due to the enhancement of the Raman signal. (2) Fluorescence quenching due to the molecule–metal chemical interaction. (3) Possibility of identifying the molecular sites of interaction with the metal by observing changes in the band frequencies and intensities.
FIG. 7. The SERS spectra of PNA after subtracting Ag-PDMS. Excitation: 514.5 nm.
FIG. 8. The micro-SERS spectra of Ag-PDMS with PNP. Excitation: 514.5 nm.
These properties are very important for obtaining the rapid and sensitive identification of different aromatic nitroderivatives; there is also a significant advantage in terms of the time needed for the analytical determination. In addition, the use of Ag nanoparticles embedded in PDMS allows for a simple detection procedure, without any need for sample manipulation, extraction from a solvent, or concentration procedures, because the pollutant molecules are capped by the polymer matrix after coming into contact with the substrate. In the future, we plan to use the present technique to identify the presence in an aqueous environment of other important pollutants, such as nitroimidazoles. These compounds are present in drugs used for farm animals (cows, poultry, and pigs) and promote genetic mutations and tumor modifications when present in aquifers. They have previously been identified using various chromatographic methods,15–17 which require complex extraction procedures. 1. U.S. Environmental Protection Agency (US EPA). Nitrophenols: Ambient Water Quality Criteria. EPA No. 440580063. Criteria and Standards Division. Office of Water Planning and Standards. Washington, DC: U.S. Environmental Protection Agency, 1980. 2. Agency for Toxic Substances and Disease Registry (ATSDR). Toxicological Profile for Nitrophenols: 2-Nitrophenol and 4-Nitrophenol. Atlanta, GA: U.S. Department of Health and Human Services, U.S. Public Health Service, 1992. 3. D.K. Richards, W.K. Shieh. ‘‘Biological Fate of Organic Priority Pollutants in the Aquatic Environment’’. Water Res. 1986. 20(9): 1077-1090. 4. R.C. Loehr, R. Krishnamoorthy. ‘‘Terrestrial Bioaccumulation Potential of Phenolic Compounds’’. Hazard. Waste Hazard. 1988. 5(2): 109-119. 5. L. Zhu, B. Lou, K. Yang, B. Chen. ‘‘Effects of Ionizable Organic Compounds in Different Species on the Sorption of p-Nitroaniline to Sediment’’. Water Res. 2005. 39(2-3): 281-288. 6. S. Schlu¨cker, Ed. Surface Enhanced Raman Scattering: Analytical, Biophysical and Life Science Applications. Weinheim, Germany: Wiley-VCH, 2011. 7. M. Muniz-Miranda. ‘‘SERS Monitoring of the Catalytic Reduction of 4-Nitrophenol on Ag-Doped Titania Nanoparticles’’. Appl. Catal. BEnviron. 2013. 146: 147-150.
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8. M. Muniz-Miranda, B. Pergolese, A. Bigotto. ‘‘Surface-Enhanced Raman Scattering and Density Functional Theory Study of 4Nitrobenzonitrile Adsorbed on Ag and Ag/Pd Nanoparticles’’. J. Phys. Chem. C. 2008. 112(17): 6988-6992. 9. M. Muniz-Miranda, B. Pergolese, A. Bigotto. ‘‘SERS and DFT Study of Nitroarenes Adsorbed on Metal Nanoparticles’’. Vib. Spectrosc. 2007. 43(1): 97-103. 10. M. Muniz-Miranda. ‘‘Adsorption Mechanism of 2-Amino,5-Nitropyrimidine on Silver Substrates, as Detected by Surface-Enhanced Raman Scattering’’. Vib. Spectrosc. 2002. 29(1-2): 229-233. 11. M. Muniz-Miranda. ‘‘pH Dependence of the Surface-Enhanced Raman Scattering of p-Nitroaniline Adsorbed on Silver Sols’’. J. Raman Spectrosc. 1997. 28(4): 205-210. 12. A. Goyal, A. Kumar, P.K. Patra, S. Mahendra, S. Tabatabaei, P.J. Alvarez, G. John, P.M. Ajayan. ‘‘In Situ Synthesis of Metal Nanoparticle Embedded Free Standing Multifunctional PDMS Films’’. Macromol. Rapid Comm. 2009. 30(13): 1116-1122. 13. T. Tanaka, A. Nakajima, A. Watanabe, T. Ohno, Y. Ozaki. ‘‘SurfaceEnhanced Raman Scattering of Pyridine and p-Nitrophenol Studied by
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14.
15.
16.
17.
Density Functional Theory Calculations’’. Vib. Spectrosc. 2004. 34(1): 157-167. D.A. Perry, H.J. Son, J.S. Cordova, L.G. Smith, A.S. Biris. ‘‘Adsorption Analysis of Nitrophenol Isomers on Silver Nanostructures by Surface-Enhanced Spectroscopy’’. J. Colloid Interf. Sci. 2010. 342(2): 311-319. R. Zeleny, S. Harbeck, H. Schimmel. ‘‘Validation of a Liquid Chromatography-Tandem Mass Spectrometry Method for the Identification and Quantification of 5-Nitroimidazole Drugs and Their Corresponding Hydroxy Metabolites in Lyophilised Pork Meat’’. J. Chromatogr. A. 2009. 1216(2): 249-256. H.-W. Sun, F.-C. Wang, L.-F. Ai. ‘‘Simultaneous Determination of Seven Nitroimidazole Residues in Meat by Using HPLC-UV Detection with Solid-Phase Extraction’’. J. Chromatogr. B. 2007. 857(2): 296-300. D. Hurtaud-Pessel, B. Delepine, M. Laurentie. ‘‘Determination of Four Nitroimidazole Residues in Poultry Meat by Liquid Chromatography–Mass Spectrometry’’. J. Chromatogr. A. 2000. 882(1-2): 89-98.
Universita` Federico II, Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse (DiSTAR), 80134 Napoli, Italy China University of Geosciences (CUGW), Department of Marine Science and Engineering, Wuhan 430074, China Universita` di Firenze, Dipartimento di Chimica ‘‘Ugo Schiff’’, Via della Lastruccia 3, 50019 Sesto Fiorentino, Italy
The increasing and urgent demand for clean water requires new approaches for identifying possible contaminants. In the present study, polymer substrates with embedded silver nanoparticles are employed to reveal the presence of traces of nitroaromatic compounds in water on the basis of the surface-enhanced Raman scattering (SERS) effect. These platforms provide an easy and sensitive method of detecting of low concentrations of these organic pollutants in contaminated water. Index Headings: Surface-enhanced Raman scattering; SERS; Silver nanoparticles; Pollutant; Aromatic nitro-derivatives; Water.
INTRODUCTION Clean water, free from toxic chemicals, is essential to both human health and economic development; this is clear when we consider its many uses, such as for drinking water, in agriculture, for zoo-technical uses, and in industrial systems. Moreover, the demand for clean water is continuously rising due to global industrialization and the socioeconomic growth of emerging countries. Unfortunately, the aquifers that are the sources of drinking water can become polluted due to atmospheric contamination, the discharge of liquid contaminants, and percolation from contaminated soils. Moreover, water pollution can cause soil contamination from the accumulation of both heavy metals and toxic or carcinogenic compounds in marine and fluvial sediments. In marine waters, as well as in flow waters and aquifers, chemical compounds (usually herbicides, pesticides, and antibacterial agents used in agriculture and zoo prophylaxis) can be present. Many of these compounds have molecular structures similar to natural products and are degraded naturally or through their own chemical or photochemical instability. But when these compounds are stable, they can alter the ecosystem, accumulating in living organisms or diffusing through the environment by volatilization, dissolution, or percolation from contaminated soils. As a consequence, the effects of the pollution can manifest themselves very far from the original contamination site. Nitrophenols are a class of pollutants that are now arousing environmental alarm as being responsible for severe damage to vegetation.1,2 These compounds can derive from combustion or, to a larger extent, from reactions in atmosphere by nitration or oxidation. 4-Nitrophenol (PNP) is one of the most Received 10 December 2013; accepted 4 February 2014. * Author to whom correspondence should be sent. E-mail: muniz@unifi. it. DOI: 10.1366/13-07428
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studied pollutants because it readily is absorbed by and accumulates in soils, dramatically modifying humus pH. Therefore, this pollutant does not degrade and contaminates flow waters, aquifers, and marine waters.3,4 Nitroanilines are aromatic compounds that also may entail severe risks to the environment and, therefore, to the human health. 4-Nitroaniline (PNA), in particular, is commonly used in the synthesis of dyes, antioxidants, and pharmaceuticals; in gum inhibitors; in poultry medicines; and as a corrosion inhibitor. The compound is toxic, is particularly harmful to all aquatic organisms, and can cause long-term damage to the environment.5 For this reason, it has been categorized as a priority pollutant by the environmental protection agencies of many countries. Among the most recent techniques that have been proposed to detect pollutants, Raman spectroscopy allows for the identification of different molecules on the basis of their vibrational bands, providing an unambiguous molecular fingerprint. However, the low sensitivity of Raman scattering, along with possible spectral interference due to fluorescence emission, impairs the use of this technique for the recognition of molecular traces. In these cases, researchers can use the surface-enhanced Raman scattering (SERS) effect,6 which enhances the Raman intensity of molecules adsorbed onto metal substrates by many orders of magnitude and promotes a drastic quenching of fluorescence. Huge magnifications of the Raman signal are observed when a molecule adheres to the nanostructure surfaces of metals with high optical reflectivity, such as Ag, Au, and Cu. The SERS enhancement factors are generally up to 107 with respect to the Raman intensities of non-adsorbed molecules. By means of experimental procedures that combine microscopy and spectral observation beyond the light diffraction limit, researchers have achieved enhancement factors of 1014–1015, thus ensuring the detection of single molecules. Thanks to its peculiar properties, SERS spectroscopy, since its discovery at the beginning of the 1970s, has achieved a leading role in the analytical investigation of very low concentrations of contaminants, allowing for spectroscopic detection at the subpicogram level. As demonstrated in the SERS spectroscopy literature, 7–11 researchers can identify aromatic nitroderivatives adsorbed onto Ag nanoparticles by observing the strong enhancements of their Raman bands. In this study, we propose using SERS platforms of Ag nanoparticles embedded into a polymer matrix to identify trace amounts of PNP and PNA in water.
0003-7028/14/6807-0784/0 Q 2014 Society for Applied Spectroscopy
APPLIED SPECTROSCOPY
FIG. 1. The UV-Vis extinction of Ag-PDMS samples with different metal densities.
EXPERIMENTAL Polydimethylsiloxane (PDMS) elastomer kits (Sylgard 184, Dow Corning) and silver benzoate (Sigma-Aldrich) were used as received. The kit contains the elastomer (PDMS) and the curing agent, which is composed of dimethyl, methylhydrogen siloxane, dimethyl siloxane, dimethylvinylated, and trimethylated silica, tetramethyltetravinyl cyclotetrasiloxane, and ethyl benzene. The samples of PNP and PNA (Sigma-Aldrich) were analytical grade, and the solutions were prepared with highpurity water from a Milli-QPLUS (Millipore) ultrapure water system. The preparation of PDMS with embedded Ag nanoparticles (Ag-PDMS) was performed following the procedure used by Goyal et al.12 The elastomer (8 g) was mixed thoroughly with the curing agent in a weight ratio of 10 : 1 and then degassed under vacuum to remove trapped air bubbles. Silver benzoate (3 mL of a 5 3 103 M solution in hexane) was added to the polymer and sonicated for 15 min to obtain a homogeneous mixture. Subsequently, the color of the mixture changed to brown. It was then cast on glass slides and cured under vacuum at room temperature. The ultraviolet visible (UV-Vis) absorption spectra of Ag-PDMS were obtained in the 200–800 nm region using a Cary 5 spectrophotometer (Varian). Macro-Raman spectra were obtained using a HG-2S monochromator (Jobin-Yvon), a cooled RCA-C31034A photomultiplier (RCA), and the 514.5 nm exciting line supplied by an Ar ion laser (coherent) emitting at 50 mW. A defocused laser beam and a rotating device were adopted to impair thermal effects. The micro-Raman spectra were measured using a RM2000 instrument (Renishaw) equipped with an Arþ laser emitting at 514.5 nm. Sample irradiation was accomplished using the 503 objective lens of a Leica microscope DMLM (Leica Microsystems). The backscattered Raman signal was fed into the monochromator through 40 lm slits and detected by an air-cooled charge-coupled detector (CCD) (2.5 cm1 per
FIG. 2.
Raman spectra of PDMS and Ag-PDMS. Excitation: 514.5 nm.
pixel) filtered by a double holographic notch filtering system (Renishaw). The spectra were calibrated with respect to a silicon wafer at 520 cm1.
SPECTROSCOPIC CHARACTERIZATION OF THE SILVER–POLYDIMETHYLSILOXANE SUBSTRATE The Ag-PDMS substrate was characterized using UVVis absorption and Raman spectroscopy (Figs. 1 and 2, respectively). The occurrence of a surface plasmon resonance band around 400 nm indicates the formation of Ag nanoparticles embedded in the polymer matrix. When we increased the content of silver benzoate and, consequently, the density of the silver nanoparticles, the absorbance center moved from 410 to 428 nm (Fig. 1). Both PDMS and silver benzoate have several strong Raman bands. The first step in SERS applications using Ag-PDMS is to recognize and eliminate the Raman bands of both the polymer and benzoate. As shown in Fig. 2, strong and sharp bands occur around 1000 and 1600 cm1, due to the ring vibrations of the benzoate and intensified by the SERS effect promoted by the Ag nanoparticles.
SURFACE-ENHANCED RAMAN SCATTERING SPECTRA OF 4-NITROPHENOL AND 4NITROANILINE Both PNP and PNA are nitroaromatic compounds that exhibit sizable solubilities in water (at 25 8C, PNP: 3.2 g/L at pH 5–6 and 5.0 g/L at pH 7; PNA: 1.9 g/L at pH 5–7). Thus, they can be considered possible pollutants of the aqueous environment (both wastewaters and aquifers) and, consequently, of soil that has come into contact with contaminated water. Both PNP and PNA have polar groups (a hydroxyl and an amino group, respectively) that can strongly interact with PDMS (as shown in Fig. 3), leaving the nitrogroups free to interact with the Ag nanoparticles embedded in the polymer matrix. Hence, both compounds can be adsorbed onto the Ag-PDMS
APPLIED SPECTROSCOPY
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FIG. 3. Interaction scheme of PNP and PNA with PDMS.
substrate, and researchers can easily detect them using the SERS effect by simply depositing drops of aqueous solution containing the compounds onto the polymer surface and drying the samples at room temperature. In addition, both compounds show a very strong Raman band in the 1300–1350 cm1 region (see Fig. 4), due to the symmetric stretching mode of the nitrogroup, where no band of benzoate and polysiloxane (constituents of the polymer matrix) occurs (Fig. 2). Consequently, the presence of these aromatic derivatives can be clearly evidenced by the occurrence of the marker band of the nitrogroup, as shown in Fig. 5. To obtain the SERS spectra in Fig. 5, it was sufficient to deposit drops of a much diluted solution of PNP or PNA onto the polymer surface. The ligand molecules that resulted were strongly bound to the Ag-PDMS matrix because we could observe the SERS spectra without loss of intensity even after washing the sample for a long time in running solvent. Finally, we subtracted the Raman signal of Ag-PDMS from the observed SERS spectra, obtaining the difference spectra shown in Figs. 6 and 7 for adsorbed PNP and PNA, respectively. These SERS spectra closely correspond to those reported in the literature for both compounds adsorbed onto Ag colloidal particles11,13,14 when the nitrogroups interact with the metal surface. By employing a rotating device for the sample and a defocused laser beam, we can obtain
SERS spectra without the occurrence of spurious bands due to thermal effects. As shown in Figs. 6 and 7, the limit of detection (LOD) for these compounds can reach concentrations of 107 M or less; consequently, this procedure can be considered a sensitive method for detecting these pollutants in water or in soil that has been in contact with contaminated water. Not only can this procedure be used to reveal the presence of nitroderivatives in water, but also to identify the specific nitrophenols and nitroanilines by the assignment of the observed SERS bands. Similar results can be obtained by using a microRaman apparatus and the same laser exciting line (514.5 nm). The laser beam is focused onto Ag aggregates under a micro-Raman spectroscope at different depths of the polymer after immersing the polymer in a PNP solution (106 M concentration). After washing the sample for a long time in running water, we achieved effective spectra up to 200 lm below the polymer surface, demonstrating that the PNP molecules are extracted from the aqueous solution and enter into the polymer matrix. However, the optimum spectrum for adsorbed PNP was acquired at the surface of the substrate (Fig. 8). When the focus was set on deeper Ag aggregates instead, the inevitable photoreaction of benzoate occurred. As shown in Fig. 8, the background
FIG. 4. Raman spectra of PNP and PNA in aqueous solutions. Excitation: 514.5 nm.
FIG. 5. The SERS spectra of Ag-PDMS with PNP and PNA. Excitation: 514.5 nm.
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FIG. 6. The SERS spectra of PNP after subtracting Ag-PDMS. Excitation: 514.5 nm.
signal of the fluorescence emission increases with the polymer depth while, in contrast, the marker band of benzoate (1000 cm1) strongly decreases. Moreover, large and broad bands appear in the high-frequency region (at 1380 and 1580 cm1) due to the Raman bands of amorphous carbon produced during the laser irradiation.
CONCLUSION We used SERS platforms composed of Ag nanoparticles embedded in a polymer matrix to reveal the presence of amounts of nitroaromatic compounds, which are dangerous pollutants in flow waters, aquifers, and fluvial and marine waters. Using SERS to detect these molecules has these advantages: (1) Very high sensitivity due to the enhancement of the Raman signal. (2) Fluorescence quenching due to the molecule–metal chemical interaction. (3) Possibility of identifying the molecular sites of interaction with the metal by observing changes in the band frequencies and intensities.
FIG. 7. The SERS spectra of PNA after subtracting Ag-PDMS. Excitation: 514.5 nm.
FIG. 8. The micro-SERS spectra of Ag-PDMS with PNP. Excitation: 514.5 nm.
These properties are very important for obtaining the rapid and sensitive identification of different aromatic nitroderivatives; there is also a significant advantage in terms of the time needed for the analytical determination. In addition, the use of Ag nanoparticles embedded in PDMS allows for a simple detection procedure, without any need for sample manipulation, extraction from a solvent, or concentration procedures, because the pollutant molecules are capped by the polymer matrix after coming into contact with the substrate. In the future, we plan to use the present technique to identify the presence in an aqueous environment of other important pollutants, such as nitroimidazoles. These compounds are present in drugs used for farm animals (cows, poultry, and pigs) and promote genetic mutations and tumor modifications when present in aquifers. They have previously been identified using various chromatographic methods,15–17 which require complex extraction procedures. 1. U.S. Environmental Protection Agency (US EPA). Nitrophenols: Ambient Water Quality Criteria. EPA No. 440580063. Criteria and Standards Division. Office of Water Planning and Standards. Washington, DC: U.S. Environmental Protection Agency, 1980. 2. Agency for Toxic Substances and Disease Registry (ATSDR). Toxicological Profile for Nitrophenols: 2-Nitrophenol and 4-Nitrophenol. Atlanta, GA: U.S. Department of Health and Human Services, U.S. Public Health Service, 1992. 3. D.K. Richards, W.K. Shieh. ‘‘Biological Fate of Organic Priority Pollutants in the Aquatic Environment’’. Water Res. 1986. 20(9): 1077-1090. 4. R.C. Loehr, R. Krishnamoorthy. ‘‘Terrestrial Bioaccumulation Potential of Phenolic Compounds’’. Hazard. Waste Hazard. 1988. 5(2): 109-119. 5. L. Zhu, B. Lou, K. Yang, B. Chen. ‘‘Effects of Ionizable Organic Compounds in Different Species on the Sorption of p-Nitroaniline to Sediment’’. Water Res. 2005. 39(2-3): 281-288. 6. S. Schlu¨cker, Ed. Surface Enhanced Raman Scattering: Analytical, Biophysical and Life Science Applications. Weinheim, Germany: Wiley-VCH, 2011. 7. M. Muniz-Miranda. ‘‘SERS Monitoring of the Catalytic Reduction of 4-Nitrophenol on Ag-Doped Titania Nanoparticles’’. Appl. Catal. BEnviron. 2013. 146: 147-150.
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Density Functional Theory Calculations’’. Vib. Spectrosc. 2004. 34(1): 157-167. D.A. Perry, H.J. Son, J.S. Cordova, L.G. Smith, A.S. Biris. ‘‘Adsorption Analysis of Nitrophenol Isomers on Silver Nanostructures by Surface-Enhanced Spectroscopy’’. J. Colloid Interf. Sci. 2010. 342(2): 311-319. R. Zeleny, S. Harbeck, H. Schimmel. ‘‘Validation of a Liquid Chromatography-Tandem Mass Spectrometry Method for the Identification and Quantification of 5-Nitroimidazole Drugs and Their Corresponding Hydroxy Metabolites in Lyophilised Pork Meat’’. J. Chromatogr. A. 2009. 1216(2): 249-256. H.-W. Sun, F.-C. Wang, L.-F. Ai. ‘‘Simultaneous Determination of Seven Nitroimidazole Residues in Meat by Using HPLC-UV Detection with Solid-Phase Extraction’’. J. Chromatogr. B. 2007. 857(2): 296-300. D. Hurtaud-Pessel, B. Delepine, M. Laurentie. ‘‘Determination of Four Nitroimidazole Residues in Poultry Meat by Liquid Chromatography–Mass Spectrometry’’. J. Chromatogr. A. 2000. 882(1-2): 89-98.
