Satellite Nanoassemblies for Excellent Extra

Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 44027−44037

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Au-Protected Ag Core/Satellite Nanoassemblies for Excellent Extra-/ Intracellular Surface-Enhanced Raman Scattering Activity Zhiqiang Zhang,*,†,§ Kazuki Bando,† Atsushi Taguchi,† Kentaro Mochizuki,† Kazuhisa Sato,‡ Hidehiro Yasuda,‡ Katsumasa Fujita,*,† and Satoshi Kawata† †

Department of Applied Physics, Osaka University, Suita, Osaka 565-0871, Japan Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Ibaraki, Osaka 567-0047, Japan § CAS Key Lab of Bio-Medical Diagnostics, Suzhou Institute of Biomedical Engineering and Technology, Chinese Academy of Sciences, 215163 Suzhou, China ‡

S Supporting Information *

ABSTRACT: Silver nanoparticles (AgNPs) and their assembled nanostructures such as core/satellite nanoassemblies are quite attractive in plasmonic-based applications. However, one biggest drawback of the AgNPs is the poor chemical stability which also greatly limits their applications. We report fine Au coating on synthesized quasi-spherical silver nanoparticles (AgNSs) with few atomic layers to several nanometers by stoichiometric method. The fine Au coating layer was confirmed by energy-dispersive X-ray spectroscopy elemental mapping and aberration-corrected high-angle annular darkfield scanning transmission electron microscopy. The optimized minimal thickness of Au coating layer on different sized AgNSs (22 nm [email protected] nm Au, 44 nm [email protected] nm Au, 75 nm [email protected] nm Au, and 103 nm [email protected] nm Au) was determined by extreme chemical stability tests using H2O2, NaSH, and H2S gas. The thin Au coating layer on AgNSs did not affect their plasmonic-based applications. The core/satellite assemblies based on Ag@Au NPs showed the comparable SERS intensity and uniformity three times higher than that of noncoated Ag core/satellites. The Ag@Au core/satellites also showed high stability in intracellular SERS imaging for at least two days, while the SERS of the noncoated Ag core/satellites decayed significantly. These spherical Ag@Au NPs can be widely used and have great advantages in plasmon-based applications, intracellular SERS probes, and other biological and analytical studies. KEYWORDS: core/satellite, nanoassemblies, Au coating, silver nanoparticles, SERS

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INTRODUCTION

Silver nanoparticles (AgNPs) have great plasmonic properties and have been extensively studied for enhancement of optical effects such as SERS.16−18 However, AgNPs are unstable under harsh chemical environments such as hydrogen peroxide (H2O2), and their localized surface plasma resonance (LSPR) dampens significantly once they are exposed to sulfide.19−21 In biomedical applications, the release of Ag ions from the bulk nanoparticles contributes to the toxicity of AgNPs for the cell.22,23 To overcome these disadvantages, AgNPs can be coated with a thin oxide layer such as TiO2 or SiO2.24−27 However, the oxide layers coated on the AgNPs usually are porous with some tiny pinholes. Recently, Ren et al. developed a pinhole-free SiO2 coating on the citrate stabilized Ag nanospheres (AgNSs) by assistance with NaBH4 treatment.28 Although the SiO2 layer coating can be perfectly coated on AgNPs, this kind of coating may hinder the direct surface

Core/satellite nanostructures assembled from plasmonic metal nanoparticles are particularly attractive in the fields of catalysis,1,2 optical sensing,3 and surface-enhanced Raman scattering (SERS)4−7 because these nanoassemblies provide multiple enhanced electromagnetic field locations (hot spots). This type of structure can generate hot electrons for catalysis under excitation light,8 sensitively respond to refractive index of the surrounding medium,9 and enhance the molecular spectroscopy by surface plasmonic resonance.10,11 The plasmonic properties of core/satellite nanoassemblies depend on several parameters such as the size of the core and/or satellite, shape of the core, composition of the core and satellite, and interparticle distance between core and satellite.12−14 From the material point of view, the silver core/satellite has much stronger electromagnetic field coupling than the gold core/satellite because of the intrinsic advantages of the silver over the gold such as strong plasmon strength or low plasmonic losses in the ultraviolet and visible regions.15 © 2017 American Chemical Society

Received: October 2, 2017 Accepted: November 24, 2017 Published: November 24, 2017 44027

DOI: 10.1021/acsami.7b14976 ACS Appl. Mater. Interfaces 2017, 9, 44027−44037

Research Article

ACS Applied Materials & Interfaces

and the thickness of Au coating layer on AgNSs with atomic layer level (Supplementary Experimental Calculations) can be controlled. Here, there are two key points for successful Au coating on synthesized polycrystalline AgNSs. One is the pH of the growth solution should be higher than 12 to overcome the galvanic replacement reaction; otherwise, productivity of perfect Ag@Au decreases significantly. Second, the Au coating solutions (2 mM of Au(SO3)22−) were added into the AgNSs solution at the last step to trigger the Au growth on AgNSs. This addition manner can keep the pH of the solution from changing too much so that any galvanic replacement reaction can be avoided. First, the monodisperse quasi-spherical AgNSs with different sizes (22−103 nm) were synthesized using the seeded growth protocol (Figure S1).47 Second, Au coating on different sized AgNSs was carried out by optimized antigalvanic method,44 and there were no pits or voids on Ag@Au NPs (Figure S2), which means the galvanic replacement reaction was suppressed in the Au coating process. Typically, the EDX mapping images of 75 nm Ag@Au NPs showed clear Ag-core/Au-shell configuration with calculated increased Au thickness (Figure 1). To confirm

modification with functional thiol molecules. The other method is to fully alloy the Au/Ag core/shell nanoparticles into single crystalline Ag/Au alloy nanoparticles by high temperature annealing with a protective SiO2 layer.29 However, the application of this method is somehow limited because it requires a complex preparation procedure, and the chemical stability of alloyed nanoparticles depends on the ratio of Ag/Au core−shell nanoparticles before thermal annealing. Au coating on AgNPs is another way to improve the chemical stability of the AgNPs while retaining their advantageous plasmonic properties. However, the direct addition of AuCl4− ions into the AgNPs solution usually causes the etching of AgNPs because of the galvanic replacement reaction between AuCl4− ions and Ag atoms,30−32 which results from the different standard reduction potential of Ag+/Ag (0.80 V vs standard hydrogen electrode, SHE) and AuCl4−/Au (0.99 V vs SHE).33 Although the galvanic replacement reaction of the pair of Ag and AuCl4− ions has been used extensively to obtain various metallic hollow and caged nanostructures30,31,34−41 this reaction should be retarded or suppressed in the process of Au coating. Recently, several groups have contributed to the Au coating on the anisotropic silver nanocrystals such as pentagonal nanorods, nanodecahedras, nanocubes, nanooctahedras, and nanoplates.42−45 In these studies, the Au layer was deposited on the crystallized Ag facets by suppressing the galvanic replacement reaction. For example, Qin et al.43 reported Au coating on Ag nanocubes with few Au atomic layers by introducing a strong reducing agent (ascorbic acid/ NaOH or NaBH4) to retard the galvanic reaction; Yin et al.44 reported Au coating on Ag nanoplates with several nanometers of Au by using a gold(I) sulfite complex (0.111 V vs SHE46) to suppress the galvanic replacement reaction. Compared with these crystalline anisotropic AgNPs, the fine Au coating on spherical Ag nanoparticles (AgNSs) was seldom reported, which may due to the fact that the synthesized AgNSs are usually polycrystalline or hybrid crystalline nanoparticles consisting of complex surface facets. Although Yin et al.44 also showed thick Au coating (∼7.5 nm) on 45 nm AgNSs, this Au layer was too thick to utilize the plasmonic property of AgNSs. Therefore, the controllable Au coating on AgNSs should be studied not only because the AgNSs have the highest shape stability and have been widely used for plasmonics research and applications but also because they were often used as a basic element for self-assembled nanostructures such as core/satellite assemblies.8 In this work, we demonstrated the finely controllable coating of Au layer with atomic layers to a few nanometers on quasi-spherical Ag nanoparticles with the diameters of 22, 44, 75, and 103 nm. The Au coating layer on AgNSs was confirmed by energy-dispersive X-ray spectroscopy (EDX) and scanning transmission electron microscopy (STEM). By chemical stability tests under harsh conditions of H2O2, NaSH, and H2S gas, the optimal thicknesses of Au coating layer on different sized AgNSs were determined. Core/ satellite assemblies were fabricated using 44 nm Ag@Au, 75 nm Ag@Au, and 103 nm Ag@Au as the cores and 22 nm Ag@Au as satellites. These core/satellite superstructures showed excellent and stable extracellular and intracellular SERS activity.

Figure 1. STEM and EDX mapping images of 75 nm Ag@Au NPs with different calculated Au thickness from 0 to 4.5 nm. All scale bars are 50 nm.

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the validation of stoichiometric calculation for controllable Au coating on AgNSs, the experimental and theoretical values of Au percentage in Ag@Au NPs with different Au thickness were compared. As shown in Figure 2, the Au percentage obtained by EDX elementary analysis matched well with the theoretical values, which indicates that the thickness of Au coating layer can be controlled by stoichiometric calculation. This stoichio-

RESULTS AND DISCUSSION Controllable Au Coating on AgNSs. The mass ratio of Au precursor to the core AgNSs is the key point to control the thickness of Au coating layer on different sized AgNSs. By stoichiometric calculation, the size of quasi-spherical AgNSs 44028


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the change in full-width at half-maximum (fwhm) of extinction spectra changed in different trends with increasing the thickness of the Au layer. For 22 nm AgNSs, the LSPR peak at 400 nm wavelength becomes broader for increased Au thickness. At the same time, a new peak appears at around 500 nm wavelength for thicker Au coating. For 44 nm AgNSs, the LSPR spectrum has a trend similar to that of 22 nm AgNSs with peak broadening and appearance of a second peak for the increased Au thickness. For larger AgNSs, they show a similar behavior in LSPR peak shift and broadening with the increase in Au thickness. For example, the 75 nm AgNSs exhibits the peak wavelength shift from 444 to 498 nm, and the 103 nm AgNSs show the peak shift from 484 to 530 nm with the increase in Au thickness. The color changes in photos of Ag@Au solutions also showed the different changes in Au coating on four AgNPs (Figure S7). The finite-difference time-domain (FDTD) simulation was used to further understand how the Au coating layer affects the optical spectrum of AgNSs. As shown in Figures S8A−D, the calculated extinction spectrum includes the scattering and absorption spectra of Ag@Au NPs and shows a size-dependent effect of different Au thickness on the AgNSs. Overall, the Au layer greatly contributes to the broadened absorption of smaller AgNSs and mainly affects the red-shift of LSPR peak of larger AgNSs. For example, for 22 nm AgNSs, the LSPR peak at 400 nm decreased in intensity with increased thickness of Au coating layer. At the wavelength of 400 nm, Au behaves as an absorptive dielectric but not as a metal. As a consequence, the presence of thin Au layer dampens plasmon oscillation of Ag NP, and the plasmon strength at the plasmon peak becomes lower. Meanwhile, the plasmon mode of Au appears at around 520 nm with increasing shell thickness. For 103 nm Ag@Au NPs, the dipole plasmon mode shifted from 485 nm to longer wavelength as Au thickness was increased. Similar to the case of 22 nm, the peak at 400 nm is dampened with the presence of Au coating. To examine the effect of the Au shell on the field enhancement of the nanoparticles, optical near-field FDTD simulations were also carried out. Typically, the 75 nm Ag nanosphere was selected as the model for demonstration. As shown in Figure S8E, the overall near-field enhancement decreases when the thickness of Au shell increases from 1 to 3 nm. The image of electromagnetic (EM) field distribution around the Ag@Au nanoparticle shows that the highest EM field is distributed at the outer surface layer of the Au shell and decays gradually toward the outside of the particle. However, it should be noticed that the highest EM field intensity for each Ag@Au nanoparticle is comparable, which indicates their similar plasmonic performance. Evaluation of Chemical Stability of Ag@Au NPs. One important issue for the Au coated AgNSs is their stability under harsh treatments such as hydrogen peroxide (H2O2), sulfide (NaSH), and hydrogen sulfide (H2S). Generally, the Ag atoms cannot be etched or escape from the inner part with a perfect Au coating layer, which depends on the quality and thickness of the Au layer. However, if there are some defects such as pinholes on the Au coating layer, the inner Ag atoms will be etched or oxidized gradually by the penetrated chemical species. Besides, the thicker Au layer affects the plasmonic properties of Ag@Au NPs, especially for smaller AgNSs. Therefore, a pinhole-free Au layer with minimum thickness is expected for Ag@Au NPs.

Figure 2. Au percentage in 75 nm Ag@Au NSs from the EDX analysis and stoichiometric calculation.

metric method can also be used for controllable Au coating on 22, 44, and 103 nm AgNSs, as shown in EDX mapping images (Figure S3−S5) and Au percentage (Figure S6). Effect of Au Coating Layer on Optical Properties of Ag@Au NPs. Experimentally, UV−vis spectrum was used to investigate the effect of different Au thickness on differently sized AgNSs. As shown in Figure 3, the shift of LSPR peak and

Figure 3. Normalized UV−vis spectrum of Au coated AgNSs with different thicknesses of the Au coating layer (upper) and the shift of LSPR peak and the change in fwhm (lower): (A) 22 nm Ag@Au, (B) 44 nm Ag@Au, (C) 75 nm Ag@Au, and (D) 103 nm Ag@Au. The histogram is the LSPR peak and the change in the fwhm of the UV− vis spectrum. The Au thickness values in each spectrum of Ag@Au NSs are calculated according to stoichiometric method. 44029


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are attributed to the decrease in extinction peak of Ag@Au NPs after 4 days, especially for the larger Ag@Au NPs. In fact, some post-treatment methods such as centrifugation48−50 can be used to separate these Au hollow nanoparticles from Ag@Au NPs to be used for other applications. For the NaSH treated Ag@Au NPs, the dampening of plasmonic AgNPs may be attributed to the sulfidation on Ag surface to form Ag2S shell.20,21,51 The SEM images (Figure S15) showed aggregated Ag@Au NPs without any morphology change, which confirmed that the damping of extinction spectrum was resulted from the sulfidation effect. Also, there is always a difference in intensity at extinction peak between the control and NaSH treated sample even for the thick Au coated AgNSs. To further understand the effect of sulfidation on Ag@ Au NPs, the 55 nm Au nanoparticles immobilized on the substrate were used for reference tests. The NaSH was also found to cause the decrease in the extinction at LSPR peak of AuNPs (Figure S16), which indicated that the sulfidation effect can also occur on the gold surface. Similarly, the sulfidation also happened in the H2S gas test, which depended on the Au thickness. These harsh chemical stability tests can help to select appropriate Ag@Au NPs with different Au thicknesses for plasmonic-based applications. For example, the results of H2S gas test indicate that the thicker Au coating on AgNSs can resist the degradation in SERS or tip-enhanced Raman scattering measurements due to H2S atmospheric environment while maintaining the plasmonic property. To choose the proper Au thickness on AgNSs for intracellular SERS application, this work mainly considers the stability under harsh H2O2 treatment because the chemical environments in living cells should be more moderate compared to those harsh conditions. In addition, there is a trade-off between Au thickness and enhancement of Raman scattering, and it would be beneficial for microscopic sensing or imaging with SERS to choose conditions that give higher enhancement. On the basis of the above consideration, the different Au thicknesses of the different sized AgNSs from H2O2 test for aqueous SERS application were chosen: 22 nm [email protected] nm Au, 44 nm Ag@ 1.8 nm Au, 75 nm [email protected] nm Au, and 103 [email protected] nm Au. Characterization of H2O 2 Treated Ag@Au NPs. Aberration-corrected high-angle annular dark-field scanning TEM (HAADF-STEM) was used to further confirm the quality of the Au coating layer on AgNSs. Because of limitations in observable thickness with HAADF-STEM, the 22 nm Ag@Au and 44 nm Ag@Au NPs, which were treated by 0.8 M H2O2, were chosen for core@shell confirmation, and only two Au thickness were taken. As shown in Figure 5, the 22 nm [email protected] nm Au NPs showed a bright shell (Figure 5A), and 3.5 nm Au coating layer apparently showed a thick shell (Figure 5B). Here, the 0.9 nm Au coating layer corresponds to about 4 atomic layers of Au. For the 44 nm [email protected] nm Au NPs (Figure 5C) and 44 nm [email protected] nm Au NPs (Figure 5D), the distribution of Au showed wrinkled mapping. Unlike the results from Qin’s and Yin’s works on single crystalline Ag nanocubes and nanoplates,43,44 these polycrystalline Ag@Au NPs showed nonuniform contrast of Au coating layer in the HAADFSTEM images. Even though the AgNSs consist of polycrystalline facets, Au coating can still provide an excellent resistance against the harsh H2O2 treatment. Besides, Figure 6 clearly shows the few atomic Au layers and Au lattices on 22 nm AgNSs. It is worth noting that the thicknesses of Au coating layers on 22 nm Ag and 44 nm

The stabilities of Ag@Au samples before and after treatment with 0.8 M of H2O2 for 4 days (Figure S9), 10 μM of NaSH for 1 day (Figure S10), and 4350 ppm of H2S gas for 2 h (Figure S11) were compared. The H2O2 and NaSH tests were carried out in nanoparticles solution, and the test for the H2S gas was performed by using Ag@Au NPs immobilized on glass substrates and incubated in H2S gas. To quantitatively evaluate the chemical stability of Ag@Au NPs, the UV−vis extinction spectra (Figure S12 and S13) were measured, and the relative intensity percentage of the spectra between the chemically treated Ag@Au NPs and the control sample was used. As shown in Figure 4, the stability of Ag@Au NPs became higher with increase in Au thickness against different chemical treatments. From the SEM images of H2O2 treated Ag@Au NPs (Figure S14), it can be seen that some Au hollow nanoparticles with pinholes exist in each Ag@Au NP if the thickness of the Au layer is too thin. Generally, these Au hollow nanoparticles came from the imperfect Au coated AgNSs and

Figure 4. Stability of (A) 22 nm Ag@Au NPs, (B) 44 nm Ag@Au NPs, (C) 75 nm Ag@Au NPs, and (D) 103 nm Ag@Au NPs after treatment with 0.8 M of H2O2 for 4 days (left), 10 μM NaSH for 1 day (middle), and 4350 ppm of H2S gas for 2 h (right). The stability here is given by the percentage value of the relative intensity ratio at the LSPR peak between the chemically treated Ag@Au and nontreated sample. 44030


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particles. Unlike previously reported methods using dithiol or cysteamine as the linker,3,5,53 our method using PLL allows further modification or functionalization of the nanoparticles’ surface with thiol-based SERS probes. Figure 7 shows the UV−vis spectra and SEM images of core/ satellites. As expected, all the core/satellites showed two resonance peaks showing the basic plasmonic mode of satellite nanoparticles around 400 nm and the plasmonic resonance coupling between core and satellites in the range of 600 to 700 nm, which was also confirmed from the color change in the scattering images. The SEM images showed that smaller satellite nanoparticles assembled on the surface of Ag core NPs. For the Ag@Au NPs core/satellites (Figures 7E−G), the longer resonance peak showed a profile much clearer than that of AgNSs core/satellites (Figures 7B−D), while the peak position shifted between 550 to 600 nm. Also, the extinction at the shorter wavelength is much lower compared with AgNSs core/ satellites. From the SEM images, it can be seen that the number of satellite nanoparticles on the core NPs is less than AgNSs core/satellites. The difference in the number of satellite nanoparticles on AgNSs and Ag@Au NPs may be attributed from the different surface charge of satellite nanoparticles. The ζ-potential measurement confirmed that the citrate capped 22 nm AgNSs have surface potential higher than that of PVPcapped Ag@Au NPs (Figure S18). Therefore, the electrostatic attractive force between the positively charged PLL on the PVP-capped core NPs and citrate-stabilized 22 nm AgNSs is higher than that on the PVP-capped 22 nm Ag@Au NPs. The SERS activities of AgNSs and Ag@Au NPs-based core/ satellite assemblies were compared at the single particle level. The low density of the core AgNSs on the substrate was controlled by short incubation time, and the distribution was confirmed by dark-field imaging (Figure S19). After adsorption of satellite nanoparticles on the core NPs, the color of the core NPs changed from green to orange. The 4-mercaptobezoic acid (p-MBA) molecules were used as the SERS probe because the thiol-based probes can form a uniform self-assembled monolayer on the surface of silver and gold nanoparticles. The SERS spectrum of p-MBA modified core/satellite assemblies was measured at 594 nm laser wavelength. Typically, the dark-field image of 75 nm Ag@ 3 nm Au core/satellites showed isolated orange colored spots (Figure 8A), and the SERS mapping image was obtained at the same position (Figure 8B). To show the SERS activity of the core/satellite assemblies, ten hot spots marked in the dark-field and SERS mapping image were randomly selected, and their SERS spectrum showed similar intensity (Figure 8C), which means the Ag@Au core/satellites showed the uniformity of SERS signal at least three times higher than that of noncoated AgNSs core/satellites. It also should be noted that the SERS spectra in Figure 8C are from the single pixel in the center of the spots, which indicated the excellent SERS activity of core/satellites. The SERS activities of different sized noncoated Ag core/ satellites and Ag@Au core/satellites nanoassemblies (44 nm Ag, 44 nm Ag@Au, 75 nm Ag, 75 nm Ag@Au, 103 nm Ag, and 103 nm Ag@Au) were evaluated under the same irradiation conditions. As shown in Figure 8D, the SERS intensity of Ag@ Au core/satellites assemblies showed values similar to those of AgNSs core/satellites assemblies except that 44 nm AgNSs core/satellites with a larger error bar and lower intensity. It should be noted that the productivity and uniformity of 44 nm AgNSs core/satellites were lower than others, which can be seen from the broadened spectrum and lower intensity at the

Figure 5. Typical HAADF-STEM images of H2O2 treated Ag@Au NPs. (A) 22 nm [email protected] nm Au, (B) 22 nm [email protected] nm Au, (C) 44 nm [email protected] nm Au, and (D) 44 nm [email protected] nm Au.

Figure 6. Magnified HAADF-STEM image of Au lattice on 22 nm Ag NSs with 4 atomic Au layers (A) and 3.5 nm Au layer (B).

AgNSs measured by HAADF-STEM were approximate to the stoichiometric calculation values. Therefore, the thickness of the Au coating layer on AgNSs can be controlled from several atomic layers to a few nanometers. SERS Activity of Core/Satellite Assemblies. The above data have clearly shown that the Au coating layer can greatly enhance the chemical stability of AgNSs but may affect their plasmonic properties. Therefore, it is worth studying how much this Au layer affects their SERS activity. However, the SERS signal from isolated single AgNSs or Ag@Au NPs is typically too weak to evaluate the SERS activity precisely. It is noticed that the core/satellites structures assembled by nanoparticles have multiple hot spots provided by the plasmonic resonance between the core nanoparticle and small satellite nanoparticles.8,52 In this work, the core/satellites nanoassemblies were made by using poly-L-lysine (PLL) as the positively charged linker to attach satellite NPs onto the core NPs, as shown in Figure 7A. The satellite NPs were not only immobilized on the surface of the core nanoparticles but also on the substrates as a background. The 44 nm [email protected] nm Au, 75 nm [email protected] nm Au, and 103 nm [email protected] nm Au nanoparticles were used for core nanoparticles, and 22 nm [email protected] nm Au nanoparticles were used for satellite nano44031


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Figure 7. (A) Preparation procedure of core/satellites (C/S) nanostructures. (B−D) and (E−G) are the UV−vis spectrum and SEM images of 44, 75, and 103 nm AgNSs and Ag@Au NPs core/satellites, respectively. The inserts in the UV−vis spectrum are the scattering photos of AgNSs core (left) and AgNSs core/satellites (right) on the glass substrates. All scale bars are 100 nm. It should be noted that these UV−vis extinction spectra of core/satellites did not include the contribution of smaller satellite NPs. The UV−vis spectra, including the core/satellites and smaller satellite NPs, are shown in Figure S17.

environment and hold potential applications greater than those of pure silver core/satellites. Intracellular SERS Imaging of Core/Satellites Assemblies. Plasmonic nanoparticles have been extensively used as SERS probes to measure some information inside the living cell54 such as pH,55−57 redox potential,58,59 superoxide anion radical,60 and the endocytosis pathway.61,62 In these applications, the chemical stability of the nanoparticle is also a vital issue for intracellular sensing. For core/satellites assemblies, there have been many studies on plasmonic optical sensors and SERS applications.1,3,5,9,63−68 However, as far as we know, there is no report demonstrating their application on intracellular SERS imaging. Four types of nanoparticles, including 75 nm Ag and Ag@Au core/satellites and 44 nm Ag and Ag@Au NPs were chosen to compare intracellular SERS stability. The immobilized 75 nm core/satellites assemblies were first modified with p-MBA molecules and then were released from the glass substrate by sonication and collected in H2O. Similar to the immobilized assemblies, the UV−vis spectrum of core/satellites solution showed second extinction peaks around 650 nm for Ag core/ satellites and 580 nm for Ag@Au core/satellites, as shown in Figure 9A. Moreover, the SEM and DF-STEM images clearly showed the asymmetric core/satellites configuration after being released from the substrates (Figures 9B−E).

range between 500 and 600 nm in the extinction spectrum (Figure 7B). Besides, the shorter error bars in SERS intensity of three different sized Ag@Au core/satellite assemblies indicated that nanoassemblies composed of Ag@Au nanospheres have better uniformity. From the view of plasmonic properties, the Au coating layer on AgNSs affects the LSPR properties, which can be seen in Figure 3 and Figure S8. Although the number of satellite nanoparticles on the Ag@Au core is less than that the case of AgNSs core/satellites (Figure 7), they showed comparable SERS intensities, indicating a higher SERS enhancement with Au coating. The SERS stability of AgNSs and Ag@Au NPs core/satellites under harsh chemical treatments was also examined. The 75 nm Ag and 75 nm Ag@Au core/satellites were treated by 0.5 M H2O2 and 0.8 M H2O2 for 5 min, respectively. Once the addition of H2O2, the 75 nm AgNSs core/satellites were gradually etched and finally disappeared, which can be seen in the dark-field imaging video (Video S1). For Au coated AgNS core/satellites, there is no change in the dark-field image, and the SERS intensity did not change after 5 min treatment of H2O2, as shown in Figure S20. The typical SERS spectrum of core/satellites before and after treated with H2O2 did not change (Figure S21). Therefore, the Ag@Au NPs core/ satellites present a high SERS stability in the harsh chemical 44032


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Figure 9. (A) UV−vis extinction spectrum of collected 75 nm Ag core/satellites (blue line) and 75 nm Ag@Au core/satellites (orange line). The inserts are the photos of collected core/satellites solutions. (B and C) Typical SEM and STEM images of collected 75 nm Ag core/satellites. (D and E) Typical SEM and STEM images of 75 nm Ag@Au core/satellites. All scale bars are 50 nm.

Figure 8. Typical dark-field image of 75 nm Ag@Au core/satellites (A), the SERS mapping image at the same position (B), and the SERS spectra of randomly selected ten hot spots (C) marked in the darkfield and SERS mapping image. The SERS spectrum is the raw data from the single pixel centered in hot spots without normalization. The pixel number of the SERS image (B) is 290 × 270. (D) Comparison of SERS intensity of different core/satellites assemblies measured under the same condition (laser wavelength: 594 nm, laser power: 0.65 mW, integration time: 500 ms). Thirty pixels were selected and averaged for the plots of SERS intensity. The error bar shows the standard deviation. Figure 10. Dark-field image and intracellular SERS mapping image (peak intensity at 1600 cm−1) of different NPs@p-MBA probes incubated with a macrophage cell (J774A.1) for different times: (A− C) 75 nm Ag core/satellites for 6, 24, and 48 h, respectively; and (D− F) 75 nm Ag@Au core/satellites for 6, 24, and 48 h, respectively. The intensity scales in all SERS mapping images are the same. All scale bars are 10 μm.

Once the p-MBA modified core/satellites nanoparticle solution was introduced to the medium in the cell dish, these nanoparticles were internalized by the cellular uptake within a few hours.69 As shown in Figure 10, the 75 nm Ag core/ satellites composed of pure AgNSs showed clear SERS intensity within 1 day, but it dropped greatly after 2 days of incubation. For Ag@Au core/satellites, their SERS intensity did not change within 2 days, which indicates that the Ag@Au core/satellites showed higher intracellular SERS stability. SERS images at 5 different areas were measured; all areas showed the same trend. The decrease in SERS signal of Ag core/satellites may be due to the desorption of p-MBA, which resulted from the destroyed Ag−S bond between p-MBA molecule and Ag atoms. The loss of Ag−S bond can be attributed to the oxidation of surface Ag atoms on AgNSs due to the complicated chemical environment inside the cell. Furthermore, the desorption of p-MBA molecules from the nanoparticles will lead to the aggregation of nanoparticles because the p-MBA also acts as a stabilizer for the nanoparticle’s stability. In fact, such an indication can be seen from the dark-field image of core/satellites incubated cell. The 75 nm Ag core/satellites showed many isolated red/orange dots at 6 h (Figure 10A). However, it showed larger and

brighter orange dots after 2 days of incubation (Figure 10C). However, the dark-field image of the 75 nm Ag@Au core/ satellites incubated cell did not change within 2 days (Figures 10D−F). To further prove this hypothesis, the intracellular SERS images of the 44 nm AgNSs (Figures S22A−C) and 44 nm Ag@Au NPs (Figures S22D−F) within 2 days and their darkfield images were measured. As expected, the 44 nm Ag@Au NPs showed stable SERS activity within 2 days, and there was no change in the dark-field images. The 44 nm AgNSs showed SERS intensity higher than that of the 44 nm Ag@Au NPs at 6 h of incubation, but its SERS signal decreased significantly after 1 day. Additionally, the color of dark-field image showed bright blue at 6 h but finally changed to yellow/orange after 2 days of incubation. The yellow/orange color should be from the larger 44033


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nm Ag seed NPs, a mixture of 30 mL of glycerol and 40 mL of H2O in a 100 mL flask was heated in an oil bath at 106 °C for 30 min. Then, 18 mg of silver nitrate dissolved in 1.0 mL of H2O was added into the above mixture under vigorous magnetic stirring. After 1 min, 2.0 mL of sodium citrate (3%) was added. The reaction mixture was stirred for another 1 h and cooled to room temperature. The obtained 22 nm Ag seed NPs was used as the seeds to produce large sized Ag nanoparticles. In a 60 mL vial, 4.6 mL of glycerol and 120 mg of PVP K30 were added into 27.0 mL of H2O and stirred for 20 min at room temperature. Then, different volumes of 22 nm AgNPs solution were added. After 20 min, 230 μL of a mixture of diamine silver complex (20 mg silver nitrate in 1 mL of water plus 220 μL of ammonium hydroxide, 28%) together with 18 mL of ascorbic acid solution (7.5 mg). The growth finished after 1 h. Au Coating on Colloidal AgNSs. The procedure of Au coating of AgNSs was performed according to the modified protocol.44 The growth solution of Au was prepared by mixing 4.87 mL of H2O, 20 μL of 0.5 M HAuCl4, 50 μL of 1.0 M NaOH, and 60 μL of 0.5 M Na2SO3. The obtained mixture was left undisturbed for 12 h. For the Au coating of 75 nm AgNPs, a 3.0 mL of synthesized AgNSs solution, 1.0 mL of 5% PVP K30, 1.0 mL of 0.1 M L-AA, 150 μL of 1.0 M NaOH, and 70 μL of 0.1 M Na2SO3 were added into a 20 mL vial. Then, the Au coating started by addition of 20−200 μL of Au growth solution. After being shaken by hand for 10 s, the final mixture was incubated in a 45 °C oven overnight. To clean the Ag@Au NPs, the mixture was centrifuged at 3000−8000 rpm for 20 min and redispersed in a 5 mL of H2O. Chemical Stability Test of Au Coated AgNSs. One and a half milliliters of cleaned Ag@Au NPs solution was diluted to 2.5 mL by H2O. To test the stability of Ag@Au NPs under the treatment of H2O2, 80 μL of 30% H2O2 was added into the 0.8 mL of diluted Ag@ Au NPs and mixed by hand shaking for a few seconds. After 4 days, the mixture was centrifuged at 3000 rpm for 20 min and redispersed into 200 μL of H2O. For the control sample, 80 μL of H2O was added into the 0.8 mL of diluted Ag@Au NPs. To test the stability of Ag@Au NPs under the treatment of NaSH, 71 μL of H2O and 9 μL of freshly prepared 1 mM NaSH was added into 0.8 mL of diluted Ag@Au NPs and mixed by hand shaking for a few seconds. After 28 h, the mixture was centrifuged at 3000 rpm for 20 min and redispersed into 200 μL of H2O. To know the effect of hydrogen sulfides on the AgNPs and AuNPs, the immobilized NPs on glass substrates were used. The 8 × 16 mm of quartz slides were cleaned by ultrasonication in acetone, ethanol, 10 M NaOH, and H2O for 10 min and then immersed into 1 mg/mL of poly-L-lysine for 20 min. After drying by nitrogen flow, 100 μL of Ag@ Au NPs solution was dropped on the one side of PLL-quartz substrate overnight, followed by rinsing with H2O and drying by argon flow. The Ag@Au decorated quartz slide was placed in a 1.5 L glass tank. Hydrogen sulfide gas was generated by reaction of 1 mL of 500 mM NaSH (0.25 mmol) with extensive 1 M sulfuric acid in a 20 mL brown vial according to the literature.70 Then, the glass tank was covered by a glass plate, and the gap between the tank and the cover was sealed by vacuum grease. Preparation of Core/Satellite Nanostructure on Glass Substrate. The commercial cell culture dish was first cleaned by oxygen plasma at 100 sccm flow rate and 75 W for 2 min. The 1 mg/ mL of PLL solution was dropped onto a cleaned cover glass and incubated for 20 min, followed by washing with pure water and drying by nitrogen gas flow. Then, the diluted NPs solution was added at the middle of the cover glass. The density of the isolated NPs on the substrate was controlled by the incubation time. The distribution of the isolated NPs was confirmed under the dark-field microscope. Then, the PLL solution was deposited on the surface of the immobilized NPs for 6 h. The solution of 22 nm AgNPs and 22 nm [email protected] nm Au was added on the PLL modified NPs overnight. After being washed with pure water, the substrates were incubated with 0.5 mM of p-MBA for 2 h. Intracellular SERS Imaging. The immobilized core/satellite assemblies modified with p-MBA were released into H2O by ultrasonication and collected at a concentration of about 2 × 1010

sized aggregates of 44 nm AgNSs because of the desorption of p-MBA. Therefore, the pure Ag nanoparticles cannot maintain their stability for a long time inside the cell. It can be concluded that the Au coated AgNSs and their assembled nanostructures have great merit in maintaining the intracellular SERS activity. Discussion on the Quality of Core/Satellites Nanoassemblies. The Ag@Au NPs core/satellites assemblies showed their attractive extra/intracellular SERS activity. However, there is still room to improve their SERS performance such as uniformity and enhancement. For example, the Ag@Au nanoparticle core/satellites in Figure 9C show that there are no satellite nanoparticles on the top of the core particle. Although this naked top may be caused by asymmetrical configuration of our substrate-based preparation protocol and/or by evaporation force induced movement of top satellites during the drying process on the TEM mesh for characterization, some effects can be done to enhance the binding force between satellites and core particle by optimization of surface chemistry. In this way, the uniformity of the core/satellites will be further improved for better performance in SERS applications. For higher enhancement of Raman scattering, it might be useful to produce more hot spots by increasing the number of satellites on the core particles by multistep assembly of satellites.

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CONCLUSIONS In summary, we demonstrated a strategy for controllable Au coating on colloidal polycrystalline spherical Ag nanoparticles (22, 44, 75, and 103 nm) with few atomic layers to several nanometers. The Au coating layer was confirmed by EDX elemental mapping and HAADF-STEM. These Ag@Au NPs showed high chemical stability under H2O2 (0.8 M for 4 days), NaSH (10 μM for 1 day), and H2S gas (4530 ppm for 2 h) with optimized minimal Au thickness (22 nm [email protected] nm Au, 44 nm [email protected] nm Au, 75 nm [email protected] nm Au, and 103 nm Ag@ 0.9 nm Au). The thin Au coating layer on AgNSs slightly changed the shape of the LSPR spectrum but did not affect their plasmonic-based applications. As a demonstration, the core/satellite assemblies from Ag@Au NPs showed comparative SERS activity with AgNSs core/satellite assemblies, and their SERS activity was still retained even after extreme oxidant treatment. The Ag@Au core/satellites showed highly stable intracellular SERS activity. Because of the facile preparation protocol, these quasi-spherical Ag@Au NPs and the core/ satellite nanoassemblies will be widely used and have great promising applications in plasmonic, intracellular SERS tracking probes, and other biological and analytical studies.

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MATERIALS AND METHODS

Materials. Silver nitrate (AgNO3, 99.9%), gold(III) chloride trihydrate (HAuCl4·3H2O, 99.9%), sodium borohydride (NaBH4, 99%), poly-L-lysine solution (0.1% w/v), L-ascorbic acid (L-AA, 99.5%), poly(vinylpyrrolidone) (PVP K30, average Mw 40 000), 4mercaptobenzoic acid (4-MBA, 99%), NaSH, and sodium sulfite (Na2SO3, 98%) were purchased from Sigma-Aldrich. Ammonium hydroxide (NH4OH, 28%), hydrogen peroxide (H2O2, 30%), sodium hydroxide (NaOH, 98%), and trisodium citrate (C6H5O7Na3·2H2O) were purchased from Wako Pure Chemical Industries, Ltd. (Japan) and used without further purification. Milli-Q H2O (18.2 MΩ·cm−1) was used for all experiments. All other reagents were of analytical grade. All glasswares were rigorously cleaned in aqua regia (3:1, HCl:HNO3) and rinsed thoroughly with Milli-Q H2O before use. Synthesis of AgNSs. The differently sized AgNSs were prepared according to the modified Lee and Meisel method.47 To synthesize 22 44034


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Dynamic Tracking of Heterogeneous Nanocatalytic Processes by Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy. Nat. Commun. 2017, 8, 1−8. (2) Xie, W.; Walkenfort, B.; Schlücker, S. Label-Free SERS Monitoring of Chemical Reactions Catalyzed by Small Gold Nanoparticles Using 3D Plasmonic Superstructures. J. Am. Chem. Soc. 2013, 135 (5), 1657−1660. (3) Ode, K.; Honjo, M.; Takashima, Y.; Tsuruoka, T.; Akamatsu, K. Highly Sensitive Plasmonic Optical Sensors Based on Gold Core− Satellite Nanostructures Immobilized on Glass Substrates. ACS Appl. Mater. Interfaces 2016, 8 (32), 20522−20526. (4) Ruan, Q.; Shao, L.; Shu, Y.; Wang, J.; Wu, H. Growth of Monodisperse Gold Nanospheres with Diameters From 20 Nm to 220 Nm and Their Core/Satellite Nanostructures. Adv. Opt. Mater. 2014, 2 (1), 65−73. (5) Chang, H.; Kang, H.; Yang, J.-K.; Jo, A.; Lee, H.-Y.; Lee, Y.-S.; Jeong, D. H. Ag Shell−Au Satellite Hetero-Nanostructure for UltraSensitive, Reproducible, and Homogeneous NIR SERS Activity. ACS Appl. Mater. Interfaces 2014, 6 (15), 11859−11863. (6) Xu, L.; Kuang, H.; Xu, C.; Ma, W.; Wang, L.; Kotov, N. A. Regiospecific Plasmonic Assemblies for in Situ Raman Spectroscopy in Live Cells. J. Am. Chem. Soc. 2012, 134 (3), 1699−1709. (7) Xiong, W.; Sikdar, D.; Yap, L. W.; Premaratne, M.; Li, X.; Cheng, W. Multilayered Core−Satellite Nanoassemblies with Fine-Tunable Broadband Plasmon Resonances. Nanoscale 2015, 7 (8), 3445−3452. (8) Xie, W.; Schlücker, S. Hot Electron-Induced Reduction of Small Molecules on Photorecycling Metal Surfaces. Nat. Commun. 2015, 6, 1−6. (9) Prasad, J.; Zins, I.; Branscheid, R.; Becker, J.; Koch, A. H. R.; Fytas, G.; Kolb, U.; Sö nnichsen, C. Plasmonic Core−Satellite Assemblies as Highly Sensitive Refractive Index Sensors. J. Phys. Chem. C 2015, 119 (10), 5577−5582. (10) Gellner, M.; Steinigeweg, D.; Ichilmann, S.; Salehi, M.; Schütz, M.; Kömpe, K.; Haase, M.; Schlücker, S. 3D Self-Assembled Plasmonic Superstructures of Gold Nanospheres: Synthesis and Characterization at the Single-Particle Level. Small 2011, 7 (24), 3445−3451. (11) Gandra, N.; Abbas, A.; Tian, L.; Singamaneni, S. Plasmonic Planet−Satellite Analogues: Hierarchical Self-Assembly of Gold Nanostructures. Nano Lett. 2012, 12 (5), 2645−2651. (12) Yoon, J. H.; Yoon, S. Probing Interfacial Interactions Using Core−Satellite Plasmon Rulers. Langmuir 2013, 29 (48), 14772− 14778. (13) Rong, Z.; Xiao, R.; Wang, C.; Wang, D.; Wang, S. Plasmonic Ag Core−Satellite Nanostructures with a Tunable Silica-Spaced Nanogap for Surface-Enhanced Raman Scattering. Langmuir 2015, 31 (29), 8129−8137. (14) Cha, H.; Yoon, J. H.; Yoon, S. Probing Quantum Plasmon Coupling Using Gold Nanoparticle Dimers with Tunable Interparticle Distances Down to the Subnanometer Range. ACS Nano 2014, 8 (8), 8554−8563. (15) Rycenga, M.; Cobley, C. M.; Zeng, J.; Li, W.; Moran, C. H.; Zhang, Q.; Qin, D.; Xia, Y. Controlling the Synthesis and Assembly of Silver Nanostructures for Plasmonic Applications. Chem. Rev. 2011, 111 (6), 3669−3712. (16) Nie, S.; Emory, S. R. Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering. Science 1997, 275 (5303), 1102−1106. (17) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Single Molecule Detection Using SurfaceEnhanced Raman Scattering (SERS). Phys. Rev. Lett. 1997, 78 (9), 1667−1670. (18) Bhunia, S. K.; Zeiri, L.; Manna, J.; Nandi, S.; Jelinek, R. CarbonDot/Silver-Nanoparticle Flexible SERS-Active Films. ACS Appl. Mater. Interfaces 2016, 8 (38), 25637−25643. (19) Chen, R.; Nuhfer, N. T.; Moussa, L.; Morris, H. R.; Whitmore, P. M. Silver Sulfide Nanoparticle Assembly Obtained by Reacting an Assembled Silver Nanoparticle Template with Hydrogen Sulfide Gas. Nanotechnology 2008, 19 (45), 455604−455612.

assemblies/mL. The cell culturing and nanoparticle uptake procedures were carried out according to our previous work.62 Briefly, the macrophage cells (J774A.1) were cultured on a glass bottom dish in a Dulbecco’s modified Eagle’s medium (DMEM) solution at 37 °C and 5% CO2 overnight, and then 50 μL of core/satellites solutions were added and incubated for 6−48 h, followed by removing the culture medium, washing with PBS, and replacing with HBSS buffer solution. For comparison, 44 nm Ag and 44 nm Ag @1.8 nm Au NPs were used. Characterization Methods. All UV−vis extinction spectra were measured using a Shimadzu UV-3600 Plus UV−vis−NIR spectrophotometer. The HAADF-TEM images were obtained on a JEOL JEMARM200F transmission electron microscope. The SEM and EDS analysis were performed on a Hitachi S-9000 field-emission scanning electron microscope (FE-SEM). ζ-potential and size of the AuNPs was measured by Malvern Zetasizer Nano-ZS90 (Malvern Instruments Ltd., UK) at 25.0 ± 0.1 °C; the surface potential fitting model was Smoluchowski model. The Raman spectra were recorded by a Raman microscope system with line-scan mode described in the literature.71 After taking the dark-field image of the sample under halogen illumination with a color CCD (DS-Fi1c, Nikon), SERS spectra were collected at the same view-field using a dry 60× objective with a numerical aperture of 0.7. A 594 nm laser with a power of 0.65 mW at the objective and 500 ms integration time for each line were used for Raman measurement. To evaluate the SERS activity of the core/ satellites assemblies, we randomly chose 30 hot spots which showed orange color in the dark-field image. It must be noted that the all spectral profiles were raw data without smoothing and fitting. For intracellular SERS line scan imaging, the integration time was set at 200 ms/line with 75 lines. We used a water immersion 60× objective with a numerical aperture of 1.27.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b14976. Experimental calculations, Tables S1 and S2 for synthesis parameters for growth of large AgNSs and Au coating, and Figures S1−S22 (PDF) Dark-field Video S1 of etching of 75 nm AgNSs core/ satellites by 0.8 M H2O2 within 3 min (20× fps) (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhiqiang Zhang: 0000-0001-5418-3584 Atsushi Taguchi: 0000-0001-5815-2339 Katsumasa Fujita: 0000-0002-2284-375X Satoshi Kawata: 0000-0003-1631-9588 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the NOF of Osaka University for technical support of SEM and EDX characterization. The authors acknowledge supports from JSPS KAKENHI (Grant 26000011), NSFC (Grant 51405483), and NSFJS (Grant BK20140377).

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REFERENCES

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