Gold Nanoparticle Self-Assembly in Saturated Phospholipid

Feb 12, 2010 - Gold Nanoparticle Self-Assembly in Saturated Phospholipid Monolayers. Alina Mogilevsky, Roman Volinsky, Yohai Dayagi, Noa Markovich ...
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Gold Nanoparticle Self-Assembly in Saturated Phospholipid Monolayers Alina Mogilevsky, Roman Volinsky, Yohai Dayagi, Noa Markovich, and Raz Jelinek* Ilse Katz Institute for Nanotechnology and Department of Chemistry, Ben Gurion University, Beer Sheva 84105, Israel Received December 19, 2009. Revised Manuscript Received January 31, 2010 Self-assembly of nanostructures on surfaces is a promising area in the emerging field of “bottom-up nanolithography”. We describe a systematic analysis of hydrophobically capped gold nanoparticle (Au NP) assemblies created within monolayers of saturated phospholipids deposited at the air/water interface. We show that the Au NPs are segregated within the mixed monolayers, forming distinct configurations. Microscopy analysis reveals that organized Au NP aggregates, including wires, rings, and “doughnut-shape” structures, are observed only within condensed-phase monolayers comprising phospholipids exhibiting longer acyl side-chains. In these monolayers, the Au NPs are localized at the edges of the condensed phospholipid domains. In addition to the pronounced effect of the phospholipid phases at the air/water interface, NP organization was found to depend upon the hydrophobic capping agents of the particles. The Au nanostructures assembled at the air/water interface can be transferred onto solid substrates, suggesting that the selfassembly monolayer approach could be exploited for practical nanoelectronic and sensing applications.

Introduction “Bottom-up”, sometime denoted as “soft”, lithography has attracted growing interest as a possible alternative for conventional lithography technologies. Bottom-up nanolithography approaches broadly encompass methods that exploit molecular self-assembly as means for creating complex nanostructures.1,2 Among the potential candidates for bottom-up nanolithography, Langmuir-Blodgett (LB) films have been proposed as a particularly promising platform.3-5 In LB systems, uniaxial compression of monolayers comprising amphiphlic molecules and/or nanoparticles (NPs) floating on an aqueous subphase induces self-assembly and long-range packing over large surface areas. Ordered monolayers formed at the air/water interface can then be transferred onto solid surfaces in their entirety, generally with high fidelity.5 LB nanolithography could, in principle, produce diverse surface nanostructures, using simple instrumentation and readily available chemical building blocks.6 Traditionally, Langmuir monolayers have been employed for studying the behavior of amphiphilic molecules, such as lipids and fatty acids, on water surfaces.7 In particular, the organization of monolayer components in two dimensions makes such systems amenable for comprehensive physicochemical investigation and structural manipulation using varied surface characterization techniques. Numerous reports have illuminated the thermodynamic profiles, phase transitions, and structural features of Langmuir monolayers and films of surfactant molecules using varied experimental techniques.7 *Corresponding author. Telephone: þ972-8-6461747. Fax: þ972-8-6472943 [email protected]. (1) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335–1338. (2) Gracias, D. H.; Tien, J.; Breen, T. L.; Hsu, C.; Whitesides, G. M. Science 2000, 289(5482), 1170–1172. (3) Acharya, S.; Hill, J. P.; Ariga, K. Adv. Mater. 2009, 21, 2959–2981. (4) Guo, Q.; Teng, X.; Rahman, S.; Yang, H. J. Am. Chem. Soc. 2003, 125, 630– 631. (5) Tao, A. R.; Huang, J.; Yang, P. Acc. Chem. Res. 2008, 41, 1662–1673. (6) Song, H.; Kim, F.; Connor, S.; Somorjai, G. A.; Yang, P. J. Phys. Chem. B 2005, 109(1), 188–193. (7) Zasadzinski, J. A.; Viswanathan, R.; Madsen, L.; Garnaes, J.; Schwartz, D. K. Science 1994, 263, 1726–1733.

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Within the field of nanoparticle self-assembly, Langmuir monolayers have been employed for deposition of gold nanoparticles (Au NPs) coated with hydrophobic substances, which allow the NPs to “float” upon the aqueous subphase.8 Au NPs have attracted great interest due to their optical properties,9 versatile synthetic routes for surface functionalization, and potential bionanotechnology applications.10 Recent studies have shown that Langmuir monolayers of amphiphilic molecules and lipids constrain hydrophobically coated Au NPs into defined structures.11 We have demonstrated that careful selection of surfactant constituents and external parameters, such as temperature and surface pressure, could produce diverse Au NP configurations at the air/water interface, including nanowires, nanoislets, and nanorods, within Langmuir monolayers.10,12 Here we present a systematic analysis of Au NP self-assembly within monolayers of saturated-chain phospholipids. Using different microscopy techniques, we detect distinct organization of the NPs, which depend upon the degree of monolayer condensation, the phospholipid chain length, and capping agents of the Au NPs.

Materials and Methods Materials. HAuCl4, tetradecylammonium bromide, sodium borohydride, oleylamine, and dodecanethiol were obtained from Sigma-Aldrich and used as obtained. Phospholipids, including 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-distearoylsn-glycero-3-phosphocholine (DSPC), 1,2-diarachidoyl-sn-glycero-3-phosphocholine (DAPC), and 1,2-dibehenoyl-sn-glycero3-phosphocholine (DBPC) were obtained from Avanti Polar Lipids (Alabaster, AL) as a powder and used as provided. Chloroform (CHCl3), hexane (C6H14), ethanol (C2H5OH), and methanol (CH3OH) were HPLC grade (Frutarom Ltd., Haifa, (8) Lin, B.; Schultz, D. G.; Lin, X.; Li, D.; Gebhardt, J.; Meron, M.; Viccaro, P. J. Thin Solid Films 2007, 10, 2615–2619. (9) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293–346. (10) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078–1081. (11) Hassenkam, T.; Norgaard, K.; Iversen, L.; Kiely, C. J.; Brust, M.; Bjornholm, T. Adv. Mater. 2002, 14, 1126–1130. (12) Philosof-Mazor, L.; Volinsky, R.; Jopp, J.; Blumberg, P.; Rapaport, H.; Marquez, V. E.; Jelinek, R. Chem. Phys. Chem 2009, 10, 2615–2619.

Published on Web 02/12/2010

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Figure 1. Experimental scheme. Au NPs are produced which are coated with hydrophobically moieties (dodecanethiol is shown). The hydrophobically capped Au NPs are interspersed with phospholipids and isothermally compressed at the air/water interface. The resultant films can be transferred onto solid substrates for microscopy analysis. Israel). Parent solutions of the phospholipids were prepared by codissolution in chloroform to a concentration of 2 mM. Glass substrates for AFM measurements were obtained from MenzelGlaser (Romikal Ltd., Beer Sheva, Israel). Synthesis of Dodecanthiol-Capped Au NPs. Synthesis of the dodecanthiol-capped Au NPs was carried out according to the Brust13 procedure with slight changes,14 based on a two-phase (toluene/water) reduction of HAuCl4 in the presence of the stabilizing ligand. Briefly, HAuCl4 (100 mg) was dissolved in water (10 mL) and transferred to toluene (40 mL) by mixing with tetradecylammonium bromide (500 mg). Dodecanethiol (0.0147 mL) was added to the vigorously stirred solution. Finally, NaBH4 (200 mg) in water (1 mL) was added. After 10 min the solution appeared dark brown, indicating the formation of Au NPs. The solution was stirred overnight to ensure completion of the reaction. The organic phase was washed with 2 M H2SO4 and water in a separation funnel and subsequently evaporated in vacuum to near dryness. The resultant dodecanethiol-capped Au NPs were precipitated by addition of ethanol (60 mL). After separation by filtration, the precipitate was washed three times with ethanol and once with 2-propanol and dried in vacuu to yield the Au NPs (34.9 mg) as a black wax. The resulting Au NP were dissolved in chloroform to produce a solution at a final concentration of 3.00 mg/mL. The Au NPs were characterized TEM. The average diameter of the particles was approximately 3 nm. Synthesis of Oleylamine-Capped Au NPs. Synthesis of the oleylamine-capped Au NPs was carried out according to the Pazos-Perez15 procedure with slight changes. HAuCl4 (20 mg) was dissolved in OA (8 mL) by sonication at room temperature until the solution turned from pale yellow, to an intense orange color, Thereafter, the solution was left undisturbed for 24 h, during which time the solution color changed again gradually from orange to pale yellow. A white precipitate also originated from OA oxidation, which was redissolved by the addition of CHCl3 (7 mL) when the reaction was finished. The last step of the Au NP was carried out in a thermostatic bath at 60 °C to speed up the reaction, followed by aging for 3 days. The resultant oleyla(13) Brust, M.; et al. J. Chem. Soc., Chem. Commun. 1994, 801–802. (14) Norgaard, K.; Weygand, M. J.; Kjaer, K.; Brust, M.; Bjornholm, T. Faraday Discuss 2004, 125, 221–233. (15) Pazos-Perez, N.; Baranov, D.; Irsen, S.; Hilgendorff, M.; Liz-Marzan, L. M.; Giersig, M. Langmuir 2008, 24, 9855–9860.

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mine-capped Au NPs were precipitated by addition of ethanol (20 mL) and hexane (4 mL). After separation by filtration, the precipitate was washed two times with ethanol and dried in vacuu to yield the Au NPs (13.8 mg) as a black wax, which was easily dissolved in chloroform (final concentration 6.8 mg/mL). The Au NPs were characterized TEM. The average diameter of the particles was approximately 10 nm. Surface-Pressure/Area Isotherms. Surface-pressure/area isotherms were recorded using a computerized Langmuir trough (model 622/D1, Nima Technology Ltd., Coventry, U.K.). The water subphase used in the Langmuir trough was doubly purified with a Barnstead D7382 water purification system (Barnstead Thermolyne Corporation, Dubuque, IA), yielding 18.3 mΩ resistivity. The surface pressure was monitored using 1 cm-wide filter paper as a Wilhelmy plate. For each isotherm experiment, the desired amount of mixture in chloroform was spread on the water subphase and equilibrated for 15 min, allowing for solvent evaporation prior to compression. Compression was carried out at a constant barrier speed of 8 cm2 min-1. Each isotherm represents three experimental runs, which were reproducible within an error of 1.0 A˚2 molecule-1. Brewster Angle Microscopy (BAM). A Brewster angle microscope (NFT, Gottingen, Germany) mounted on a Langmuir film balance was used to observe the microscopic structures in situ. The light source of the BAM was a frequency-doubled Nd: YAG laser with a wavelength of 532 nm and 20-70 mW primary output power in a collimated beam. The BAM images were recorded with a CCD camera. The scanner objective was a Nikon superlong working distance objective with nominal 10 magnification and a diffraction limited lateral resolution of 2 μm. The images were corrected to eliminate side ratio distortion originating from a nonperpendicular line of vision of the microscope. Transmission Electron Microscopy (TEM). For TEM analysis, films at the desired surface pressures were transferred horizontally onto 400 mesh copper Formvar/carbon grids (Electron Microscopy Sciences, Hatfield, PA). TEM images were recorded on a Jeol JEM-1230 transmission electron microscope (JEOL Ltd., Tokyo, Japan) operating at 120 kV. Atomic Force Microscopy (AFM). For AFM analysis, films at the desired surface pressures were transferred horizontally onto freshly cleaved mica. The phospholipid templates and alkylthiol capping moieties were subsequently removed by plasma etching Langmuir 2010, 26(11), 7893–7898

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Figure 2. Thermodynamic and Brewster angle microscopy (BAM) analyses. Surface-area/pressure isotherms and BAM images recorded on water subphase at 25 °C. In all isotherms, solid curve = pure phospholipid and broken curve = mixed Au NP/phospholipid monolayer (0.5 mol % Au NPs). Key: (A) DMPC; (B) DPPC; (C) DSPC; (D) DAPC; (E) DBPC. BAM images shown were recorded when monolayers were compressed to approximately 20 mN/m (arrows indicate point in the isotherm). Bar corresponds to 100 μm for all pictures. for approximately 30 min, (Ultraviolet ozone cleaning system, T10  10/GES/E UVCOS INC., Montgomeryville, PA). AFM images were obtained with a 100 μm scanner using a Dimensions Langmuir 2010, 26(11), 7893–7898

3100 instrument (Veeco, United States). Microfabricated Sicantilevers - NSC15 (Mikromasch, United States) with integrated pyramidal tips were used. The 512 pixel x 512 pixel images were DOI: 10.1021/la9047903

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taken in a noncontact mode with a scan size of up to 80 μm, at a scan rate of 0.250-1 Hz.

Results Figure 1 depicts the experimental scheme. Au NPs coated with hydrophobic capping agents were deposited at the air/water interface, interspersed within phospholipid molecules.11 The mixed monolayers were isothermally compressed and films were extracted for ex situ microscopy analysis at different pressures to assess the Au NP organization. The saturated phosphatydilcholine phopspholipids selected for this study exhibit acyl chains of varying-length, and consequently form different phases at the air/ water interface at room temperature: Dimirystoylphosphatidylcholine (DMPC, 14 carbons in each of the saturated acyl chains) exists in a liquid-expanded monolayer in the entire surfacepressure range,16 while dipalmitoylphosphatidylcholine (DPPC, 16 carbons) exhibits a transition between a liquid-expanded and liquid-condensed phase.11 Distearoylphosphatidylcholine (DSPC, 18 carbons), diarachinoylphosphatidylcholine (DAPC, 20 carbons), and dibehenoylphosphatidylcholine (DBPC, 22 carbons) all form condensed monolayers immediately after deposition at the air/water interface at room temperature.17-20 Figure 2 compares the surface-pressure/area isotherms of the pure phospholipid monolayers and mixed dodecanethiol-capped Au-NP/phospholipid monolayers, respectively, and also shows the corresponding Brewster angle microscopy (BAM) images of the monolayers. Figure 2 indicates that, in all phospholipid systems, the inclusion of the dodecanethiol-capped Au NPs within the monolayers of the phospholipids examined slightly shifted the isotherms to higher surface pressures, confirming incorporation of the NPs within the phospholipid monolayers. Figure 2 also demonstrates that the overall shapes and slopes of the pure phospholipid isotherms were generally retained after Au NP addition, suggesting that incorporation of the Au NPs did not interfere with the physical properties and phase transitions of the phospholipid monolayers (i.e., the Au NPs were immiscible within the phospholipid monolayers. The BAM images in Figure 2, which illuminate the macroscale organization of film constituents at the air/water interface,21 highlight differences in Au organization among the phospholipid monolayers analyzed. In general, the brighter domains observed in the BAM images correspond to the more reflective regions populated by the Au NPs.22 The BAM data in Figure 2 reveal significant differences between the bright snake-like regions of the Au NPs assemblies in the DMPC or DPPC monolayers (Figure 2A,B), and the broad, seemingly diffusive, sheaths of Au NPs in case of DSPC, DAPC, and DBPC (Figure 2C-E). Also, the reflective Au aggregates in the Au NP/DMPC monolayer (Figure 2A) appear somewhat thinner and more entangled than the corresponding thicker gold domains in the Au NP/ DPPC monolayer (Figure 2B). Overall, the BAM images indicate that the phospholipids clearly affect the assembly of the Au NPs at the air/water interface. The pronounced difference between the BAM data recorded (16) Volinsky, R.; Gaboriaud, F.; Berman, A.; Jelinek, R. J. Phys. Chem. B 2002, 106, 9231–9236. (17) Dynarowicz-Latka, P.; Rosilio, V.; Boullanger, P.; Fontaine, P.; Goldmann, M.; Baszkin, A. Langmuir 2005, 21, 11941–11948. (18) Bouffioux, O.; Berquand, A.; Eeman, M.; Paquot, M.; Dufrene, Y. F.; Brasseur, R.; Deleu, M. Biochim. Biophys. Acta 2007, 1768, 1758–1768. (19) Zhao, L.; Feng, S. J. Colloid Interface Sci. 2004, 74, 55–68. (20) Hollinshead, C. M.; Harvey, R. D.; Barlow, D. J.; Webster, J. R.; Hughes, A. V.; Weston, A.; Lawrence, M. J. Langmuir 2009, 25, 4070–4077. (21) Henon, S.; Meunier, J. J. Rev. Sci. Instrum. 1991, 64, 936–939. (22) Hansen, C. R.; Westerlund, F.; Moth-Poulsen, K.; Ravindranath, R.; Valiyaveettil, S.; Bjornholm, T. Langmuir 2008, 24, 3905–3910.

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Figure 3. Au NP/DMPC and Au NP/DPPC films. TEM images of (A) Au NP/DMPC film and (B) Au NP/DPPC film. The films were transferred from the air/water interface at a surface pressure of 20 mN/m. The bar represents 1 μm. The bar in the insets represents 50 nm.

for the monolayers comprising Au NPs and DMPC or DPPC (Figure 2A-B) on the one hand, and DSPC, DAPC, and DBPC on the other hand (Figure 2C-E) might be ascribed to the liquidexpanded monolayers formed by the shorter-acyl-chain phospholipids (DMPC and DPPC),16,23 compared to the liquid-condensed monolayer phase in case of the latter three phospholipids.17-19 Transmission electron microscopy (TEM) and atomic force microscopy (AFM) experiments depicted in Figures 3-6 further probe the differences in Au NP organization within the different phospholipid monolayers, and expose interesting structural features of the Au NP assemblies in the nanoscale. The TEM data confirm that the distribution of the Au NPs is highly dependent upon the phospholipid molecules codeposited in the monolayer, and support the proposition that the Au NP patterns formed at the air/water interface are closely associated with the phospholipid monolayer phases. Figure 3 depicts TEM images of monolayers comprising dodecanethiol-capped Au NPs codeposited with DMPC and DPPC, respectively. Similar to previously reported data,11 Figure 3 shows that the Au NPs form randomly oriented aggregates when deposited and isothermally compressed with either DMPC or DPPC. Furthermore, the TEM results in Figure 3 show that adjacent dodecanethiol-capped Au NPs, which generally exhibit spherical shapes following synthesis,14 appear merged into irregularly shaped particles in both DMPC and DPPC monolayers. Significantly different film structures were recorded when the dodecanethiol-capped Au NPs were mixed and isothermally compressed with phospholipids exhibiting longer saturated acyl chains;DSPC (Figure 4), DAPC (Figure 5A), and DBPC (Figure 5B). Figure 4(A,B) depict representative TEM images of Au NP patterns formed in Au NP/DSPC Langmuir monolayers. Figure 4 shows that the Au NPs aggregated around circularly shaped particle-free spaces in the films. The circular regions most likely comprise condensed lipid domains which restrict the Au NPs into adjacent areas. Previous studies have indeed detected liquid-condensed circularly shaped domains within compressed DSPC monolayers.20 The TEM results in Figure 4 (23) Duncan, S. L.; Larson, R. G. Biophys. J. 2008, 94, 2965–2986.

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Figure 4. Au NP/DSPC films. Representative TEM patterns of Au NPs embedded in templates of DSPC. Key: (A) film transferred at a surface pressure of 10 mN/m; (B) 20 mN/m; (C) 50 mN/m. The bar represents 50 nm.

Figure 6. Au NPs coated with oleylamine. TEM images showing the Au NPs assemblies in mixed monolayers: (A) DAPC; (B) DBPC. Patterns were transferred from the air/water interface at a surface pressure of 20 mN/m. The bar represents 50 nm.

Figure 5. Au NP/DAPC and Au NP/DBPC films. TEM images: (A) Au NP/DAPC films transferred from the air/water interface at a surface pressure of (i) 20 mN/m and (ii) 50 mN/m; (B) Au NP/ DBPC films transferred from the air/water interface at a surface pressure of (i) 20 and (ii) 50 mN/m (the bar represents 50 nm); (C) AFM images of films transferred at 20 mN/m of (i) Au NP/DAPC and (ii) Au NP/DBPC.

indicate that the Au NPs were either distributed over relatively large film areas in between the condensed phospholipid spaces (Figure 4A), or formed rather isolated aggregates (Figure 4B,C). The latter patterns were more abundant in monolayers isothermally compressed to higher surface pressures, suggesting that surface pressure is an imprtant parameter contributing to the organization of the Au NPs in the films. Figure 5 presents self-assembled Au NP structures formed within Langmuir monolayers of DAPC (Figure 5A) and DBPC (Figure 5B). The TEM images in Figure 5A,B reveal dramatic differences of Au NP organization compared to both DMPC and DPPC (Figure 3), as well as compared to the Au NP/DSPC films (Figure 4). Specifically, Figure 5 demonstrates that DAPC and Langmuir 2010, 26(11), 7893–7898

DBPC induce formation of extremely thin Au NP “wires”, essentially comprising “single file” assemblies of the nanoparticles. These features were reproducible, for the Au NPs mole ratio, temperature, and surface pressure employed. Figure 5 further indicates that similar thin Au NP assemblies were formed in the mixed monolayers with both DAPC and DBPC regardless of surface pressure employed for film transfer, appearing either at the edges of large irregularly shaped enclosed curved areas, or forming round-shape domains. Similar to the Au NP/DSPC monolayers discussed above, the enclosed domains surrounded by the Au NPs most likely correspond to liquid-condensed phospholipid areas within the monolayers.20 The atomic force microscopy (AFM) images in Figure 5C provide a striking visual depiction of typical circular gold domains formed in the DAPC and DBPC monolayers. In particular, the AFM experiment demonstrates that the organized Au assemblies formed in the Langmuir trough were retained after transfer from the water surface onto solid substrates and subsequent removal of the organic substances, including lipids and nanoparticles’ capping agents, by plasma etching. This result implies that the phospholipid-templating approach might be employed for creation of metallic surface patterns for practical applications. While Figures 2-5 illuminate the effects of the different phospholipids on Au NP organization, we also examined the structural consequences of varying the hydrophobic capping agents of the Au NPs (Figure 6). The TEM images in Figure 6 depict typical film configurations of Au NPs coated with oleylamine15 instead of dodecanethiol, codeposited with DAPC (Figure 6A) or DBPC (Figure 6B). The synthesized oleylaminecapped Au NPs exhibited larger diameters compared to their dodecanethiol-capped conterparts.24 Similar to the dodecanethiol-capped Au NPs depicted in Figure 5, the oleylaminecoated Au NPs assemble around circularly shaped Au-free domains within the DAPC or DBPC films (Figure 6). However, (24) Hiramatsu, H.; Frank, O. E. Chem. Mater. 2004, 16, 2509–2511.

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in contrast to the narrow, single-file NP organization recorded in the films comprising dodecanethiol-capped Au NPs (Figure 5), the oleylamine-coated Au NPs form larger aggregates around the condensed circular phospholipid domains (Figure 6).

Discussion This study presents a systematic investigation of self-assembled hydrophobically coated Au NPs in Langmuir monolayers of saturated phospholipids. Surface-pressure/area isothermal analysis indicates that the Au NPs were incorporated within the phospholipid monolayers without significantly altering their compression properties and phase transitions at the air/water interface (Figure 2). This result is consistent with immiscibility and segregation between the different Au NP aggregates and the phospholipid domains, observed in the TEM experiments (Figures 3-6). Indeed, the microscopy experiments reveal that the Au NPs were not uniformly interspersed within the phospholipid monolayers, but rather assembled into distinct configurations. Importantly, the TEM results in Figures 3-6 suggest that condensed domains within the phospholipid monolayers constitute the most significant factor affecting Au NP organization at the air/water interface. Thus, Au NPs form random aggregates in monolayers of the liquid-expanded DMPC monolayer (Figure 3A). Poorly organized aggregates of Au NPs similarly appeared within DPPC monolayers (Figure 3B), either before the phase transition of the phospholipid (in which DPPC exists in a liquid expanded monolayer) or after (liquid condensed). This result suggests that the initial fluid phase of DPPC determines the rather random pattern of Au NP aggregates, which is not significantly altered following the monolayer transition into a liquid-condensed phase. The most dramatic surface features of the dodecanethiolcapped Au NPs were observed in monolayers of the longer-chain phospholipids DSPC, DAPC, and DBPC (Figures 4 and 5). These phospholipids do not exhibit liquid-expanded phases, but rather display a liquid-condensed organization throughout the entire surface-pressure range (Figure 2C-E). The TEM and AFM experiments featured in Figures 4 and 5 suggest that the structures of the Au NP assemblies were essentially shaped by the network of condensed phospholipid domains within the films. Specifically, the Au NPs assembled around enclosed film domains, most likely comprising condensed-phase phospholipids. Furthermore, the “sintering” among adjacent Au NPs which was prevalent in the monolayers of DMPC and DPPC (Figure 3) was largely absent in

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the monolayers of the longer-chain phospholipids, suggesting that the fluid monolayer phases of DMPC and DPPC promote NP fusion, most likely through facilitating mobility of the NPs with respect to one another. It should be noted that differences in Au NP assembly were detected between DSPC on the one hand (Figure 4) and DAPC or DBPC on the other hand (Figure 5). Specifically, while the Au NPs in the DSPC monolayer featured aggregates comprising large number of nanoparticles formed around circularly shaped domains (Figure 4), in both DAPC and DBPC monolayers the Au NPs were organized in single-file arrangements at the edges of condensed phospholipid domains (Figure 5). This difference might be ascribed to an initial liquid-expanded phase that seemed to exist in the mixed Au NP/DSPC monolayer, leading to the slight shoulder at lower pressures in the surface-pressure isotherm of Au NP/DSPC (Figure 2C).25 Experiments conducted with a different capping agent of the Au NPs (oleylamine rather than dodecanethiol, Figure 6) further support the proposition that interactions between the hydrophobic layer of the Au NPs and the acyl chains of the phospholipid molecules within the monolayer constitute a crucial driving force in Au NP organization. Specifically, the oleylamine-coated NPs formed aggregates covering large areas which enclosed circular domains when mixed and compressed with DAPC and DBPC (Figure 6), in contrast to the single column NP organization of the dodecanethiol-capped Au NPs in monolayers comprising of the same phospholipids (Figure 5). In conclusion, this study demonstrates that hydrophobically capped Au NPs form distinct nanostructures in Langmuir monolayers of saturated phospholipids. The main determinant affecting Au NP organization appears to be the formation and shapes of condensed phospholipid domains within the films, although the chemical interactions between the capping agents and the phospholipids play a structural role too. This study underlines the use of lipid monolayers to shape gold nanostructures at the air/ water interface. The results also point to the possibility for employing Au NPs as a vehicle for exposing internal condensed-domain configurations within monolayers of amphiphilic molecules. Acknowledgment. We are grateful to J. Jopp and R. Golan for help with AFM experiments, and R. Jeger for help with TEM. (25) Barlow, W. A. Langmuir Blodgett Films; Elsevier: New York, 1980; Vol. 1.

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