Self-Assembly of Poly(ethylenimine)-Capped Au Nanoparticles at a

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Langmuir 2008, 24, 7178-7183

Self-Assembly of Poly(ethylenimine)-Capped Au Nanoparticles at a Toluene-Water Interface for Efficient Surface-Enhanced Raman Scattering Kwan Kim,†,* Hyang Bong Lee,† Ji Won Lee,† Hyoung Kun Park,† and Kuan Soo Shin‡,* Department of Chemistry, Seoul National UniVersity, Seoul 151-742, Korea and Department of Chemistry, Soongsil UniVersity, Seoul 156-743, Korea ReceiVed March 8, 2008. ReVised Manuscript ReceiVed April 25, 2008 Branched poly(ethylenimine) (PEI)-capped Au nanoparticles are prepared at room temperature using PEI as the reductant of hydrogen tetrachloroaurate (HAuCl4). The size of Au nanoparticles, ranging from 10 to 70 nm, is readily controlled by varying the relative amount of PEI used initially versus HAuCl4. The PEI-capped Au nanoparticles are further demonstrated to be assembled into a large area of 2-D aggregates at a toluene-water interface either by heating the mixture or by adding benzenethiol to the toluene phase at room temperature. Both films are quite homogeneous, but Au nanoparticles appear to be more closely packed in the film assembled via the mediation of benzenethiol. The optical property of the PEI-capped Au films is controlled by the amount of benzenethiol added to the toluene phase. The obtained large area of PEI-capped Au film exhibits strong SERS activity of benzenethiol and also exhibits a very intense SERS spectrum of 4-nitrobenzenethiol via a place-exchange reaction that takes place between benzenethiol and 4-nitrobenzenethiol. Because the proposed method is cost-effective and is suitable for the mass production of diverse Au films irrespective of the shapes of the underlying substrates, it is expected to play a significant role in the development of optical nanotechnology especially for surface plasmon-based analytical devices.

Introduction The development of synthesis protocols for nanostructured materials with tunable physicochemical properties is an important goal in nanotechnology. Among the numerous nanostructured materials, gold and silver nanoparticles, in particular, have received considerable attention because of their unique optical properties, and their applications in the areas of mircoelectronics, nonlinear optics, DNA sequencing, and surface-enhanced Raman scattering have been demonstrated.1–5 Accordingly, it is very desirable to achieve precise control over the size, shape, and dispersion of those nanoparticles.6 The widely used Brust method consists of a biphasic system where a reducing agent such as NaBH4, along with an appropriate capping agent in an organic phase, is used to generate Au and Ag nanoparticles.7 Recently, amine-functionalized molecules have been frequently used as both the reductant and stabilizer.8–10 Tertiary amine adducts can induce Ag nanoparticles to form from organometallic Ag complexes.11,12 Au nanoclusters can be formed using aminophe* Corresponding authors. Tel: +82-2-8806651 (K.K.); +82-2-8200436 (K.S.S.). Fax: +82-2-8891568 (K.K.); +82-2-8244383 (K.S.S.). E-mail: [email protected] (K.K.); [email protected] (K.S.S.). † Seoul National University. ‡ Soongsil University. (1) Fendler, J. H. Nanoparticles and Nanostructured Films; Wiley-VCH: Weinhem, Germany, 1998. (2) Andrew, T. T.; Mirkin, C. A.; Letsinger, R. L. Science 2000, 289, 1757. (3) Duan, G.; Cai, W.; Luo, Y.; Li, Y.; Lei, Y. Appl. Phys. Lett. 2006, 89, 181918. (4) Miranda, M. M.; Pergolese, B.; Bigotto, A.; Giusti, A. J. Colloid Interface Sci. 2007, 314, 540. (5) Schwartzberg, A. M.; Grant, C. D.; Wolcott, A.; Talley, C. E.; Huser, T. R.; Bogomolni, R.; Zhang, J. Z. J. Phys. Chem. B 2004, 108, 19191. (6) Zhang, Z.; Han, M. J. Mater.Chem. 2003, 13, 641. (7) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 7, 801. (8) Chen, C. C.; Hsu, C. H.; Kuo, P. L. Langmuir 2007, 23, 6801. (9) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (10) Sun, X.; Dong, S.; Wang, E. Langmuir 2005, 21, 4710. (11) Chaki, N. K.; Sudrik, S. G.; Sonawaneb, H. R.; Vijayamohanan, K. Chem. Commun. 2002, 1, 76. (12) Yamamoto, M.; Nakamoto, M. J. Mater. Chem. 2003, 13, 2064.

noxyethylether as a reducing agent at an aqueous-chloroform interface. Aslam et al.13 reported that Au nanoparticles are produced via the formation of a Au-amine complex, followed by its thermal decomposition. The thermal decomposition of the Au-amine complex was supposed to occur so rapidly that the gold nanoparticles formed in water were protected immediately by amine molecules, avoiding any further aggregation.14 Poly(ethylenimime) (PEI) is a cationic polymer used as a versatile vector for cell transfection.15–17 Recalling the reductive capability of amines, PEI-capped Au nanoparticles would then be able to form even in living cells. Recently, Wang and his colleagues10,18,19 showed that Au (or Ag) nanoparticles can be prepared in a single process that involves only heating an aqueous solution of a metal precursor and polyamine. The reductive capability of amines has been known for a long time, but the detailed mechanism of how Au and Ag nanoparticles are formed by amines has not yet been clarified. The amine-based syntheses of Au nanoparticles are usually accomplished at 60-100 °C.10,18,19 Sun et al.,20 however, have recently reported that the PEI-capped Au nanoparticles could also be prepared at room temperature. Curiously, it was found that increasing the molar ratio of PEI to gold precursor led to increasing particle size. In pursuit of the possible formation of Au nanoparticles in cells under ambient conditions, we have reexamined in this work whether Au nanoparticles can indeed be formed at room temperature using PEI as the reductant. In fact, we have demonstrated that PEI-capped Au nanoparticles can be formed at room temperature. Furthermore, the size and optical properties (13) Aslam, M.; Fu, L.; Su, M.; Vijayamohanan, K.; Dravid, V. P. J. Mater. Chem. 2004, 14, 1795. (14) Sun, X.; Jiang, X.; Dong, S.; Wang, E. Macromol. Rapid Commun. 2003, 24, 1024. (15) Bieber, T.; Elsa¨sser, H. Biotechniques 2001, 30, 74. (16) Godbey, W. T.; Wu, K. K.; Mikos, A. G. Med. Sci. 1999, 96, 5177. (17) Boussif, O.; Lezoualc’h, F.; Zenta, M. A.; Mergny, M. D.; Scherman, D.; Demeneix, B.; Behr, J. P. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 7297. (18) Sun, X.; Dong, S.; Wang, E. Polymer 2004, 45, 2181. (19) Sun, X.; Dong, S.; Wang, E. Macromol. 2004, 37, 7105. (20) Sun, X.; Dong, S.; Wang, E. J. Colloid Interface Sci. 2005, 288, 301.

10.1021/la800733x CCC: $40.75  2008 American Chemical Society Published on Web 06/20/2008

Self-Assembly of PEI-Capped Au Nanoparticles

of PEI-stabilized Au nanoparticles can be controlled by changing the molar ratio of the reactants. Another important purpose of this work is to assemble homogeneous 2-D films consisting of positively charged Au nanoparticles for the development of multifunctional optical devices. Kumar et al.21 have observed that aromatic molecules in the organic phase bind strongly with aqueous gold nanoparticles. This process leads to the immobilization of gold nanoparticles in the form of a highly localized film at the interface. Recently, it was reported that nanoparticles synthesized in an aqueous medium can be self-assembled into 2-D arrays at liquid-liquid interfaces, which is induced by the destabilization of nanoparticles.22,23 Destabilization was achieved by adding alcohol to an aqueous nanoparticle suspension. When a particle is hydrophilic, it can be trapped at liquid-liquid interfaces by bringing the contact angle close to 90°. Reincke et al.24 have found that ethanol forces the contact angle of citrate-stabilized Au nanoparticles with a heptane-water interface close to 90°, which causes these nanoparticles to form a monolayer film. Suzuki et al.25 also investigated the method for the preparation of 2-D films of Au nanospheres and nanorods at a hexane-water interface by adding polar solvents such as acetonitrile and methanol and demonstrated that Au nanorods with a high aspect ratio are more effective in SERS enhancement. Similar methodology for synthesizing 2-D arrays of Au nanorods at a hexane-water interface and thickness-dependent SERS of the adsorbed molecules have also been reported by Yun at el.26 More recently, Lee et al.27 showed that C60 can induce the self-assembly of gold nanoparticles at water-oil interfaces. However, when salts such as NaClO4 were added to the Au solution, coagulation occurred in the bulk solution.28 Most recently, Park et al.29 reported the self-assembly of highly ordered nanoparticle monolayers at a water-hexane interface with 1-dodecanethiol in the hexane layer. The adsorption of 1-dodecanethiol to the nanoparticle surface caused the conversion of the electrostatic repulsive force to a van der Waals interaction. We have discovered, in this report, that the PEI-capped Au nanoparticles can be fabricated into a 2-D film by adding toluene to the colloidal solution. To the best of our knowledge, however, this is the first report on the formation of a larger area of homogeneous Au nanoparticle films at the toluene-water interface simply by heating the mixtures. For the case of a PEIcapped Au solution, 2-D nanoaggregates were not fabricated by the addition of polar solvents (acetonitrile, methanol, or ethanol) or by the addition of C60 or salts. This is obviously due to the presence of cationic polyelectrolytes (PEI) around the Au nanoparticles. Moreover, another unique phenomenon observed in this study is that the Au nanoparticle film layer creeps up the glass wall of the vial after adding benzenethiol to the toluene phase. This phenomenon indicates that the driving force to form an interfacial monolayer is very strong. The optical property of the Au films can be controlled by the amount of benzenethiol added to the toluene phase. The Au films have also been (21) Kumar, A.; Mandal, S.; Mathew, S. P.; Selvakannan, P. R.; Mandale, A. B.; Chaudhari, R. A.; Sastry, M. Langmuir 2002, 18, 6478. (22) Duan, H.; Wang, D.; Kurth, D. G.; Mohwald, H. Angew. Chem., Int. Ed. 2004, 43, 5639. (23) Ki, Y. J.; Huang, W. J.; Sun, S. G. Angew. Chem., Int. Ed. 2006, 45, 2537. (24) Reincke, F.; Hickey, S. G.; Kegel, W. K.; Vanmaekelbergh, D. Angew. Chem., Int. Ed. 2004, 43, 458. (25) Suzuki, M.; Niidome, Y.; Terasaki, N.; Inoue, K.; Kuwahara, Y.; Yamada, S. Jpn. J. Appl. Phys. 2004, 43, L554. (26) Yun, S.; Park, Y.-K.; Kim, S. K.; Park, S. Anal. Chem. 2007, 79, 8584. (27) Lee, K. Y.; Cheong, G. W.; Han, S. W. Colloids Surf., A 2006, 275, 79. (28) Suzuki, M.; Niidome, Y.; Kuwahara, Y.; Terasaki, N.; Inoue, K.; Yamada, S. J. Phys. Chem. B 2004, 108, 11660. (29) Park, Y. K.; Yoo, S. H.; Park, S. Langmuir 2007, 23, 10505.

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demonstrated to be very surface-enhanced Raman scattering active. The application prospects of the PEI-capped Au nanoparticles are thus expected to be very great in the areas of nanoscience and nanotechnology.

Experimental Procedures Hydrogen tetrachloroaurate (HAuCl4, 99.99%), branched poly(ethylenimine) (PEI, MW ∼25 kDa), benzenethiol (BT, 99+%), 4-nitrobenzenethiol (4-NBT, 80%), rhodamine 6G (R6G, 99%), poly(allylamine) hydrochloride (PAH, MW ∼70 kDa), poly(vinylpyrrolidone) (PVP, ∼40 kDa), and tetraethylenepentamine (TEP, 97%) were purchased from Aldrich and used as received. Other chemicals, unless specified, were reagent grade, and highly purified water, with a resistivity greater than 18.0 MΩ cm (Millipore Milli-Q system), was used in preparing the aqueous solutions. To prepare PEI-capped Au nanoparticles, 25 mL of a 1.4 mM aqueous HAuCl4 solution was mixed with 0.4-1.0 mL of 1% (w/w) PEI and then stirred vigorously at room temperature for 16 h. The size of the Au nanoparticles was controlled by the amount of PEI added to the reaction mixture. When the reaction was carried out in ethanol, about 80 h was needed to obtain similar products. The reacted mixture was then ultracentrifuged and filtered. The precipitate was washed with copious amounts of deionized water. The PEIcapped Au nanoparticles thus obtained were redispersed in water (or ethanol) and gently sonicated before transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectroscopy were carried out. A fairly homogeneous film was formed at the toluene-water interface simply by heating the mixture of toluene and PEI-capped Au suspension to a boil. A more homogeneous film was formed at the toluene-water interface, at room temperature, as thiol molecules such as benzenethiol (BT) were added to the toluene phase. The large area of the 2-D film was able to form on a separate glass substrate immersed in the mixture, or the interfacial layer could be transferred to a glass substrate as a Langmuir-Blodgett film. UV-vis spectra were obtained with a SINCO S-2130 UV-vis absorption spectrometer. TEM images were taken on a JEM-200CX transmission electron microscope at 200 kV. FE-SEM images were obtained with a JSM-6700F FE-SEM operated at 5.0 kV. XRD patterns were obtained on a Rigaku model D/Max-3C powder diffractometer using Cu KR radiation. XPS measurements were carried out with a VG Scientific ESCALAB MK II spectrometer using Mg KR X-rays as the light source. Infrared spectra were recorded using a Bruker IFS 113v FTIR spectrometer equipped with a globar light source and a liquid-N2-cooled wide-band mercury cadmium telluride detector. To record the transmission spectra, a CaF2 window was used, and a total of 256 scans were performed at a resolution of 2 cm-1. The Happ-Genzel apodization function was used in Fourier transforming the interferograms. Raman spectra were obtained using a Renishaw Raman system model 2000 spectrometer. The 632.8 nm line from a 17 mW He/Ne laser (Spectra Physics model 127) was used as the excitation source. Raman scattering was detected over 180° with a Peltier cooled (-70 °C) charge-coupled device (CCD) camera (400 pixels × 600 pixels). The data acquisition time was usually 30 s, and the measured intensity was normalized with respect to that of a silicon wafer at 520 cm-1.

Results and Discussion Figure 1 shows the evolution of UV-vis spectra of a gold colloid from the mixture of aqueous HAuCl4 and PEI solution at 25 °C: in this specific case, the reaction mixture consisted of 25 mL of 1.4 mM HAuCl4 and 0.7 mL of 1% (w/w) PEI. Initially, the aqueous AuCl4- solution shows one distinct band at 292 nm caused by the ligand (π)-to-metal (σ*) charge-transfer transition (LMCT).30 When PEI is added to the aqueous HAuCl4 solutions, the 292 nm band disappears immediately. Much the same behavior (30) Torigoe, K.; Esumi, K. Langmuir 1992, 8, 59.

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Figure 1. UV-vis spectra obtained as a function of time (time interval ) 1 h) after the addition of PEI to an aqueous HAuCl4 solution at 25 °C. The inset shows the absorbance at 515 nm vs the incubation time.

Figure 3. (a) XRD and (b) XPS spectra of PEI-capped Au nanoparticles prepared from a reaction mixture consisting of 25 mL of 1.4 mM HAuCl4 and 0.7 mL of 1% (w/w) PEI.

Figure 2. UV-vis spectra and corresponding TEM images taken after mixtures containing 25 mL of 1.4 mM aqueous HAuCl4 solution and (a) 0.9, (b) 0.7, and (c) 0.5 mL of a 1% (w/w) aqueous PEI solution were stirred for 16 h at room temperature.

was observed when PAH was added to the HAuCl4 solution. This indicates that the π-to-σ* LMCT is effectively quenched by cationic polyelectrolytes.31 In the presence of PEI, however, a new band developed at 515 nm for up to 16 h (inset of Figure 1) that could be attributed to the surface plasmon resonance (SPR) band of Au nanoparticles.32 A similar SPR band is not identified at all in the presence of PAH, suggesting that the reductive capability of PEI is much greater than that of PAH; the weaker reductivity of PAH is supposed to be associated with the characteristics of primary amines contained. Having confirmed the formation of Au nanoparticles by PEI, we took more UV-vis spectra using mixtures of AuCl4- and PEI in different proportions. As can be seen in the upper panel of Figure 2, an absorption band appeared at ∼520 nm for all mixtures that is due to the SPR of Au nanoparticles. A close examination reveals that the peak maximum shifts gradually, from 511 to 537 nm, as the amount of PEI is decreased. The SPR (31) Gangopadhayay, A. K.; Chakravorty, A. J. Chem. Phys. 1961, 35, 2206. (32) Mulvaney, P. Langmuir 1996, 12, 788.

peak shift observed must be attributed to the size effect of Au nanoparticles. In general, smaller metal nanoparticles are obtained when a larger amount of reductant is used.14 In agreement with this expectation, the size of Au nanoparticles was found to be determined by the amount of PEI used initially with respect to that of AuCl4-. The lower panel of Figure 2 shows the TEM images of Au nanoparticles corresponding to the UV-vis spectra labeled a, b, and c in the upper panel of Figure 2, respectively; their average sizes are 10 ( 4, 27 ( 7, and 70 ( 19 nm, respectively. Clearly, when the concentration of PEI is low, larger particles are formed. This is in sharp contrast to the result of Sun et al.20 They found that increasing the molar ratio of PEI to gold precursor leads to increasing particle size. Their opposite result might be due to the use of concentrated HAuCl4 and PEI solutions. Supposedly when the concentration of HAuCl4 is high, Au particles formed by the reductive action of PEI may be positioned to be too close to form larger aggregates before they are stabilized by capping with PEI; relatively larger Au particles will then form as the molar ratio of PEI to gold precursor is increased. The formation of Au nanoparticles by the action of PEI can also be confirmed by the XRD pattern (Figure 3a) and the XPS spectra (Figure 3b). The several distinct XRD peaks in Figure 3a can be assigned to the reflections from the 111, 200, 220, and 311 planes of the face-centered-cubic gold particles.14 Apart from the XRD data, the XPS peaks at 83.7 and 87.4 eV in Figure 3b can be assigned, respectively, to the 4f7/2 and 4f5/2 peaks of zero-valent Au. The binding energy of the Au 4f core level is in agreement with the XPS spectra of alkylamine-capped gold nanoparticles as reported by Leff, Brandt, and Heath.9 Any AuCl4is expected to give rise to XPS peaks for Au(III) in the form of a doublet at 87 eV (4f7/2) and 92 eV (4f5/2), but the absence of any peak around 92 eV indicates that no Au(III) species exist in the PEI-capped Au nanoparticles. This does not necessarily mean that all AuCl4- ions are reduced completely by PEI. As can be seen in Figure 1, a shoulder band at 380 nm was present even after the 520 nm band attained a plateau level. The shoulder band completely disappears when the reaction mixture is heated to 50 °C, however. The absorbance at 520 nm is then intensified by up to 20%. This indicates that although a fair amount of unreacted AuCl4- is present in the reaction mixture at room

Self-Assembly of PEI-Capped Au Nanoparticles

Figure 4. Consecutive photographs of the Au nanoparticle film layer creeping up the glass wall of the vial after adding benzenethiol to the toluene phase.

temperature, their PEI complexes remain in dissolved states during the centrifugation to obtain PEI-capped Au nanoparticles. Tetraethylenepentamine (TEP) has the same monomeric unit as PEI. We have examined whether TEP can also be used as a reductant. Although less effective than PEI, TEP was able to reduce HAuCl4 to produce zero-valent gold atoms. However, the gold particles that were formed were severely aggregated because TEP could not function as a stabilizer. To obtain stabilized Au nanoparticles, a polymeric stabilizer such as poly(vinylpyrrolidone) (PVP) has to be added to the reaction medium.33,34 Again, this clearly supports the dual roles of PEI as both a reductant and stabilizer. In fact, the PEI-capped Au nanoparticles prepared in this work remained stable for at least 2 months without any obvious colloidal aggregation. Having confirmed the one-pot synthesis of PEI-capped Au nanoparticles, we sought an easy method to assemble them into well-ordered 2-D arrays for diverse practical applications. The assembly of nanoparticles into 2-D arrays is usually difficult to achieve because of their uncontrolled coagulation. One promising approach would be the creation of a 2-D arrangement of nanoparticles at the liquid-liquid interfaces. We succeeded in fabricating 2-D nanoaggregates after adding a toluene phase to the PEI-capped Au solution. A fairly homogeneous film was formed at the toluene-water interface simply by heating the mixture of toluene and PEI-capped Au solution to a boil. A more homogeneous film was formed at the toluene-water interface, this time at room temperature, as thiol molecules such as benzenethiol (BT) were added to the toluene phase. As can be seen in Figure 4, a reddish Au solution turns into a blue-colored film at the toluene-water interface as well as on the bottle wall upon adding BT to the toluene phase. The fact that the filmed layer creeps up the glass walls of the vial indicates that the driving force to form an interfacial monolayer is very strong. Furthermore, if one considers that the monolayer reaches up the vertical wall of the vial to a height of a few centimeters without breaking into pieces, then the films must be robust. The film is able to form on a separate glass substrate immersed in the mixture, or the interfacial layer can be transferred to a glass substrate as a Langmuir-Blodgett film. The Au film formed on a glass by adding BT shows a UV-vis absorption band at 695 nm, as can be seen in Figure 5a. For reference, the UV-vis spectrum of the PEI-capped Au solution is reproduced in Figure 5b. Supposedly because of the strong interparticle plasmon coupling caused by the close packing of Au nanoparticles,35 the SPR band in Figure (33) Carotenuto, G.; Nicolais, L. Composites, Part B 2004, 35, 385. (34) Selva, S. T.; Ono, Y.; Nogami, M. Mater. Lett. 1998, 37, 156.

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Figure 5. (a) UV-vis spectrum of a Au film formed at the toluene-water interface by adding benzenethiol. (b) UV-vis spectrum of a PEI-capped Au sol solution.

Figure 6. FE-SEM images of PEI-capped Au films formed at the toluene-water interface (a) by heat treatment and (b) by adding benzenethiol to the toluene phase. The inset in b shows the TEM image of the Au film formed by adding benzenethiol to the toluene phase.

5a has been red shifted by as much as 175 nm from that in aqueous solution. Figure 6a,b shows the FE-SEM images of PEI-capped Au films formed at the toluene-water interface by heat treatment and by the addition of BT, respectively. The inset in Figure 6b is the TEM image showing the Au film formed at the toluene-water interface by adding benzenethiol to the toluene phase. Both films are quite homogeneous, but Au nanoparticles appear to be more closely packed in the film assembled via the mediation of benzenethiol. The detailed mechanism of how PEIcapped Au nanoparticles coagulate at the toluene-water interface (35) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735.

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Figure 7. UV-vis spectra of PEI-capped Au films formed at the toluene-water interface by adding increasing amounts of benzenethiol to the toluene phase. Specifically, PEI-capped Au nanoparticles were initially prepared from a reaction mixture consisting of 25 mL of 1.4 mM HAuCl4 and 0.7 mL of 1% (w/w) PEI. After centrifugation, the reaction product was redispersed in 25 mL of water, and then 25 mL of toluene was poured over the aqueous solution. Subsequently, (a) 0.6, (b) 0.7, (c) 0.8, and (d) 1.0 mL of benzenethiol were added to the toluene phase.

by heat treatment is a matter of conjecture. Surface tension, electrostatic repulsion, and van der Waals attractive forces must act favorably for PEI-capped Au nanoparticles to have a thermodynamically stable state at the interface. Uncharged Au nanoparticles are supposed to favor the toluene phase, though the hydrophilic PEI prefers to remain in the water. Upon heating the mixture of toluene and PEI-capped Au sol, the amphiphilic particles might have overcome the kinetic barrier sufficiently to move to the biphasic interface and form 2-D aggregates. The kinetic barrier would also be overcome by the adsorption of thiol molecules onto the surfaces of Au nanoparticles. Because of the repulsive force among the PEI-capped Au nanoparticles when there is no thiol in the toluene layer, the number of nanoparticles at the interface is low. However, the presence of benzenethiol in the toluene layer converts the electrostatic repulsive force into a van der Waals interaction by forming organic coating layers around the Au nanoparticles when they are adsorbed to the interface. Energetically, it must be favorable for thiol-bound Au nanoparticles to move toward the toluene phase, but the particles cannot penetrate into the phase because of the presence of polyelectrolytes accompanying them. When the Au film is formed by heat treatment, its thickness hardly varies. In contrast, when the Au film is formed by the addition of BT, its thickness is subtly dependent on the amount of BT added to the toluene phase. This is clearly seen in Figure 7, in which the absorbance obviously increases upon increasing the amount of BT, whereas the position of the SPR band is gradually red shifted. The latter observation suggests that optically tuned Au films are readily fabricated by varying the amount of BT added to the toluene phase. All of the Au films fabricated in this work were very surfaceenhanced Raman scattering (SERS) active.36 Very intense SERS spectra of BT were observed by using 632.8 nm radiation as the excitation source, as shown in Figure 8a.37 The spot-to-spot variation of the peak intensities was within 6%, and the batchto-batch variation was within 9% (Figure S1 in Supporting Information). This illustrates that the Au film is microscopically smooth. When the BT-adsorbed Au film was soaked in other thiol solutions (36) Moskovits, M. ReV. Mod. Phys. 1985, 57, 783. (37) Joo, T. H.; Kim, M. S.; Kim, K. J. Raman Spectrosc. 1987, 18, 57.

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Figure 8. (a) SERS spectrum of benzenethiol adsorbed on a PEI-capped Au film corresponding to Figure 7c. The excitation wavelength was 632.8 nm. (b) SERS spectrum taken after a place-exchange reaction was allowed to occur between benzenethiol (BT) and 4-nitrobenzenethiol (4-NBT) for 3 h in a 10 mM ethanol solution of 4-NBT. Vibrational modes due to benzenethiol are labeled with stars. The excitation wavelength was 632.8 nm.

such as 4-nitrobenzenethiol (4-NBT), a place-exchange reaction took place between BT and 4-NBT, exhibiting a very intense SERS spectrum of 4-NBT as in Figure 8b; the distinct peak at 1340 cm-1 is due to the symmetric stretching vibration of the nitro group of 4-NBT.38 We have to mention that no Raman peak is observed using 514.5 nm radiation, however. Recalling the electromagnetic enhancement mechanism of SERS,39 this is not surprising at all. This implies that the intense Raman signal in Figure 8 originated from the large local electromagnetic fields caused by the resonant surface plasmons of the Au film. That is, the observed excitation wavelength dependence is correlated with the UV-vis absorption profile in Figure 7. Additionally, the SERS spectra of BT adsorbed on the Au films with different particle sizes prepared from different amounts of PEI solution (shown in Figure 2) as well as those on Au films prepared with different amounts of BT added to the toluene phase (shown in Figure 7) have also been measured. It was found that Au films with the largest average size of 70 ( 19 nm and the Au film formed by the addition of 1.0 mL of BT demonstrated the strongest SERS intensity (Figures S2 and S3 in Supporting Information). We noticed, quite separately, that the SERS peaks in Figure 8a (normalized with respect to a silicon band at 520 cm-1) are ∼20% weaker than the SERS peaks of BT assembled on µAg powders. Recalling the fact that the surface enhancement factor (EF) of µAg powders is ∼106,40 the EF value of the PEIcapped Au film is ∼1 × 105. Also, the SERS activity of the PEI-capped Au film formed by heat treatment is comparable to that formed by adding BT to the toluene phase. It may not be surprising to observe that the SERS signal of rhodamine 6G (R6G) on the thermally formed Au film is stronger than that on the electrochemically prepared Au substrate (Figure S4 in Supporting Information).

Conclusions Branched PEI has been confirmed to be an efficient agent for the preparation of stabilized Au nanoparticles at room temperature. The size of the Au nanoparticles can be controlled by varying the initial amount of PEI used. The PEI-capped Au nanoparticles thus prepared are stable for several months (38) Skadtchenko, B. O.; Aroca, A. Spectrochim. Acta, Part A 2001, 57, 1009. (39) Jiang, J.; Bosnik, K.; Maillard, M.; Brus, L. J. Phys. Chem. B 2003, 107, 9964. (40) Kim, K.; Lee, H. S.; Kim, N. H. Anal. Bioanal. Chem. 2007, 388, 81.

Self-Assembly of PEI-Capped Au Nanoparticles

without aggregation. It was also demonstrated that PEI-capped Au nanoparticles could be assembled into homogeneous 2-D aggregates at a toluene-water interface. The optical properties of the Au films could be controlled by the quantity of aromatic thiols added to the toluene phase. The PEI-capped Au films were microscopically smooth and were shown to be highly SERS-active substrates. Considering the fact that PEI has long been used as a versatile vector for gene and oligonucleotide transfer to cells, in culture as well as in vivo, PEI-capped Au nanoparticles are expected to be useful not only for biolabeling and cell recognition but also in medical diagnostics or as gene carriers. In addition, because the method of formation of robust Au films is cost-effective and is suitable for the mass production of diverse Au films irrespective of the shapes of the underlying

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substrates, the method will be useful in the development of plasmon-based analytical devices, specifically SERS-based biosensors. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (grants R01-2006-00010017-0 and R11-2007-012-02002-0). K.S.S. also was supported by the Soongsil University Research Fund. Supporting Information Available: SERS spectra of benzenethiol and rhodamine 6G on PEI-capped Au films. All spectra were obtained using a He/Ne laser at 632.8 nm as the excitation source, and the SERS intensities were normalized with respect to that of a silicon wafer at 520 cm-1. This material is available free of charge via the Internet at http://pubs.acs.org. LA800733X