Didodecyldimethylammonium Bromide Lipid Bilayer-Protected Gold

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Didodecyldimethylammonium Bromide Lipid Bilayer-Protected Gold Nanoparticles: Synthesis, Characterization, and Self-Assembly Lixue Zhang,†,‡ Xuping Sun,†,‡ Yonghai Song,†,‡ Xiue Jiang,†,‡ Shaojun Dong,*,† and Erkang Wang*,† State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, China ReceiVed October 20, 2005. In Final Form: December 21, 2005 Didodecyldimethylammonium bromide (DDAB) lipid bilayer-protected gold nanoparticles (AuNPs), which were stable and hydrophilic, were synthesized by in situ reduction of HAuCl4 with NaBH4 in an aqueous medium in the presence of DDAB. As-prepared nanoparticles were characterized by UV-vis spectra, transmission electron microscopy, dynamic light scattering analysis, and X-ray photoelectron spectroscopy. All these data supported the formation of AuNPs. Fourier transform infrared spectroscopy (FTIR) and differential thermal analysis/thermogravimetric analysis data revealed that DDAB existed in a bilayer structure formed on the particle surface, resulting in a positively charged particle surface. The FTIR spectra also indicated that the DDAB bilayer coated on the surface of AuNPs was probably in the ordered gel phase with some end-gauche defects. On the basis of electrostatic interactions between such AuNPs and anionic polyelectrolyte poly(sodium 4-styrenesulfonate) (PSS), we successfully fabricated (PSS/AuNP)n multilayers on a cationic polyelectrolyte poly(ethylenimine) coated indium tin oxide substrate via the layer-by-layer self-assembly technique and characterized as-formed multilayers with UV-vis spectra and atomic force microscopy.

Introduction Nanosized particles of noble metals hold promise for use as advanced materials with novel electronic, optical, and thermal properties as well as catalytic properties due to their potential applications in the fields of physics, chemistry, biology, medicine, and material science and their different interdisciplinary fields,1 and therefore, the synthesis and characterization of metal nanoparticles have attracted considerable attention from a fundamental and practical point of view.2 To date, a variety of methods have been developed to prepare metal nanoparticles, and many reviews are now available.3 However, chemical reduction of gold salts is still a general route. Since nanoparticles tend to be fairly unstable in solution, special precautions have to be taken to avoid their aggregation or precipitation. The most common strategy is the use of a protective agent, which not only prevents their aggregation but also results in functionalized particles.4 During the past few years, bilayer-protected nanostructures have attracted increasing research efforts due to their water solubility, biocompatibility, and potential applications in many fields. After Sastry and co-workers demonstrated that nanoscale curvature enables the formation of interdigitated bilayers of fatty acid on a colloidal silver particle surface,5 a wealth of papers * To whom correspondence should be addressed. Fax: 86-431-5689711. E-mail: [email protected]. E-mail: [email protected]. † Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences. (1) (a) Fendler, J. H., Ed. Nanoparticles and Nanostructured Films; VCH: Weinheim, Germany, 1998. (b) Klabunde, K. J., Ed. Nanoscale Matrials in Chemistry; VCH: Weinheim, Germany, 2001. (2) (a) Kiwi, J.; Gratzel, M. J. Am. Chem. Soc. 1979, 101, 7214. (b) Schmid, G. Chem. ReV. 1992, 92, 1709. (c) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (3) (a) Roucoux, A.; Schulz, J.; Patin, H. Chem. ReV. 2002, 102, 3757. (b) Daniel; M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (4) Rao, N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P. Chem. Soc. ReV. 2000, 29, 27. (5) Patil, V.; Mayya, K. S.; Pradhan, S. D.; Sastry, M. J. Am. Chem. Soc. 1997, 119, 9281.

appeared on the successful preparation of water-soluble, bilayerprotected nanostructures, including noble metal nanoparticles,6-12 metal nanorods,13,14 magnetic nanoparticles,15 and semiconductor quantum dots.16 In previous studies, the formation of bilayerprotected nanoparticles usually involved multiple steps. Recently, Sastry and co-workers have developed a simple one-step preparative method of cetyltrimethylammonium bromide bilayerprotected gold nanocrystals of various morphologies using an aqueous foam as a template.11 Didodecyldimethylammonium bromide (DDAB), a simple artificial cationic lipid, was widely used in the study of vesicles and other biomemebrane models.17-19 DDAB has also been used as a protective reagent for the formation of monolayer-protected gold nanoparticles (AuNPs) in an organic medium in our group.20 More recently, Zhu et al. have synthesized L-R-dipalmitoylphosphatidylcholine (DPPC) monolayer-capped gold nanoparticles at a water/toluene interface by a one-step process.21 Compared with those lipid monolayer-protected nanoparticles dispersed in (6) Patil, V.; Sastry, M. Langmuir 1998, 14, 2707. (7) Sastry, M.; Mayya, K. S.; Patil, V. Langmuir 1998, 14, 5921. (8) Lala, N.; Chittiboyina, A. G.; Chavan, S. P.; Sastry, M. Colloids Surf., A 2002, 205, 15. (9) Swami, A.; Kumar, A.; Sastry, M. Langmuir 2003, 19, 1168. (10) Cheng, W. L.; Dong, S. J.; Wang, E. K. Electrochem. Commun. 2002, 4, 412. (11) Mandal, S.; Arumugam, S. K.; Adyanthaya, S. D.; Pasricha, R.; Sastry, M. J. Mater. Chem. 2004, 14, 43. (12) Fan, H. Y.; Yang, K.; Boye, D. M.; Sigmon, T.; Malloy, K. J.; Xu, H. F.; Lopez, G. P.; Brinker, C. J. Science 2004, 304, 567. (13) Nikoobakht, B.; El-sayed, M. A. Langmuir 2001, 17, 6368. (14) Gole, A.; Orendorff, C. J.; Murphy, C. J. Langmuir 2004, 20, 7117. (15) Shen, L.; Laibinis, P. E.; Hatton, T. A. Langmuir 1999, 15, 447. (16) Fan, H. Y.; Leve, E. W.; Scullin, C.; Gabaldon, J.; Tallant, D.; Bunge, S.; Boyle, T.; Wilson, M. C.; Brinker C. J. Nano Lett. 2005, 5, 645. (17) Rupert, L. A. M.; Hoekstra, D.; Engberts, J. B. F. N. J. Am. Chem. Soc. 1985, 107, 2628. (18) Caboi, F.; Monduzzi, M. Langmuir 1996, 12, 3548. (19) Subramanian, G.; Hjelm, P. P.; Deming, T. J.; Smith, G. S.; Li, Y.; Safinya, C. R. J. Am. Chem. Soc. 2000, 122, 26. (20) Han, X. J.; Cheng, W. L.; Zhang, Z. L.; Dong, S. J.; Wang, E. K. BBAs Bioenergetics 2002, 1556, 273. (21) Zhu, H. F.; Tao, C.; Zheng, S. P.; Li, J. B. Colloids Surf., A 2005, 257258, 411.

10.1021/la052822l CCC: $33.50 © 2006 American Chemical Society Published on Web 02/09/2006

DDAB Lipid Bilayer-Protected Gold Nanoparticles

an organic phase, however, better biocompatibility and more potential applications can be expected when the lipid bilayerprotected nanoparticles are dispersed in water. To the best of our knowledge, there is no work reporting the preparation of DDAB lipid bilayer-protected nanoparticles. In this study, we develop a facile one-step method for the preparation of very stable, DDAB lipid bilayer-protected AuNPs in an aqueous medium by in situ chemical reduction of HAuCl4 with NaBH4 in the presence of DDAB. The formation of DDAB-protected gold nanoparticles was evidenced by UV-vis spectra, transmission electron microscopy (TEM), dynamic light scattering (DLS) analysis, and X-ray photoelectron spectroscopy (XPS). The structure of DDAB capped on the gold nanoparticles was determined by Fourier transform infrared spectroscopy (FTIR) and differential thermal analysis/thermogravimetric analysis (DTA/TGA), and it was found that the AuNPs were protected by the DDAB bilayer in the ordered gel phase with some end-gauche defects and thus had a positively charged surface. In addition, the UV-vis and atomic force microscopy (AFM) studies showed that the asprepared AuNPs can be fabricated into (PSS/AuNP)n multilayers on a cationic poly(ethylenimine) (PEI)-coated indium tin oxide (ITO) substrate via the layer-by-layer (LBL) self-assembly technique,22 which should be due to electrostatic interactions between such AuNPs and anionic polyelectrolyte poly(sodium 4-styrenesulfonate) (PSS). Experimental Section Chemicals. HAuCl4, NaBH4, PEI (90%, Mw ≈ 15000), and PSS (Mw ≈ 70000) were purchased from Aldrich (America). DDAB was purchased from Acros (Belgium). All reagents were used as received without further purification. The water used was purified through a Millipore system. Synthesis of DDAB Lipid Bilayer-Protected AuNPs. The DDAB lipid bilayer-protected gold nanoparticles were prepared according to the following procedure: First, 375 µL of 0.048 M HAuCl4 aqueous solution and 60 mL of ∼1.4 mM DDAB aqueous solution were mixed together under vigorous stirring. Then 200 µL of freshly prepared 0.4 M NaBH4 aqueous solution was added to the former stirred solution drop by drop. The as-formed wine-colored solution was stored at room temperature for further characterization. Preparation of Samples for Different Kinds of Characterizations. For the LBL self-assembly experiment, the DDAB lipid bilayerprotected AuNP solution was centrifuged two times at 14000 rpm/ min at room temperature for 20 min to remove the uncoordinated lipid molecules from solution as much as possible. The top solution was decanted off, and the bottom fraction of AuNPs was saved. The resulting heavily concentrated solution of AuNPs was then diluted to a suitable concentration for the LBL self-assembly experiment. The transparent ITO glass was sonicated in acetone for 5 min, followed by rinsing with water, ultrasonic agitation in concentrated NaOH in 1:1 (v/v) water/ethanol, rinsing further with water, immersing in CHCl3 for 10 min, and drying with a high-purity nitrogen stream. Before assembly, a precursor film of PEI was deposited onto a clean ITO surface by adsorption of PEI from its 1 mg mL-1 aqueous solution (for 20 min). The ITO/PEI substrate was then immersed in the polyanionic PSS solution (1 mg mL-1) for 20 min, giving an ITO/PEI/PSS substrate. After being thoroughly rinsed with water and dried with a nitrogen stream, the as-formed substrate was transferred into the DDAB lipid bilayer-protected AuNP solution for 30 min, producing an ITO/PEI/PSS/AuNP substrate. For the construction of AuNP multilayers, the modified ITO glass was then alternately dipped into PSS and AuNP solutions until the desired numbers of layers were obtained. The resulting films were used to record UV-vis spectra and AFM images to follow the deposition process. Samples for TEM characterization were prepared (22) Decher, G., Schlenoff, J. B., Eds. Multilayer Thin Films; Wiley-VCH: New York, 2002.

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Figure 1. UV-vis spectrum of the DDAB-protected gold nanoparticles dispersed in water. The inset shows the photographs of the as-prepared gold nanopaticles before (A) and after (B) the addition of chloroform. by placing a drop of the as-prepared gold colloidal sample on a carbon-coated copper grid and then drying at room temperature. For XPS characterization, the sample was prepared by depositing a drop of the as-prepared colloidal solution onto a clean glass slide and then drying in air. The removal of water from the colloidal solution by heating to about 90 °C in a nitrogen atmosphere and then drying under vacuum gave a dry powder of DDAB lipid bilayer-protected AuNPs. Both the pure DDAB powder and the as-prepared AuNP powder were then mixed with KBr to press into transparent disks. The prepared disks were used to get the FTIR results. For the DTA/ TGA experiments, the samples were the pure DDAB powder and the as-prepared AuNP powder. Measurements. UV-vis spectra of the DDAB lipid bilayerprotected AuNPs in aqueous solution and the ITO glass modified with DDAB lipid bilayer-protected AuNPs were recorded using a CARY 500 Scan UV-vis-NIR spectrophotometer. TEM measurements were made on a JEOL 2010 transmission electron microscope operated at an accelerating voltage of 200 KV. The DLS experiment was conducted with a vertically polarized He-Ne laser (DAWN EOS, Wyatt Technology) at a fixed scattering angle of 90° and at a constant temperature of 25 °C. X-ray photoelectron spectra were collected on an ESCLAB MKII using Mg KR radiation as the exciting source. The charging calibration was performed by referring C1s to the binding energy at 284.6 eV. All the FTIR measurements of pure DDAB powder and the as-prepared AuNP powder were acquired using a Bruker Vertex 70 FTIR spectrometer operated at a 4 cm-1 resolution. DTA/TGA experiments were performed with a TA SDT2960 instrument at a heating rate of 7 °C/min in a nitrogen environment. AFM images were taken by using a Nanoscope IIIa instrument from Digital Instruments (Santa Barbara, CA) at room temperature under ambient conditions. All images were recorded in the tapping mode using standard silicon nitride tips. The surface was scanned at 1-2 Hz with 256 lines per image resolution.

Results and Discussion The formation of gold nanoparticles was first evidenced by UV-vis spectra of the colloidal gold solution thus formed, as shown in Figure 1. The peak of the plasmon resonance band is located at 521 nm, which indicates the formation of gold nanoparticles.23 The inset (Figure 1) shows the photograph of the as-prepared colloidal aqueous solution (photo A). After being mixed with chloroform, it can be seen that the AuNPs are still maintained in the aqueous phase (inset B of Figure 1), indicating (23) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, T.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706.

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Figure 3. XPS spectrum of Au 4f of the as-prepared DDAB-protected gold nanoparticles deposited on the glass slide.

Figure 4. FTIR spectra of the DDAB-protected AuNPs (a) and pure DDAB (b). All spectra were obtained in the form of KBr plates.

Figure 2. Typical TEM image (A), corresponding size distribution histogram (B), and DLS data (C) of the DDAB-protected gold nanoparticles.

that the as-prepared gold nanoparticles are hydrophilic. Considering the long-term stability (this colloidal solution can be stored for at least five months without observable aggregation under air conditions), we consider that DDAB serves as an effective protective reagent for gold nanoparticles. A TEM image provides further evidence to support the formation of gold nanoparticles, as shown in Figure 2A. It is clearly seen that most of the gold nanoparticles are spherical in shape and well-separated from each other. It is reported that aggregation of gold nanoparticles results in the occurrence of longitudinal plasmon resonance;24 however, the UV-vis spectrum shows the absence of longitudinal plasmon resonance, also indicating no particle aggregates exist in the same preparation.

Figure 2B shows the corresponding particle size distribution histogram of these gold particles. It is found that the particles range from 2 to 16 nm in size and mainly consist of particles about 4-6 nm in diameter. The size distribution of such nanoparticles was further examined by a DLS experiment (Figure 2C). DLS data suggest that the as-formed gold nanoparticles mainly range from 7 to 11 nm in the aqueous solution. Obviously, the particle size by DLS analysis is larger than the particle size by TEM analysis, which may be attributed to the formation of DDAB-protected gold nanoparticles. To further confirm the formation of Au atoms from HAuCl4, XPS was used to examine the change in oxidation states for Au after the reaction had occurred. Figure 3 shows the XPS spectrum of gold nanoparticles thus formed. The spectrum shows two peaks centered at binding energies of 83.2 and 86.9 eV, which can be assigned to the Au 4f7/2 peak and Au 4f5/2 peak, respectively, confirming the formation of metal Au.25 (24) Yu, Y.; Chang, S.; Lee, C.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661.

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Figure 5. (A) DTA data recorded for the DDAB-protected AuNPs (curve 1) and pure DDAB (curve 2). (B) TGA data recorded for the DDAB-protected AuNPs (curve 1) and pure DDAB (curve 2) (top part) and their derivates for the DDAB-protected AuNPs (curve 1) and pure DDAB (curve 2) (bottom part).

To prove that as-prepared gold nanoparticles are indeed protected by DDAB, we performed FTIR measurements. Figure 4 shows the FTIR spectra of DDAB-protected AuNPs (a) and pure DDAB (b). At the high-frequency region (as shown in Figure 4A), the IR spectrum of DDAB-protected AuNPs (curve a) is similar to that of pure DDAB (curve b). It is well-known that the symmetric and asymmetric stretching vibrations of methylene can be used as an indicator of the ordering of the alkyl chains and higher energies of stretching vibrations correspond to more gauche defects.26-29 The symmetric and asymmetric stretching vibrations of methylene of DDAB coated on AuNPs are located at 2854 and 2924 cm-1, respectively, and have little or no shift to higher frequencies than those of DDAB (Figure 4A), indicating that there is possibly a small increase of the number of gauche defects for DDAB coated on AuNPs compared to that for pure DDAB. The little or no shift of vibrational frequencies also indicates the high surface coverage of DDAB on the gold nanoparticles,30 which can be attributed to the fact that the higher surface coverage restricts the liberty of movement of the methylene chains more seriously than lower surface coverage. In the low-frequency region (as shown in Figure 4B), for AuNPs the intensities of C-N+ stretching bands of DDAB molecules at 919, 941, and 966 cm-1 decrease clearly and some new bands at 826, 1026, and 1097 cm-1 are formed. Peaks at 826, 1026, and 1097 cm-1 are probably the stretching modes of C-N+ to the gold surface.13 The characteristic C-H‚‚‚metal vibration has not been observed, strongly indicating the absence of C-H binding to the gold surface.31 The peaks located at 721 and 1467 cm-1 for both DDAB-protected AuNPs and pure DDAB (25) Jaramillo, T. F.; Baeck, S.; Cuenya, B. R.; McFarland, E. W. J. Am. Chem. Soc. 2003, 125, 7148. (26) Naselli, C.; Rabolt, J. F.; Swalen, J. D. J. Chem. Phys. 1985, 82, 2136. (27) Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43, 437. (28) Hostetle, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604. (29) Cheng, W. L.; Dong, S. J.; Wang, E. K. Langmuir 2003, 19, 9434. (30) Kung, K. H. S.; Hayes, K. F. Langmuir 1993, 9, 263.

are assigned to the rocking mode and scissoring motion of the methylene chains, respectively. Though the methyl symmetric bending vibration of DDAB-protected AuNPs (at 1378 cm-1) is shifted to higher frequency than that of pure DDAB (at 1374 cm-1), it is relatively insensitive to the conformation for the methylene chains because this mode is independent of other motions.28 Compared with that of the pure DDAB, the peak at 890 cm-1 for DDAB-protected AuNPs, which is assigned to the in-plane methyl rocking mode, decreases obviously. Meanwhile, the peak settled at 945 cm-1 increases clearly, implying that DDAB-protected AuNPs have more higher energy end-gauche defects, characteristic of a single gauche C-C bond on the end of an all-trans methylene chain, than pure DDAB.28 An obvious spectral change is also observed at the C-C stretching vibration region. We could observe both the trans C-C bond (RT) and gauche C-C bond (RG) modes located at 1131 and 1082 cm-1, respectively, in the spectrum of DDAB-protected AuNPs. The presence of a gauche C-C stretching vibration in the spectra is indicative of either a near-surface defect or an internal kink in the DDAB lipid bilayers.28,29 Additionally, the peak observed at 1347 cm-1 for DDAB-protected AuNPs also means the presence of a chain end-gauche defect.28 The FTIR data reveal that some end-gauche defects appear when DDAB is tethered to the gold nanoparticles, which is possibly due to the higher curvature of the gold nanoparticles. However, the presence of twisting-rocking or wagging progression bands (1258 and 1277 cm-1) in curve a is strong evidence that the microenvironment of the DDAB bilayers protected on the gold nanoparticles are probably in the crystal state,28 which may be attributed to the following two reasons: (1) DDAB itself is an instinctly double-tailed structure, and the packing parameter (p) of DDAB is 1/2 < p < 1, which is critical to the formation (31) Manner, W. L.; Bishop, AR.; Girolami, G. S.; Nuzzo, R. G. J. Phys. Chem. B 1998, 102, 8816.

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Figure 6. UV-vis absorption spectra of PEI/(PSS/AuNP)n multilayers on the ITO substrate with different numbers of PSS/AuNP layers: from (a) to (d), 1, 2, 3, and 4, successively. The inset shows the absorbance at 521 nm versus the number of layers.

of a vesicle structure.32 (2) The high surface coverage of DDAB on the gold nanoparticles restricts the liberty of the DDAB molecules. It is reported that Br- of CTAB can be strongly adsorbed on the gold surface, the cationic CTAB headgroups surround the Br- layer, and the corresponding hydrocarbon chains are extended outward, resulting in the formation of stable, CTAB monolayer-protected, hydrophobic gold nanoparticles dispersed in an organic phase.29 However, in the present study, the resulting gold nanoparticles are dispersed in an aqueous medium, and therefore, it is energetically favorable to form the DDAB bilayer structure by virtue of the van der Waals hydrophobic force of the double tails of DDAB, which leads to the formation of hydrophilic gold nanoparticles. On the basis of these observations, we suggest that the gold nanoparticles are capped by a DDAB bilayer, which is probably in the ordered gel phase with some end-gauche defects. In fact, considering the actual thickness of the DDAB bilayer (about 1.93 nm),33 the size of the DDABprotected nanoparticles obtained from the DLS experiment is nearly equal to the sum of the TEM result and double thickness of the DDAB bilayer, indicating that a DDAB bilayer is indeed formed on gold nanoparticles. DTA/TGA was used to further investigate the structure of DDAB coated on the surface of AuNPs. Figure 5A corresponds to the DTA data. TGA plots and their derivates are shown as the top part and bottom part of Figure 5B, respectively. In all parts of the figure, curve 1 corresponds to the data recorded for the DDAB-protected AuNPs and curve 2 to the data obtained from pure DDAB. The DTA curve of pure DDAB shows five endothermic peaks located at 64.6, 77.6, 173.3, 222.0, and 265.8 °C successively. However, for DDAB-protected AuNPs, there are six endothermic peaks at 58.1, 74.4, 168.5, 221.8, 261.4, and 291.2 °C successively. Peaks at 64.6 °C (Figure 5A, curve 2) and 58.1 °C (Figure 5A, curve 1) should be the main phase transition temperatures (Tm) of pure DDAB and DDAB coated on the AuNPs, respectively. At Tm, the double hydrocarbon chains of DDAB are from the gel (or ordered) phase transformed into the liquid crystalline (or fluid) phase. The mechanism of peaks at 77.6 °C (Figure 5A, curve 2) and 74.4 °C (Figure 5A, curve 1) is not very clear yet, and the two peaks may be correlated to the change of the (32) Recoul, F.; Duboil, M.; Zemb, T.; Plusquellec, D. Eur. Phys. J. B 1998, 4, 333. (33) Einaga, Y.; Sato, O.; Iyoda, T.; Fujishima, A.; Hashimoto, K. J. Am. Chem. Soc. 1999, 121, 3745.

Figure 7. AFM images of the ITO/PEI/PSS substrate (A), ITO/ PEI/(PSS/AuNP)1 films (B), and ITO/PEI/(PSS/AuNP)4 multilayers (C). (D) is the height distribution histogram corresponding to (B).

orderliness of the small headgroups of DDAB. Peaks settled at 173.3 °C (Figure 5A, curve 2) and 168.5 °C (Figure 5A, curve 1) should be the melting temperature of pure DDAB and DDAB coated on the AuNPs, respectively, at which the electrostatic interactions among the headgroups of DDAB are destroyed.34 All three peaks of DDAB-protected AuNPs are smaller than those of pure DDAB, which should be attributed to the higher disorderliness when DDAB is tethered to the gold nanoparticles.

DDAB Lipid Bilayer-Protected Gold Nanoparticles

Above the melting temperature, every endothermic process is accompanied by a corresponding weight loss process. From the TGA curve and its derivate of pure DDAB, we could find two major weight losses with relevant loss peaks at 222 and 263 °C. The first peak starts at 173.3 °Csthe melting temperature of pure DDABsand is assigned to desorption of the DDAB molecules in monomer form. The second one at 263 °C is attributed to desorption of the DDAB molecules in the aggregated form. The TGA data show nearly complete weight loss at 400 °C. However, for DDAB-protected AuNPs, there are three major weight losses with related peaks at 222, 259, and 291 °C and one minor weight loss begins from the temperature rising process, which is due to the loss of moisture entrapped in the DDABprotected AuNP powder.9 The first peak at 222 °C starts at 168.5 °C, which is the melting temperature of the DDAB-protected AuNP powder, and it should be attributed to desorption of the DDAB molecules in the monomer form in the DDAB-protected AuNP powder. The second peak located at 259 °C should be assigned to desorption of the DDAB molecules in aggregated form as well as the outer layer of the DDAB bilayer coated on the AuNPs. The third band at 291 °C should be related to desorption of the DDAB molecules which are bonding to the AuNPs.13 The weight loss process of DDAB-protected gold nanoparticles is very similar to that of cationic surfactant bilayerprotected gold nanorods reported by El-Sayed.13 The DTA/TGA data further confirm that the DDAB coated on AuNPs in aqueous solution is in bilayer form. Also note that about 20% of the mass of the AuNP powder corresponding to metal Au and salts remains until 400 °C. We also fabricated AuNP-containing multilayers on an ITO glass via the layer-by-layer self-assembly technique. Figure 6 shows the UV-vis spectra of such a multilayer with increasing n of (PSS/AuNP)n on an ITO/PEI substrate. It can be seen that an absorption of the ITO/PEI/(PSS/AuNP)n multilayers occurs at 521 nm, and a second one begins to appear at about 600 nm and strengthens with sequential deposition. The absorbance at 521 nm corresponding to the surface plasmon resonance band of DDAB-protected AuNPs is observed to increase linearly with the number of layers. This phenomenon indicates that a regular and uniform multilayer film has been structured.35 The absorbance beginning at around 600 nm should be attributed to the coupled plasmon resonance of gold nanoparticles which were in close contact when the polyelectrolyte and nanoparticles were assembled.36 It is expected that the electrostatic interactions between the negatively charged PSS and positively charged AuNPs are (34) Sui, S. F. Molecular Membrane Biology ( in Chinese); Higher Education Press: Beijing, 2003; p 247. (35) Araki, K.; Wagner, M. J.; Wrighton, M. S. Langmuir 1996, 12, 5393. (36) Shipway, A. N.; Lahav, M.; Gabai, R.; Willner, I. Langmuir 2000, 16, 8789.

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responsible for the successful creation of such multilayer structures. Note that no AuNPs are adsorbed on the ITO substrate modified with a PEI monolayer, providing another piece of evidence to support the formation of positively charged, DDAB bilayer-protected AuNPs. AFM experiments were conducted to characterize the morphologies of the ITO/PEI/PSS substrate (Figure 7A), ITO/PEI/ (PSS/AuNP)1 films (Figure 7B), and ITO/PEI/(PSS/AuNP)4 multilayers (Figure 7C). Because of the deposition of PEI and PSS polyelectrolyte layers, the ITO substrate exhibits a relatively smooth surface morphology with a root-mean-squared roughness of 1.65 nm (Figure 7A). After the immobilization of one layer of DDAB lipid bilayer-protected AuNPs, the ITO becomes rougher with a roughness of 2.32 nm. Lateral dimensions are distorted due to the convolution of the AFM tip. The crosssectional contour plots of surface topography give particle heights mostly of 7-10 nm for the immobilized gold nanoparticles (Figure 7D). This result is quite consistent with the DLS data. From Figure 7C, we can see that the surface morphology of the ITO/ PEI/(PSS/AuNP)4 film is dominantly comprised of individual, uniformly distributed nanoparticles and some aggregated clusters of AuNPs. The root-mean-squared roughness of the ITO/PEI/ (PSS/AuNP)4 surface is 5.68 nm, which indicates the surface becoming much rougher. In addition, it can be seen that, with an increasing number of layers, the density of gold nanoparticles is enhanced, which is consistent with the result of the UV-vis experiment.

Conclusion In conclusion, very stable, lipid-protected gold nanoparticles dispersed in a water medium can be easily prepared by in situ chemical reduction of HAuCl4 in DDAB aqueous solution. When the preparation is carried out in a water medium, a DDAB bilayer structure is expected to form on the particle surface, which gives particles with a hydrophilic surface. It is found that the DDAB molecules on the surface of gold nanoparticles are indeed in the bilayer structure and the DDAB bilayer is probably in the ordered gel phase with some end-gauche defects. The DDAB lipid bilayerprotected nanoparticles have also been successfully fabricated into multilayers on the substrate via alternative assembly with a negatively charged polyelectrolyte. Acknowledgment. This work was supported by the National Natural Science Foundation of China via Grant Nos. 20335040 and 20275036 and National Key Basic Research Development Project 2001 CB5102. LA052822L