Article pubs.acs.org/Langmuir

Mussel-Adhesive-Inspired Fabrication of Multifunctional Silver Nanoparticle Assemblies Shuqiang Xiong,† Yan Wang,*,‡ Jing Zhu,‡ Junrong Yu,*,† and Zuming Hu† †

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials and ‡College of Material Science and Engineering, Donghua University, 201620 Shanghai, PR China S Supporting Information *

ABSTRACT: The assembly of metal nanoparticles (NPs) has attracted a great deal of attention recently because of their collective properties that could not be exhibited by individual NPs. Here a one-step approach was reported for the fabrication of spherical silver NP assemblies (AgNAs). The formation of AgNAs simply included the stirring of silver ammonia and 3,4-dihydroxy-L-phenylalanine (DOPA) in aqueous solution at room temperature, in which DOPA acted as a reductant for AgNPs first because of its reducing ability and then directed the assembly of AgNPs into AgNAs. The AgNAs exhibited hierarchical structure with controllable sizes ranging from 180 to 610 nm by adjusting the concentrations of reagents. The two individual components, AgNPs and polyDOPA, also allowed AgNAs with multiple functions as demonstrated in this study of durable catalytic activity, high SERS sensitivity, and good antioxidant properties. The thin polyDOPA layer coated on AgNAs further offered the opportunity to modify the surface of AgNAs. The results presented here may provide a green and facile approach to designing multifunctional NP assemblies.

1. INTRODUCTION Metal nanoparticles (NPs) have attracted considerable interest in recent years because of their unique properties. It is well known that the properties of NPs, such as their catalytic capability, magnetic response, and surface plasmon resonance (SPR), are closely related to their composition, size, and morphology; therefore, tremendous efforts have been devoted to the surface functionalization and shape control of NPs.1,2 Recently, the assembly of NPs has become a fascinating subjects in nanoscience because the assemblies of NPs have demonstrated collective properties that are generally different from those of individual NPs.3,4 For instance, the near-field coupling of surface plasmons between adjacent plasmonic NPs (gold or silver NPs) could generate “hot spots” that are important for surface-enhanced Raman scattering (SERS);5 the assemblies of iron oxide NPs exhibit an improved response to magnetic fields and contrast enhancement in magnetic resonance imaging in comparison to individual NPs because of the much higher magnetization per cluster;6 for catalysis applications, whereas individual NPs tend to lose their activity during the reaction process due to aggregation, the preformed assemblies might not,7 to name just a few. It is these obvious advantages that have stimulated research in developing new strategies for creating NP assemblies or designing NP assemblies with novel functionalities. Among the assemblies of NPs, spherical assemblies are especially attractive because of their uniform sizes and dispersibility in solvents, which could facilitate their solution© XXXX American Chemical Society

based applications and processing into devices as compared to assemblies on substrates.8 Bottom-up self-assembly from individual NPs represents the main approach to the construction of such superstructures. However, assembling individual NPs with desired sizes and properties is a challenging task because it is needed to precisely tailor the interparticle interactions.9 To this end, several types of small molecules or polymers containing metal chelating groups have been introduced into the bulk solution of NPs to induce specific interactions, such as hydrogen bonding,10 host−guest attractions,11 or even covalent bonds12 among individual NPs to direct their assembly. In addition, several elaborate techniques, such as microemulsion13 and solvethermal-14 or microfluidicbased assembly,15 have also been developed to synthesize NP assemblies. However, the delicate design and synthesis of molecules or polymers that can stimulate NPs with specific surface ligands, with strict control of the global assembling environment of NPs, are necessary to organize the NPs while avoid their uncontrollable aggregation,16 which increases the synthetic effort and complexity for the fabrication of such assemblies. Biomacromolecules, which could strongly interact with NPs via hydrophobic or electrostatic forces, constitute another kind of naturally occurring materials for the assembly of NPs.17 For example, Kotov’s group recently reported a selfReceived: March 5, 2015 Revised: April 26, 2015

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2.2. Fabrication of AgNAs. In a typical procedure for preparing AgNAs, 0.197 g of DOPA was first dissolved in 90 mL of deionized water. In parallel, silver ammonia ([Ag(NH3)2]+) solution was prepared by adding ammonium hydroxide to a AgNO3 solution (0.019 g AgNO3 dissolved in 10 mL of deionized water) dropwise until a transparent solution was obtained. Then DOPA solution was poured into the [Ag(NH 3 ) 2 ] + solution. Thus, the effective concentration of DOPA in the reaction solution was approximately 2 mg/mL, and the molar ratio of Ag+ to DOPA was 1:9. The solution changed from transparent to pink and then to green in a few minutes, suggesting the oxidation of DOPA and the reduction of Ag+. The reaction was allowed to proceed for 5 h with mild stirring at ambient temperature. AgNAs were then obtained by centrifugation and three washings with deionized water. 2.3. Characterization. Transmission electron microscopy (TEM) images were obtained with a JEOL2100F. Fourier transform infrared (FTIR) spectra were recorded on a PerkinElmer Paragon 1000 FTIR spectrometer. X-ray powder diffraction (XRD) was performed on a D8 Advance X-ray diffractometer (Bruker, Germany). Thermogravimetric analysis (TGA) was performed in air with a PerkinElmer TGA2050 instrument at a heating rate of 10 °C/min. UV−vis spectra of AgNAs were recorded with a LambdaA 35 UV−vis spectrometer. The catalytic activity of AgNAs was tested by the reduction of 4nitrophenol (4-NP) with NaBH4 as the reductant in the presence of AgNAs. In a typical procedure, 1.5 mL of aqueous 4-NP solution (1 mM) and 1.5 mL of freshly prepared NaBH4 solution (100 mM) were mixed in a cuvette, and then 0.3 mg of AgNAs was added to the mixture as a catalyst. The reduction process was monitored by recording the time-dependent UV−vis spectra of the mixture. For SERS measurements, rhodamine 6G (R6G) at different concentrations was mixed with an aqueous solution of AgNAs (∼0.04 mg/mL) and stirred for 1 h, and then the mixture was dropped on the silica substrate and dried in air. Raman spectra were recorded with a Renishaw micro-Raman spectrometer (inVia-Reflex) equipped with a holographic grating of 1800 lines mm−1 and a He−Ne laser (532 nm) as the excitation source. The free radical scavenging activity of AgNAs was evaluated by DPPH assay. In detail, fresh DPPH solution (0.1 mM) in 95% ethanol was prepared before use. Then 2 mL of an aqueous dispersion of AgNAs was mixed with 2 mL of DPPH solution, and the total amount of AgNAs was varied from 5 to 60 μg in each dispersion. The scavenging activity was evaluated by monitoring the absorbance decrease at 516 nm after it was stored in the dark for 20 min. The DPPH radical scavenging activity was calculated as I = [1 − (Asample − Ablank)/Acontrol] × 100%, where Acontrol is the absorbance of DPPH solution without samples, Asample is the absorbance of the samples mixed with DPPH solution, and Ablank is the absorbance of the samples themselves without DPPH solution.

limiting self-assembly process of inorganic NPs intermediated by proteins.18,19 The use of such proteins not only permitted the organization of size-controllable spherical assemblies by the balance of attractive and repulsive forces between building blocks but also realized bionic particles that show multiple functions from each component, which provided a costeffective, facile, and green approach to the fabrication of NP assemblies with diverse promising applications. Herein, inspired by the recent advances in manipulating NPs with biomolecules,17 we reported a facile one-step, bottom-up method for the synthesis of spherical silver nanoparticle assemblies (AgNAs) from silver salts and 3,4-dihydroxy-Lphenylalanine (DOPA). Silver nanoparticles (AgNPs) are a kind of intriguing nanomaterial with good optical and catalytic properties. The assemblies of AgNPs have also been pursued for their potential applications in various fields.20−22 For example, Cho et al. successfully prepared AgNAs by using copolymer micelles as soft templates and demonstrated their use as highly sensitive SERS substrates20 by adopting an oil-inwater emulsion approach reported previously.13 Dintinger et al. obtained spherical AgNAs and showed their intriguing optical properties due to collective resonance.21 In parallel, DOPA is a kind of biological molecule presented in the mussel adhesive proteins (MAPs) and is primarily responsible for the excellent adhesive characteristics of MAPs.23 Because of the ability to self-polymerize to form melanin-like polymer films on nearly all substrates under alkaline or oxidative conditions, DOPA24 and its analogues (such as dopamine25 and norepinephrine26) have been widely studied for surface modification. The reducing and chelating capabilities of such melanin-like polymer to metal ions have stimulated research on their composites with NPs as well.27 Although there were quite a few papers in the literature reporting the use of DOPA (or dopamine) to synthesize AgNP assemblies,28,29 the process exclusively involved the formation of a melanin-like polymer layer on substrates and the subsequent deposition of AgNPs. To the best of our knowledge, no work on the direct formation of spherical AgNAs by DOPA or its analogues from molecular precursors has been reported. In this study, spherical AgNAs were prepared simply by stirring a silver ammonia complex and DOPA in aqueous solution at room temperature without any reductant or structure-directing agent or strictly controlled reaction conditions. It was found that the AgNAs had hierarchical structures with tunable sizes ranging from about 180 to 610 nm by adjusting the reactant concentrations, while optical characterizations suggested the excitation of collective resonance in AgNAs. Because of the unique assembled structures of AgNAs from AgNPs and polyDOPA, the AgNAs had multiple functions as demonstrated in this study of durable catalytic activity, high SERS sensitivity, and good antioxidant properties. In addition, the coated active polyDOPA layer was not only helpful in maintaining the integrated structure of AgNAs but also offered the possibility for further surface modification.

3. RESULTS AND DISCUSSION 3.1. Fabrication and Characterizations of AgNAs. In fact, the initial purpose of this study is the employment of DOPA as both a reductant and a molecular building block for the construction of AgNPs/polyDOPA composite particles in which AgNPs might be distributed in continuous polyDOPA that serves as a spherical matrix because of its self-polymerization ability. Such an approach has already been adopted in the fabrication of Pd/polypyrrole30 and Au/poly(ethylene imine) composite particles.31 The composite particles showed durable catalytic properties30 or held promise in the application of metamaterials31 as demonstrated before. We chose DOPA as the organic phase on the basis of the consideration that polyDOPA was biocompatible with additional antioxidant functionality, which could make composite particles with multiple functions in addition to the catalytic, optical, or antibacterial properties of AgNPs. The reducibility of DOPA was thought to reduce silver ions to AgNPs first,27 and then the

2. EXPERIMENTAL SECTION 2.1. Materials. 3,4-Dihydroxy-L-phenylalanine (DOPA) was purchased from J&K Chemical Technology. Ammonium hydroxide (25−28%) and silver nitrate (AgNO3) were obtained from Sinopharm Chemical Reagent Co., Ltd. (SCRC). 1,1-Diphenyl-2-picrylhydrazyl radical (DPPH) was purchased from Sigma (USA). Deionized water was used for all experiments. All reagents were used as received without further purification. B

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imental Section was indeed the assembly of AgNPs (AgNAs) rather than our assumed structure that AgNPs were distributed in polyDOPA particles as reported previously.30,31 Figure 1a demonstrates the formation of spherical AgNAs. The sizes of AgNAs were then calculated from about 100 particles based on TEM images, as presented in Figure 1b, from which we can see that the average size of AgNAs is about 180 nm. Interestingly, the amplified images (Figure 1c−e) indicated that the AgNAs had a quasi-core/shell structure with a compact internal core and a less compact external shell. The cores were assemblies of a large number of small particles, and some cavities due to the steric hindrance between adjacent AgNPs could also be observed whereas the shells were composites that consisted of small particles embedded in amorphous materials, which were conceivably the polyDOPA. A closer inspection of the AgNAs (Figure 1e) revealed that the AgNPs that constructed the AgNAs were about 10 nm in diameter, and an amorphous thin layer of polyDOPA was coated on the AgNAs, which contributed to the good dispersibility of AgNAs in water (Figure S1) and could also protect AgNAs from destruction and facilitate their surface modification.35 Figure 1f exhibiting lattice fringes with a distance of 0.24 nm, corresponding to the (111) planes of Ag, verified that the AgNAs were indeed composed of assembled AgNPs.36 In a control experiment, we have directly mixed the aqueous solutions of AgNO3 and DOPA (molar ratio of 1:9) for comparison. However, precipitates formed soon after mixing. TEM images (Figure S2) showed that irregularly shaped Ag particles with a wide size distribution ranging from tens to hundreds nanometers were formed, suggesting the uncontrollable reaction between Ag+ and DOPA. Several measurements were carried out to further characterize the composition and structure of AgNAs, as shown in Figure 2. Figure 2a presented the FTIR spectra of AgNAs and pure polyDOPA. In the fingerprint region, polyDOPA showed no distinguishable peaks due to its complex structure, which was consistent with previous studies.37,38 The peaks at about 2926 cm−1 represented the stretching vibrations of C−H bonds in polyDOPA, and the peaks centered at around 3429 cm−1 were ascribed to −NH or −OH bonds, revealing their hydrophilic nature. The spectrum of AgNAs was similar to that of polyDOPA, suggesting that the surface functional groups of AgNAs were dominated by the coated polyDOPA, which was in agreement with the observation in Figure 1e. The XRD pattern of AgNAs (Figure 2b) comprised an amorphous peak and several crystallization peaks. The amorphous peak at about 24.1° was attributed to the peak of polyDOPA if compared to the XRD pattern of neat polyDOPA. The peaks at about 37.6, 43.8, 64.6, 77.3, and 81.6° were assigned to the (111), (200), (220), (311), and (222) planes of face-centered cubic (fcc) silver, demonstrating that AgNAs were indeed

cohesive forces generated by the self-polymerization process of DOPA32 were expected to embed the small AgNPs into larger polyDOPA particles. Another important aspect of this work was the choice of [Ag(NH3)2]+ as the silver source because the reduction potential for [Ag(NH3)2]+/Ag in aqueous solution (+0.373 V) is lower than that of the Ag+/Ag system (+0.7996 V), which means that the reduction process of [Ag(NH3)2]+ by DOPA could proceed in a milder way.33 Moreover, the NH3 molecules released from [Ag(NH3)2]+ during the formation of AgNPs could promote the self-polymerization of DOPA in return because the hydroxyl groups in DOPA could be deprotonated in alkaline solution and the abstraction of hydrogen atoms from deprotonated DOPA by O2 is the ratedetermining step for the oxidative polymerization of DOPA or its analogues.34 However, as observed in TEM images (Figure 1), the morphology of particles prepared according to the Exper-

Figure 1. TEM images (a, c−f) of AgNAs under different magnifications and (b) the statistics of diameters of AgNAs.

Figure 2. FTIR spectra (a), XRD patterns (b), and TGA curves (c) of AgNAs and polyDOPA. C

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method could also be applied to quickly prepare AgNPs in an environmentally friendly manner. The sizes of AgNPs at this stage were also consistent with those observed in Figure 1. After reaction for 1 h (Figure 3c,d), the small AgNPs were assembled into larger particles. However, at this stage, the particles had only a core composed of assembled AgNPs, whereas no distinguishable composite shell was formed around the core, suggesting that the assembly of AgNPs was faster than the formation of the composite shell. With a further prolonged reaction time of 5 h (Figure 3e,f), a composite shell that consisted of AgNPs and polyDOPA was formed around the core. Comparing the sizes of AgNAs with reaction times of 1 and 5 h, we can deduce that the sizes of AgNAs were mainly determined by the assembly process of AgNPs because the cores of AgNAs in Figure 3e were similar in size to those in Figure 3c. These images also questioned whether the remaining small number of AgNPs in solution were copolymerized with DOPA to form a composite shell on AgNAs because most of the AgNPs in solution had been assembled. However, after 24 h of reaction time, the morphology of AgNAs seemed to be unchanged as compared to AgNAs with a reaction time of 5 h. This could be explained by the fact that O2 in solution was gradually consumed by the oxidative polymerization of DOPA after 5 h or less, so increasing the reaction time resulted in a much slower polymerization of DOPA and had no significant effect on the morphology of AgNAs.41 The formation process and the evolution of optical properties of AgNAs were also monitored by UV−vis spectra, as presented in Figure 4. After 5 min of reaction, a peak at

Figure 4. UV−vis spectra of AgNAs at different reaction times.

about 458 nm appeared, corresponding to the SPR band of AgNPs. However, it is noted that the peak intensity was weak and the full width at half-maximum (fwhm) was broad, which we attributed to the effect of the surrounding medium, because the adsorbed DOPA or polyDOPA on AgNPs significantly changed the interface conditions of AgNPs.42 The peak position gradually bathochromically shifted to longer wavelength at prolonged reaction time, and the fwhm further broadened, indicating the formation of assemblies and increased sizes as the reaction proceeded, which induced a plasmonic coupling effect in the AgNPs,31,43 resulting in the excitation of collective resonance in AgNAs. The peak position of AgNAs at a reaction time of 5 h (538 nm) only slightly shifted to 546 nm at a reaction time of 24 h, suggesting that 5 h was long enough to complete the reaction, which was consistent with the TEM characterizations (Figure 3). Moreover, in addition to the characteristic absorption peak of DOPA at about 280 nm, a shoulder at 305 nm, which was attributed to the formation of quinone, was established as the reaction

Figure 3. TEM images of AgNAs at different reaction times: (a, b) 5 min, (c, d) 1 h, (e, f) 5 h, and (g, h) 24 h. D

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Figure 5. Proposed mechanism for the formation of AgNAs.

were thus defined. In the final stage, the preformed AgNAs served as substrates and the free DOPA together with the remaining small number of AgNPs in the alkaline solution (provided by the NH3 released from the reduction of [Ag(NH3)2]+) underwent further oxidation polymerization to form a composite shell on AgNAs. In parallel to the formation of AgNAs, it is rational that free DOPA could also have selfpolymerized into polyDOPA aggregates in solution in the second and third steps, which have been removed by the washing process, which also explained why the AgNAs contained only a small amount of the organic phase. However, the detailed mechanism might be more complicated than our proposed mechanism because of the complexity of the polymerization of DOPA and the multiple interactions among different compounds, which deserve further investigation. To gain further insight into the formation process, we have investigated the effects of synthetic parameters on the morphology of AgNAs. First, we fixed the concentration of [Ag(NH3)2]+ at 1.1 mM while varying the molar ratio of [Ag(NH3)2]+ to DOPA, and the TEM images of the resulting particles are shown in Figure 6. When the molar ratio was 1:2 (Figure 6a,b), some aggregated AgNPs instead of well-defined AgNAs were obtained, suggesting that the oxidized DOPA around AgNPs was not enough to link AgNAs together and that excess DOPA in solution did not participate in the assembly process. As the molar ratio decreased to 1:4 (Figure 6c,d), AgNAs with sizes and morphology similar to those in Figure 1, except for the thinner outermost polyDOPA layers, were obtained. However, when the molar ratio further decreased to 1:13, AgNAs agglomerated (Figure 6e) and some AgNAs were wrapped together by the outer polyDOPA, forming a cucurbit-like particle (Figure 6f), indicating that excess DOPA was adverse to preparing well-dispersed AgNAs. This group of experiments illustrated that DOPA was primarily responsible for the formation of AgNAs. However, as the ratio of DOPA reached a critical value, it influenced only the shell but could hardly influence the sizes of AgNAs. Then we increased the concentration of [Ag(NH3)2]+ to 2.8 and 5.6 mM while keeping the molar ratio of [Ag(NH3)2]+ and

proceeded, suggesting that DOPA was oxidized soon after mixing with silver salts. The gradually increased absorbance from the whole UV to the visible light range indicated the increased amount of melanin-like polyDOPA with increasing reaction time. Obviously, DOPA plays a vital role in the formation of AgNAs because traditionally used reducing and stabilizing agents have not been reported to induce the assembly of AgNPs. From the above characterizations, we tentatively divide the formation process of AgNAs into three steps, as shown in Figure 5. In the first step, it is reasonable that DOPA could reduce silver ions to AgNPs because of the reducibility of catechol groups in DOPA.44 This step proceeded quickly as the nucleation and growth of silver crystals was usually complete within several minutes.45 The UV−vis characterizations also support this because the SPR band of AgNPs and characteristic peak of oxidized DOPA were both established within 10 min (Figure 4). The following assembly of AgNPs is thought to be the key in the formation process. Although it is not clear what the driving force for the assembly of AgNPs is at this moment, recent reports on the investigation of the polymerization mechanism and structure of polydopamine, which are still under debate, might shed some light on this point. It was proposed that under oxidative or alkaline conditions dopamine could be oxidized to form dopaquinone. The subsequent intramolecular Michael addition and further oxidation then occurred on these compounds, yielding indole-containing compounds that were extremely reactive in forming eumelanin-like aggregates via covalent or noncovalent interactions.32,46−48 In our case, as DOPA and [Ag(NH3)2]+ were mixed together, some DOPA close to [Ag(NH3)2]+ was thought to be quickly oxidized by silver ions as described in the first step and then were located around the reduced AgNPs because of their chelating ability.28,29 Oxidized DOPA underwent a fast ring-closing reaction to form indole compounds with higher reactivity than the free DOPA in the solution.32,48 Then AgNPs interacted with each other by covalent or noncovalent interactions through the surrounding indole compounds. Because most of the AgNPs were assembled into larger particles, the assembly ceased and the sizes of AgNAs E

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Figure 6. TEM images of AgNAs obtained with different molar ratios of [Ag(NH3)2]+ and DOPA: (a, b) 1:2; (c, d) 1:4; and (e, f) 1:13. The concentration of [Ag(NH3)2]+ was 1.1 mM.

DOPA at 1:9. From the TEM images (Figure 7), it can be seen that increasing concentrations of reagents resulted in increased particle sizes. When the concentrations of [Ag(NH3)2]+ were 2.8 (Figure 7a−d) and 5.6 mM (Figure 7e−h), AgNAs with average sizes of about 320 and 610 nm were obtained, demonstrating that the sizes of AgNAs were controlled by the number of AgNPs preformed in the solution, which was in agreement with our proposed mechanism. Interestingly, the shell thicknesses of AgNAs were both in the range of 30−40 nm despite their different sizes (Figure 7c,g), and TGA characterizations (Figure S3) revealed that the content of organic phases in AgNAs decreased from 17.5 wt % in AgNAs with a size of 180 nm to 12.1 wt % in AgNAs with a size of 320 nm to 8.7 wt % in AgNAs with a size of 610 nm. Such phenomena suggested that the assembly of AgNPs was faster than the polymerization of polyDOPA shells on AgNAs, and the organic polyDOPA was mainly located on the outer shell of AgNAs; the smaller content of polyDOPA in larger AgNAs was simply due to their smaller surface areas compared to that of smaller AgNAs, which resulted in less deposition of polyDOPA. These controlled experiments also suggested that although the chemistry involved in the reaction was not exactly understood, our proposed formation process of AgNAs was reasonable. In addition, we have also monitored the formation process of AgNAs with an average size of 610 nm by TEM (Figure S4). The images showed that the formation process of larger AgNAs was similar to that of smaller ones. 3.2. Potential Applications of AgNAs. Because AgNAs are composed of small AgNPs and polyDOPA, they should

Figure 7. TEM images and statistics of diameters of AgNAs synthesized with [Ag(NH3)2]+ concentrations of (a−d) 2.8 mM and (e−h) 5.6 mM. The molar ratio of [Ag(NH3)2]+ to DOPA was 1:9.

exhibit multiple functions from both components. Therefore, we have tested the possibility of AgNAs as catalysts, SERS substrates, and antioxidant agents. AgNAs with an average diameter of 180 nm were used for these tests. To evaluate the catalytic performance of AgNAs, the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) was chosen as a model reaction. To make the reaction follow pseudo-first-order kinetics, the initial concentration of NaBH4 was set to be much higher than that of 4-NP.49 As monitored by UV−vis spectra (Figure 8a), the reduction immediately occurred upon the addition of a trace amount (0.3 mg) of AgNAs, the absorption peak of 4-NP at 400 nm gradually disappeared, and a new peak at 295 nm was progressively established, demonstrating the reduction of 4-NP and the formation of 4-AP. After 60 min, the solution become colorless, indicating the complete conversion of 4-NP. Figure 8b showed the linear plot of ln(At/A0) against reaction time, where At is the absorbance of the solution at time t and A0 is the absorbance of the solution at reaction time t = 0, from which we can determine that the rate constant k was 0.0327 min−1. F

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Figure 8. (a) UV−vis spectra of 4-nitrophenol reduced by sodium borohydride in the presence of AgNAs. (b) Plot of ln(At/A0) against reaction time with AgNAs and cAgNPs as catalysts. (c) Conversion of 4-nitrophenol as a function of cycle with AgNAs and cAgNPs as catalysts.

For comparison, we have also prepared traditionally used citrate-capped AgNPs (cAgNPs) with sizes in the range of 10− 20 nm (Figure S5) as catalysts under the same experimental conditions, except for the small amount of cAgNPs (0.06 mg) used for catalysis because of the fast reaction rate; the plot of ln(At/A0) against reaction time is also shown in Figure 8b. However, despite the smaller amount of cAgNPs used, the apparent k (0.105 min−1) was still higher than that of AgNAs. The true activity parameter (κ), which represents the ratio of k to the mass of catalyst, was about 1.817 S−1 g−1 for AgNAs and 29.17 S−1 g−1 for cAgNPs. Such results suggested that the polyDOPA layers on AgNAs might retard the contact of reagents with catalysts. Nevertheless, the κ of AgNAs was still comparable to that of some AgNPs supported on a spherical substrate.50 Because of the high cost and limited supply of noble metals and the possible pollution of waste NPs, it is important to develop reusable catalysts. Therefore, the reusability of AgNAs was tested (Figure 8c). The concentrations of reagents and catalysts were increased by 20-fold to shorten the reaction time. In each cycle, the sample was diluted 20-fold for UV−vis measurements to determine the conversion of 4-NP after 8 min of reaction followed by centrifugation and rinsing, and then the obtained catalysts were dispersed into deionized water for the next cycle of catalysis. The results of the test demonstrated that the AgNAs can be reused in 10 successive reactions with conversions of ∼100% within 8 min of reaction time, suggesting the polyDOPA on the surface of AgNAs could act as a protective layer to prevent the structure of AgNAs from being destroyed. The conversion obviously started to decrease after ∼12 cycles, and TEM images indicated that the fusion of AgNPs had occurred after these cycles of catalysis, which diminished the surface areas of AgNAs (Figure S6). In sharp contrast, the conversion with cAgNPs as catalysts significantly decreased to 64.0% after the first cycle of catalysis, with a further decrease to 14.4% in the fourth cycle of catalysis. The results clearly demonstrated the advantages of our AgNAs for use as reusable catalysts. The plasmonic coupling effect between adjacent AgNPs also indicated the possible use of our AgNAs in SERS applications. To demonstrate this, we have carried out SERS measurements by using R6G as a model analyte. As shown in Figure 9a, when R6G (10−3 M) was directly deposited on a silica substrate, only a featureless background was obtained. In contrast, wellresolved and intense Raman signals of R6G,51 that is, the band of aromatic C−H bonding at 1194 cm−1 and the peaks of aromatic C−C stretching vibrations at 1280, 1358, 1508, and 1648 cm−1, were observed at the same concentration with

Figure 9. (a) SERS spectra of R6G (10−3 M) on AgNAs and the silica substrate. (b) SERS spectra of R6G with different concentrations on AgNAs.

AgNAs as the sensor platform, demonstrating the good SERS effect of our AgNAs. To evaluate the detection limit of R6G on AgNAs, we measured the SERS spectra of R6G at different concentrations from 10−5 to 10−12 M, as shown in Figure 9b. Generally, the intensity of signals decreased with the decreasing concentration of R6G; however, the peaks of R6G could still be clearly observed at concentrations as low as 10−10 M, suggesting the high SERS sensitivity of our AgNAs. Because polyDOPA was a good antioxidant agent, stable DPPH radicals were used to evaluate the radical scavenging activity (RSA) of the AgNAs.52 We found that pure polyDOPA was not able to form regularly shaped particles under alkaline conditions (Figure S7); therefore, polydopamine particles (PDAP) with diameters similar to those of AgNAs (the synthesis of PDAP can be found in our previous work38), which have a chemical structure similar to that of polyDOPA and have been demonstrated to be good antioxidant agents recently,32 were used as a control. Generally, AgNAs have a better RSA than does PDAP of the same dosage (Figure 10). For example,

Figure 10. DPPH radical scavenging activity of AgNAs and polydopamine particles with similar sizes. G

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as the dosage of AgNAs was 30 μg, the RSA of AgNAs achieved the maximum value of about 74.2%, and the RSA of PDAP was only 29.8%. Further increasing dosage of AgNAs resulted in no further increases in RSA; this is due to the fact that AgNAs has strong absorbance in the visible light region whereas the RSA was also calculated by the absorbance of solution in the same region. Even if the DPPH radical has been fully eliminated, the solution of AgNAs could also absorb light with a wavelength of 516 nm. Therefore, we can deduce that 30 μg of AgNAs was enough to completely remove DPPH radicals whereas PDAP could not. The better RSA can be attributed to the higher activity of polyDOPA compared to that of polydopamine52 and the rougher surface of AgNAs, which could offer more available reactions sites to react with DPPH. Such results suggested that the AgNAs could also be used as antioxidant agents in the field of biology or packaging. Furthermore, as compared to some previously reported nanoparticle assemblies, the large number of reactive groups on the surface of AgNAs offered an opportunity for surface modification. As a demonstration, we have functionalized the AgNAs with 1-octadecanethiol because the thiol groups could react with the catechol/quinine groups32 of polyDOPA layers on AgNAs. The results demonstrated the change in hydrophilic AgNAs to hydrophobic AgNAs after surface modifications (Figure S8).

AUTHOR INFORMATION

Corresponding Authors

*Tel: +86-21-67792944. Fax: +86-21-67792945. E-mail: wy@ dhu.edu.cn. *Tel: +86-21-67792944. Fax: +86-21-67792945. E-mail: yjr@ dhu.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (no. 51203019), the National 973 Project of China (no. 2011CB606103), the National 863 Project of China (no. 2012AA03A212), and Fundamental Research Funds for the Central Universities (no. 2232014D3-25) for support.

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REFERENCES

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4. CONCLUSIONS We presented a one-step bioinspired approach to the synthesis of spherical silver nanoparticle assemblies. In this approach, the silver precursor [Ag(NH3)2]+ was first reduced by DOPA, and then the oxidized compounds of DOPA on reduced AgNPs were expected to direct the assembly of AgNPs. The NH3 molecules released by the reduction process adversely promoted the oxidation polymerization of excess DOPA to form AgNPs/polyDOPA composite shells on AgNAs. The sizes of AgNAs were tunable from 180 to 610 nm simply by adjusting the reactant concentrations. Because of the unique structures and compositions of AgNAs, they showed durable catalytic activity, high SERS sensitivity, and good antioxidant properties. In addition, the coated polyDOPA layers on AgNAs not only maintained the integrated structure of AgNAs but also permitted facile surface modification on AgNAs. Besides the potential applications presented here, it is anticipated that our AgNAs could also find applications in metamaterials or antibacterial agents as well.

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Article

ASSOCIATED CONTENT

S Supporting Information *

Dispersions of AgNAs in water with different concentrations, TEM images of Ag particles prepared by directly mixing aqueous solutions of AgNO3 and DOPA, TGA curves of AgNAs synthesized with different [Ag(NH3)2]+ concentrations at a fixed molar ratio of [Ag(NH3)2]+ to DOPA, TEM images of AgNAs (average size ∼610 nm) at different reaction times, TEM images of citrate-capped AgNPs, TEM image of AgNAs after 12 cycles of catalysis, TEM images of polyDOPA prepared by oxidation polymerization without [Ag(NH3)2]+, and spectra and dispersion state of AgNAs and surface-modified AgNAs. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b00820. H

DOI: 10.1021/acs.langmuir.5b00820 Langmuir XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.langmuir.5b00820 Langmuir XXXX, XXX, XXX−XXX