Rapid recognition of isomers of monochlorobiphenyls at trace levels by surface-enhanced Raman scattering using Ag nanorods as a substrate

Nano Research, Jun 2010

Isomers and homologues of organic pollutants are hard to distinguish-especially in trace amounts-due to the similarities in their physical and chemical properties. We report here that by identifying the Raman characteristics of isomers of monochlorobiphenyls, these compounds can be recognized, even at trace levels, by using the surface-enhance Raman scattering method with silver nanorods as a substrate. When dissolved in acetone, 2-, 3-, and 4-chlorobiphenyls were detected at a concentration of 10−8 mol/L, at which their characteristic Raman peaks were visible. This study may provide a fast, simple, and sensitive method for the detection and recognition of organic pollutants such as polychlorinated biphenyls.

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Rapid recognition of isomers of monochlorobiphenyls at trace levels by surface-enhanced Raman scattering using Ag nanorods as a substrate

Qin Zhou 0 Ye Yang 0 Jie Ni 0 Zhengcao Li 0 Zhengjun Zhang ( 0 0 Advanced Materials Laboratory, Department of Materials Science and Engineering, Tsinghua University , Beijing 100084, China Isomers and homologues of organic pollutants are hard to distinguishespecially in trace amountsdue to the similarities in their physical and chemical properties. We report here that by identifying the Raman characteristics of isomers of monochlorobiphenyls, these compounds can be recognized, even at trace levels, by using the surface-enhance Raman scattering method with silver nanorods as a substrate. When dissolved in acetone, 2-, 3-, and 4-chlorobiphenyls were detected at a concentration of 10-8 mol/L, at which their characteristic Raman peaks were visible. This study may provide a fast, simple, and sensitive method for the detection and recognition of organic pollutants such as polychlorinated biphenyls. 1. Introduction Persistent organic pollutants (POPs), e.g., dioxins, and polychlorinated biphenyls (PCBs), etc., are harmful and have polluted almost everywhere in the world [1]. The removal of these pollutants, which has aroused great research interest in recent years, requires techniques that are able to detect these compounds even at trace levels. This is because even in trace amounts in the environment, they can be accumulated at high dosage in human bodies through foods (vegetables, plants, animals, etc.) and cause severe health problems when they exceed the critical dose [13]. Currently, a combination of high-resolution gas chromatography and mass spectrometry is widely used as a powerful means for the detection of these compounds; this method is, however, expensive and time-consuming, and is not always able to distinguish isomers [47]. Materials with dimensions on the nanometer scale exhibit many interesting properties and may find opportunities in the detection of trace amounts of organic pollutants [811]. For example, using nanostructures of noble metals (Cu, Ag, and Au) as the substrate, some organic species have been detected in trace amounts by surface-enhanced Raman scattering (SERS) [1218]. The advantages of SERS are its high sensitivity, simplicity, fast detection speed, as well as its capability in the recognition of compounds. Therefore it is of interest to investigate the possibility of using SERS in detection/recognition of POPs such as PCBs. The difficulty of using SERS for PCBs is that they are insoluble in water, the solvent normally used in SERS measurements [19, 20]. Besides, little is known about the characteristics of their Raman spectra, which are essential for the recognition of isomers. Here, we report an experimental and theoretical study of the Raman spectra of biphenyl and monochlorobiphenyl isomers, from which the characteristics of their Raman spectra were obtained, and then describe the detection and recognition of these compounds in trace amounts using aligned Ag nanorods as the substrate. 2. Experimental procedure The Raman spectra of biphenyl, and 2-, 3-, and 4-chlorobiphenyls were measured with a Renishaw Raman 100 spectrometer using a 633 nm HeNe laser as the excitation source at room temperature. Powders of these compounds are commercially available from the AccuStandard Company. Simulation of the Raman spectra was performed by means of density functional theory using the Gaussian 03 program package in order to better understand the vibrational modes observed, and define the fingerprints of these compounds. For the SERS measurements, powders of chlorobiphenyls were dissolved in acetone to concentrations from 104 to 1010 mol/L. The substrates were Ag nanorods prepared by electron beam deposition. The deposition of the Ag nanorods has been described elsewhere [21]. A small volume of each solution (~ 0.5 L) was dropped on the surface of Ag nanorods, and acetone was blown away using a nitrogen flow. 3. Results and discussion Figures 1(a)1(d) show the measured Raman spectra Figure 1 Raman spectra of (a) biphenyl; (b) 2-chlorobiphenyl; (c) 3-chlorobiphenyl; (d) 4-chlorobiphenyl, measured using powders commercially available from the AccuStandard Company of biphenyl, and 2-, 3-, and 4-chlorobiphenyl, respectively. Each material has strong peaks at ~3065, 1600, 1280, 1030, and 1000 cm1, demonstrating the common features of biphenyl and its derivatives. One may also notice differences between the Raman spectra of the four materials. For example: (1) biphenyl, 3- and 4-chlorobiphenyl all have a strong Raman peak around 1276 cm1, while the corresponding peak for 2-chlorobiphenyl is at ~1297 cm1; (2) biphenyl has a strong peak at 738 cm1, 2- and 4-chlorobiphenyl have strong peaks at ~760 cm1, but there are no significant peaks for 3-chlorobiphenyl in this region; (3) both 2- and 3-chlorobiphenyl have strong peaks around ~680 cm1, while biphenyl and 4-chlorobiphenyl have no visible peak nearby; (4) only 2-chlorobiphenyl has a strong peak at ~432 cm1. The above features might be used to detect and distinguish between biphenyl, and 2-, 3, and 4-chlorobiphenyl. To gain a clear understanding of these features, we performed simulations using density functional theory with the Gaussian 03 program package. The simulations were carried out by Beckes three-parameter hybrid method using the LeeYangParr correlation functional (B3LYP), and the LANL2DZ basis set [22]. Gaussian View was used to input data visually. The bond length of the benzene ring was set to be 1.409 , the bond length between C and H atoms was set to be 1.088 , and the bond length between C and Cl atoms was set to be 1.760 . Table 1 lists the major vibrational modes of the four materials obtained by the above simulations. The common features in their experimental Raman spectra at ~3065, 1600, 1280, 1030, and 1000 cm1 (see Fig. 1), can be attributed to the CH stretching mode (~3100 cm1), the ring CCC stretching mode (~1650 cm1), the CC bridge bond stretching mode (~1280 cm1), the CH bending in-plane mode (1050 to 1100 cm1), and the CCC trigonal breathing mode (~1000 cm1), respectively. Replacement of the H by Cl atom results in different changes in the three CCC bending (ring deformation) in-plane modes (with calculated Raman shifts of 760 cm1, 680 cm1, and 460 cm1) for 2-, 3-, and 4-chlorobiphenyl. Figures 2(a)2(c) show the most intense CCC bending in-plane mode for 2-, 3-, and 4-chlorobiphenyl, respectively. For 2-chlorobiphenyl, the 25 direction CCC bending at 460 cm1 is the strongest, the 36 direction CCC bending at 680 cm1 is slightly weaker, while the 14 direction bending at 760 cm1 is weak. For 3-chlorobiphenyl, the 25 direction bending mode is negligible, the 36 direction bending mode is strong, while the 14 direction bending mode is weak. For 4-chlorobiphenyl, the 25 direction bending mode is negligible, the 36 direction bending mode is weak, while the 14 direction bending mode is strong. These results are in agreement with the experimental measurements (Fig. 1), and suggest that the intensities of the CCC bending inplane modes can be used to distinguish between the three isomers. The substrate used in the SERS measurements was Ag nanorods prepared by the electron beam deposition technique [21]. Figure 3 shows a typical scanning electron microscope (SEM) image of the Ag nanorods, taken with an FEI SEM (Quanta 200 FEG) working at 20 kV. Powders of 2-, 3-, and 4-chlorobiphenyl were dissolved in acetone and diluted to concentrations Table 1 Major simulated vibrational modes for 2-, 3-, and 4-chlorobiphenyl 359 34 50 22 44 4 8 0 351 38 62 15 27 23 7 0 Figure 2 The most intense ring deformation in-plane modes for the three isomers of chlorobiphenyl: (a) 2-chlorobiphenyl; (b) 3-chlorobiphenyl; (c) 4-chlorobiphenyl ranging from 104 to 1010 mol/L. A small volume (0.5 L) of these solutions was dropped on Ag nanorods and the acetone was blown away using a gentle nitrogen flow. Figures 4(a)4(c) show the SERS spectra of the 2-, 3-, and 4-chlorobiphenyl, respectively, at various concentrations. The accumulation time of each spectrum was fixed at 30 s per 100 cm1, and we used only 10% laser power (0.47 mW) to avoid radiation damage. The characteristic Raman peaks are all clearly Figure 3 A typical SEM image showing the morphology of Ag nanorods used as the SERS substrate in this study observed for each of the three isomers even at a concentration of 108 mol/L, suggesting that the SERS technique is able to detect chlorobiphenyls even at such a low concentration. Figure 4(d) shows a comparison of the SERS spectra of the three isomers at a concentration of 106 mol/L. At this concentration the three spectra show the common features mentioned above around 1600, 1280, 1030, and 1000 cm1 (the region around 3100 cm1 was not scanned). The spectra also clearly show the individual characteristic Raman peaks for the three isomers, i.e., the difference in the CCC bending (ring deformation) in-plane modes caused by the Cl atom replacement. The spectra suggest that by using Ag nanorods as substrates, the SERS technique is capable of detecting chlorobiphenyls at trace levels and is capable of recognizing the isomers at low concentrations. 4. Concluding remarks We have demonstrated a simple method to detect and recognize the isomers of chlorobiphenyls, even in trace amounts, by using the SERS technique with Ag nanorods as substrates, based on an understanding of Raman characteristics of these compounds. This method might be also applicable to the detection of PCBs, which is crucial for the effective removal of these hazardous substances. Figure 4 SERS spectra of (a) 2-chlorobiphenyl; (b) 3-chlorobiphenyl; (c) 4-chlorobiphenyl at concentrations from 104 to 1010 mol/L in acetone. (d) A comparison of the SERS spectra of the three isomers at a concentration of 106 mol/L Acknowledgements The authors are grateful for financial support from the National Natural Science Foundation of China (No. 50931002), and the National Basic Research Program of China (973 program, No. 2007CB936601). Open Access: This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.


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Qin Zhou, Ye Yang, Jie Ni, Zhengcao Li, Zhengjun Zhang. Rapid recognition of isomers of monochlorobiphenyls at trace levels by surface-enhanced Raman scattering using Ag nanorods as a substrate, Nano Research, 2010, 423-428, DOI: 10.1007/s12274-010-0001-0