DOI: 10.1021/la902354e 12425Langmuir 2009, 25(21), 12425–12428 Published on Web 09/28/2009

pubs.acs.org/Langmuir

© 2009 American Chemical Society

One-Pot Preparation ofDendrimer-GoldNanoparticleHybrids in aDipolar

Aprotic Solvent

Marco Stemmler, Fernando D. Stefani,*,† Stefan Bernhardt, Roland E. Bauer, Maximilian Kreiter,Klaus M€ullen, and Wolfgang Knoll‡

Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany. †Present address:Ludwig-Maximilians-Universit€at M€unchen, Physik Department, Photonics and Optoelectronics Group andCeNS, Amalienstrasse 54, D-80799 Munich, Germany. ‡Present address: Austrian Research Centers GmbH,

Donau-City-Strasse 1, A-1220 Vienna, Austria

Received July 1, 2009. Revised Manuscript Received September 15, 2009

Wepresent a simple one-pot, one-stepmethod to obtain stable and nearlymonodisperse gold nanoparticles in dipolaraprotic solvents. Novel thiomethyl-functionalized polyphenylene dendrimers are used to control the growth andstabilize the nanoparticles in suspension. The dendrimer functionalized gold nanoparticles have an average size ofroughly 10 nm and are stable in suspension for several weeks. The stability in dipolar aprotic solvents and the greatfunctionalization flexibility offered by the dendrimers make these metal/dendrimer hybrid systems promising forapplications such as nanophotonics, molecular electronics, and sensing.

Introduction

Because of their interesting size-dependent optical, electronic,and catalytic properties, gold nanoparticles (AuNPs) are amongthe oldest and best-studied nanoscale materials.1 Numerousmethods to prepare particles with diameter ranging from 1 nmto several micrometers are documented.2-5 All chemical prepara-tion methods of AuNPs consist of nucleation and growth of goldclusters by reduction of a gold salt. The challenge in this kind ofsynthesis is the growth to a controlled size and shape, thestabilization in suspension, and the surface functionalization ofthe nanoparticles. This is usually achieved by introducing acapping agent. Although a large number of capping agents havebeen investigated, only a handful of them lead to stable nano-particles with narrow size distributions. For water-solubleAuNPs, the most used procedures are variations of the classicTurkevich-Frens citrate reduction route.6,7 The surface functio-nalization ofAuNPswith organicmolecules is crucial for their useas building blocks in novel nano-objects with potential applica-tion in plasmonics andmolecular electronics.8,9 For this reason, itis desirable to obtain AuNPs stable in different organic solvents.Most hydrophobic particles are prepared by borohydride reduc-tion in an organic solvent in the presence of thiol-cappingligands using either a two-phase liquid/liquid system or a suitable

single-phase solvent.10-18 The multifunctionality and well-de-fined macromolecular structure of dendrimers renders them asinteresting candidates for capping agents. Water-soluble dendri-mers have been successfully used to control the size, stability, andsolubility of small metal nanoparticles with diameters smallerthan roughly 4 nm.19-23 Such metal-dendrimer systems may betransferred to organic solvents via multistep phase transferprocesses.22 The interaction of thiol-terminated dendrimerswith Au atoms has been demonstrated and used to investigateAu55 clusters.

24,25 In this paper, we present a simple one-pot andone-step method to obtain stable Au NPs in dipolar aproticsolvents with narrow size distributions. The method relies onnovel thiomethyl-functinoalized polyphenylene dendrimers thatcontrol the growth of the particles and stabilize them in thesolvent.

*Corresponding author. E-mail: fernando.stefani@physik.uni-muenchen.de.(1) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104(1), 293–346.(2) Schmid, G.; Pfeil, R.; Boese, R.; Bandermann, F.;Meyer, S.; Calis, G.H.M.;

Vandervelden, W. A. Chem. Ber./Recl. 1981, 114(11), 3634–3642.(3) Goia, D. V.;Matijevic, E.Colloids Surf., A: Physicochem. Eng. Aspects 1999,

146(1-3), 139–152.(4) Jana,N. R.; Gearheart, L.;Murphy, C. J.Langmuir 2001, 17(22), 6782–6786.(5) Hussain, I.; Brust, M.; Papworth, A. J.; Cooper, A. I.Langmuir 2003, 19(11),

4831–4835.(6) Turkevich, J.; Stevenson, P. C.; Hillier, J.DiscussionsOf The Faraday Society

1951, No.11, 55.(7) Frens, G. Nature (London), Phys. Sci. 1973, 241(105), 20–22.(8) Maier, S. A.; Brongersma,M. L.; Kik, P.G.;Meltzer, S.; Requicha, A. A.G.;

Atwater, H. A. Adv. Mater. 2001, 13(19), 1501–1505.(9) Hipps, K. W. Science 2001, 294(5542), 536–537.(10) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem.

Soc., Chem. Commun. 1994, No.7, 801–802.(11) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B.

Chem.;Eur. J. 1996, 2(3), 359–363.

(12) Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; Harris, J. E.; Vachet, R. W.;Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish,G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14(1), 17–30.

(13) Wuelfing, W. P.; Gross, S. M.; Miles, D. T.; Murray, R.W. J. Amer. Chem.Soc. 1998, 120(48), 12696–12697.

(14) Bartz, M.; Kuther, J.; Nelles, G.; Weber, N.; Seshadri, R.; Tremel, W.J. Mater. Chem. 1999, 9(5), 1121–1125.

(15) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar,I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32(5), 397–406.

(16) Alivisatos, A. P.; Johnsson, K. P.; Peng, X. G.;Wilson, T. E.; Loweth, C. J.;Bruchez, M. P.; Schultz, P. G. Nature 1996, 382(6592), 609–611.

(17) Templeton, A. C.; Wuelfing, M. P.; Murray, R. W. Acc. Chem. Res. 2000,33(1), 27–36.

(18) Kanaras, A. G.; Kamounah, F. S.; Schaumburg, K.; Kiely, C. J.; Brust, M.Chem. Commun. 2002, No.20, 2294–2295.

(19) Balogh, L.; Tomalia, D. A. J. Am. Chem. Soc. 1998, 120(29), 7355–7356.(20) Balogh, L.; Swanson, D. R.; Tomalia, D. A.; Hagnauer, G. L.; McManus,

A. T. Nano Lett. 2001, 1(1), 18–21.(21) Crooks, R. M.; Lemon, B. I.; Sun, L.; Yeung, L. K.; Zhao, M. Q.

Dendrimer-encapsulated metals and semiconductors: Synthesis, characterization,and applications. In Dendrimers Iii: Design, Dimension, Function; V€ogtle, F., Ed.;Springer: Berlin/Heidelberg, 2001; Vol. 212, pp 81-135.

(22) Crooks, R.M.; Zhao,M. Q.; Sun, L.; Chechik, V.; Yeung, L.K.Acc. Chem.Res. 2001, 34(3), 181–190.

(23) Taubert, A.; Wiesler, U.M.; M€ullen, K. J.Mater. Chem. 2003, 13(5), 1090–1093.

(24) Schmid, G.; Emmrich, E.; Majoral, J. P.; Caminade, A. M. Small 2005, 1(1), 73–75.

(25) Schmid, G.; Meyer-Zaika, W.; Pugin, R.; Sawitowski, T.; Majoral, J. P.;Caminade, A. M.; Turrin, C. O. Chem.;Eur. J. 2000, 6(9), 1693–1697.

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12426 DOI: 10.1021/la902354e Langmuir 2009, 25(21), 12425–12428

Letter Stemmler et al.

Methylsulfanyl Functional Dendrimers

Polyphenylene dendrimers made by iterative Diels-Alderreaction steps of terminal ethynyl groups and functionalizedtetraphenylclyclopentadienones were recently introduced andfurther developed.26 The strongly interlocked pentaphenylben-zene branching points in these structures provide high stiffnessand shape persistence.27,28 Compared to dendrimer/metal com-posite materials investigated earlier, these polyphenylene dendri-mers bear several advantages in view ofAuNP stabilization. Sinceno backfolding is possible, a maximum peripheral exposition ofthe surface functionalities is achieved, resulting in a precisecontrol of the solubility properties of these macromoleculesand, consequently, of the obtained AuNP/dendrimer hybrids.

Moreover, the well-defined size and nanoporosity of the poly-phenylene dendrimers allow the build up of gold-compositematerials with potential applications in sensorics.29,30 For thiswork, we developed three dendrimers (Figure 1; see synthesisdetails in the Supporting Information), and their performance asstabilizing capping agents for Au NPs was investigated. Dendri-mers B and C were designed to interact with the growing Aunanoparticles bymeans ofmethylsulfanyl groups close to or at theperiphery. Dendrimer A served as a reference; since it has nomethylsulfanyl groups, no interaction with the goldwas expected.

Preparation of the Dendrimer-Stabilized AuNPs inTetrahydrofuran (THF)

The AuNPs were synthesized by direct reduction of HAuCl4(THF) with NaBH4 (H2O) in the presence of the dendrimer

Figure 1. Second-generation functional polyphenylene dendrimers used in this study. A contains nomethylsulfanyl groups; B contains eightmethylsulfanyl functions in the scaffold; C: polyphenylene dendrimer with sixteen methylsulfanyl groups at the periphery.23

Figure 2. Monitoring the formation of Au NPs by extinction spectroscopy. Extinction vs wavelength of (a) HAuCl4(THF) and HAuCl4-(THF) þNaBH4(aq); (b) octanethiol(THF), octanethiol(THF) þHAuCl4(THF), and octanethiol(THF) þHAuCl4(THF) þNaBH4(aq);(c,d,e) dendrimers A, B and C, respectively, in THF, and after the successive addition of HAuCl4(THF) and HAuCl4(THF)þNaBH4(aq).(f) Color photographs of the Au NP colloid created with dendrimer B and of the unstable precipitate produced with dendrimer A.

(26) Bauer, R. E.; Grimsdale, A. C.; M€ullen, K. Functionalised polyphenylenedendrimers and their applications. In Functional Molecular Nanostructures;Schl€uter, A. D., Ed.; Springer: Berlin/Heidelberg, 2005; pp 253-286.(27) Brocorens, P.; Zojer, E.; Cornil, J.; Shuai, Z.; Leising, G.; M€ullen, K.;

Bredas, J. L. Synth. Met. 1999, 100(1), 141–162.(28) Wind, M.; Saalwachter, K.; Wiesler, U. M.; M€ullen, K.; Spiess, H. W.;

Solid-state, N. M. R. Macromolecules 2002, 35(27), 10071–10086.

(29) Krasteva, N.; Besnard, I.; Guse, B.; Bauer, R. E.; M€ullen, K.; Yasuda, A.;Vossmeyer, T. Nano Lett. 2002, 2(5), 551–555.

(30) Krasteva, N.; Fogel, Y.; Bauer, R. E.; Mullen, K.; Joseph, I.; Matasuzawa,N.; Yasuda, A.; Vossmeyer, T. Adv. Funct. Mater. 2007, 17, 881.

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Stemmler et al. Letter

molecules A, B, and C (THF); further details are provided in theSupporting Information. Control experiments were done by redu-cingHAuCl4 (THF) in the presence of octanethiol (THF) and in theabsence of any capping agent. A concentrated solution of NaBH4

was used in order to keep the volume of added H2O negligible.The evolution of the reduction reactions and the formation of

the gold clusters were followed by monitoring the wavelength-dependent extinction of the suspension. The extinction spectra ofthe product of the reduction of HAuCl4 with NaBH4 in theabsence of any capping agent and in the presence of octanethiolare displayed in Figure 2a,b, respectively. The extinction spectraof pure dendrimer, of the dendrimer/HAuCl4 mixture, and of theproduct of the reduction reaction with NaBH4 are shown inFigure 2c,d,e for dendrimers A, B, and C, respectively. Thereactions in the absence of any capping agent or in the presenceof octanethiol, as well as of dendrimer A, produce black, unstableprecipitate (Figure 2f), and their extinction spectra are ratherfeatureless (Figure 2a,b,c). No stable AuNPs are formed in thesecases. Only the reaction in the presence of the thiomethyl-functionalized dendrimers B and C produce a stable red colloid(Figure 2f) with an absorption maximum around 520-530 nm,which is characteristic for nearly spherical AuNPs.32 The stabilityof these red colloids was also proven by ultraviolet/visible (UV/vis) spectroscopy; no change on the extinction of a suspension ofdendrimer-stabilized AuNPs was observed after 2 weeks.

From these control experiments, the most interesting ones arethe reduction of HAuCl4 in the presence of dendrimer A and inthe presence of octanethiol because they prove the specificefficiency of the methylsulfanyl-dendrimers B and C; i.e., neitherthe dendrimer structure nor the thiol-functionality alone aresufficient to produce stable AuNPs.

In order to obtain an estimate of the nanoparticle size distribu-tion, the product of each reaction was deposited on a siliconsubstrate and imaged by scanning electronmicroscopy (SEM; seeSupporting Information for more details). In agreement with theUV/vis measurements, the SEM study confirms that the colloidsstabilized with dendrimer B or C are composed of individual,nearly spherical Au nanoparticles (Figure 3a,b).We used custom-madeparticle size analysis software to analyze the SEM images. Abrightness threshold was applied to differentiate the Au particlesfrom the silicon background. The threshold was set to a bright-ness value halfway between the background level and the averagenanoparticle brightness so as to obtain, on average, a halfintensity particle size. An ellipse is fitted to each Au particle,and their major and minor axes are determined. On average, theAuNPs show a difference between their major and minor axis ofless than 30%. The distribution of AuNPs sizes (average dia-meter) prepared with dendrimers B and C are shown inFigures 3c,d, respectively. It should be noted that the SEM imageshave a resolution of 1.8 nm, which sets the limit for theAuNP sizeaccuracy.

The results indicate that, due to the strong Au-sulfur interac-tion, the thiomethyl-functionalized dendrimers B and C bind tothe growing gold clusters, controlling their size and stabilizingthem in suspension in THF bymeans of their soluble branches. Inorder to confirm that the thiomethyl-functionalized dendrimers Band C are attached to the surface of the AuNPs, we make use ofthe fluorescence of the dendrimers. The fluorescence emission offluorophores is quenched when they are in close proximity to aAuNP.33Figure 4a shows the fluorescence spectra of dendrimerCas the reduction reaction was conducted.

After addition of HAuCl4, the fluorescence emission decreasesin intensity and shifts to the red as a result of the strongabsorption

Figure 3. Size distribution of the different AuNPs. (a,b) SEM images of the nanoparticles produced with dendrimers B and C, respectively.On the right, intensity profiles of two nanoparticles (green) with the corresponding size determined as the full-width half maximum of aGaussian fit to the signal (black). (c,d) Size distributions of the AuNPs obtainedwith dendrimers B andC. The average size and the standarddeviation are shown.

(31) Bernhardt, S.; Baumgarten,M.;Wagner,M.;Mullen, K. J. Am. Chem. Soc.2005, 127(35), 12392–12399.(32) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer

Verlag: Berlin, 1995; Vol. 25.

(33) Dulkeith, E.; Morteani, A. C.; Niedereichholz, T.; Klar, T. A.; Feldmann,J.; Levi, S. A.; van Veggel, F. C. J. M.; Reinhoudt, D. N.;Moller, M.; Gittins, D. I.Phys. Rev. Lett. 2002, 89(20), 203002.

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of HAuCl4 up to roughly 350 nm. Upon the addition of NaBH4,Au nanoparticles start to form and, although HAuCl4 is con-sumed, the fluorescence decreases further (Figure 4a left). Thisfluorescence quenching clearly shows that the thiomethyl-den-drimer molecules are closely bound to the AuNP surface. Addi-tion of NaBH4 in excess leads to degradation of the particles. Thegold precipitates from suspension in the form of a black unstableprecipitate, and the thiomethyl-dendrimers are released into thesolution. The fluorescence emission is nearly fully recovered(Figure 4a right). An analogous result is obtained with dendrimerB. Figure 4b displays the integrated fluorescence emission ofdendrimersB andC, normalized to the fluorescence after additionof HAuCl4, as a function of the molar ratio of NaBH4/HAuCl4.In both cases, the fluorescence reaches a minimum at NaBH4/HAuCl4= 3, which corresponds to the full reduction of the Au3þ

ions, and recovers almost totally for sufficient excess of HAuCl4.

Conclusions

We have demonstrated that thiomethyl-functionalized dendri-mer molecules allow the one-step, one-pot preparation of AuNPs

in dipolar aprotic solvents. The obtained nanoparticles present arather narrow size distribution centered at 10 nmand are stable asindividuals in the solvent for several weeks. The availability ofstable dendrimer-nanoparticle complexes in organic solventsopens the door to novel synthetic routes to combine organicmolecules with metallic nanoparticles. The dendrimer-AuNPhybrids presented here are promising nano-building-blocks withpotential impact in plasmonics, sensing, molecular electronics,and nanophotonic applications.

Acknowledgment. This work was partly supported by theBundesministerium f€ur Bildung und Forschung (Grant No.03N8702) and by German Science Foundation (SFB 625).

Supporting Information Available: Details on the materi-als, physical, and analytical methods, the synthesis of thescaffold-functionalized polyphenylene dendrimers, andthe preparation of the Au nanoparticles. This infor-mation is available free of charge via the Internet at http://pubs.acs.org.

Figure 4. Dendrimer fluorescence emission during the nanoparticle synthesis. (a) Fluorescence emission spectra of dendrimer C. Left:particle formation process. Pure dendrimer C (gray), dendrimer CþHAuCl4 (1:16; green), dendrimer CþHAuCl4 þNaBH4 (1:16:16 and1:16:48; blue). Right: over-reduction leading to Au precipitation. Dendrimer C þHAuCl4 þNaBH4 (1:16:96, 1:16:160, 1:16:256; red). Theoriginal spectrumof the pure dendrimer C is shown for comparison (gray). (b) Integrated fluorescence emission of dendrimer B andC duringthenanoparticle synthesis.Data are normalized to the fluorescence emissionafter the additionofHAuCl4. Thehigh fluorescencedatapoint atzero corresponds to the emission before the addition of HAuCl4. The dashed lines are guides to the eye.

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