Silver

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In-Plate and On-Plate Structural Control of Ultra-Stable Gold/Silver Bimetallic Nanoplates as Redox Catalysts, Nanobuilding Blocks, and Single-Nanoparticle Surface-Enhanced Raman Scattering Probes Ju-Hwan Oh,† Hyunku Shin,‡ Jong Yun Choi,† Hee Won Jung,† Yeonho Choi,*,‡,§ and Jae-Seung Lee*,† †

Department of Materials Science and Engineering, ‡Department of Bio-convergence Engineering, and §School of Biomedical Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea S Supporting Information *

ABSTRACT: Noble metal bimetallic nanomaterials have attracted a great deal of attention owing to the strong correlation between their morphology and chemical and physical properties. Even though the synthetic strategies for controlling the shapes of monometallic nanomaterials such as gold (Au) and silver (Ag) are well-developed, limited advances have been made with Au/Ag bimetallic nanomaterials to date. In this work, we demonstrate a highly complex in-plate and on-plate structural control of Au/Ag bimetallic nanoplates (Au/ AgBNPLs) in contrast to conventional, simply structured, 1D and 2D, branched, and polyhedral nanomaterials. The polymer used in the synthesis of seeds plays a critical role in controlling the structure of the Au/AgBNPLs. The Au/AgBNPLs exhibit exceptionally high chemical stability against various chemical etchants and a versatile catalytic reactivity with biologically and environmentally relevant chemical species. Significantly, the reversible assembly formation of the Au/AgBNPLs is demonstrated by carrying out the surface-functionalization of the materials with thiol DNA, emphasizing the potential applications of the Au/AgBNPLs in various diagnostic and therapeutic purposes. Finally, the surface-enhanced Raman scattering (SERS) properties of the Au/ AgBNPLs are experimentally and theoretically investigated, demonstrating a substantial potential of the Au/AgBNPLs as singlenanoparticle SERS probes. Electron microscopy, UV−vis spectroscopy, selected area electron diffraction (SAED), and energydispersive X-ray (EDX) spectroscopy are employed to analyze the structure and composition of the Au/AgBNPLs at the atomic level. KEYWORDS: nanoparticle, bimetallic, DNA, SERS, catalysis, chemical stability, gold, silver

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INTRODUCTION

absorption intensity originating from the nanosized Au and Ag.8−15 To date, a number of Au/Ag bimetallic nanostructures have been intensively investigated owing to their structure-dependent chemical properties and applications. In order to explore the potential applications of Au/Ag nanomaterials based on their shape-dependent properties, their structural control needs to be investigated. Several Au/Ag bimetallic nanostructures with complex morphologies including wire,16−18 rod,19−21 plate,16,22,23 frame, 24−26 hollow,27,28 and core−shell29,30 structures have been demonstrated. Their synthesis, however, is limited with the narrow metallic composition ranges (one at ∼10 mol % related to the other),22 initial structures of sacrificial templates,27−30 electropotential-dependent pairing of elements when galvanic replacement is involved,24,27,29,30 and cumbersome synthetic procedures in need of structure-directing agents during the growth.16,18,20 Most of all, these structures are

Noble metal plasmonic nanomaterials are of significant interest owing to their unique chemical and physical properties which are determined by the size, shape, and metallic composition of the nanomaterials. In addition to pure gold (Au) and silver (Ag) nanomaterials, their bimetallic nanostructures have attracted particular attention as they exhibit a wide range of excellent properties over the monometallic nanostructures.1,2 For example, Au/Ag bimetallic nanomaterials were taken advantage of to activate the catalytic oxidation of carbon monoxide at an unusually low temperature and the catalytic reduction of nitroaromatic compounds with a higher efficiency than the single-component nanomaterials.3,4 In certain cases, Au/Ag nanomaterials exhibited a three-photon luminescence and enhanced photoacoustic properties stronger than those of the monometallic Au and Ag nanoparticles.5,6 The dual-energy mammography contrast properties of these bimetallic materials were used in in vivo imaging of tumors.7 Au/Ag nanomaterials also exhibited a high chemical stability, biocompatibility, photothermal effects, fluorescence, and strong plasmon © XXXX American Chemical Society

Received: August 5, 2016 Accepted: September 22, 2016

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DOI: 10.1021/acsami.6b09803 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

5 mL) to an aqueous mixture composed of trisodium citrate (2.5 mM, 5 mL), polymer (0.05 g/L, 250 μL), and sodium borohydride (10 mM, 300 μL) at a rate of 2 mL/min with stirring. After the seed preparation, a deep yellow color was observed. Subsequently, the assynthesized seed solution was injected into the reducing solution composed of L-ascorbic acid (10 mM, 75 μL) and pure water (5 mL). Finally, 3 mL of an aqueous solution containing AgNO3 and HAuCl4 ([AgNO3] + [HAuCl4] = 0.5 mM) was continuously injected into the growth solution at a rate of 2 mL/min with stirring. The usage of the reducing solution and the precursor solution for the growth of the seeds was identical regardless of the types of the seeds and was considered to be the common growth condition. After the growth (∼5 min), the trisodium citrate solution (25 mM, 0.5 mL) was added into the final mixture for protecting the surface of the Au/AgBNPLs. The as-synthesized Au/AgBNPLs were washed with a TWEEN 20 solution (0.01%) three times by repeated centrifugation and redispersion. Catalytic Activity. Catalytic oxidation of horseradish peroxidase (HRP)-substrates was carried out by combining an HRP-substrate (0.01 M, 200 μL), H2O2 (30 wt %, 30 μL), and buffer components (670 μL; 100 mM phosphate buffer, pH 7.4, for ODA and OPD and 200 mM citrate buffer, pH 3.8, for ABTS). Subsequently, the mixture was combined with 100 μL of nanoparticle solution followed by incubating at 40 °C for 1 h. The oxidation reaction was monitored using UV−vis spectroscopy. Catalytic reduction of 4-NPh was carried out by injecting the Au/AgBNPL solution (30 μL) into an aqueous solution containing 4-NPh (25 mM, 100 μL), NaBH4 (88 mM, 100 μL), and H2O (770 μL). The color of the solution changed from yellow to colorless as the reaction progressed. The changes in absorbance of the mixture were monitored using UV−vis spectroscopy. Synthesis of DNA-Au/AgBNPL. DNA-Au/AgBNPLs were synthesized following a procedure described in existing literature.34,35 In brief, monothiol DNA sequences (DNA-1 and DNA-2) were deprotected with dithiothreitol (0.10 M; phosphate buffer (0.17 M, pH 8.0)), and purified using a NAP-5 column. The deprotected DNA sequences were combined with the Au/AgBNPL solution. The mixture was buffered with phosphate (pH 7.4, 10 mM phosphate, 0.15 M NaCl, 0.01% SDS) and incubated at 25 °C for 12 h. To remove the unconjugated free DNA, the mixture was centrifuged at 8000 rpm for 20 min. The supernatant was then discarded, and the remaining DNA-Au/AgBNPLs were redispersed in a phosphate buffer (10 mM PB, 0.15 M NaCl, and 0.01% TWEEN 20). The washing procedure was repeated three times. After the purification, the obtained DNA-Au/AgBNPLs were stored at 4 °C until further use. Reversible Assembly of DNA-Au/AgBNPLs. Two complementary DNA-Au/AgBNPLs were combined in a buffered solution (pH 7.4, 10 mM phosphate, and 0.01% TWEEN 20) and were allowed to hybridize at various concentrations of NaCl (0.15, 0.30, 0.45, and 0.60 M). The hybridization was monitored by measuring the extinction at 615 nm every 1 min with homogeneous stirring. The reversible dehybridization of the hybridized DNA-Au/AgBNPLs was analyzed by measuring the changes in extinction at 615 nm from 25 to 65 °C. SERS Characterization. To prepare a single nanoparticle SERS substrate, cover glass was cleaned using piranha solution, rinsed with ultrapure water and ethanol, and dried with N2 gas. Subsequently, it was incubated in 3-mercaptopropyl-trimethoxy-silane (1 mM) ethanoic solution for 1 h, rinsed with ethanol, and dried with N2 gas. The nanoparticles were deposited on the substrate by dropping the nanoparticle solution onto the substrate. The number of the nanoparticles on the substrate was controlled by changing the treatment time. After being rinsed with ultrapure water, the resultant substrate was immersed in BDT (1 mM) ethanoic solution for 2 days, rinsed with ethanol, and dried with N2 gas. The dark-field images and SERS spectra were obtained using a dark-field microscope and spectrometer with a 50× objective lens (NA: 0.55). The incident 785 nm laser power was 10 mW, and the acquisition time was 1 min. Specifically, we situated only one single nanoparticle at the center of the laser spot (diameter =1.740 μm) which is at the center of an 8 μm × 8 μm area, by comparing an image of spectrometer to the real-time

simple, well-defined, and either completely single- or polycrystalline. Recently, Skrabalak et al. reported the formation of radially grown complex structures composed of both single- and polycrystalline domains based on the coreduction of multiple metal precursors in the presence of “already-grown” nanostructures with desired shapes as initial seeds.31 The final architecture of the resultant bimetallic nanomaterials, however, still strongly depends on the initial compositions and structures of the seeds and templates, the preparation of which requires substantial efforts. Herein, we report the synthesis of Au/Ag bimetallic nanoplates (Au/AgBNPLs) with novel and unprecedented structures based on a polymer-mediated synthesis approach using seeds and a reducing solution without employing any additional structure-directing agents (SDAs) during the growth procedure. We demonstrate that the type and concentration of the polymeric materials used ultimately determine the final “inplate” and “on-plate” structures of the nanomaterials, which are rarely achieved with other conventional methods. The surface of the Au/AgBNPLs is highly stable against chemical etching, is catalytic for both oxidation and reduction reactions, and can be densely conjugated with monothiol DNA, indicating the multifunctional properties of these materials useful in further applications. The surface-enhanced Raman scattering (SERS) properties of the Au/AgBNPLs are highly attractive, which is strongly correlated with the in-plate porous structures. A threestep growth mechanism is suggested based on the observation of the growth of the Au/AgBNPLs.

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EXPERIMENTAL SECTION

Materials and Instruments. Silver nitrate (cat. no. 204390), gold chloride trihydrate (cat. no. 520918), sodium citrate tribasic dihydrate (cat. no. S4641), poly(sodium 4-styrenesulfonate) (PSSS, cat. no. 434574; MW 1 000 000), polyvinylpyrrolidone (PVP, cat. no. 234257; MW 29 000), poly(ethylenimine) (PEI) solution (cat. no. 482595; MW 1200), diethylaminoethyl-dextran hydrochloride (DEAD-dextran, cat. no. D9885), poly(acrylic acid) sodium salt (PA) solution (cat. no. 416037; MW 15 000), pluronic F-108 (cat. no. 542342; MW 14 600), sodium borohydride (cat. no. 480886), L-ascorbic acid (cat. no. A5960), hydrogen peroxidase (cat. no. H1009), o-phenylenediamine (OPD, cat. no. P23938), o-dianisidine (ODA, cat. no. D9143), 2,2′azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS, cat. no. A1888), 4-nitrophenol (4-NPh, cat. no. 241326), dithiothreitol (cat. no. 43815), 1,4-benzendithiol (BDT, cat. no. 763969), and the chemicals used for buffer preparation were purchased from SigmaAldrich (Milwaukee, WI, USA). Ultrapure water from a Millipore Direct-Q3 system (18.2 MΩ·cm; Millipore (Billerica, MA, USA)) was used in the experiments. All the solutions were freshly prepared before use. Thiol DNA sequences (DNA-1: 5′ HS-A10-ATTATCACT 3′; DNA-2: 5′ HS-A 10-AGTGATAAT 3′) were purchased from GenoTech Corp. (Daejeon, Republic of Korea). NAP-5 columns were purchased from GE Healthcare (Buckinghamshire, United Kingdom). UV−vis extinction and absorbance spectra were measured using an Agilent 8453 UV−vis spectrophotometer equipped with a Peltier temperature controller (Agilent Technologies, Santa Clara, CA, USA). The morphologies of the obtained nanomaterials were observed using transmission electron microscopy (TEM) with a Tecnai F20 (FEI, Hillsboro, Oregon USA, operated at 200 kV), Talos F200X (FEI, operated at 200 kV), and Titan themis3 (FEI, operated at 300 kV). The SERS spectra were obtained using a dark-field microscope (Carl Zeiss, Germany) equipped with a spectrometer (Princeton instruments, Acton, MA, USA). Synthesis of Au/AgBNPLs. Au/AgBNPLs were synthesized using a seed-mediated growth method in combination with the coreduction of AuCl4− and Ag+.32,33 Initially, the seed nanoparticles were synthesized by continuously injecting the AgNO3 solution (0.5 mM, B


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ACS Applied Materials & Interfaces dark field image and obtained the SERS spectrum of a single nanoparticle at a time.

reported values for Au, Ag, and Au/Ag alloys.36 We also investigated the crystallinity of the Au/AgBNPLs using selected area electron diffraction (SAED) patterns. A SAED pattern typical of that of other Au, Ag, and Ag/Ag alloy materials was obtained (Figure 1e), indicating that the Au/AgBNPLs are crystalline. The UV−vis spectrum of the Au/AgBNPLs exhibited the maximum extinction at a characteristic wavelength (λMAX of 615 nm) as shown in Figure 1f.37 However, spheroidal Au/Ag bimetallic nanoparticles with a similar diameter and composition were reported to exhibit the maximum extinction at ∼495 nm.38 The color of the Au/AgBNPL solution appeared as deep blue, exhibiting intense optical properties owing to surface plasmon resonance (SPR). Besides, their narrow spectral curve also implies the monodispersity of the particles, as demonstrated by the TEM image shown in Figure 1a. Moreover, by considering the strong correlation between the structures and optical properties of the plasmonic nanoparticles, we investigated how to control the structure of the Au/AgBNPLs by introducing PSSS at different concentrations during the seed synthesis. Specifically, we synthesized two types of PSSS-AgSeeds at two more different concentrations of PSSS (12 and 0.12 mg/L) and grew them into anisotropic structures under the common growth condition (seed solution volume = 20 μL, Figures 1g,h). At a higher [PSSS] (12 mg/L), the size of the Au/AgBNPLs slightly decreased, accompanied by a disappearance of the central bump and reduction of the pore sizes and numbers (Figures 1g and S1). This observation demonstrates the crucial role of the polymer during the seed synthesis process with respect to the polymer concentration. The shape-determining role of the polymer was further investigated at a lower concentration. As [PSSS] decreased to 0.12 mg/L, we observed a bump composed of multiple subdomains, which is large enough to cover the entire surface of the original planar structure (Figure 1h). Interestingly, similar structures were also obtained with AgSeeds synthesized without any polymers (see Figure S2), indicating that the polymers are functional only above a certain threshold concentration. Assuming that the shape-directing role of polymers would be more pronounced with a synthetic system, nanostructures were synthesized with only one of the precursors. When only Au precursor was present in the growth solution, only isotropic structures were produced (data not shown) even with the seeds synthesized at a high [PSSS] (12 mg/L). Therefore, we grew the PSSS-AgSeeds synthesized at a low [PSSS] (0.0012 mg/L) in a growth solution containing only Ag+. This experimental design excludes the effects of AgCl formation and Au/Ag coreduction, allowing us to examine the results primarily occurring from the interactions of the polymer and Ag. Interestingly, multiple thin and linear cracks that were radially oriented were observed in Ag nanoplates, exhibiting structural similarity with respect to the distribution of the inplate pores of the Au/AgBNPLs (Figure 1i). The interactions of the polymer and the AgSeeds during the seed synthesis, even though still ambiguous, might cause the formation of polymerinduced defects that will develop into pores.33 In addition, we conducted a similar series of synthesis and characterization of the Au/AgBNPLs grown from PVP-AgSeeds because PVP is one of the most widely employed polymers in nanomaterial synthesis owing to its attractive specific interactions with Au and Ag atoms.39−43 The resultant Au/AgBNPLs exhibited very similar structures and properties to those from the PSSSAgSeeds, demonstrating the versatility of the synthetic method for the Au/AgBNPLs in this work (see Figure S3). Finally, in

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RESULTS AND DISCUSSION Overview of the Synthesis of Au/AgBNPLs. The synthesis of the Au/AgBNPLs was carried out using a typical seed-mediated growth procedure by initially preparing tiny Ag seeds (AgSeeds). Importantly, a polymeric material of interest at a desired concentration was combined with the Ag precursor (AgNO3) during its reduction, resulting in the formation of polymer-incorporated AgSeeds (polymer-AgSeeds), which played a critical role in determining the structures after the growth of the seeds. The seeds were further grown in a reducing solution containing only a reductant and Au and Ag precursors (final [AuCl4−] = 0.075 mM and final [Ag+] = 0.1125 mM; AuCl4−/Ag+ = 2:3) without using any shapedirecting agents, which remarkably simplifies the synthetic system and minimizes any undesired effects of other chemicals that are commonly used in the synthesis and stabilization of nanomaterials (Scheme 1). Scheme 1. Schematic Illustration for the Growth of PolymerAgSeed into Bimetallic Nanostructures with In-Plate Pores and/or On-Plate Bumps

Structural Control of Au/AgBNPLs Using PSSS. In order to control the structures of the final grown Au/AgBNPLs, we employed various polymers to induce the formation of seeds that are capable of anisotropically growing. PSSS was first chosen and incorporated into AgSeeds (final [PSSS] = 1.2 mg/ L) owing to its ability in directing the anisotropic growth of monometallic Ag nanoparticles.32,33 Surprisingly, nanoplates containing two distinctive structural characteristics, multiple inplate pores and a single on-plate bump, were obtained with 20 μL of PSSS-AgSeeds (Figure 1a). In order to investigate the detailed structure of the Au/AgBNPLs, a high-resolution TEM (HRTEM) image was analyzed (Figure 1b). The overall particle diameter was determined to be ∼60 nm, and there was an isotropic bump having a diameter of ∼30 nm at the center of the particle. Interestingly, we also observed that several 1D pores (length ≈ 10 nm and width ≈ 2 nm) radially stretched out in-plate from the center. The presence of pores was further confirmed with a scanning TEM (STEM) image (Figure 1c), where a significantly dark contrast of the pores convincingly implies that they are in-plate. The STEM image also clearly indicates that the pores are stretched even under the bump. We also analyzed the peripheral area of a pore using HRTEM (Figure 1d). Importantly, neither ordered nor disordered atoms were observed in the pore area, clearly demonstrating that the pores exhibit open ends on both the facial sides of the nanoplates and that the pores are not closed chambers. The interplanar distance of the lattice of the nanoplates is determined to be 0.254 nm, which matches with the previously C


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Figure 1. (a) TEM image of Au/AgBNPLs grown from PSSS-AgSeeds synthesized at 1.2 mg/L [PSSS]. (b) HRTEM and (c) STEM images of an identical Au/AgBNPL. (d) HRTEM image of the peripheral area of a pore with atomic resolution. The gap between yellow lines indicates interplanar distance of the lattice structure of Au, Ag, or Au/Ag alloy (0.254 nm). (e) SAED pattern of the Au/AgBNPLs in panel a. (f) UV−vis spectrum and (inset) a photo of an Au/AgBNPL solution. (g and h) Au/AgBNPLs grown from PSSS-AgSeeds synthesized at (g) 12 and (h) 0.12 mg/L [PSSS]s. (i) Silver nanoplates grown from PSSS-AgSeeds synthesized at a lower [PSSS].

metal precursors or from the planar to the central area of the Au/AgBNPLs during or after the growth.44,45 A linescan analysis was further conducted using energy-dispersive X-ray spectroscopy (EDX) along an imaginary line over pores and a bump as shown in Figure 3a (Figure 3e,f). Figure 3e shows the high stepwise signal intensity from Au around the central area of the Au/AgBNPL. The changes in atomic percent exhibit nonflat, fluctuating signal intensity around the pore area, while higher proportion of Au is observed in the central bump area (Figure 3f). This observation implies that the bump is layerstructured and is mainly composed of Au. We also conducted the elemental analysis of the Au/AgBNPLs grown from the PVP-AgSeeds and obtained similar results including the even distribution of Au and Ag (see Figure S4). Growth Mechanism of Au/AgBNPLs. To investigate the growth mechanism of the Au/AgBNPLs, we carefully monitored the initial growth of the PSSS-AgSeeds by controlling their growth with increasing amounts of the precursors from 5 to 50% using TEM. The UV−vis spectra of the intermediate Au/AgBNPLs exhibited an increase in both extinction and λMAX, clearly demonstrating the gradual growth of the Au/AgBNPLs as the amount of the precursors increased (Figure 4a). We further investigated the intermediate Au/ AgBNPLs using high-angle annular dark-field (HAADF)-STEM and elemental mapping analysis (Figure 4b). Interestingly, the distribution of gold and silver is almost identical during the entire growth procedure, indicating that the coreduction of their precursors keeps occurring after the initial galvanic

order to confirm that the central darker pattern in Figure 1c is truly a 3D bump and not an area rich in gold in a flat planar nanostructure, we obtained several TEM images of an Au/ AgBNPL that was tilted at various angles from −30 to +70° with an increment of 10° and presented corresponding 3D modeling images accordingly (Figure 2).

Figure 2. TEM images of an Au/AgBNPL that is tilted at various angles from −30 to +70° with an increment of 10° are demonstrated with corresponding 3D modeling images.

Elemental Analysis of Au/AgBNPLs Based on PSSS. In order to reveal the bimetallic nature of the Au/AgBNPLs, the elemental distribution of a representative nanoplate containing the distinctive on-plate bump and in-plate pores was investigated by elemental mapping (Figure 3). Au and Ag were evenly distributed all over the nanoplate, and the porous and bumpy structures were clearly distinguished by sharp contrasts (Figure 3a−d). The atomic ratio of Au/Ag was determined to be 13:7, indicating that the Au/AgBNPLs were obtained mainly via simultaneous coreduction of the two metal precursors. We also consider the possibility of atomic diffusion of Ag atoms from the PSSS-AgSeeds to surrounding reduced D


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Figure 3. (a) STEM image and EDX elemental mapping images of an Au/AgBNPL grown from a PSSS-AgSeed with respect to (b) Au, (c) Ag, and (d) Au and Ag. (e and f) EDX linescans along the yellow line on the Au/AgBNPL in panel a in terms of absolute intensity (e) and atomic % (f). Figure 4. Monitoring the growth of a PSSS-AgSeed by controlling the precursor concentration. (a) UV−vis spectra obtained from the intermediate Au/AgBNPLs grown at various precursor concentrations (5, 10, 15, 20, 25, 30, 40, and 50%), (inset) Photo of the solutions containing the corresponding intermediate Au/AgBNPLs. (b) HAADF-STEM images and the corresponding elemental distribution images of the obtained nanostructures. Scale bar of 40 nm shown in the right-top side of panel b is the same, except for the one obtained at 5% of the precursors. (c) Schematic illustration for the growth mechanism of the Au/AgBNPL from a PSSS-AgSeed.

replacement reaction. Based on our observation, we propose the following three-step growth mechanism for the Au/ AgBNPLs beginning from the PSSS-AgSeeds (Figure 4c): (1) Initial formation of bimetallic ring/frame nanostructures at the sacrifice of the PSSS-AgSeeds is observed. Galvanic reaction seems to be responsible for the disappearance of the PSSSAgSeeds. (2) The multiple planar fragments grow laterally with multiple radial in-between cracks, while the multiple pieces of protrusion appear around the hole which was originally occupied by the PSSS-AgSeed. (3) As the growth further proceeds, the enlarged planar fragments are merged together at the outer boundary, while keeping the internal former cracks, or in-plate pores. The central protrusion keeps growing into a larger on-plate bump at the center. We also monitored the growth of PVP-AgSeeds. Interestingly, very similar structural evolution was observed during the growth (see Figure S5), indicating that both PSSS and PVP chemically function in a similar way for the growth of the nanostructures. The ratio of both precursors was also investigated with PVP-AgSeeds, resulting in the distinctive inplate and on-plate nanostructures of the Au/AgBNPLs only at a specific ratio of AuCl4− and Ag+ (see Figure S6). Effect of Other Polymers on Structures of Au/ AgBNPLs. The strong structure-directing abilities of PSSS and PVP encouraged us to investigate the role of other polymers under similar synthetic conditions. Four commercially available polymers, namely, DEAE-dextran, PEI, PA, and F-108, were employed to synthesize polymer-AgSeeds. The AgSeeds synthesized with DEAE-dextran and PEI grew into irregularly

shaped nanoparticles, which is presumably attributed to the positive charge of the polymers. The electrostatic repulsion between the polymers having positive charges and Ag+ would interrupt the formation of polymer-AgSeeds, as observed in the previous work, resulting in the formation of AgSeeds unaffected by the polymers, eventually leading to an isotropic growth (Figure 5a,b).33 In contrast to the cationic polymers, however, the AgSeeds synthesized with negatively charged or neutral polymers (PA: negative; F-108: neutral) grew into structures containing in-plate pores and an on-plate bump (Figure 5c,d), similar to the ones grown from the PSSS and PVP-AgSeeds. In fact, the effects of charges on the polymer on the final structure of the grown bimetallic nanomaterials was similarly observed with Ag nanoplates grown from AgSeeds prepared with a large library of polymers, indicating the determinant role of the polymer charge.33 In addition, we obtained the TEM images of the 4 types of AgSeeds synthesized with 3 different polymers (PVP, PSSS, and PEI) and without a polymer (see Figure S7). E


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Figure 5. (a−d) TEM images of nanostructures grown from AgSeeds synthesized with (a) DEAE-dextran, (b) PEI, (c) PA, and (d) F-108.

Figure 6. TEM images of the Au/AgBNPLs (a) before and (b and c) after the reaction with 5 M H2O2 for (b) 72 h and (c) 2 weeks. (d) UV−vis spectra obtained from the resultant nanostructures in panels a−c and Ag nanoplates after the reaction with 5 M H2O2.

In spite of the distinctive final grown structures, the 4 types of the AgSeeds similarly exhibit quasi-spherical structures with an average diameter of ∼10 nm regardless of the types of polymers. We also grew the AgSeeds synthesized without a polymer into larger structures in the presence of PSSS at the same final concentration during the growth and obtained only irregularly shaped nanoparticles (see Figure S8), indicating that the presence of polymers is important only during the AgSeed synthesis and is ineffective during the growth. Chemical Stability of Au/AgBNPLs. We also investigated the chemical stability of the Au/AgBNPLs. As monometallic Ag nanoplates were reported to be easily etched by oxidants, particularly by H2O2, their applications in diagnostics and therapeutics are strictly restricted under oxidative conditions. To solve this problem, the spherical Au/Ag alloy nanoparticles have been suggested as alternatives for monometallic Ag counterparts, since they possess similar optical properties but far better chemical stabilities.9 Therefore, we expected that our Au/AgBNPLs (Figure 6a) would exhibit enhanced resistance against oxidative etching by H2O2, which was systematically examined in aqueous media. This result would be particularly important because the chemical stability of “anisotropic” Au/Ag bimetallic nanoparticles is demonstrated for the first time. Initially, we immersed the Au/AgBNPLs in 5 M H2O2 aqueous solution, allowing them to react for 72 h. The in-plate pores and on-plate bumps remained intact (Figure 6b), indicating that the chemical stability of the Au/AgBNPLs is higher than that of the spherical Au/Ag alloy nanoparticles.8,9 The bimetallic nanoparticles still maintained their structural characteristics without any discernible deformation occurring even after the prolonged H2O2 reaction time period (2 weeks; Figure 6c). In addition, the analysis of constant UV−vis spectra of the Au/AgBNPLs obtained after the incubation with H2O2 also confirms their enhanced chemical stability (Figure 6d). Monometallic Ag nanoplates, however, were not able to survive in the H2O2 solution even for a few seconds, as evidenced by the almost flat UV−vis spectrum obtained (Figure 6d). Catalytic Properties of Au/AgBNPLs. As observed in Figure 6d, the low chemical stability of the monometallic Ag

nanomaterials could be significantly disadvantageous, particularly in the presence of etchants or acids. Even though Ag nanomaterials exhibit interesting catalytic properties,46,47 their chemical instability limits their applications as a catalyst in a variety of fields. Recently, He et al. demonstrated that the alloy formation of Ag with Pt or Pd effectively led to an improvement in the chemical stability of the products against acidic conditions, consequently leading to the successful oxidative catalytic performance in the presence of H2O2.48,49 After confirming the exceptionally high stability of our Au/ AgBNPLs against oxidative etching with H2O2 (Figure 6), we hypothesized that our nanostructures would also exhibit substantial catalytic properties for the oxidation of colorimetric substrates of enzyme-linked immunosorbent assay (ELISA). Importantly, we investigated the peroxidase-like activity of our bimetallic nanoparticles in the oxidation of most commonly used HRP substrates such as OPD, ODA, and ABTS in the presence of H2O2. The catalytic oxidation of HRP-substrates was monitored by measuring the increase in the absorption at λMAX (OPD: 415 nm, ODA: 425 nm, and ABTS: 420 nm) using UV−vis spectroscopy (Figure 7a). The changes in the absorbance at λMAX were also monitored as a function of time for 60 min, which demonstrated almost a linear increase in the case of OPD and ODA and even a plateau in case of ABTS (Figure 7b). The color changes of the oxidized substrates observed after 60 min were clearly visible to the naked eye from colorless to brown for ODA, yellow for OPD, and blue for ABTS (Figure 7b, inset). The colors of the substrates did not change in the absence of the Au/AgBNPLs, clearly demonstrating that the oxidation can be catalyzed only in the presence of the Au/AgBNPLs. In addition to the oxidative catalytic properties, we further evaluated the reductive catalytic activity of the Au/AgBNPLs with 4-NPh as the substrate which is associated with environmental pollution.50,51 As the reduction took place, the UV−vis spectra of 4-NPh exhibited a gradual decrease in absorbance, an optical signature of the reduction of 4-NPh (Figure 7c). This spectral change at 400 nm was monitored as a function of time (Figure 7d). The color change occurring from yellow to colorless was also observed F


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their morphologies using TEM before and after performing the DNA-conjugation (Figures 8a,b). Interestingly, the shapes of

Figure 7. (a) UV−vis spectra of the HRP-substrates (ABTS, OPD, and ODA) before and after their oxidation catalyzed by the Au/ AgBNPLs. (b) Absorbance changes were monitored at λMAX of the oxidized HRP-substrates as a function of time. (inset) Photo of the HRP-substrate solutions under oxidative conditions without and with the Au/AgBNPLs. (c) UV−vis spectral changes of 4-NPh as its reduction catalytically proceeds by the Au/AgBNPLs. (d) Absorbance change of 4-NPh was monitored at 400 nm during the reduction as a function of time. (inset) Photo of the 4-NPh solutions under reductive conditions without and with the Au/AgBNPLs.

Figure 8. TEM images of Au/AgBNPLs (a) before and (b) after functionalization with DNA. (c) Thermal melting transitions of hybridized DNA-Au/AgBNPLs at different [Na+]’s. (inset) Plot of Tm as a function of the [Na+].

the Au/AgBNPLs remained unchanged after the reaction with thiol DNA, indicating the high chemical stability of the Au/ AgBNPLs as observed with their resistance against other chemical reactions.9 The Au/AgBNPLs prepared in this study are highly valuable owing to their optical properties controlled in the visual range similar to monometallic Ag nanostructures and high stability that is attributed to the bimetallic formation of Ag and Au. To further examine the presence and functionality of DNA, two complementary DNA-Au/AgBNPL conjugates (DNA-1 and DNA-2) were hybridized at 25 °C. As the hybridization proceeded, the color of the reaction mixture changed from blue to almost colorless, exhibiting a concomitant redshift and a decrease in intensity in the UV−vis spectra (see Figure S11). Subsequently, the assembled DNA-Au/AgBNPLs were reversibly disassembled as the temperature increased, exhibiting sharp melting transitions (full width at half-maximum of the first derivative ≈ 2.2 °C, Figure 8c). These cooperative melting properties evidently indicate that the DNA strands are densely conjugated on the bimetallic nanoparticle surfaces, as observed with the monometallic Au and Ag nanoparticle−DNA conjugates.59,60 In addition, the melting temperature increased as [NaCl] increased, clearly showing that the assembly properties were precisely controlled by changing the binding properties of the DNA-Au/AgBNPLs.60 Surface Enhanced Raman Scattering Properties of Au/AgBNPLs. Finally, we investigated the SERS properties of the Au/AgBNPLs. In general, nanostructures with an internal nanogap are highly attractive for the enhancement of SERS signals, because they do not need to irreversibly aggregate or to be individually placed in close proximity to an Au thin film.61−64 The in-plate pores of the Au/AgBNPLs are only a few nanometers wide, offering great potential to sufficiently enhance SERS signals as an ideal single-nanoparticle platform. To evaluate the effect of the in-plate pores on the SERS

with bare eyes (Figure 7d, inset). As observed in the case of the catalytic oxidation, the reduction did not proceed in the absence of Au/AgBNPLs, indicating that the bimetallic nanoparticles are truly catalytic as if they were artificial enzymes.52 The catalytic properties of the Au/AgBNPLs were further compared with those of other mono- and bimetallic nanoparticles with various shapes and sizes. Specifically, we prepared 3 types of nanoparticles including Au/Ag nanospheres, Ag nanoplates, and Au nanoplates (see Figure S9)53−55 and conducted catalytic oxidation of OPD using the Au/ AgBNPLs, Au/Ag nanospheres, Ag nanoplates, and Au nanoplates as catalysts (see Figure S10). Interestingly, the Au/AgBNPLs exhibited the most excellent catalytic activity which is much better than those of the Au and Ag nanoplates and was still slightly better than the Au/Ag bimetallic nanospheres. We presume that the high stability, bimetallic composition, and specific crystalline structures potentially with high-energy facets of the Au/AgBNPLs must be the key factors to enhance their catalytic properties.48,49 DNA Functionalization and Reversible Assembly Properties of Au/AgBNPLs. In addition, we demonstrate that the surface of the Au/AgBNPLs could be functionalized with thiol DNA. To the best of our knowledge, the surface conjugation of highly anisotropic Au/Ag bimetallic nanoparticles with thiol DNA has been rarely investigated.56−58 Moreover, the chemical stability of the bimetallic nanostructures against thiol functionality is an important issue, considering that the anisotropic Ag nanomaterials are typically susceptible to oxidative etching induced by thiol compounds.59 In order to examine the chemical stability of the Au/AgBNPLs against thiols and their ability to be surface-tailored, we functionalized the Au/AgBNPLs with thiol DNA and examined G


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molecules in the effective excitation volume. We measured INormal by measuring the Raman signal of 0.1 M BDT solution in methanol. The Raman peak at 1555 cm−1 was chosen to calculate the EF. To estimate NSERS, we first calculated the surface area of each particle by using geometries in COMSOL Multiphysics. The surface areas are 8526 nm3 for an Au/ AgBNPL, 9536 nm3 for an Ag nanoplate, and 20 088 nm3 for an Au nanosphere. We assumed that the molecules form a close-packed monolayer on the nanoparticle with a molecular footprint of 0.19 nm2.66 To estimate NNormal, the effective excitation volume was considered as a cylinder and calculated as 14.5 × 10−12 m2. Then, we could calculate NNormal using the molecular concentration, effective excitation volume, and Avogadro constant. Finally, the Raman EFs of the nanoparticles were calculated as 5.3 × 105 for an Au/AgBNPL, 2.5 × 105 for an Ag nanoplate, and 6.9 × 103 for an Au nanosphere. The calculated EF for Au/AgBNPL seems modest, but it represents the averaged value over the entire surface of a single particle. The actual enhancement of the Raman signal takes place at the in-plate nanopores with high local electromagnetic field and thus is estimated to be extremely high. To further explain quantitative data, we simulated the enhancement of electromagnetic fields with respect to each type of nanoparticles, particularly their shapes (see Figure S14). The Au nanospheres rarely exhibited the local enhancement, and the Ag nanoplates produced higher local field enhancement only around their sharp tips, which can explain their minor effect on the SERS enhancement.67,68 Importantly, the local electromagnetic field enhancement was theoretically predicted inside the in-plate pores of the Au/AgBNPLs, resulting in evenly distributed hot spots over the nanoplates. Even though the enhancement of the individual pore is not as high as that of the tips of the Ag nanoplates, the presence of multiple pores and their larger surface area would significantly increase the probability of the Raman signal enhancement of the Au/ AgBNPLs.69

enhancement, we prepared comparable nanoparticles that did not have nanogap structures, such as Ag nanoplates and Au nanospheres, and compared the single-nanoparticle SERS spectra of BDT (Figure 9a) adsorbed on each type of

Figure 9. (a) SERS spectra obtained from the each nanoparticle (Au/ AgBNPL, Ag nanoplate, and Au nanoparticle) which exposed to 1 mM 1,4-benzendithiol (BDT). Each spectrum is an average spectrum of 6 different single nanoparticles. (b) Quantitative comparison of SERS signals at 1555 cm−1. All spectra were obtained under 10 mW 785 nm laser and the acquisition time was 60 s. The error bars represent standard error.

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CONCLUSIONS This work is particularly important because it demonstrates for the first time the on-plate and in-plate structural control of nanomaterials simply based on the polymer-dependent synthesis of the seeds. The synthetic procedure employs neither multiple growth steps nor independent galvanic replacement reactions. The Au/AgBNPLs have also demonstrated an exceptionally high chemical stability and capability as artificial enzymes for common HRP-substrates and an environmental pollutant. The densely loaded DNA on the Au/AgBNPLs implies the potential applications of the Au/AgBNPLs as smart nanoprobes and drug carriers for sensing and therapeutic purposes. Significantly, the SERS enhancement by the Au/ AgBNPLs demonstrates that they could be used as singlenanoparticle SERS imaging probes for use in monitoring intracellular interactions and detecting chemically and biologically important target molecules.70−72

nanoparticles individually dispersed on a glass substrate (see Figure S12). Among the three types of nanoparticles, the Au/ AgBNPLs exclusively exhibited the intense SERS properties at multiple Raman shift frequencies, indicating that the in-plate pores of the Au/AgBNPLs played an essential role as effective nanogaps for the SERS enhancement. In contrast, the Ag nanoplates and the Au nanospheres exhibited only limited SERS enhancement. We further quantitatively analyzed the maximum intensity of the three SERS spectra at 1555 cm−1 (Figure 9b). Although the Raman signal intensity enhanced by the Au nanospheres was almost negligible, the Ag nanoplates exhibited SERS enhancement to some extent, which is still half as high as that of the Au/AgBNPLs. It is noticeable that despite the overlapping of the excitation wavelength (785 nm) with the absorption band of the Ag nanoplates (see Figure S13) the Au/ AgBNPLs still exhibit a stronger Raman signal than the Ag nanoplates do. For quantification of the SERS enhancements, we calculated the SERS enhancement factors (EFs) using the following equation:65

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ASSOCIATED CONTENT

* Supporting Information S

EF = (ISERS × NNormal)/(INormal × NSERS)

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b09803. TEM/STEM images of various polymer-AgSeeds and their grown nanostructures, UV spectra of Au/AgBNPLs, catalytic properties of various bi- and monometallic

where ISERS is the intensity of the SERS signal of 1,4benzenedithiol, INormal is the intensity of the Raman signal of 1,4-benzenedithiol, NSERS is the number of the adsorbed target molecules on each nanoparticle, and NNormal is the number of H


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nanoparticles, and simulated electromagnetic field distribution and dark-field images of nanoparticles (PDF)

(12) Wu, P.; Gao, Y.; Zhang, H.; Cai, C. Aptamer-Guided Silver− Gold Bimetallic Nanostructures with Highly Active Surface-Enhanced Raman Scattering for Specific Detection and Near-Infrared Photothermal Therapy of Human Breast Cancer Cells. Anal. Chem. 2012, 84 (18), 7692−7699. (13) Zhang, N.; Si, Y.; Sun, Z.; Chen, L.; Li, R.; Qiao, Y.; Wang, H. Rapid, Selective, and Ultrasensitive Fluorimetric Analysis of Mercury and Copper Levels in Blood Using Bimetallic Gold−Silver Nanoclusters with “Silver Effect”-Enhanced Red Fluorescence. Anal. Chem. 2014, 86 (23), 11714−11721. (14) Padmos, J. D.; Langman, M.; MacDonald, K.; Comeau, P.; Yang, Z.; Filiaggi, M.; Zhang, P. Correlating the Atomic Structure of Bimetallic Silver−Gold Nanoparticles to Their Antibacterial and Cytotoxic Activities. J. Phys. Chem. C 2015, 119 (13), 7472−7482. (15) Rajendra, R.; Bhatia, P.; Justin, A.; Sharma, S.; Ballav, N. Homogeneously-Alloyed Gold−Silver Nanoparticles as per Feeding Moles. J. Phys. Chem. C 2015, 119 (10), 5604−5613. (16) Qian, Z.; Park, S.-J. Silver Seeds and Aromatic Surfactants Facilitate the Growth of Anisotropic Metal Nanoparticles: Gold Triangular Nanoprisms and Ultrathin Nanowires. Chem. Mater. 2014, 26 (21), 6172−6177. (17) Mayer, M.; Scarabelli, L.; March, K.; Altantzis, T.; Tebbe, M.; Kociak, M.; Bals, S.; García de Abajo, F. J.; Fery, A.; Liz-Marzán, L. M. Controlled Living Nanowire Growth: Precise Control over the Morphology and Optical Properties of AgAuAg Bimetallic Nanowires. Nano Lett. 2015, 15 (8), 5427−5437. (18) Crespo, J.; Lopez-de-Luzuriaga, J. M.; Monge, M.; Elena Olmos, M.; Rodriguez-Castillo, M.; Cormary, B.; Soulantica, K.; Sestu, M.; Falqui, A. The spontaneous formation and plasmonic properties of ultrathin gold-silver nanorods and nanowires stabilized in oleic acid. Chem. Commun. 2015, 51 (93), 16691−16694. (19) Huang, J.; Zhu, Y.; Liu, C.; Zhao, Y.; Liu, Z.; Hedhili, M. N.; Fratalocchi, A.; Han, Y. Fabricating a Homogeneously Alloyed AuAg Shell on Au Nanorods to Achieve Strong, Stable, and Tunable Surface Plasmon Resonances. Small 2015, 11 (39), 5214−5221. (20) Ye, X.; Jin, L.; Caglayan, H.; Chen, J.; Xing, G.; Zheng, C.; Doan-Nguyen, V.; Kang, Y.; Engheta, N.; Kagan, C. R.; Murray, C. B. Improved Size-Tunable Synthesis of Monodisperse Gold Nanorods through the Use of Aromatic Additives. ACS Nano 2012, 6 (3), 2804− 2817. (21) Zhang, W.; Goh, H. Y. J.; Firdoz, S.; Lu, X. Growth of Au@Ag Core−Shell Pentatwinned Nanorods: Tuning the End Facets. Chem. Eur. J. 2013, 19 (38), 12732−12738. (22) Kim, M.; Lee, K. Y.; Jeong, G. H.; Jang, J.; Han, S. W. Fabrication of Au-Ag Alloy Nanoprisms with Enhanced Catalytic Activity. Chem. Lett. 2007, 36 (11), 1350−1351. (23) Gao, C.; Lu, Z.; Liu, Y.; Zhang, Q.; Chi, M.; Cheng, Q.; Yin, Y. Highly Stable Silver Nanoplates for Surface Plasmon Resonance Biosensing. Angew. Chem., Int. Ed. 2012, 51 (23), 5629−5633. (24) McEachran, M.; Keogh, D.; Pietrobon, B.; Cathcart, N.; Gourevich, I.; Coombs, N.; Kitaev, V. Ultrathin Gold Nanoframes through Surfactant-Free Templating of Faceted Pentagonal Silver Nanoparticles. J. Am. Chem. Soc. 2011, 133 (21), 8066−8069. (25) Lu, X.; Au, L.; McLellan, J.; Li, Z.-Y.; Marquez, M.; Xia, Y. Fabrication of Cubic Nanocages and Nanoframes by Dealloying Au/ Ag Alloy Nanoboxes with an Aqueous Etchant Based on Fe(NO3)3 or NH4OH. Nano Lett. 2007, 7 (6), 1764−1769. (26) Bai, T.; Tan, Y.; Zou, J.; Nie, M.; Guo, Z.; Lu, X.; Gu, N. AuBr2−-Engaged Galvanic Replacement for Citrate-Capped Au−Ag Alloy Nanostructures and Their Solution-Based Surface-Enhanced Raman Scattering Activity. J. Phys. Chem. C 2015, 119 (51), 28597− 28604. (27) Lu, X.; Tuan, H.-Y.; Chen, J.; Li, Z.-Y.; Korgel, B. A.; Xia, Y. Mechanistic Studies on the Galvanic Replacement Reaction between Multiply Twinned Particles of Ag and HAuCl4 in an Organic Medium. J. Am. Chem. Soc. 2007, 129 (6), 1733−1742. (28) Jang, H.; Min, D.-H. Spherically-Clustered Porous Au−Ag Alloy Nanoparticle Prepared by Partial Inhibition of Galvanic Replacement

AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NRF funded by the Korean government, MSIP (NRF-2016R1A5A1010148, 2015M3A9D7031015, and NRF-2015R1C1A1A01053865). J.S. L. thanks Dr. Hionsuck Baik and Ms. Suhyun Park at the Korea Basic Science Institute (KBSI; Seoul, Republic of Korea) for their great help with the TEM work.

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