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Hierarchical Ag mesostructures for single particle SERS substrate
Applied Surface Science 393 (2017) 197–203
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Hierarchical Ag mesostructures for single particle SERS substrate Minwei Xu ∗ , Yin Zhang School of Science, MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, Xi’an Jiaotong University, Xi’an, 710049, China
a r t i c l e
i n f o
Article history: Received 1 July 2016 Received in revised form 19 September 2016 Accepted 3 October 2016 Available online 5 October 2016 Keywords: Hierarchical Silver Seed-mediated growth Mesostructure SERS
a b s t r a c t Hierarchical Ag mesostructures with highly rough surface morphology have been synthesized at room temperature through a simple seed-mediated approach. Electron microscopy characterizations indicate that the obtained Ag mesostructures exhibit a textured surface morphology with the flower-like architecture. Moreover, the particle size can be tailored easily in the range of 250–500 nm. For the growth process of the hierarchical Ag mesostructures, it is believed that the self-assembly mechanism is more reasonable rather than the epitaxial overgrowth of Ag seed. The oriented attachment of nanoparticles is revealed during the formation of Ag mesostructures. Single particle surface enhanced Raman spectra (sp-SERS) of crystal violet adsorbed on the hierarchical Ag mesostructures were measured. Results reveal that the hierarchical Ag mesostructures can be highly sensitive sp-SERS substrates with good reproducibility. The average enhancement factors for individual Ag mesostructures are estimated to be about 106 . © 2016 Elsevier B.V. All rights reserved.
1. Introduction Surface enhanced Raman scattering (SERS) is a powerful analytical tool for chemical and biological sensing applications. During the last decade, the potential applications of sensitive SERS for molecular identification have attracted considerable interests. However, one feature which has limited its practical application is the difficulty involved in producing uniform, highly sensitive and reproducible SERS substrates [1,2]. Among the many different SERS substrates that have been thoroughly investigated, synthesis of Ag-based SERS substrates seems to be particularly interesting because Ag exhibits the best SERS activity compared with other metal substrates. The main enhancement mechanism for SERS is believed to the electromagnetic mechanism [3–6], where the socalled hotspots at the nanoscaled cavities, interstitial sites or void spaces provide strongly enhanced electromagnetic fields. Theoretical and experiment investigations reveal that the SERS activity is very sensitive to their morphologies. The Ag mesostructures with complex morphology, which have rough surface, interstitial sites or sharp tips, are likely to exhibit higher SERS activity [7–9]. During the past decade, shape-controlled synthesis of Ag mesostructures has advanced remarkably and it is now achievable to generate the hotspots for highly sensitive SERS substrates. For example, Xia et al. reported a water-based method to prepare highly branched Ag nanostructures. Because of their high
∗ Corresponding author. E-mail address: [email protected] (M. Xu). http://dx.doi.org/10.1016/j.apsusc.2016.10.011 0169-4332/© 2016 Elsevier B.V. All rights reserved.
surface areas and many sharp edges, such highly branched Ag nanostructures could serve as great substrates for SERS detection of low concentration analytes [10]. Ren et, al devoloped a electrochemical preparation of Ag nanothorns with the addition of thiourea. The SERS activity of the unique Ag nanothorns is about 20 folds stronger than that of the Ag nanoparticles [11]. Xu et al. reported an acid-directed synthesis of hierarchical assemblies of Ag nanostructures. The obtained structures with highly roughened surfaces can be highly sensitive SERS plarforms and show no polarization dependency of the incident laser [12]. Jiang fabricated the multipod-shaped Ag structures through anisotropic growth and the high SERS activity was also demonstrated [13]. To date, a variety of Ag mesostructures with high SERS activity have been reported. However, most of these strategies are limited to attain high tenability in a brad range of size. Moreover, in many cases, the unavoidable addition of surfactant, capping agents or polymeric templates may make the synthetic procedure and purification process more complex. It is inevitable that a thin layer of impurity molecules will reside on the surface of the final products and the adsorbed molecule can interfere with the SERS signal of the target molecule. Therefore, producing uniform, highly sensitive and reproducible SERS substrates remains a great challenge, particularly in terms of size control and morphology design with a surfactant-free procedure. On the other hand, the bulk Ag aggregates, which is formed by the aggregation of Ag nanostructures or ordered arrays, are usually applied as SERS substrates [14–16]. However, the controlled aggregation of Ag nanostructures is really a difficult task and the hotspots are formed randomly. Thus, such SERS substrates may
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reveal poor controllability and reproducibility. Moreover, these bulk Ag aggregates are too large to be used as SERS substrates in biological systems. Therefore, colloidal Ag nanocrystals are regarded as the alternative SERS substrates and the Raman scattering observed from single particles without the formation of hotspots via aggregates are more desirable. Thus, it is further important to perform the single particle SERS (sp-SERS) experiments, which will further expand the application of SERS [17–20]. Recently, Xia and Yang reported the sp-SERS performances for Ag nanocubes, etched octahedral and octapods. Modest enhanced factors are obtained in the range from 104 to 105 [21,22]. However, reports on sp-SERS of Ag mesostructures are relatively rare and further improvement should be achieved to augment the sensitivity of an individual Ag mesostructure. In this article, we reported the synthesis of hierarchical Ag mesostructures via a seed-mediated approach [23]. The resulting Ag mesostructures revealed the hierarchical flower-like architecture, in which the hotspots can be easily achieved even without the aggregation. They exhibited the nearly monodispersed sizes and the diameter of the Ag mesostructures could be controlled by adjusting the dosage of Ag seeds. The growth mechanism for the hierarchical Ag mesostructures were proposed and the oriented attachment of nanoparticles was revealed during the formation of Ag mesostructures. The obtained hierarchical Ag mesostructures can service as the sp-SERS substrates. The average SERS enhancement factors for an individual Ag mesoflower could be estimated to be as high as ∼106 , making them very appealing as SERS sensors or SERS-active tags.
2. Experimental 2.1. Materials Silver nitrate (AgNO3 ), ascorbic acid (Vc), trisodium citrate dehydrate (C6 H5 Na3 O7 ·H2 O) were analytical grade and used as received. Deionized water (Millipore) was used in all preparations. The Vc solutions was freshly prepared before use.
Table 1 Summarized sample denotations and corresponding reaction conditions together with their size and morphology properties.a Sample
Ag seed
Vc
AgNO3
Average size
Particle morphology
#1 #2 #3
1.0 ml 0.5 ml 0.2 ml
2.5 ml 3.0 ml 3.3 ml
1 ml 1 ml 1 ml
∼250 nm ∼360 nm ∼500 nm
Flower-like architectures Flower-like architectures Flower-like architectures
a Vc is used as reducing agent. The concentration of Vc and AgNO3 is 10 and 100 mM, respectively. The reaction is kept at room temperature.
2.4. Characterization The X-ray powder diffraction (XRD) investigation was carried out by using a Bruke D8-Advance diffractometer. UV–vis absorption spectra were obtained with a Hitachi U-4100 UV–vis spectrophotometer at room temperature under ambient conditions. Scanning electron microscopy (SEM) images and energy-dispersive X-ray (EDS) analyses were characterized on a JEOL JSM-7100F microscope. Transmission electron microscopy (TEM and HRTEM) observations were carried out on a JEOL JEM-2100 instrument. 2.5. Surface enhanced Raman spectroscopy Crystal Violet (CV) was used as the Raman probe for the SERS measurements. For preparation of SERS substrates, 10 l of the diluted Ag mesostructures in ethanol solution was carefully dropped on the cleaned Si substrate (10 mm × 10 mm). After the solution was completely dried in air, 100 l CV solution (10−8 M) was uniformly dropped on the substrate, and dried in air. Raman measurements were conducted with a confocal microprobe Raman spectrometer (LabRAM HR800, HORIBA JOBIN YVON). Radiation of 633 nm from a He-Ne laser was used for excitation. The laser beam was focused on the sample in a size of about 1 m. An extremely low incident laser power (decreased by a D2 attenuation piece, about 0.04 mW) was used in order to minimize heating effects during the Raman measurements and the acquisition time was fixed at 10 s. 3. Results and discussion
2.2. Preparation of Ag seeds
3.1. Synthesis of hierarchical Ag mesostructures
The Ag seeds were synthesized according to the reduction of Ag+ by sodium citrate. Typically, an aqueous solution of 5 mM AgNO3 (200 ml) was brought to boiling. Subsequently, 0.4 g sodium citrate was dissolved in 20 ml of H2 O and added dropwise in to the boiled AgNO3 solution. The mixed solution was kept on boiling for about 1 h. The as-obtained greenish yellow colloid was kept at 5 ◦ C, which served as the seeds for Ag mesostructures.
The original Ag seeds were prepared according to a typical citrate reduction method, in which the silver ion could be reduced to metal silver due to the reducibility of sodium citrate in the boiling water [24]. The representative TEM image reveal that the resultant Ag seeds are irregular nanoparticles with an average size of about 60 nm (Supplementary material, Fig. S1). Fig. S1c shows the UV–vis spectra of the as-synthesized Ag seeds and an absorption maximum near 420 nm was observed. It can be diluted easily as the photographs shown in Fig. S1c inset. With dilution by 10-, 25-, 50- 100- fold, the seed solution become transparent with the color changed from greenish yellow to light yellow. Moreover, the obtained Ag seed possesses good stability, which could be kept in fridge for several weeks without the precipitation. For the synthesis of hierarchical Ag mesostructures, a certain amount of Ag seed and fresh prepared Vc solution were mixed together. Then, the AgNO3 solution was added rapidly and the Ag mesostructures were formed in just a few seconds. The chemical reaction for the reduction of AgNO3 by Vc can be described as follows [25]:
2.3. Synthesis of hierarchical Ag mesostructures In a typical synthesis, a certain amount of Ag seed was diluted into 3.5 ml by a fresh prepared Vc solution (10 mM). Then, 1 ml of a 100 mM AgNO3 solution was added rapidly. Additions were made by micropipette and the reaction was conducted at room temperature without stirring. The original light yellow color turned into black immediately, which indicated the formation of Ag mesostructures. The mixed solution was allowed to age for several hours and the as-synthesized precipitate was separated by centrifugation, washed numerous times with deionized water and stored in ethanol for further characterization. While the essential conditions were kept the same, some other control experiments were also performed. The main reaction parameters and corresponding results were summarized in Table 1.
C6 H8 O6 + 2Ag+ → C6 H6 O6 + 2Ag + 2H+ (1) Fig. 1 displays the morphologies and microstructure of the obtained hierarchical Ag mesostructures with different sizes. Large quantity and good uniformity of flower-like Ag mesostructures
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Fig. 1. SEM, TEM images and size distribution of the obtained hierarchical Ag mesostructures via the seed-mediated approach with the assistance of (a, b, c) 1 ml, (d, e, f) 0.5 ml and (g, h, i) 0.2 ml Ag seed, respectively.
are achieved via this seed-mediated method. It is found that the obtained Ag mesostructures show the hierarchical structure, which are assembled by nanopetals. Moreover, further investigate from the SEM and TEM images reveals that each petal is further composed of many nanoparticles. The size of these Ag mesostructures can be controlled by adjusting the dosage of Ag seeds while all other parameters are kept constant. The main reaction parameters and corresponding results are summarized in Table 1. When the Ag seed was varied from 1 ml to 0.5 ml and 0.2 ml, the diameters of the resulting Ag mesostructures were roughly 250, 360 and 500 nm, respectively. For comparison, direct reduction of Ag+ by Vc (no extra Ag seeds) is also studied (Fig. S2). The absence of Ag seed leads to the rough microspheres with a huge size of ∼1800 nm. The selected area electron diffraction (SAED) pattern as shown in Fig. S3 reveals that the obtained Ag mesostructures have a polycrystalline structure. It demonstrates that the petals seem to assemble randomly, which exhibit independent orientation. The crystal structure and phase composition of these Ag mesostructures are also characterized by X-ray diffraction. Fig. 2 shows the typical XRD patterns of the as-prepared Ag samples. Four peaks can be observed, which correspond to diffractions from the (111), (200), (220), and (311) planes of the face-centered-cubic (fcc) Ag phase (JCPDS 04-0783). The sharp peaks and identical intensity ratios imply that the Ag mesostructures with different size are all well crystallized. In addition, as compared to the XRD pattern of Ag mesostructure obtained in the absence of Ag seed (Fig. S2c), the broad diffraction peaks indicate that the obtained Ag mesostructures with the assistance of Ag seeds are in small sizes. Furthermore, the EDS result (Fig. S4)
Fig. 2. XRD patterns of the as-obtained hierarchical Ag mesostructures.
reveals that silver is the dominant element while a trace of silicon originating form the substrate for SEM and EDS analysis is found. 3.2. The possible growth process for hierarchical Ag mesostructures During the seed-growth process, the reduction of metal ions in solution always results in two competing processes: epitaxial
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Fig. 3. TEM images (a, b) of the Ag mesoflowers of about 500 nm (sample #3) and the HRTEM image (c) originated from the marked area in (b). Inset is the FFT pattern of the two particles which indicates that the adjacent particles share the same crystallographic orientation.
Fig. 4. Schematic illustration for the formation of hierarchical Ag mesostructures during the seed-mediated growth processes.
growth of seeds and self-nucleation [26]. Generally, a slow reaction rate as well as a low concentration is favorable for homogeneous growth, while a fast rate leads to the self-nucleation process. In this work, the seed growth process is limited due to the fast reac-
tion rate and the self-nucleation followed with the seed-mediated self-ssembly is more reasonable for understanding the formation mechanism. Fig. 3 shows the TEM images of the Ag mesostructures obtained with 0.2 ml Ag seeds (sample #3), confirming that
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each petal is further composed of many tiny particles. The grey parts show the Ag petals lied horizontally while the dark areas correspond to the Ag petals stand vertically. More interestingly, the HRTEM images recorded from the junction of two particles indicate that lattice fringes are aligned between the adjacent nanoparticles. The attached nanoparticles share the similar crystallographic orientation, which is indicated by the solid lines shown in Fig. 3c. This result indicates that the oriented attachment [27] assembling plays an important role on the formation of the Ag mesostructures. The inset of Fig. 3c presents the Fast Fourier Transform (FFT) patterns. It demonstrates the single crystal nature for the two particles. The dotted line shows the junction area of the adjacent nanoparticles, in which some defects can also be founded. To further understand the formation of the nanopetals, the Ag mesostructures with different sizes are also studied by TEM analysis. Fig. S5 reveals the similar phenomenon in the Ag mesostructures with smaller size (sample #1). Clear and continuous lattice-fringe demonstrates that the adjacent particles share the similar crystallographic oriention. The primary particles are orderly attached with each other to form the petals. Accordingly, the possible growth process for the formation of flower-like Ag mesostructures was proposed. Such a growth process involves a fast nucleation and oriented aggregation of primary particles, which can be schematically illustrated in Fig. 4. In the early stage of reaction, primary particles were formed from the self-nucleation due to the fast reduction of AgNO3 by Vc (Fig. 4b). Then, the Ag seeds began to grow by randomly aggregating with the primary particles, which became the core of the flower-like structure. The random assemble was not homogeneous and there should be many irregular and randomly arranged protrusions on the core (Fig. 4c). The random assemble at the early stage results that the whole Ag mesoflower reveals the polycrystalline nature. As the reaction proceeded, some of the protrusions continued to grow through the self-assemble of nanoparticles along the identical direction to form the Ag sheets (oriented attachment, Fig. 4d). Finally, no nanoparticle remained and the sample was composed entirely of flower-like mosestructure. Actually, the formation of primary Ag particles and the self-assembly growth could be finished in a short time. Then, with the decreasing of the concentration, the reduction rate is reduced and the Ostwald ripening [28] is carried out for the further growth of the Ag mesostructures. To further understand the above processes, the hydroquinone with weak reducibility is also introduced to the seed-mediated process for Ag mesostructures. As shown in Fig. S6, hierarchical Ag mesostructures with flower-like morphology can also be attained. However, much different from the Ag mesostructures obtained by Vc reduction, these Ag petals are perfect nanosheets, which show the smooth texture. It is believed that the weak reducibility of hydroquinone can extend the Ostwald ripening processes and the smooth surfaces of Ag nanopetals are obtained due to the prolonged Ostwald ripening processes. 3.3. Single particle SERS properties of the hierarchical Ag mesostructures Fig. S7 reveals the Raman spectra of the blank Ag mesostructures. It is found that the aggregation of the blank Ag mesostructures (sample #1) exhibits the Raman signals and all these peaks can be ascribed to the Raman frequencies for sodium citrate [29]. These results demonstrate that the surface of the obtained Ag mesostructures would be inevitably covered by citrate although most of the citrate can be washed away. However, these Raman signals of citrate are really weaker than the Raman signals of the Crystal Violet (CV) when the aggregation Ag was used as Raman substrates (Fig.S8). Moreover, for single blank Ag particle, very weak Raman signals were detected (Fig. S7). All these results
201
Fig. 5. Reproducibility of the hierarchical Ag mesostructures with the size of about 250 nm (sample #1). The sp-SERS spectra were collected from randomly selected individual Ag particles and the spectra with higher intensity or none intensity were discarded.
Table 2 Observed wavenumbers (cm−1 ) of crystal violet and their assignment. Wavenumber (cm−1 )
Vibrational Assignment
812 1170 1377 1620
Ring C H bend (out of plane) Ring of C H bend (in plane) N phenyl stretching Ring C C stretching
revealed that the adsorbed citrate would cause less disturbance to the SERS detection of CV molecules. Motivated by the complex structural feature of the obtained hierarchical Ag mesostructures, the sp-SERS properties of the asprepared Ag mesostructures are investigated while CV was used as a probe molecule. Herein, sp-SERS performance is studied from individual particles without the formation of hotspots via aggregates. The individual Ag mesostructure on the Si substrate is selected randomly and the interparticle spacing should be large enough to ensure only single particles are covered. Fig. 5 shows the typical sp-SERS spectra of CV molecules adsorbed on Ag mesostructures (sample #1). The Raman signals at the frequencies (cm−1 ) of 812, 1170, 1377 and 1620 are clearly observed. According to the literatures [30,31], the vibrational assignments of the observed Raman signals are listed in Table 2. It is believed that the hierarchical Ag mesostructures are demonstrated as highly sensitive SERS substrates because extremely low concentration of CV solution (10−8 ) was used for SERS detection. The unique hierarchical architecture not only ensures the large surface area, but also lead to the formation of nanocavity areas between the adjacent Ag petals. There are numerous hot spots associated with the nanocavity structure, which contribute to the greatly enhanced Raman signal. Fig. 5 shows the single particle Raman spectra collected from the randomly selected Ag mesostructures. Almost all the Ag particles show the SERS activity, in which the uniform Raman intensity reveals the high reproducibility of the Ag mesostructures for sp-SERS substrate. The typical sp-SERS spectra of CV molecules adsorbed on Ag mesostructures with different sizes are shown in Fig. 6. Compared to the Raman signal of the bare silicon, all of the obtained Ag mesostructures with different sizes exhibit the enhanced SERS activity. The average EFs for individual Ag mesostructures were calculated using the expression EF = (Isurface /Isolution )*(Nsolution /Nsurface ), [32–35] where Isurface and Isolution are the SERS and normal signal intensities respectively, and Nsolution and Nsurface correspond to the number of molecules probed in a standard solution and on the substrate, respectively. Herein,
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Fig. 6. (a) Typical sp-SERS spectra and (b) EFs of the hierarchical Ag mesostructures with different sizes (sample #1, #2 and #3), respectively.
The hierarchical Ag mesostructures with different sizes exhibit the similar sp-SERS performance due to the similar morphology (Fig. 6b). The bands at about 1620 cm−1 in Fig. 6a are applied and the average enhancement factors for individual Ag mesostructures with different sizes were estimated to be ∼106 (the details for calculation are shown in Supplementary material). It should be pointed out that only molecules located at the hotspots are contributing significantly to the Raman signals since the enhanced electromagnetic fields are highly localized. Moreover, it is difficult to ensure that the Ag particle was located at the focus point during the Raman detection. Usually, more Nsurface are estimated and the focus problem will cause less Isurface . Thus, the real EFs for these hierarchical Ag mesostructures should be larger than the obtained values. Actually, it is reported that EFs as low as 107 are sufficient for the detection of SERS signals from single molecules [36]. Compared with the reported EFs of single nanocubes, octahedra and octapods [21,22]. The obtained large EFs indicate the as-prepared hierarchical Ag mesostructures has high efficiency as sp-SERS substrates and might have potential application in SERS based technology. 4. Conclusions In summary, the hierarchical Ag mesostructures with highly roughened surfaces were synthesized via a seed-mediated approach. The growth mechanism for the hierarchical Ag mesostructures were proposed and the oriented attachment of nanoparticles was revealed during the formation of Ag mesostructures. More important, these Ag mesostructures present the high production, tailorable size as well as the roughened surfaces. When they were used as the SERS substrates, the average enhancement factors for an individual Ag mesostructures could be estimated to be as high as 106 , which can be highly sensitive SERS substrates. Acknowledgement This work was supported by the China Postdoctoral Science Foundation (2015M570828). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2016.10. 011.
References [1] R.A. Tripp, R.A. Dluhy, Y.P. Zhao, Novel nanostructures for SERS biosensing, Nano Today 3 (2008) 31–37. [2] J.F. Li, Y.F. Huang, Y. Ding, Z.L. Yang, S.B. Li, X.S. Zhou, F.R. Fan, W. Zhang, Z.Y. Zhou, D.Y. Wu, B. Ren, Z.L. Wang, Z.Q. Tian, Shell-isolated nanoparticle-enhanced Raman spectroscopy, Nature 464 (2010) 392–395. [3] X.M. Qian, S.M. Nie, Single-molecule and single-nanoparticle SERS: from fundamental mechanisms to biomedical applications, Chem. Soc. Rev. 37 (2008) 912–920. [4] K.I. Yoshida, T. Itoh, V. Biju, M. Ishikawa, Y. Ozaki, Spectral shapes of surface-enhanced resonance Raman scattering sensitive to the refractive index of media around single Ag nanoaggregates, Appl. Phys. Lett. 95 (2009) 263104. [5] Z.L. Zhang, Y.Q. Wen, Controllable aggregates of silver nanoparticle induced by methanol for surface-enhanced Raman scattering, Appl. Phys. Lett. 101 (2012) 173109. [6] C.F. Tian, C.H. Ding, S.Y. Liu, S.C. Yang, X.P. Song, B.J. Ding, Z.Y. Li, J.X. Fang, Nanoparticle attachment on silver corrugated-wire nanoantenna for large increases of surface-enhanced Raman scattering, ACS Nano 5 (2011) 9442–9449. [7] W.H. Hsiao, H.Y. Chen, Y.C. Yang, Y.L. Chen, C.Y. Lee, H.T. Chiu, Surface-enhanced Raman scattering imaging of a single molecule on urchin-like silver nanowires, ACS Appl. Mater. Interfaces 3 (2011) 3280–3284. [8] M.F. Zhang, A.W. Zhao, H.Y. Guo, D.P. Wang, Z.B. Gan, H.H. Sun, D. Li, M. Li, Green synthesis of rosettelike silver nanocrystals with textured surface topography and highly efficient SERS performances, Cryst. Eng. Comm. 13 (2011) 5709–5717. [9] X.S. Shen, G.Z. Wang, X. Hong, W. Zhu, Nanospheres of silver nanoparticles: agglomeration, surface morphology control and application as SERS substrates, Phys. Chem. Chem. Phys. 11 (2009) 7450–7454. [10] Y.L. Wang, P.H.C. Camargo, S.E. Skrabalak, H.C. Gu, Y.N. Xia, A facile, water-based synthesis of highly branched nanostructures of silver, Langmiur 24 (2008) 12042–12046. [11] Y.M. Fang, Z.B. Lin, Y.M. Zeng, W.K. Chen, G.N. Chen, J.J. Sun, B. Ren, Z.Q. Tian, Facile electrochemical preparation of Ag nanothorns and their growth mechanism, Chem. Eur. J. 16 (2010) 6766–6770. [12] B. Zhang, P. Xu, X.M. Xie, H. Wei, Z.P. Li, N.H. Mack, X.J. Han, H.X. Xu, H.L. Wang, Acid-directed synthesis of SERS-active hierarchical assemblies of silver nanostructures, J. Mater. Chem. 21 (2011) 2495–2501. [13] T. Liu, D.S. Li, D.R. Yang, M.H. Jiang, Fabrication of flower-like silver structures through anisotropic growth, Langmuir 27 (2011) 6211–6217. [14] C.W. Cheng, B. Yan, S.M. Wong, X.L. Li, W.W. Zhou, T. Yu, Z.X. Shen, H.Y. Yu, H.J. Fan, Fabrication and SERS performance of silver-nanoparticle-decorated Si/ZnO nanotrees in ordered arrays, ACS Appl. Mater. Interfaces 2 (2010) 1824–1828. [15] Y. Hirai, H. Yabu, Y. Matsuo, K. Ijiro, M. Shimomura, Arrays of triangular shaped pincushions for SERS substrates prepared by using self-organization and vapor deposition, Chem. Commun. 46 (2010) 2298–2300. [16] J. Yan, X.J. Han, J.J. He, L.L. Kang, B. Zhang, Y.C. Du, H.T. Zhao, C.K. Dong, H.L. Wang, P. Xu, Highly sensitive surface-enhanced Raman spectroscopy (SERS) platforms based on silver nanostructures fabricated on polyaniline membrane surfaces, ACS Appl. Mater. Interfaces 4 (2012) 2752–2756. [17] C. Hrelescu, T.K. Sau, A.L. Rogach, F. Jackel, J. Feldmann, Single gold nanostars enhance Raman scattering, Appl. Phys. Lett. 94 (2009) 153113.
M. Xu, Y. Zhang / Applied Surface Science 393 (2017) 197–203 [18] H.Y. Liang, Z.P. Li, W.Z. Wang, Y.S. Wu, H.X. Xu, Highly surface-roughened flower-like silver nanoparticles for extremely sensitive substrates of surface-enhanced Raman scattering, Adv. Mater. 21 (2009) 4614–4618. [19] J.X. Fang, S.Y. Liu, Z.Y. Li, Polyhedral silver mesocages for single particle surface-enhanced Raman scattering-based biosensor, Biomaterials 32 (2011) 4877–4884. [20] J.X. Fang, S. Lebedkin, S.C. Yang, H. Hahn, A new route for the synthesis of polyhedral gold mesocages and shape effect in single-particle surface-enhanced Raman spectroscopy, Chem. Commun. 47 (2011) 5157–5159. [21] J.M. McLellan, Z.Y. Li, A.R. Siekkimem, Y.N. Xia, The SERS activity of a supported Ag nanocube strongly depends on Its orientation relative to laser polarization, Nano Lett. 7 (2007) 1013–1017. [22] M.J. Mulvihill, X.Y. Ling, J. Henzie, P.D. Yang, Anisotropic etching of silver nanoparticles for plasmonic structures capable of single-particle SERS, J. Am. Chem. Soc. 132 (2010) 268–274. [23] M.W. Xu, Y. Zhang, Seed-mediated approach for the size-controlled synthesis of flower-like Ag mesostructures, Mater. Lett. 130 (2014) 9–13. [24] P.C. Lee, D. Meisel, Adsorption and surface-enhanced Raman of dyes on silver and gold sols, J. Phys. Chem. 86 (1982) 3391–3395. [25] L.H. Lu, A. Kobayashi, K. Tawa, Y. Ozaki, Silver nanoplates with special shapes: controlled synthesis and their surface plasmon resonance and surface-enhanced Raman scattering properties, Chem. Mater. 18 (2006) 4894–4901. [26] Q. Zhang, Y.X. Hu, S.R. Guo, J. Goebl, Y.D. Yin, Seeded growth of uniform Ag nanoplates with high aspect ratio and widely tunable surface plasmon bands, Nano Lett. 10 (2010) 5037–5042. [27] R.L. Penn, J.F. Banfield, Morphology development and crystal growth in nanocrystalline aggregates under hydrothermal conditions: insights from titania, Geochim. Cosmochim. Acta 63 (1999) 1549–1557. [28] T. Sugimoto, Preparation of monodispersed colloidal particles, Adv. Colloid Interface Sci. 28 (1987) 65–108.
203
[29] C.H. Munro, W.E. Smith, M. Garner, J. Clarkson, P.C. White, Characterization of the surface of a citrate-reduced colloid optimized for use as a substrate for surface-enhanced resonance Raman scattering, Langmuir 11 (1995) 3712–3720. [30] E.J. Liang, X.L. Ye, W. Kiefer, Surface-enhanced Raman spectroscopy of crystal violet in the presence of halide and halate ions with near-infrared wavelength excitation, J. Phys. Chem. A 101 (1997) 7330–7335. [31] A. Kudelski, Raman studies of rhodamine 6G and crystal violet sub-monolayers on electrochemically roughened silver substrates: do dye molecules adsorb preferentially on highly SERS-active sites? Chem. Phys. Lett. 414 (2005) 271–275. [32] J.X. Fang, S.Y. Du, S. Lebedkin, Z.Y. Li, R. Kruk, M. Kappes, H. Hahn, Gold mesostructures with tailored surface topography and their self-assembly arrays for aurface-enhanced Raman spectroscopy, Nano Lett. 10 (2010) 5006–5013. [33] W. Cai, B. Ren, X.Q. Li, C.X. She, F.M. Liu, X.W. Cai, Z.Q. Tian, Investigation of surface-enhanced Raman scattering from platinum electrodes using a confocal Raman microscope: dependence of surface roughening pretreatment, Surf. Sci. 406 (1998) 9–22. [34] E.C.L. Ru, E. Blackie, M. Meyer, P.G. Etchegoin, Surface enhanced Raman scattering enhancement factors: a comprehensive study, J. Phys. Chem. C 11 (2007) 13794–13803. [35] E.Z. Tan, P.G. Yin, T.T. You, H. Wang, L. Gao, Three dimensional design of large-scale TiO2 nanorods scaffold decorated by silver nanoparticles as SERS sensor for ultrasensitive malachite green detection, ACS Appl. Mater. Interfaces 4 (2012) 3432–3437. [36] L. Su, W.Z. Jia, D.P. Manuzzi, L.C. Zhang, X.P. Li, Z.Y. Gu, Y. Lei, Highly sensitive surface-enhanced Raman scattering using vertically aligned silver nanopetals, RSC Adv. 2 (2012) 1439–1443.
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Hierarchical Ag mesostructures for single particle SERS substrate Minwei Xu ∗ , Yin Zhang School of Science, MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, Xi’an Jiaotong University, Xi’an, 710049, China
a r t i c l e
i n f o
Article history: Received 1 July 2016 Received in revised form 19 September 2016 Accepted 3 October 2016 Available online 5 October 2016 Keywords: Hierarchical Silver Seed-mediated growth Mesostructure SERS
a b s t r a c t Hierarchical Ag mesostructures with highly rough surface morphology have been synthesized at room temperature through a simple seed-mediated approach. Electron microscopy characterizations indicate that the obtained Ag mesostructures exhibit a textured surface morphology with the flower-like architecture. Moreover, the particle size can be tailored easily in the range of 250–500 nm. For the growth process of the hierarchical Ag mesostructures, it is believed that the self-assembly mechanism is more reasonable rather than the epitaxial overgrowth of Ag seed. The oriented attachment of nanoparticles is revealed during the formation of Ag mesostructures. Single particle surface enhanced Raman spectra (sp-SERS) of crystal violet adsorbed on the hierarchical Ag mesostructures were measured. Results reveal that the hierarchical Ag mesostructures can be highly sensitive sp-SERS substrates with good reproducibility. The average enhancement factors for individual Ag mesostructures are estimated to be about 106 . © 2016 Elsevier B.V. All rights reserved.
1. Introduction Surface enhanced Raman scattering (SERS) is a powerful analytical tool for chemical and biological sensing applications. During the last decade, the potential applications of sensitive SERS for molecular identification have attracted considerable interests. However, one feature which has limited its practical application is the difficulty involved in producing uniform, highly sensitive and reproducible SERS substrates [1,2]. Among the many different SERS substrates that have been thoroughly investigated, synthesis of Ag-based SERS substrates seems to be particularly interesting because Ag exhibits the best SERS activity compared with other metal substrates. The main enhancement mechanism for SERS is believed to the electromagnetic mechanism [3–6], where the socalled hotspots at the nanoscaled cavities, interstitial sites or void spaces provide strongly enhanced electromagnetic fields. Theoretical and experiment investigations reveal that the SERS activity is very sensitive to their morphologies. The Ag mesostructures with complex morphology, which have rough surface, interstitial sites or sharp tips, are likely to exhibit higher SERS activity [7–9]. During the past decade, shape-controlled synthesis of Ag mesostructures has advanced remarkably and it is now achievable to generate the hotspots for highly sensitive SERS substrates. For example, Xia et al. reported a water-based method to prepare highly branched Ag nanostructures. Because of their high
∗ Corresponding author. E-mail address: [email protected] (M. Xu). http://dx.doi.org/10.1016/j.apsusc.2016.10.011 0169-4332/© 2016 Elsevier B.V. All rights reserved.
surface areas and many sharp edges, such highly branched Ag nanostructures could serve as great substrates for SERS detection of low concentration analytes [10]. Ren et, al devoloped a electrochemical preparation of Ag nanothorns with the addition of thiourea. The SERS activity of the unique Ag nanothorns is about 20 folds stronger than that of the Ag nanoparticles [11]. Xu et al. reported an acid-directed synthesis of hierarchical assemblies of Ag nanostructures. The obtained structures with highly roughened surfaces can be highly sensitive SERS plarforms and show no polarization dependency of the incident laser [12]. Jiang fabricated the multipod-shaped Ag structures through anisotropic growth and the high SERS activity was also demonstrated [13]. To date, a variety of Ag mesostructures with high SERS activity have been reported. However, most of these strategies are limited to attain high tenability in a brad range of size. Moreover, in many cases, the unavoidable addition of surfactant, capping agents or polymeric templates may make the synthetic procedure and purification process more complex. It is inevitable that a thin layer of impurity molecules will reside on the surface of the final products and the adsorbed molecule can interfere with the SERS signal of the target molecule. Therefore, producing uniform, highly sensitive and reproducible SERS substrates remains a great challenge, particularly in terms of size control and morphology design with a surfactant-free procedure. On the other hand, the bulk Ag aggregates, which is formed by the aggregation of Ag nanostructures or ordered arrays, are usually applied as SERS substrates [14–16]. However, the controlled aggregation of Ag nanostructures is really a difficult task and the hotspots are formed randomly. Thus, such SERS substrates may
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reveal poor controllability and reproducibility. Moreover, these bulk Ag aggregates are too large to be used as SERS substrates in biological systems. Therefore, colloidal Ag nanocrystals are regarded as the alternative SERS substrates and the Raman scattering observed from single particles without the formation of hotspots via aggregates are more desirable. Thus, it is further important to perform the single particle SERS (sp-SERS) experiments, which will further expand the application of SERS [17–20]. Recently, Xia and Yang reported the sp-SERS performances for Ag nanocubes, etched octahedral and octapods. Modest enhanced factors are obtained in the range from 104 to 105 [21,22]. However, reports on sp-SERS of Ag mesostructures are relatively rare and further improvement should be achieved to augment the sensitivity of an individual Ag mesostructure. In this article, we reported the synthesis of hierarchical Ag mesostructures via a seed-mediated approach [23]. The resulting Ag mesostructures revealed the hierarchical flower-like architecture, in which the hotspots can be easily achieved even without the aggregation. They exhibited the nearly monodispersed sizes and the diameter of the Ag mesostructures could be controlled by adjusting the dosage of Ag seeds. The growth mechanism for the hierarchical Ag mesostructures were proposed and the oriented attachment of nanoparticles was revealed during the formation of Ag mesostructures. The obtained hierarchical Ag mesostructures can service as the sp-SERS substrates. The average SERS enhancement factors for an individual Ag mesoflower could be estimated to be as high as ∼106 , making them very appealing as SERS sensors or SERS-active tags.
2. Experimental 2.1. Materials Silver nitrate (AgNO3 ), ascorbic acid (Vc), trisodium citrate dehydrate (C6 H5 Na3 O7 ·H2 O) were analytical grade and used as received. Deionized water (Millipore) was used in all preparations. The Vc solutions was freshly prepared before use.
Table 1 Summarized sample denotations and corresponding reaction conditions together with their size and morphology properties.a Sample
Ag seed
Vc
AgNO3
Average size
Particle morphology
#1 #2 #3
1.0 ml 0.5 ml 0.2 ml
2.5 ml 3.0 ml 3.3 ml
1 ml 1 ml 1 ml
∼250 nm ∼360 nm ∼500 nm
Flower-like architectures Flower-like architectures Flower-like architectures
a Vc is used as reducing agent. The concentration of Vc and AgNO3 is 10 and 100 mM, respectively. The reaction is kept at room temperature.
2.4. Characterization The X-ray powder diffraction (XRD) investigation was carried out by using a Bruke D8-Advance diffractometer. UV–vis absorption spectra were obtained with a Hitachi U-4100 UV–vis spectrophotometer at room temperature under ambient conditions. Scanning electron microscopy (SEM) images and energy-dispersive X-ray (EDS) analyses were characterized on a JEOL JSM-7100F microscope. Transmission electron microscopy (TEM and HRTEM) observations were carried out on a JEOL JEM-2100 instrument. 2.5. Surface enhanced Raman spectroscopy Crystal Violet (CV) was used as the Raman probe for the SERS measurements. For preparation of SERS substrates, 10 l of the diluted Ag mesostructures in ethanol solution was carefully dropped on the cleaned Si substrate (10 mm × 10 mm). After the solution was completely dried in air, 100 l CV solution (10−8 M) was uniformly dropped on the substrate, and dried in air. Raman measurements were conducted with a confocal microprobe Raman spectrometer (LabRAM HR800, HORIBA JOBIN YVON). Radiation of 633 nm from a He-Ne laser was used for excitation. The laser beam was focused on the sample in a size of about 1 m. An extremely low incident laser power (decreased by a D2 attenuation piece, about 0.04 mW) was used in order to minimize heating effects during the Raman measurements and the acquisition time was fixed at 10 s. 3. Results and discussion
2.2. Preparation of Ag seeds
3.1. Synthesis of hierarchical Ag mesostructures
The Ag seeds were synthesized according to the reduction of Ag+ by sodium citrate. Typically, an aqueous solution of 5 mM AgNO3 (200 ml) was brought to boiling. Subsequently, 0.4 g sodium citrate was dissolved in 20 ml of H2 O and added dropwise in to the boiled AgNO3 solution. The mixed solution was kept on boiling for about 1 h. The as-obtained greenish yellow colloid was kept at 5 ◦ C, which served as the seeds for Ag mesostructures.
The original Ag seeds were prepared according to a typical citrate reduction method, in which the silver ion could be reduced to metal silver due to the reducibility of sodium citrate in the boiling water [24]. The representative TEM image reveal that the resultant Ag seeds are irregular nanoparticles with an average size of about 60 nm (Supplementary material, Fig. S1). Fig. S1c shows the UV–vis spectra of the as-synthesized Ag seeds and an absorption maximum near 420 nm was observed. It can be diluted easily as the photographs shown in Fig. S1c inset. With dilution by 10-, 25-, 50- 100- fold, the seed solution become transparent with the color changed from greenish yellow to light yellow. Moreover, the obtained Ag seed possesses good stability, which could be kept in fridge for several weeks without the precipitation. For the synthesis of hierarchical Ag mesostructures, a certain amount of Ag seed and fresh prepared Vc solution were mixed together. Then, the AgNO3 solution was added rapidly and the Ag mesostructures were formed in just a few seconds. The chemical reaction for the reduction of AgNO3 by Vc can be described as follows [25]:
2.3. Synthesis of hierarchical Ag mesostructures In a typical synthesis, a certain amount of Ag seed was diluted into 3.5 ml by a fresh prepared Vc solution (10 mM). Then, 1 ml of a 100 mM AgNO3 solution was added rapidly. Additions were made by micropipette and the reaction was conducted at room temperature without stirring. The original light yellow color turned into black immediately, which indicated the formation of Ag mesostructures. The mixed solution was allowed to age for several hours and the as-synthesized precipitate was separated by centrifugation, washed numerous times with deionized water and stored in ethanol for further characterization. While the essential conditions were kept the same, some other control experiments were also performed. The main reaction parameters and corresponding results were summarized in Table 1.
C6 H8 O6 + 2Ag+ → C6 H6 O6 + 2Ag + 2H+ (1) Fig. 1 displays the morphologies and microstructure of the obtained hierarchical Ag mesostructures with different sizes. Large quantity and good uniformity of flower-like Ag mesostructures
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Fig. 1. SEM, TEM images and size distribution of the obtained hierarchical Ag mesostructures via the seed-mediated approach with the assistance of (a, b, c) 1 ml, (d, e, f) 0.5 ml and (g, h, i) 0.2 ml Ag seed, respectively.
are achieved via this seed-mediated method. It is found that the obtained Ag mesostructures show the hierarchical structure, which are assembled by nanopetals. Moreover, further investigate from the SEM and TEM images reveals that each petal is further composed of many nanoparticles. The size of these Ag mesostructures can be controlled by adjusting the dosage of Ag seeds while all other parameters are kept constant. The main reaction parameters and corresponding results are summarized in Table 1. When the Ag seed was varied from 1 ml to 0.5 ml and 0.2 ml, the diameters of the resulting Ag mesostructures were roughly 250, 360 and 500 nm, respectively. For comparison, direct reduction of Ag+ by Vc (no extra Ag seeds) is also studied (Fig. S2). The absence of Ag seed leads to the rough microspheres with a huge size of ∼1800 nm. The selected area electron diffraction (SAED) pattern as shown in Fig. S3 reveals that the obtained Ag mesostructures have a polycrystalline structure. It demonstrates that the petals seem to assemble randomly, which exhibit independent orientation. The crystal structure and phase composition of these Ag mesostructures are also characterized by X-ray diffraction. Fig. 2 shows the typical XRD patterns of the as-prepared Ag samples. Four peaks can be observed, which correspond to diffractions from the (111), (200), (220), and (311) planes of the face-centered-cubic (fcc) Ag phase (JCPDS 04-0783). The sharp peaks and identical intensity ratios imply that the Ag mesostructures with different size are all well crystallized. In addition, as compared to the XRD pattern of Ag mesostructure obtained in the absence of Ag seed (Fig. S2c), the broad diffraction peaks indicate that the obtained Ag mesostructures with the assistance of Ag seeds are in small sizes. Furthermore, the EDS result (Fig. S4)
Fig. 2. XRD patterns of the as-obtained hierarchical Ag mesostructures.
reveals that silver is the dominant element while a trace of silicon originating form the substrate for SEM and EDS analysis is found. 3.2. The possible growth process for hierarchical Ag mesostructures During the seed-growth process, the reduction of metal ions in solution always results in two competing processes: epitaxial
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Fig. 3. TEM images (a, b) of the Ag mesoflowers of about 500 nm (sample #3) and the HRTEM image (c) originated from the marked area in (b). Inset is the FFT pattern of the two particles which indicates that the adjacent particles share the same crystallographic orientation.
Fig. 4. Schematic illustration for the formation of hierarchical Ag mesostructures during the seed-mediated growth processes.
growth of seeds and self-nucleation [26]. Generally, a slow reaction rate as well as a low concentration is favorable for homogeneous growth, while a fast rate leads to the self-nucleation process. In this work, the seed growth process is limited due to the fast reac-
tion rate and the self-nucleation followed with the seed-mediated self-ssembly is more reasonable for understanding the formation mechanism. Fig. 3 shows the TEM images of the Ag mesostructures obtained with 0.2 ml Ag seeds (sample #3), confirming that
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each petal is further composed of many tiny particles. The grey parts show the Ag petals lied horizontally while the dark areas correspond to the Ag petals stand vertically. More interestingly, the HRTEM images recorded from the junction of two particles indicate that lattice fringes are aligned between the adjacent nanoparticles. The attached nanoparticles share the similar crystallographic orientation, which is indicated by the solid lines shown in Fig. 3c. This result indicates that the oriented attachment [27] assembling plays an important role on the formation of the Ag mesostructures. The inset of Fig. 3c presents the Fast Fourier Transform (FFT) patterns. It demonstrates the single crystal nature for the two particles. The dotted line shows the junction area of the adjacent nanoparticles, in which some defects can also be founded. To further understand the formation of the nanopetals, the Ag mesostructures with different sizes are also studied by TEM analysis. Fig. S5 reveals the similar phenomenon in the Ag mesostructures with smaller size (sample #1). Clear and continuous lattice-fringe demonstrates that the adjacent particles share the similar crystallographic oriention. The primary particles are orderly attached with each other to form the petals. Accordingly, the possible growth process for the formation of flower-like Ag mesostructures was proposed. Such a growth process involves a fast nucleation and oriented aggregation of primary particles, which can be schematically illustrated in Fig. 4. In the early stage of reaction, primary particles were formed from the self-nucleation due to the fast reduction of AgNO3 by Vc (Fig. 4b). Then, the Ag seeds began to grow by randomly aggregating with the primary particles, which became the core of the flower-like structure. The random assemble was not homogeneous and there should be many irregular and randomly arranged protrusions on the core (Fig. 4c). The random assemble at the early stage results that the whole Ag mesoflower reveals the polycrystalline nature. As the reaction proceeded, some of the protrusions continued to grow through the self-assemble of nanoparticles along the identical direction to form the Ag sheets (oriented attachment, Fig. 4d). Finally, no nanoparticle remained and the sample was composed entirely of flower-like mosestructure. Actually, the formation of primary Ag particles and the self-assembly growth could be finished in a short time. Then, with the decreasing of the concentration, the reduction rate is reduced and the Ostwald ripening [28] is carried out for the further growth of the Ag mesostructures. To further understand the above processes, the hydroquinone with weak reducibility is also introduced to the seed-mediated process for Ag mesostructures. As shown in Fig. S6, hierarchical Ag mesostructures with flower-like morphology can also be attained. However, much different from the Ag mesostructures obtained by Vc reduction, these Ag petals are perfect nanosheets, which show the smooth texture. It is believed that the weak reducibility of hydroquinone can extend the Ostwald ripening processes and the smooth surfaces of Ag nanopetals are obtained due to the prolonged Ostwald ripening processes. 3.3. Single particle SERS properties of the hierarchical Ag mesostructures Fig. S7 reveals the Raman spectra of the blank Ag mesostructures. It is found that the aggregation of the blank Ag mesostructures (sample #1) exhibits the Raman signals and all these peaks can be ascribed to the Raman frequencies for sodium citrate [29]. These results demonstrate that the surface of the obtained Ag mesostructures would be inevitably covered by citrate although most of the citrate can be washed away. However, these Raman signals of citrate are really weaker than the Raman signals of the Crystal Violet (CV) when the aggregation Ag was used as Raman substrates (Fig.S8). Moreover, for single blank Ag particle, very weak Raman signals were detected (Fig. S7). All these results
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Fig. 5. Reproducibility of the hierarchical Ag mesostructures with the size of about 250 nm (sample #1). The sp-SERS spectra were collected from randomly selected individual Ag particles and the spectra with higher intensity or none intensity were discarded.
Table 2 Observed wavenumbers (cm−1 ) of crystal violet and their assignment. Wavenumber (cm−1 )
Vibrational Assignment
812 1170 1377 1620
Ring C H bend (out of plane) Ring of C H bend (in plane) N phenyl stretching Ring C C stretching
revealed that the adsorbed citrate would cause less disturbance to the SERS detection of CV molecules. Motivated by the complex structural feature of the obtained hierarchical Ag mesostructures, the sp-SERS properties of the asprepared Ag mesostructures are investigated while CV was used as a probe molecule. Herein, sp-SERS performance is studied from individual particles without the formation of hotspots via aggregates. The individual Ag mesostructure on the Si substrate is selected randomly and the interparticle spacing should be large enough to ensure only single particles are covered. Fig. 5 shows the typical sp-SERS spectra of CV molecules adsorbed on Ag mesostructures (sample #1). The Raman signals at the frequencies (cm−1 ) of 812, 1170, 1377 and 1620 are clearly observed. According to the literatures [30,31], the vibrational assignments of the observed Raman signals are listed in Table 2. It is believed that the hierarchical Ag mesostructures are demonstrated as highly sensitive SERS substrates because extremely low concentration of CV solution (10−8 ) was used for SERS detection. The unique hierarchical architecture not only ensures the large surface area, but also lead to the formation of nanocavity areas between the adjacent Ag petals. There are numerous hot spots associated with the nanocavity structure, which contribute to the greatly enhanced Raman signal. Fig. 5 shows the single particle Raman spectra collected from the randomly selected Ag mesostructures. Almost all the Ag particles show the SERS activity, in which the uniform Raman intensity reveals the high reproducibility of the Ag mesostructures for sp-SERS substrate. The typical sp-SERS spectra of CV molecules adsorbed on Ag mesostructures with different sizes are shown in Fig. 6. Compared to the Raman signal of the bare silicon, all of the obtained Ag mesostructures with different sizes exhibit the enhanced SERS activity. The average EFs for individual Ag mesostructures were calculated using the expression EF = (Isurface /Isolution )*(Nsolution /Nsurface ), [32–35] where Isurface and Isolution are the SERS and normal signal intensities respectively, and Nsolution and Nsurface correspond to the number of molecules probed in a standard solution and on the substrate, respectively. Herein,
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Fig. 6. (a) Typical sp-SERS spectra and (b) EFs of the hierarchical Ag mesostructures with different sizes (sample #1, #2 and #3), respectively.
The hierarchical Ag mesostructures with different sizes exhibit the similar sp-SERS performance due to the similar morphology (Fig. 6b). The bands at about 1620 cm−1 in Fig. 6a are applied and the average enhancement factors for individual Ag mesostructures with different sizes were estimated to be ∼106 (the details for calculation are shown in Supplementary material). It should be pointed out that only molecules located at the hotspots are contributing significantly to the Raman signals since the enhanced electromagnetic fields are highly localized. Moreover, it is difficult to ensure that the Ag particle was located at the focus point during the Raman detection. Usually, more Nsurface are estimated and the focus problem will cause less Isurface . Thus, the real EFs for these hierarchical Ag mesostructures should be larger than the obtained values. Actually, it is reported that EFs as low as 107 are sufficient for the detection of SERS signals from single molecules [36]. Compared with the reported EFs of single nanocubes, octahedra and octapods [21,22]. The obtained large EFs indicate the as-prepared hierarchical Ag mesostructures has high efficiency as sp-SERS substrates and might have potential application in SERS based technology. 4. Conclusions In summary, the hierarchical Ag mesostructures with highly roughened surfaces were synthesized via a seed-mediated approach. The growth mechanism for the hierarchical Ag mesostructures were proposed and the oriented attachment of nanoparticles was revealed during the formation of Ag mesostructures. More important, these Ag mesostructures present the high production, tailorable size as well as the roughened surfaces. When they were used as the SERS substrates, the average enhancement factors for an individual Ag mesostructures could be estimated to be as high as 106 , which can be highly sensitive SERS substrates. Acknowledgement This work was supported by the China Postdoctoral Science Foundation (2015M570828). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2016.10. 011.
References [1] R.A. Tripp, R.A. Dluhy, Y.P. Zhao, Novel nanostructures for SERS biosensing, Nano Today 3 (2008) 31–37. [2] J.F. Li, Y.F. Huang, Y. Ding, Z.L. Yang, S.B. Li, X.S. Zhou, F.R. Fan, W. Zhang, Z.Y. Zhou, D.Y. Wu, B. Ren, Z.L. Wang, Z.Q. Tian, Shell-isolated nanoparticle-enhanced Raman spectroscopy, Nature 464 (2010) 392–395. [3] X.M. Qian, S.M. Nie, Single-molecule and single-nanoparticle SERS: from fundamental mechanisms to biomedical applications, Chem. Soc. Rev. 37 (2008) 912–920. [4] K.I. Yoshida, T. Itoh, V. Biju, M. Ishikawa, Y. Ozaki, Spectral shapes of surface-enhanced resonance Raman scattering sensitive to the refractive index of media around single Ag nanoaggregates, Appl. Phys. Lett. 95 (2009) 263104. [5] Z.L. Zhang, Y.Q. Wen, Controllable aggregates of silver nanoparticle induced by methanol for surface-enhanced Raman scattering, Appl. Phys. Lett. 101 (2012) 173109. [6] C.F. Tian, C.H. Ding, S.Y. Liu, S.C. Yang, X.P. Song, B.J. Ding, Z.Y. Li, J.X. Fang, Nanoparticle attachment on silver corrugated-wire nanoantenna for large increases of surface-enhanced Raman scattering, ACS Nano 5 (2011) 9442–9449. [7] W.H. Hsiao, H.Y. Chen, Y.C. Yang, Y.L. Chen, C.Y. Lee, H.T. Chiu, Surface-enhanced Raman scattering imaging of a single molecule on urchin-like silver nanowires, ACS Appl. Mater. Interfaces 3 (2011) 3280–3284. [8] M.F. Zhang, A.W. Zhao, H.Y. Guo, D.P. Wang, Z.B. Gan, H.H. Sun, D. Li, M. Li, Green synthesis of rosettelike silver nanocrystals with textured surface topography and highly efficient SERS performances, Cryst. Eng. Comm. 13 (2011) 5709–5717. [9] X.S. Shen, G.Z. Wang, X. Hong, W. Zhu, Nanospheres of silver nanoparticles: agglomeration, surface morphology control and application as SERS substrates, Phys. Chem. Chem. Phys. 11 (2009) 7450–7454. [10] Y.L. Wang, P.H.C. Camargo, S.E. Skrabalak, H.C. Gu, Y.N. Xia, A facile, water-based synthesis of highly branched nanostructures of silver, Langmiur 24 (2008) 12042–12046. [11] Y.M. Fang, Z.B. Lin, Y.M. Zeng, W.K. Chen, G.N. Chen, J.J. Sun, B. Ren, Z.Q. Tian, Facile electrochemical preparation of Ag nanothorns and their growth mechanism, Chem. Eur. J. 16 (2010) 6766–6770. [12] B. Zhang, P. Xu, X.M. Xie, H. Wei, Z.P. Li, N.H. Mack, X.J. Han, H.X. Xu, H.L. Wang, Acid-directed synthesis of SERS-active hierarchical assemblies of silver nanostructures, J. Mater. Chem. 21 (2011) 2495–2501. [13] T. Liu, D.S. Li, D.R. Yang, M.H. Jiang, Fabrication of flower-like silver structures through anisotropic growth, Langmuir 27 (2011) 6211–6217. [14] C.W. Cheng, B. Yan, S.M. Wong, X.L. Li, W.W. Zhou, T. Yu, Z.X. Shen, H.Y. Yu, H.J. Fan, Fabrication and SERS performance of silver-nanoparticle-decorated Si/ZnO nanotrees in ordered arrays, ACS Appl. Mater. Interfaces 2 (2010) 1824–1828. [15] Y. Hirai, H. Yabu, Y. Matsuo, K. Ijiro, M. Shimomura, Arrays of triangular shaped pincushions for SERS substrates prepared by using self-organization and vapor deposition, Chem. Commun. 46 (2010) 2298–2300. [16] J. Yan, X.J. Han, J.J. He, L.L. Kang, B. Zhang, Y.C. Du, H.T. Zhao, C.K. Dong, H.L. Wang, P. Xu, Highly sensitive surface-enhanced Raman spectroscopy (SERS) platforms based on silver nanostructures fabricated on polyaniline membrane surfaces, ACS Appl. Mater. Interfaces 4 (2012) 2752–2756. [17] C. Hrelescu, T.K. Sau, A.L. Rogach, F. Jackel, J. Feldmann, Single gold nanostars enhance Raman scattering, Appl. Phys. Lett. 94 (2009) 153113.
M. Xu, Y. Zhang / Applied Surface Science 393 (2017) 197–203 [18] H.Y. Liang, Z.P. Li, W.Z. Wang, Y.S. Wu, H.X. Xu, Highly surface-roughened flower-like silver nanoparticles for extremely sensitive substrates of surface-enhanced Raman scattering, Adv. Mater. 21 (2009) 4614–4618. [19] J.X. Fang, S.Y. Liu, Z.Y. Li, Polyhedral silver mesocages for single particle surface-enhanced Raman scattering-based biosensor, Biomaterials 32 (2011) 4877–4884. [20] J.X. Fang, S. Lebedkin, S.C. Yang, H. Hahn, A new route for the synthesis of polyhedral gold mesocages and shape effect in single-particle surface-enhanced Raman spectroscopy, Chem. Commun. 47 (2011) 5157–5159. [21] J.M. McLellan, Z.Y. Li, A.R. Siekkimem, Y.N. Xia, The SERS activity of a supported Ag nanocube strongly depends on Its orientation relative to laser polarization, Nano Lett. 7 (2007) 1013–1017. [22] M.J. Mulvihill, X.Y. Ling, J. Henzie, P.D. Yang, Anisotropic etching of silver nanoparticles for plasmonic structures capable of single-particle SERS, J. Am. Chem. Soc. 132 (2010) 268–274. [23] M.W. Xu, Y. Zhang, Seed-mediated approach for the size-controlled synthesis of flower-like Ag mesostructures, Mater. Lett. 130 (2014) 9–13. [24] P.C. Lee, D. Meisel, Adsorption and surface-enhanced Raman of dyes on silver and gold sols, J. Phys. Chem. 86 (1982) 3391–3395. [25] L.H. Lu, A. Kobayashi, K. Tawa, Y. Ozaki, Silver nanoplates with special shapes: controlled synthesis and their surface plasmon resonance and surface-enhanced Raman scattering properties, Chem. Mater. 18 (2006) 4894–4901. [26] Q. Zhang, Y.X. Hu, S.R. Guo, J. Goebl, Y.D. Yin, Seeded growth of uniform Ag nanoplates with high aspect ratio and widely tunable surface plasmon bands, Nano Lett. 10 (2010) 5037–5042. [27] R.L. Penn, J.F. Banfield, Morphology development and crystal growth in nanocrystalline aggregates under hydrothermal conditions: insights from titania, Geochim. Cosmochim. Acta 63 (1999) 1549–1557. [28] T. Sugimoto, Preparation of monodispersed colloidal particles, Adv. Colloid Interface Sci. 28 (1987) 65–108.
203
[29] C.H. Munro, W.E. Smith, M. Garner, J. Clarkson, P.C. White, Characterization of the surface of a citrate-reduced colloid optimized for use as a substrate for surface-enhanced resonance Raman scattering, Langmuir 11 (1995) 3712–3720. [30] E.J. Liang, X.L. Ye, W. Kiefer, Surface-enhanced Raman spectroscopy of crystal violet in the presence of halide and halate ions with near-infrared wavelength excitation, J. Phys. Chem. A 101 (1997) 7330–7335. [31] A. Kudelski, Raman studies of rhodamine 6G and crystal violet sub-monolayers on electrochemically roughened silver substrates: do dye molecules adsorb preferentially on highly SERS-active sites? Chem. Phys. Lett. 414 (2005) 271–275. [32] J.X. Fang, S.Y. Du, S. Lebedkin, Z.Y. Li, R. Kruk, M. Kappes, H. Hahn, Gold mesostructures with tailored surface topography and their self-assembly arrays for aurface-enhanced Raman spectroscopy, Nano Lett. 10 (2010) 5006–5013. [33] W. Cai, B. Ren, X.Q. Li, C.X. She, F.M. Liu, X.W. Cai, Z.Q. Tian, Investigation of surface-enhanced Raman scattering from platinum electrodes using a confocal Raman microscope: dependence of surface roughening pretreatment, Surf. Sci. 406 (1998) 9–22. [34] E.C.L. Ru, E. Blackie, M. Meyer, P.G. Etchegoin, Surface enhanced Raman scattering enhancement factors: a comprehensive study, J. Phys. Chem. C 11 (2007) 13794–13803. [35] E.Z. Tan, P.G. Yin, T.T. You, H. Wang, L. Gao, Three dimensional design of large-scale TiO2 nanorods scaffold decorated by silver nanoparticles as SERS sensor for ultrasensitive malachite green detection, ACS Appl. Mater. Interfaces 4 (2012) 3432–3437. [36] L. Su, W.Z. Jia, D.P. Manuzzi, L.C. Zhang, X.P. Li, Z.Y. Gu, Y. Lei, Highly sensitive surface-enhanced Raman scattering using vertically aligned silver nanopetals, RSC Adv. 2 (2012) 1439–1443.