shell nanoparticles

Polymer 54 (2013) 485e489

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Conjugated polymer mediated synthesis of nanoparticle clusters and core/shell nanoparticles Ping Xu a, b, **, Kuan Chang a,1, Young Il Park b, 2, Bin Zhang a,1, Leilei Kang a,1, Yunchen Du a,1, Rashi S. Iyer c, Hsing-Lin Wang b, * a b c

Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China C-PCS, Los Alamos National Laboratory, Los Alamos, NM 87545, USA B-7, Los Alamos National Laboratory, Los Alamos, NM 87545, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 October 2012 Received in revised form 26 November 2012 Accepted 2 December 2012 Available online 6 December 2012

Two water soluble conjugated polymers, poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and ammonium ion stabilized poly(phenylene vinylene) (P2), are found to be able to reduce noble metal ions to zero-valent metals via a direct chemical deposition technique. Au nanoparticle clusters can be obtained through reduction of Au3þ ions by PEDOT:PSS and the electronic coupling between them can be controlled by HAuCl4 concentration. Core/shell Ag/polymer nanostructures are prepared from reduction of Agþ ions by P2, which have a ppb detection limit for 4-MBA using surfaceenhanced Raman spectroscopy (SERS). This conjugated polymer mediated synthesis of metal nanoparticles may open a new avenue for fabricating nanomaterials and nanocomposites with tunable optical properties that are dominated by their structure and electronic coupling between nanoparticles. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Conjugated polymer Poly(3,4-ethylene dioxythiophene) Poly(phenylene vinylene)

1. Introduction Metal nanostructures have revealed potentials toward optical, electronic, sensing and biomedical applications due to their sizedependent and quantum confined photophysical properties [1]. Various synthetic platforms, including chemical, physical and even biological approaches, have been developed to achieve size and morphology control of the metal nanoparticles [2,3]. Among them, solution chemistry processes are commonly used for synthesizing noble metal nanocrystals with well-defined shapes and structures wherein polyol [4], sodium citrate [5], borohydride [6,7], ascorbic acid [8], aldehyde and hydrazine [9] have been used as reducing agents. However, solution chemistry methods typically consume a large amount of organic solvents and hazardous chemicals that are considered environmentally unfriendly. In the past few years, we have demonstrated that a conjugated polymer with a lower reduction potential than a metal ion can reduce the metal ion into zero-valent metal. Polyaniline (PANI),

* Corresponding author. Tel.: þ1 505 6656811. ** Corresponding author. Department of Chemistry, Harbin Institute of Technology, Harbin 150001, China. Tel.: þ86 451 86413702; fax: þ86 451 86418750. E-mail addresses: [email protected] (P. Xu), [email protected] (H.-L. Wang). 1 Tel.: þ86 451 86413702. 2 Tel.: þ1 505 6656811. 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2012.12.003

polypyrrole (PPy), and PANI-PPy copolymers have been used as reducing agents to prepare Ag, Au, Pt and Pd nanostructures [10e 15]. Our recent work shows that even AgeAu alloys can be formed by sequential deposition at PANI surfaces [16]. Of particular interest is that monomers of some conjugated polymers, e.g. 3,4ethylene dioxythiophene and aniline, can also react with noble metal salts (HAuCl4) to produce metal-polymer nanocomposites through a one-step process [17e20]. Those metal-polymer nanocomposites produced by the conjugated polymer mediated technique can be promising as catalysts in organic synthesis [13e15] and sensitive platforms in surface-enhanced Raman spectroscopy (SERS) [21e26]. Here, we demonstrate two kinds of water soluble conjugated polymers (Scheme 1), poly(3,4-ethylene dioxythiophene) :poly(styrene sulfonate) (PEDOT:PSS) and ammonium ion stabilized poly(phenylene vinylene) (P2), that can be used as reducing agents to synthesize metal nanoparticles. The advantage of this technique lies in the ease of obtaining water soluble metal/polymer nanocomposites via a direct chemical reduction route, which is not accessible by using other conducting polymers like PANI and PPy. Morphology and size of the Au nanostructures produced through chemical reduction by PEDOT:PSS can be easily tuned by the HAuCl4 concentration. Reduction of AgNO3 by P2 can lead to metal/ polymer core/shell structures. We believe the conjugated polymer mediated synthesis of metal nanoparticles can open a new avenue

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3. Results and discussion

Scheme 1. Schematic illustration of the conjugated polymer mediated synthesis of Au nanoparticle clusters by PEDOT:PSS and Ag/polymer core/shell nanoparticles by P2.

for fabricating nanomaterials and nanocomposites with tunable optical properties that are dominated by their structure and electronic coupling between nanoparticles. 2. Experimental 2.1. Materials Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) in water (1 wt%, Baytron P), HAuCl4 (99% Aldrich) and AgNO3 (99.9999% Aldrich) are used as received. Ammonium ion stabilized poly(phenylene vinylene) (P2) is prepared according to a previous work [28]. 2.2. Synthesis For the preparation of metal nanoparticles by PEDOT:PSS, 1 ml of as-obtained PEDOT:PSS solution was firstly diluted in 2 ml of H2O. Then, 1 ml of HAuCl4 solution or AgNO3 solution was added. The solutions were manually mixed and then kept undisturbed. Here, HAuCl4 solutions of different concentrations were used. For the preparation of metal nanoparticles by P2, an aqueous P2 solution with a concentration of 0.5 mg/ml was made as a stock solution. 1 ml of P2 solution and 1 ml 10 mM AgNO3 solution or HAuCl4 solution were mixed together and kept undisturbed for 6 h. 2.3. Characterization The morphology of the prepared metal nanoparticles was imaged on a JEOL 3000F transmission electron microscope (TEM) by dropping the metal nanoparticle solution onto a carbon coated TEM grid. UVevis spectra were recorded on a UVevisible transmission absorption spectroscopy (Varian Cary 300). The Ag/polymer core/shell nanoparticles were dispersed in MBA ethanol solution of different concentrations for 30 min before the surfaceenhanced Raman scattering (SERS) response was determined. The SERS spectra were recorded on a Kaiser Raman spectrometer through a 20 (0.50 NA) microscope objective, coupled with a liquid-nitrogen-cooled charge-coupled device (CCD) detector (wavelength: 785 nm). The incident laser power was kept at 2 mW and total accumulation times of 5 s were employed.

In a typical procedure, 1 ml of PEDOT:PSS solution (Baytron P) was diluted in 2 ml of H2O, and then 1 ml HAuCl4 solution was added. The solutions were manually mixed and then kept undisturbed. The reaction solution was then kept undisturbed. PEDOT:PSS under TEM is micron sized sheet-like structures (See Fig. S1). The as-synthesized Au nanoparticles can be separated from the reaction system by centrifugation. At a medium speed (3000 rpm), the PEDOT:PSS will sink to the bottom, and the Au nanoparticles can be collected from the supernatant. Fig. 1 shows the TEM images of the Au nanoparticles produced by mixing PEDOT:PSS solution with HAuCl4 solution of various concentrations at a reaction period of 2 h. With a HAuCl4 concentration 10 mM, only scattered Au nanoparticles are produced (See Fig. S2). Here, one can see Au nanoparticles with an average diameter of 20 nm and very narrow size distribution with a HAuCl4 concentration of 10 mM (Fig. 1a). Increasing the HAuCl4 concentration to 20 mM has resulted in Au nanoparticle clusters (Fig. 1b), resembling the PtePd structures produced through a chemical route with ascorbic acid as reducing agent [27]. The high resolution TEM image inset in Fig. 1b shows that these cluster structures are actually ensembles of Au nanorods with w2 nm in width, and they are well crystallized along the (111) plane. An increase in the concentration of HAuCl4 to 30 mM leads to Au nanospheres (w100 nm in size) consisting of close packed Au nanorods (Fig. 1c). However, a HAuCl4 concentration 50 mM will produce bigger Au nanospheres, with a very broad size distribution. Moreover, Au nanoparticles that form the nanospheres are even more aggregated and it is not easy to distinguish the individual nanoparticles (Fig. 1d). Actually, HAuCl4 can react immediately with PEDOT:PSS upon mixing, and the reaction between PEDOT and HAuCl4 was tracked by time-dependent UVevis spectra. It can be seen that PEDOT:PSS solution only displays a very broad absorption band in the region of 400e800 nm, with no distinct absorption peak (Fig. 2). As we mix the PEDOT:PSS with 20 mM HAuCl4 solution, an absorption peak corresponding to Au nanoparticles can be found at 540 nm in just 1 min, indicating that reduction of Au3þ ions by PEDOT follows a very fast reaction pathway. As the reaction proceeds, the Au absorption peak intensifies coupling with a slightly red-shifted lmax, presumably due to the increase in nanoparticle concentration and size. After 100 min, the absorption peak stabilized at 552 nm, and thus the reaction is then terminated at a time period of 2 h. Inset in Fig. 2 shows the normalized UVevis spectra of the Au nanoparticles prepared by using HAuCl4 solution of different concentrations at a reaction time of 2 h. It is apparent that with increase in the HAuCl4 solution, the size of the obtained Au nanoparticles becomes larger, as manifested by the red shifts in the UVe vis spectra, which is also consistent with the TEM results. It is important to note that the Au nanoparticle clusters obtained at 20 mM exhibit a relatively narrow and slightly red-shifted UVevis spectrum, suggesting that these Au naoparticles have a relatively weak electronic coupling between them. The strong coupling between Au nanoparticles emerges when the concentration reaches >30 mM, as manifested by fairly broad and heavily red-shifted absorption spectra. We believe such diference is due to Au nanoparticle clusters that are loosely bounded by polymer (Fig. 1b) as compared to some nanoparticles that are almost fused together with minimum insulation by PEDOT (Fig. 1c, d). The timedependent UVevis spectra of samples obtained at various concentrations can be found in Fig. S3eS5. PEDOT:PSS can also be used to reduce Agþ ions to Ag nanoparticles, which is found to process slower due to a lower reduction potential of Agþ/Ag (0.799 V), as compared to that of Au growth (reduction potential of Au3þ/Au ¼ 1.50 V). Unfortunately, shape and size control of the Ag

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Fig. 1. TEM images of Au nanoparticles produced with water soluble PEDOT:PSS as the reducing agent and at different HAuCl4 concentrations: (a) 10 mM, (b) 20 mM, (c) 30 mM and (d) 50 mM.

nanoparticles has not been successfully achieved yet (See Fig. S6). The ease of size and shape control of the Au nanoparticles but not the Ag nanoparticles may result from the difference in nucleation and growth of these two metals when using PEDOT:PSS as the reducing agent.

Fig. 2. Time-dependent UVevis spectra recording the reaction between PEDOT:PSS and HAuCl4 with a concentration of 20 mM. Inset shows the normalized UVevis spectra of the Au nanoparticles prepared from different HAuCl4 concentrations at a reaction time of 2 h.

Poly(phenylene vinylene), a conjugated polymer having been extensively used in fabricating optical and electronic devices, is also found to be an reducing agent for preparing nanostructured metals. The ammonium salt on the P2 side chain renders its solubility in water [28]. After P2 and AgNO3 were mixed together, we observed a fairly slow reaction as the solution color changes from red to dark red in a few hours. Fig. 3 shows the TEM images of the Ag nanoparticles produced by mixing 1 ml of 0.5 mg/ml P2 solution and 1 ml 10 mM AgNO3 solution for 6 h. To our surprise, the final product is core/shell nanoparticles with 50 nm Ag nanoparticle core and 5 nm P2 shell. Several Ag nanoparticles can be linked together by the polymer to form chain-like structures (Fig. 3a). While, one can also find separated individual Ag nanoparticles capped by P2 (Fig. 3b). Structure of Ag nanoparticles produced from reduction of Agþ by P2 is independent of Agþ ion concentration. We believe the formation of core/shell structures using P2 while irregular Ag particles using PEDOT:PSS should be ascribed to the chemical nature of these two water soluble polymers as reducing agents. P2 applied during the synthesis is actually a molecular dispersion, which will wrap around the Ag nuclei to form a passive layer upon reaction with Agþ ions, leading to the core/shell structure. However, PEDOT:PSS is a dispersion with micron sized sheetlike structures (See Fig. S1), which only reduces the Agþ ions upon contact, a process separating from the PEDOT:PSS and unable to form nanostructures hybridizing with the polymer. The UVevis absorption of Ag/P2 core/shell nanoparticles has one peak at 420 nm which is attributed to the plasmon absorption of Ag nanoparticles and the absorption for P2 at 450 nm is likely overwhelmed by the plasmon of Ag nanoparticles (Fig. 3c). The P2

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Fig. 3. (a, b) TEM images of the Ag nanoparticles produced with water soluble P2 as the reducing agent. Reaction time: 6 h (c) UVevis spectra of P2 and P2-Ag. (d) SERS spectra of 4MBA of different concentrations on the core/shell Ag nanoparticles.

structure has been modified after the reaction process as FT-IR spectrum of Ag/P2 coreeshell nanoparticles suggests that the peak centers around 1620 cm1, which is assigned to the asymmetric stretching of C]C double bond of P2 has become significantly broadened, presumably due to oxidation of P2 and the interaction between P2 and Ag nanoparticles (See Fig. S7). This core/shell Ag/P2 structure can be a very sensitive SERS substrate for the detection of chemical molecules. As shown in Fig. 3d, a detection sensitivity of 108 M towards the target molecule 4-mercaptobenzoic acid (4-MBA) can be reached. The as-prepared Ag/polymer nanoparticles may have similar functionalities to the shell-isolated nanoparticles [29,30]. Reaction of P2 with HAuCl4 leads to Au nanoparticles with polymer tails, but not core/shell structures (See Fig. S8). More work needs to be done to control the structure of the prepared Au nanoparticles. 4. Conclusion In summary, we have demonstrated here utilization of two water soluble conjugated polymers, poly(styrene sulfonate) stabilized poly(3,4-ethylene dioxythiophene) and ammonium stabilized poly(phenylene vinylene), as reducing agents for the preparation of nanoparticle clusters and core/shell nanoparticles, respectively. Morphologies of the Au nanoparticles prepared from PEDOT:PSS and the electronic coupling between them can be controlled by HAuCl4 concentration. Core/shell Ag/polymer nanostructures prepared from P2 have a ppb detection limit for 4-MBA using SERS technique. The formation of different Ag and Au structures using P2 and PEDOT:PSS should be ascribed to the chemical nature of these two water soluble polymers and difference in nucleation and

growth nature of Au and Ag. This conjugated polymer mediated synthesis might offer a novel pathway for preparing structural and functional nanocomposites. Acknowledgments HLW acknowledges the financial support from BES Office of Science, Biomaterials program, and the Laboratory Directed Research and Development (LDRD) fund under the auspices of DOE. PX thanks the support from Natural Science Foundation of China (No. 21101041, 21203045, 21003029, 21071037, 91122002), Fundamental Research Funds for the Central Universities (Grant No. HIT.NSRIF. 2010065 and 2011017, HIT.BRETIII. 201223), and Director’s Postdoctoral Fellowship from LANL. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymer.2012.12.003. References [1] Skrabalak SE, Au L, Li XD, Xia YN. Nature Protocols 2007;2(9):2182e90. [2] Xiong YJ, Xia YN. Advanced Materials 2007;19(20):3385e91. [3] Murphy CJ, Gole AM, Stone JW, Sisco PN, Alkilany AM, Goldsmith EC, et al. Accounts of Chemical Research 2008;41(12):1721e30. [4] Sun YG, Xia YN. Science 2002;298(5601):2176e9. [5] Dick K, Dhanasekaran T, Zhang ZY, Meisel D. Journal of the American Chemical Society 2002;124(10):2312e7. [6] Brust M, Walker M, Bethell D, Schiffrin DJ, Whyman R. Journal of the Chemical Society, Chemical Communications 1994;7:801e2. [7] Mallin MP, Murphy CJ. Nano Letters 2002;2(11):1235e7.

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