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Formation of Wormlike Aggregates of Fluorocarbon-Hydrocarbon Hybrid Surfactant by Langmuir-Blodgett Transfer and Alignment of Gold Nanoparticles Yukishige Kondo,*,†,‡ Hiroshi Fukuoka,† Shuichi Nakano,† Kohei Hayashi,† Tatsuya Tsukagoshi,† Mutsuyoshi Matsumoto,‡,§ and Norio Yoshino*,†,‡ Department of Industrial Chemistry, Tokyo UniVersity of Science, 12-1 Ichigaya-funagawara, Shinjuku, Tokyo 162-0826, Japan, DiVision of Colloid and Interface Science, Tokyo UniVersity of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan, and Department of Materials Science and Technology, Tokyo UniVersity of Science, 2641 Yamazaki, Noda, Chiba 278-8501, Japan ReceiVed February 22, 2007. In Final Form: April 17, 2007 A novel anionic fluorocarbon-hydrocarbon hybrid surfactant (SS-Hyb-Na+) with a disulfide group has been synthesized from 11-bromo-1-undecanal and perfluorohexylethyl iodide via three steps. The Langmuir-Blodgett (LB) transfer of the 1:100 (mol/mol) mixed monolayer of SS-Hyb-Na+ and stearyl alcohol (C18OH) formed on an aqueous solution containing a cationic polymer, poly(diallyldimethylammonium chloride) (PDDA+Cl-) onto a hydrophobic silicon wafer yields the formation of wormlike aggregates consisting of SS-Hyb-/PDDA+ polyion complexes. It is found that the aggregates align along the withdrawal direction of the wafer substrate. When the wafer on which the wormlike aggregates exist is immersed into the dispersion of gold nanoparticles (Au NPs) prepared by the citrate reduction method, Au NPs align along the wormlike structures. Even though the surface of the wafer is placed either vertical or parallel to the monolayer compression direction during the LB transfer, the one-dimensional (1D) array of Au NPs is observed along the withdrawal direction of the wafer. This indicates that the wormlike aggregates of SS-Hyb-/ PDDA+ complexes are aligned during the LB transfer, and the aligned aggregates behave as a scaffold in the 1D array of Au NPs.
Introduction The alignment of metal nanoparticles (NPs) has been vigorously attempted because one-dimensional (1D)-arrayed NPs can be used as waveguides, quantum dot wires, and precursors of conductive nanowires1-3 and therefore can be applied to photonic nanodevices, nanoelectronics, and chemical sensors. It is wellknown that DNA molecules are good candidates to obtain a 1D array of gold colloidal particles since anionic DNA molecules behave as a scaffold for positively charged gold particles.4,5 However, the direction of the aligned particles obtained by using DNA molecules cannot be controlled. Thus, the direction control of a 1D array of particles is undoubtedly an essential technique for the fabrication of future nanoarchitectures. The research groups of Teranishi and Brust have prepared ditched substrates and succeeded in aligning NPs along the ditches.1,6 Recently, Yang and his co-worker demonstrated a method for preparing a 1D array of NPs by using the contact line of the substrate/water/air interface.7 Our research group has thus far been involved in the synthesis of a new type of amphiphile that has a fluorocarbon chain and * To whom correspondence should be addressed. ci.kagu.tus.ac.jp (Y.K.);
[email protected] (N.Y.). † Department of Industrial Chemistry. ‡ Division of Colloid and Interface Science. § Department of Materials Science and Technology.
a hydrocarbon chain in a molecule, that is, a hybrid surfactant,8-11 and reported their unique properties such as the emulsification of the ternary mixture of hydrocarbon oil/fluorocarbon oil/water,8 intramicellar phase separation between the fluorocarbon and hydrocarbon chains,12 and an unusually long lifetime of the micelles.13 In the course of the research, another interesting property of a dimeric hybrid surfactant (SS-Hyb-Na+) has been found. Here, we report the alignment of wormlike aggregates on hydrophobic substrates along the withdrawal direction of the substrate during the Langmuir-Blodgett (LB) transfer of a mixed monolayer of SS-Hyb-Na+ and stearyl alcohol (C18OH) on an aqueous subphase containing a cationic polymer [poly(diallyldimethylammonium) chloride: PDDA+Cl-]. Further, we demonstrated the alignment of gold NPs (Au NPs) using the aligned wormlike structures as a scaffold. Besides the method of Teranishi, Brust, and Yang, the results described here will provide us with an alternative method to control the direction of the metal NP alignment.
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(1) Hutchinson, T. O.; Liu, Y.-P.; Kiely, C.; Kiely, C. J.; Brust, M. AdV. Mater. 2001, 13, 1800-1803. (2) Simon, U.; Scho¨n, G.; Schmid, G. Angew. Chem., Int. Ed. 1993, 32, 250254. (3) Maier, S. A.; Kik, P. G.; Atwater, H. A.; Meltzer, S.; Harel, E.; Koel, B. E.; Requicha, A. A. G. Nat. Mater. 2003, 2, 229-232. (4) Wang, G.; Murray, R. W. Nano Lett. 2004, 4, 95-101. (5) Braun, G.; Inagaki, K.; Estabrook, R. A.; Wood, D. K.; Levy, E.; Cleland, A. N.; Strouse, G. F.; Reich, N. O. Langmuir 2005, 21, 10699-10701. (6) Teranishi, T.; Sugawara, A.; Shimizu, T.; Miyake, M. J. Am. Chem. Soc. 2002, 124, 4210-4211. (7) Huang, J.; Tao, A. R.; Connor, S.; He, R.; Yang, P. Nano Lett. 2006, 6, 524-529.
10.1021/la700522e CCC: $37.00 © 2007 American Chemical Society Published on Web 05/01/2007
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Figure 2. AFM image (2.5 µm × 2.5 µm) of the wafer on which the 1:100 (mol/mol) mixed monolayer of SS-Hyb-Na+ and C18OH was transferred at a surface pressure of 10 mN m-1.
Figure 1. Surface pressure (π)-area per molecule (A) isotherms for SS-Hyb-Na+ and the mixtures of C18OH and SS-Hyb-Na+ on the aqueous solutions containing PDDA+Cl- (PDDA+Cl-: 0.019 wt %) at 20 °C.
Results and Discussion SS-Hyb-Na+ was synthesized by the disulfidation14 of a hybrid alcohol prepared from the Grignard reaction15,16 of perfluorohexylethyl iodide with 11-bromo-1-undecanal, followed by sulfation16,17 with a SO3/pyridine complex. SS-Hyb-Na+ has a dimeric structure connecting two hybrid surfactants with a disulfide group. Details of the synthesis are described in the Supporting Information. Figure 1 shows the surface pressure (π)-area per molecule (A) isotherms for pure SS-Hyb-Na+ and the mixtures of SSHyb-Na+ and C18OH on aqueous subphases containing 0.019 wt % (1 mM monomer unit) of PDDA+Cl- at 20 °C. SS-Hyb-Na+ formed a stable Langmuir monolayer with a collapse pressure of 58 mN m-1. The onset point of the surface pressure was 1.9 nm2, and the area per molecule of SS-Hyb-Na+ was 1.5 nm2 at 10 mN m-1. Irrespective of the mixture ratio, all the mixtures of SS-Hyb-Na+/C18OH also formed stable monolayers with a collapse pressure of 58 mN m-1. The mixture of SS-Hyb-Na+/ C18OH with a molar mixture ratio of 1:100 also gave a stable monolayer; the π-A isotherm (not shown in Figure 1) of the mixture was identical to that of pure C18OH. A hydrophobic silicon wafer surface-modified with octadecyltrimethoxysilane (OTS) was immersed into an aqueous solution containing 0.019 wt % PDDA+Cl- as the surface of the (8) Yoshino, N.; Hamano, K.; Omiya, Y.; Kondo, Y.; Ito, A.; Abe, M. Langmuir 1995, 11, 466-469. (9) Kondo, Y.; Yoshino, N. Curr. Opin. Colloid Interface Sci. 2005, 10, 8893. (10) C¸ alik, P.; Ileri, N.; Erdinc¸ , B. I.; Aydogan, N.; Argun, M. Langmuir 2005, 21, 8613-8619. (11) Aydogan, N.; Aldis, N. Langmuir 2006, 22, 2088-2033. (12) Ito, A.; Kamogawa, K.; Sakai, H.; Hamano, K.; Kondo, Y.; Yoshino, N.; Abe, M. Langmuir 1997, 13, 2935-2942. (13) Kondo, Y.; Miyazawa, H.; Sakai, H.; Abe, M.; Yoshino, N. J Am. Chem. Soc. 2002, 124, 6516-6517. (14) Mureau, N.; Guittard, F.; Ge´ribaldi, S. Tetrahedron Lett. 2000, 41, 28852889. (15) Liang, X.; Bols, M. J. Chem. Soc., Perkin Trans. 1 2002, 503-508. (16) Miyazawa, H.; Kondo, Y.; Yoshino, N. J. Oleo Sci. 2005, 54, 167-178. (17) Miyazawa, H.; Igawa, K.; Kondo, Y.; Yoshino, N. J. Fluorine Chem. 2003, 124, 189-196.
wafer was placed vertical to the monolayer compression direction of the LB instrument. The 1:100 mixture of SS-Hyb-Na+/C18OH was spread on the subphase, and the resultant monolayer was transferred onto the wafer by withdrawing the wafer at a surface pressure of 10 mN m-1 (stroke speed: 4 mm min-1). The transfer ratio was 0.1, which was very low. Figure 2 is the atomic force microscopy (AFM) image of the obtained wafer. As shown in the figure, a number of wormlike aggregates having diameters in the range of 30-50 nm were observed along the withdrawal direction. To date, there have been very few reports concerning the formation of wormlike structures in LB films. Boury et al.18 previously reported wormlike aggregates in LB films prepared from the monolayers of poly(D,L-lactide-co-glycolide) (PLA25GA50). However, the LB film needs to be aged for 3 months in order to obtain wormlike aggregates, and no alignment of the aggregates has been observed. Although Ulvenlund et al.19 also found the formation of wormlike aggregates in LB films of poly-γ-methyl-L-glutamate (pMeE) deposited on mica, no ordering of the aggregates has been observed. Therefore, we believe that the formation of the wormlike aggregates along a given direction is a unique finding. The wafer on which the wormlike aggregates exist was immersed for 30 s into a dispersion of Au NPs (diameter: 15 nm) prepared by the citrate reduction method.20 Figure 3a-c shows scanning electron microscopy (SEM) images of the wafer after the immersion. It should be noted that the NPs are aligned one-dimensionally along the stroke direction employed for the LB transfer. The particle line length was ∼2 µm, and its width was in the range of 15-20 nm, corresponding to the diameter of the Au NPs. Not all of the aligned particles were in contact with each other, and gaps of less than 5 nm were observed in some locations (Figure 3c). The LB film of a pure SS-Hyb-Na+ monolayer was prepared by the same procedure and conditions that were used for the 1:100 monolayer of SS-Hyb-Na+/C18OH. The transfer ratio was almost unity. The Au NPs were adsorbed over the whole surface of the LB film (Figure 4). An X-ray photoelectron spectroscopy (XPS) measurement of the LB film showed that the ratio of N, S, Na, and Cl atoms was 0.43:1:0.00:0.00. Assuming that an SS-Hyb- molecule having two negative charges forms a complex (18) Boury, F.; Saulnier, P.; Proust, J. E.; Panaı¨otov, I.; Ivanova, T.; Postel, C.; Abillon, O. Colloids Surf., A: Physicochem. Eng. Aspects 1999, 155, 117129. (19) Gillgren, H.; Stenstam, A.; Ardhammar, M.; Norde´n, B.; Sparr, E.; Ulvenlund, S. Langmuir 2002, 18, 462-469. (20) Vakarelski, I. U.; Higashitani, K. Langmuir 2006, 22, 2931-2934.
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Figure 3. SEM images of the wafers after being immersed into the dispersions of Au NPs. The arrows show the withdrawal direction of the substrates employed when the 1:100 mixed monolayer of SS-Hyb-Na+ and C18OH was transferred onto the wafer: (a-c) the substrate surface was placed vertical to the monolayer compression direction during the LB transfer, and (d) the substrate surface was placed parallel to the compression direction. Bar lengths are (a) 400 nm, (b) 500 nm, (c) 40 nm, and (d) 400 nm.
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Figure 5. Relationship between the area per molecule (A) and the mixture ratio of SS-Hyb-Na+/C18OH at a fixed surface pressure.
electrostatically with two (PDDA+)units, where (PDDA+)unit represents the monomer unit of PDDA+ having a single positive charge, the calculated values are N:S:Na:Cl ) 0.5:1:0:0. The observed values were in good agreement with the calculated ones, although the number of N atoms is slightly small. This result reveals that one SS-Hyb- molecule complexes with two (PDDA+)units to produce insoluble polyion complexes on the aqueous subphase. Hence, the adsorption of Au NPs over the LB film shown in Figure 4 will result from the adsorption of NPs onto the disulfide groups in SS-Hyb-/PDDA+ polyion complexes.21,22 In contrast, the pure C18OH monolayer was hardly transferred onto the hydrophobic silicon wafer by withdrawing the wafer at 10 mN m-1, thereby yielding a transfer ratio of 0.1, which was the same as that of the 1:100 mixed monolayer of SS-Hyb-Na+/C18OH. Moreover, no adsorption of Au NPs onto the withdrawn wafer was observed. From the AFM observation (Figure 2), the thickness (height) of the wormlike aggregates ranged from 1.2 to 1.7 nm. Assuming all-trans conformations,
the molecular length of SS-Hyb-Na+ (from the S atom of the disulfide group to the C atom adjacent to the sulfate group) is calculated to be around 1.5 nm, which is identical to the thickness of the wormlike aggregates. When combined, these results suggest that the wormlike aggregates consist of SS-Hyb-/PDDA+ polyion complexes and that the 1D array of Au NPs as seen in Figure 3 is brought about by the adsorption of the NPs onto the disulfide groups in the aligned aggregates. The alignment of the wormlike aggregates is essential for the formation of a 1D array of the NPs. Considering the width of the wormlike aggregate, its structure may consist of a bundle of the polyion complexes or a chain of globular polyion complexes. Besides the formation of the wormlike aggregates, small grainlike aggregates with a diameter of ca. 50 nm and a height of around 2.5 nm were observed in the AFM image shown in Figure 2. It might be possible that these aggregates are C18OH phases that were barely transferred onto the wafer. Figure 5 shows the relationship between the mixture ratio of SS-Hyb-Na+/C18OH (mole fraction of SS-Hyb-Na+) and A at a given surface pressure estimated from Figure 1. At both 10 and 20 mN m-1, A increased linearly with the mole fraction of SSHyb-Na+, indicating that the SS-Hyb-/PDDA+ polyion complex and C18OH are ideally miscible or phase-separated in the mixed monolayer. SS-Hyb-Na+ has two long fluorocarbon chains that are immiscible with hydrocarbons, and therefore the ideal miscibility between the SS-Hyb-/PDDA+ complex and C18OH is not expected.23 We previously investigated the micelle structure of fluorocarbon-hydrocarbon hybrid surfactants and reported that the fluorocarbon chains are phase-separated from the hydrocarbon chains inside the micelles.12 The electrostatic association of SS-Hyb- molecules with PDDA+ molecules leads to a two-dimensional (2D) concentration, namely, the 2D micelle formation of SS-Hyb- molecules along the backbone of PDDA+ on the aqueous subphase. Therefore, even in a single SS-Hyb-/ PDDA+ polyion complex molecule, the fluorocarbon chains will be immiscible with the hydrocarbon chains as we observed for hybrid surfactant micelles. If the fluorocarbon chains twodimensionally surround the hydrocarbon chains in the polyion complex molecule, the polyion complex will be phase-separated from the C18OH Langmuir monolayer, even at a significantly
(21) Yonezawa, T.; Yasui, K.; Kimizuka, N. Langmuir 2001, 17, 271-273. (22) Daniel, M.-C.; Didier, A. Chem. ReV. 2004, 104, 293-346.
(23) Funasaki, N. In Mixed Surfactant Systems; Ogino, K., Abe, M., Eds.; Marcel Dekker: New York, 1992; p 145.
Figure 4. SEM image of the LB film prepared from pure SSHyb-Na+ followed by immersion into the dispersion of Au NPs with a diameter of 15 nm. Bar length is 400 nm.
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low concentration of SS-Hyb-, that is, a mixture ratio of 1:100. The dispersion stability in the C18OH Langmuir monolayer of the polyion complex having a structure surrounded by the fluorocarbon chains will be poor because of the high line tension24 between the fluorocarbon chains (C6F13) in SS-Hyb- molecules and the hydrocarbon chains (C18H37) in C18OH molecules or water molecules. It is considered that this dispersion instability led to the aggregation of the polyion complexes, that is, the formation of a wormlike structure having diameters in the range of 30-50 nm as can be seen in Figure 2. In other words, the hybridity of SS-Hyb-Na+ will play an important role in the wormlike aggregate formation. The 1D array of Au NPs along the withdrawal direction of the substrate is observed at a surface pressure of 1 mN m-1 as well as 10 mN m-1, which implies that SS-Hyb- molecules form wormlike aggregates even at a diluted surface concentration. This is because PDDA+ molecules electrostatically concentrate SS-Hyb- molecules on the backbones of PDDA+ molecules, and the resultant polyion complexes aggregate on the aqueous subphases. In order to study whether the alignment of the wormlike aggregates along the withdrawal direction of the substrate occurred during the LB transfer, we immersed the hydrophobic wafer into the aqueous subphase such that the surface of the wafer was parallel to the monolayer compression direction, and the 1:100 mixed monolayer of SS-Hyb-Na+/C18OH was then transferred by withdrawing the wafer at a surface pressure of 10 mN m-1. Figure 3d shows the SEM image of the wafer after being immersed into the Au NP dispersion. As observed in the figure, the alignment of the NPs along the withdrawal direction was observed again. This indicates that the alignment of the wormlike aggregates on the wafer is induced during the LB transfer. On the other hand, when the 1:10 mixed monolayer of SS-Hyb-Na+/C18OH was transferred onto the hydrophobic wafer at a surface pressure of 10 mN m-1, the 2D network of Au NPs was obtained independent of the placement of the wafer toward the monolayer compression direction. It seems that the number of wormlike aggregates increases with increasing SS-Hyb- concentration, and, consequently, the wormlike aggregates become networked. It is important to understand that most of the wormlike aggregates are discrete, not networked, on the aqueous subphase in order to obtain a 1D array of NPs along the withdrawal direction. In addition, C18OH might play a critical role in the alignment of wormlike aggregates. During the LB transfer, it is possible that C18OH molecules slip down the substrate against the withdrawal direction because C18OH is hardly transferred. The slippingdown of C18OH molecules might produce a downward flow on the substrate. When these discrete wormlike aggregates are transferred onto the hydrophobic substrate, the downward flow of C18OH molecules might align the wormlike aggregates along the withdrawal direction. (24) Iimura, K.; Shiraku, T.; Kato, T. Langmuir 2002, 18, 10183-10190.
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In this study, we showed the alignment of wormlike aggregates along a given direction by LB transfer. To the best of our knowledge, this will be the first report with regard to the direction control of the alignment of the wormlike aggregates. Moreover, we demonstrated a 1D array of Au NPs by using the wormlike structures as a scaffold. Further studies to shed light on the mechanism of the alignment and formation of the wormlike aggregates are in progress. Experimental Measurements. A Filgen LB-400 (Aichi, Japan) instrument (Kuhn type) was used to measure the π-A isotherms. SS-Hyb-Na+ was dissolved into a 0.9:0.1 (v/v) mixed solution of chloroform (HPLC grade, Kanto Chemical) and methanol (HPLC grade, Kanto Chemical), and the resultant solution was applied onto Milli-Q pure water containing 0.019 wt % PDDA+Cl- (Mw 200 000-350 000: Aldrich Chemical) at 20 °C. After 30 min, the π-A isotherms were measured at a compression speed of 2.7 nm2 min-1molecule-1. For C18OH (TCI) and the mixtures of SS-Hyb-Na+ and C18OH, their π-A isotherms were measured using the same procedure and conditions as those of SS-Hyb-Na+ except that the developing solvent of C18OH was pure chloroform. SEM and XPS measurements were performed with a Hitachi S-5000 microscope and a JEOL 7100 instrument, respectively. The ratio of the N, S, Na, and Cl atoms in the LB film was obtained by integrating the bands of N1s, S2p, Na2s, and Cl2p, respectively, in the XPS spectrum. Fabrication of LB Films. OTS (Aldrich Chemical) was distilled before use under a N2 atmosphere. The fresh OTS was dissolved into chloroform, and the resultant solution was applied onto pure water to obtain an OTS monolayer at 20 °C. After 30 min, the monolayer was compressed up to a surface pressure of 10 mN m-1 and then transferred by withdrawing the silicon wafer (stroke speed of 4 mm min-1) washed according to the method described in a previous paper.25 After the LB deposition, the wafer was heated at 120 °C for 10 min. The static contact angle of water on the OTSmodified silicon wafer was 98.1 ( 1.7°, indicating that the wafer surface is hydrophobic. The OTS-modified wafer was immersed into an aqueous subphase containing 0.019 wt % PDDA+Cl- such that the surface of the wafer was vertical to the monolayer compression direction. The monolayer of SS-Hyb-Na+, C18OH, or their mixtures was compressed at a compression speed of 2.7 nm2 min-1molecule-1 and then transferred by withdrawing the wafer at a speed of 4 mm min-1.
Acknowledgment. The authors are grateful to Mr. T. Hasumi (Tokyo University of Science) and Dr. H. Shibata (Tokyo University of Science) for the AFM observation. Financial support was provided by the Ministry of Education, Culture, Sports Science and Technology (MEXT) in Japan (Grant-in-Aid for Young Scientists (B) 17750189). Supporting Information Available: Details of the synthesis of SS-Hyb-Na+ and the preparation of Au NPs. This information is available free of charge via the Internet at http://pubs.acs.org. LA700522E (25) Tsukagoshi, T.; Kondo, Y.; Yoshino, N. Colloids Surf., B: Biointerfaces 2007, 54, 101-107.
