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Self-assembly Ag nanoparticle monolayer film as SERS Substrate for pesticide detection
Applied Surface Science 270 (2013) 292–294
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Self-assembly Ag nanoparticle monolayer film as SERS Substrate for pesticide detection Li Zhang School of Chemistry & Life Science, Anhui Key Laboratory of Spin Electron and Nanomaterials (Cultivating Base), Suzhou University, SuZhou 234000, PR China
a r t i c l e
i n f o
Article history: Received 15 November 2012 Received in revised form 26 December 2012 Accepted 4 January 2013 Available online 11 January 2013 Keywords: Self-assembly SERS Ag nanoparticle Methyl-parathion
a b s t r a c t A self-assembled protocol is introduced to provide effective platforms for the fabrication of ordered Ag nanosized monolayer film. The assembled Ag nanosized monolayer film was characterized using scanning electronic microscopy and surface-enhanced Raman scattering (SERS). The results show that the assembled SERS substrate own excellent Raman enhancement and reproducibility. The synthesized SERS-active substrate was further used to detect methyl-parathion, and the limitation of detection can reach 10−7 M. © 2013 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Surface enhanced Raman scattering (SERS) spectrum becomes a useful tool in many areas, such as environment, security and analytical chemistry [1–4]. For several years, a great deal of SERSactive substrates have been obtained, most of which are made from pure metallic nanostructures, in particular Ag and Au with different shapes [5–10]. Now days, the research on self-assembly of nanostructures has gained wide attention because of their promising potential in mechanical, optical, electronic and magnetic applications due to the uniquely characteristic of regular arrangement compared with disordered and irregular nanostructures [11–13]. Various methods and technologies have been used to assemble nanostructures, such as solvent evaporation [13,14], in situ formation at interfaces [15–17], the Langmuir–Blodgett (LB) technique [18–20], layer-bylayer assembly [21], convective assembly [22], and spin-casting [23]. However, the above approaches have some restrictions and shortcomings which limit their extensive application, such as long processing time, unique special equipment, or low throughput. In this study, we introduce a simple self-assembled protocol to provide effective platforms for the fabrication of large-area ordered Ag nanosized monolayer film. The results show that the assembled monolayer film with excellent Raman enhancement, which has been used to detect pesticide.
2.1. Preparation of SERS-active substrates
E-mail address: [email protected] 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.014
SERS-active substrates were prepared by chemical assembly of Ag nanoparticles on the 3-aminopropyltrimethoxysilane (APTMS)functionalized ITO surfaces. Ag nanoparticles were synthesized according to the typical protocol: 200 mL of AgNO3 aqueous solution (2.91 × 10−4 M) was heated to boiling under vigorous stirring, and 1.3 mL of 1% trisodium citrate aqueous solution was added. The color of the solution changed notably within the first several minutes. A gray-colored solution was obtained finally and was ready for chemical assembly. The detail process to prepare the assembled substrates followed the literature process with a slight modification [24]. Glass slides were ultrasonically cleaned in deionized water, isopropyl alcohol, acetone, and ultrapure water for 15 min, respectively, followed by cleaning in H2 O/H2 O2 (30%)/NH4 OH (5:1:1) for 30 min. The slides were further cleaned by sonication in ultrapure water (Milli-Q) for 20 min and immersed into a 2% (volume concentration) APTMS aqueous solution for 12 h at room temperature for functionalization. Afterward, the substrates were rinsed with ultrapure water and annealed at 110 ◦ C in an oven to remove the loosely bound or physisorbed APTMS molecules to avoid the aggregation of the nanoparticles due to the existence of excess amount of APTMS when the functionalized glass substrate is immersed in the Ag colloid solution. After that, the substrates were immersed into the 50-nm Ag colloid solution for 8 h to obtain a SERS substrate of assembled Ag nanoparticle film on the glass surface by APTMS.
L. Zhang / Applied Surface Science 270 (2013) 292–294
293
Fig. 1. SEM images of Ag nanoparticles on the glass slide (A) without assembled and (B) with assembled.
2.2. Characterization The morphology and structure of the products were observed by an FEI Sirion 200 field-emission scanning electronic microscopy (FESEM; Eindhoven, the Netherlands). SERS spectra were carried out on a LabRAM HR800 confocal microscope Raman system (Horiba Jobin Yvon). The excitation wavelength was 532 nm laser. The microscope objective for laser illumination and signal collection was of long working distance (8 mm) with 50 magnifications and a numerical aperture of 0.5. 3. Results and discussion The morphology of the substrates with assembled Ag nanoparticles on the glass slides surface was characterized by SEM are shown in Fig. 1B. It can be seen from the images that Ag nanoparticles of <50 nm are distributed fairly uniformly on the slides surface. The Ag nanoparticles are highly monodispersive and close-packed in its nanosized monolayer film, the average interspace between Ag nanoparticles is about 5 nm. Fig. 1A displayed the SEM images of Ag nanoparticles without self-assembly. There are many aggregations and blank which are not fit for uniform SERS substrate. The first key element to a new SERS-active substrate is the sensitivity. To evaluate the SERS performance of the Ag monolayer film, we chose R6G act as probe molecular. As shown in Fig. 2, the
Ag monolayer film substrate exhibits good enhancement ability, The detection limit of the assemble different Ag nanostructures is 10−10 M shown in Fig. 2, however, the detection limit of Ag NPs without self-assembly substrate for comparison is 10−8 M (data not shown). In this paper, the results further confirmed that the nanoparticles assembled method was an effective method for the increase the signal of SERS. The reproducibility of the substrate is another important factor for SERS detection. Fig. 3 shows that high reproducibility of the assembly SERS signals. Significantly, each spot showed distinctive Raman intensity, thus revealing excellent capability to enhance the Raman signals of the R6G molecules. The strong SERS signals suggest the presence of a high density of the so-called “hot spots” over the assembly, resulting in high reproducibility. To further assess the reproducibility of the SERS signals, the intensity of the main vibration of R6G from 45 spots SERS datum was shown in Fig. 4. To get a statistically meaningful result, the relative standard deviation (RSD) of the Raman intensity of the carbon skeleton stretching modes was calculated. A reasonable mechanism for the superior SERS performance may be attributed to the narrow interparticle gap among the adjacent Ag NPs, which played a key role in promoting cooperative plasmon mode and SERS enhancement. Fig. 4 shows that RSDs of the Raman vibrations at 1362 cm−1 were 15.49%. The RSD of the band, of less than 20%, demonstrated further that
Raman Intensity(a.u.)
2000
1500 10-8M 10-9M
1000
10-10M 10-11M
500
0 600
800
1000
1200
1400
-1
1600
1800
Raman Shift(cm ) Fig. 2. SERS spectra of different concentrations of R6G aqueous solution collected on the Ag nanosized monolayer film. The acquisition time is 2 s.
Fig. 3. SERS spectra of 10−8 M R6G in the 5 spots collected on the Ag monolayer film.
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C O stretching Raman peak, centered at 1264 cm−1 , was used as a quantitative evaluation of methyl parathion pesticides. The limit of detection was determined to be 10−7 M. 4. Conclusion In summary, self-assembly of densely arranged Ag nanostructures monolayer film to fabricate highly active SERS substrates has been demonstrated. The SERS signals collected on the Ag nanostructures assembly coated with R6G molecules showed large enhancement and high sensitivity for Raman detection. This substrate can be used for rapidly detecting low concentration methyl parathion molecule.Acknowledgements This work is supported by the Key Projects of National Science Foundation of China (No. 21271136) and the Important Project of Anhui Provincial Education Department (KJ2010ZD09). Fig. 4. RSD of R6G aqueous solution (at 1 × 10−8 M) in the 45 spots SERS line-scan spectra collected on the Ag monolayer film.
Fig. 5. SERS spectra obtained for different concentrations of methyl parathion. Curves 1–4 corresponding to 10−7 to 10−4 M.
the as-prepared substrate was suitable as a highly sensitive SERS substrate, which is similar the former studies [25,26]. Due to its high sensitivity and rich structure information for molecules, SERS show great potential application in sensing of various chemical and biological molecules. Recently, environmental problems grasp more and more attention to the organic pollutants related to wastewater and polluted food. Organic pollutants, such as organophosphate methyl-parathion, can accumulate in living organisms and result in negative effects including carcinogenicity and acute toxicity. Consequently, it is necessary to devote into the environmental problems. Fig. 5 shows SERS spectra for different concentrations of methyl parathion pesticide. Variations in the Raman peaks at 1597, 1397, 1326, 1246, 1165, and 860 cm−1 were monitored at different concentrations [27]. The intensities of the Raman peaks increased concomitantly with the increase in the concentration of methyl parathion pesticides. In particular, the
References [1] Y. Zhou, J. Chen, L. Zhang, L.B. Yang, European Journal of Inorganic Chemistry (2012) 3176–3182. [2] L.B. Yang, L. Ma, G.Y. Chen, L.H. Liu, Z.Q. Tian, Chemistry: A European Journal 16 (2010) 12683–12693. [3] L.B. Yang, H.L. Liu, J. Wang, F. Zhou, Z.Q. Tian, J.H. Liu, Chemical Communications 47 (2011) 3583–3585. [4] X.H. Li, G.Y. Chen, L.B. Yang, Z. Jin, J.H. Liu, Advanced Functional Materials 20 (2010) 2815–2824. [5] K. Qian, H.L. Liu, L.B. Yang, J.H. Liu, Nanoscale 4 (2012) 6449–6454. [6] Y.J. Ye, H.L. Liu, L.B. Yang, J.H. Liu, Nanoscale 4 (2012) 6442–6448. [7] Q.Q. Ding, H.L. Liu, L.B. Yang, J.H. Liu, Journal of Materials Chemistry 22 (2012) 19932–19939. [8] Y.M. Ma, H.L. Liu, K. Qian, L.B. Yang, J.H. Liu, Journal of Colloid and Interface Science 386 (2012) 451–455. [9] L.B. Yang, Z.Y. Bao, Y.C. Wu, J.H. Liu, Journal of Raman Specroscopy 43 (2012) 848–856. [10] H.L. Liu, L.B. Yang, H.W. Ma, Z.M. Qi, J.H. Liu, Chemical Communications 47 (2011) 9360–9362. [11] B.A. Korgel, Nature Materials 6 (2007) 551–552. [12] B.A. Korgel, Nature Materials 9 (2010) 701–703. [13] T.P. Bigioni, X.M. Lin, T.T. Nguyen, E.I. Corwin, T.A. Witten, H.M. Jaeger, Nature Materials 5 (2006) 265–270. [14] T. Ming, X.S. Kou, H.J. Chen, T. Wang, H.L. Tam, K.W. Cheah, J.Y. Chen, J.F. Wang, Angewandte Chemie International Edition 47 (2008) 9685–9690. [15] H. Xia, D. Wang, Advanced Materials 20 (2008) 4253–4256. [16] F. Reincke, S.G. Hickey, W.K. Kegel, D. Vanmaekelbergh, Angewandte Chemie International Edition 43 (2004) 458–462. [17] H.W. Duan, D.Y. Wang, D.G. Kurth, H. Mohwald, Angewandte Chemie International Edition 43 (2004) 5639–5642. [18] A.R. Tao, J.X. Huang, P.D. Yang, Accounts of Chemical Research 41 (2008) 1662–1673. [19] L.J. Cote, F. Kim, J.X. Huang, Journal of American Chemical Society 131 (2009) 1043–1049. [20] J.W. Liu, J.H. Zhu, C.L. Zhang, H.W. Liang, S.H. Yu, Journal of American Chemical Society 132 (2010) 8945–8952. [21] S. Srivastava, N.A. Kotov, Accounts of Chemical Research 41 (2008) 1831–1841. [22] L. Malaquin, T. Kraus, H. Schmid, E. Delamarche, H. Wolf, Langmuir 3 (2007) 11513–11521. [23] S. Coe-Sullivan, J.S. Steckel, W.K. Woo, M.G. Bawendi, V. Bulovic, Advanced Functional Materials 15 (2005) 1117–1124. [24] M.D. Li, Y. Cui, M.X. Gao, J. Luo, B. Ren, Z.Q. Tian, Analytical Chemistry 80 (2008) 5118–5125. [25] Q. Shao, R.H. Que, M.W. Shao, L. Cheng, S.T. Lee, Advanced Functional Materials 22 (2012) 2067–2070. [26] R.H. Que, M.W. Shao, S.J. Zhuo, C.Y. Wen, S.D. Wang, S.T. Lee, Advanced Functional Materials 21 (2011) 3337–3343. [27] D. Lee, S. Lee, G.H. Seong, J. Choo, E.K. Lee, D.G. Gweon, S. Lee, Applied Spectroscopy 60 (2006) 373–378.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Self-assembly Ag nanoparticle monolayer film as SERS Substrate for pesticide detection Li Zhang School of Chemistry & Life Science, Anhui Key Laboratory of Spin Electron and Nanomaterials (Cultivating Base), Suzhou University, SuZhou 234000, PR China
a r t i c l e
i n f o
Article history: Received 15 November 2012 Received in revised form 26 December 2012 Accepted 4 January 2013 Available online 11 January 2013 Keywords: Self-assembly SERS Ag nanoparticle Methyl-parathion
a b s t r a c t A self-assembled protocol is introduced to provide effective platforms for the fabrication of ordered Ag nanosized monolayer film. The assembled Ag nanosized monolayer film was characterized using scanning electronic microscopy and surface-enhanced Raman scattering (SERS). The results show that the assembled SERS substrate own excellent Raman enhancement and reproducibility. The synthesized SERS-active substrate was further used to detect methyl-parathion, and the limitation of detection can reach 10−7 M. © 2013 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Surface enhanced Raman scattering (SERS) spectrum becomes a useful tool in many areas, such as environment, security and analytical chemistry [1–4]. For several years, a great deal of SERSactive substrates have been obtained, most of which are made from pure metallic nanostructures, in particular Ag and Au with different shapes [5–10]. Now days, the research on self-assembly of nanostructures has gained wide attention because of their promising potential in mechanical, optical, electronic and magnetic applications due to the uniquely characteristic of regular arrangement compared with disordered and irregular nanostructures [11–13]. Various methods and technologies have been used to assemble nanostructures, such as solvent evaporation [13,14], in situ formation at interfaces [15–17], the Langmuir–Blodgett (LB) technique [18–20], layer-bylayer assembly [21], convective assembly [22], and spin-casting [23]. However, the above approaches have some restrictions and shortcomings which limit their extensive application, such as long processing time, unique special equipment, or low throughput. In this study, we introduce a simple self-assembled protocol to provide effective platforms for the fabrication of large-area ordered Ag nanosized monolayer film. The results show that the assembled monolayer film with excellent Raman enhancement, which has been used to detect pesticide.
2.1. Preparation of SERS-active substrates
E-mail address: [email protected] 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.014
SERS-active substrates were prepared by chemical assembly of Ag nanoparticles on the 3-aminopropyltrimethoxysilane (APTMS)functionalized ITO surfaces. Ag nanoparticles were synthesized according to the typical protocol: 200 mL of AgNO3 aqueous solution (2.91 × 10−4 M) was heated to boiling under vigorous stirring, and 1.3 mL of 1% trisodium citrate aqueous solution was added. The color of the solution changed notably within the first several minutes. A gray-colored solution was obtained finally and was ready for chemical assembly. The detail process to prepare the assembled substrates followed the literature process with a slight modification [24]. Glass slides were ultrasonically cleaned in deionized water, isopropyl alcohol, acetone, and ultrapure water for 15 min, respectively, followed by cleaning in H2 O/H2 O2 (30%)/NH4 OH (5:1:1) for 30 min. The slides were further cleaned by sonication in ultrapure water (Milli-Q) for 20 min and immersed into a 2% (volume concentration) APTMS aqueous solution for 12 h at room temperature for functionalization. Afterward, the substrates were rinsed with ultrapure water and annealed at 110 ◦ C in an oven to remove the loosely bound or physisorbed APTMS molecules to avoid the aggregation of the nanoparticles due to the existence of excess amount of APTMS when the functionalized glass substrate is immersed in the Ag colloid solution. After that, the substrates were immersed into the 50-nm Ag colloid solution for 8 h to obtain a SERS substrate of assembled Ag nanoparticle film on the glass surface by APTMS.
L. Zhang / Applied Surface Science 270 (2013) 292–294
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Fig. 1. SEM images of Ag nanoparticles on the glass slide (A) without assembled and (B) with assembled.
2.2. Characterization The morphology and structure of the products were observed by an FEI Sirion 200 field-emission scanning electronic microscopy (FESEM; Eindhoven, the Netherlands). SERS spectra were carried out on a LabRAM HR800 confocal microscope Raman system (Horiba Jobin Yvon). The excitation wavelength was 532 nm laser. The microscope objective for laser illumination and signal collection was of long working distance (8 mm) with 50 magnifications and a numerical aperture of 0.5. 3. Results and discussion The morphology of the substrates with assembled Ag nanoparticles on the glass slides surface was characterized by SEM are shown in Fig. 1B. It can be seen from the images that Ag nanoparticles of <50 nm are distributed fairly uniformly on the slides surface. The Ag nanoparticles are highly monodispersive and close-packed in its nanosized monolayer film, the average interspace between Ag nanoparticles is about 5 nm. Fig. 1A displayed the SEM images of Ag nanoparticles without self-assembly. There are many aggregations and blank which are not fit for uniform SERS substrate. The first key element to a new SERS-active substrate is the sensitivity. To evaluate the SERS performance of the Ag monolayer film, we chose R6G act as probe molecular. As shown in Fig. 2, the
Ag monolayer film substrate exhibits good enhancement ability, The detection limit of the assemble different Ag nanostructures is 10−10 M shown in Fig. 2, however, the detection limit of Ag NPs without self-assembly substrate for comparison is 10−8 M (data not shown). In this paper, the results further confirmed that the nanoparticles assembled method was an effective method for the increase the signal of SERS. The reproducibility of the substrate is another important factor for SERS detection. Fig. 3 shows that high reproducibility of the assembly SERS signals. Significantly, each spot showed distinctive Raman intensity, thus revealing excellent capability to enhance the Raman signals of the R6G molecules. The strong SERS signals suggest the presence of a high density of the so-called “hot spots” over the assembly, resulting in high reproducibility. To further assess the reproducibility of the SERS signals, the intensity of the main vibration of R6G from 45 spots SERS datum was shown in Fig. 4. To get a statistically meaningful result, the relative standard deviation (RSD) of the Raman intensity of the carbon skeleton stretching modes was calculated. A reasonable mechanism for the superior SERS performance may be attributed to the narrow interparticle gap among the adjacent Ag NPs, which played a key role in promoting cooperative plasmon mode and SERS enhancement. Fig. 4 shows that RSDs of the Raman vibrations at 1362 cm−1 were 15.49%. The RSD of the band, of less than 20%, demonstrated further that
Raman Intensity(a.u.)
2000
1500 10-8M 10-9M
1000
10-10M 10-11M
500
0 600
800
1000
1200
1400
-1
1600
1800
Raman Shift(cm ) Fig. 2. SERS spectra of different concentrations of R6G aqueous solution collected on the Ag nanosized monolayer film. The acquisition time is 2 s.
Fig. 3. SERS spectra of 10−8 M R6G in the 5 spots collected on the Ag monolayer film.
294
L. Zhang / Applied Surface Science 270 (2013) 292–294
C O stretching Raman peak, centered at 1264 cm−1 , was used as a quantitative evaluation of methyl parathion pesticides. The limit of detection was determined to be 10−7 M. 4. Conclusion In summary, self-assembly of densely arranged Ag nanostructures monolayer film to fabricate highly active SERS substrates has been demonstrated. The SERS signals collected on the Ag nanostructures assembly coated with R6G molecules showed large enhancement and high sensitivity for Raman detection. This substrate can be used for rapidly detecting low concentration methyl parathion molecule.Acknowledgements This work is supported by the Key Projects of National Science Foundation of China (No. 21271136) and the Important Project of Anhui Provincial Education Department (KJ2010ZD09). Fig. 4. RSD of R6G aqueous solution (at 1 × 10−8 M) in the 45 spots SERS line-scan spectra collected on the Ag monolayer film.
Fig. 5. SERS spectra obtained for different concentrations of methyl parathion. Curves 1–4 corresponding to 10−7 to 10−4 M.
the as-prepared substrate was suitable as a highly sensitive SERS substrate, which is similar the former studies [25,26]. Due to its high sensitivity and rich structure information for molecules, SERS show great potential application in sensing of various chemical and biological molecules. Recently, environmental problems grasp more and more attention to the organic pollutants related to wastewater and polluted food. Organic pollutants, such as organophosphate methyl-parathion, can accumulate in living organisms and result in negative effects including carcinogenicity and acute toxicity. Consequently, it is necessary to devote into the environmental problems. Fig. 5 shows SERS spectra for different concentrations of methyl parathion pesticide. Variations in the Raman peaks at 1597, 1397, 1326, 1246, 1165, and 860 cm−1 were monitored at different concentrations [27]. The intensities of the Raman peaks increased concomitantly with the increase in the concentration of methyl parathion pesticides. In particular, the
References [1] Y. Zhou, J. Chen, L. Zhang, L.B. Yang, European Journal of Inorganic Chemistry (2012) 3176–3182. [2] L.B. Yang, L. Ma, G.Y. Chen, L.H. Liu, Z.Q. Tian, Chemistry: A European Journal 16 (2010) 12683–12693. [3] L.B. Yang, H.L. Liu, J. Wang, F. Zhou, Z.Q. Tian, J.H. Liu, Chemical Communications 47 (2011) 3583–3585. [4] X.H. Li, G.Y. Chen, L.B. Yang, Z. Jin, J.H. Liu, Advanced Functional Materials 20 (2010) 2815–2824. [5] K. Qian, H.L. Liu, L.B. Yang, J.H. Liu, Nanoscale 4 (2012) 6449–6454. [6] Y.J. Ye, H.L. Liu, L.B. Yang, J.H. Liu, Nanoscale 4 (2012) 6442–6448. [7] Q.Q. Ding, H.L. Liu, L.B. Yang, J.H. Liu, Journal of Materials Chemistry 22 (2012) 19932–19939. [8] Y.M. Ma, H.L. Liu, K. Qian, L.B. Yang, J.H. Liu, Journal of Colloid and Interface Science 386 (2012) 451–455. [9] L.B. Yang, Z.Y. Bao, Y.C. Wu, J.H. Liu, Journal of Raman Specroscopy 43 (2012) 848–856. [10] H.L. Liu, L.B. Yang, H.W. Ma, Z.M. Qi, J.H. Liu, Chemical Communications 47 (2011) 9360–9362. [11] B.A. Korgel, Nature Materials 6 (2007) 551–552. [12] B.A. Korgel, Nature Materials 9 (2010) 701–703. [13] T.P. Bigioni, X.M. Lin, T.T. Nguyen, E.I. Corwin, T.A. Witten, H.M. Jaeger, Nature Materials 5 (2006) 265–270. [14] T. Ming, X.S. Kou, H.J. Chen, T. Wang, H.L. Tam, K.W. Cheah, J.Y. Chen, J.F. Wang, Angewandte Chemie International Edition 47 (2008) 9685–9690. [15] H. Xia, D. Wang, Advanced Materials 20 (2008) 4253–4256. [16] F. Reincke, S.G. Hickey, W.K. Kegel, D. Vanmaekelbergh, Angewandte Chemie International Edition 43 (2004) 458–462. [17] H.W. Duan, D.Y. Wang, D.G. Kurth, H. Mohwald, Angewandte Chemie International Edition 43 (2004) 5639–5642. [18] A.R. Tao, J.X. Huang, P.D. Yang, Accounts of Chemical Research 41 (2008) 1662–1673. [19] L.J. Cote, F. Kim, J.X. Huang, Journal of American Chemical Society 131 (2009) 1043–1049. [20] J.W. Liu, J.H. Zhu, C.L. Zhang, H.W. Liang, S.H. Yu, Journal of American Chemical Society 132 (2010) 8945–8952. [21] S. Srivastava, N.A. Kotov, Accounts of Chemical Research 41 (2008) 1831–1841. [22] L. Malaquin, T. Kraus, H. Schmid, E. Delamarche, H. Wolf, Langmuir 3 (2007) 11513–11521. [23] S. Coe-Sullivan, J.S. Steckel, W.K. Woo, M.G. Bawendi, V. Bulovic, Advanced Functional Materials 15 (2005) 1117–1124. [24] M.D. Li, Y. Cui, M.X. Gao, J. Luo, B. Ren, Z.Q. Tian, Analytical Chemistry 80 (2008) 5118–5125. [25] Q. Shao, R.H. Que, M.W. Shao, L. Cheng, S.T. Lee, Advanced Functional Materials 22 (2012) 2067–2070. [26] R.H. Que, M.W. Shao, S.J. Zhuo, C.Y. Wen, S.D. Wang, S.T. Lee, Advanced Functional Materials 21 (2011) 3337–3343. [27] D. Lee, S. Lee, G.H. Seong, J. Choo, E.K. Lee, D.G. Gweon, S. Lee, Applied Spectroscopy 60 (2006) 373–378.