Research ArticleNanoporous Ag-Au Bimetallic Triangular NanoprismsSynthesized 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, China2Department of Mechanical Engineering, Khalifa University of Science and Technology, Abu Dhabi 127788, UAE3Department of Chemical Engineering, Khalifa University of Science and Technology, Abu Dhabi 127788, UAE4International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an JiaotongUniversity, Shaanxi 710049, China5College 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 AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

Galvanic replacement is a versatile method of converting simple noble metallic nanoparticles into structurally more complexporous multimetallic nanostructures. In this work, roughened nanoporous Ag-Au bimetallic triangular nanoprisms (TNPs) aresynthesized by galvanic replacement between smooth Ag triangular plates and AuCl−4 ions. Transmission electron microscopeand the elementary mapping measurements show that numerous protrusions and pores are formed on the {111} facets, and Agand Au atoms are homogeneously distributed on the triangular plates. Due to the additional “hot spots” generated by thesurface plasmon coupling of the newly formed protrusions and pores, the roughened nanoporous Ag-Au TNP aggregatesdemonstrate a higher surface-enhanced Raman scattering enhancement factor (seven times larger) and better reproducibilitythan that of smooth Ag triangular particle aggregates. These synthesized roughened nanoporous Ag-Au bimetallic TNPs are apromising candidate for the applications in analytical chemistry, biological diagnostics, and photothermal therapy due to theirexcellent 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 propertiescan be effectively tuned by the size [5, 6], shape [7, 8],chemical compositions of nanostructures [9, 10], and eventhe dielectric environment [11, 12]. In particular, when twonanostructures are brought together, the SP coupling canresult in an enormous electromagnetic field enhancementin the gap of two nanostructures, which is known as “hotspot” [13–16]. This giant local field has demonstrated variousinteresting 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 locatedin 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 designedfor plasmonic applications, such as metallic roughenedsurface [29–32], nanoparticle aggregates [33–35], periodicnanostructures [36], and core/shell nanoparticles [37–39].Nanotriangles, in particular, have strong electromagneticfield enhancements at their sharp corners, which haveattracted great attention after the pioneering work of Jinet al. [40]. However, most of the previous works focusedon the nanotriangles with atomically smooth surface andsolely chemical composition (Ag or Au) [41–44]. Besidesthe sharp corners, it is still anticipated to improve theplasmonic performance by synthesizing a triangle structure

HindawiJournal of NanomaterialsVolume 2018, Article ID 1263942, 7 pageshttps://doi.org/10.1155/2018/1263942

with a roughened surface and multimetallic compositions,such as Ag-Au alloy, which is excellent in both near-fieldenhancement and biocompatibility [45–47].

In this work, roughened nanoporous Ag-Au bimetallictriangular nanoprisms (TNPs) are synthesized by galvanicreplacement. Firstly, Ag triangular plates with the ultra-smooth surface are synthesized by a solvothermal method.Then, roughened nanoporous Ag-Au bimetallic TNPs areobtained by a galvanic displacement reaction. After addinga certain amount of HAuCl4, the {111} facets of Ag TNPsare preferentially corroded and forming numerous nanome-ter size protrusions and pores. The elementary mappingsdemonstrate that the Ag and Au atoms are homogeneouslydistributed in the as-prepared bimetallic nanoplates. TheSERS measurements demonstrate that the roughened nano-porous Ag-Au bimetallic TNPs can have a superior plas-monic activity and reproducibility to the Ag triangularplates with a smooth surface, which can be due to the addi-tional “hot spots” generated by the surface plasmon couplingof the newly formed protrusions and pores. These nanopor-ous Ag-Au bimetallic TNPs, simultaneously having excellentplasmonic properties and good biocompatibility, are a prom-ising candidate for the application in the fields of biomedicaland 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 surfacewere synthesized by a solution-based solvothermal methodwithN,N-dimethylformamide (DMF) as solvent and a reduc-ing agent [42, 43]. Briefly, 25mL DMF solution of 50mMpolyvinyl pyrrolidone (PVP, MW≈ 30,000 gmol−1) wasprepared in a 40mL autoclave at room temperature. Then,2mL 1mM AgNO3 was added dropwise. As the addition of

AgNO3, the clear and colorless DMF solution generallyturned dark brown, which indicated the formation of smallAg nanoparticles. Then, the uniform dark brown solutionwas sealed and heated at 140°C for 8 h. The final product(Ag triangular nanoplates) was washed three times withethanol and redispersed in deionized water. Furthermore,400μL of the Ag triangular nanoplate colloid (1mg/mL)was added into 10mL PVP aqueous solution (2mg/mL)and stirred for 10min at 100°C. Then, 0.2mMHAuCl4 aque-ous solution (8mL) was injected. Keep reacting 30min at100°C, Ag-Au bimetallic TNPs can be obtained via galvanicreplacement. The sample was centrifuged and washed withsaturated NaCl aqueous solution to remove AgCl and thenwith water several times to remove PVP and NaCl. Theultraviolet-visible-near infrared (UV-Vis-NIR) absorptionspectra were obtained using a Lambda 900 UV/VIS/NIRspectrometer at the wavelength range from 300 to 1600nm.The surface morphologies of the Ag triangular nanoplatesand Ag-Au bimetallic TNPs were characterized using a trans-mission electron microscope (TEM) (Hitachi 7650 workingat 80 kV and JEOL 2010F working at 200 kV). SERS measure-ments were performed by an inVia Renishaw Raman spec-trometer at the excitation of 532nm. The excitation powerwas 140μW. The acquisition time was set at 10 s.

3. Results and Discussion

Figure 1(b) shows the TEM image of the as-prepared sil-ver TNPs. They have average edge length about 284 nm.From the inset, we can see that the nanoplate has verysharp corners and smooth surface. The selected-area electrondiffraction (SAED) and X-ray diffraction (XRD) patterns(Figure S1) confirm that the synthesized nanoplates arewell crystallized with top and bottom surfaces boundedby triangular {111} facets. As the addition of HAuCl4

AgNO3{111}

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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-preparednanoporous Ag-Au bimetallic TNPs.

2 Journal of Nanomaterials

solution, the Ag crystal will be corroded by galvanicdisplacement reaction.

3Ag + AuCl−4⟶Au + 3AgCl + Cl− 1

It is known that, for a face-centered cubic structure inthe absence of surfactants, the low-index crystallographicplane {110} is more active than the other two planes:{100} and {111}, because of the descending surface energiesγ{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 muchlarger than the {100} plane. Therefore, AuCl−4 ions will pref-erentially react with Ag atoms on the {111} plane. Owing tothe Kirkendall effect [46, 49], Ag-Au bimetallic nanoprismswith numerous nanometer size protrusions and pores canbe obtained, that is, roughened nanoporous Ag-Au bimetallicTNPs, as shown in Figure 1(c). It is interesting to know thatthe average particle size of TNPs is almost unaffected before(~284nm) and after (~289nm) the galvanic displacement(see Figure S2), which also confirmed that the formationsof nanoporous Ag-Au alloyed nanoprisms are based on thegalvanic displacement by using the Ag nanoprisms as aframe. The XRD pattern from these nanoporous Ag-AuTNPs is shown in Figure S3. By comparison with the XRDof smooth Ag nanoplates, besides the prominent {111}diffraction peak, the peaks from {200}, {220}, {311}, and{222} facets are also observed, which also indicates thecorrosion of {111} facets and formation of protrusions. Tofurther illustrate the Ag-Au bimetallic TNP compositions,elementary mapping and energy dispersive spectroscopy(EDS) were performed. As shown in Figure 2, themorphology of nanoporous TNPs can be well reproducedby the elementary imaging of Ag and Au, respectively. TheEDS line scan profiles reveal that the atomic ratio of Ag toAu in the Ag-Au nanoprism is about 1 : 1.8. These resultsdemonstrate that the Ag and Au atoms are homogenouslydistributed in the as-prepared bimetallic nanoprism.

Figure 3 shows the UV-Vis-NIR absorption spectra of thesynthesized nanoplates. For the Ag TNPs, three absorptionpeaks can be observed, that is, 350 nm, 390nm, andbroadband in the region from 620 to 1300 nm. The peaks at350nm and 390nm 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 highlydependent on the nanoplate morphology; the nonhomoge-neous size distributions and truncation degree of theangles make it broadband, which can be identified fromFigure 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 prominentredshift. These optical property evolutions also reflect thecorrosion of {111} facets and the retention of triangularnanoprism shape, forming a roughened nanoporous Ag-Auplate. The red-shift of in-plane dipole mode is due to thesurface plasmon coupling of the newly formed protrusionsand pores.

In light of the sharp corners and rough surfaces, thisAg-Au bimetallic prisms can be an excellent candidatefor SERS application. A typical crystal violet (CV) moleculesolution (1× 10−8M) was used as a probe to study the SERSperformance of Ag nanoplates with a smooth surface androughened nanoporous Ag-Au bimetallic TNPs. Figure 4(a)shows the schematic structure of CV molecule and theRaman spectrum detected from CV powder. The Raman fin-gerprints can be clearly identified at 1174 cm−1, 1361 cm−1,1586 cm−1, and 1619 cm−1, which are mainly from the

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Ag-LAu-L

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

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

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“benzene” vibrations [50]. Figure 4(b) presents the SERSspectra detected from Ag nanoplate aggregate (blue line)and the Ag-Au TNP aggregate (red line). By comparison withthe spectrum from powder, it is found that the Raman signalis greatly enhanced, but the observed wavenumbers are littlechanged, which indicates that the CV molecules arephysisorbed on the metallic surface. Interestingly, we alsonotice that the SERS from the roughened porous Ag-AuTNP aggregate is much stronger than that from the smoothAg nanoplate aggregates. To confirm this phenomenon, tenmore aggregates were detected and the SERS reproducibilityfrom the TNP aggregates was evaluated. The SERS spectrafrom the nanoporous Ag-Au TNP aggregates and Ag TNPaggregates 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 about20% (Figure 4(c)) and the RSD of Ag TNP aggregates isabout 28% (Figure 4(d)). The RSD of 1619 cm−1 and1174 cm−1 is 20% and 24% for Ag-Au TNPs and 33% and41% for Ag nanoplates, which are shown in Figure S4.These results indicate that the roughened Ag-Au TNPs cangive larger and more reproducible SERS intensity. Thisphenomenon can be understood by the fact that denser“hot spots” can be generated on the surface of roughenednanoporous Ag-Au TNP aggregates, due to the surfaceplasmon coupling of the TNPs and the newly formedprotrusions and pores. This dense “hot spot” distributioncan conceal the fluctuation of SERS intensity due to theuneven molecule absorption and “hot spot” quality, thusdemonstrating a better reproducibility [51]. Furthermore,

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Figure 4: The SERS reproducibility of Ag TNP aggregates and Ag-Au TNP aggregates. (a) The Raman spectrum from CV powder. (b) TheSERS spectra of CV from nanoporous Ag-Au TNP aggregates (red line) and Ag nanoplate aggregates (blue line). (c) Raman signal intensity at1586 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 Journal of Nanomaterials

by the equation of SERS enhancement factor EF = ISERS/IBulk × NBulk/NSERS [52, 53], we can calculate that theEF1586 cm−1

Ag triangles and EF1586 cm−1Ag‐Au triangles are 7× 106 and 5× 107,

respectively. Here, we should emphasize that the improvedEF is mainly resulted from the additional “hot spots”created on the roughened surface by the surface plasmoncoupling of the newly formed protrusions and pores,instead of the enlarged surface area. That is because, onlywhen the molecules located in the “hot spots” can theRaman scattering be effectively enhanced. These roughenednanoporous Ag-Au bimetallic TNPs with excellentplasmonic performances (high enhancement and goodreproducibility) and well biocompatibility are certainlyuseful for the practical applications, such as the chemicalanalysis, biological diagnostics, and photothermal therapy.

4. Conclusion

In summary, roughened nanoporous Ag-Au bimetallic TNPswere synthesized by galvanic displacement reaction betweenAg TNPs and AuCl−4 ions. The elementary mapping showedthat the Ag and Au atoms were homogeneously distributedon the Ag-Au TNPs with the Ag to Au atomic ratio about1 : 1.8. Promisingly, the SERS of CV from these roughenednanoporous Ag-Au TNP aggregates exhibited seven timeshigher intensity than that from smooth Ag TNP aggregates,which is due to the extra “hot spots” generated by the surfaceplasmon coupling of the newly formed protrusions andpores. These high dense “hot spots” distributed on the nano-porous Ag-Au TNP aggregates can also conceal the SERSfluctuation induced by the uneven molecule adsorption and“hot spot” quality, thus giving a better SERS reproducibility.Collectively, due to high plasmonic performances and inher-ent biocompatibility of Au element, all the results of thepresent study prove that the synthesized biocompatiblenanoporous Ag-Au bimetallic TNPs can be a potential candi-date in the field of analytical chemistry, biological diagnos-tics, photothermal therapy, and so on.

Data Availability

The selected-area electron diffraction pattern of singleflat-lying Ag nanoprism, XRD pattern of Ag nanoprism,XRD pattern of roughened Ag-Au alloyed TNPs, and theSERS reproducibility of Ag nanoprism and Ag-Au TNPaggregates with Raman signal intensity data used to supportthe findings of this study are included within the supple-mentary information file.

Conflicts of Interest

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

Acknowledgments

The authors acknowledge financial support from the KeyProgram in the Youth Elite Support Plan in Universities ofAnhui Province, China (Grant no. gxyqZD2016244).

Supplementary Materials

Figure S1: (a) the selected-area electron diffraction pattern ofone flat-lying Ag TNP with its top surface perpendicular tothe electron beam, in which the spots enclosed in therectangle, triangle, and circle correspond to the {220},{422}, and 1/3{422} Bragg reflections of face-centered cubicsilver, respectively. The existence of {220} reflection indicatesthat the Ag TNP was single crystals with {111} planes as thebasal planes. (b) XRD pattern of Ag nanoplate. Figure S2:the statistics of the average size of TNPs before and afterthe galvanic displacement. (a) The TEM image of theas-prepared Ag TNP with sharp corners and smooth sur-face; (b) the TEM image of the as-prepared nanoporousAg-Au TNPs; (c) the edge length statistics of Ag TNPsindicated in (a), the average size is about 284nm withrelative standard deviation (RSD) 10%; (d) the edge lengthstatistics of Ag-Au TNPs indicated in (b), the average size isabout 289nm with relative standard deviation (RSD) 11%.Figure S3: XRD pattern of roughened Ag-Au alloyed TNPs.Figure S4: the SERS reproducibility of Ag TNP aggregatesand Ag-Au TNP aggregates. The SERS spectra of CV fromnanoporous Ag-Au TNPs (a) and Ag triangular aggregates(b). Raman signal intensity at 1618 cm−1 in SERS spectrafrom Ag-Au TNP aggregates (c) and Ag triangular aggre-gates (d). Raman signal intensity at 1174 cm−1 in SERSspectra from Ag-Au TNPs (e) and Ag TNP aggregates (f).(Supplementary Materials)

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