Tip-Selective Growth of Silver on Gold Nanostars for Surface

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Functional Inorganic Materials and Devices

Tip-Selective Growth of Silver on Gold Nanostars for Surface Enhanced Raman Scattering Weiqing Zhang, Jie Liu, Wenxin Niu, Heng Yan, Xianmao Lu, and Bin Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19328 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 23, 2018

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ACS Applied Materials & Interfaces

Tip-Selective Growth of Silver on Gold Nanostars for Surface Enhanced Raman Scattering Weiqing Zhang,†,‡,§ Jie Liu,‡,§ Wenxin Niu,‡ Heng Yan,‡ Xianmao Lu*,‡,¶, and Bin Liu*,‡ †

Tianjin Key Laboratory of Advanced Functional Porous Materials, Institute for New Energy

Materials & Low-Carbon Technologies, Tianjin University of Technology, Tianjin 300384, China. ‡

Department of Chemical and Biomolecular Engineering, National University of Singapore,

Singapore 117585 ¶

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing

100083, China KEYWORDS: gold-silver, bimetallic nanostructures, anisotropic growth, interior nanogaps, hot spots, surface enhanced Raman scattering

ABSTRACT: Nanogaps as “hot spots” with highly localized surface plasmon can generate ultrastrong electromagnetic fields. Superior to exterior nanogaps obtained via aggregation and selfassembly, interior nanogaps within Au and Ag nanostructures give stable and reproducible surface-enhanced Raman scattering (SERS) signals. However, the synthesis of nanostructures

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with interior hot spots is still challenging due to the lack of high-yield strategies and clear design principles. Herein, gold-silver nanoclusters (Au-Ag NCs) with multiple interior hot spots were fabricated as SERS platforms via selective growth of Ag nanoparticles on the tips of Au nanostars. Furthermore, the interior gap sizes of Au-Ag NCs can be facilely tuned by changing the amount of AgNO3 used. Upon 785 nm excitation, single Au-Ag NC350 exhibits 43-fold larger SERS enhancement factor and the optimal signal reproducibility relative to single Au nanostar. The SERS enhancement factors and signal reproducibility of Au-Ag NCs increase with the decrease of gap sizes. Collectively, the Au-Ag NCs could serve as a flexible, reproducible, and active platform for SERS investigation.

INTRODUCTION Engineering anisotropy in noble metal nanoparticles is a powerful strategy to tailor the properties and functions for various important applications, including catalysis, optic devices, sensing and imaging.1-3 For optical properties, anisotropic growth of metal nanostructures can produce richer plasmonic modes relative to spherical nanoparticles.4-8 For catalytic properties, anisotropic growth of metal nanostructures can offer high densities of catalytically active sites due to the formation of atomic steps, ledges, and kinks.9-13 More significantly, anisotropic growth of bimetallic nanostructures can either skillfully integrate two properties within one nanoarchitecture or highly enhance the original properties.14-18 For example, the selective deposition of Pd and Pt on the partial surface of Au structures can utilize the plasmonic properties of Au to enhance the catalytic activities of Pd and Pt.19-21


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Efforts have been devoted to the synthesis of anisotropic bimetallic nanostructures via the selective growth of second materials on template nanostructures. Manipulating of the reaction kinetics is widely used for selective growth of the second metal.22-26 For examples, Huang et al reported the selective growth of Ag at both one end and two ends of Au nanorods or decahedrons by changing pH of the reaction solutions.26 Xia et al. synthesized Pd-Ag bimetallic nanostructures including hybrid dimers, nonconcentric and concentric core-shell structures by simply controlling the injection rate of AgNO3 and reaction temperature.23,25 Besides, selective molecular capping, with molecules unevenly attached to the surface of metal nanoparticles, has a striking effect on the selective growth of the second metal.27-30 In this regard, Horswell et al. reported a selective deposition of Pt on Au nanorods by using CO to selectively block the Au surface.28 Wang et al. realized anisotropic overgrowth of the second metal on Au nanorods by site-selective silica coating.27 Anisotropic Au-Ag bimetallic nanostructures are prominent surface-enhanced Raman scattering (SERS) platforms owing to three reasons.31-33 Firstly, Au-Ag bimetallic nanostructures can integrate the advantages of both materials within one nanostructure: high plasmonic effect of Ag and good stability of Au.14,34 Secondly, bimetallic system has greater flexibility for fine-tune the shape, size and surface morphology of the nanomaterials.4,26,35,36 Thirdly, the peak of SPR for bimetallic nanostructures can be tuned to the optimal position to yield strong SERS activity.18,3740

Among the anisotropic Au and Ag nanoparticles, nanostructures with nanogaps have become a

new concern due to ultra-strong localized electromagnetic fields. This is because the nanogaps formed between Au and Ag nanoparticles (termed as ‘‘hot spots’’) can produce highly localized surface Plasmon coupling.41-43 Now, it is widely believed that exterior nanogaps formed via selfassembly or aggregation of nanoparticles have the problem of poor reproducibility, making the


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quantitative Raman signals unreliable.42,43 To address this challenge, interior nanogaps between two shells in one nanoparticle have been obtained via wet-chemical methods.31,41,44,45 In some of these works, the size of interior nanogaps can also be controlled. For instances, Duan et al. synthesized spherical core-shell Au nanoparticles with different nanogaps by taking advantage of amphiphilic block copolymers as hard template. The polymer brushes with Raman dyes reduced the localized metal precursor and were encoded in the nanogaps.41 Recently, our group synthesized double-shelled Au/Ag hollow nanoboxes with Raman reporter enclosed within controlled interior nanogaps (from 1 to 16 nm) via a two-step galvanic replacement reaction approach.44 Studies have proved that these nanostructures with interior nanogaps could generate strong and reproducible SERS signals. However, these promising SERS platforms with interior nanogaps have shown limitation for practical applications, primarily because of the low-yield synthetic strategies and unclear design principles. Herein, we report that by using a synthesized ﬂuorescent small molecule 2,5-bis[(1-(ethyl5-(1,2-dithiolan-3-yl)pentanoate)-3,3-dimethyl-2,3dihydroindole-2-ylidene)methyl]cyclobutendiylium-1,3-diolate (named M1) as a selective capping agent and also a SERS reporter, flower-like Au-Ag nanoclusters (NCs) with multiple opened interior nanogaps can be synthesized via selective growth of Ag on the tips of Au nanostars. Firstly, Au nanostars were incubated with M1 molecules, which were unevenly covered on the surface of Au nanostars via strong Au-S bonds.46,47 Subsequently, the reduced Ag atoms selectively deposited at the tips of Au nanostars, where the Au surface is uncovered by M1 molecules. With the selective growth of Ag nanoparticles on each tip of Au nanostars, multiple interior nanogaps among the branches appeared, leading to multiple hot spots. Meanwhile, the M1 molecules were located within these hot spots by adsorbing on the surface of Ag nanoparticles. By changing the amount of AgNO3, the gap sizes among the branches can be


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facilely tuned. Upon 785 nm excitation, the SERS enhancement factor from Au-Ag NCs increases with the decrease of gap sizes. The enhancement factor of single Au-Ag NC with the smallest gap size is 43-fold larger than that of singe Au nanostars. RESULTS AND DISCUSSION Au nanostars capped with hexadecyltrimethylammonium bromide were chosen as the growth templates for three reasons. Firstly, the morphologies of Au nanostar, including its size, tip number, and tip length, are highly tunable by varying the size and concentration of seed.48 Secondly, the synthesis of Au nanostar is relatively straightforward with good reproducibility. Thirdly, the different curvature between tips and the other parts of Au nanostars is beneficial to the selective adsorption of capping agents.49-51 These offer great flexibility for the development of Au-Ag bimetallic systems. As shown in Figure 1, electrically-neutral M1 molecule has a molecular mass of 863 g/mol and two lipoic acid functional groups. When Au nanostar was incubated with M1 at 40 °C for 3 hours (step i), the thick part of branches and cores with low surface curvature can adsorb more M1 molecules than the tips of Au nanostars which have small surface area and high curvature. This is because that the molecular packing density of organothiol ligands on highly curved surfaces is lower than that on planar surfaces.50-54 More importantly, due to the multivalent interactions between M1 and Au, as well as the large binding footprint size of M1, the formed M1 molecule layer would efficiently block the interaction between Ag ions and Au nanostar.27 In step ii, Ag ions in the growth solution was reduced by ʟ(+)-ascorbic acid when the reaction was initiated by the addition of 28% NH4OH. The reduced Ag atoms could be selectively deposited on the sparsely covered tips of Au nanostars rather than the surfaces fully covered with M1 molecules. Interestingly, with the expansion of Ag nanoparticles, multiple interior nanogaps among the branches were generated. Meanwhile, M1 as


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reporter molecules were trapped within these nanogaps through adsorption on the surface of Ag nanoparticles.

Figure 1. Preparation of Au-Ag NCs with interior gaps via selective growth of Ag on the tips of Au nanostars. i) selective capping of M1 molecules on the core and thick branches of Au nanostars, leaving the sharp tips unoccupied, ii) selective growth of Ag nanoparticles at the tips of Au nanostars. As shown in Figure S1, the average core size and branch length of Au nanostars are 42 and 27 nm, respectively. The average number of branches is 7. After selective growth of Ag nanoparticles on the tips of Au nanostar, the shape of Au-Ag bimetallic nanostructures resembles nanocluster. Figure 2A and B show the TEM images of Au-Ag NCs prepared by adding 225 µL of 10 mM AgNO3. As revealed from the TEM images, each Au-Ag NC is composed of one Au nanostar with a Ag hat on each tip. The Au-Ag NCs have an average particle size of 85±5 nm. The Ag nanoparticles on each tip have an average diameter of 27±8 nm. In addition, observable nanogaps between Ag nanoparticles can be found and the size of the gaps can be estimated to range from 10 nm to 15 nm. The elemental distribution in the Au-Ag NCs is characterized by energy dispersive X-ray spectroscopy analysis, which further reveals that each NC is composed


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of one Au core and several Ag nanoparticles (Figure 2C-E). Once the M1 molecule is absent, the deposition of Ag atoms started from the spherical core of Au nanostar and expanded outwards, resulting in Au@Ag core-shell spherical structures (Figure S2). In this case, the selective deposition of Ag on the tips of Au nanostar disappeared, strongly indicating the effect of selective adsorption of M1 molecules on the Au nanostar surface. Once M1 molecules are selectively adsorbed on the surface of Au nanostar, the adsorbed surface is passivated by forming strong Au-S bonds between Au and M1 molecules. It is well-known that the second metal atoms prefer to deposit on the bare or weakly bound surfaces relative to the surface strongly bounded with a high concentration of capping agents.22,55,56 Therefore, Ag atoms are readily deposited on the uncovered tip surface of Au nanostars. To further verify the selective capping effect of M1 molecules on Au nanostars, two commercial molecules, lipoic acid and 1,6-hexanedithiol, have been used as ligands for the deposition of Ag. Lipoic acid, which is the precursor for the synthesis of M1 molecule, provides the same function group with M1. Since each M1 molecule contains two lipoic acid group, the addition amount of lipoic acid is two time larger than that of M1. As shown in Figure S3, there is no selective deposition of Ag on the tips of Au nanostars. It can be found that the Ag atoms uniformly deposited on the whole surface of Au nanostars, including on the branches and cores of nanostars. Another representative ligand with similar carbon chain length and two thiol function groups, 1,6-hexanedithiol, was used to replace M1 molecules. A similar deposition manner of Ag was observed with that in the presence of lipoic acid (Figure S4). The nonselective deposition of Ag is probably attributed to the fact that lipoic acid and 1,6-hexanedithiol molecules have a weaker ability to form a robust protective layer on the surface of branches and cores of Au nanostars as compared to that of M1 molecules.


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Figure 2. TEM images of (A) and (B) Au-Ag NCs with interior hot spots, (D) and (E) EDX elemental mapping of Ag and Au for the Au-Ag NC shown in (C). To verify the universality of this method, another similar shape of gold nanoparticle, Au trisoctahedron, was selected as growth template. As shown in Figure S5A, Au trisoctahedron has high-curvature corners and the average diameter is 96±5 nm. When Au trisoctahedron was used as the template to go through the same procedure of M1 incubation and Ag deposition (Figure S5, Path 1), Ag atoms also selectively coat on the relatively sharp corners of Au trisoctahedra due to the selective molecular capping effect of M1 (shown in Figure S5B). Simultaneously, a control experiment without M1 adsorption before Ag coating was conducted (Figure S5, Path 2). Under this condition, Au@Ag core-shell nanostructures were obtained (Figure S5C). The results indicate that our synthetic strategy is widely applicable to gold nanoparticles with high-curvature ends/corners/tips. These results prove that M1 could serve as a soft template for the development of anisotropic systems.


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It is worth noting that the size controllability of plasmonic nanogaps is important for SERS because gap size can significantly affect the Raman signal intensity or even the configuration and position of Raman probe molecules.43 The interior gap sizes of Au-Ag NC in this work can be facilely tuned via changing the adding amount of AgNO3. When 75 µL of 10 mM AgNO3 was added, the branch tips of Au nanostar were capped with small particles and the gap sizes between Ag nanoparticles are large (Figure 3A-C, denoted as Au-Ag NC75). The TEM and SEM images show that the deposition of Ag happened only at the sharp tips of the Au nanostar rather than on the side surface of branches. Increasing the amount of AgNO3 (10 mM) from 75 to 225 µL, the Ag nanoparticles grew larger, and the space among the branches of Au nanostar decreased due to the expansion of Ag nanoparticles in radius direction (Figure 3D-F, denoted as Au-Ag NC225). When 350 µL of AgNO3 was introduced into the growth solution, the size of Ag nanoparticles became larger, leading to smaller gap size among the branches (Figure 3G-I, denoted as Au-Ag NC350). As compared to the previous reports on different strategies to control the gap size, the current method is simple and straightforward.41,44


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Figure 3. TEM and SEM images of Au-Ag NCs with different gap sizes obtained by adding different amount of 10 mM AgNO3: A-C) 75 µL, D-F) 225 µL, and G-I) 350 µL. Localized surface plasmon resonance (LSPR), as one of the most important optical properties of Au and Ag nanocrystals, is strongly dependent on their structure and composition. As shown in Figure 4A, two LSPR bands can be clearly observed from the Au-Ag NCs (curve c-d). The LSPR band located at 713 nm was corresponding to the tip mode of Au nanostars; the other band at 420 nm is derived from the presence of Ag nanoparticles, which matches that of Ag nanoparticles with an average diameter of 23 nm. As compared to the LSPR bands of Ag nanoparticles (curve a) and Au nanostars (curve b), the two LSPR peaks of Au-Ag NC became broad due to the plasmonic hybridization of the tips of Au nanostar and Ag nanoparticles. It is known that Au nanostar is an excellent SERS platform since the sharp tips of Au nanostar give


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strong electric near field enhancement due to tip plasmon modes.1,17,48,57,58 Although the tips of Au nanostar as intrinsic hot spots disappeared after hatting Ag nanoparticles, the number of nanogaps among Au nanostar branches was increased, leading to new SERS hot spots.

Figure 4. (A) UV-Vis-NIR extinction of (a) Ag nanoparticles, (b) Au nanostars, (c) Au-Ag NC75, (d) Au-Ag NC225, (e) Au-Ag NC350; (B) SERS spectra obtained from (a) Au nanostars, (b) the mixture of Au nanostars and Ag nanoparticles, (c) Au-Ag NC75, (d) Au-Ag NC225, (e) Au-Ag NC350. The excitation wavelength was 785 nm. The SERS evaluation analysis was firstly conducted based on overall SERS intensity of the samples in colloid form. The SERS spectra were obtained at 785 nm excitation, which matches the LSPR bands of the Au nanostar and Au-Ag NCs. For all measurement, the concentration of the colloidal samples (1.0×1011 particles/mL) was confirmed from the particle distribution profile via Nanosight (Figure S6). As revealed from SERS spectra in Figure S7 and 4B, all Raman peaks obtained from M1 powder can be observed from colloidal Au-Ag NC samples. Clearly, the AuAg NCs gave much stronger SERS enhancement than Au nanostars. It is accepted that Ag produces larger SERS enhancement than Au due to more effective plasmonic behavior of Ag.57,59 In addition, the metal surface area directly decides the adsorbed number of SERS probe molecule. Therefore, metal materials and the amount of M1 molecules adsorbed on the particle


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surface should be excluded. For comparison, the mixture of Au nanostars and 33 nm Ag nanoparticles (Figure S8) was selected as another control substrate. The concentration of Au nanostars was kept the same with that of Au-Ag NCs and the mixed ratio was 1 to 7 based on the average tip number of Au nanostars. Comparing curve b and d in Figure 4B, SERS enhancement of Au-Ag NC225 was much larger than that of the mixture of Au nanostars and Ag nanoparticles. Since the average size of Ag nanoparticles in the mixture was similar with that of Ag nanoparticles grown on the tips of Au-Ag NC225, the mixture of Au nanostars and Ag nanoparticles possesses the same materials and very similar surface area with Au-Ag NC225. Hence, such great enhancement can be primarily attributed to the different types of SERS hot spots between Au-Ag NCs and the mixture of Au nanostars and Ag nanoparticles. In the mixture of Au nanostars and Ag nanoparticles, the sharp tips of Au nanostar served as SERS hot spots. Based on the mechanism of selective molecular capping, there should be trace number of M1 molecules adsorbed on the tips of Au nanostar. Consequently, the Raman signal from M1 molecules did not benefit from the highly localized surface plasmon on the tips of Au nanostars, resulting in relatively weak SERS intensity. In contrast, the tips of Au nanostar were selectively covered by Ag nanoparticles in Au-Ag NCs, and the nanogaps among branches served as new SERS hot spots. The newly generated nanogaps in Au-Ag NCs were more effective for enhancing the Raman signal of M1. To further investigate the structure superiority of Au-Ag NCs as SERS substrate, singleparticle SERS spectra of Au-Ag NC and Au nanostar were measured (Figure S9-12). As shown in Figure S13, the SEM image and optical images of single particle match very well, confirming the focus of laser on a single particle during the SERS measurements. Figure 5A-C show the typical SERS spectra obtained from a single particle of Au-Ag NC75, Au-Ag NC225, and Au-Ag


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NC350, respectively. The Raman peak at 1199 cm-1 was selected as reference peak for the enhancement factor calculation (more details can be found in Supporting Information). The SERS enhancement factors were calculated based on the normal Raman intensity of 10 mM M1 solution (Figure S14). The number of M1 adsorbed on the surface of particles was estimated with the aid of Nanosight and UV-Vis spectroscopy (Figure S15). As shown in Figure 5D, Au-Ag NCs produce much larger enhancement factors than both of Au nanostars and silver coated gold nanostar (named Au@Ag). Specifically, the enhancement factors of Au-Ag NC350 were 1.91×107, which was 43 and 35-fold enhancement relative to those of Au nanostar (4.43×105) and Au@Ag (5.43×105), respectively. This result further proved that the superior SERS enhancement from Au-Ag NC350 was mainly attributed to their interior hot spots. In addition, the enhancement factors of Au-Ag NCs increased along with decreasing of their nanogap sizes (curves c-e in Figure 5D). This observation is consistent with the findings of the previous studies because the electromagnetic field is strongly dependent on gap size.41,44 To clarify, the gap size of Au-Ag nanoclusters is difficult to quantify due to three-dimensional complex nanostructures. To illustrate the signal reproducibility of each SERS platform, SERS spectra were collected from several single particles (Figure S9-12). It was found that single Au-Ag NC350 can generate the minimum intensity distribution of SERS spectra, indicating that the Au-Ag NC350 exhibits the optimal SERS performance toward both intensity and reproducibility.


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Figure 5. SERS spectra of M1 from single Au-Ag NC: (A) Au –Ag NC75, B) Au-Ag NC225, and C) Au-Ag NC350. The insets are the SEM images of one single particle as examples, the scale bar is 20 nm. D) The SERS enhancement factors on individual Au-Ag NCs, and Au nanostar (Au NS) for the peak located at 1199 cm-1. The excitation wavelength is 785 nm. CONCLUSION Au-Ag NCs with opened interior nanogaps were obtained via selective growth of Ag on the tips of gold nanostar in the presence of M1. The roles of M1 molecules are both selective molecular capping and SERS reporter. The gap sizes between branches of Au nanostar can be facilely tuned by changing the introduced amount of AgNO3. The experiment results show that the Au-Ag NCs exhibit much larger SERS enhancement factor and better signal reproducibility relative to the control nanostructures, including Au nanostar, the mixture of Au nanostar and Ag nanoparticles,


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and Au@Ag core-shell nanoparticles. Furthermore, the SERS EFs of Au-Ag MCs increases significantly with the decrease of gap sizes. Single-particle SERS analysis shows that single AuAg NC350 gives 43-fold SERS enhancement and the optimal reproducibility for M1 molecules relative to Au nanostar. Together with this wide applicable synthetic strategy, Au-Ag NCs could serve as promising SERS platform for sensitive, reliable single-particle-level SERS investigations. EXPERIMENTAL SECTION Materials. 2,3,3-Trimethyl-3H-indolenine (Sigma-Aldrich), 2-bromoethanol (Sigma-Aldrich), 3,4-dihydroxy-3-cyclobutene-1,2-dione (Sigma-Aldrich), lipoic acid (Sigma-Aldrich, and Adamas), 1,6-hexanedithiol (Alfa), gold (III) chloride trihydrate (HAuCl4·3H2O, Alfa Aesar), silver

nitrate

(AgNO3,

hexadecyltrimethylammonium

Sigma-Aldrich), bromide

ʟ-(+)-ascorbic

(Sigma-Aldrich),

acid

(Sigma-Aldrich),

polyvinylpyrrolidone

(average

Mw=55000, Sigma-Aldrich) and ammonium hydroxide (NH4OH, 28%, Sigma-Aldrich) were used as received without further purification. Ultrapure deionized water (Barnstead, 18.2 MΩ/cm) was used throughout the experiments. Characterization. NMR spectra were collected on a Bruker Avance 500 NMR spectrometer (500 MHz for 1H and 125 MHz for 13C). Transmission electron microscopy (TEM) images, high-resolution TEM (HRTEM) images, and energy dispersive X-ray spectroscopy (EDX) spectra were acquired using a JEOL JEM-2100F and Tecnai G2 Spirit TWIN operating at 200 kV and 120 kV, repectively. Scanning electron microscopy (SEM) images were taken using a JEOL JSM-6700F operating at 20 kV. All UV-Vis spectra were recorded on a UV-1800 spectrophotometer (Shimadzu) at room temperature. The particle concentration of each sample was confirmed using Nanosight LM10-HS with a laser output of 60 mW at 405 nm. Surface-


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enhanced Raman studies were performed on an XploRA PLUS Raman microscope (Horiba/JY, France). The excitation source was a 785 nm laser, and the power on the sample was about 0.1 mW. A 100 × magnification long working distance (8 mm) objective was used to focus the laser onto the sample and to collect the scattered light. Synthesis of M1. M1 was synthesized in four steps starting from 2,3,3-trimethyl-3Hindolenine (Scheme S1). In brief, N-alkylation of 2,3,3-trimethyl-3H-indolenine was reacted with 2-bromoethanol in acetonitrile at 80 °C to give 1-(2-hydroxyethyl)-2,3,3-trimethyl-3Hindolium bromide 1 in 62% yield. Deprotonation of 1 in sodium hydroxide aqueous solution afforded 9,9,9α-Trimethyl-2,3,9,9α-tetrahydro-oxazolo[3,2-hyindole 2 in 93% yield, which was treated with squaric acid in a mixture of toluene and butanol to give 2,5-bis[(1-(2-Hydroxyethyl)3,3-dimethyl-2,3-dihydroindole-2-ylidene)methyl]cyclobutendiylium-1,3-diolate 3 in 51% yield through dehydration reaction. Conjugation between 3 and lipoic acid at room temperature yielded 4 (M1) in the presence of dicyclocarbodiimide and 4-(dimethylamino)pyridine. M1 can be easily dissolved in polar solvents (e.g. methanol, ethanol, dimethyl sulfoxide) due to the presence of positive charges. The chemical structures of all the compounds were verified by nuclear magnetic resonance (NMR). Synthesis of Au-Ag NCs with Multiple Interior SERS Hot Spots. The synthesis of Au nanostars can be found in supporting information. The growth of Au nanostar was a rapid process, indicated by the immediate color change of the solution from light yellow to dark blue after the simultaneous addition of AgNO3 and ʟ-(+)-ascorbic acid. To synthesize Au-Ag NCs, 2.5 mL of Au nanostar dispersed within 1 mM hexadecyltrimethylammonium bromide was transferred into a 15-mL centrifuge tube. 25 µL of 0.1 mM M1 solution was added to the Au nanostar dispersion, which was kept at 40 °C for 3 hours. Then various amounts of 10 mM


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AgNO3, 0.1 M ʟ-(+)-ascorbic acid and 28% NH4OH were added to different samples in sequence, resulting in various amount of Ag coating. The volume ratio among AgNO3, ʟ-(+)ascorbic acid and NH4OH was kept at 50:5:1. For example, 250 µL of 10 mM AgNO3 was coupled with 25 µL of 0.1 M ʟ-(+)-ascorbic acid and 5 µL of 28% NH4OH. The solution was thoroughly mixed after the addition of each chemical. It was left undisturbed for 5 min until the color of solution was stabilized at dark green. For control reactions, the same procedure was conducted by using lipoic acid and 1,6-hexanedithiol as capping agents instead of M1. Ag nanoparticles were synthesized through the same procedure expect the absence of Au nanostars into the solution. Preparation of single particle SERS Substrates. 10 µL of Au nanosheets solution was dropped on the cleaned silicon wafer. After drying under nitrogen stream, 10 µL of colloidal particle solution was drop on the substrate. Then the particle solution was blow away by nitrogen and the substrate is ready for Raman studies.

ASSOCIATED CONTENT Supporting Information. Additional TEM images, the concentration profile of Au-Ag MCs, SERS spectra and photograph and the corresponding SEM images of Au-Ag MCs dispersed on silicon substrate for the SERS study, and the details for synthesis of Au seeds and calculation of SERS enhancement factors. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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* E-mail: [email protected], [email protected] Author Contributions § These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We are thankful for financial support from the Tianjin Municipal Science and Technology Commission (16JCYBJC41600), the National Natural Science Foundation of China (21402219) and the National University of Singapore (R279-000-482-133).

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Tip-Selective Growth of Silver on Gold Nanostars for Surface

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