Immobilization of the Nanoparticle Monolayer onto Self-Assembled

Kathryn E. Knowles , Mario Tagliazucchi , Michał Malicki , Nathaniel K. Swenson , and Emily A. ... Zhangquan Peng, Thomas Walther, and Karl Kleinerma...
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Langmuir 2004, 20, 5-10

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Immobilization of the Nanoparticle Monolayer onto Self-Assembled Monolayers by Combined Sterically Enhanced Hydrophobic and Electrophoretic Forces Zhangquan Peng, Xiaohu Qu, and Shaojun Dong* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, People’s Republic of China Received June 20, 2003. In Final Form: October 31, 2003 The immobilization of surface-derivatized gold nanoparticles onto methyl-terminated self-assembled monolayers (SAMs) on gold surface was achieved by the cooperation of hydrophobic and electrophoretic forces. Electrochemical and scanning probe microscopy techniques were utilized to explore the influence of the SAM’s structure and properties of the nanoparticle/SAM/gold system. SAMs prepared from 1-decanethiol (DT) and 2-mercapto-3-n-octylthiophene (MOT) were used as hydrophobic substrates. The DT SAM is a closely packed and organized monolayer, which can effectively block the underlying gold and inhibit a variety of solution species including organic and inorganic molecules from penetrating, whereas the MOT monolayer is poorly packed or disorganized (because of a large difference in dimension between the thiophene head and the alkylchain tail) and permeable to many organic probes in aqueous solution but not to inorganic probes. Thus, the MOT monolayer provides a more energetically favorable hydrophobic surface for the penetration and adsorption of organic species than the DT monolayer. This hypothesis is supported by experiments in which the density of hydrophobically immobilized nanoparticles on the MOT SAM is much larger than that on the DT SAM. The results also suggest new approaches for modification of macroscopic surfaces with nanoscopic particles.

Introduction Organized, surface-confined monolayers are commonly employed to assign desired chemical or physical properties to surfaces.1 A variety of interfacial properties including wetting,2 adhesion,3 adsorption,4 catalysis,5 and electron transfer6 can be tuned by self-assembled monolayers (SAMs) on the solid surface. With respect to electrochemistry, self-assembly has become one of the major approaches for designing selective electrodes and sensors for electroanalytical applications.7 The requirements of SAMs for such applications include chemical and electrochemical inertness and, at the same time, a strong attachment onto inert conducting surfaces, for example, gold and platinum, while maintaining the ability to functionalize the electrode/SAM/electrolyte interface. * Corresponding author. Fax: [email protected].

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(1) (a) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Boston, 1991. (b) Kuhn, H.; Ulman, A. In Thin Films; Ulman A., Ed.; Academic Press: New York, 1995; Vol. 20. (c) Characterization of Organic Thin Films; Ulman, A., Ed.; Boston, 1995. (d) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) (a) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 5897. (b) Bain, C. D.; Whitesides, G. M. Adv. Mater. 1989, 4, 522. (c) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (d) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Am. Chem. Soc. 1990, 112, 570. (e) SondagHuethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1992, 8, 2560. (f) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (g) Evans, S. D.; Sharma, R.; Ulman, A. Langmuir 1991, 7, 156. (h) Walczak, M. M.; Chung, C.; Stole, S. M.; Widrig, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2370. (3) (a) Thomas, R. C.; Houston, J. E.; Crooks, R. M.; Kim, T.; Michalske, T. A. J. Am. Chem. Soc. 1995, 117, 3830. (b) Frisbie, C. D.; Rozsnayai, L. F.; Noy, A.; Wrighton, M. S.; Lieber, C. M. Science 1994, 265, 2071. (c) Rubinstein, I.; Rishpon, J.; Sabatini, E.; Redondo, A.; Gottesfeld, S. J. Am. Chem. Soc. 1990, 112, 6135. (d) Sabatini, E.; Redondo, A.; Rishpon, J.; Rudge, A.; Rubinstein, I.; Gottesfeld, S. J. Chem. Soc., Faraday Trans. 1993, 89, 287. (4) (a) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 1164. (b) Delamarche, E.; Sundarababu, G.; Biebuyck, H.; Michel, B.; Gerber, C.; Sigrist, H.; Wolf, H.; Ringsdorf, H.; Xanthopoulos, N.; Mathieu, H. J. Langmuir 1996, 12, 1997. (c) Dimilla, P. A.; Folkers, P.; Biebuyck, H. A.; Ha¨rter, R.; Lo´pez, G. P.; Whitesides, G. M. J. Am. Chem. Soc. 1994, 116, 2225.

These have made monolayers of alkanethiols on gold the most extensively employed SAM systems because of their stability and high degree of organization. A relatively complete picture of the self-assembly process and the resulting structure of the alkanethiol SAMs have emerged over the last two decades.1d Although a great many achievements have been made on the organized SAM systems, less attention is paid to the preparation and application of the disorganized SAMs. More recently, a new trend of SAM investigation was the disorganized or selective monolayers on the solid surface.8 For example, Markovic and Mandler9 have reported a novel and generic concept for assembling selective interfaces, in which a hydrophobic disorganized brush-type octadecylsilane monolayer was formed on an indium-tin oxide surface showing promising application in electroanalytical chemistry. Our group has prepared an inherently disorganized 2-mercapto-3-n-octylthiophene (MOT) SAM by introducing an extraordinary functional group (thiophene) into the alkanethiol, and during the (5) (a) Collman, J. P.; Ennis, M. S.; Offord, D. A.; Cheng, L. L.; Griffin, J. H. Inorg. Chem. 1996, 35, 1751. (b) Sato, Y.; Itoigawa, H.; Uosaki, K. Bull. Chem. Soc. Jpn. 1993, 66, 1032. (c) Hong, H.-G. Electrochim. Acta, 1997, 42, 2319. (d) Doron, A.; Portnoy, M.; Lion-Dagan, M.; Katz, E.; Willner, I. J. Am. Chem. Soc. 1996, 118, 8937. (e) Hutchison, J. E.; Postlethwaite, T. A.; Chen, C.; Hathcock, K. W.; Ingram, R. S.; Ou, W.; Linton, R. W.; Murray, R. W.; Tyvoll, D. A.; Chang, L. L.; Collman, J. P. Langmuir 1997, 13, 2143. (f) Zak, J.; Yuan, H.; Ho, M.; Woo, L. K.; Porter, M. D. Langmuir 1993, 9, 2772. (g) Katz, E.; Schmidt, H.-L. J. Electroanal. Chem. 1994, 368, 87. (h) Lorenzo, E.; Sanchez, L.; Pariente, F.; Tirado, J.; Abruna, H. D. Anal. Chim. Acta 1995, 309, 79. (i) Kunitake, M.; Akiyoshi, K.; Kawatana, K.; Nakashima, N.; Manabe, O. J. Electroanal. Chem. 1990, 292, 277. (j) Maskus, M.; Abruna, H. D. Langmuir 1996, 12, 4455. (k) Upadhyay, D. N.; Yegnaraman, V.; Rao, G. P. Langmuir 1996, 12, 4249. (l) Pierrat, O.; Bourdillon, C.; Moiroux, J.; Laval, J.-M. Langmuir 1998, 14, 1692. (6) (a) Janek, R. P.; Fawcett, W. R.; Ulman, A. Langmuir 1998, 14, 3011. (b) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1992, 96, 2657. (c) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1993, 97, 6233. (d) Bard, A. J.; Abruna, H. D.; Chidsey, C. E. D.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, M.; Melroy, O.; Murray, R. W.; Porter, M. D.; Soriaga, M. P.; White, H. S. J. Phys. Chem. 1993, 97, 7147. (e) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekkler: New York, 1996; Vol. 19, Chapter 2. (f) Li, T. T.-T.; Liu, H. Y.; Weaver, M. J. J. Am. Chem. Soc. 1984, 106, 1233.

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monolayer formation process (such as self-assembly), the molecular defects within the monolayer form simultaneously.10a,b The obtained MOT monolayer is loosely packed and permeable to hydrophobic molecules but not to highly hydrated species, while its alkanethiol counterpart [CH3(CH2)9SH, ab. 1-decanethiol, DT] with a similar molecular length demonstrated its excellent blocking properties and is not permeable to both hydrophilic and hydrophobic molecules. It presents us an advantage that the loosely packed MOT monolayer can provide a much larger hydrophobic driving force (sterically enhanced hydrophobic effect) for the penetration and adsorption of organic species. One example employing the hydrophobicity of the MOT monolayer has been demonstrated by us, where a phospholipid monolayer deposited on the MOT SAM shows an enhanced rate of bilayer formation and a better stability of the bilayer structure. Specifically, the bilayer takes an interdigitating conformation and is robust enough to endure prolonged electrochemical experiments.10c In this paper, we report that the sterically enhanced hydrophobicity can be employed to effectively organize gold nanoparticles onto solid supports. Although the organization of nanoparticles on solid supports has been achieved by different methods, including nanoparticle crystallization,11 the Langmuir-Blodgett technique,12 chemical cross-linkage,13 the self-assembly technique,14 electrophoretic15 and electrostatic16 interactions, DNA hybridization,17 and so forth, the hydrophobic interaction is less noted and employed to organize nanoparticles.18 Because surface hydrophobicity plays an important role in many biological systems, nanoparticle organization by such types of interaction can find significance in designing complicated multilayer structures with promising applications in biomolecular electronics. Here, the gold (7) (a) Flink, S.; van Veggel, F.; Reinhoudt, D. N. J. Phys. Chem. B 1999, 103, 6515. (b) Shen, H.; Mark, J. E.; Seliskar, C. J.; Mark, H. B.; Heineman, W. R. J. Solid State Electrochem. 1997, 1, 241. (c) Turyan, I.; Mandler, D. Anal. Chem. 1997, 69, 894. (d) Wang, J.; Wu, H.; Angnes, L. Anal. Chem. 1993, 65, 1893. (e) Takehara, K.; Takemura, H.; Ide, Y. Electrochim. Acta 1994, 39, 817. (f) Doblhofer, K.; Figura, J.; Fuhrhop, J.-H. Langmuir 1992, 8, 1811. (g) Doron, A.; Katz, E.; Tao, G.; Willner, I. Langmuir 1997, 13, 1783. (h) Malem, F.; Mandler, D. Anal. Chem. 1993, 65, 37. (i) Turyan, I.; Mandler, D. Isr. J. Chem. 1997, 37, 225. (j) Li, J.; Ding, L.; Wang, E.; Dong, S. J. Electroanal. Chem. 1996, 414, 17. (8) Mirsky, V. M. Trends Anal. Chem. 2002, 21, Nos. 6 and 7. (9) (a) Markovic, I.; Mandler, D. J. Electroanal. Chem. 2000, 484, 194. (b) Markovic, I.; Mandler, D. J. Electroanal. Chem. 2001, 500, 453. (10) (a) Peng, Z.; Dong, S. Langmuir 2001, 17, 4904. (b) Peng, Z.; Wang, J.; Wang, E.; Dong, S. J. Electrochem. Soc. 2003, 150, E197. (c) Peng, Z.; Tang, J.; Han, X.; Wang, E.; Dong, S. Langmuir 2002, 18, 4834. (11) (a) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (b) Schmid, G.; Pugin, R.; Sawitowski, T.; Simon, U.; Marler, B. Chem. Commun. 1999, 1303. (c) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 37, 397. (12) (a) Remacle, F.; Collier, C. P.; Markovich, G.; Heath, J. R.; Banin, U.; Levine, R. D. J. Phys. Chem. B 1998, 102, 7727. (b) Chen, S. Langmuir 2001, 17, 6664. (13) (a) Chen, S. Adv. Mater. 2000, 12, 186. (b) Novak, J. P.; Feldheim, D. L. J. Am. Chem. Soc. 2000, 122, 186. (c) Leibowitz, F. L.; Zhang, W.; Maye, M. M.; Zhong, C. Anal. Chem. 1999, 71, 5076. (d) Chan, E. W. L.; Yu, L. Langmuir 2002, 18, 311. (14) (a) Kiely, C. J.; Fink, J.; Brust, M.; Bethell, D.; Schiffrin, D. J. Nature 1998, 396, 444. (b) Connolly, S.; Fullam, S.; Korgel, B.; Fitzmaurice, D. J. Am. Chem. Soc. 1998, 120, 2969. (c) Aherne, D.; Rao, S. N. J. Phys. Chem. B 1999, 1037, 1821. (d) Zamborini, F. P.; Hicks, J. F.; Murray, R. W. J. Am. Chem. Soc. 2000, 122, 4514. (15) (a) Giersig, M.; Mulvaney, P. J. Phys. Chem. 1993, 97, 6334. (b) Giersig, M.; Mulvaney, P. Langmuir 1993, 9, 3408. (16) (a) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148. (b) Doron, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313. (17) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (18) Aslam, M.; Mulla, I. S.; Vijayamohanan, K. Langmuir 2001, 17, 7487.

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nanoparticles are surface-derivatized with a hydrophobic monolayer, and two kinds of methyl-terminated SAMs from DT and MOT are employed as the hydrophobic substrates. When the MOT monolayer-coated substrate, with a droplet of nanoparticle-containing organic solution (e.g., chloroform) on its surface, is immersed into an aqueous electrolyte solution and followed by potential control, excess nanoparticles and organic solvent on the substrate surface are allowed to electromigrate, or diffuse away, leaving a monolayer of particles to self-assemble on the MOT SAM surface, while on the DT monolayer very few nanoparticles have been immobilized. This difference in the ability of nanoparticles to immobilize is attributed to the difference of the alkylchain packing of the MOT and DT SAMs. Our results also suggest new approaches for modification of macroscopic surfaces with nanoscopic particles. Experimental Section Chemicals. MOT was synthesized as previously described.10a Ferrocene-terminated thiols (FcCO2C8SH) were synthesized as described in the literature.19 DT and NaClO4 were purchased from Aldrich and used as received. Ethanol was from Beijing Chemicals and of analytical reagent grade. All solutions were prepared using ultrapure water purified with a Millipore-Q+ system (18.2 MΩ). Synthesis of Surface-Derivatized Nanoparticles. The surface-derivatized gold nanoparticles were synthesized by a two-step procedure: (i) According to Natan et al.’s method with a slight modification,20 0.8 mL of 1.0% sodium citrate and 0.1 mL of 0.075% sodium borohydride were blended into 100-mL aqueous solutions. The mixture was then rapidly added into 100-mL solutions of 2.0% HAuCl4 with vigorous stirring. After further stirring for 2 h, the color of the obtained solution turned from weak yellow to rose, indicating the formation of the gold colloids. (ii) The gold nanoparticles in aqueous solutions were then surfacederivatized with thiols by a phase-transfer procedure. In brief, 10 mL or more of the prepared gold colloidal solutions just described were mixed with a 10-mL chloroform solution containing 1 mM mixed thiols of MOT and FcCO2C8SH with ∼5:1 mole ratio. Upon mixing, the surface-derivatization and the transfer of modified nanoparticles into the chloroform phase took place, consequently. Then, the chloroform phase was separated and washed by large amounts of water at least 10 times to remove the citrate in the chloroform phase. Finally, the organic phase was diluted by chloroform to 20 mL and kept at 4 °C for later use. Preparation and Pretreatment of the Gold Bead Electrode. The gold bead electrode was prepared by melting the end of a gold wire (diameter 1.2 mm, purity 99.99%) in a hydrogenoxygen flame.21 Before melting, the gold wire was cleaned by a three-step procedure: (1) etching in aqua regia for about 3 s, (2) heating in a 30% HNO3 solution at 80 °C for about 30 min, and (3) sonicating in ultrapure water for 15 min. (WARNING! Hydrogen-oxygen mixtures are powerful explosive gases and should be used in a well-ventilated closet or room. Both aqua regia and 30% HNO3 are very corrosive and must be used with extreme caution; they react violently with organic material, and workers should wear anticorrosive gloves when dealing with them.) During the melting process, the developing gold bead was also submitted to the cleaning procedure several times. The gold bead obtained after melting was about 2.5 mm in diameter. On the well-prepared gold bead surface, several Au(111) facets would appear and could be observed by unaided eyes. One of the (111) facets was directly used as a scanning probe microscopy scanning plane without further polishing in our lab.22 Before the electrochemical measurements, the gold bead was annealed in a (19) Chidesy, C. D. E.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (20) (a) Grabar, K. C.; Brown, K. R.; Keating, C. K.; Stranick, S. J.; Tang, S.; Natan, M. J. Anal. Chem. 1997, 69, 471. (b) Lyon, L. A.; David, J.; Pena, D. J.; Natan, M. J. J. Phys. Chem. B 1999, 103, 5826. (21) (a) Clavilier, J.; Armand, D.; Wu, B. L. J. Electroanal. Chem. 1982, 135, 159. (b) Sawaguchi, T.; Yamada, T.; Okinaka, Y.; Itaya, K. J. Phys. Chem. 1995, 99, 14149. (22) Tang, Z.; Liu, S.; Wang, E.; Dong, S. Langmuir 2000, 16, 4946.

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hydrogen-oxygen flame near 600 °C for 30 s and then quickly immersed into ultrapure water saturated with hydrogen. Finally, the tail rod connecting the gold bead was carefully sealed with Parafilm (Chicago, IL, U.S.A.). Upon preparation, the electrode was rinsed with deionized water and ethanol, immediately immersed into ethanol solutions containing about 1 mM MOT or DT overnight, and then rinsed carefully with ethanol and water to remove the nonchemisorbed species. Instruments. Transmission electron microscopy (TEM) measurements were carried out on a JEOL model 2010 instrument operated at an accelerating voltage of 200 kV after dispersing gold nanoparticles on a copper grid precoated with a thin carbon film. Low-current scanning tunneling microscopy (LC-STM) images were obtained in air with a Nanoscope IIIa microscope (Digital Instruments, Inc.). Tapping-mode atomic force microscopy (TP-AFM) was conducted with a SPI3800N microscope (Seiko Instruments, Inc.). Electrochemical measurements were carried out with an Autolab PG30 electrochemical analyzer system (Eco Chemie B. V., The Netherlands). A conventional three-electrode system was used throughout. Potentials were recorded and reported versus a saturated calomel electrode as the reference. The working electrode was a SAM-coated gold bead electrode with a droplet of particle-containing chloroform on its surface, which was formed by dipping the electrode into the particle-containing chloroform solution, then transferred into an electrochemical cell containing 0.1 M NaClO4, immediately. The counter electrode was a large platinum foil, and the measurements were carried out at 20 ( 1 °C.

Results and Discussion MOT and DT SAMs on Gold. Previously, we have reported a systematic investigation of the MOT SAMs on the gold electrode by electrochemical techniques.10a,b Experimental results showed that many organic molecules (such as dopamine, 1,2-naphthoquinone, and 1,2-naphthoquninone-4-sulfonic sodium) can more easily penetrate into the SAMs than aqueous ionic probes [such as Fe(CN)63-/4- and Ru(NH3)63+/4+] especially when they are in their noncharged state (e.g., dopamine in weak base solutions). Pinhole mapping by alternating current impedance measurements in the presence of an aqueous redox couple indicated that the MOT monolayer has a very low ratio of measurable pinholes. Measurements indicate that the MOT monolayer is not a totally compact one, although its apparent coverage is as large as that of long-chain alkanethiols. A possible explanation for the peculiar interfacial properties is that there exists a large difference in dimension between the head (thiophene thiolate) and the tail (alkylchain) groups of the adsorbates, which results in a loose packing of the alkylchains of the monolayer. SAMs from DT, which have been studied extensively, have quite different properties.23 Briefly, the DT SAM is an organized and insulating monolayer, which is closely packed and can block a variety of solution species, including organic and inorganic molecules, from penetration. Figure 1 shows cyclic votammograms of MOT (solid curve) and DT (dotted curve) monolayer-covered gold electrodes in a solution containing 1 mM catechol at pH 7. On the MOT monolayer-covered electrode, masstransfer-limited currents are observed, indicating that the catechol molecules can penetrate into the MOT monolayer. While on the DT monolayer, there is only background current observed in the scan range -0.2 to +0.7 V, showing excellent blocking properties to solution species. For comparison, the cyclic voltammogram on the bare gold electrode in the same electrolyte solutions is also presented in the Figure 1 inset. These results suggest that the packing of the MOT on gold is loose and that there exists a hydrophobic micro-environment within the MOT SAM that is large enough to accommodate the organic species from the electrolyte solution.

Figure 1. Cyclic voltammograms for 1 mM catechol/0.1 M KCl on bare (inset), MOT (solid curve), and DT (dotted curve) monolayer-covered gold electrodes; the scan rate is 0.1 V/s.

To study the surface structures of SAMs of MOT and DT, LC-STM imaging was carried out. Figure 2 shows STM images of MOT (A, 47 × 47 nm) and DT (B, 34 × 34 nm) on Au(111) facets on the gold bead electrode. The magnification of the STM image of the DT SAM is high enough to clarify the sizes of the domains and the individual thiols within the domain. All the images show the typical features of a gold surface covered with an alkanethiol SAM such as pits and domain boundaries.24 The depth of the pits was about 0.24 nm, which is close to the monatomic step height of the Au(111) surface, and, therefore, these pits were assigned to the vacancy islands of the gold surface,25 while in the case of the MOT SAM we did not get the image with molecular resolution, although we performed the LC-STM experiments on over 20 different samples of MOT SAMs on Au(111). We think that it is the flexibility of the alkylchain of the MOT SAM that prevents us from getting the STM images with molecular resolution. In general, the greater the mismatch between the van der Waals radii of the tail and those of the headgroups and between these quantities and the substrate lattice parameters, the greater the tendency for the SAM to deviate from a well-arranged structure to exhibit structural disorder and defects.26 In the MOT SAMs in this work, the functional groups of the adsorbates may affect the structure of the monolayer in important ways. For instance, if the functional groups with a relatively large dimension set the spacing of the chains, we would expect that it is not necessary for the chains to tilt to present a close and commonly oriented state as a long-chain alkanethiol does, and the interchain van der Waals interactions may be weak. If the spacing is too large, the chains may be poorly packed. These loosely packed chains may be in a liquidlike state at room temperature and can be convolved with the STM tip, which makes the acquisition of the high-resolution imaging of the MOT SAM very difficult at room temperature. Nanoparticle Assembly. The surface-derivatized gold nanoparticles prepared by the previously described experimental procedure were characterized by TEM. Figure 3 shows the electron micrographs obtained from gold (23) (a) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660. (b) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (c) Creager, S. E.; Steiger, C. Langmuir 1995, 11, 1852. (24) Poirier, G. E. Chem. Rev. 1997, 97, 1117 and references therein. (25) (a) Schonenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611. (b) McDermott, C. A.; McDermott, M. T.; Green, J.-B.; Porter, M. D. J. Phys. Chem. 1995, 99, 13257. (26) (a) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (b) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (c) Zehner, R. W.; Sita, L. R. Langmuir 1997, 13, 2973.

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Figure 3. Transmission electron micrographs and histogram for the particle size distribution obtained from surface-derivatized gold nanoparticles dispersed on an electron microscope grid from the dilute particle solution.

Figure 2. (A) Topographical LC-STM image (47 × 47 nm) of the MOT SAM on the Au(111) surface: setpoint is 4 pA; bias voltage is 1.1 V. (B) Topographical LC-STM image (34 × 34 nm) of the DT SAM on the Au(111) surface: setpoint is 4 pA; bias voltage is 1.6 V.

nanoparticles dispersed on an electron microscope grid from the particle solution. A fairly uniform distribution of well-dispersed particles can be seen in the Figure 3 inset, and the size distribution of the particles is in the range of 22 ( 5 nm. Because the surface-derivatized gold nanoparticles synthesized by us only disperse well in organic solution because of their surface hydrophobicity, the strategy for exploring hydrophobic interaction to make possible the assembly of the nanoparticles onto a hydrophobic SAM must be developed. The immobilization of gold nanoparticles consists of two consecutive steps: (i) The MOT or DT monolayer-coated gold bead electrode was dipped into the particle-containing chloroform solution for about 1 min. The hydrophobic nature of both the substrate surface and the chloroform solvent makes sure of the effective

adherence of a chloroform droplet on the electrode surface when the electrode was taken out from the organic solution. (ii) The electrode with the droplet on its surface was immersed into an aqueous 0.1 M NaClO4 electrolyte solution, followed by prolonged potential cycling between -0.2 and +0.8 V. In this potential range, the redox moieties on the particle surface can be oxidized leading to a highly charged particle surface, and excess nanoparticles on the electrode surface are allowed to electromigrate or diffuse away leaving a monolayer of nanoparticles to self-assemble on the hydrophobic SAM surface (Scheme 1). It is necessary to point out that before immersing the electrode into the aqueous 0.1 M NaClO4 electrolyte there should be a very thin film of chloroform coating the substrate surface, which prevents the nanoparticles from congregating into densely packed agglomerates. From our observation, the rate of the volatilization of the chloroform droplet on the hydrophobic surface is very fast (within 15 s). If the electrode with chloroform on its surface was thoroughly dried in air, a very weak electrochemical response of the electroactive nanoparticles can be observed when performing cyclic voltammetry in the NaClO4 electrolyte. That is, only part of the ferrocene moieties can be addressed in the potential scan, and the electroactive nanoparticles conglomerate into a thick multilayer on the electrode surface. The principle of the immobilization comes from the fundamentals of intermolecular and surface forces, which extensively exist in biological and colloidal sciences.27 At potentials higher than 0.6 V, the ferrocene moieties on the particle surface are electrochemically oxidized into ferriceniums leading to the formation of a highly positively charged particle surface. The Coulomb repulsion between the particles will dramatically increase because of the high density of the surface charge and exceed their van der Waals attraction, resulting in an increased mean distance between particles. From a thermodynamics viewpoint, the change in free energy on transferring an (27) (a) Isrealachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1992. (b) Science 2002, 295, 2935 (Special Issues on Supermolecular Chemistry and Self-Assembly).

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Scheme 1. Hydrophobic Immobilization of a Nanoparticle Surface Modified with MOT and FcCO2C8SH onto a MOT SAM on Golda

a The alkylchains of the modified nanoparticle are interdigitating with those of loosely the packed SAM monolayer. This pictures does not intend to depict a lack of chain orientation of the SAM or the stoichiometry of the modified nanoparticle.

ion from a medium of low dielectric constant 1 to one of high dielectric constant 2 is negative, that is, energetically favorable, and equal to ∆G ) -(69z2/a)(1/1 - 1/2) kJ mol-1, where a is given in nanometers.28 Thus, if 1 mol of monovalent cations and anions are transferred from chloroform ( ) 5) into water ( ) 78), the gain in the molar free energy will be negative; assuming a ) 0.14 nm for both cations and anions, the ∆G ) -2(69/0.14)(1/5 1/78) ≈ -200 kJ mol-1. Similarly, the positively charged ferriceniums on the particle surface are also favorable surrounded by a medium with large dielectric constant just because of the decrease in free energy, which provides a thermodynamic basis for the highly charged particles to leave from a medium with a smaller dielectric constant to a more polar one, such as water. On the other hand, if the SAM-coated gold electrode can provide enough surface attraction force for the nanoparticle adsorption, some particles will stick to the electrode surface even if they are in a charged state. In our system, there are three factors favorable for the immobilization of a nanoparticle onto the SAM-coated substrate: (i) The van der Waals attraction of the particleelectrode surface pair is greater than that of the particleparticle pair. Qualitatively, the nonretarded van der Waals interaction energy of the particle-particle pair is W ) -(A/6D)[R1R2/(R1 + R2)] and for the particle-surface pair is W ) -AR/6D, where A is the Hamaker constant, R is the radius of the particles, and D is the distance between the particles or between the particle and the surface.27a In our system, it is reasonable to assume that the Hamaker constants are the same for both the particle-particle pair and the particle-surface pair. Obviously, the (dW/dD)p-p (28) Bockris, J. O’M.; Reddy, A. K. N. Modern Electrochemistry 1: Ionics, 2nd ed.; Plenum: New York, 1998; Chapter 2.

) AR/12D2 is less than the (dW/dD)p-s ) AR/6D2. (ii) The Coulomb repulsion between the substrate and the charged particles is less than that between the particles because both the delocalization of the net charge on the gold electrode surface and the screen of the charge by the MOT monolayer will decrease the repulsion between the particle and the surface. (iii) The loosely packed alkylchains of the hydrophobic MOT monolayer provides a sterically enhanced hydrophobic effect for the interdigitating between the alkylchains of the MOT monolayer and the particles, which is advantageous for the anchoring of the particles. By using the enhanced hydrophobic effect, we have fabricated the hybrid bilayer membrane by adsorption of a phospholipid layer on the MOT monolayer.10c When the alkylchains are anchored on the nanoparticle surface by using the same thiols of MOT as those on the bulk gold surface, it is envisioned that the alkylchains on the nanoparticle surface are also loosely packed and that if these surface-modified particles are in contact with the MOT monolayer, the alkylchains on the particle surface will interdigitate with those of the underlying SAM under certain conditions. Again, the sterically enhanced hydrophobic effect proved to be a determining factor in effectively immobilizing the nanoparticles, where the number of hydrophobically immobilized nanoparticles on MOT is much more than that on the DT monolayer (vide infra). Immobilization of the particles on the MOT and DT SAM surfaces by the combined forces was monitored by cyclic voltammetry. The cyclic voltammograms (Figure 4A) recorded on the MOT-coated electrode at potentials between -0.2 and +0.8 V at 100 mV/s in 0.1 M NaClO4 show a successive decrease in both the anodic and the cathodic peak currents, indicating the redox activity of the surface-derivatized nanoparticles and their depletion

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Langmuir, Vol. 20, No. 1, 2004

Letters

Figure 4. Cyclic voltammogram taken at 0.1 V/s in 0.1 M NaClO4 using MOT (A) and DT (B) SAM-coated gold bead electrodes with a droplet of particle-containing chloroform on their surfaces as the working electrode. The inset in part A shows the zoom-in of the last potential cycle between 0.3 and 0.7 V.

process at the electrode/solution interface. But even after the prolonged potential cycling, there are some particles confined on the MOT SAM surface (Figure 4A, inset). For comparison purposes, a cyclic voltammogram is also shown for the DT SAM-coated electrode pretreated by the same procedure as that of the MOT electrode. Figure 4B shows a different voltammetric response. The rate of the anodic peak current decreased much faster than that on the MOT SAM-coated electrode, and after the 10th potential cycle the anodic peaks disappeared totally and the shape of the following cyclic voltammograms became constant. The disappearance of the redox response of the ferrocene/ ferricenium indicated that the surface-derivatized gold nanoparticles cannot be anchored at the DT SAM/ electrolyte interface when they are charged. Immobilization of the nanoparticles onto MOT and DT SAMs was also characterized by TP-AFM. After the potential scan in an electrochemical cell, the gold bead electrode was then rinsed with water and dried in a vacuum desiccator at room temperature overnight. Figure 5A shows an AFM image of gold nanoparticles immobilized onto MOT SAMs, where the immobilized nanoparticles have average diameter of about 28 nm, a value slightly larger than the diameter obtained by TEM. If we take into account the thickness of the protecting layer and convolution effects between the AFM tip and the sample, the difference in the diameter was acceptable.20a However, in the case of the DT SAM-coated electrode, very few nanoparticles were observed on the monolayer surface from the AFM images (Figure 5B), proving that the closely packed and organized alkylchains of the DT SAM are less favorable than those of the MOT SAM for the immobilization of nanoparticles. Conclusions We have prepared gold nanoparticles surface-derivatized with an electroactive protecting monolayer and tried

Figure 5. TP-AFM images of the surface-derivatized gold nanoparicles on MOT (A) and DT (B) SAM-coated Au(111) facets of the gold bead electrode.

to organize the particles onto two kinds of SAMs of MOT and DT. The strong affinity of the hydrophobic nanoparticles to the MOT monolayer is attributed to the sterically enhanced hydrophobic effect of the MOT SAM, which is absent in the DT SAM system. The fabrication is easy because both the SAM preparation and the formation of the nanoparticle adlayer involve self-assembly processes. Furthermore, this methodology can be applied to immobilize other materials of choice. We believe this methodology will provide an exciting range of tailored substrates for studies in harnessing the properties of nanoparticles and other materials on novel devices and applications in nanotechnology. Acknowledgment. This project was supported by the National Natural Science Foundation of China (No. 20275037). LA035088J