- Email: [email protected]
A green approach for the synthesis of silver dendrites and their superior SERS performance
Accepted Manuscript Title: A green approach for the synthesis of silver dendrites and their superior SERS performance Authors: Jing Tang, Mo Yu, Tao Jiang, Enyan Wang, Chunli Ge, Zhirong Chen PII: DOI: Reference:
S0030-4026(17)30184-5 http://dx.doi.org/doi:10.1016/j.ijleo.2017.02.041 IJLEO 58857
To appear in: Received date: Revised date: Accepted date:
1-9-2016 11-2-2017 12-2-2017
Please cite this article as: Jing Tang, Mo Yu, Tao Jiang, Enyan Wang, Chunli Ge, Zhirong Chen, A green approach for the synthesis of silver dendrites and their superior SERS performance, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2017.02.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A green approach for the synthesis of silver dendrites and their superior SERS performance Jing Tang1, Mo Yu1, Tao Jiang2, Enyan Wang1, Chunli Ge1, and Zhirong Chen1, 1
Institute of Physics, Ningbo University of Technology, Ningbo 315016, P.R. China
2
Department of Microelectronic Science and Engineering, Faculty of Science, Ningbo
University, Ningbo 315211, P.R. China.
Corresponding
author: Zhirong Chen; E-mail: [email protected]
Abstract Silver dendrites were prepared using a facile in situ reduction method. The structures and morphologies of the products were investigated using a scanning electron microscope and an UV‒vis spectrometer. The as-synthesized silver dendrites shown significant surface-enhanced Raman scattering (SERS) activity for 4-mercaptobenzoic acid and a detection limitation of 1 × 10-8 M was obtained. The proposed reason for their superior SERS performance is estimated to be the contribution of the large amount of branches and tips, where enhanced electromagnetic fields exist. Such novel dendrite structures may find further application in the practical biological medicine detection. Keywords: Silver dendrites; SERS; hot spots Introduction Surface enhanced Raman scattering (SERS) has generated extensive research interest during the past few decades due to their wide application in photothermal remedy, chemical sensing and immune detection[1-3]. The advantages of SERS in these fields are its high sensitivity and specificity in trace detection of analytes at extremely low concentrations even down to the single molecular level. Such an amazing detection ability is mainly due to the fact that the Raman signal of target molecules can be enhanced by several orders of magnitude near the roughened surface of noble metal
nanoparticles[4-6]. Although the detailed mechanism of SERS has not been fully understood, an electromagnetic mechanism is widely accepted[7,8]. The electromagnetic mechanism is based on an excited electromagnetic field between adjacent metal nanostructures with very small nanogaps even down to several nanometers. These areas with particularly large electromagnetic fields are called “hot spots”. One critical feature of an effective SERS substrate is the presence of enough “hot spots”, which arise from the electromagnetic field around the metallic nanostructures with large curvature[9,10]. In this regard, architectural control of metallic nanostructures with specific morphologies is very important for the success of SERS approaches toward future clinical detections. Up to now, various noble metal nanoparticles with novel structures and morphologies have been employed to produce such highly dense electromagnetic field, including nanostars, nanocubes, nanoflowers and nanourchins[11-14]. However, the application of these nanostructures in SERS has been limited to some extent due to the poor SERS signal homogeneity. Therefore, it is still a big challenge to develop other versatile nanostructure for better SERS performance. Among the noble metal, silver is an ideal candidate for SERS applications owing to its big optical cross-section in the visible region. Particularly, hierarchical silver nanostructures have stimulated significant
research interest due to their unique physicochemical properties and potential applications in photonics, electronics and catalysis[15-20]. However, the synthesis strategy of hierarchical nanostructures is usually very complexes, and various organic surfactants have to be added, which is not environment friendly. Furthermore, the organic surfactants used for morphology and structure adjusting might strongly adhere onto the samples leading to the heterogeneous impurities or significant interference, thus limit their potential applications such as SERS, sensing and catalysis[21,22]. Recently, there is a tendency for the nanotechnology to shift from traditional condition to relative ‘‘green’’ strategy[23]. Consequently, there have been increasing interests in applying green principles to produce noble metal nanoparticles. In this study, silver dendrites were prepared using a facile in situ reduction method without any organic surfactant. The structures and morphologies of the products were investigated using a scanning electron microscope and an UV‒vis spectrometer. Due to the existence of a large amount of “hot spots” around their branches and tips, the silver dendrites exhibited significant SERS activity for 4-mercaptobenzoic acid (4MBA) and a detection limitation as low as 1 × 10-8 M was achieved. Compared to the silver nanoparticles synthesized by simply reducing the reduction time, the dendrites showed a much intense SERS signal with nearly three times enhancement. Such novel silver dendrite structures may find further
application in the practical biological medicine detection. 2. Experimental Silver dendrites were prepared by a facial in situ reduction technique without any template or organic additive. Single crystalline silicon wafer was ultrasonically rinsed with acetone, methanol and deionized water in sequence. After the ultrasonic treatment, the silicon wafer was dipped into 5% HF aqueous solution for 2 min to form H‒terminated surfaces. Then the wafer was immediately immersed into a solution containing 4.8 M HF and 0.02 M AgNO3 for 20 min to form silver dendrites. After that, the substrate was rinsed with deionized water to remove the extra silver cation and then dried by a gentle flow of argon gas. The sizes and morphologies of the products were observed by an S‒4800 FESEM. The optical absorption spectra of the products were recorded with a Nicolet 6700 UV‒vis spectrometer. The SERS properties of the samples were examined by a Raman spectrometer using a 785‒nm semiconductor laser as the excitation source. All the analyses were performed at room temperature. 3. Result and discussion A low-magnification FESEM image of the as-obtained silver products in Fig. 1a reveals that the obtained architecture is a one-dimensional chain consisting of many long secondary branches that are oriented to the trunks. In a high-magnification FESEM image of the
sample in Fig. 1b, it can be seen these secondary branches have developed into many uniform tilted shorter ones with the length of 150 to 250 nm. The UV‒vis absorption spectrum of synthesized silver dendrites is shown in Fig. 2a. As can be seen from this figure, the sample shows asymmetric absorption band with a main absorbance peak occurring at 423 nm. The absorbance band arises from the SPR of silver dendrites. The chemical composition of the sample was analyzed by the energy dispersive spectrum (EDS) under FESEM as provided in Fig. 2b. Besides the peaks of element silica from the silicon substrates, the peaks of silver are dominant in all the spectra, indicating the formation of pure silver dendrites. To investigate the detection capabilities of the silver dendrites, 4MBA was chosen as the target molecule. Figure 3 illustrates the measured SERS signals from 4MBA decorated silver dendrites with different concentration. Some characteristic bands of 4MBA were observed with strong intensities, suggesting the exceptional SERS enhancement. Two dominant peaks at 1076 and 1586 cm−1 in the spectra are assigned to the ring breathing modes. The Raman band at 849 cm−1 is attributed to the COO- bending mode (δ(COO-)) and that at 1144 cm−1 is attributed to a mixed mode (13β(CCC) + ν(C‒S) + ν(C‒COOH)). Besides, the one at 1432 cm−1 is ascribed to the νs(COO-) stretching mode[24]. The spectral intensities and resolutions decrease with the diluting of the
molecule solution and the peaks at about 1076 and 1586 cm−1 still appeared clearly at as low as 1 × 10-8 M, indicating the detection limitation[25]. The SERS properties of silver dendrites are significant relevant to the amount of the branches and tips and their aggregation degree. These branches and tips can form the so called lightning rod effect and result in greater localized field enhancement. Especially, high density of ‘‘hot spots’’ will appear in numerous nanogaps formed by overlapped branches to generate significant SERS signals, which can contribute to the low detection limitation. The reproducibility of silver dendrites was also evaluated using R6G molecules. As shown in Fig. 4a, the characteristic Raman peaks of R6G can be obviously observed from 200 to 2000 cm−1. The peak at 229 cm−1 occurs from the Ag‒N stretching mode. The band at 570 cm−1 is assigned to the torsional vibration of the carbon skeleton. The peaks at about 614 and 772 cm−1 result from an in-plane bending mode of the C‒C–C ring and an out-of-plane bending motion of C–H of the xanthene skeleton. The peaks at 1312, 1365, 1509 and 1650 cm−1 are ascribed to the C–C stretching modes[26]. The intensity of the peak at 1365 cm−1 was then chosen as a parameter to character the homogeneity of the SERS signals. Figure 4b represents the measured SERS signals from twenty different probe sites, which were randomly selected on the glass slide covered with R6G modified silver dendrites. It can be found that the intensities of SERS signals on different sites of
silver substrate are nearly consistent. The relative standard deviation of the peak intensity was calculated to be only 4.76%. The small dispersion of detection signals within 10% confirms the advantages of the silver dendrites as reliable and reproducible SERS substrates. The time stability of the Ag nanostructures is one of the critical aspects for their SERS application. Therefore, a time-dependent experiment was conducted for the as-prepared silver dendrites. One sample was prepared and separated into three pieces that were all exposed to air. The SERS intensity of R6G gradually decreased after 10 days and 30 days, however, the major peak position did not changed as shown in Fig. 4c. Consequently, the silver dendrites cannot be storied for too long time due to the oxidation of silver. To further prove the priority of silver dendrites as SERS substrates, their SERS performance was compared with silver nanoparticles synthesized by simply changing the reduction conditions. If the silica wafer was immersed in the reduction solution (4.8 M HF and 0.02 M AgNO3) for only 1 min, silver nanoparticles formed instead of dendrites as shown in Fig. 5a. As it is can be seen, the diameters of the nanoparticles distribute between 50 and 100 nm and they connect with each other tightly to form a silver film on the surface of silica wafer. The SERS performances of these two structures were compared in Fig. 5b. The silver dendrites exhibited an extremely higher intense signal than that of nanoparticles
with an enhanced ratio of nearly 3. The following equation was then used to compare the enhancement factors (EFs) of these two nanostructures: EF = (ISERS/Ibulk) × (Nbulk/NSERS) The ISERS and Ibulk are the intensities of the same Raman band for the SERS and bulk molecule Raman spectra. The Nbulk and NSERS represent the number of 4MBA molecules on the silver nanoparticles and dendrites. However, the intensity of the peak at 1076 cm−1 was only used to roughly calculate the EF value without exactly knowing the number of molecules on the samples. The Raman spectrum of 4MBA was used to calculate the factors of Nbulk and Ibulk. Base on the concentration of the applied 4MBA solution as 1 mM and the illuminated volume as 1.5 µm3 under the Raman spectrometer, the Nbulk was determined to be 9.0 × 1010. For the calculation of the NSERS, the illuminated area and total surface area of the sample substrate should both be considered. The laser spot was calculated as 0.1 µm2 based on the numerical aperture of objective lens and the excitation wavelength. After dripping 5 µl of 4MBA (1 mM) solution onto the substrate, we assumed that the probe molecules were uniformly distributed on the substrate with an area of 2 cm2. Therefore, NSERS was calculated as 3.0 × 108. Fig. 5b shows the SERS spectra of 4MBA collected from the silver dendrite and nanoparticle substrates as well as the normal Raman spectra of 4MBA recorded from the ethanol solution under the same condition. The integrated intensities of Raman peak at
1078 cm−1 are 1.4 ×106, 5.6 × 105 and 6.0 × 102 for the SERS and normal Raman spectra, respectively. As a result, the EF value of the silver dendrites and nanoparticles were calculated to be 7.0 × 105 and 2.8 × 105, respectively. Such novel silver dendrites synthesized without any organic additives may find its potential application in future biological immunoassay. 4. Conclusions In conclusion, silver dendrites were prepared by a facile in situ reduction method. The dendrite morphology of the product was confirmed by SEM images. The as-synthesized silver dendrites shown significant SERS activity for 4MBA and a detection limitation of 1 × 10-8 M was obtained. The proposed reason for their superior SERS performance is attributed ‘‘hot spots’’ in numerous nanogaps formed by overlapped branches and tips. A low relative standard deviation of only 4.76% was obtained by using R6G as a target molecule. The silver dendrites further showed an extremely higher intense signal than that of nanoparticles with an enhanced ratio of nearly 3. The EF value of the silver dendrites and nanoparticles were also calculated to be 7.0 × 105 and 2.8 × 105, respectively. Acknowledgements This work was supported by the Ningbo University of Technology (Grant Nos. 2013001) and the Foundation of Zhejiang Educational
Commission (Grant Nos. Y201430403 and Y201430419).
References [1] M. Aioub, M.A. El-Sayed, J. Am. Chem. Soc. 138 (2016) 1258. [2] H.K. Choi, W.H. Park, C.G. Park, H.H. Shin, K.S. Lee, Z.H. Kim, J. Am. Chem. Soc. 138 (2016) 4673. [3] Y. Wei, Y.Y. Zhu, M.L. Wang, Optik 127 (2016) 7902. [4] G.Q. Liu, Z.Q. Liu, Y.H. Chen, K. Huang, L. Li, F.L. Tang, L.X. Gong, Y. Hu, X.N. Zhang, Optik 124 (2013) 5124. [5] C.S. Ambaye, G.P. Zhang, Optik 126 (2015) 1894. [6] L.A. Nafie, J. Raman Spectrosc. 41 (2010) 1566. [7] S.L. Qi, X.Y. Shen, Z.W. Lin, G.F. Tian, D.Z. Wu, R.G. Jin, Nanoscale 5 (2013) 12132. [8] C.S.C. Jean, A.A. Rômulo, C.S. Antonio, M.R. Liane, S.S. Paulo, L.A.T. Márcia, C. Paola, Phys. Chem. Chem. Phys. 11 (2009) 7491. [9] D. Radziuk, H. Moehwald, Phys. Chem. Chem. Phys. 17 (2015) 21072. [10] Y.F. Wang, Z.M. Xiao, Nanosci. Nanotechnol. Lett. 5 (2013) 1302. [11] A.G. Leis, J.V.G. Ramos, S.S. Cortes, J. Phys. Chem. C 33 (2013) 7791. [12] M. Rycenga, M.H. Kim, P.H.C. Camargo, C. Cobley, Z.Y. Li, Y.N. Xia, J. Phys. Chem. A 113 (2009) 3932. [13] L.F. Wu, W.W. Wu, X.L. Jing, J.L. Huang, D.H. Sun, T.O. Wubah, H.Y. Liu, H.T. Wang, Q.B. Li, Ind. Eng. Chem. Res., 52 (2013) 5085. [14] X.S. Wang, D.P. Yang, P. Huang, M. Li, C. Li, D. Chen, D.X. Cui, Nanoscale 4 (2012) 7766. [15] Y. Li, C.C. Li, S.O. Cho, G.T. Duan, W.P. Cai, Langmuir 23 (2007) 9802.
[16] A. Dandapat, T.K. Lee, Y.M. Zhang, S.K. Kwak, E.C. Cho, D.H. Kim, ACS Appl. Mater. Interfaces 7 (2015) 14793. [17] L.L. Kang, P. Xu, D.T. Chen, B. Zhang, Y.C. Du, X.J. Han, Q. Li, H. L. Wang, J. Phys. Chem. C 117 (2013) 10007. [18] Y.F. Chan, C.X. Zhang, Z.L. Wu, D.M. Zhao, W. Wang, Appl. Phys. Lett. 102 (2013) 183118. [19] B. Zhao, Y. Lu, C. Zhang, Y. Fu, S. Moeendarbari, S. R. Shelke, Y. Liu, Y. Hao, Appl. Surf. Sci. 387 (2016) 431. [20] C.Y. Zhang, Y. Lu, B. Zhao, Y.W. Hao, Y.Q. Liu, Appl. Surfa. Sci. 377 (2016) 167. [21] Q.S. Ferreira, S.W. Silva, C.M.B. Santos, G.C. Ribeiro, L.R. Guilherme, P.C. Morais, J. Raman Spectrosc. 46 (2015) 765. [22] S. Tokonami, N. Morita, K. Takasaki, N. Toshima, J. Phys. Chem. C 114 (2010) 10336. [23] R. Ciriminna, M. Pagliaro, Org. Process Res. Dev. 17 (2013) 1479. [24] T. Jiang, L. Zhang, J. Zhou, Analyst 139 (2014) 5893. [25] D. Li, J. Liu, H. Wang, C.J. Barrow, W. Yang, Chem. Comm. 52 (2016) 10968. [26] V.S. Tiwari, T. Oleg, G.K. Darbha, W. Hardy, J.P. Singh, P.C. Ray, Chem. Phys. Lett. 446 (2007) 77.
Figure captions
Fig. 1 SEM images of silver dendrites: (a) and (b).
Fig. 2 Absorption (a) and EDS (b) spectra of silver dendrites.
Fig. 3 Raman spectra of 4MBA molecules with different concentration from 1 × 10-8 to 1 × 10-3 M on silver dendrites.
Fig. 4 Raman spectra of R6G on silver dendrites (a), the peak intensities of the ten measured sites at the band of 1365 cm−1 (b) and Raman spectra of R6G on silver dendrites after storage for different time (c).
Fig. 5 SEM image of silver nanoparticles (a) and Raman spectra of 4MBA on silver dendrites, silver nanoparticles and bulk 4MBA solution (b).
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
19
S0030-4026(17)30184-5 http://dx.doi.org/doi:10.1016/j.ijleo.2017.02.041 IJLEO 58857
To appear in: Received date: Revised date: Accepted date:
1-9-2016 11-2-2017 12-2-2017
Please cite this article as: Jing Tang, Mo Yu, Tao Jiang, Enyan Wang, Chunli Ge, Zhirong Chen, A green approach for the synthesis of silver dendrites and their superior SERS performance, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2017.02.041 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A green approach for the synthesis of silver dendrites and their superior SERS performance Jing Tang1, Mo Yu1, Tao Jiang2, Enyan Wang1, Chunli Ge1, and Zhirong Chen1, 1
Institute of Physics, Ningbo University of Technology, Ningbo 315016, P.R. China
2
Department of Microelectronic Science and Engineering, Faculty of Science, Ningbo
University, Ningbo 315211, P.R. China.
Corresponding
author: Zhirong Chen; E-mail: [email protected]
Abstract Silver dendrites were prepared using a facile in situ reduction method. The structures and morphologies of the products were investigated using a scanning electron microscope and an UV‒vis spectrometer. The as-synthesized silver dendrites shown significant surface-enhanced Raman scattering (SERS) activity for 4-mercaptobenzoic acid and a detection limitation of 1 × 10-8 M was obtained. The proposed reason for their superior SERS performance is estimated to be the contribution of the large amount of branches and tips, where enhanced electromagnetic fields exist. Such novel dendrite structures may find further application in the practical biological medicine detection. Keywords: Silver dendrites; SERS; hot spots Introduction Surface enhanced Raman scattering (SERS) has generated extensive research interest during the past few decades due to their wide application in photothermal remedy, chemical sensing and immune detection[1-3]. The advantages of SERS in these fields are its high sensitivity and specificity in trace detection of analytes at extremely low concentrations even down to the single molecular level. Such an amazing detection ability is mainly due to the fact that the Raman signal of target molecules can be enhanced by several orders of magnitude near the roughened surface of noble metal
nanoparticles[4-6]. Although the detailed mechanism of SERS has not been fully understood, an electromagnetic mechanism is widely accepted[7,8]. The electromagnetic mechanism is based on an excited electromagnetic field between adjacent metal nanostructures with very small nanogaps even down to several nanometers. These areas with particularly large electromagnetic fields are called “hot spots”. One critical feature of an effective SERS substrate is the presence of enough “hot spots”, which arise from the electromagnetic field around the metallic nanostructures with large curvature[9,10]. In this regard, architectural control of metallic nanostructures with specific morphologies is very important for the success of SERS approaches toward future clinical detections. Up to now, various noble metal nanoparticles with novel structures and morphologies have been employed to produce such highly dense electromagnetic field, including nanostars, nanocubes, nanoflowers and nanourchins[11-14]. However, the application of these nanostructures in SERS has been limited to some extent due to the poor SERS signal homogeneity. Therefore, it is still a big challenge to develop other versatile nanostructure for better SERS performance. Among the noble metal, silver is an ideal candidate for SERS applications owing to its big optical cross-section in the visible region. Particularly, hierarchical silver nanostructures have stimulated significant
research interest due to their unique physicochemical properties and potential applications in photonics, electronics and catalysis[15-20]. However, the synthesis strategy of hierarchical nanostructures is usually very complexes, and various organic surfactants have to be added, which is not environment friendly. Furthermore, the organic surfactants used for morphology and structure adjusting might strongly adhere onto the samples leading to the heterogeneous impurities or significant interference, thus limit their potential applications such as SERS, sensing and catalysis[21,22]. Recently, there is a tendency for the nanotechnology to shift from traditional condition to relative ‘‘green’’ strategy[23]. Consequently, there have been increasing interests in applying green principles to produce noble metal nanoparticles. In this study, silver dendrites were prepared using a facile in situ reduction method without any organic surfactant. The structures and morphologies of the products were investigated using a scanning electron microscope and an UV‒vis spectrometer. Due to the existence of a large amount of “hot spots” around their branches and tips, the silver dendrites exhibited significant SERS activity for 4-mercaptobenzoic acid (4MBA) and a detection limitation as low as 1 × 10-8 M was achieved. Compared to the silver nanoparticles synthesized by simply reducing the reduction time, the dendrites showed a much intense SERS signal with nearly three times enhancement. Such novel silver dendrite structures may find further
application in the practical biological medicine detection. 2. Experimental Silver dendrites were prepared by a facial in situ reduction technique without any template or organic additive. Single crystalline silicon wafer was ultrasonically rinsed with acetone, methanol and deionized water in sequence. After the ultrasonic treatment, the silicon wafer was dipped into 5% HF aqueous solution for 2 min to form H‒terminated surfaces. Then the wafer was immediately immersed into a solution containing 4.8 M HF and 0.02 M AgNO3 for 20 min to form silver dendrites. After that, the substrate was rinsed with deionized water to remove the extra silver cation and then dried by a gentle flow of argon gas. The sizes and morphologies of the products were observed by an S‒4800 FESEM. The optical absorption spectra of the products were recorded with a Nicolet 6700 UV‒vis spectrometer. The SERS properties of the samples were examined by a Raman spectrometer using a 785‒nm semiconductor laser as the excitation source. All the analyses were performed at room temperature. 3. Result and discussion A low-magnification FESEM image of the as-obtained silver products in Fig. 1a reveals that the obtained architecture is a one-dimensional chain consisting of many long secondary branches that are oriented to the trunks. In a high-magnification FESEM image of the
sample in Fig. 1b, it can be seen these secondary branches have developed into many uniform tilted shorter ones with the length of 150 to 250 nm. The UV‒vis absorption spectrum of synthesized silver dendrites is shown in Fig. 2a. As can be seen from this figure, the sample shows asymmetric absorption band with a main absorbance peak occurring at 423 nm. The absorbance band arises from the SPR of silver dendrites. The chemical composition of the sample was analyzed by the energy dispersive spectrum (EDS) under FESEM as provided in Fig. 2b. Besides the peaks of element silica from the silicon substrates, the peaks of silver are dominant in all the spectra, indicating the formation of pure silver dendrites. To investigate the detection capabilities of the silver dendrites, 4MBA was chosen as the target molecule. Figure 3 illustrates the measured SERS signals from 4MBA decorated silver dendrites with different concentration. Some characteristic bands of 4MBA were observed with strong intensities, suggesting the exceptional SERS enhancement. Two dominant peaks at 1076 and 1586 cm−1 in the spectra are assigned to the ring breathing modes. The Raman band at 849 cm−1 is attributed to the COO- bending mode (δ(COO-)) and that at 1144 cm−1 is attributed to a mixed mode (13β(CCC) + ν(C‒S) + ν(C‒COOH)). Besides, the one at 1432 cm−1 is ascribed to the νs(COO-) stretching mode[24]. The spectral intensities and resolutions decrease with the diluting of the
molecule solution and the peaks at about 1076 and 1586 cm−1 still appeared clearly at as low as 1 × 10-8 M, indicating the detection limitation[25]. The SERS properties of silver dendrites are significant relevant to the amount of the branches and tips and their aggregation degree. These branches and tips can form the so called lightning rod effect and result in greater localized field enhancement. Especially, high density of ‘‘hot spots’’ will appear in numerous nanogaps formed by overlapped branches to generate significant SERS signals, which can contribute to the low detection limitation. The reproducibility of silver dendrites was also evaluated using R6G molecules. As shown in Fig. 4a, the characteristic Raman peaks of R6G can be obviously observed from 200 to 2000 cm−1. The peak at 229 cm−1 occurs from the Ag‒N stretching mode. The band at 570 cm−1 is assigned to the torsional vibration of the carbon skeleton. The peaks at about 614 and 772 cm−1 result from an in-plane bending mode of the C‒C–C ring and an out-of-plane bending motion of C–H of the xanthene skeleton. The peaks at 1312, 1365, 1509 and 1650 cm−1 are ascribed to the C–C stretching modes[26]. The intensity of the peak at 1365 cm−1 was then chosen as a parameter to character the homogeneity of the SERS signals. Figure 4b represents the measured SERS signals from twenty different probe sites, which were randomly selected on the glass slide covered with R6G modified silver dendrites. It can be found that the intensities of SERS signals on different sites of
silver substrate are nearly consistent. The relative standard deviation of the peak intensity was calculated to be only 4.76%. The small dispersion of detection signals within 10% confirms the advantages of the silver dendrites as reliable and reproducible SERS substrates. The time stability of the Ag nanostructures is one of the critical aspects for their SERS application. Therefore, a time-dependent experiment was conducted for the as-prepared silver dendrites. One sample was prepared and separated into three pieces that were all exposed to air. The SERS intensity of R6G gradually decreased after 10 days and 30 days, however, the major peak position did not changed as shown in Fig. 4c. Consequently, the silver dendrites cannot be storied for too long time due to the oxidation of silver. To further prove the priority of silver dendrites as SERS substrates, their SERS performance was compared with silver nanoparticles synthesized by simply changing the reduction conditions. If the silica wafer was immersed in the reduction solution (4.8 M HF and 0.02 M AgNO3) for only 1 min, silver nanoparticles formed instead of dendrites as shown in Fig. 5a. As it is can be seen, the diameters of the nanoparticles distribute between 50 and 100 nm and they connect with each other tightly to form a silver film on the surface of silica wafer. The SERS performances of these two structures were compared in Fig. 5b. The silver dendrites exhibited an extremely higher intense signal than that of nanoparticles
with an enhanced ratio of nearly 3. The following equation was then used to compare the enhancement factors (EFs) of these two nanostructures: EF = (ISERS/Ibulk) × (Nbulk/NSERS) The ISERS and Ibulk are the intensities of the same Raman band for the SERS and bulk molecule Raman spectra. The Nbulk and NSERS represent the number of 4MBA molecules on the silver nanoparticles and dendrites. However, the intensity of the peak at 1076 cm−1 was only used to roughly calculate the EF value without exactly knowing the number of molecules on the samples. The Raman spectrum of 4MBA was used to calculate the factors of Nbulk and Ibulk. Base on the concentration of the applied 4MBA solution as 1 mM and the illuminated volume as 1.5 µm3 under the Raman spectrometer, the Nbulk was determined to be 9.0 × 1010. For the calculation of the NSERS, the illuminated area and total surface area of the sample substrate should both be considered. The laser spot was calculated as 0.1 µm2 based on the numerical aperture of objective lens and the excitation wavelength. After dripping 5 µl of 4MBA (1 mM) solution onto the substrate, we assumed that the probe molecules were uniformly distributed on the substrate with an area of 2 cm2. Therefore, NSERS was calculated as 3.0 × 108. Fig. 5b shows the SERS spectra of 4MBA collected from the silver dendrite and nanoparticle substrates as well as the normal Raman spectra of 4MBA recorded from the ethanol solution under the same condition. The integrated intensities of Raman peak at
1078 cm−1 are 1.4 ×106, 5.6 × 105 and 6.0 × 102 for the SERS and normal Raman spectra, respectively. As a result, the EF value of the silver dendrites and nanoparticles were calculated to be 7.0 × 105 and 2.8 × 105, respectively. Such novel silver dendrites synthesized without any organic additives may find its potential application in future biological immunoassay. 4. Conclusions In conclusion, silver dendrites were prepared by a facile in situ reduction method. The dendrite morphology of the product was confirmed by SEM images. The as-synthesized silver dendrites shown significant SERS activity for 4MBA and a detection limitation of 1 × 10-8 M was obtained. The proposed reason for their superior SERS performance is attributed ‘‘hot spots’’ in numerous nanogaps formed by overlapped branches and tips. A low relative standard deviation of only 4.76% was obtained by using R6G as a target molecule. The silver dendrites further showed an extremely higher intense signal than that of nanoparticles with an enhanced ratio of nearly 3. The EF value of the silver dendrites and nanoparticles were also calculated to be 7.0 × 105 and 2.8 × 105, respectively. Acknowledgements This work was supported by the Ningbo University of Technology (Grant Nos. 2013001) and the Foundation of Zhejiang Educational
Commission (Grant Nos. Y201430403 and Y201430419).
References [1] M. Aioub, M.A. El-Sayed, J. Am. Chem. Soc. 138 (2016) 1258. [2] H.K. Choi, W.H. Park, C.G. Park, H.H. Shin, K.S. Lee, Z.H. Kim, J. Am. Chem. Soc. 138 (2016) 4673. [3] Y. Wei, Y.Y. Zhu, M.L. Wang, Optik 127 (2016) 7902. [4] G.Q. Liu, Z.Q. Liu, Y.H. Chen, K. Huang, L. Li, F.L. Tang, L.X. Gong, Y. Hu, X.N. Zhang, Optik 124 (2013) 5124. [5] C.S. Ambaye, G.P. Zhang, Optik 126 (2015) 1894. [6] L.A. Nafie, J. Raman Spectrosc. 41 (2010) 1566. [7] S.L. Qi, X.Y. Shen, Z.W. Lin, G.F. Tian, D.Z. Wu, R.G. Jin, Nanoscale 5 (2013) 12132. [8] C.S.C. Jean, A.A. Rômulo, C.S. Antonio, M.R. Liane, S.S. Paulo, L.A.T. Márcia, C. Paola, Phys. Chem. Chem. Phys. 11 (2009) 7491. [9] D. Radziuk, H. Moehwald, Phys. Chem. Chem. Phys. 17 (2015) 21072. [10] Y.F. Wang, Z.M. Xiao, Nanosci. Nanotechnol. Lett. 5 (2013) 1302. [11] A.G. Leis, J.V.G. Ramos, S.S. Cortes, J. Phys. Chem. C 33 (2013) 7791. [12] M. Rycenga, M.H. Kim, P.H.C. Camargo, C. Cobley, Z.Y. Li, Y.N. Xia, J. Phys. Chem. A 113 (2009) 3932. [13] L.F. Wu, W.W. Wu, X.L. Jing, J.L. Huang, D.H. Sun, T.O. Wubah, H.Y. Liu, H.T. Wang, Q.B. Li, Ind. Eng. Chem. Res., 52 (2013) 5085. [14] X.S. Wang, D.P. Yang, P. Huang, M. Li, C. Li, D. Chen, D.X. Cui, Nanoscale 4 (2012) 7766. [15] Y. Li, C.C. Li, S.O. Cho, G.T. Duan, W.P. Cai, Langmuir 23 (2007) 9802.
[16] A. Dandapat, T.K. Lee, Y.M. Zhang, S.K. Kwak, E.C. Cho, D.H. Kim, ACS Appl. Mater. Interfaces 7 (2015) 14793. [17] L.L. Kang, P. Xu, D.T. Chen, B. Zhang, Y.C. Du, X.J. Han, Q. Li, H. L. Wang, J. Phys. Chem. C 117 (2013) 10007. [18] Y.F. Chan, C.X. Zhang, Z.L. Wu, D.M. Zhao, W. Wang, Appl. Phys. Lett. 102 (2013) 183118. [19] B. Zhao, Y. Lu, C. Zhang, Y. Fu, S. Moeendarbari, S. R. Shelke, Y. Liu, Y. Hao, Appl. Surf. Sci. 387 (2016) 431. [20] C.Y. Zhang, Y. Lu, B. Zhao, Y.W. Hao, Y.Q. Liu, Appl. Surfa. Sci. 377 (2016) 167. [21] Q.S. Ferreira, S.W. Silva, C.M.B. Santos, G.C. Ribeiro, L.R. Guilherme, P.C. Morais, J. Raman Spectrosc. 46 (2015) 765. [22] S. Tokonami, N. Morita, K. Takasaki, N. Toshima, J. Phys. Chem. C 114 (2010) 10336. [23] R. Ciriminna, M. Pagliaro, Org. Process Res. Dev. 17 (2013) 1479. [24] T. Jiang, L. Zhang, J. Zhou, Analyst 139 (2014) 5893. [25] D. Li, J. Liu, H. Wang, C.J. Barrow, W. Yang, Chem. Comm. 52 (2016) 10968. [26] V.S. Tiwari, T. Oleg, G.K. Darbha, W. Hardy, J.P. Singh, P.C. Ray, Chem. Phys. Lett. 446 (2007) 77.
Figure captions
Fig. 1 SEM images of silver dendrites: (a) and (b).
Fig. 2 Absorption (a) and EDS (b) spectra of silver dendrites.
Fig. 3 Raman spectra of 4MBA molecules with different concentration from 1 × 10-8 to 1 × 10-3 M on silver dendrites.
Fig. 4 Raman spectra of R6G on silver dendrites (a), the peak intensities of the ten measured sites at the band of 1365 cm−1 (b) and Raman spectra of R6G on silver dendrites after storage for different time (c).
Fig. 5 SEM image of silver nanoparticles (a) and Raman spectra of 4MBA on silver dendrites, silver nanoparticles and bulk 4MBA solution (b).
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Figure 5
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