Constructing Metal Nanoparticle Multilayers with Polyphenylene

*Z.L.: e-mail, [email protected]; M.Y.: e-mail, [email protected]; L.W.: e-mail, [email protected]. Tel/Fax: +86 371 67781205. Info icon. Your current c...
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Constructing Metal Nanoparticle Multilayers with Polyphenylene Dendrimer/Gold Nanoparticles via “Click” Chemistry Huiqiang Li,†,‡ Zhanxian Li,*,† Linzhi Wu,† Yuna Zhang,† Mingming Yu,*,† and Liuhe Wei*,† †

The College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, People’s Republic of China Department of Radiology, Henan Provincial People’s Hospital, Zhengzhou 450003, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Multilayer films composed of azide-functional polymer and polyphenylene dendrimer-stabilized gold nanoparticles with alkynes in their peripheries have been fabricated using a layer-by-layer (LBL) approach via “click” chemistry. This method permits facile covalent linking of the polymer/ nanoparticle interlayers in the mixture of DMF and water, which provides a general and powerful technique for preparing uniform nanoparticle (NP) thin films. The deposition process is linearly related to the number of bilayers as monitored by UV−vis spectroscopy. The multilayer structure and morphology have been characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and contact angle.

nanoparticles were usually introduced into the hybrid film by electronic or other weak interactions, which were susceptible to disassembly under some solution conditions (e.g., salt, pH) and could not offer higher stability. In this paper, a highly efficient LbL assembly technique was reported, which was based on “click” chemistry to construct composite gold nanoparticles/polymer films. The combination of click chemistry and LbL assembly can offer a number of significant advantages,22 such as high yield in mild conditions, high stability, and wide application. The two clickable species, polymer and gold nanoparticles, were first synthesized via reversible addition−fragmentation chain transfer polymerization (RAFT) and the Brust-Schiffrin method, respectively. In LbL, the initial polymer deposition on the quartz slide modified with PEI was achieved by electronic interactions. And after that, click coupling between gold nanoparticles and the polymer was repeated continuously until the desired numbers of “bilayers” were obtained (as shown in Scheme 1). This technique provides a high efficient method to form composite nanoparticles/polymer films of controlled thickness.

1. INTRODUCTION During the past few decades, layer-by-layer (LbL) self-assembly technique in constructing multilayers has attracted considerable attention.1 Compared with other assembly methods, LbL selfassembly technique can improve the accuracy of thin film deposition and allow precise thickness control at the nanoscale level. The LbL assembly process was based on the alternating interaction of oppositely charged species, positively and negatively charged polyelectrolytes,2,3 and other building blocks such as dendrimers,4 DNA,5 or polypeptides.6 Nowadays, it also has successfully been extended to different kinds of driving forces, such as hydrogen bonding interaction7 and covalent bonding.8 With the development of this technique, there is a more widespread application in the coating and encapsulation of colloid particles,9,10 including constructing multilayers of various species on planar subtracts.11 Various assembly approaches have been employed to fabricate well-ordered films comprised of polymers and nanocrystals or various other inorganic materials. For the LbL assembly of thin films, most combinations including nanoparticles (NPs), nanosheets, nanowires, and nanotubes with polymers have been used.12−14 Since the preparation of such films allows the combination of various polymers or organic molecules with a variety of nanoparticle cores, it is possible to open broad perspectives for this technique in a wide range of applications such as biological, chemical, and electrochemical sensing15−17 or the construction of optoelectronic nanodevices.18 Nanomaterials usually possess size- and shapedependent optical, magnetic, electronic, and catalytic properties.19,20 Chemical stability, easy preparation, and known surface chemistry are main advantages of using gold nanoparticles for the LbL assembly process.21 However, gold © 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. PdCl2, 4-iodoaniline, trimethylsiylacetylene, copper(I) iodide, triphenylphosphine, hydrogen tetrachloroaurate (III) trihydrate (HAuCl4·3H2O), poly(ethyleneimine) (PEI, Mn = 25000), tetrabutylammonium fluoride (Bu4N+F−, 1 M solution in THF) and 4-vinylbenzyl chloride (VBC) were purchased and used without further purification. tBA was purified by passing through a Received: October 18, 2012 Published: February 27, 2013 3943

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2.2. Synthesis. The synthetic procedures of the assembled products are listed in Scheme 2, Scheme 3, and the Supporting Information. Compound 1 was synthesized following the description in a previous report.23,24 Benzyl dithiobenzoate (BDB) was prepared following the method described by John Chiefari and co-workers.25 2.2.1. Synthesis of G2SH-Coated Gold Nanoparticles (G2SHAuNPs). G2SH-AuNPs were prepared following the Brust’s two-phase reaction procedure.26 All glassware was cleaned in aqua regia (Caution! Aqua regia is corrosive and should be handled with care) and rinsed with deionized water before use. Chloroauric acid (HAuCl4·3H2O) (50.2 mg, 120 mmol) was dissolved in 1.5 mL deionized water. To this solution, a toluene (1 mL) solution of tetra-noctylammonium bromide (53.5 mg) was added and the mixture was vigorously shaken. The yellow aqueous solution quickly cleared, and the toluene phase turned orange-brown as a result of the transformation of AuCl4− with tetraoctylammonium cations. The organic phase was isolated and a toluene (2.1 mL) solution of G2SH (92 mg) was added. The resulting solution was stirred for 10 min at room temperature, and a freshly prepared aqueous solution (1.3 mL) of sodium borohydride (18.9 mg) was added under argon at 0 °C. The toluene phase immediately turned from orange-brown to red-brown with the nanoscopic gold nanoparticles formed. The now very dark solution was further stirred at 0 °C for 1 h and at room temperature for 20 h. The organic phase was collected and concentrated on a rotary

Scheme 1. Schematic Illustration of the LbL Process Between G2SH-AuNPs-Alk and P(VBA-AA)

basic alumina column to remove the inhibitor. All other reagents were local commercial products that were used as received.

Scheme 2. Synthesis of Polyphenylene Dendrimers Terminated with Different Numbers of Functional Ethynyl via the Combination of the Diels-Alder Cycloaddition and Desilylation Reaction

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Scheme 3. Synthesis of P(VBA-AA)

evaporator. Then, the G2SH-coated Au nanoparticles were precipitated from the toluene phase by addition of 5 mL ethanol. This mixture was cooled at −10 °C for 4 h. The Au nanoparticles, which in precipitate form appeared as a black powder, were collected by centrifugation and characterized by FT-IR, UV−vis spectroscopy, and TEM. Yield: 85%. 2.2.2. Synthesis of Alkyne-Containing Gold Nanoparticles G2SHAuNPs-Alk. To a stirred THF (3 mL) solution of G2SH-AuNPs (0.0275 g), 1 M of tetrabutylammonium fluoride (Bu4N+F−) in THF (0.8 mL) was added. The resulting reaction solution was heated to reflux. After stirring for 4 h, the mixture was concentrated on a rotary evaporator. Then, G2SH-AuNPs-Alk were precipitated from the THF phase by addition of ca. 5 mL ethanol. This mixture was cooled for 4 h at ca. −10 °C. G2SH-AuNPs-Alk was collected by centrifugation and characterization by FT-IR, UV−vis spectroscopy, and TEM. Yield: 80%. 2.2.3. Synthesis of P(VBA-AA). P(VBA-AA) was synthesized in two steps as follows. Initial reactants were mixed at a 70:30:1 molar ratio of 4-vinylbenzyl chloride (1.5235 g), tert-butyl acrylate (0.9367 g), and a RAFT reagent (0.0256) (BDB). Ten wt % 2,2′-azobisisobutyronitrile (AIBN, 5.2 mg) relative to the RAFT reagent was also added. The solution was deoxygenated by five freeze−pump−thaw cycles and sealed and placed in a preheated oil bath at 75 °C. The polymerization was quenched after 10 h by cooling and exposing it to air. The product was dialyzed for 24 h to remove excess monomer. Furthermore, the polymer was stirred overnight with sodium azide (1.150 g) at 40 °C and then stirred overnight with trifluoroacetic acid (TFA, 7 mL). The final product was dialyzed again for 24 h, freeze-dried, and characterized by FT-IR (KBr, cm−1): 3430 and 1700 (−COO−), 2111 (−N3). Molecular weight: 20000 (Mn). PDI: 1.29. Yield: 71%. 2.3. Preparation of P(VBA-AA)/G2SH-AuNPs-Alk Films via “Click” Chemistry. The multilayers were prepared by a modified method.27 A typical procedure was described as follows. A quartz slide was first boiled in a freshly prepared piranha solution (98% H2SO4/ 30% H2O2, 7/3 (v/v); Caution! Piranha solution is highly corrosive; it reacts violently with organic materials and should be handled with extreme caution) for 30 min at 70 °C and rinsed thoroughly with deionized water. The slide was then sonicated with a mixture of isopropanol and water [1/1 (v/v)] for 20 min and finally heated to 60 °C for 20 min in RCA solution [water/hydrogen peroxide/ammonia, 5/1/1 (v/v/v)]. A thin layer of poly(ethyleneimine) (PEI) was then absorbed on the quartz slide, constituting the substrate for subsequent LbL assembly. LbL assembly was performed by sequentially exposing the quartz slide to the solutions of P(VBA-AA) and G2SH-AuNPs-Alk in DMF, containing sodium ascorbate and copper sulfate under argon for 24 h, with a DMF rinsing after deposition of each layer. When the first layer was formed, polymer [P(VBA-AA)] solution without a catalyst was used, since the polymers were absorbed on the quartz slide by electrostatic interaction. The next layers were constructed by click chemistry. The dipping solutions were prepared from the following stock solutions: (a) P(VBA-AA) in DMF (16 mg·mL−1), (b) G2SHAuNPs-Alk in DMF (10 mg·mL−1), (c) DMF, (d) copper sulfate (0.5 mg·mL−1), and sodium ascorbate (1.0 mg·mL−1) in deionized water. P(VBA-AA) and G2SH-AuNPs-Alk dipping solutions were made up in

a constant volume ratio of 6:0.5:0.25 (c):(a):(d) and 6:0.1:0.25 (c): (b):(d), respectively. To prevent oxidation of the copper, freshly prepared catalyst solutions (d) were used in each deposition. After each deposition step, the quartz slide was dried in a stream of argon and sequentially characterized by UV−vis spectra. 2.4. Measurements. FT-IR spectra were recorded on a NEXUS470 spectrometer at frequencies ranging from 400 to 4000 cm−1. 1H NMR and 13C NMR spectra were performed at room temperature on a DRX-400 spectrometer. The apparent molecular weights and polydispersities (Mw/Mn) of polymers were determined on an Agilent LC 1200 gel permeation chromatograph (GPC) equipped with Agilent PL columns with THF as the eluent (1.0 mL/min). UV−vis absorption spectra were measured on a Persee TU-1901 spectrophotometer at room temperature. The scanning conditions were as follows: a scanning rate of 50 nm/min, a response time of 1 s, and a bandwidth of 2 nm. The TEM images were recorded by JEM-2100/ INCA OXFORD TEM (JEOL/OXFORD), operating at 200 kV and having a point resolution of 0.19 nm. A diluted toluene solution of G2SH-AuNPs or G2SH-AuNPs-Alk was dropped onto the surface of 300 mesh Formvar-carbon film-coated copper grids, followed by drying at room temperature for 24 h. The size distribution was estimated by Image J. XPS analysis was performed on a ESCALab220iXL spectrometer. AFM images were acquired by nanoscope with a silicon tip mounted on a cantilever operating at a resonant frequency of 300 kHz and scan rate of 1 Hz. Samples were analyzed over a 2.0 × 2.0 μm sample area at a resolution of 512 × 512 pixels, and images were produced by NanoScope 6.11r1. The contact angle was recorded on SL200B.

3. RESULTS AND DISCUSSION Synthesis and Characterization of G2SH. Secondgeneration polyphenylene dendrimers (G2SH) functionalized with sulfhydryl and ethynyl groups were used to stabilize the gold nanoparticles. The dendrimers were synthesized mainly in two steps, the preparation of G2 and its sequent chemical modification (as illustrated in Scheme 2). The synthesis of G2 was primarily based on two reactions: the [4 + 2] Diels−Alder cycloaddition of tetrasubstituted cyclopentadienones to ethlynes and the deprotection of triisopropylsilyl (TiPS)substituted alkynes. First, the synthesis of G2 was carried out following a divergent synthetic protocol.28 With the employment of a [4 + 2] Diels−Alder cycloaddition procedure of the TiPS-protected ethynyl-substituted cyclopentadienone branching unit 1 to 4-(aminophenylethynyl) acetylene 2, the first generation dendrimer G1 with TiPS-ethynyl groups was obtained in 89% yield. After removal of the TiPS groups in G1 with THF solution of tetra-n-butylammonium fluoride (TBAF) refluxing for 4 h, dendrimer 3 without ethynyl groups was obtained in 85% yield. The resulting ethynyl groups were further reacted with branching unit 1, and the second generation dendrimer G2 (82%) was achieved. Second, in 3945

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order to provide stabilizer for the preparation of gold nanoparticles, sulfhydryl group was subsequently introduced into dendrimer G2. Condensation of this aminopolyphenylene (G2) with 6-bromohexanoyl chloride in the presence of triethylamine (TEA) was performed and afforded 4. Then, bromide 4 was converted to polyphenylene disulfide (G2SH) via thioesterification with potassium thioacetate and subsequent base deprotection of 5. Preparation and Characterization of G2SH Stabilized Gold Nanoparticles. Gold nanoparticles were synthesized by the Brust-Schiffrin method, and a series of Au/S feed ratio was conducted. When the Au/S ratio was set at 1:1, no obvious surface plasmon absorption peak around 500 nm was observed in the UV−vis spectra, which implies that quite small particles were formed.29 This was confirmed by TEM observation (the as-prepared nanoparticles had a mean diameter of 1.7 nm with a standard deviation of 0.02 nm (Figure 1a). As the Au/S ratio

Figure 2. TEM images G2SH-AuNPs with the ratio of AuCl4−/thiol = 3 after storage at 4 °C for 6 months in air. The scale bar is 20 nm.

G2SH-AuNPs-Alk. G2SH-AuNPs with triisopropylsilyl (TiPS)-protected ethynyl groups in their peripheries were deprotected by tetrabutylammonium fluoride. After 4 h refluxing, G2SH-AuNPs-Alk was obtained by precipitating a reaction mixture into cold ethanol twice. The precipitate was redissolved back into DMF to form a stable reddish colloid. The deprotected Au nanoparticles were first characterized by UV−vis spectra. As shown in Figure 3, both of G2SH-Au NPs

Figure 1. TEM images and size distributions of G2SH-AuNPs with the ratio of AuCl4−/thiol = 3. Scale bars in (a) and (b) are 20 nm.

was increased, a continuous increase of the absorption peak toward higher energy was observed, due to the collective oscillation of the electrons in the conduction band of the gold atoms, under optical excitation.4 The increase of the absorbance peaks was consistent with the increase of the size of the nanoparticles by augmenting the Au/S molar ratio from 2 to 4. The morphology and size of the gold particles were further characterized by TEM. In order to obtain a good statistic for the size distribution, as many particles as possible were counted for each sample. From the TEM images, it can be seen that uniform nanoparticles with the shape of spheres were obtained and the size of the particles prepared by varing Au/S from 1 to 5 did not change a lot, which is consistent with the UV−vis observations (Figure 1 and Figure S1 of the Supporting Information). The diameters of the particles are consistent with the calculation results (Figure S2 of the Supporting Information). In addition, when the feed ratio of Au/S was set at 20, small and uniform gold nanoparticles also can be obtained. No obvious precipitate appeared after the toluene solutions of these gold nanoparticles were reserved in exclusion of light for half a year at 4 °C (Figure 2). It means that the shape-persistent polyphenylenes can efficiently prevent the gold particles from growing bigger and aggregating together.

Figure 3. UV−vis absorption spectra for multilayer hybrid films assembled on quartz with an increasing bilayer number. The inset is the absorbance as a function of bilayer number monitored at 520 nm.

and G2SH-AuNPs-Alk show a typical absorption peak around 520 nm, which are from the surface plasmon resonance of gold particles. This peak is consistent with previous observations.30 In general, thermal treatment of composite nanoparticles in solutions31,32 or solid state33 will induce them aggregating into bigger structures. The reasons can be ascribed to two consecutive processes,34 thermally driven desorption of the ligands from the gold surface and the reshaping or resizing that minimizes the chemical potentials. TGA analysis (Figure S3 of the Supporting Information) shows that gold nanoparticles did 3946

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transfer agent, followed by converting chloride to azide (Figure S5 of the Supporting Information). The obtained yellowish polymer had a molecular weight of 20000 (Mn) with a polydispersity of 1.28. Gold Nanoparticle−Polymer Multilayers. Hybrid films were fabricated by an LbL assembly technique, and the LbL process was achieved by click chemistry. The initial deposition of the polymer P(AA-VBC)-AZ was performed via electrostatic interactions, and polymer P(AA-VBC)-AZ containing both azido and carboxyl groups was synthesized. After the first deposition of the polymer, the alkyne-containing gold nanoparticles could easily be absorbed via a click coupling reaction.35 Click coupling between the polymer and G2SH-AuNPs-Alk was repeated continuously until the desired numbers of “bilayers” were obtained. Figure 3 shows the absorption spectra of the multilayer built up on quartz slides. The gold colloids have a surface plasmon absorption band at about 520 nm, which was used to monitor the growth of the layers and the assembly process of the polymer/G2-AuNPs-Alk film. As revealed in the inset of Figure 3, the absorbance at the surface plasmon band increased linearly with the number of bilayers, which indicated a progressive and uniform deposition process of the multilayer, suggesting that the amount of each absorbed bilayer in the assembly process was essentially the same. The peak centered at 260 nm, corresponding to the complex formation between copper and the carboxyl group,36,37 grows in a linear manner. As a comparison, the results without a catalyst showed no progressive deposition after two bilayers and the absorption did not increase, indicating that the mutilayer film was formed by a click coupling reaction. X-ray photoelectron spectroscopy (XPS) was employed to probe interlayer interactions (Figure S6 of the Supporting Information). In addition to the peaks assigned to C1s (284.5 eV) and O1s (531.0 eV). After deposition of the gold particle layers, unambiguous Au 4f7/2 and 4f5/2 were observed at 84.1 and 87.7 eV, respectively. These are typical values for Au0,38 indicating the existence of Au nanoparticles in the films. The disappearance of the peak at 405 eV assigned to azide conformed the formation of triazole linkages in the multilayers.39 The changes of the topography for each layer of deposition on silicon slides were investigated by the tapping mode atomic force microscopy. The scan size was 2 μm for the image. In previous work, 40 although no correlation between the dendrimer generation and the measured film thicknesses was observed after the dendrimer was introduced into the films, the change of the height on the film’s surface could be detected. Figure 6 shows the topographic images of the following steps of the buildup process: successively chemical deposition based on click chemistry between the G2SH-AuNPs-Alk nanoparticles of 2.0 nm average diameters (Figure 4) and the azide-functional polymer, P(AA-VBC)-AZ. When the polymer P(AA-VBC)-AZ was deposed on the silicon slide, the surface was relatively smooth (rms 0.68 nm) and little etching was observed. After the gold nanoparticles G2SH-AuNPs-Alk were introduced into the multilayer films by click chemistry, the surface roughness was found to induce significant roughening (rms 7.9 nm). It indicates that the polyphenylene dendrimer-capped gold nanoparticles were successfully connected to the polymer P(AA-VBC)-AZ. In the next deposition, the surface roughness was determined with no obvious change in topography (rms 7.1 nm).

not lose weight until the temperature was increased to about 180 °C. The TEM image and the size distribution histograms (Figure 4) show that deprotected AuNPs have spherical shapes

Figure 4. TEM images and size distributions of G2SH-AuNPs with the ratio of AuCl4−/thiol = 3 (a) before and (b) after deprotection. Scale bars in (a) and (b) are 100 nm; those in the inset images in (a) and (b) are 20 nm.

and monodisperse sizes. G2SH prevents the particle growth at a relatively low temperature.34 G2SH-Au NPs average size is similar with that of before protection, which is consistent with the UV−vis spectra (Figure S4 of the Supporting Information). The characteristic peak of gold nanoparticles does not show an obvious shift after TiPS groups deprotected, even after the reaction solution was refluxed at 70 °C for 4 h, indicating that the diameter of gold particles does not change much after deprotection. Figure 5 shows FT-IR spectra of G2-AuNPs and G2-AuNPsAlk, the peak at 2152 cm−1 is from the C−H stretching vibrate

Figure 5. FT-IR spectra of (a) G2-AuNPs and (b) G2-AuNPs-Alk.

of a TiPS-protected alkyne group, and the peak at 2100 cm−1 is due to the bare alkyne group. 2. Azide-Functional Polymer, P(AA-VBC)-AZ. The azide functional polymer, P(AA-VBC)-AZ, based on acrylic acid and 4-vinylbenzyl chloride was conveniently synthesized by a reversible addition−fragmentation chain transfer (RAFT) polymerization using benzyl dithiobenzoate (BDB) as chain 3947

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by UV−vis spectroscopy, AFM, and the contact angle. From these characterizations, we can see that the deposition process is linearly related to the number of bilayers. Linear growth of the multilayer was monitored by the progressive increase of optical absorbance at 520 nm from the surface plasmon band of G2SH-AuNPs and at 260 nm, corresponding to the complex formation between Au and the carboxyl group. In summary, via RAFT and the Brust-Schiffrin method, azide functional polymer and alkyne functional gold nanoparticles, which were stabled by polyphenylene dendrimers, were prepared. Bilayers were obtained through click coupling between polyphenylene dendrimer-capped gold nanoparticles and P(VBA-AA) polymers. On the basis of repeated click chemistry, corresponding composite gold nanoparticles/polymer films were constructed. This strategy can provide a highly efficient method to form composite nanoparticles/polymer films of controlled thickness, especially the systems that cannot be fabricated using traditional LbL process, such as noncharged polymers and nanoparticles. The composite nanoparticles/ polymer films have potential applications as vapor−sorption and electrical chemiresistors for polyphenylene dendrimer’s 3D structure, containing frozen empty voids in their interior and their activated tunneling process for charge transport.

Figure 6. AFM images of the multilayer hybrid films assembled on silicon. (a) P(VBA-AA), (b) P(VBA-AA)-(G2SH-AuNPs-Alk), and (c) P(VBA-AA)-(G2SH-AuNPs-Alk)-P(VBA-AA).

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Figure 7 shows contact angles of the multilayer hybrid films assembled on silicon. After the deposition of polymer P(AA-

ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Z.L.: e-mail, [email protected]; M.Y.: e-mail, [email protected]. cn; L.W.: e-mail, [email protected]. Tel/Fax: +86 371 67781205.

Figure 7. Contact angle of the multilayer hybrid films assembled on silicon. (a) P(VBA-AA), (b) P(VBA-AA)-(G2SH-AuNPs-Alk), and (c) P(VBA-AA)-(G2SH-AuNPs-Alk)-P(VBA-AA).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the National Science Foundation of China (Grants 50903075 and 50873093).

VBC)-AZ on the silicon slide, the surface tended to be hydrophilic (60.8°). The surface changed with a tendency to be hydrophobic (87.4°), while the gold nanoparticles G2SHAuNPs-Alk were deposed, which was caused by less hydrophilic polyphenylene dendrimers (G2SH). When the polymer P(AAVBC)-AZ was introduced into the multilayer films again, the contact angle came down to 69.7°. The alteration of the contact angle on the multilayer surface was consistent with the transformation of chemical properties of the surface component during the process of deposition.

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REFERENCES

(1) Srivastava, S.; Kotov, N. A. Composite Layer-by-Layer (LBL) Assembly with Inorganic Nanoparticles and Nanowires. Acc. Chem. Res. 2008, 41, 1831−1841. (2) Lvov, Y.; Decher, G.; Mohwald, H. Assembly, Structural Characterization, And Thermal Behavior of Layer-by-Layer Deposited Ultrathin Films of Poly(Vinyl Sulfate) and Poly(Allylamine). Langmuir 1993, 9, 481−486. (3) Decher, G. Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites. Science 1997, 277, 1232−1237. (4) Liu, Z. L.; Wang, X. D.; Wu, H. Y.; Li, C. X. Silver Nanocomposite Layer-by-Layer Films Based on Assembled Polyelectrolyte/Dendrimer. J. Colloid Interface Sci. 2005, 287, 604−611. (5) Lvov, Y.; Decher, G.; Sukhorukov, G. Assembly of Thin-Films by Means of Successive Deposition of Alternate of DNA and Poly(allylamine). Macromolecules 1993, 26, 5396−5399. (6) Boulmedais, F.; Ball, V.; Schwinte, P.; Frisch, B.; Schaaf, P.; Voegel, J. C. Buildup of Exponentially Growing Multilayer Polypeptide Films with Internal Secondary Structure. Langmuir 2003, 19, 440− 445.

4. CONCLUSIONS Polyphenylene dendrimer-capped gold nanoparticles with naked alkynes in their peripheries were synthesized based on the reduction of chloroauric acid (HAuCl4·3H2O) by sodium borohydride using polyphenylene dendrimer (G2SH) as the stabilizing thiol ligand. UV−vis spectroscopy, TEM, and FT-IR results show that the particles have quasispherical shapes and monodisperse sizes, and the ligands on their surface exist as alkynes. For fabrication of the multilayer hybrid films, another clickable species, P(VBA-AA), was synthesized by RAFT. G2SH-AuNPs of 2.0 nm average diameters were used for the construction of layer-by-layer self-assembled multilayer hybrid films. The polymer/gold cluster hybrid films were characterized 3948

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dx.doi.org/10.1021/la400397q | Langmuir 2013, 29, 3943−3949