Journal of Nanomaterials
Volume 2018 (2018), Article ID 1263942, 7 pages
https://doi.org/10.1155/2018/1263942

Research Article

Nanoporous Ag-Au Bimetallic Triangular Nanoprisms Synthesized by Galvanic Replacement for Plasmonic Applications

Hongmei Qian,¹ Shoaib Anwer,² G. Bharath,³ Shahid Iqbal,⁴ and Lijuan Chen⁵

¹Department of Architecture and Civil Engineering, West Anhui University, Anhui 237012, China
²Department of Mechanical Engineering, Khalifa University of Science and Technology, Abu Dhabi 127788, UAE
³Department of Chemical Engineering, Khalifa University of Science and Technology, Abu Dhabi 127788, UAE
⁴International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, China
⁵College of Materials and Chemical Engineering, West Anhui University, Anhui 237012, China

Correspondence should be addressed to Hongmei Qian; hmqian0621@163.com

Received 11 April 2018; Accepted 3 June 2018; Published 27 June 2018

Academic Editor: Rajesh R. Naik

Copyright © 2018 Hongmei Qian et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Galvanic replacement is a versatile method of converting simple noble metallic nanoparticles into structurally more complex porous multimetallic nanostructures. In this work, roughened nanoporous Ag-Au bimetallic triangular nanoprisms (TNPs) are synthesized by galvanic replacement between smooth Ag triangular plates and ions. Transmission electron microscope and the elementary mapping measurements show that numerous protrusions and pores are formed on the {111} facets, and Ag and Au atoms are homogeneously distributed on the triangular plates. Due to the additional “hot spots” generated by the surface plasmon coupling of the newly formed protrusions and pores, the roughened nanoporous Ag-Au TNP aggregates demonstrate a higher surface-enhanced Raman scattering enhancement factor (seven times larger) and better reproducibility than that of smooth Ag triangular particle aggregates. These synthesized roughened nanoporous Ag-Au bimetallic TNPs are a promising candidate for the applications in analytical chemistry, biological diagnostics, and photothermal therapy due to their excellent plasmonic performances and good biocompatibility.

1. Introduction

Noble metal nanostructures supporting the surface plasmons (SPs) have attracted great scientific and technological interest [1–4]. Due to the nanoscale confinement, the SP properties can be effectively tuned by the size [5, 6], shape [7, 8], chemical compositions of nanostructures [9, 10], and even the dielectric environment [11, 12]. In particular, when two nanostructures are brought together, the SP coupling can result in an enormous electromagnetic field enhancement in the gap of two nanostructures, which is known as “hot spot” [13–16]. This giant local field has demonstrated various interesting applications, such as the plasmonic catalysis [17, 18], sensing [19, 20], photothermal therapy [21, 22], and surface-enhanced Raman scattering (SERS) [23–25]. For instance, the Raman signal from the molecules located in the “hot spots” can be magnified up to 108–14 times, which makes the single molecule Raman detection possible [26–28]. So far, various nanostructures have been designed for plasmonic applications, such as metallic roughened surface [29–32], nanoparticle aggregates [33–35], periodic nanostructures [36], and core/shell nanoparticles [37–39]. Nanotriangles, in particular, have strong electromagnetic field enhancements at their sharp corners, which have attracted great attention after the pioneering work of Jin et al. [40]. However, most of the previous works focused on the nanotriangles with atomically smooth surface and solely chemical composition (Ag or Au) [41–44]. Besides the sharp corners, it is still anticipated to improve the plasmonic performance by synthesizing a triangle structure with a roughened surface and multimetallic compositions, such as Ag-Au alloy, which is excellent in both near-field enhancement and biocompatibility [45–47].

In this work, roughened nanoporous Ag-Au bimetallic triangular nanoprisms (TNPs) are synthesized by galvanic replacement. Firstly, Ag triangular plates with the ultrasmooth surface are synthesized by a solvothermal method. Then, roughened nanoporous Ag-Au bimetallic TNPs are obtained by a galvanic displacement reaction. After adding a certain amount of HAuCl₄, the {111} facets of Ag TNPs are preferentially corroded and forming numerous nanometer size protrusions and pores. The elementary mappings demonstrate that the Ag and Au atoms are homogeneously distributed in the as-prepared bimetallic nanoplates. The SERS measurements demonstrate that the roughened nanoporous Ag-Au bimetallic TNPs can have a superior plasmonic activity and reproducibility to the Ag triangular plates with a smooth surface, which can be due to the additional “hot spots” generated by the surface plasmon coupling of the newly formed protrusions and pores. These nanoporous Ag-Au bimetallic TNPs, simultaneously having excellent plasmonic properties and good biocompatibility, are a promising candidate for the application in the fields of biomedical and chemical analysis and sensing, and so on.

2. Experimental

The synthesis procedure is shown in Figure 1(a). Single-crystalline Ag nanoplates with atomically smooth surface were synthesized by a solution-based solvothermal method with N,N-dimethylformamide (DMF) as solvent and a reducing agent [42, 43]. Briefly, 25 mL DMF solution of 50 mM polyvinyl pyrrolidone (PVP, MW ≈ 30,000 g mol⁻¹) was prepared in a 40 mL autoclave at room temperature. Then, 2 mL 1 mM AgNO₃ was added dropwise. As the addition of AgNO₃, the clear and colorless DMF solution generally turned dark brown, which indicated the formation of small Ag nanoparticles. Then, the uniform dark brown solution was sealed and heated at 140°C for 8 h. The final product (Ag triangular nanoplates) was washed three times with ethanol and redispersed in deionized water. Furthermore, 400 μL of the Ag triangular nanoplate colloid (1 mg/mL) was added into 10 mL PVP aqueous solution (2 mg/mL) and stirred for 10 min at 100°C. Then, 0.2 mM HAuCl₄ aqueous solution (8 mL) was injected. Keep reacting 30 min at 100°C, Ag-Au bimetallic TNPs can be obtained via galvanic replacement. The sample was centrifuged and washed with saturated NaCl aqueous solution to remove AgCl and then with water several times to remove PVP and NaCl. The ultraviolet-visible-near infrared (UV-Vis-NIR) absorption spectra were obtained using a Lambda 900 UV/VIS/NIR spectrometer at the wavelength range from 300 to 1600 nm. The surface morphologies of the Ag triangular nanoplates and Ag-Au bimetallic TNPs were characterized using a transmission electron microscope (TEM) (Hitachi 7650 working at 80 kV and JEOL 2010F working at 200 kV). SERS measurements were performed by an inVia Renishaw Raman spectrometer at the excitation of 532 nm. The excitation power was 140 μW. The acquisition time was set at 10 s.

(a)

(b)

(c)

Figure 1: The synthesis of Ag TNPs and Ag-Au TNPs. (a) The schematic of synthesizing Ag nanoplates and Ag-Au bimetallic TNPs. (b) The TEM image of the as-prepared Ag nanoplates with sharp corners and smooth surface. (c) The TEM image of the as-prepared nanoporous Ag-Au bimetallic TNPs.

3. Results and Discussion

Figure 1(b) shows the TEM image of the as-prepared silver TNPs. They have average edge length about 284 nm. From the inset, we can see that the nanoplate has very sharp corners and smooth surface. The selected-area electron diffraction (SAED) and X-ray diffraction (XRD) patterns (Figure S1) confirm that the synthesized nanoplates are well crystallized with top and bottom surfaces bounded by triangular {111} facets. As the addition of HAuCl₄ solution, the Ag crystal will be corroded by galvanic displacement reaction.

It is known that, for a face-centered cubic structure in the absence of surfactants, the low-index crystallographic plane {110} is more active than the other two planes: {100} and {111}, because of the descending surface energies γ{110} > γ{100} > γ{111} [48]. However, in our experiment, PVP is used as a surfactant during the nanoplate synthesis, which preferentially covers the {110} facets. Additionally, the exposed area of the {111} plane on Ag TNPs is much larger than the {100} plane. Therefore, ions will preferentially react with Ag atoms on the {111} plane. Owing to the Kirkendall effect [46, 49], Ag-Au bimetallic nanoprisms with numerous nanometer size protrusions and pores can be obtained, that is, roughened nanoporous Ag-Au bimetallic TNPs, as shown in Figure 1(c). It is interesting to know that the average particle size of TNPs is almost unaffected before (~284 nm) and after (~289 nm) the galvanic displacement (see Figure S2), which also confirmed that the formations of nanoporous Ag-Au alloyed nanoprisms are based on the galvanic displacement by using the Ag nanoprisms as a frame. The XRD pattern from these nanoporous Ag-Au TNPs is shown in Figure S3. By comparison with the XRD of smooth Ag nanoplates, besides the prominent {111} diffraction peak, the peaks from {200}, {220}, {311}, and {222} facets are also observed, which also indicates the corrosion of {111} facets and formation of protrusions. To further illustrate the Ag-Au bimetallic TNP compositions, elementary mapping and energy dispersive spectroscopy (EDS) were performed. As shown in Figure 2, the morphology of nanoporous TNPs can be well reproduced by the elementary imaging of Ag and Au, respectively. The EDS line scan profiles reveal that the atomic ratio of Ag to Au in the Ag-Au nanoprism is about 1 : 1.8. These results demonstrate that the Ag and Au atoms are homogenously distributed in the as-prepared bimetallic nanoprism.

Figure 2: Atomic percentage profile of Ag-Au TNPs calculated from the relative counts in the EDS line scan, as indicated by the red line in the scanning transmission electron microscopy (STEM) image in the inset. The inset shows the morphology of Ag-Au TNPs imaged by STEM, Ag element, and Au element, respectively.

Figure 3 shows the UV-Vis-NIR absorption spectra of the synthesized nanoplates. For the Ag TNPs, three absorption peaks can be observed, that is, 350 nm, 390 nm, and broadband in the region from 620 to 1300 nm. The peaks at 350 nm and 390 nm should be attributed to the weak out-of-plane dipole and quadrupole resonance, respectively. The absorption in 620–1300 nm is from the intense in-plane dipole resonance. This in-plane dipole mode is highly dependent on the nanoplate morphology; the nonhomogeneous size distributions and truncation degree of the angles make it broadband, which can be identified from Figure 1(b). Interestingly, for the nanoporous Ag-Au TNPs, the out-of-plane dipole and quadrupole modes disappear, and the in-plane dipole mode demonstrates a prominent redshift. These optical property evolutions also reflect the corrosion of {111} facets and the retention of triangular nanoprism shape, forming a roughened nanoporous Ag-Au plate. The red-shift of in-plane dipole mode is due to the surface plasmon coupling of the newly formed protrusions and pores.

Figure 3: The UV-Vis-NIR absorption spectra of the as-prepared colloids of Ag and Ag-Au TNPs. The inset shows the photograph of the colloids of Ag and Ag-Au TNPs.

In light of the sharp corners and rough surfaces, this Ag-Au bimetallic prisms can be an excellent candidate for SERS application. A typical crystal violet (CV) molecule solution (1 × 10⁻⁸ M) was used as a probe to study the SERS performance of Ag nanoplates with a smooth surface and roughened nanoporous Ag-Au bimetallic TNPs. Figure 4(a) shows the schematic structure of CV molecule and the Raman spectrum detected from CV powder. The Raman fingerprints can be clearly identified at 1174 cm⁻¹, 1361 cm⁻¹, 1586 cm⁻¹, and 1619 cm⁻¹, which are mainly from the “benzene” vibrations [50]. Figure 4(b) presents the SERS spectra detected from Ag nanoplate aggregate (blue line) and the Ag-Au TNP aggregate (red line). By comparison with the spectrum from powder, it is found that the Raman signal is greatly enhanced, but the observed wavenumbers are little changed, which indicates that the CV molecules are physisorbed on the metallic surface. Interestingly, we also notice that the SERS from the roughened porous Ag-Au TNP aggregate is much stronger than that from the smooth Ag nanoplate aggregates. To confirm this phenomenon, ten more aggregates were detected and the SERS reproducibility from the TNP aggregates was evaluated. The SERS spectra from the nanoporous Ag-Au TNP aggregates and Ag TNP aggregates are shown in Figure S4. Taking the 1586 cm⁻¹ mode as an example, the relative standard deviation (RSD) of SERS from nanoporous Ag-Au TNP aggregates is about 20% (Figure 4(c)) and the RSD of Ag TNP aggregates is about 28% (Figure 4(d)). The RSD of 1619 cm⁻¹ and 1174 cm⁻¹ is 20% and 24% for Ag-Au TNPs and 33% and 41% for Ag nanoplates, which are shown in Figure S4. These results indicate that the roughened Ag-Au TNPs can give larger and more reproducible SERS intensity. This phenomenon can be understood by the fact that denser “hot spots” can be generated on the surface of roughened nanoporous Ag-Au TNP aggregates, due to the surface plasmon coupling of the TNPs and the newly formed protrusions and pores. This dense “hot spot” distribution can conceal the fluctuation of SERS intensity due to the uneven molecule absorption and “hot spot” quality, thus demonstrating a better reproducibility [51]. Furthermore, by the equation of SERS enhancement factor [52, 53], we can calculate that the and are 7 × 10⁶ and 5 × 10⁷, respectively. Here, we should emphasize that the improved is mainly resulted from the additional “hot spots” created on the roughened surface by the surface plasmon coupling of the newly formed protrusions and pores, instead of the enlarged surface area. That is because, only when the molecules located in the “hot spots” can the Raman scattering be effectively enhanced. These roughened nanoporous Ag-Au bimetallic TNPs with excellent plasmonic performances (high enhancement and good reproducibility) and well biocompatibility are certainly useful for the practical applications, such as the chemical analysis, biological diagnostics, and photothermal therapy.

(a)

(b)

(c)

(d)

Figure 4: The SERS reproducibility of Ag TNP aggregates and Ag-Au TNP aggregates. (a) The Raman spectrum from CV powder. (b) The SERS spectra of CV from nanoporous Ag-Au TNP aggregates (red line) and Ag nanoplate aggregates (blue line). (c) Raman signal intensity at 1586 cm⁻¹ in SERS spectra from Ag-Au TNP aggregates. (d) Raman signal intensity at 1586 cm⁻¹ in SERS spectra from Ag TNP aggregates.

4. Conclusion

In summary, roughened nanoporous Ag-Au bimetallic TNPs were synthesized by galvanic displacement reaction between Ag TNPs and ions. The elementary mapping showed that the Ag and Au atoms were homogeneously distributed on the Ag-Au TNPs with the Ag to Au atomic ratio about 1 : 1.8. Promisingly, the SERS of CV from these roughened nanoporous Ag-Au TNP aggregates exhibited seven times higher intensity than that from smooth Ag TNP aggregates, which is due to the extra “hot spots” generated by the surface plasmon coupling of the newly formed protrusions and pores. These high dense “hot spots” distributed on the nanoporous Ag-Au TNP aggregates can also conceal the SERS fluctuation induced by the uneven molecule adsorption and “hot spot” quality, thus giving a better SERS reproducibility. Collectively, due to high plasmonic performances and inherent biocompatibility of Au element, all the results of the present study prove that the synthesized biocompatible nanoporous Ag-Au bimetallic TNPs can be a potential candidate in the field of analytical chemistry, biological diagnostics, photothermal therapy, and so on.

Data Availability

The selected-area electron diffraction pattern of single flat-lying Ag nanoprism, XRD pattern of Ag nanoprism, XRD pattern of roughened Ag-Au alloyed TNPs, and the SERS reproducibility of Ag nanoprism and Ag-Au TNP aggregates with Raman signal intensity data used to support the findings of this study are included within the supplementary information file.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

The authors acknowledge financial support from the Key Program in the Youth Elite Support Plan in Universities of Anhui Province, China (Grant no. gxyqZD2016244).

Supplementary Materials

Figure S1: (a) the selected-area electron diffraction pattern of one flat-lying Ag TNP with its top surface perpendicular to the electron beam, in which the spots enclosed in the rectangle, triangle, and circle correspond to the {220}, {422}, and 1/3{422} Bragg reflections of face-centered cubic silver, respectively. The existence of {220} reflection indicates that the Ag TNP was single crystals with {111} planes as the basal planes. (b) XRD pattern of Ag nanoplate. Figure S2: the statistics of the average size of TNPs before and after the galvanic displacement. (a) The TEM image of the as-prepared Ag TNP with sharp corners and smooth surface; (b) the TEM image of the as-prepared nanoporous Ag-Au TNPs; (c) the edge length statistics of Ag TNPs indicated in (a), the average size is about 284 nm with relative standard deviation (RSD) 10%; (d) the edge length statistics of Ag-Au TNPs indicated in (b), the average size is about 289 nm with relative standard deviation (RSD) 11%. Figure S3: XRD pattern of roughened Ag-Au alloyed TNPs. Figure S4: the SERS reproducibility of Ag TNP aggregates and Ag-Au TNP aggregates. The SERS spectra of CV from nanoporous Ag-Au TNPs (a) and Ag triangular aggregates (b). Raman signal intensity at 1618 cm⁻¹ in SERS spectra from Ag-Au TNP aggregates (c) and Ag triangular aggregates (d). Raman signal intensity at 1174 cm⁻¹ in SERS spectra from Ag-Au TNPs (e) and Ag TNP aggregates (f). (Supplementary Materials)

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