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Surface-enhanced Raman scattering spectroscopy of dendrimer-entrapped gold nanoparticles
Surface & Coatings Technology 228 (2013) S137–S141
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Surface-enhanced Raman scattering spectroscopy of dendrimer-entrapped gold nanoparticles Yan He, Lizhen Yang, Qiang Chen ⁎ Laboratory of Plasma Physics and Materials, Beijing Institute of Graphic Communication, Beijing 102600, China
a r t i c l e
i n f o
Available online 20 July 2012 Keywords: Nano-Au PAMAM Self-assembly SERS
a b s t r a c t In this paper we report the surface-enhanced Raman scattering (SERS) spectroscopy, which is used to detect the probe in chemical and biological analysis based on the colloidal gold (Au) plasmonic resonance spectroscopy. The Au nano-particles (NP) synthesized by the hydroxylamine seed mediated growing method with controllable sizes of 30 nm, 50 nm to 90 nm, respectively, are employed to study the assembled nano-Au thickness effect. For the purpose of high sensitivity of SERS the dendrimer polyamido amine (PAMAM) is also used. The results reveal that SERS efficiency depended on the Au NPs size and the generation of dendrimer PAMAM. The highest intensity in the spectroscopy is achieved in 50 nm Au NPs assembled Si substrates, which increases significantly along with the generations of PAMAM. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiments
Surface-enhanced Raman scattering (SERS) spectroscopy has received a great deal of attention for its utility as a sensitive technique for chemical and biomedical analysis [1–5]. It is a kind of this abnormal spectrum whose enhancement effect is base on rough surface or granular system in nanometer scale. Because of high detection sensitivity, it can detect the adsorption molecule on metal surface and give the structure of surface molecules [6–9]. Theoretically, the enhancement effect is attributed to two models, chemical mechanism (CM) and electromagnetic mechanism (EM) [10]. The EM model caused from surface plasmon resonance of metal electrons relates to the size, shape and the collective structure of the metal nanoparticles [11,12]. It means that the preparation of SERS active substrates plays a crucial role on SERS application [13]. Thus, in the past ten years, in order to obtain a certain roughness surface to improve SERS activity, all sorts of orderly metal NPs array were trialed. An ideal SERS substrate will be uniform, reproducible, clean, and high sensitivity. However, until now only self-assembled Au NPs is considered as the ideal one [14]. In this paper we report a novel SERS technology based on dendrimer PAMAM-entrapped Au nanoparticles and surface-assembled growth on substrates with pyridine as probe. The influence of PAMAM generation and synthesized Au NPs size on the spectroscopy is emphasized. The possible reason of the enhanced signal is explained in this work.
2.1. Synthesis of Au NPs
⁎ Corresponding author. Tel.: +86 10 6026 1099. E-mail address: [email protected] (Q. Chen). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.07.006
The Au NPs in diameters of ca. 30 nm, 50 nm, and 90 nm were synthesized by hydroxylamine seed mediated growth method. Before the Au NPs synthesis the ca. 23 nm gold seeds were prepared through the reduction reaction of chloroauric acid by citric acid sodium. The protocol of the seed and various sizes of Au NPs synthesis is that [15]: firstly rapidly mixing the 1.5 ml, 1% (m/v) citric acid sodium into 100 ml, 1% (m/v) boiling chloroauric acid solution; then stirring and condensing the solution in boiling state for 15 min, and cooling down with stirring; secondly the 25 mM hydroxylamine hydrochloride was added to 20 ml, 23 nm Au seed solution; after stirring for several minutes, 20.8 mM HAuCl4 solution was added. Through controlling the concentrations of hydroxylamine hydrochloride and HAuCl4 by the dropping speed, Au NPs with diameter in 30 nm, 50 nm and 90 nm can be synthesized facilely. 2.2. Preparation of SERS-active substrates Before assembling Au NP on substrates Si wafers were washed in aqua regia solution (HCl: HNO3 = 3:1 v/v) for 3 min and then rinsed in ultrapure water for over three times. The wafers were further cleaned in acid solution (H2O2:H2SO4= 1:4 (in volume)) and boiling for 10 min, then rinsed again in ultrapure water for three times. The cleaned Si substrates were then immersed in a 1% (m/v) PAMAM methanol solution for 24 h to assemble PAMAM, and then rinsed three times by methanol to remove the excess PAMAM from surface. After that the substrates were dried at 100 °C for 2 h in an oven.
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Fig. 3. The UV-vis spectrum of nano-Au prepared in various citric acid sodium concentrations.
times. The XploRA laser microscopic confocal Raman Spectrometer (Horiba Jobin Yovn, France) was used to detect the signal in wavelengths of 532 nm and 638 nm, respectively, which will quantify substrate sensitivity. 3. Results and discussion Fig. 1. The protocol of SERS active substrate preparation.
In sequence, the aminated Si substrates were dipped into Au NPs colloidal suspension for overnight to self-assemble Au nanoparticles layer. After fetching out from solution, the substrates were rinsed with ultrapure water for three times and dried at 100 °C for 30 min. The protocol of SERS substrate preparation by surface-assembly method is shown in Fig. 1. Besides the SERS measurement, the atomic force microscope (AFM, Veeco diInnova), and scanning electron microscope (SEM, Zeiss ULTRA-plus) were also used to analyze the morphology of substrates assembled Au NPs layer in this paper. 2.3. SERS measurement The detailed process of SERS measurement is shown in Fig. 2. For subsequent SERS measurement pyridine was used as probe. The active substrates were immersed in pyridine solution for 24 h and then dried at room temperature after rinsing with ultrapure water for several
As shown in Fig. 3, controllable diameter of Au NPs could be recognized through the variation of citric acid sodium concentration. Regardless the sol stability and nanoparticle size, the 2 ml of citric acid sodium was added, and 23 nm in diameter Au can be obtained. Subsequently, the Au NPs in diameters of 30 nm, 50 nm and 90 nm were synthesized by this hydroxylamine seed mediated growth method [16] through increasing the hydroxylamine hydrochloride and HAuCl4 concentrations, i.e. varying the dropping speed to control the nano-Au diameter. In Fig. 4 the UV‐vis spectrum shows the peaks caused from Au nanoparticles surface plasmon resonance that appears at 522 nm, 525 nm, 535 nm and 548 nm, respectively. The red-shifting of the wavelength infers the increase of Au NPs size. Transmission electron microscope (TEM) in Fig. 5 confirms the nano-Au size and reveals that Au NPs are in irregular spherical or multi-faceted sphere. Based on the process in Fig. 1 the SERS substrates were prepared. It is known that PAMAM can provide a great amount of terminal amino groups, thus PAMAM assisted SERS substrate shall exhibit a unique advantage compared to the pure nanoparticle assembled ones.
Fig. 2. The schematic of a—the modifications of Si substrate; b—surface-assembly of Au NPs on Si substrate.
Y. He et al. / Surface & Coatings Technology 228 (2013) S137–S141
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Fig. 4. UV-vis spectrum of Au NPs in diameters of 30 nm, 50 nm and 90 nm, respectively with seed Au of 23 nm.
Fig. 7. Raman spectrum in various concentrations of probe pyridine based on Au NPs SERS substrate.
SEM images in Fig. 6 display the substrate configurations after self-assembled Au seed, 30 nm, and 50 nm of nano-Au, respectively. As seen the substrates assembled with Au NPs of 30 nm and 50 nm are blurry due to the residual organic pyridine or solvent layer covering on particles after the substrate rinsed. Fig. 7 shows the spectrum in various concentration of probe based on 50 nm Au NPs assembled SERS substrate. The wavelength of laser is 638 nm, power focused on samples is 1 mW, the exposure time is 50 s and each measurement runs for 2 cycles. It is found that the limit concentration of pyridine is 1 × 10 −5 M. Fig. 8 exhibits SERS spectrum variation with Au NPs diameter. One can see the peaks corresponding to the C\H symmetrical breath vibration at 1009 cm−1, the C\H face vibration at 1035 cm−1, and ring telescopic vibration at 1597 cm−1 all visibly appeared in the spectrum. It is noticed that the intensity obtained in 50 nm Au NPs SERS substrate is
the highest. The signals in 30 nm and 90 nm assembled substrates are relatively low. The reason is that at 50 nm Au thickness the laser can excite more photonics due to the surface plasmon resonance that happened in the metal [17]. Besides the Au NPs sizes affect the Raman intensity, the uniformity of nanoparticles assembled on the surface also plays an important role. It is indispensable for the capped Au NPs, dispersing them uniformly on the substrate and avoiding the aggregation for SERS. The dendrimer PAMAM is one of the efficient reagents to cap the nano-Au. The different generations of PAMAM that affected the SERS are then investigated. Fig. 9 shows the AFM images of assembled Au NPs assisted by different generations of PAMAM on substrates. With the increase of PAMAM generation form 1.0 G to 5.0 G, the surface roughness of SERS substrates increased from 0.59 nm to 1.31 nm. This is because, under the same
Fig. 5. TEM images of a—23 nm seed Au NPs; and synthesized Au NPs b—50 nm; c—90 nm.
Fig. 6. SEM images of Au NPs: (a) seed; (b) 30 nm; (c) 50 nm.
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Fig. 8. Raman spectrum of various diameters of Au NPs SERS substrate with pyridine as probe molecule.
Fig. 10. Raman spectrum of PAMAM-functionalized SERS substrate with pyridine as probe molecule.
conditions of electrostatic assembly Au NPs, the content of the terminal amino groups increases along with the PAMAM generations, which leads to the SERS substrates adsorb more Au NPs. The activity of SERS substrates assisted by PAMAM dendrimer is measured at 532 nm excitation laser, when the laser power is 0.5 mW, the exposure time is 20 s, and each measurement is taken for 2 cycles. In Fig. 10 the Raman spectrum shows the intensity is remarkably increased along with the generations of dendrimer PAMAM. The reason is the high generation of PAMAM adsorbing more nano-Au on the surface and leads to a rough surface formation which increases the probe signal. So we summarize that the dendrimer PAMAM can improve not only the Au NPs dispersion on the surface, but the excitation signal.
4. Conclusions A simple SERS active substrate is prepared by electrostatically assisted PAMAM-functionalized surface-assembly of Au NPs in this paper. The SERS efficiency of substrates assembled by Au NPs is investigated through Au NP size and the generation of PAMAM. It is found that substrate assembled with 50 nm Au NPs generated the strongest enhancement signal. With the increase of PAMAM generations, the surface roughness is increased, which leads to the high sensitive of Raman signal. It results that PAMAM-functionalized surface-assembly method shall be an attractive substrate, and can be used in a new targeted molecular detection.
Fig. 9. AFM images (5 × 5 μm) of PAMAM modified Au NPs SERS substrate. (a–e) 1 .0 G, 2.0 G, 3.0 G, 4.0 G and 5.0 G, respectively.
Y. He et al. / Surface & Coatings Technology 228 (2013) S137–S141
Acknowledgments This work was financially supported NSFC (no. 11175024), Beijing Natural Science Foundation (no.1112012), 2011BAD24B01, KM 201110015008, KM 201010015005 and PHR20110516. References [1] Zhi-Yuan Li, Younan Xia, Nano Lett. 10 (2010) 243. [2] Jixiang Fang, Shuya Du, Sergei Lebedkin, Zhiyuan Li, Robert Kruk, Manfred Kappes, Horst Hahn, Nano Lett. 10 (2010) 5006. [3] Prashant K. Jain, Kyeong Seok Lee, Ivan H. El-Sayed, Mostafa A. El-Sayed, J. Phys. Chem. B 110 (2006) 7238. [4] Jaemoon Yang, Kilho Eom, Eun-Kyung Lim, et al., Langmuir 24 (2008) 12112. [5] Q.M. Yu, G. Golden, Langmuir 23 (2007) 8659.
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[6] Arne Wittstock, Jurgen Biener, Marcus Baumer, Phys. Chem. Chem. Phys. 12 (2010) 12919. [7] Xiangyang Shi, Suhe Wang, Sasha Meshinchi, Mary E. Van Antwerp, Xiangdong Bi, Inhan Lee, James R. Baker Jr., Small 3 (2007) 1245. [8] Xiaodong Cao, Yongkang Ye, Songqin Liu, Anal. Biochem. 417 (2011) 1. [9] Janina Kneipp, Harald Kneipp, William L. Rice, Katrin Kneipp, Anal. Chem. 77 (2005) 2381. [10] M. Sackmann, A. Materny, J. Raman Spectrosc. 37 (2006) 305. [11] E.N. Esenturk, A.R.H. Walker, J. Raman Spectrosc. 40 (2009) 86. [12] P.P. Fang, J.F. Li, Z.Q. Tian, et al., J. Raman Spectrosc. 39 (2008) 1679. [13] Jian qiang Hu, Yong Zhang, Bin Ren, et al., Chin. J. Light Scatt. 15 (2003) 63. [14] Qianqian Su, Xiaoyuan Ma, Jian Dong, Caiyun Jiang, Weiping Qian, ACS Appl. Mater. Interfaces 3 (2011) 1873. [15] G. Frens, Nat. Phys. Sci. 241 (1973) 20. [16] Yan hui Xu, Cui Yan, Bi ju Liu, Bin Ren, Wei Shi, Chin. J. Light Scatt. 21 (2009) 295. [17] Senfang Sui, Caide Xiao, Jun Yang, In: Surface plasmon resonance-based biosens, Shanghai: Shanghai Science and Technology Press, 2008, p. 45.
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Surface-enhanced Raman scattering spectroscopy of dendrimer-entrapped gold nanoparticles Yan He, Lizhen Yang, Qiang Chen ⁎ Laboratory of Plasma Physics and Materials, Beijing Institute of Graphic Communication, Beijing 102600, China
a r t i c l e
i n f o
Available online 20 July 2012 Keywords: Nano-Au PAMAM Self-assembly SERS
a b s t r a c t In this paper we report the surface-enhanced Raman scattering (SERS) spectroscopy, which is used to detect the probe in chemical and biological analysis based on the colloidal gold (Au) plasmonic resonance spectroscopy. The Au nano-particles (NP) synthesized by the hydroxylamine seed mediated growing method with controllable sizes of 30 nm, 50 nm to 90 nm, respectively, are employed to study the assembled nano-Au thickness effect. For the purpose of high sensitivity of SERS the dendrimer polyamido amine (PAMAM) is also used. The results reveal that SERS efficiency depended on the Au NPs size and the generation of dendrimer PAMAM. The highest intensity in the spectroscopy is achieved in 50 nm Au NPs assembled Si substrates, which increases significantly along with the generations of PAMAM. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiments
Surface-enhanced Raman scattering (SERS) spectroscopy has received a great deal of attention for its utility as a sensitive technique for chemical and biomedical analysis [1–5]. It is a kind of this abnormal spectrum whose enhancement effect is base on rough surface or granular system in nanometer scale. Because of high detection sensitivity, it can detect the adsorption molecule on metal surface and give the structure of surface molecules [6–9]. Theoretically, the enhancement effect is attributed to two models, chemical mechanism (CM) and electromagnetic mechanism (EM) [10]. The EM model caused from surface plasmon resonance of metal electrons relates to the size, shape and the collective structure of the metal nanoparticles [11,12]. It means that the preparation of SERS active substrates plays a crucial role on SERS application [13]. Thus, in the past ten years, in order to obtain a certain roughness surface to improve SERS activity, all sorts of orderly metal NPs array were trialed. An ideal SERS substrate will be uniform, reproducible, clean, and high sensitivity. However, until now only self-assembled Au NPs is considered as the ideal one [14]. In this paper we report a novel SERS technology based on dendrimer PAMAM-entrapped Au nanoparticles and surface-assembled growth on substrates with pyridine as probe. The influence of PAMAM generation and synthesized Au NPs size on the spectroscopy is emphasized. The possible reason of the enhanced signal is explained in this work.
2.1. Synthesis of Au NPs
⁎ Corresponding author. Tel.: +86 10 6026 1099. E-mail address: [email protected] (Q. Chen). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.07.006
The Au NPs in diameters of ca. 30 nm, 50 nm, and 90 nm were synthesized by hydroxylamine seed mediated growth method. Before the Au NPs synthesis the ca. 23 nm gold seeds were prepared through the reduction reaction of chloroauric acid by citric acid sodium. The protocol of the seed and various sizes of Au NPs synthesis is that [15]: firstly rapidly mixing the 1.5 ml, 1% (m/v) citric acid sodium into 100 ml, 1% (m/v) boiling chloroauric acid solution; then stirring and condensing the solution in boiling state for 15 min, and cooling down with stirring; secondly the 25 mM hydroxylamine hydrochloride was added to 20 ml, 23 nm Au seed solution; after stirring for several minutes, 20.8 mM HAuCl4 solution was added. Through controlling the concentrations of hydroxylamine hydrochloride and HAuCl4 by the dropping speed, Au NPs with diameter in 30 nm, 50 nm and 90 nm can be synthesized facilely. 2.2. Preparation of SERS-active substrates Before assembling Au NP on substrates Si wafers were washed in aqua regia solution (HCl: HNO3 = 3:1 v/v) for 3 min and then rinsed in ultrapure water for over three times. The wafers were further cleaned in acid solution (H2O2:H2SO4= 1:4 (in volume)) and boiling for 10 min, then rinsed again in ultrapure water for three times. The cleaned Si substrates were then immersed in a 1% (m/v) PAMAM methanol solution for 24 h to assemble PAMAM, and then rinsed three times by methanol to remove the excess PAMAM from surface. After that the substrates were dried at 100 °C for 2 h in an oven.
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Fig. 3. The UV-vis spectrum of nano-Au prepared in various citric acid sodium concentrations.
times. The XploRA laser microscopic confocal Raman Spectrometer (Horiba Jobin Yovn, France) was used to detect the signal in wavelengths of 532 nm and 638 nm, respectively, which will quantify substrate sensitivity. 3. Results and discussion Fig. 1. The protocol of SERS active substrate preparation.
In sequence, the aminated Si substrates were dipped into Au NPs colloidal suspension for overnight to self-assemble Au nanoparticles layer. After fetching out from solution, the substrates were rinsed with ultrapure water for three times and dried at 100 °C for 30 min. The protocol of SERS substrate preparation by surface-assembly method is shown in Fig. 1. Besides the SERS measurement, the atomic force microscope (AFM, Veeco diInnova), and scanning electron microscope (SEM, Zeiss ULTRA-plus) were also used to analyze the morphology of substrates assembled Au NPs layer in this paper. 2.3. SERS measurement The detailed process of SERS measurement is shown in Fig. 2. For subsequent SERS measurement pyridine was used as probe. The active substrates were immersed in pyridine solution for 24 h and then dried at room temperature after rinsing with ultrapure water for several
As shown in Fig. 3, controllable diameter of Au NPs could be recognized through the variation of citric acid sodium concentration. Regardless the sol stability and nanoparticle size, the 2 ml of citric acid sodium was added, and 23 nm in diameter Au can be obtained. Subsequently, the Au NPs in diameters of 30 nm, 50 nm and 90 nm were synthesized by this hydroxylamine seed mediated growth method [16] through increasing the hydroxylamine hydrochloride and HAuCl4 concentrations, i.e. varying the dropping speed to control the nano-Au diameter. In Fig. 4 the UV‐vis spectrum shows the peaks caused from Au nanoparticles surface plasmon resonance that appears at 522 nm, 525 nm, 535 nm and 548 nm, respectively. The red-shifting of the wavelength infers the increase of Au NPs size. Transmission electron microscope (TEM) in Fig. 5 confirms the nano-Au size and reveals that Au NPs are in irregular spherical or multi-faceted sphere. Based on the process in Fig. 1 the SERS substrates were prepared. It is known that PAMAM can provide a great amount of terminal amino groups, thus PAMAM assisted SERS substrate shall exhibit a unique advantage compared to the pure nanoparticle assembled ones.
Fig. 2. The schematic of a—the modifications of Si substrate; b—surface-assembly of Au NPs on Si substrate.
Y. He et al. / Surface & Coatings Technology 228 (2013) S137–S141
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Fig. 4. UV-vis spectrum of Au NPs in diameters of 30 nm, 50 nm and 90 nm, respectively with seed Au of 23 nm.
Fig. 7. Raman spectrum in various concentrations of probe pyridine based on Au NPs SERS substrate.
SEM images in Fig. 6 display the substrate configurations after self-assembled Au seed, 30 nm, and 50 nm of nano-Au, respectively. As seen the substrates assembled with Au NPs of 30 nm and 50 nm are blurry due to the residual organic pyridine or solvent layer covering on particles after the substrate rinsed. Fig. 7 shows the spectrum in various concentration of probe based on 50 nm Au NPs assembled SERS substrate. The wavelength of laser is 638 nm, power focused on samples is 1 mW, the exposure time is 50 s and each measurement runs for 2 cycles. It is found that the limit concentration of pyridine is 1 × 10 −5 M. Fig. 8 exhibits SERS spectrum variation with Au NPs diameter. One can see the peaks corresponding to the C\H symmetrical breath vibration at 1009 cm−1, the C\H face vibration at 1035 cm−1, and ring telescopic vibration at 1597 cm−1 all visibly appeared in the spectrum. It is noticed that the intensity obtained in 50 nm Au NPs SERS substrate is
the highest. The signals in 30 nm and 90 nm assembled substrates are relatively low. The reason is that at 50 nm Au thickness the laser can excite more photonics due to the surface plasmon resonance that happened in the metal [17]. Besides the Au NPs sizes affect the Raman intensity, the uniformity of nanoparticles assembled on the surface also plays an important role. It is indispensable for the capped Au NPs, dispersing them uniformly on the substrate and avoiding the aggregation for SERS. The dendrimer PAMAM is one of the efficient reagents to cap the nano-Au. The different generations of PAMAM that affected the SERS are then investigated. Fig. 9 shows the AFM images of assembled Au NPs assisted by different generations of PAMAM on substrates. With the increase of PAMAM generation form 1.0 G to 5.0 G, the surface roughness of SERS substrates increased from 0.59 nm to 1.31 nm. This is because, under the same
Fig. 5. TEM images of a—23 nm seed Au NPs; and synthesized Au NPs b—50 nm; c—90 nm.
Fig. 6. SEM images of Au NPs: (a) seed; (b) 30 nm; (c) 50 nm.
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Fig. 8. Raman spectrum of various diameters of Au NPs SERS substrate with pyridine as probe molecule.
Fig. 10. Raman spectrum of PAMAM-functionalized SERS substrate with pyridine as probe molecule.
conditions of electrostatic assembly Au NPs, the content of the terminal amino groups increases along with the PAMAM generations, which leads to the SERS substrates adsorb more Au NPs. The activity of SERS substrates assisted by PAMAM dendrimer is measured at 532 nm excitation laser, when the laser power is 0.5 mW, the exposure time is 20 s, and each measurement is taken for 2 cycles. In Fig. 10 the Raman spectrum shows the intensity is remarkably increased along with the generations of dendrimer PAMAM. The reason is the high generation of PAMAM adsorbing more nano-Au on the surface and leads to a rough surface formation which increases the probe signal. So we summarize that the dendrimer PAMAM can improve not only the Au NPs dispersion on the surface, but the excitation signal.
4. Conclusions A simple SERS active substrate is prepared by electrostatically assisted PAMAM-functionalized surface-assembly of Au NPs in this paper. The SERS efficiency of substrates assembled by Au NPs is investigated through Au NP size and the generation of PAMAM. It is found that substrate assembled with 50 nm Au NPs generated the strongest enhancement signal. With the increase of PAMAM generations, the surface roughness is increased, which leads to the high sensitive of Raman signal. It results that PAMAM-functionalized surface-assembly method shall be an attractive substrate, and can be used in a new targeted molecular detection.
Fig. 9. AFM images (5 × 5 μm) of PAMAM modified Au NPs SERS substrate. (a–e) 1 .0 G, 2.0 G, 3.0 G, 4.0 G and 5.0 G, respectively.
Y. He et al. / Surface & Coatings Technology 228 (2013) S137–S141
Acknowledgments This work was financially supported NSFC (no. 11175024), Beijing Natural Science Foundation (no.1112012), 2011BAD24B01, KM 201110015008, KM 201010015005 and PHR20110516. References [1] Zhi-Yuan Li, Younan Xia, Nano Lett. 10 (2010) 243. [2] Jixiang Fang, Shuya Du, Sergei Lebedkin, Zhiyuan Li, Robert Kruk, Manfred Kappes, Horst Hahn, Nano Lett. 10 (2010) 5006. [3] Prashant K. Jain, Kyeong Seok Lee, Ivan H. El-Sayed, Mostafa A. El-Sayed, J. Phys. Chem. B 110 (2006) 7238. [4] Jaemoon Yang, Kilho Eom, Eun-Kyung Lim, et al., Langmuir 24 (2008) 12112. [5] Q.M. Yu, G. Golden, Langmuir 23 (2007) 8659.
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[6] Arne Wittstock, Jurgen Biener, Marcus Baumer, Phys. Chem. Chem. Phys. 12 (2010) 12919. [7] Xiangyang Shi, Suhe Wang, Sasha Meshinchi, Mary E. Van Antwerp, Xiangdong Bi, Inhan Lee, James R. Baker Jr., Small 3 (2007) 1245. [8] Xiaodong Cao, Yongkang Ye, Songqin Liu, Anal. Biochem. 417 (2011) 1. [9] Janina Kneipp, Harald Kneipp, William L. Rice, Katrin Kneipp, Anal. Chem. 77 (2005) 2381. [10] M. Sackmann, A. Materny, J. Raman Spectrosc. 37 (2006) 305. [11] E.N. Esenturk, A.R.H. Walker, J. Raman Spectrosc. 40 (2009) 86. [12] P.P. Fang, J.F. Li, Z.Q. Tian, et al., J. Raman Spectrosc. 39 (2008) 1679. [13] Jian qiang Hu, Yong Zhang, Bin Ren, et al., Chin. J. Light Scatt. 15 (2003) 63. [14] Qianqian Su, Xiaoyuan Ma, Jian Dong, Caiyun Jiang, Weiping Qian, ACS Appl. Mater. Interfaces 3 (2011) 1873. [15] G. Frens, Nat. Phys. Sci. 241 (1973) 20. [16] Yan hui Xu, Cui Yan, Bi ju Liu, Bin Ren, Wei Shi, Chin. J. Light Scatt. 21 (2009) 295. [17] Senfang Sui, Caide Xiao, Jun Yang, In: Surface plasmon resonance-based biosens, Shanghai: Shanghai Science and Technology Press, 2008, p. 45.