Porous AuâAg Nanospheres with High-Density and Highly Accessible

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Porous Au-Ag Nanospheres with High-Density and Highly Accessible Hotspots for SERS Analysis Kai Liu, Yaocai Bai, Lei Zhang, Zhongbo Yang, Qikui Fan , Haoquan Zheng, Yadong Yin, and Chuanbo Gao Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b00868 • Publication Date (Web): 18 May 2016 Downloaded from http://pubs.acs.org on May 19, 2016

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Porous Au-Ag Nanospheres with High-Density and Highly Accessible Hotspots for SERS Analysis Kai Liu,† Yaocai Bai,‡ Lei Zhang,† Zhongbo Yang,§ Qikui Fan,† Haoquan Zheng,£ Yadong Yin,‡ and Chuanbo Gao*,†

†

Center for Materials Chemistry, Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi'an, Shaanxi 710054, China.

‡

Department of Chemistry, University of California, Riverside, California 92521, United States.

§

Chongqing Key Laboratory of Multi-scale Manufacturing Technology, Chongqing Institute of

Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China. £

Department of Materials and Environmental Chemistry, Stockholm University, Stockholm 10691, Sweden.

KEYWORDS: Porous nanoparticles, plasmonic nanoparticles, dealloying process, hotspots, surfaceenhanced Raman scattering

ABSTRACT: Colloidal plasmonic metal nanoparticles have enabled surface-enhanced Raman scattering (SERS) for a variety of analytical applications. While great efforts have been made to create hotspots for amplifying Raman signals, it remains a great challenge to ensure their high density and accessibility for improved sensitivity of the analysis. Here we report a dealloying process for the

1 ACS Paragon Plus Environment

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fabrication of porous Au-Ag alloy nanoparticles containing abundant inherent hotspots, which were encased in ultrathin hollow silica shells so that the need of conventional organic capping ligands for stabilization is eliminated, producing colloidal plasmonic nanoparticles with clean surface and thus high accessibility of the hotspots. As a result, these novel nanostructures show excellent SERS activity with an enhancement factor of ~1.3×107 on a single particle basis (off-resonant condition), promising high applicability in many SERS-based analytical and biomedical applications.

Nanoparticles (NPs) of Au and Ag are able to produce intense electromagnetic field in their vicinity due to their localized surface plasmon resonance (LSPR), which can further enable surface-enhanced Raman scattering (SERS) for many analytical applications.1-2 The sensitivity of the SERS analysis relies on the so-called “hotspots” where the electromagnetic field is extremely strong for amplifying Raman scattering of target molecules.1 Many efforts have been made to construct such hotspots, typically by synthesizing Au/Ag nanoparticles with rough surface,3-4 sharp tips,5-9 and inter-10-14 or intra-particle nanogaps.15-19 However, for practical applications it remains highly desirable to fabricate hotspots of a high density by rational design of Au/Ag nanostructures for further improved SERS performance. On the other hand, conventional colloidal plasmonic nanoparticles need to be stabilized by organic capping ligands (or surfactants), which however prevent the hotspots from being accessed by most analytes and therefore greatly suppress the SERS activity.20 The capping ligands are, however, indispensable in the typical procedures for the synthesis of nanoparticles as a stable colloid.21 Therefore, it becomes another great opportunity for further enhancement of SERS activity if one can develop new synthetic strategies to ensure a clean and highly accessible nanoparticle surface while maintaining their colloidal dispersity for broad applicability, e.g., in SERS analysis in vitro or live cells. In this work, we report the fabrication of colloidal porous Au-Ag alloy nanoparticles with built-in


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hotspots of high density and accessibility. The nanoporous Au-Ag particles were synthesized by leaching less-stable Ag from fully alloyed Au-Ag nanospheres, with remaining Au-rich species reconstructing into a nanoporous framework, which produces effective coupling of the LSPR and extreme field enhancement for Raman signal amplification. Our strategy resembles the technique that was involved in the preparation of the non-plasmonic Raney nickel catalyst,22 nanoporous Au “leaf”,2328

and Au-Ag nanocages and nanoframes.29-31 While the nanoporous Au leaf might be SERS active, its

macroscopic dimension constitutes a major problem for its broader use in biomedical applications. Although the efforts in reducing the dimension of the nanoporous Au have been reported, the porous Au nanoparticles are usually supported ones obtained by first patterning Au-Ag alloy nanostructures on a substrate followed by a chemical or electrochemical dealloying process, which may make it difficult to achieve large-scale production for many practical applications.32-39 The synthesis of porous metal nanoparticles as a stable colloid has met challenges, with only limited success in non-plasmonic Pt-based materials40-42 and plasmonic Au nanoparticles with low porosity.43-44 On the other hand, the nanoporosity in the Au-Ag nanocages and nanoframes was under less control for achieving optimal SERS activity, probably due to the difficulty in producing fully-alloyed and tunable Au-Ag nanoboxes as a starting material by the galvanic replacement. Thus, the current work establishes a robust route to highly porous and colloidal plasmonic nanoparticles with well-controlled porosity, peculiar optical property and excellent SERS activity. To further improve the SERS activity, we also incorporated a novel strategy to ensure the high accessibility of the built-in hotspots by avoiding the use of conventional organic capping ligands for colloidal stabilization. An ultrathin hollow silica shell was designed to encase the porous nanoparticles and maintain their colloidal stability. Thanks to the porous nature of the ultrathin silica shell, the inherent hotspots are easily accessible by guest molecules. Combining the advantageous features of high-density hotspots and highly accessible surfaces, the resulting porous alloy nanoparticles showed excellent SERS activity and promising applicability in various fields.


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Figure 1. Synthesis of p-AuAg@void@SiO2 yolk/shell NPs by a dealloying method. (a) A scheme illustrating the synthesis route. (b–f) TEM images of the intermediates, including Au@Ag@SiO2 (b), AuAg alloy@SiO2 (c–e, with decreasing thickness of the silica shells), and the final pAuAg@void@SiO2 NPs after dealloying at 0 °C (f). The size of the p-AuAg NPs was ~61 nm. The arrow in the inset of (e) indicates a defect in the thin silica shell. (g) p-AuAg@void@SiO2 NPs with p-AuAg NPs of a large size (~160 nm).

In a typical synthesis (Figure 1a), Au-Ag alloy NPs were first prepared by a silica-protected annealing process.45 Briefly, Au@Ag@SiO2 core/shell NPs were obtained by stoichiometric seeded growth of Ag on Au NPs,46-47 followed by coating of the nanospheres by a silica layer. The transmission electron microscopy (TEM) image shows clear contrast of the three sequential components in a


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core/shell configuration in individual nanospheres (Figure 1b). These nanospheres were then subjected to high-temperature annealing (950 °C), which converted Au@Ag core/shell NPs into the fully alloyed ones accompanied by disappearance of the Au@Ag boundary and improved single crystallinity (Figure 1c and Supporting Information Figure S2). The silica shells were significantly densified due to silicate condensation and collapse of their intrinsic micropores during the annealing process. As a result, layerby-layer stripping of the silicate species was achieved when the AuAg alloy@SiO2 NPs were etched by an alkaline solution (Figure 1d–e). By controlling the etching time, an ultrathin layer of silica (< 5 nm) was eventually formed on the AuAg alloy NPs, with some defects or openings present on the shells (Figure 1e). While ultrathin silica shells are challenging to synthesize, the controlled etching process developed in this work represents an alternative yet effective way to conventional sol-gel methods.48 The resulting AuAg alloy@SiO2 NPs encased in ultrathin silica shells were subsequently dealloyed with concentrated nitric acid, which partially dissolved the less-stable Ag from the AuAg alloy NPs. As a result, the remaining Au-rich species reconstructed into a highly porous alloy network (p-AuAg). The dealloying process was made possible by the porous ultrathin silica shells, which not only facilitated convenient diffusion of nitric acid to the alloy particles but also prevented the metal nanoparticles from irreversible aggregation in the concentrated nitric acid. It is interesting to note that with the silica shell for reference, a clear volume contraction of the metal has been observed during the dealloying process, consistent with a previous report,38 which led to a p-AuAg@void@SiO2 yolk/shell nanostructure (Figure 1f). The average size of the p-AuAg NPs was ~61 nm, which can be well tuned by controlling the size of the alloy nanospheres for particular applications (see Figure 1g for p-AuAg NPs of ~160 nm).


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Figure 2. Structural characterization of the p-AuAg NPs dealloyed at 0 °C. (a–c) A TEM image, and slices cut out from the reconstructed tomogram of an individual p-AuAg NP. Inset: an illustration showing the cutting position of the nanoparticle. (d–e) HRTEM images of two individual p-AuAg NPs, showing single crystallinity. Inset: electron diffraction patterns recorded on a whole nanoparticle basis.

The porous structure of the p-AuAg NPs was investigated by three-dimensional electron tomography and high-resolution TEM (HRTEM) (Figure 2). The tomogram reconstructed from the title series (Figure S3) shows that nanopores are present throughout the entire nanoparticle, which are interconnected to form a bicontinous structure (Figure 2a–c, Video S1). It is interesting to note that an individual p-AuAg NP is a single crystal, as determined by the HRTEM and the corresponding electron diffractions (Figure 2d–e). The two p-AuAg NPs were aligned along the [011] and [001] zone axis, respectively, which shows lattice fringes aligning along the same direction, leading to a single set of diffractions in the electron diffractions. Minor deviations of the lattice fringes can be also observed


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occasionally. The single crystallinity can be verified from particle to particle by electron diffraction, indicating that during the reconstruction the Au-Ag ligaments inherited the original single crystallinity of the Au-Ag alloy nanospheres (Figure S2), and the ligaments were grown in an epitaxial manner on a whole nanoparticle basis. The p-AuAg@void@SiO2 NPs are free of capping ligands confirmed by Fourier transform infrared spectroscopy (FTIR) (Figure S4). Although polyvinylpyrrolidone (PVP) was involved in the initial wet-chemistry synthesis, it easily underwent pyrolysis during the annealing process at 950 °C. While capping ligands were absent, the nanoparticles formed a highly stable colloid due to the thin silica shells, which served as an inorganic pseudo-surfactant to provide charges and thus repulsive force between the nanoparticles, with ζ-potential measured to be –18.1 mV. Dynamic light scattering (DLS) demonstrated Gaussian size distribution for all nanoparticles involved in the synthesis, confirming their uniform size and colloidal property (Figure 3a).

Figure 3. (a) Size distributions of the AuAg alloy@SiO2, AuAg alloy@SiO2(thin) and pAuAg@void@SiO2 NPs measured by DLS. (b) UV-vis-NIR spectra of the Au@Ag@SiO2, AuAg alloy@SiO2 and p-AuAg@void @SiO2 NPs. Inset: digital photos of the respective sols.


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The evolution of the optical properties of the metal nanoparticles during the alloying and dealloying processes was further investigated by UV-vis-near IR (NIR) spectroscopy (Figure 3b). The Au@Ag@SiO2 and AuAg alloy@SiO2 NPs showed strong LSPR with maximal extinction appearing at 475 and 445 nm, respectively, exhibiting a bright yellow color. After dealloying, the resulting pAuAg@void@SiO2 NPs displayed distinct optical property, showing a dark bluish color. The main extinction band appeared at ~1280 nm of the wavelength. It can be attributed to the dipolar LSPR resonance of the p-AuAg NPs, which underwent significant red shift compared with that of solid nanospheres (~540 nm) due to the presence of abundant nanopores. Besides, it is observed that some Au-Ag alloy nanoparticles can survive the dealloying process (Figure S5). They may possess high Au/Ag ratios by chance and thus are much stable, which well explains the weak LSPR band at ~510 nm of the wavelength (Figure 3b).

Figure 4. (a–c) TEM images of the p-AuAg NPs obtained by dealloying at 0 (a), 20 (b) and 40 °C (c), respectively. (d) Atomic percentage of Au and Ag elements in the p-AuAg NPs measured by EDS. (e)


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Plots of the particle size and width of the AuAg ligaments as a function of the dealloying temperature. (f) UV-vis-NIR spectra of the p-AuAg NPs.

Because dealloying is basically a kinetically controlled process, it becomes possible to tune the porosity of the p-AuAg NPs and thus their optical property by many synthesis parameters, such as the dealloying temperature. Figure 4a–c shows the TEM images of the p-AuAg NPs after dealloying at 0, 20 and 40 °C, respectively, which confirm the high porosity for all samples. HRTEM images confirm that the p-AuAg NPs are also single crystalline with only minor lattice deviations (Figure S6). Energy dispersive X-ray spectroscopy (EDS) suggests that these porous nanoparticles are composed of an alloy of Au and Ag with Au being a majority (Figure S7), and the Au/Ag ratio rises with temperature, indicating an increasing degree of Ag erosion at an elevated temperature (Figure 4d). The difference in the reaction kinetics has enabled formation of nanoparticles with different size and porous properties. At an elevated temperature, the width of the Au-Ag ligaments increased while the overall size of the porous nanoparticles shrank (Figure 4e). It is assumed that at a high temperature, etching of Ag was in a high rate, which made it difficult to form stable Au islands at their original position, leading to a shrinkage of the overall particle size. The remaining Au species of high concentration grew on a relatively small population of the pre-existing Au islands, giving rise to nanoframes of a large feature size. As the LSPR is sensitive to the size of the nanospheres, the nanopores and the ligaments, the extinction band of the p-AuAg NPs therefore varies with the dealloying temperature. As shown in Figure 4f, the LSPR band shifted from 1280 to 990 and 830 nm when the dealloying temperature was elevated from 0 to 20 and 40 °C, respectively. The convenient tuning of the LSPR makes it possible for the p-AuAg NPs to achieve optimal activity in many plasmonic applications.


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Figure 5. Simulated near-field electromagnetic field distributions of the p-AuAg NPs irradiated by an incident plane wave with wavelength of (a) 532 nm, (b) 633 nm, (c) 785 nm, and (d) 818 nm (onresonance condition), respectively. All simulations were performed in Lumerical FDTD Solutions.

To reveal the optical properties of the p-AuAg NPs and confirm the formation of hotspots, distributions of the near-field electromagnetic field were investigated by finite-difference time-domain (FDTD) simulations (Figure 5). For simplification, a porous Au nanosphere with size of 60 nm was constructed as a model (Figure S8), which was surrounded by air, and irradiated by a plane wave of different wavelengths. The model porous nanosphere displays an LSPR band at 818 nm of the wavelength (Figure S8), and therefore is analogous to a p-AuAg NP which was dealloyed at 40 °C. It is clear that at all irradiation wavelengths investigated, the near-field electromagnetic fields (described by E2/E02) in the nanopores are highly intense compared with the incident field (Figure 5a–d). The


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nanopores thus resemble resonators to focus the incident electromagnetic field and afford high-density hotspots for SERS analysis, even when the plasmons are off-resonant under short-wavelength irradiations (Figure 5a–b, irradiation wavelength: 532 and 633 nm). The excitation of the nanopores by a broad range of the wavelength may account for the high background in the extinction spectrum of the p-AuAg NPs (Figure 3b, 4f). However, with increasing irradiation wavelength, the maximal electromagnetic field enhancement increases dramatically, which promises significantly enhanced SERS activity. At long irradiation wavelengths, besides the inside nanopores, the local electromagnetic field gets stronger over the outer surface of the p-AuAg NP (Figure 5c–d, irradiation wavelength: 785 and 818 nm). The electromagnetic field over the outer surface becomes extremely intense when the irradiation wavelength coincides with the LSPR wavelength (818 nm, Figure 5d), with the field distribution confirming the dipolar-mode resonance in the extinction spectrum (Figure 3b, 4f).

Figure 6. SERS activities of the p-AuAg@void@SiO2 NPs in comparison with Au and p-AuAg NPs.


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(a) Raman signals of CV (10–6 M) recorded from silicon substrates deposited with respective NPs (size: ~51 nm; dealloying temperature: 40 °C). (b) Raman signals recorded from aqueous solutions of CV (10–6 M) with different suspensions of the NPs (size: ~51 nm; dealloying temperature: 40 °C). (c) Raman signals of CV (10–6 M) recorded from a single particle (size: ~160 nm; dealloying temperature: 0 °C) deposited on a silicon substrate. (d) Images of the single particle for SERS analysis, observed by SEM (left) and optical microscope equipped on the Raman spectrophotometer (right).

Combining the hotspots predicted by the FDTD simulations and the clean surface rendered by the silica-shell protection, the p-AuAg@void@SiO2 NPs are expected to show excellent SERS activity in detecting molecules of interest in low abundance, which has been confirmed in our experiment (Figure 6). PVP-stabilized Au NPs and p-AuAg NPs of the same size and concentration were investigated as control samples for comparison. Substrate-based SERS was first evaluated (Figure 6a). A known amount of the samples was deposited on a silicon substrate, followed by drying a model analyte, crystal violet (CV), on the substrate prior to SERS analysis. Compared with the Au NPs, the substrate fabricated with p-AuAg NPs showed much enhanced Raman signals, which can be attributed to the partial adsorption of CV on the inherent hotspots of the p-AuAg NPs. The substrate of pAuAg@void@SiO2 NPs showed extremely intense Raman signals, with intensities reaching ~20 and ~2.5 fold of those from the Au and p-AuAg substrates, respectively, which emphasized the critical role of the nanopores and the clean surface in affording optimal SERS activities. Due to the nanoporosity, the p-AuAg@void@SiO2 NPs are capable of enriching molecules of interest from an aqueous solution, which makes it possible to detect analytes directly from its solutions. As shown in Figure 6b, when nanoparticles of the same concentration were introduced into a solution of CV, SERS signals from both Au and p-AuAg NPs were very weak, which can be attributed to the insufficient density of the hotspots and the presence of PVP on their surface, respectively. By clear contrast, the Raman signals from the solution containing p-AuAg@void@SiO2 NPs again were particularly high, reaching ~17


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fold of those from the Au and p-AuAg NPs. In addition, the superior activity of the pAuAg@void@SiO2 NPs enables single-particle SERS analysis (Figure 6c–d). While nearly no signals or only weak signals can be detected from a single Au NP or a p-AuAg NP, the signals from a single p-AuAg@void@SiO2 NP (p-AuAg: 160 nm) showed well-resolved Raman scattering with extremely high intensity. The enhancement factor was estimated to be 1.3×107 (Supporting Information), which exceeds many earlier reports, especially those of colloid ones (Table S1).3, 6, 36, 43, 49-54 It is worth noting that the enhancement factor was obtained under irradiation of a 633 nm laser, which is in fact an offresonant condition for the p-AuAg@void@SiO2 NPs (Figure S10). An even larger enhancement factor is envisioned when a laser of a long wavelength is employed, which promises wide use of the pAuAg@void@SiO2 NPs as a single-particle probe for detecting molecules of interest with excellent sensitivity. All demonstrations confirmed high activity of the p-AuAg@void@SiO2 NPs in SERS applications due to the high-density hotspots and their convenient accessibility. In summary, we have developed a robust dealloying method to synthesize highly porous Au-Ag alloy nanoparticles with an ultrathin hollow silica shell in place of conventional capping ligands for stabilization. The high porosity of the nanoparticles enables significant coupling of the LSPR and thus abundant inherent hotspots. The clean surface ensured by the ultrathin hollow silica shell makes these hotspots readily accessible by target molecules. Both unique features contribute to the high sensitivity of the material in detecting molecules of interest by SERS, and promise high applicability in many SERS-based analytical and biomedical applications. In addition, this novel material may find use in drug delivery and photothermal therapeutics due to their nanometer dimension, porous structure and optical activity in the near-IR range of the spectrum.

ASSOCIATED CONTENT Supporting Information.


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Supporting Information Available: Electron tomography (video), experimental details, calculation of the enhancement factor, additional FTIR, UV-vis-NIR, TEM, HRTEM, EDS mapping, FDTD simulation, SERS results, and a summary of typical enhancement factors reported in literature. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS C.G. acknowledges support from the National Natural Science Foundation of China (21301138), the startup fund, and operational fund for the Center for Materials Chemistry from Xi’an Jiaotong University. Y.Y. acknowledges support from the U.S. National Science Foundation (CHE-1308587). Z.Y. acknowledges support from Fundamental & Advanced Research Project of Chongqing, China (cstc2013jcyjC00001). The authors thank Prof. Liqing Huang at School of Science, Xi'an Jiaotong University for help with FDTD simulations. REFERENCES 1. Schlucker, S. Angew. Chem. Int. Ed. 2014, 53, 4756–4795. 2. Nie, S.; Emory, S. R. Science 1997, 275, 1102–1106. 3. Wang, H.; Halas, N. J. Adv. Mater. 2008, 20, 820–825. 4. You, H.; Ji, Y.; Wang, L.; Yang, S.; Yang, Z.; Fang, J.; Song, X.; Ding, B. J. Mater. Chem. 2012, 22, 1998–2006. 5. Rodríguez-Lorenzo, L.; Álvarez-Puebla, R. A.; Pastoriza-Santos, I.; Mazzucco, S.; Stéphan, O.;


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