Satellite Nanoassemblies for Excellent Extra

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Au-Protected Ag Core/Satellite Nanoassemblies for Excellent Extra-/Intracellular SERS Activity Zhiqiang Zhang, Kazuki Bando, Atsushi Taguchi, Kentaro Mochizuki, Kazuhisa Sato, Hidehiro Yasuda, Katsumasa Fujita, and Satoshi Kawata ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14976 • Publication Date (Web): 24 Nov 2017 Downloaded from http://pubs.acs.org on November 25, 2017

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Au-Protected Ag Core/Satellite Nanoassemblies for Excellent Extra-/Intracellular SERS 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

Key words: Core-satellite, Nanoassemblies, Au coating, Silver Nanoparticles, SERS

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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 EDX elemental mapping and aberration-corrected high-angle annular dark-field scanning TEM (HAADF-STEM). 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 such as 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 with three-times higher uniformity than that of non-coated Ag core/satellites. The Ag@Au core/satellites also showed the high stability in intracellular SERS imaging for at least two days, while the SERS of the non-coated 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.

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Introduction 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 filed locations (“hot-spots”). This type of structure can generate “hot-electron” for catalysis under excitation light,8 sensitively response to refractive index of 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 point view of material, the silver core/satellite has much stronger electromagnetic filed 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 ultraviolet and visible region.15 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) will damp 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 layer of oxide layer such as TiO2 or SiO2.24-27 However, the oxide layer coated on the AgNPs usually are porous and there are some tiny pin-holes. Recently, Ren et al. developed a pinhole-free SiO2 coating on the citrate stabilized Ag nanospheres (AgNSs) by assistant with NaBH4 treatment.28 Although the SiO2 layer coating can be perfectly coated on AgNPs, this kind

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of coating may hinder the direct surface 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 complex preparation procedure and 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 atoms30-32, 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.

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Compared with these crystalline anisotropic AgNPs, the fine Au coating on spherical Ag nanoparticles (AgNSs) was seldom reported, which may due to that the synthesized AgNSs are usually polycrystalline or hybrid crystalline nanoparticles consisted 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 is quite worth to be studied not only because the AgNSs have the highest shape stability and have been widely used for plasmonics research and applications, but also 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 few nanometers on quasi-spherical Ag nanoparticles with the diameters of 22 nm, 44 nm, 75 nm, and 103 nm. The Au coating layer on AgNSs were confirmed by EDX and STEM. By chemical stability tests under harsh condition of H2O2, NaSH, and H2S gas, the optimal thickness of Au coating layer on different sized AgNSs were determined. Core/satellite assemblies were fabricated by using 44 nm Ag@Au, 75 nm Ag@Au, 103 nm Ag@Au as the core and the 22 nm Ag@Au as satellites. This core/satellite superstructures showed excellent and stable extracellular and intracellular SERS activity.

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 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. The one is the pH of the growth solution should be higher than 12 to

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overcome the galvanic replacement reaction, otherwise productivity of perfect Ag@Au decreased significantly. Second, the Au coating solutions (2 mM of Au(SO3)2 2−) was added into the AgNSs solution at the last step to trigger the Au growth on AgNSs. This addition manner can keep the pH of solution unchanged too much so that any galvanic replacement reaction can be avoided. First, the monodisperse quasi-spherical AgNSs with different sizes (22 nm to 103 nm) were synthesized by seeded growth protocol (Figure S1).47 Secondly, Au coating on different sized AgNSs were carried out by optimized anti-galvanic method44 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 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 stoichiometric method can also be used for controllable Au coating on 22 nm, 44 nm, and 103 nm AgNSs, as shown in EDX mapping images (Figure S3-S5) and Au percentage (Figure S6).

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

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

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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 different sized AgNSs. As shown in Figure 3, the shift of LSPR peak and the change in full-width at half-maximum (FWHM) of extinction spectra changed in different trends with increasing the thickness of Au layer. For 22 nm AgNSs, the LSPR peak at 400 nm in wavelength becomes broader for increased Au thickness. At the same time, a new peak appears at around 500 nm in wavelength for thicker Au coating. For 44 nm AgNSs, the LSPR spectrum has a similar trend to 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 the increase of Au thickness. For example, the 75 nm AgNSs exhibits the peak wavelength shift from 444 nm to 498 nm and the 103 nm AgNSs show the peak shift from 484 nm to 530 nm with the increase of 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 FDTD simulation was used to further understand how the Au coating layer affects the optical spectrum of AgNSs. As shown in Figure S8A-D, the calculated extinction spectrum includes the scattering and absorption spectra of Ag@Au NPs showed 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 metal. As a consequence, the presence of thin Au layer dumps plasmon oscillation of Ag NP, and the plasmon strength at plasmon peak becomes lower. Meanwhile the plasmon mode of Au appears at around 520 nm with increasing shell thickness. For 103 nm

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Ag@Au NPs, the dipole plasmon mode is shifting from 485 nm to longer wavelength as Au thickness is increased. Similar to the case of 22 nm, the peak at 400 nm is dumped with the presence of Au coating.

Figure 3. The normalized UV-vis spectrum of Au coated AgNSs with different thickness of 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, (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. To examine the effect of the Au shell on the field enhancement of the nanoparticles, optical near-field FDTD simulation were also carried out. Typically, the 75 nm Ag nanosphere was

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selected as model for demonstration. As shown in Figure S8E, the overall near-field enhancement decreases when the thickness of Au shell increases from 1 nm 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 towards the outside of 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 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 affect 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. The stability of Ag@Au samples before and after treated with 0.8 M of H2O2 for 4 days (Figure S9), 10 µM of NaSH for 1 day (Figure S10), and 4350 ppm H2S gas for 2 hours (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 of Au

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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 NPs if the thickness of Au layer is too thin. Generally, these Au hollow nanoparticles came from the imperfect Au coated AgNSs and attribute to the decrease of extinction peak of Ag@Au NPs after 4 days, especially for the larger Ag@Au NPs. In fact, some post-treatment methods such as centrifugation methods48-50 can be used to separate these Au hollow nanoparticles from Ag@Au NPs and to be used for other applications. For the NaSH treated Ag@Au NPs, the damping of plasmonic of AgNPs may attribute 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 of 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 depends on the Au thickness. These harsh chemical stability tests can help to select appropriate Ag@Au NPs with the different Au thickness 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 TERS measurements due to H2S atmospheric environment with 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 since the chemical environments in living

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cells should be more moderate compared to those harsh conditions. In addition, there is a tradeoff between Au thickness and enhancement of Raman scattering, and it would be beneficial for microscopic sensing or imaging with SERS to choose conditions that gives higher enhancement. Based on above consideration, the different Au thickness of the different sized AgNSs from H2O2 test for aqueous SERS application was chosen, that is, 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.

Figure 4. The 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 treated with 0.8 M of H2O2 for 4 days (left), 10 µM NaSH for 1 day (middle), and 4350 ppm H2S gas for 2 hours (right). The stability here is given

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by the percentage value of the relative intensity ratio at LSPR peak between the chemically treated Ag@Au and non-treated sample. Characterization of H2O2 Treated Ag@Au NPs. The 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 limitation in observable thickness with HAADFSTEM, 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 bright shell (Figure 5A) and 3.5 nm Au coating layer apparently showed thick shell (Figure 5B). Here, the 0.9 nm Au coating layer corresponds to about 4 atomic layer 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 non-uniform 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 thickness of Au coating layers on 22 nm Ag and 44 nm 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 few nanometers.

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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, (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). SERS Activity of Core/Satellite Assemblies. Above data have clearly shown that the Au coating layer can greatly enhance the chemical stability of AgNSs, but it may affect their plasmonic properties. Therefore, it is worth to study 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

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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 nanoparticles. Unlike previously reported methods using dithiol or cysteamine as the linker3,5,53, our method using PLL allows further modification or functionalization of the nanoparticles’ surface with thiol based SERS probes.

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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 nm, 75 nm, 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 were shown in Figure S17. 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 nm 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 (Figure 7E-G), the longer resonance peak showed much clear profile than that of AgNSs core/satellites (Figure 7B-D) while the peak position shifted between 550 nm 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 satellites 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 zeta potential measurement confirmed that the citrate capped 22 nm AgNSs have higher surface potential 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 PVP-capped 22 nm Ag@Au NPs.

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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 were 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 was used as 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 were 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 non-coated 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.

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Figure 8. Typical dark-field image of 75 nm Ag@Au core/satellites (A) and the SERS mapping image at the same position (B), and the SERS spectra of randomly selected ten hot spots (C) marked in the dark-field 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) The 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). 30 pixels were selected and averaged for the plots of SERS intensity. The error bar shows the standard deviation. The SERS activities of different sized non-coated 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

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Ag@Au) were evaluated under the same irradiation condition. As shown in Figure 8D, the SERS intensity of Ag@Au core/satellites assemblies showed similar values to 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 broaden spectrum and lower intensity at the range between 500 nm 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 nanoasemblies composed of Ag@Au nanospheres have better uniformity. From the view of plasmonic properties, the Au coating layer on AgNSs affect the LSPR properties which had been seen in Figure 3 and Figure S8. Although the number of satellites nanoparticles on the Ag@Au core is less than the case of AgNSs core/satellites (Figure 7), they showed the 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 AgNSs 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 environment and hold great potential applications than pure silver core/satellites.

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Intracellular SERS Imaging of Core/Satellites Assemblies. Plasmonic nanoparticles have been extensively used as SERS probes to measure some information inside the living cell,54 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 sensor or SERS applications.1,3,5,9,63-68 However, as far as we known, there is no report demonstrated their application on intracellular SERS imaging.

Figure 9. (A) The 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) are typical SEM and STEM images of collected 75 nm Ag core/satellites. (D) and (E) are typical SEM and STEM images of 75 nm Ag@Au core/satellites. All scale bars are 50 nm.

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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 released from the substrates (Figure 9B-E). Once the p-MBA modified core/satellites nanoparticles solution was introduced to the medium in cell dish, these nanoparticles were internalized by the cellular uptake within 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’ 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 of areas showed the same trend. The decrease of SERS signal of Ag core/satellites may 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 attribute to the oxidation of surface Ag atoms on AgNSs because of 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 acted as stabilizer for 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 hours (Figure 10A). However, it showed larger and brighter orange dots after 2 days’ incubation

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(Figure 10C). However, the dark-field image of 75 nm Ag@Au core/satellites incubated cell did not change within 2 day (Figure 10D-F).

Figure 10. Dark-field image and intracellular SERS mapping image (peak intensity at 1600 cm-1) of different NPs@p-MBA probes incubated with macrophage cell (J774A.1) for different time. (A-C) 75 nm Ag core/satellites for 6 hours, 24 hours, and 48 hours, respectively. (D-F) 75 nm Ag@Au core/satellites for 6 hours, 24 hours, and 48 hours, respectively. The intensity scale in all SERS mapping images are same. All scale bars are 10 µm. To further prove this hypothesis, the intracellular SERS images of the 44 nm AgNSs (Figure S22A-C) and 44 nm Ag@Au NPs (Figure S22D-F) within 2 days and their dark-field images were measured. As expected, the 44 nm Ag@Au NPs showed stable SERS activity within 2 days and there was no change in dark-field images. For 44 nm AgNSs, it showed higher SERS intensity than 44 nm Ag@Au NPs at 6 hours’ incubation, but its SERS signal decreased significantly after 1 day. Additionally, the color of dark-field image showed bright blue at 6 hours, but finally changed to yellow/orange after 2 days’ incubation. The yellow/orange color

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should be from the larger sized aggregates of 44 nm AgNSs because of the desorption of pMBA. 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 About the Quality of Core/Satellites Nanoassemblies. The Ag@Au NPs core/satellites assemblies had shown 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 shows that there are no satellite nanoparticles on the top of 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. By 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.

Conclusions In summary, we demonstrated a strategy for controllable Au coating on colloidal polycrystalline spherical Ag nanoparticles (22 nm, 44 nm, 75 nm, 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 hours) with optimized minimal

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Au thickness (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). The thin Au coating layer on AgNSs slightly changed the shape of 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.

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), 4-mercaptobenzoic acid (4MBA, 99%), NaSH, and sodium sulfite (Na2SO3, 98%) were purchase 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 glass wares were rigorously cleaned in aqua regia (3:1, HCl: HNO3) and rinsed thoroughly with MilliQ H2O before use. Synthesis of AgNSs. The different sized AgNSs were prepared according to the modified Lee and Meisel method.47 To synthesis 22 nm Ag seed NPs, a mixture of 30 mL glycerol and 40 mL

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H2O in a 100 mL flask was heated in an oil bath with 106°C for 30 min. Then, 18 mg silver nitrate dissolved in 1.0 mL H2O was added into above mixture under vigorous magnetic stirring. After 1 min, 2.0 mL sodium citrate (3%) was added. The reaction mixture was stirred for another 1 h and cooled down 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 glycerol and 120 mg PVP K30 were added into 27.0 mL 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 diamine silver complex (20 mg silver nitrate in 1 mL water plus 220 µL ammonium hydroxide 28%), together with 18 mL 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 4.87mL 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 a handshaking for 10 seconds, the final mixture was incubated in a 45 °C oven for 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. 1.5 mL 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 few seconds. After 4 days, the mixture was centrifuged at 3000 rpm for 20 min, and redispersed into 200 µL H2O. For the control sample, 80 µL of H2O was added into the 0.8 mL

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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 fresh prepared 1 mM NaSH was added into 0.8 mL of diluted Ag@Au NPs, and mixed by hand shaking for few seconds. After 28 hours, the mixture was centrifuged at 3000 rpm for 20 min, and redispersed into 200 µL H2O. To know the effect of hydrogen sulfides on the AgNPs and AuNPs, the immobilized NPs on glass substrates were used. The 8 mm × 16 mm of quartz slides were cleaned by ultra-sonication in acetone, ethanol, 10 M NaOH, and H2O for 10 min, then immersed into 1 mg/mL of poly-Llysine 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 for overnight, followed by rinsing with H2O and dried 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 of 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 dried 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 incubated 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

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added on the PLL modified NPs for overnight. After washing 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 which modified with p-MBA were released into H2O by ultra-sonication and collected at a concentration of about 2 × 1010 assemblies/ml. The cell culturing and nanoparticles 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 for overnight, then 50 µL of core/satellites solutions were added and incubated for six hours to 48 hours, followed by removing the culture medium and 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 performed on a JEOL JEM-ARM200F transmission electron microscope. The SEM images and EDS analysis were performed on a Hitachi S-9000 field-emission scanning electron microscope (FE-SEM). Zeta potential and size of the AuNPs was measured by Malvern Zetasizer Nano-ZS90 (Malvern instruments Ltd., U.K.) 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 viewfield 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

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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. Supporting Information Experimental calculations, and Table S1-S2 for synthesis parameters for growth of large AgNSs and Au coating, and Figure 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)

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge the NOF of Osaka University for technical support of SEM and EDX characterization. The authors acknowledge supports from JSPS KAKENHI (26000011), NSFC (51405483), and NSFJS (BK20140377).

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