Combinatorial Polymer Library Approach for the Synthesis of Silver

Article pubs.acs.org/cm

Combinatorial Polymer Library Approach for the Synthesis of Silver Nanoplates Byung-Ho Kim,†,‡ Ju-Hwan Oh,†,‡ Sang Hun Han,‡ Young-Ji Yun,‡ and Jae-Seung Lee*,‡ ‡

Department of Materials Science and Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul, Republic of Korea 136-713 S Supporting Information *

ABSTRACT: We employ a combinatorial library approach to control the shape of the silver nanostructures by regulating the structure of their seeds. Twenty-four polymers are investigated in detail with respect to their functionalities, chemical structures, molecular weights, and charges, each of which turns out to play significant roles in synthesizing high-quality silver nanoplates at room temperature in a fast manner. A mechanism depicting the exceptionally stable seed structures ‘stitched’ by polymer threads is proposed, which clearly explains the experimental observations. KEYWORDS: combinatorial polymer library, silver nanoplates, seed-mediated growth, surface plasmon resonance, defect structure ‘seeds,’ owing to the high surface energy and consequent instability of the plate-like seeds.15,33 To shift this unfavorable chemical equilibrium to the facilitated formation of plate-like seeds, polyhedral seeds were either not allowed to take shape from the beginning by slow kinetics25 or etched away by corrosive chemicals.22,23 Recently, the presence of a charged polymer in the seed production step was discovered to influence the seed structure for the production of AgNPLs, which, however, did not provide much elaborate information on precisely how the polymer worked.32 Moreover, the shape evolution of silver nanoplates were monitored systematically, providing evidence for better understanding of their synthetic nature.34 Herein, we present a combinatorial approach to chemically modulate the seed structures for the synthesis of monodisperse and size-controllable AgNPLs. To build up the chemical library, we employed 24 synthetic and natural polymers with a variety of chemical functionalities, molecular structures, and average molar masses, each further categorized by its positive, negative, or neutral charge (Figure 1). The investigation via screening the polymeric materials elucidates what fundamentally controls the stable defect formation of plate-like silver seeds during their early developing stages, which results in high-quality AgNPLs with distinctive chemical and physical properties in a straightforward manner.

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he past decade has witnessed an explosion of advances in synthesizing various anisotropic silver nanomaterials, such as nanosized polyhedrons,1,2 nanowires,3−5 nanorods,6−8 and branched nanostructures.9 These nanomaterials have been widely investigated for various applications, mainly owing to their unique chemical and physical properties, which depend on the shapes and sizes.10−12 In particular, planar silver nanostructures with triangular, hexagonal, and discoidal shapes, or silver nanoplates (AgNPLs), are substantially attractive because of their tunable surface plasmon resonance (SPR), electromagnetic-field enhancement for various spectroscopic techniques, and the possibility of functionalizing their surfaces for various applications.13−16 To develop synthetic strategies for the AgNPLs, desirable features include (1) highly monodispersed planar nanostructure, (2) narrow size distribution, and (3) high reproducibility. Thorough investigation of the AgNPLs has resulted in numerous advances in their synthesis, such as photochemical reactions,17−20 thermal reactions,21,22 oxidative etching,23 decreased kinetics of growth control,24,25 reverse micelles or ultrasonication in organic solvents,26−28 biosynthesis using alga extracts,29 seed-mediated growth,30−32 and combinations of these in certain cases. Despite the vast efforts devoted to achieve these three goals, however, the results have often turned out to be merely mixtures of planar silver nanostructures with undesirable spheroidal particles. Obviously, the existence of such isotropic, nonplanar nanoparticles significantly jeopardizes the highly anisotropic chemical and physical properties of AgNPLs, leading to the failure of further fundamental investigations, and practical applications. This morphological diversity of the resultant nanomaterials obtained from the AgNPL synthesis mainly stems from the thermodynamic preference of polyhedral structures to plate-like ones with suitable defects during the formation of nuclei, or © 2012 American Chemical Society

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RESULTS AND DISCUSSION To investigate the effect of the polymers for the seed structure, we first conducted the synthesis of AgNPLs using silver seeds prepared with each polymer and analyzed their structures using Received: August 31, 2012 Revised: October 15, 2012 Published: October 23, 2012 4424

dx.doi.org/10.1021/cm3028115 | Chem. Mater. 2012, 24, 4424−4433

Chemistry of Materials

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Figure 1. Chemical structures of the 24 polymers that are screened for their ability to control the seed structures for the synthesis of silver nanoplates. Cationic polymers (A to J) are demonstrated in the green box, while neutral (K to R) and anionic (S to X) polymers are displayed in the red and blue boxes, respectively.

soluble in water, as the synthesis of AgNPLs was carried out in an aqueous system (vide inf ra). The synthesis of AgNPLs in this work was carried out following the typical seed-mediated growth method, only except that the seeds were synthesized with the given polymer at a certain concentration (Scheme 1). For the seed preparation, a mixture solution of trisodium citrate, sodium borohydride, and a given polymer was prepared, to which aqueous silver nitrate was slowly added. In a typical synthesis, the color of the final mixture turned yellow, indicative of the formation of tiny silver seed particles. Significantly, in contrast to the seeds synthesized without polymers, these seeds were

transmission electron microscopy (TEM). Before the synthesis, 24 polymers were carefully selected to constitute a polymer library that encompasses a variety of chemically functional moieties (Figure 1). They are primarily classified into three categories in regard to their charges (10 cationic, 6 anionic, and 8 neutral polymers) and further subdivided based on molecular weights, hydrophilic/hydrophobic properties, polarities, and arrangement of blocks in case of di- and triblock copolymers. We also employed polyvinylpyrrolidones (PVPs) with 4 different molecular weights, which are popularly known as powerful structure-directing reagents for the shape control of silver nanomaterials.21,28,35 Particularly, we classified them as positively charged polymers (G, H, I, and J in Figure 1, green box with deep green background) in spite of their actual net neutral charge.36,37 This classification is because of their partial positive charges with quasi-cationic properties associated with the unshared electron pair of nitrogen and much stronger attractive interactions with anionic molecules than positive or neutral ones.38−42 The library also comprises polymers having branched or chain structures, aromatic or aliphatic (cyclic/ linear) monomers, strong or weak ionic strength, and synthetic or natural sources. It also contains biological polymers such as polysaccharides and polypeptides. Importantly, this highly diverse nature of the polymer library is expected to provide a number of possible chemical interactions between the functional moieties of the polymers and the embryonic silver atom clusters during the early development of silver seeds, leading to their controlled defect structures.43−45 All of the polymers are

Scheme 1. Schematic Illustration of the Polymer Library Approach for the Synthesis of Silver Nanoplates Based on the Seed-Mediated Growth Method

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Figure 2. UV−vis spectra of the seeds synthesized with (A) cationic, (B) neutral, and (C) anionic polymers and their grown structures (D: cationic, E: neutral, and F: anionic polymers). Note that the spectra are not normalized in order to express the exact extinction from the concentrations and extinction coefficients of the seeds and grown structures under the same synthetic conditions.

stable at 4 °C for at least one month, after which they exhibited the same properties and resultant nanoparticles grown (Figure 1S, see the Supporting Information). For the growth of AgNPLs, the seed solution was combined with silver precursor (AgNO3) and reductant (ascorbic acid). The 25 types of seeds (24 types of seeds each with one of the 24 polymers and control seed devoid of any polymers) appeared almost indistinguishable with bare eyes. As the growth of the AgNPLs took place, however, the color of the reaction mixture gradually changed, depending on their homogeneity and the size of the AgNPLs. To spectroscopically analyze the optical properties of the seeds and corresponding AgNPLs, we first obtained the UV− vis spectra of the 25 types of seeds (Figures 2A to 2C) and the AgNPLs obtained from those seeds (Figures 2D to 2F) and grouped them by the charge of the associated polymers for comparison (cationic polymers: Figures 2A and 2D; neutral polymers: Figures 2B and 2E; and anionic polymers: Figures 2C and 2F). For quantitative analysis, we obtained both the wavelengths where the maximum plasmon absorption of the seeds and AgNPLs takes place (λMAX) and the full width at halfmaximum (fwhm) of the spectra (Table 1S, see the Supporting Information). Generally, the seeds with cationic polymers resulted in mainly one surface plasmon band around 400 nm, indicative of isotropic silver nanostructures (Figure 2A). In most cases, however, the spectra were substantially broad or almost flat, which suggests that the products were composed of highly polydispersed silver nanoparticles with various sizes, possibly up to several micrometers. For example, the spectra of the seeds with A, D, E, and F are almost flat, indicating that the resultant seed solutions were actually almost ‘seedless’. Meanwhile, B and C resulted in nanosized seeds with SPR, but with UV−vis spectra that were still broader than that of the control seeds (Y in Figure 2C). Interestingly, the tendency of cationic polymers to produce the seeds exhibiting broad UV− vis spectra is also observed with the smallest and largest PVPs (G and J). Over all, only two PVPs with intermediate molecular weights (H and I) resulted in seeds exhibiting sharp plasmon bands among the ten positively charged polymers examined

(from A to J). The dependence of the particle shape on the PVP length was also reported in conventional AgNPL synthesis, proving the general importance of the polymer length in nanomaterial synthesis.46 Unlike the seeds synthesized with the cationic polymers, those with the neutral polymers exhibited nonflat spectra in all cases (Figure 2B). The broadness of the spectra, however, varied significantly depending on the chemical components of the polymers. For example, the seeds with polyethylene glycol (PEG; K) exhibited a remarkably broad curve with a lowest maximum extinction at 388 nm, while other block copolymers containing both PEG and additional hydrophobic groups such as propylene glycols (PPG; P and R), alkyl chains (O), or benzene rings (M) produced seeds exhibiting relatively sharp plasmon bands. This contrast indicates that the combination of hydrophilic and hydrophobic blocks in a polymer molecule tend to make a strong affirmative contribution to the formation of nanosized silver seeds in high yield. Interestingly, however, the seeds with triblock copolymers with larger proportion of hydrophobic blocks (PPG) such as L (PPG-PEG-PPG) exhibited broader spectra with weaker extinction. We also observed that P, the shortest among the three PEG-PPG-PEGtype triblock copolymers (P, Q, and R), exhibited very broad seed spectra with low extinction and asymmetry. Note that this observation of how the length of structurally similar polymers affects the seed formation is analogous to what we observed with PVPs (vide supra). Six anionic polymers were further examined for the seed synthesis. Most of the anionic polymers produced seeds with clear, sharp plasmon bands with a maximum extinction at ∼393 nm (Figure 2C). Significantly, this result was obtained regardless of the anionic moieties, such as carboxylate, phosphate, sulfate, or sulfonate. Only the seeds with T resulted in almost flat spectra. T is a copolymer that consists of two different monomers arranged in an alternative way and may exhibit different properties from those of other block copolymers. In fact, T is the only copolymer among the anionic polymers in our work, possibly indicating that anionic homopolymers provide more favorable conditions for the silver 4426



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Figure 3. TEM images of the grown silver nanostructures from the seeds synthesized with each polymer (A to X) and with no polymer (Y). Most of the nanoplates have truncated triangular shapes, while a few of those are discoidal. The spheroidal particles, most of which are observed in (A), (B), (C), (D), (E), (F), and (G), are much smaller and appear darker than the AgNPLs. No particulate nanostructures are observed in (A), but rather larger microstructures are obtained. The size and size distribution of the nanostructures are quantified (Table 2S, see the Supporting Information).

Although the UV−vis spectra of the silver seeds synthesized with various polymers and their corresponding AgNPLs provided certain clues for how the polymers control the overall optical properties and size distributions of the seeds, only limited information can be inferred, inaccurately if any, for the larger silver nanomaterials grown. To further investigate how the polymers induce growth of the AgNPLs by controlling the structures of the seeds, the actual AgNPL morphologies and the proportion of undesirable impure spheroidal particles were further analyzed by TEM. Figure 3 demonstrates the 25 representative TEM images of the silver nanomaterials grown from each of the 25 types of the seeds. Significantly, the TEM images show dramatic differences in the proportion of the spheroidal nanoparticles in regard to the types and functions of the polymers employed. In particular, A resulted in nonparticulate structures, and the grown product from the seeds with C contains a few giant triangular or hexagonal nanoplates whose edge length is over 1 μm (Figures 3A and 3C). Most of the cationic polymers (B, D, E, and F), however, led to the formation of a majority of spheroidal nanoparticles, as inferred from their UV−vis spectra (Figure 2D). Interestingly, the quasicationic PVPs (H, I, and J) generally induced the formation of a majority of AgNPLs, with an exception of the shortest PVP (G), whose result is just a mixture of nanoparticles with various shapes. In contrast, the neutral and anionic polymers (K to X)

seeds to have defined nanometer sizes. Interestingly, unlike the seeds with other anionic polymers, the silver seeds with W exhibit the maximum extinction at 422 nm, which is slightly red-shifted compared to the plasmon bands of the others. The effect of the polymers on the seed structures was evaluated by the optical properties of their grown structures. For this purpose, we allowed the seeds to grow into larger silver nanostructures and obtained their UV−vis spectra (Figures 2D, E, and F). In general, the seeds that exhibit intense plasmon bands also grew up into nanostructures exhibiting intense optical properties at or over 650 nm, which is the range of typical λMAX of AgNPLs. Meanwhile, several polymers resulted in optically active seeds but inactive grown structures with little SPR, such as B, C, U, and V. In other words, the grown structures with intense plasmon bands are a sufficient condition for the corresponding seeds with strong plasmon bands, but the reverse is not always true. This result demonstrates that more distinctive selectivity of polymers for the seed growth into AgNPLs is obtained by observing the UV−vis spectra of the grown structures. Moreover, various λMAX values in Figures 2D, E, and F indicate various concentrations of the seeds from the same initial concentration of Ag+ (0.24 mM), which demonstrates the fundamentally different interactions of the polymers with Ag+ and the seeds during the seed synthesis. 4427



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the same capability, we first chose two representative polymers from each of the neutral and anionic polymer libraries (R and W) and synthesized the AgNPLs with three different sizes using different amounts of the seeds (Figures 5A to 5F). It was clear that the AgNPLs grew larger with fewer seeds, and smaller with more seeds, indicating that the seeds whose structures are controlled by the specific polymers still exhibit the ordinary growth mechanism of other types of seeds. We further chose another representative polymer from the cationic polymer library (H) and obtained the UV−vis spectra of the AgNPLs grown from 10 different amounts of the seed solution from 5 to 600 μL (Figure 5G). All of the spectra exhibit a major plasmon band for the longitudinal absorption (in-plane dipole), which gradually blue-shifts from 850 to 450 nm as the seed solution volume increases from 5 to 600 μL (Figure 5H), as well as a minor extinction consistently around 400 nm owing to the transverse absorption (in-plane and out-of-plane quadrupoles).47 These spectra are further strong evidence to demonstrate the systematic size control of the AgNPLs by changing the amount of the seeds. The broadness of the transverse (quadrupole) bands was quantitatively obtained by measuring their fwhm, demonstrating the typical sharper plasmon bands of the smaller AgNPLs (Figure 5H, inset), as observed in previous studies on the size control of AgNPLs.31 Polymers, surfactants, or halide ions during the growth of nanomaterials make a significant contribution to shape control, mainly by protecting specific facets and preventing their growth.48 Strictly speaking, the growth condition in this work also includes a certain amount of the given polymer from the seed solution containing 12 mg/L of the polymer. Although the final concentration of the polymer in the growth solution is extremely low (typically 0.22 mg/L) and is inconsistent depending on the amount of the seed solution, their possible role as a protecting ligand during the growth of the AgNPLs needs to be verified. To prove this hypothesis, we synthesized the control seeds (seeds synthesized with no polymer) and allowed them to grow in the presence of two representative polymers that exhibited high selectivity of the AgNPL formation (R and W), respectively. Importantly, the concentration of the polymer in the growth solution was determined to be either the same as that of our growth conditions (typically 0.22 mg/L) or far higher and the same as that of our seed synthesis conditions (12 mg/L). Figure 6 demonstrates the TEM images of the grown nanomaterials in the presence of each polymer at two different concentrations. In all cases, the result is a mixture of AgNPLs and spheroidal nanoparticles, analogous to the structures grown without any polymers from the control seeds (Figure 2Y). This result clearly demonstrates the critical function of the polymers on the formation of the seed structures during the synthesis of the seeds and excludes the possibility of the polymers to block specific facets of the seeds for their growth control.32 To further determine precisely how the polymers affect the seed structure formation, we controlled the stoichiometry of silver nitrate and the polymer. Specifically, we chose W as a model polymer and synthesized the silver seeds with W at six different concentrations of 0.012, 0.6, 1.2, 6, 24, and 180 mg/L, spanning 4 orders of magnitude. Considering 12 mg/L as 1X, which is the concentration of polymers in a typical seed synthesis, the varied concentrations of W correspond to 0.001X, 0.05X, 0.1X, 0.5X, 2X, and 15X. When the seeds were synthesized with the reduced concentration of W down to 0.001X, they grew into nanostructures that were still planar, yet

appear to provide favorable conditions during the seed synthesis for their growth into the AgNPLs. We also observed the product grown from the control seeds (no polymer), which does not exhibit any preference for the AgNPL formation (Figure 3Y). The effect of the polymers on the structures of the seeds was further quantitatively analyzed by counting the number of the AgNPLs and other impure particles to obtain their proportions. For reasonable statistical analysis of each grown product, we obtained several TEM images for each grown silver nanomaterial associated with each polymer (A to Y) from multiple batches, identified all of the individual nanomaterials whose total number was over 300, and compared the ratio of AgNPLs to spheroidal nanoparticles (Figure 4). Among the 24

Figure 4. The ratio of the number of the AgNPLs to the number of the spheroidal nanoparticles, which are grown from the seeds synthesized with each polymer (A to X) or no polymer (Y). Each color of the bars indicates the charge of the polymers (green: cationic, red: neutral, and blue: anionic). In case that the ratio of the AgNPLs is larger than ∼19 (over 95% of AgNPLs), we consider it to be extremely large and do not draw it to scale (H, O, R, S, and W).

polymers, only H, O, R, S, and W exhibited exceptionally excellent selectivity for the formation of AgNPLs. Interestingly, two of them are neutral (O and R), one is quasi-cationic (H), and the other two are anionic (S and W), which clearly demonstrates that neutral and negative charges of the polymers are preferred to control the structures of the seeds to induce the selective formation of AgNPLs. Note that both neutral polymers (O and R) are amphiphilic and contain polyethylene glycol blocks. We also observed a very clear length-dependence of the similarly structured homopolymers (H, I, and J) and block copolymers (P, Q, and R) on the AgNPL formation, indicating that the polymer length plays a significant role in controlling the structure of the seeds. While the neutral and anionic polymers demonstrated that each of them induced at least more than half of the grown nanoparticles to be AgNPLs, most of the cationic polymers failed to regulate the seeds to induce anisotropic planar structures. The ratio of the AgNPLs grown from the control seeds (Y) is about 0.5, indicating that the presence of the neutral or anionic polymers highly enhances the formation of AgNPLs. In contrast, the cationic polymers significantly deteriorate the selective formation of AgNPLs, resulting in ratios of AgNPLs that are close to zero. One of the advantages of the seed-mediated growth is the capability to control the size of the grown structures simply by controlling the concentration of the seeds. In order to investigate if the AgNPLs described in this work also have 4428



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Figure 5. The TEM images of the AgNPLs grown from the various amounts of the seeds synthesized with R (A: 10 μL, B: 300 μL, and C: 1000 μL of the seeds) and W (D: 1 μL, E: 80 μL, and F: 1000 μL of the seeds). Note that the size of the grown structures gradually increases as the amount of the seeds decreases. (G) UV−vis spectra of the grown structures from various amounts of the seeds synthesized with H. (H) The wavelength where the maximum extinction takes place (λMAX) in the UV−vis spectra of (G), and the full width at half-maximum (fwhm) of the spectra is drawn as a function of the seed volume (inset).

From the result obtained with a series of concentrations of W in Figure 7, we noted three important observations: (O1) in all cases a majority of AgNPLs were synthesized, although their shapes, sizes, and proportions varied depending on the concentration of W; (O2) more spheroidal nanoparticles were obtained at the higher concentration of W; and (O3) the UV−vis spectra of the seeds remained almost the same at 2X and 15X of W. These interesting observations, in association with (O4) the long-term stability of the seeds (vide supra), provide a convincing outline concerning how these polymers induce the seed growth into AgNPLs (Scheme 2A). At the beginning of the seed formation, the silver cations are expected to locate in close proximity to the anionic/neutral polymer molecules owing to the attractive electrostatic and van der Waals interactions.49−52 These interactions would lead to the increased local concentration of silver cations, followed by their concomitant rapid reduction by NaBH4. Eventually, we expect the formation of silver seed nanoparticles with the incorporated linear polymer molecules, partly embedded inside of the particles and partly exposed, as if the particles are “stitched” by the polymer “thread,” instead of their surfaces being modified with it (Scheme 2A). According to our hypothesis, it is at this stage that the previously discussed chemical and structural diversity of each polymer plays a significant role in stitching the seeds to exhibit controlled defects and grow into planar

irregularly sized and disfigured, such as trapezoids with a few cracks (Figure 7A). The size distribution was further improved with the increased concentration of W to 0.05X, while the cracks were still observed in almost every plate (Figure 7B). Eventually, the uniformity of the AgNPLs was obtained at 0.1X, 0.5X, and 1X of W (Figures 7C, 7D, and 3W). As we increased the concentration of W to 2X and 15X, however, we obtained a considerable number of spheroidal nanoparticles along with the AgNPLs (Figures 7E and 7F). To understand how the concentration of W leads to the change in size and concentration of the silver seeds, we investigated the optical properties of the seeds synthesized at different concentrations of W (from 0.001X to 15X) by monitoring their UV−vis spectra (Figure 7G). Interestingly, each concentration of W resulted in a dramatic difference in extinction, λMAX, and broadness of the plasmon band. As the concentration of W increased from 0.001X to 2X, λMAX substantially increased from 389 to 453 nm with gradually reduced extinctions, indicating that the seed size increased but its concentration decreased (Figure 7G, inset). As the concentration of W further increased to 15X, however, both λMAX and extinction remained almost the same, demonstrating that the seeds synthesized with W at higher concentrations than 1X were of similar sizes at similar concentrations. 4429



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Figure 6. The TEM images of the silver nanostructures grown with polymers from the seeds synthesized without any polymers. The seeds were synthesized with either R (A and B) or W (C and D) and further grown in the growth solutions containing different concentrations of polymers (A and C: 0.22 mg/L, B and D: 12 mg/L). In all cases, both AgNPLs and spheroidal nanoparticles are evenly observed. Figure 7. The silver nanostructures grown from the seeds synthesized at various concentrations of W (A: 0.012 mg/L, B: 0.6 mg/L, C: 1.2 mg/L, D: 6 mg/L, E: 24 mg/L, and F: 180 mg/L) and the UV−vis spectra of the seeds (G). All six structures are grown from 10 μL of the seed solution. Note that the diameter of the spheroidal nanoparticles is around 80 nm, while that of the planar structures is around 250 nm or larger. The λMAX of the spectra in (G) is shown as a function of the concentration of W in the inset of (G).

structures. This proposed mechanism can explain the exceptional long-term stability of the seeds (O4) protected by the incorporated anionic/neutral polymers that are irremovable under ambient chemical conditions.53 Importantly, these ‘stitched’ seeds not only are stable for an extended amount of time (one month) but also survive under high ionic strength conditions up to 0.5 M NaNO3 (Figure 2S, see the Supporting Information). Furthermore, this ‘stitched seed’ model explains how the stoichiometry of the polymer to Ag+ could lead to the corresponding nanostructures grown in Figure 7. Based on the UV−vis spectra of the seeds (Figure 7G), the size and concentration of the seeds remained almost the same at 2X and 15X of W (O3), which allows a consistent limit in the total amount of polymer that can be incorporated into the entire seeds at higher concentration of W, because only a limited number of stitching polymer molecule would be allowed per seed. Consequently, the extra polymers would simply interact with the stitched seeds and would be obstacles to their anisotropic growth by covering the fully stitched seeds, such that they do not grow into AgNPLs (Scheme 2B). This reasoning explains why more spheroidal nanoparticles were obtained at higher concentration of W at 15X (O2). Once stitched fully or in part without any extra polymer, however, seeds grow into AgNPLs whose shape could depend on the concentration of W (O1). In sharp contrast, cationic polymers would not be able to locally concentrate the silver cations because of their repulsion, resulting in the formation of the unstitched seeds that are unlikely to grow into AgNPLs (Scheme 2C). While the embedment of the polymers into silver seeds is supported by the salt-stability and long-term stability of the seeds (Figures 1S and 2S, see the Supporting Information), the exposure of the polymers out of the seeds requires further verification. To provide a convincing evidence for the exposure

of the polymers, we took advantage of the programmable assembly of DNA-plasmonic nanoparticle conjugates and their concomitant color change owing to the distance-dependent optical properties.54−56 We first synthesized the silver seeds stitched by polythymine (5′ T100 3′) sequences (T100-AgSeeds) following our experimental procedure (Figure 8, a) and obtained their planar nanostructures after their growth (Figure 3S, see the Supporting Information), as typically observed with other anionic polymers. We further combined the T100-AgSeeds with excess amount of gold nanoparticles conjugated with monothiol polyadenine (5′ HS-A20 3′) sequences (A20-AuNPs) and allowed them to hybridize for 4 h (Figure 8, b). Significantly, we observed the precipitated aggregates in the mixture of T100-AgSeeds and A20-AuNPs, strongly indicative of the exposed and functional T100 sequences out of the silver seeds and their DNA−DNA interconnect formation with A20AuNPs. Moreover, the color of the mixture solution was originally orange (red from gold nanoparticles + yellow from silver seeds) but changed to red after hybridization, owing to the excess amount of unhybridized A20-AuNPs in the supernatant (Figure 8, b). Once heated, however, the aggregates thermally dehybridized with a color change of the solution back to orange, which demonstrates that the aggregates were formed via the reversible assembly of T100AgSeeds and A20-AuNPs.57,58 To examine if the interactions between T100-AgSeeds and A20-AuNPs are sequence-specific, 4430



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Scheme 2. Scheme Depicting the Synthetic Mechanism of the ‘Stitched Seeds’ and Their Controlled Defects by the Polymer ‘Threads’: (A) Anionic Polymers in an Appropriate Amount, (B) Anionic Polymers in an Excessive Amount, and (C) Cationic Polymers

the seeds for their growth into AgNPLs in the context of a seed-mediated growth strategy. By statistical analysis of the electron microscopic results, we identified four important chemical properties of the polymers as the determining parameters for the structure control of the seeds and their subsequent anisotropic growth under the conditions studied: (1) the negative or neutral charge, (2) appropriate length, (3) amphiphilic structures, and (4) preference of PEG over PPG groups. Furthermore, our proposed model of the ‘polymerstitched’ seeds and their growth answers the questions that arise from our experimental observations. Obviously, the effect of other experimental conditions such as concentrations of nonpolymeric regents, reaction temperature, and pH of reaction mixture needs to be investigated for comprehensive understanding in the context of the ‘polymer-stitched’ model. Significantly, this work provides a general combinatorial strategy to control the shape of silver nanostructures into other anisotropic architectures, by screening libraries composed of other various types of polymers. Considering the recent success of several biomimetic approaches for nanomaterial synthesis, other biologically active polymeric materials such as proteins and viruses, or supramolecular assembly of elaborately designed chemicals, would be highly productive candidates with great potential.59,60

Figure 8. A photograph showing three nanoparticle solutions: a) the silver seeds stitched by polythymine (T100-AgSeeds), b) T100-AgSeeds combined with A20-AuNPs, and c) T100-AgSeeds combined with gold nanoparticles conjugated with noncomplementary sequences (5′ HSA10-AGTGATAAT 3′). Note that the hybridized T100-AgSeeds with A20-AuNPs are precipitated in b) and designated with a red circle. In contrast, the mixture of T100-AgSeeds and noncomplementary DNAAuNP conjugates remain the same without exhibiting any hybridization properties.

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EXPERIMENTAL SECTION

Materials. Trisodium citrate dihydrate (cat.# S4641), L-ascorbic acid (cat.# A5960), silver nitrate (AgNO3, cat.# 204390), sodium borohydride (NaBH4, cat.# 480886), poly-L-lysine hydrobromide (cat.# P0879), poly(ethyleneimine) solution (cat.# 482595), DEAEdextran hydrochloride (cat.# D9885), poly(diallyldimethylammonium chloride) solution (cat.# 409014), poly-L-histidine (cat.# P9386), chitosan (cat.# 448869), poly(acrylic acid, sodium salt) solution (cat.# 416037), poly(2-hydroxyethyl methacrylate) (cat.# 192066), RNA from torula yeast (cat.# R6625), dextran sulfate sodium salt from Leuconostoc spp. (cat.# 31404), poly(ethylene glycol) (cat.# 202444), Brij 58 (cat.# P5884b), Triton X-100 (cat.# X100), poly(styrene-altmaleic acid) sodium salt solution (cat.# 662631), poly(propylene glycol) (cat.# 202312), polyvinylpyrrolidone (cat.# 856452; M.W. 10,000, cat.# 234257; M.W. 29,000, cat.# 856568; M.W. 55,000, cat.# 81440; M.W. 360,000), poly(sodium 4-styrenesulfonate) solution (cat.# 434574; M.W. 1,000,000), L64 (cat.# 435449) P123 (cat.# 435465), F108 (cat.# 542342), and poly(propylene glycol)-blockpoly(ethylene glycol)-block-poly(propylene glycol) (cat.# 435481) were purchased from Sigma-Aldrich (MO, USA). DNA sequences (5′ T100 3′, 5′ HS-A20 3′, and 5′ HS-A10-AGTGATAAT 3′) were

we also prepared a mixture of T100-AgSeeds and AuNPs conjugated with noncomplementary DNA sequences (Figure 8, c). The mixture of T100-AgSeeds and noncomplementary DNAAuNP conjugates, however, did not exhibit any color change from orange nor produce any precipitates (Figure 8, c), indicating that the hybridization of T100-AgSeeds and A20AuNPs is sequence-specific. While these experimental observations well explains our proposed mechanism, further spectroscopic analysis would be required to clarify the structure of the polymer-seed conjugates at an atomic level.

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CONCLUSIONS We have established a polymer library composed of 24 diverse polymers with various chemical and physical properties and screened them to identify the ones to control the structure of 4431



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(7) Pietrobon, B.; McEachran, M.; Kitaev, V. ACS Nano 2009, 3, 21− 26. (8) Hu, J. Q.; Chen, Q.; Xie, Z. X.; Han, G. B.; Wang, R. H.; Ren, B.; Zhang, Y.; Yang, Z. L.; Tian, Z. Q. Adv. Funct. Mater. 2004, 14, 183− 189. (9) Han, S. H.; Park, L. S.; Lee, J.-S. J. Mater. Chem. 2012, 22, 20223−20231. (10) Wiley, B.; Sun, Y.; Xia, Y. Acc. Chem. Res. 2007, 40, 1067−1076. (11) Sau, T. K.; Rogach, A. L.; Jackel, F.; Klar, T. A.; Feldmann, J. Adv. Mater. 2010, 22, 1805−1825. (12) Elechiguerra, J. L.; Reyes-Gasga, J.; Yacaman, M. J. J. Mater. Chem. 2006, 16, 3906−3919. (13) Pastoriza-Santos, I.; Liz-Marzan, L. M. J. Mater. Chem. 2008, 18, 1724−1737. (14) Wiley, B.; Sun, Y. G.; Mayers, B.; Xia, Y. N. Chem.Eur. J. 2005, 11, 454−463. (15) Lu, X. M.; Rycenga, M.; Skrabalak, S. E.; Wiley, B.; Xia, Y. N. Annu. Rev. Phys. Chem. 2009, 60, 167−192. (16) Kim, J. Y.; Lee, J.-S. Chem. Mater. 2010, 22, 6684−6691. (17) Jin, R. C.; Cao, Y. W.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901−1903. (18) Zhang, J. A.; Langille, M. R.; Mirkin, C. A. J. Am. Chem. Soc. 2010, 132, 12502−12510. (19) Callegari, A.; Tonti, D.; Chergui, M. Nano Lett. 2003, 3, 1565− 1568. (20) Rocha, T. C. R.; Winnischofer, H.; Westphal, E.; Zanchet, D. J. Phys. Chem. C 2007, 111, 2885−2891. (21) Sun, Y. G.; Mayers, B.; Xia, Y. N. Nano Lett. 2003, 3, 675−679. (22) Metraux, G. S.; Mirkin, C. A. Adv. Mater. 2005, 17, 412−415. (23) Wu, X. M.; Redmond, P. L.; Liu, H. T.; Chen, Y. H.; Steigerwald, M.; Brus, L. J. Am. Chem. Soc. 2008, 130, 9500−9506. (24) Washio, I.; Xiong, Y. J.; Yin, Y. D.; Xia, Y. N. Adv. Mater. 2006, 18, 1745−1749. (25) Xiong, Y. J.; Washio, I.; Chen, J. Y.; Cai, H. G.; Li, Z. Y.; Xia, Y. N. Langmuir 2006, 22, 8563−8570. (26) Maillard, M.; Giorgio, S.; Pileni, M. P. J. Phys. Chem. B 2003, 107, 2466−2470. (27) Jiang, L. P.; Xu, S.; Zhu, J. M.; Zhang, J. R.; Zhu, J. J.; Chen, H. Y. Inorg. Chem. 2004, 43, 5877−5883. (28) Pastoriza-Santos, I.; Liz-Marzan, L. M. Nano Lett. 2002, 2, 903− 905. (29) Xie, J. P.; Lee, J. Y.; Wang, D. I. C.; Ting, Y. P. ACS Nano 2007, 1, 429−439. (30) Chen, S. H.; Carroll, D. L. Nano Lett. 2002, 2, 1003−1007. (31) Chen, S. H.; Carroll, D. L. J. Phys. Chem. B 2004, 108, 5500− 5506. (32) Aherne, D.; Ledwith, D. M.; Gara, M.; Kelly, J. M. Adv. Funct. Mater. 2008, 18, 2005−2016. (33) Lofton, C.; Sigmund, W. Adv. Funct. Mater. 2005, 15, 1197− 1208. (34) Goebl, J.; Zhang, Q.; He, L.; Yin, Y. D. Angew. Chem., Int. Ed. 2012, 51, 552−555. (35) Sun, Y. G.; Xia, Y. N. Adv. Mater. 2003, 15, 695−699. (36) Xu, Y.; Teraoka, I.; Senak, L.; Wu, C. S. Polymer 1999, 40, 7359−7366. (37) Majhi, P. R.; Moulik, S. P.; Burke, S. E.; Rodgers, M.; Palepu, R. J. Colloid Interface Sci. 2001, 235, 227−234. (38) Chiu, C. Y.; Yen, Y. J.; Kuo, S. W.; Chen, H. W.; Chang, F. C. Polymer 2007, 48, 1329−1342. (39) Frank, H. P.; Barkin, S.; Eirich, F. R. J. Phys. Chem. 1957, 61, 1375−1380. (40) Murata, M.; Arai, H. J. Colloid Interface Sci. 1973, 44, 475−480. (41) Takagish, T.; Kuroki, N. J. Polym. Sci., Polym. Chem. 1973, 11, 1889−1901. (42) Pankratov, A. N.; Borodulin, V. B.; Chaplygina, O. A. J. Coord. Chem. 2004, 57, 665−675. (43) Xiong, Y.; Washio, I.; Chen, J.; Sadilek, M.; Xia, Y. N. Angew. Chem., Int. Ed. 2007, 46, 4917−4921.

purchased from GenoTech Corp. (Daejeon, Republic of Korea). The polymers are designated as suggested by the manufacturer’s nomenclature. NAP-5 columns were purchased from GE Healthcare (NJ, USA). Synthesis of the AgNPLs. Before the synthesis, RNA was further purified using a NAP-5 column to remove insoluble impurities and salts. For the preparation of the seed particles, aqueous solutions of trisodium citrate (5 mL, 2.5 mM), a given polymer (0.25 mL, 0.5 g/L), and NaBH4 (0.3 mL, 10 mM, freshly prepared) were combined. Subsequently, an aqueous solution of AgNO3 (5 mL, 0.5 mM) was injected into the mixture solution at a rate of 2 mL/min with vigorous stirring. Consequently, a yellow seed solution was obtained in 5 min. The seed solution (typically 0.15 mL) and an ascorbic acid solution (0.075 mL, 10 mM) were added to pure water (5 mL), into which the aqueous AgNO3 solution (3 mL, 0.5 mM) was injected from a syringe drop by drop by gravity with vigorous stirring. A gradual color change of the reaction mixture took place, indicating the growth of AgNPLs. When the color change stopped, a trisodium citrate solution (0.5 mL, 25 mM) was spiked into the mixture to stabilize the AgNPLs. UV−vis Spectroscopy. The UV−vis spectra of the silver seeds and the AgNPLs were obtained at 25 °C from 300 to 1500 nm using an Agilent 8453 spectrophotometer. Transmission Electron Microscopy (TEM). Samples for TEM were prepared by placing a 2.5 μL drop of the sample-containing solution onto a carbon-coated Formvar copper grid (400 mesh, Electron Microscopy Sciences). The grids were allowed to dry in a convection oven. TEM images were obtained using TECNAI G2 F30ST (FEI) operated at 300 kV.

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ASSOCIATED CONTENT

* Supporting Information S

UV−vis spectra and TEM images of the silver nanostructures grown from aged seeds, the spectra of the stitched and control silver seeds at various salt concentrations, and a TEM image of silver nanostructures grown from silver seeds synthesized with T100. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions †

These authors equally contributed to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the second stage of the Brain Korea 21 Project in 2012 and the National Research Foundation of Korea (grant # 2012R1A1A2A10042814). We thank Dr. Hionsuck Baik and Ms. Jinhwa Oh at the Korea Basic Science Institute (KBSI; Seoul, Republic of Korea) for their help with the TEM work.

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REFERENCES

(1) Sun, Y.; Xia, Y. Science 2002, 298, 2176−2179. (2) Langille, M. R.; Zhang, J.; Personick, M. L.; Li, S.; Mirkin, C. A. Science 2012, 337, 954−957. (3) Hong, B. H.; Bae, S. C.; Lee, C. W.; Jeong, S.; Kim, K. S. Science 2001, 294, 348−351. (4) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 617−618. (5) Sun, Y. G.; Gates, B.; Mayers, B.; Xia, Y. N. Nano Lett. 2002, 2, 165−168. (6) Zhang, J.; Langille, M. R.; Mirkin, C. A. Nano Lett. 2011, 11, 2495−2498. 4432



Article

(44) Rocha, T. C. R.; Zanchet, D. J. Phys. Chem. C 2007, 111, 6989− 6993. (45) Zeng, J.; Tao, J.; Li, W. Y.; Grant, J.; Wang, P.; Zhu, Y. M.; Xia, Y. N. Chem.–Asian J. 2011, 6, 376−379. (46) Junior, A. M.; de Oliveira, H. P. M.; Gehlen, M. H. Photochem. Photobiol. Sci. 2003, 2, 921−925. (47) Millstone, J. E.; Hurst, S. J.; Metraux, G. S.; Cutler, J. I.; Mirkin, C. A. Small 2009, 5, 646−664. (48) Cathcart, N.; Frank, A. J.; Kitaev, V. Chem. Commun. 2009, 7170−7172. (49) Abdel-Mohsen, A. M.; Hrdina, R.; Burgert, L.; Krylova, G.; Abdel-Rahman, R. M.; Krejcova, A.; Steinhart, M.; Benes, L. Carbohydr. Polym. 2012, 89, 411−422. (50) Chia, K. K.; Cohen, R. E.; Rubner, M. F. Chem. Mater. 2008, 20, 6756−6763. (51) Huang, H. Z.; Yang, X. R. Biomacromolecules 2004, 5, 2340− 2346. (52) Wei, D. W.; Sun, W. Y.; Qian, W. P.; Ye, Y. Z.; Ma, X. Y. Carbohydr. Res. 2009, 344, 2375−2382. (53) Lim, D. K.; Kim, I. J.; Nam, J. M. Chem. Commun. 2008, 5312− 5314. (54) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607−609. (55) Storhoff, J. J.; Lazarides, A. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L.; Schatz, G. C. J. Am. Chem. Soc. 2000, 122, 4640− 4650. (56) Lee, J.-S.; Lytton-Jean, A. K. R.; Hurst, S. J.; Mirkin, C. A. Nano Lett. 2007, 7, 2112−2115. (57) Jin, R.; Wu, G.; Li, Z.; Mirkin, C. A.; Schatz, G. C. J. Am. Chem. Soc. 2003, 125, 1643−1654. (58) Park, H. G.; Joo, J. H.; Kim, H. G.; Lee, J.-S. J. Phys. Chem. C 2012, 116, 2278−2284. (59) Gugliotti, L. A.; Feldheim, D. L.; Eaton, B. E. Science 2004, 304, 850−852. (60) Coppage, R.; Slocik, J. M.; Briggs, B. D.; Frenkel, A. I.; Naik, R. R.; Knecht, M. R. ACS Nano 2012, 6, 1625−1636.

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Combinatorial Polymer Library Approach for the Synthesis of Silver

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