Self-Assembly of Gold Nanorods Induced by Intermolecular

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Self-Assembly of Gold Nanorods Induced by Intermolecular Interactions of Surface-Anchored Lipids Hiroshi Nakashima,* Kazuaki Furukawa, Yoshiaki Kashimura, and Keiichi Torimitsu Materials Science Research Laboratory, NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan ReceiVed January 30, 2008. ReVised Manuscript ReceiVed March 21, 2008 Surface-modified gold nanorods (Au NRs) with 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (DPPTE) were synthesized, and their self-assembled structures on a silicon substrate were observed using a scanning electron microscope (SEM). The Au NR-DPPTE complex formed characteristic one- and two-dimensional self-assemblies induced by intermolecular interactions of surface-anchored lipids via simple drying process. The interparticle distance between neighboring NRs was uniform at around 5.0 nm, which was consistent with the thickness of the lipid bilayer. Furthermore, we observed the anisotropic configurations of the NR complex, preferentially oriented in a lateral or perpendicular fashion, in a two-dimensional assembled structure dependent on the interfacial hydrophilicity or hydrophobicity of the silicon surface.

Introduction Gold nanorods (Au NRs) have a fascinating optical property, whose origin is localized surface plasmon resonance, and that is sensitively dependent on their shape, size, and the local dielectric environment.1–4 Techniques have been developed for the synthesis and characterization of dispersed NRs,5,6 thus providing the next challenge of patterning and ordering these nanoscale materials into assembled structures.7–10 The controlled selfassembly of Au NRs on a suitable surface has many potential applications in science and technology. It is vital for studying fundamental optoelectronic properties arising from collective interactions in an ordered state, and for incorporating nanomaterials into enhanced spectroscopy,11–13 (bio)chemical sensing,14–16 and nanophotonic, electronic, and plasmonic devices.17–20 Several strategies for nanofabrication, especially nonlithographic techniques, have already been reported for * To whom correspondence should be addressed. Tel: +81 46 240 3559. Fax: +81 46 270 2364. E-mail: [email protected]. (1) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257–264. (2) Murphy, C. T.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857–13870. (3) Link, S.; Mohamed, M. B.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3073–3077. (4) Spru¨nken, D. P.; Omi, H.; Furukawa, K.; Nakashima, H.; Sychugov, I.; Kobayashi, Y.; Torimitsu, K. J. Phys. Chem. C 2007, 111, 14299–14306. (5) Gao, J.; Murphy, C. J. Chem. Mater. 2005, 17, 3668–3672. (6) Sau, T. K.; Murphy, C. J. Langmuir 2004, 20, 6414–6420. (7) Sau, T. K.; Murphy, C. J. Langmuir 2005, 21, 2923–2929. (8) Nikoobakht, B.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 8635–8640. (9) Kumar, V. R. R.; Samal, A. K.; Sreeprasad, T. S.; Pradeep, T. Langmuir 2007, 23, 8667–8669. (10) Jana, N. R.; Gearheart, L. A.; Obare, S. O.; Johnson, C. J.; Edler, K. J.; Mann, S.; Murphy, C. J. J. Mater. Chem. 2002, 12, 2909–2912. (11) Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. A 2003, 107, 3372– 3378. (12) Kawamura, G.; Yang, Y.; Nogami, M. Appl. Phys. Lett. 2007, 90, 2619081–3. (13) Orendorff, C. J.; Gearheart, L.; Jana, N. R.; Murphy, C. J. Phys. Chem. Chem. Phys. 2006, 8, 165–170. (14) Wang, H.; Huff, T. B.; Zweifel, D. A.; He, W.; Low, P. S.; Wei, A.; Cheng, J.-X. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 15752–15756. (15) Takahashi, H.; Niidome, Y.; Niidome, T.; Kaneko, K.; Kawasaki, H.; Yamada, S. Langmuir 2006, 22, 2–5. (16) Chen, C.-C.; Lin, Y.-P.; Wang, C.-W.; Tzeng, H.-C.; Wu, C.-H.; Chen, Y.-C.; Chen, C.-P.; Chen, L.-C.; Wu, Y.-C. J. Am. Chem. Soc. 2006, 128, 3709– 3715. (17) 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.

assembling, patterning, and integrating NRs on a solid surface. Simple solvent evaporation under appropriate conditions and the Langmuir-Blodgett (LB) method at a water/air interface have been used for ordering of NRs.7–10,21 In addition, the seedmediated growth of Au NRs directly on an NH2-functionalized Si surface has provided accurate NR alignment.22 Functionalized Au NRs with specific molecular groups can lead to precisely controlled self-assembled structures, namely a programmed self-assembly. This approach is beneficial for building blocks of NRs via physical and chemical affinities (covalent or noncovalent van der Waals, hydrophobic, or electrostatic interactions). For example, hydrophobic Au NRs whose exteriors are coated with octadecyltrimethoxysilane (OTMS) have been used to fabricate highly ordered NR arrays including a parallel configuration or the hexagonal packing of NRs in a lamellar structure.23 The silica coating of Au NRs with a controllable silica shell thickness has been developed, and the functionalization of the Au NRs with OTMS can be achieved through silane coupling on the nanorod surface.24 These NRs stabilized with long hydrophobic alkyl chains make it possible for the NRs to be transferred into nonpolar organic solvents. This leads to a novel nanofabrication technique, for example, the efficient and customized fabrication of NR building blocks in a controllable manner by combining the hydrophobic interaction and a gentle drying process. On the other hand, in the presence of adipic acid, adjacent Au NRs were linked to each other via electrostatic interaction, resulting in two-dimensional (2-D) ordering associated with the pH effect.25 Additionally, end-toend assemblies of Au NRs have recently been observed in several (18) Fe´lidj, N.; Aubard, J.; Le´vi, G.; Krenn, J. R.; Schider, G.; Leitner, A.; Aussenegg, F. R. Phys. ReV. B 2002, 66, 245407-1–7. (19) Imura, K.; Nagahara, T.; Okamoto, H. J. Phys. Chem. B 2004, 108, 16344– 16347. (20) Sonnichsen, C.; Alivisatos, A. P. Nano Lett. 2005, 5, 301–304. (21) Kim, F.; Kwan, S.; Akana, J.; Yang, P. J. Am. Chem. Soc. 2001, 123, 4360–4361. (22) Mieszawska, A. J.; Slawinski, G. W.; Zamborini, F. P. J. Am. Chem. Soc. 2006, 128, 5622–5623. (23) Mitamura, K.; Imae, T.; Saito, N.; Takai, O. J. Phys. Chem. B 2007, 111, 8891–8898. (24) Pastoriza-Santos, I.; Pµerez-Juste, J.; Liz-Marzµan, L. M. Chem. Mater. 2006, 18, 2465–2467. (25) Orendorff, C. J.; Hankins, P. L.; Murphy, C. J. Langmuir 2005, 21, 2022– 2026.

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systems such as biotin-capped Au NRs-streptavidin,26 mercaptocarboxylic acid-coupled Au NRs,27 cysteine- or glutathionemodified Au NRs,28 and crown ether-modified Au NRs.29 It is generally accepted that programmed self-assembly with functionalized NRs can be an efficient means of nanofabrication owing to its simplicity, versatility, and low cost. However, most of the above methods only provide ordered structures with a limited area, and do not allow us to control the design or interparticle distance in the NR architecture. In addition, despite their unique characteristics, the application of Au NRs in the bioscience field is limited owing to the presence of a cationic detergent, which is used as a stabilizer for the NRs. Therefore, it is important to prepare functionalized Au NRs bearing specific biocompatible groups, and flexibly control their self-assembled structures through biomimetic, programmed intermolecular interactions. This will lead to the development of a highly sensitive biomolecular detection platform that outputs the changes in the local environment surrounding the NRs by employing their sensitive optical properties. Here we describe the synthesis and self-assembly of novel functionalized Au NRs coupled with 1,2dipalmitoyl-sn-glycero-3-phosphothioethanol (DPPTE). The Au NR-DPPTE complex produced characteristic one-dimensional (1-D) and two-dimensional (2-D) self-assemblies on a Si substrate via a controlled drying process. Additionally, the NR complex in the 2-D array was oriented with either a lateral or a perpendicular configuration, depending on the interfacial hydrophobicity or hydrophilicity of the Si substrate. These diverse self-assembled features are achieved by using an NR complex that is soluble in organic solvent.

Experimental Section Preparation of Au NRs and Au NR-DPPTE Complex. The Au NRs were prepared by combining chemical reduction of HAuCl4 and a subsequent photoirradiation method with slight modifications (Supporting Information).30,31 The NRs (average aspect ratio: 3.9) were well dispersed in water without appreciable aggregation. It is known that a bilayer of cetyltrimethylammonium bromide (CTAB) stabilizes the NRs and disturbs the aggregation. The coupling of Au NRs with DPPTE (Avanti Polar Lipids, Inc.) was performed in two systems: Complex Dispersed in Water. DPPTE was dispersed in water with sonication for a few minutes. The DPPTE aqueous solution (0.1-1.0 mM, 200 µL) was added to a Au NR solution (300 µg mL-1, 200 µL) and then the mixture was vigorously stirred for 1 h at room temperature. The obtained products were purified by centrifuging them in a tube for 10 min at 10 000 rpm and 10 °C to precipitate the solid, and then the supernatant was removed. The residue was redispersed in an appropriate volume of deionized (DI) water. This purification process was repeated twice to remove excess CTAB and DPPTE. Complex DissolVed in Chloroform. A chloroform solution of DPPTE (0.1-1.0 mM, 200 µL) was added to a Au NR aqueous solution, and then the two-phase system was vigorously stirred for 1 h at room temperature. After the stirring, the color of the NRs was transferred to the organic phase owing to the formation of Au NR-DPPTE complex, and CTAB detached from the NRs was kept in the aqueous phase by forming an oil-in-water (o/w)-type emulsion. (26) Caswell, K. K.; Wilson, J. N.; Bunz, U. H. F.; Murphy, C. J. J. Am. Chem. Soc. 2003, 125, 13914–13915. (27) Thomas, K. G.; Barazzouk, S.; Ipe, B. I.; Joseph, S. T. S.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 13066–13068. (28) Sudeep, P. K.; Joseph, S. T. S.; Thomas, K. G. J. Am. Chem. Soc. 2005, 127, 6516–6517. (29) Nakashima, H.; Furukawa, K.; Kashimura, Y.; Torimitsu, K. Chem. Commun. 2007, 1080–1082. (30) Niidome, Y.; Nishioka, K.; Kawasaki, H.; Yamada, S. Chem. Commun. 2003, 2376–2377. (31) Kim, F.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2002, 124, 14316– 14317.

The organic phase was extracted and purified by the above-mentioned process using chloroform instead of water. Finally, precipitates of Au NR-DPPTE complex were redispersed in an appropriate volume of chloroform. Self-Assembly of Au NR-DPPTE Complex on a Si Substrate. For 1-D Self-Assembly. A 5 µL portion of chloroform solution of the Au NR-DPPTE complex was dropped on a Si(111) substrate, and immediately moderate spin coating was employed at 100 rpm for 90 s for drying and assembling the NR complex. For 2-D Self-Assembly. A 5 µL portion of chloroform solution of the Au NR-DPPTE complex was cast on a hydrophilic or hydrophobic Si(111) substrate, and then the solvent was evaporated at room temperature in air. Several amounts of chloroform were dropped on the sample and simultaneously annealed at 60 °C. This drying process was repeated three times in order to prompt rearrangement of the NR complex.

Results and Discussion We prepared Au NR-DPPTE complex dispersed in water or in chloroform. As regards the preparation of the complex in chloroform, when the chloroform solution of DPPTE was mixed with the NR aqueous solution, the color of the aqueous phase was immediately transferred to the organic phase, indicating that the Au NRs become soluble in chloroform via coupling with DPPTE. The DPPTE should exchange rapidly with CTAB and attach themselves covalently to the NR surface as a result of the higher affinity of the thiol group for gold. We also confirmed that, when only chloroform was added to the initial Au NR solution, the NRs remained in the aqueous phase because there was no surface-ligand exchange. We measured the zeta potential for the NR-DPPTE complex samples dispersed in water. The zeta potential of the initial Au NRs was 24.0 mV. This value originates from the CTAB molecules adsorbed on the surface of the NRs. After coupling with DPPTE, the zeta potential of the NR complex became negative. The value decreased as the concentration of the added DPPTE increased (-30.7 mV at 0.1 mM to -53.0 mV at 1.0 mM, Supporting Information). These results indicate that the adsorbed cationic CTAB molecules were effectively replaced by the anionic DPPTE molecules, and the covering ratio of DPPTE on the NR surface increased with the DPPTE concentration. Figure 1 shows the absorption spectra of the Au NR-DPPTE complex in water and in chloroform. The initial Au NRs in an aqueous solution exhibited two plasmon absorption maxima around 702 and 521 nm, corresponding to the longitudinal mode along the long axis and the transverse mode perpendicular to the long axis. When we added a 0.1 mM DPPTE aqueous solution to the NRs, the peak maximum of the longitudinal mode was red-shifted to 726 nm, and the peak profile was broadened (Figure 1a). The spectral change was related to the formation of Au NR aggregates, which was induced by the intermolecular interactions between surface-anchored lipids. This may trigger the coupling of the plasmon absorbance as a result of the NRs’ proximity to each other. When we increased the concentration of the added DPPTE solution, the top of the longitudinal peak was slightly blue-shifted (Figure 1a and inset) and the broadened peak was somewhat diminished. It is important to note that, according to Gluodenis et al., a blue shift in the longitudinal plasmon band of 50-150 nm is expected for 100% of NRs interacting sideto-side in solution.32 Thus, the spectral change at a high DPPTE concentration might be caused by the side-to-side arrangement of some of the NR complex in an aggregated state owing to a greater amount of lipids being attached to the sides of the NRs. Another possibility is that lipids formed a bilayer on the surface (32) Gloudenis, M.; Foss, C. A. J. Phys. Chem. B 2002, 106, 9484–9489.

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Figure 2. SEM images of 1-D self-assembly of the Au NRs coupled with 1.0 mM of DPPTE on Si(111) substrate. Panels a and b are at different magnifications.

Figure 1. Absorption spectrum changes of the Au NR complex (a) in water and (b) in chloroform upon addition of various concentrations of DPPTE. Absorption intensities were normalized at λmax of the longitudinal peak. (Inset): peak wavelength at λmax of the longitudinal mode as a function of added DPPTE concentration.

of individual NRs at high DPPTE concentrations, which may lead to an increase in monodispersed NRs in solution. We performed a control experiment using an analogue lipid, 1,2dipalmitoyl-sn-glycero-3-phosphocholine (DPPC, no thiol group), under identical experimental conditions. The changes in the absorption spectra of the Au NRs with DPPC in water were negligible, regardless of the DPPC concentration (Supporting Information), indicating that the coupling reaction between the NRs and DPPC did not occur as a result of the lack of terminal thiol groups. On the other hand, the spectra profiles of the Au NR-DPPTE complex in chloroform remained largely unchanged, irrespective of the DPPTE concentration (Figure 1b). This indicates that individual particles of the NR complex are relatively free in chloroform, resulting from the good solubility of the lipids in chloroform. The slight red shift of the longitudinal mode in chloroform against the initial result for NRs in water was attributed to the difference in the dielectric constant of the surrounding medium.1–3 We can successfully fabricate 1-D and 2-D NR nanoarchitectures in a controllable manner when we deposit the chloroformsoluble NR-DPPTE complex on a solid surface. The combination of solvent evaporation from the sample drop and interfacial

hydrophilicity or hydrophobicity led to a variety of self-assembled features. It is important to note that we did not obtain an organized structure from water-dispersed Au NR complexes, regardless of the introduced DPPTE concentration (Supporting Information). The organization in the self-assembled structure is, in part, determined by the balance of van der Waals, capillary, surface tension, and other forces. The formation of an organized assembly with a stable structure is possible only when the collective interaction energy operating among the participating NRs is sufficient to overcome the effect of the entropy loss caused by ordering. As for the water-dispersed NR complex, assembly via the drying process was greatly affected by the strong interfacial force of water, which resulted in disordered NR structures in the aggregated state. Therefore, it is essential in our case that the approach using the evaporation of organic solvent allows the controllable building of blocks of the NR complex on a solid surface. In Figure 2, the solvent evaporation by means of spin coating upon deposition of the sample led to the 1-D self-assembly of the Au NR complex with side-to-side configurations. When we take the absorption spectra in chloroform into consideration, the NR complex is relatively isolated in an isotropic state. The complex was assembled during the drying process where the driving force for the side-to-side ordering is mainly the intermolecular interactions between surface-anchored lipids. The cooperative interfacial forces upon evaporation are also associated with the side-to-side ordering owing to the favorable translational entropy. In addition, the spin-coating operation reduces the collective density of the ordered NRs, leading to a 1-D configuration on the substrate. Interestingly, after chloroform was dropped on the substrate where the NR complex was initially deposited, and simultaneously dried at 60 °C, the sample exhibited a widespread, 2-D selfassembly of NRs. Furthermore, we observed anisotropically orientated NRs in a 2-D assembly, dependent on the hydrophilic or hydrophobic nature of the Si surface. On the hydrophilic Si surface (contact angle of water: 25.4°, Supporting Information) shown in Figure 3, the NR complex in the 2-D array is orientated laterally to the substrate. The distance between neighboring NRs

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Figure 3. SEM images of two-dimensionally and laterally orientated self-assembly of the Au NRs coupled with 1.0 mM of DPPTE on a hydrophilic Si(111) surface through repeated solvent evaporation and thermal annealing. Panels a and b are at different magnifications.

was uniform at around 5.0 nm (average value estimated from 50 interparticle distances in the scanning electron microscope (SEM) image), which is approximately equal to the thickness of the lipid bilayer (≈ 4.7 nm).33 A similar interparticle spacing of the NRs at regular intervals is also distinguished in the 1-D assembly (Figure 2). 2-D ordering with both side-to-side and end-to-end packing is induced by the reorganization of the NR complex. The repeated supply of chloroform prompts a suitable arrangement of the surface-anchored DPPTE in the interparticle gap, which spontaneously leads to reordering of the NR complex without any overlapping between adjacent NRs. The annealing at 60 °C may also assist the reorganization toward the 2-D array since analogue DPPC lipid changes from the gel phase to the liquid crystalline phase at around 41 °C.34 We consider that, in principle, a perfectly unidirected 2-D ordering of the NRs can be achieved if the defects caused by spherical particles (byproduct) are removed. We also investigated the formation of the assembled structures of the NR complex in a chloroform system in relation to the introduced DPPTE concentration (Figure 4). The NR complex prepared with 1.0 or 0.5 mM of DPPTE exhibited 2-D ordering in a monolayer on a hydrophilic Si substrate. The defect region of the arrayed NRs in 1.0 mM of DPPTE appeared to be smaller than that in 0.5 mM of DPPTE, and the assembly became larger in 1.0 mM of DPPTE. In contrast, when we introduced 0.1 mM of DPPTE, 2-D ordering could not be obtained, and only local side-to-side connections were observed. These results may be attributed to the DPPTE coverage ratio on the NR surface, which contributes to the frequency of the interparticle interactions between NRs. Therefore, the lipid concentration plays a critical role in NR assembly. As a control experiment, we synthesized a spherical Au nanoparticle (NP)-DPPTE complex dispersed in chloroform and confirmed that these assembled structures formed on a hydrophilic Si substrate when we used an identical evaporation process to that employed with the Au NR-DPPTE system (33) Rinia, H. A.; Kik, R. A.; Demel, R. A.; Snel, M. M. E.; Killian, J. A.; Eerden, J. P. J. M.; Kruijff, B. Biochemistry 2000, 39, 5852–5858. (34) Ghosh, Y. K.; Indi, S. S.; Bhattacharya, S. J. Phys. Chem. B 2001, 105, 10257–10265.

Figure 4. SEM images of the self-assembly of the Au NR-DPPTE complex, which is dependent on the DPPTE concentration on a hydrophilic Si(111) surface. The complex was prepared with (a) 1.0 mM, (b) 0.5 mM, and (c) 0.1 mM of DPPTE in a chloroform system.

(Supporting Information). As a result, the Au NP complex was arrayed in a monolayer at regular intervals of 5.1 nm (estimated from 100 interparticle distances in the SEM image) and partly formed hexagonal packing structures. This indicates that our method is useful for building blocks of nanoparticles with different aspect ratios. In contrast, on a hydrophobic Si substrate (contact angle of water: 70.6°), the SEM image in Figure 5a at a tilt angle of 0° exhibited a great number of circular shapes around 10 nm in size, which are different from the byproduct spherical nanoparticles (20-40 nm). We consider the circular dots to be images of the NRs from a longitudinal direction where the NRs are arranged perpendicularly to the substrate, namely, they are standing on the substrate. In fact, when the sample was tilted at an angle of 45° (Figure 5b,c), we observed that the Au NR complex is upright on the hydrophobic surface. The analogue self-assembled behavior of hydrophobic NRs with OTMS has been reported.23 In this previous report, the NRs at a low concentration were recumbent in a 2-D assembly, whereas both upright and recumbent NRs were observed at a high concentration. Similarly, in our case, the process of evaporating chloroform on a hydrophobic surface

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Letters Scheme 1. Illustration of Surface Modification of Au NR with DPPTE and 1-D and 2-D Self-Assembled Configurations of the NR Complex on a Solid Surface

Figure 5. SEM images of two-dimensionally and perpendicularly oriented self-assembly of the Au NRs coupled with 1.0 mM of DPPTE on a hydrophobic Si(111) surface through repeated solvent evaporation and thermal annealing. Images were observed at tilt angles of (a) 0° and (b,c) 45° (panels b and c show images at different places).

might produce a highly condensed NR-DTTPE complex. In the evaporation process on a hydrophobic surface, the NR complex could avoid coming into direct contact with the surface owing to the chloroform medium since nonpolar chloroform has good affinity with a hydrophobic surface. The contact between the NRs and the surface may be related to entropy loss. As the evaporation progressed, the NR complex was maintained in a chloroform droplet and was gradually condensed without being deposited on the substrate, which led to the formation of upright NRs on the substrate, assisted by the intermolecular interaction between anchored lipids. The upright NR configuration may maintain a lower potential energy than the lateral configuration. This is because the entire longitudinal side of the NRs is surrounded by the lipid bilayer with adjacent NRs, and the contact area of the NRs facing the substrate or air is minimized. Consequently, the stable configuration of the NR complex through the drying process is determined by a combination of various interfacial factors such as a hydrophilic/hydrophobic nature, a chemical affinity with solvent, interfacial tension, and spontaneous intermolecular interactions between anchored molecules, although the details of the mechanism remain unclear. However, as regards the 2-D self-assembly of the Au NRs, our results constitute the first example of precise control of the interparticle spacing and of anisotropic orientation in a lateral or perpendicular fashion dependent on the interfacial hydrophilicity or hydrophobicity. In summary, we have described 1-D and 2-D self-assembly of Au NRs coupled with lipids. The self-assembled configurations of the complex were dependent on the drying method and the hydrophilic or hydrophobic nature of the Si surface. The surfacedirected lateral or perpendicular assembly of Au NRs can be

envisioned as a unique approach to the fabrication of tailored nanoarchitectures. It is essential that flexible, noncovalent interactions and the solubility of surface-anchored lipids in organic solvents permit a variety of self-assembled configurations. The lipid-modified NRs will function as anisotropic-shaped colorimetric reporters in living cells since the NR complex itself is biocompatible and sensitive to the changes in the local environment surrounding the NR. Furthermore, highly ordered NRs on a solid surface will be utilized for surface enhanced Raman scattering or fluorescence substrates. This is because plasmonic coupling is generated between NRs in close proximity, which results in huge local electromagnetic field enhancements, namely, “hot spots”. Our results for the control of the self-assembly and interparticle spacing of the NRs will provide a guide for hot spot engineering. Supporting Information Available: Detailed Au NR synthesis, the zeta potential for Au NR-DPPTE complex in water, preparation of hydrophilic and hydrophobic silicon substrate, the absorption spectra of the control experiments, SEM images of the assembled structures in a spherical Au nanoparticle-DPPTE complex, and SEM images of the assemblies of Au NR-DPPTE complex in water system versus the DPPTE concentration. This material is available free of charge via the Internet at http://pubs.acs.org. LA8003189