Gold Nanoparticle Patterns on Polymer Filmsin the Presence of Poly(amidoamine)

Dendrimers†

Jongok Won,* Kyo Jin Ihn,‡ and Yong Soo Kang§

Department of Applied Chemistry, Sejong University,98 Gunja, Gwangjin, Seoul, 143-747, Korea, Department of

Chemical Engineering, Kangwon National University,192-1, Hyoja2, Chunchon, Kangwon, 200-701, Korea, and

Center for Facilitated Transport Membranes, Korea Instituteof Science and Technology, P.O. Box 131,

Cheongryang, Seoul, 130-650, Korea

Received April 11, 2002. In Final Form: July 11, 2002

IntroductionThin-films comprising metal nanoparticles have at-

tracted considerable attention.1 This paper describes anew procedure for preparing nanoparticle patterns.

Gold particle patterns on the surface of polymers in thepresence of dendrimers were prepared. Low-generation(G3 and G4) poly(amidoamine) (PAMAM) dendrimer wasused as a stabilizer for the formation of particles as wellas an anchor to attach the dendrimer/gold particles to thesurface of polymer substrates. We have previously shownthat dendrimers can be immobilized on the surface ofpolymer films by a simple plasma technique.2 By pre-loading dendrimers coordinated with suitable metal ionsand then reducing them into metal particles by ultraviolet(UV) exposure, patterns of dendrimer/metal particlesattached to the polymer substrate were obtained (Scheme1). Patterned surfaces with regularly sized and spacedfeatures on the nanometer-scale level are desirable formany potential applications. This procedure to preparemetal particle patterns attached to the polymeric sub-strates provides an opportunity to control the particle sizeand interparticle spacing, due to the characteristics ofdendrimers without aggregation. Metal nanoparticle filmshave usually been fabricated by the deposition of metalcolloids on inorganic substrates such as glass, silicon,indium-tin oxide glass, and metals.1,3,4 This simplepreparation method of patterned dendrimer/metal par-ticles immobilized on polymeric substrates may have atremendous organic photovoltaic application potential dueto the advantages of polymer materials (plasticity, easeof processing, and light weight).

Dendrimers, branched macromolecules, were used asa template for the particle formation and/or a protectiveagent for the agglomeration of metal particles.5-9 Manymetal particles were prepared in the presence of den-

drimers by photoreduction4 and by the addition of chemicalreductants.6-9 A variety of dendrimer/metal nanoparticleswere prepared in the presence of poly(amidoamine)dendrimers having surface amino, hydroxyl, or persub-stituted sugar groups.5,8,10-13 As a result of this research,patterned dendrimer/gold nanoparticles immobilized onpolymeric substrates were prepared.

Experimental Section

Third- and fourth-generation amine-terminated PAMAMdendrimers of 20 and 10 wt % in methanol, respectively (D;Aldrich Chemical Co.), HAuCl4‚3H2O (Aldrich Chemical Co.),and maleic anhydride (Junsei) were purchased and used asreceived. Poly(dimethylsiloxane) (PDMS; model no. SSPM100;Specialty Silicone Products, Inc., NY) and poly(ethylenetereph-thalate) (PET; Yul-Chon Co., Seoul) films were used as substrates.Dilute aqueous solutions of PAMAM dendrimers were mixedwith aqueous solutions of HAuCl4 at controlled stoichiometries.

Plasma treatment was carried out using a R300A radiofrequency generator (Autoelectronic, Seoul), operating at 13.56MHz and set at 50 W. Maleic anhydride (100 mg) was placed ina plasma reactor with polymer (PDMS or PET) films in order tointroduce succinic anhydride groups on the polymer films. Thefilm was glow-discharged for 1 min under a pressure of 0.02 Torrin a bell-jar type reactor. Inductively coupled plasma wasgenerated by a circular coil connected to a radio frequencygenerator. HAuCl4-dendrimer solution ([HAuCl4] was fixed tobe 5.08 × 10-6 mol/mL; [NH2]/[Au] ) 1.8:1) was subsequentlyreacted with succinic anhydrides on the polymer-AH film tomake polymer-AH-D/Au+ composite films. Because of thedependence of sizes on the reduction rates, we have used HAuCl4-dendrimer solutions that were as fresh as possible. The polymer-AH-D/Au+ composite film was annealed at 120 °C for 1 h toinduce the chemical reaction between the anhydrides and aminesof dendrimers. For UV irradiation of the composite film, an 8 Wlamp (VL-4.LC) supplied by Vilber Lourmat, France, was used.The sample was taken into the chamber and exposed to UVradiation at a distance of 7 mm from the lamp for 1 h. The intensityat λ ) 253.7 nm was 2070 µW/cm2. Any loosely bound materialwas removed from the composite films by rinsing in deionizedwater and sonication for 30 min. The samples were stored in avacuum before characterization.

UV-vis spectra of solutions before and after the reduction ofmetal ionsweremeasuredwithaUVspectrophotometer (Hewlett-Packard 8452A). Contact angles were measured by a contactangle meter (CAM-MICRO, Tantec Inc., IL). The samples of goldparticles in dendrimer solutions were prepared by mounting adrop of the solutions on a carbon-coated Cu grid and allowing thedrop to dry under dark. The morphology of the samples wascharacterized by transmission electron microscopy (TEM; JEOL,JEM-2010). TEM samples of patterned dendrimer/gold particlesattached to the surface of PET films were prepared by a replicationmethod. The carbon replica film containing gold particle filmwas mounted on a copper grid for TEM. A thin cross section ofthe film with approximately 50 nm thickness was prepared byan ultramicrotome, model Ultracut-R made by Leica, using adiamond knife at room temperature. The PET film containinggold particles was cut into an appropriate size and molded withepoxy resin (Epon-812 of SPI). The thin-sectioned film was

* To whom correspondence should be addressed. Tel: +82-2-3408-3230. Fax: +82-2-462-9954. E-mail: jwon@sejong.ac.kr.

† Presented at the EUPOC2001, Gargnano, Italy, May 2001.‡ Kangwon National University.§ Korea Institute of Science and Technology.(1) For example: Musick, M. D.; Kesting, C. D.; Lyon, L. A.; Botsko,

S. L.; Pena, D. J.; Holliway, W. D.; McEvoy, T. D.; Richardson, J. N.;Natan, M. J. Chem. Mater. 2000, 12, 2869.

(2) Cha, B. J.; Kang, Y. S.; Won, J. Macromolecules 2001, 34, 6631.(3) For example: Chechik, V.; Crooks, R. M. Langmuir 1999, 15,

6364.(4) Bar, G.; Rubin, S.; Cutts, R. W.; Taylor, T. N.; Zawodzinski, T.

A., Jr. Langmuir 1996, 12, 1172.(5) Esumi, K.; Suzuki, A.; Aihara, N.; Usui, K.; Torigoe, K. Langmuir

1998, 14, 3157.(6) Zhao, M.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1998, 120,

4877.(7) Balogh, L.; Tomalia, D. A. J. Am. Chem. Soc. 1998, 120, 7355.

(8) Garcia, M. E.; Baker, L. A.; Crooks, R. M. Anal. Chem. 1999, 71,256.

(9) Zhao, M.; Crooks, R. M. Adv. Mater. 1999, 11, 217.(10) Esumi, K.; Suzuki, A.; Yamahira, A.; Torigoe, K. Langmuir 2000,

16, 2604.(11) Esumi, K.; Hosoya, T.; Suzuki, A.; Torigoe, K. Langmuir 2000,

16, 2978.(12) Grohn, F.; Bauer, B. J.; Akpalu, Y. A.; Jackson, C. L.; Amis, E.

J. Macromolecules 2000, 33, 6042. Grohn, F.; Kim, G.; Bauer, B. J.;Amis, E. J. Macromolecules 2001, 34, 2179.

(13) Zhao, M.; Crooks, R. M. Chem. Mater. 1999, 11, 3379.

8246 Langmuir 2002, 18, 8246-8249

10.1021/la020344y CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 09/11/2002

mounted on a copper grid, and carbon was evaporated beforeTEM observation.

Results and Discussion

We prepared HAuCl4-dendrimer solutions with dif-ferent mole ratios of surface amine group of the dendrimerto gold salts (e.g., 10, 8, 7.2, 5, 3.6, 1.8, and 0.9) underdark. The pH of the solution varies from 7.0 to 2.7 whenthe mole ratio of surface amine group of the dendrimerto gold salts is 7.2-0.9, respectively. The solutions showedvarying colors with the mole ratio of surface amine group

to metal ion in aqueous solution before UV irradiation.There was no precipitation observed by the eye before orafter UV irradiation, and this suggests that the dendrimeracts as a very effective protective agent for formation andcontrol of the size of gold nanoparticles. With a molarratio of surface amine groups to HAuCl4 of 3.6:1 (i.e., [NH2]/[Au] ) 3.6), the color of the solution became red, while theyellow color lasted for a long time when the mole ratiowas 1.8. The results are clearly shown by UV-visabsorption spectra. Parts a and b of Figure 1 show theabsorbance spectra of HAuCl4-dendrimer solutions stored

Figure 1. UV-vis spectra of aqueous solutions of HAuCl4-dendrimer (G3) solutions (a) before and (b) after UV irradiation. (c)TEM micrographs of Au particles formed in the presence of G3 dendrimers after UV irradiation. Molar ratio of surface amine groupof G3 and HAuCl4 ) 1.8:1. [HAuCl4] ) 2.8 × 10-7 mol/dm3.

Scheme 1

Notes Langmuir, Vol. 18, No. 21, 2002 8247

for 1 day at room temperature before and after UVirradiation, respectively. Before UV irradiation, theabsorption bands at 290 and 520 nm for a ligand to metalcharge transfer and a plasmon band of gold particles,respectively, were shown in the HAuCl4-G3 dendrimersolutions at the mole ratio of 3.6. However, there was noobservation of the gold plasmon peak in the solution atthe mole ratio of 1.8. This trend is similar to that ofHAuCl4-G4 dendrimer solutions. After UV irradiation,the plasmon peaks were clearly observed in both solutions.In general, we observed that the light yellow color of thesolution lasted for several months when the mole ratio of[NH2]/[Au] was relatively low, but the color of the solutionchanged with time when the ratio was high enough,manifesting the importance of the mole ratio of [NH2]/[Au] in the amine-terminated PAMAM dendrimer for theformation of metal particles. The number of the ratio usedhere was the guideline since we refer to the number ofend amine groups only as a reference value, although thedendrimer contains an equivalent number of inner,tertiary amine groups that can also affect the wholephenomenon.

Figure 1c shows a TEM micrograph of gold particles inthe solution at the mole ratio of 1.8 after UV irradiation.Although the sizes of G3 and G4 PAMAM dendrimersdetermined by small-angle X-ray scattering are 3.3 and3.5 nm, respectively, the size of gold particles obtainedhere is ca. 10.6 ( 3.6 nm, which is comparable to the sizeof gold particles prepared by other groups,5,8 implying thatthe gold particles are stabilized by multiple, surface-

adsorbed dendrimers, not encapsulated within singledendrimers.8

The formation of multiple dendrimer association withmetal particles can be understood because lower genera-tion dendrimers cannot provide the template for the metalparticle but are instead attached to the metal surface andact as stabilizers.

With the dependence on the mole ratio of [NH2]/[Au] ofthe reduction behavior, we also considered that the lonepair electrons of the amide group play an important rolein the reduction. However, there is no distinct differencein the amide bonds between the dendrimer and gold/dendrimer composite in Fourier transform infrared (FT-IR) spectra. A similar discussion has been reported forthe G4, G5, and sugar ball dendrimer/gold particles,8,11

explaining that gold particles are formed and present onthe external surface of the dendrimers.

Polymer substrates such as PDMS and PET films wereplasma-treated in the presence of maleic anhydride inorder to introduce reactive succinic anhydride groups onthe surfaces of polymer substrates.2,15-17 The attenuatedtotal reflection (ATR) FT-IR spectra of succinic anhydride

(14) Prosa, T. J.; Bauer, B. J.; Amis, E. J.; Tomalia, D. A.;Scherrenberg, R. J. Polym. Sci., Polym. Phys. 1997, 35, 2913.

(15) Gaboury, S. R.; Urban, M. W. Langmuir 1993, 9, 3225; 1994, 10,2289.

(16) Ryan, M. E.; Hynes, A. M.; Badyal, J. P. S. Chem. Mater. 1996,8, 37.

(17) Park, Y. S.; Kang, Y. S.; Won, J. J. Appl. Polym. Sci. 2002, 83,2369.

Figure 2. (a) Dendrimer/gold image pattern on a PET substrate obtained by UV exposure through a photomask and TEM micrographsof (b) the surface and (c) a cross section of the sample.

8248 Langmuir, Vol. 18, No. 21, 2002 Notes

modified polymer films showed three bands at 1850, 1782,and 1730 cm-1, attributable respectively to the asymmetricand symmetric anhydride CdO stretching modes ofsuccinic anhydride and the CdO stretching modes of thecarboxyl group resulting most likely from the ring-openingreaction of anhydride on the surface.2 The succinicanhydride groups on the surface were subsequentlyreacted with amine groups of PAMAM dendrimers con-taining gold salts, resulting in a light yellow, transparentpolymer film. Upon exposure to UV light, this became ahighly transparent red film. The gold plasmon peak at521 nm was observed for polymer-AH-D-Au films byUV-vis spectrometry. We prepared different HAuCl4-dendrimer solutions and composite polymer films ofdendrimer/gold particles and observed the plasmon peakshift in both the solutions and films with the mole ratioand the generation of dendrimers. No clear differencebetween gold surface-plasmon bands was observed be-tween the plasmon peaks of the solutions and the films.

The surface characteristics for the dendrimer/goldcomposite films were measured by contact angle. Thecontact angles of water of pristine PET and PDMS filmswere ca. 71° and 104°, respectively. They decreased to therange of 30-40° with the loading amount of dendrimer/gold particles, meaning that the surface became hydro-philic. They were maintained even after sonication indeionized water for 30 min, demonstrating the strongadherence of gold particles on the polymeric substratesurface.

The gold particles were effectively formed by photore-duction and afforded finely resolved gold particle patternson a polymer substrate. A PET-AH-D film containing

Au ions (PET-AH-D/Au+) was irradiated with UVthrough a photomask. As shown in Figure 2a, the imagepattern was obtained. The dark area is the portion exposedby UV light through the photomask. The TEM image ofthe surface for the patterned sample of Figure 2a is shownin Figure 2b. To observe the shape of the gold particlesgrown in the surface of the PET film along the verticaldirection of the surface, a cross section of the sample wasprepared by ultramicrotomy (Figure 2c). As can be seen,the gold particles were well separated on the surface ofthe polymer film and showed a broad size distribution asshown in the solution (Figure 1c). Also, the multiple goldparticle layers were attached to the polymer substrates.

We prepared a gold nanoparticle pattern attached topolymer substrates for imaging by photomasking. Sincemetal nanoparticles in the presence of dendrimer are ofparticular interest for their potential applications suchas catalysts, the dendrimer/gold particles immobilized onpolymer substrates have an advantage of preventingaggregation. Moreover, this strategy and the resultsdiscussed above can be useful in the development of newimaging or nanowire materials.

Acknowledgment. This research is based upon worksupported by the Ministry of Science and Technology ofKorea through the Creative Research Initiatives Program.K.J.I. acknowledges the financial support by Grant No.97-0502-0301-3 from the Basic Research Program of theKorea Science & Engineering Foundation.

LA020344Y

Notes Langmuir, Vol. 18, No. 21, 2002 8249