Polyimide Composite Thin Film

10102

Langmuir 2007, 23, 10102-10108

Multilayered Gold-Nanoparticle/Polyimide Composite Thin Film through Layer-by-Layer Assembly Fengxiang Zhang† and M. P. Srinivasan* Department of Chemical and Biomolecular Engineering, the National UniVersity of Singapore, Singapore 117576 ReceiVed December 4, 2006. In Final Form: April 26, 2007 A novel type of composite thin film consisting of gold nanoparticles (AuNPs) and polymide (PI) was fabricated through layer-by-layer (LBL) assembly. To fabricate such films, bare AuNPs and a poly (amic acid) bearing pendant amine groups, namely, amino poly (amic acid) or APAA, were synthesized and assembled in an LBL fashion. Without any organic encapsulation layer on their surface, AuNPs were bound directly to APAA chains at the amine sites; X-ray photoelectron spectroscopy study suggested that the binding was based on a combined effect of metal-ligand coordination and electrostatic interaction, with the former dominating over the latter. An approximately linear growth of the film started from the second layer of AuNP as revealed by the UV-vis spectroscopy, and the degree of particle aggregation was higher in the first AuNP layer than in the subsequent layers due to the differences in the density of binding sites. The resultant assembly was heated to imidize the APAA, thereby creating a robust composite structure.

Introduction Composite films containing gold nanoparticles (AuNPs) dispersed in an organic matrix have attracted much attention in recent years due to their electrical, catalytic, sensing, electrochemical properties. Such films are often built by layer-by-layer (LBL) assembly. For example, colloidal AuNPs encapsulated by poly(diallyldimethylammonium chloride) have been selfassembled into multilayer films in an LBL fashion with anionic poly s-119; the two components are bound together through electrostatic attraction.1 Similarly, multilayer films of AuNPs (protected by sodium citrate) and a water-soluble poly (vinylimidazole) complex with osmium (4,4′-dimethyl-2, 2′bipyridine) chloride were deposited on a glass carbon electrode via electrostatic LBL assembly.2 Due to the nature of ionic bonding, one drawback of the above electrostatic assemblies lies in their vulnerability to the possible pH changes in their working environment or medium. Another type of AuNP composite film is exemplified by one prepared via LBL assembly using dodecylamine-stabilized AuNPs as the main building blocks and hexadecanedithiol or poly (propyleneimine) dendrimer as the linking agent; the dodecylamine ligand was exchanged with thiol or amino dendrimer during film assembly.3 According to Isaacs et al., these films may lack stability compared with polymerlinked nanoparticle assemblies due to the much smaller number of linking groups in dendrimer or dithiol than in the polymer linkers.4 Apart from the above approaches, hybrid structures containing AuNPs was also achieved through in situ formation of AuNPs in a polymer network using chemical or electrochemical methods. For instance, electrochemical polyaniline (PANI)/Au composite was obtained by AuCl4- reduction into a previously * Corresponding author. E-mail: [email protected]. Tel: 65-65162171. † Present address: Institute of Materials Research and Engineering, 3 Research Link, Singapore, 117602. (1) Liu, Y.; Wang, Y.; Claus, R. O. Chem. Phys. Lett. 1998, 298, 315. (2) Qian, L.; Gao, Q.; Song, Y. H.; Li, Z. A.; Yang, X. R. Sens. Actuators, B 2005, 107, 303. (3) Joseph, Y.; Krasteva, N.; Besnard, I.; Guse, B.; Rosenberger, M.; Wild, U.; Schlogl, R.; Krustev, R.; Yasuda, A.; Vossmeyer, T. Faraday Discuss. Chem. Soc. 2004, 125, 77. (4) Isaacs, S. R.; Choo, H.; Ko, W. B.; Shon, Y. S. Chem. Mater. 2006, 18, 107.

electrochemically deposited PANI film, producing a nearly uniform dispersion of Au particles with 150-300 nm diameters.5 In order to ensure long-term stability of the AuNP composite films, matrix materials with adequate robustness are highly desired; in this regard, polyimides will be advantageous over most other polymers. Polyimides are well-known highperformance materials possessing attractive properties such as excellent thermal stability, high mechanical strength, low moisture uptake, favorable dielectric properties, good chemical resistance, and so on.6-8 They have found extensive applications in microelectronic industry9 and membrane fabrication.10 Polyimides are also frequently utilized to make polymer composites, alloys and blends for interesting combinations of physical properties between polyimide and guest species such as chromophores,11 electroactive polymers,12 magnetic particles13 and silver acetate.14 However, there is rather scarce work reported on the fabrication of AuNP/polyimide nanocomposites, apart from the ion implantation technique to obtain AuNP embedded in polyimide films.15 (5) Kinyanjui, J. M.; Hanks, J.; Hatchett, D. W.; Smith, A.; Josowica, M. J. Electrochem. Soc. 2004, 151, D113. (6) Sroog, C. E. Polyimides Prog. Polym. Sci. 1991, 16, 561. (7) Verbicky, J. W. Encyclopedia of Polymer Science and Engineering, 2nd ed.; Wiley-Interscience: New York, 1988; Vol. 12, p364. (8) Ghosh, M. K.; Mittal, K. L. Polyimides: Fundamentals and Applications; Marcel Dekker: New York, 1996. (9) (a) Miwa, T. J. Photopolym. Sci. Technol. 2001, 14, 29. (b) Sidorov, V.; Shai, A.; Ritter, D.; Paz, Y. Surf. Coat. Technol. 1999, 122, 214. (c) Kuntman, A.; Kuntman, H. Microelectron. J. 2000, 31, 629. (d) Savinskii, N. Proc. SPIEInt. Soc. Opt. Eng. 2004, 5401, 136. (10) (a) Claudia, S. B.; Koros, W. J. J. Membr. Sci. 1999, 155, 145. (b) Kawakami, H.; Mikawa, M.; Nagaoka, S. J. Appl. Polym. Sci. 1996, 62, 965. (11) (a) Kenis, P. J. A.; Oscar, F. J. N.; Niek, F. H.; Johan, F. J. E.; David, N. R.; Benno, H. M. H.; Cornelis, P. J. M. V. Chem. Mater. 1997, 9, 596. (b) Yokoyama, S.; Kakimoto, M.; Imai, Y.; Yamada, T.; Kajikawa, K.; Takezoe, H.; Fukuda, A. Thin Solid Films 1996, 273, 254. (12) (a) Wang, J.; Srinivasan, M. P. Synth. Met. 1999, 105, 1. (b) Tieke, B.; Gabriel, W. Polymer 1990, 31, 20. (c) Zhang, F.; Srinivasan, M. P. Thin Solid Films 2005, 479, 95. (13) Liu, Y.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 2114. (14) Southward, R. E.; Thompson, D. S.; Thompson, D. W.; Clair, A. K. St. Chem. Mater. 1997, 9, 1691. (15) Ferna´ndez, Cd. J.; Manera, M. G.; Spadavecchia, J.; Maggioni, G.; Quaranta, A.; Mattei, G.; Bazzan, M.; Cattaruzza, E.; Bonafini, M.; Negro, E.; Vomiero, A.; Carturan, S.; Scian, C.; Della Mea, G.; Rella, R.; Vasanelli, L.; Mazzoldi, P. Sens. Actuators, B 2005, 111-112, 225.

10.1021/la0635045 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/24/2007

Gold-Nanoparticle/Polyimide Composite Thin Film

Langmuir, Vol. 23, No. 20, 2007 10103

In this work, multilayered composite films comprising polyimide and AuNPs were formed by LBL assembly using a freshly prepared sol of naked AuNPs and an in-house synthesized amino poly (amic acid), or APAA; thermal treatment after LBL assembly was performed for imidization purpose. Since there is no organic encapsulation layer on their surface, AuNPs can bind directly to the APAA chains in the assembling process, and the binding was found to be mainly based on coordination chemistry, which is advantageous over electrostatic interaction or van der Waals force in terms of robustness of the composite structures formed. Experimental Section Materials. Hydrogen tetrachloroaurate (III) trihydrate (HAuCl4· 3H2O) (g99.9%, Aldrich; corrosive, handle with care), sodium citrate (meeting USP testing specifications, Sigma) and methanol (g99.5%, Merck) were used as received. 3,3′,4,4′-Tetraaminobiphenyl (TAB) (g99%, Sigma) was stored below 4 °C and used without further purification. 4,4′-Hexafluoroisopropylidene)diphthalic anhydride (6FDA) (99%, Aldrich) was recrystallized from acetic anhydride (g99.0%, Fluka). N-Methyl-2-pyrrolidone (NMP, g 99.5%) was distilled immediately before use. p-Aminophenyltrimethoxysilane (APhS, 90%, containing other isomers) was purchased from Gelest, stored in a desiccator under vacuum and used without further purification. Toluene (g99%, Merck) was distilled over sodium before use. N,N-dimethylacetamide (DMAc) (g99%, Merck) was purified by distillation. (NMP, toluene and DMAc are all highly toxic, and should be handled with care.) Silicon (100) wafers (Wellbond Manufacturing Services Pte Ltd., Singapore) were 0.6 mm thick, p-doped, polished on one side and had a natural oxide layer. Synthesis of Gold Nanoparticles (AuNP). AuNP was prepared according to the method reported by Grabar et al.16 Specifically, 0.012 g of HAuCl4·3H2O was dissolved in 30 mL of deionized (DI) water (with a resistivity of 18.2 MΩ cm) giving a 1 mM solution, which was then refluxed at ca. 115 °C for 10 min. To the refluxing solution was added 3 mL of 38.8 mM sodium citrate also dissolved in DI water. The color of the mixture evolved gradually from gray to purple and finally to wine red. Refluxing was continued for another 10 min and then was stopped to let the resulting solution cool to room temperature. Synthesis of Amino Poly(Amic Acid), or APAA, and Its Imidization. A 0.3350 g sample of TAB was dissolved in 18 mL of freshly distilled NMP; 0.6683 g of 6FDA was then added slowly while stirring and under nitrogen purge. The system was stirred in the fume cupboard overnight at room temperature. The resulting mixture was poured into a copious amount of stirred methanol; after stirring for 4 h, the precipitate was collected by filtering, washed repeatedly with methanol, collected by filtering, and then vacuumdried overnight at 60 °C. White-colored APAA powder soluble in both DMAc and NMP was obtained. For imidization, a small amount of APAA was cured under nitrogen atmosphere first at 100 °C for 90 min and then at 350 °C for another 90 min. After such curing, the powder became yellow and insoluble in both DMAc and NMP. Composite Film Construction. Silicon wafers and glass slides were treated with a 7/3 (v/v) mixture of concentrated sulfuric acid and 30% hydrogen peroxide (caution: this “piranha” solution reacts violently with many organic materials and should be handled with extreme care) and rinsed with deionized water and methanol; they were then immersed in a 3 mM APhS solution in toluene at room temperature for 2 h. Subsequently, the substrates were rinsed copiously with toluene, sonicated in toluene for 10 min, rinsed again with toluene, methanol, and blown dry with compressed nitrogen. The treated silicon wafers and glass slides were immersed in the freshly prepared AuNP solution for 1 h, rinsed thoroughly with DI water and then baked at 120 °C for 10 min. Next, the samples were immersed in a 5 mM DMAc solution of APAA (also freshly prepared) (16) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735.

Figure 1. TEM image of gold nanoparticles. for 1 h, rinsed repeatedly with DMAc and methanol, and blown dry with nitrogen. The above two steps were performed alternately until 7 cycles were completed. Finally, the assembly was thermally cured at 200 °C under vacuum for 2 h. Characterization. The synthesized AuNPs were studied by transmission electronic micrograph (JEM-2010, JEOL). The electron beam accelerating voltage of the microscope was set at 200 kV. The sample was prepared by depositing a drop of AuNP sol onto a copper grid coated with carbon film. Fourier transform infrared spectroscopy (FTIR) spectra of the synthesized APAA and API powder were obtained in air on a BioRad FTIR Model-400 spectrophotometer by accumulating 16 scans at a resolution of 4 cm-1. Pressed pellets of KBr containing APAA or API were used as samples, and pure KBr pellet as background. The sample compartment was purged with nitrogen. APAA, API, and the assembled composite films at different assembly steps were analyzed by X-ray photoelectron spectroscopy (XPS). Dried APAA and API powders and the assembled films (on Si substrate) were glued onto XPS sample studs using double-sided tape. The analyses were made on a Kratos Analytical AXIS HSi spectrometer with a monochromatized Al KR X-ray source (1486.6 eV photons) at a constant dwell time of 100 ms and a pass-energy of 40 eV. The core-level signals were obtained at a photoelectron takeoff angle of 90° (with respect to the sample surface). All binding energies were referenced to the C1s hydrocarbon peak at 284.6 eV. In curve fitting, the full width at half-maximum (FWHM) for the Gaussian peaks was maintained constant for all components in a particular spectrum. Surface morphologies were investigated using a Nanoscope III atomic force microscope (AFM). All images were collected in air using the tapping mode and a monolithic silicon tip. The drive frequency was 330 ( 50 kHz, and the voltage was between 3.0 and 4.0 V. The drive amplitude was about 300 mV and the scan rate was 0.5-1.0 Hz. UV-visible absorption spectra were recorded on a Shimadzu UV-3101 PC scanning spectrophotometer operating at a resolution of 1 nm. Pure water was used as the background for AuNP sol spectroscopy recording; in the case of assembled films, a bare glass slide was used as the background.

Results and Discussion An aqueous sol of gold nanoparticles (AuNPs) was prepared by reducing HAuCl4·3H2O, a precursor for AuNPs, with sodium citrate; after reduction, the citrate ions in the sol may help to prevent particle aggregation. The AuNPs obtained had an average diameter of 11.6 ( 2.3 nm based on the particle size shown in the TEM picture (Figure 1); this result is similar to literature reported sizes of AuNPs synthesized using the same method.16,17

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Figure 2. UV-vis spectra of gold nanoparticles at different time points. Scheme 1. Route for APAA Synthesis and Its Imidization

Figure 3. FTIR spectra of (A) amino poly (amic acid) and (B) amino polyimide.

The particles were fairly stable over prolonged storage, as shown by UV-vis spectroscopy in Figure 2, in which the as-prepared, 1-day- and 2-day-aged samples all showed surface plasmon absorption at a wavelength of 522 nm with absorption bands of identical shape; this band verifies the presence of individual, instead of aggregated particles.18 The stability of the particles was also indicated by the observation that the inner surface of the beaker holding gold sol for several days did not show any trace of color. Amino poly (amic acid) (APAA) and amino polyimide (API) were synthesized from 6FDA and 3,3′,4,4′-tetraaminebiphenyl (TAB), following the route shown in Scheme 1. Because TAB is in excess in the polymerization system (2.4 mmol of TAB and 1.5 mmol of 6FDA, the latter added slowly to the former; see experimental section), 6FDA will condense with TAB preferentially in the way that only one amine on each phenyl ring of TAB is consumed, and the other amine remains free during polymerization since steric hindrance will inhibit reaction at adjacent amines. Resulting from the above manner of polymerization, virtually no or insignificant degree of cross-linking (17) Mayya, K. S.; Caruso, F. Langmuir 2003, 19, 6987. (18) Jiang, C.; Markutsya, S.; Tsukruk, V. V. Langmuir 2004, 20, 882.

Figure 4. XPS spectra of (A) amino poly (amic acid) and (B) amino polyimide.

occurred during polymerization, and the resultant APAA is soluble in organic solvents such as DMAc, NMP, N,N-dimethylformide and tetrahydrofuran, and moderately soluble in water; after imidization, it became insoluble in all of the above solvents. The FTIR spectra of both APAA and its imidized product (API) are shown in Figure 3 (curves A and B, respectively). Absorption bands for amine (N-H stretching) are well defined in both spectra, i.e., 3363 and 3231 cm-1 in spectrum A, and 3367 and 3204 cm-1 in spectrum B.19 These bands show that amino groups are still available after polymerization and imidization, and further corroborate that the extent of crosslinking in polymerization, if any, is minimal and has not affected the availability of the amine groups. The presence of imide functionality in API is confirmed by the bands at 1760 cm-1 (symmetric CdO stretch) and 1726 cm-1 (asymmetric CdO stretch) in spectrum B.20,19(c) (19) (a) Szafran, M.; Kowalczyk, I.; Katrusiak, A. J. Mol. Struct. 2006, 786, 25. (b) White, L. D.; Tripp, C. P. J. Colloid Interface Sci. 2000, 232, 400. (c) Albrecht, W.; Seifert, B.; Weigel, T.; Schossig, M.; Hollander, A.; Groth, T.; Hilke, R. Macromol. Chem. Phys. 2003, 204, 510.

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Figure 5. Tapping mode AFM images of the AuNP/APAA assembly at different steps: (A) APhS-modified substrate; (B) first layer AuNP; (C) first layer APAA; (D) second layer AuNP; (E) seventh layer AuNP. In each micrograph, the left half is a height image and the right is a phase image. All the images have a scan size of 2 µm × 2 µm.

N1s core-level XPS spectra (Figure 4) further confirm the presence of amino groups and provide a comparison on their abundance relative to other nitrogen-containing moieties in both APAA and API. Spectrum A is the N1s spectrum for APAA, which is fitted into primary amine, amide and protonated amine components with binding energy values of 398.7, 399.8, and 401.2 eV, respectively.21 The amine and amide components possess almost equal areas, implying that they carry the same percentage, and this agrees well with the expected structure of APAA (Scheme 1). The N1s core-level spectrum for API (spectrum B) is also fitted into three components: primary amine at 398.8 eV, imide at 400.5 eV,22 and protonated amine at 401.3 eV, the first two having a peak area ratio of 0.9:1. This ratio is also consistent with the expected structure of API molecule. In both spectra, the peak component for protonated amine is evidently small compared with the other two components, demonstrating that ammonium carboxylate formation is minor during APAA synthesis, due to the weakness of poly (amic acid). The amine-terminated substrate for the composite film construction was prepared by modifying silicon wafer or glass slide surfaces with p-aminophenyltrimethoxysilane (APhS); the resultant substrate featured a uniform surface morphology, which can be seen from the AFM image in Figure 5A. AuNPs were adsorbed from a freshly prepared aqueous solution onto the amineterminated substrate, leading to the formation of a dense layer (20) (a) Pai, I. T.; Leu, I. C.; Hon, M. H. J. Micromech. Microeng. 2006, 16, 2192. (b) Sullivan, D. M.; Bruening, M. L. J. Membr. Sci. 2005, 248, 161. (21) (a) Schick, G. A.; Sun, Z. Langmuir 1994, 10, 3105. (b) Zhang, F.; Srinivasan, M. P. Langmuir 2004, 20, 2309. (22) (a) Zhao, W. W.; Boerio, F. J. Surf. Interface Anal. 1998, 26, 316. (b) Jia, Z.; Srinivasan, M. P. Colloids Surf., A 2005, 257-258, 183.

Figure 6. XPS wide scans for the AuNP/APAA assembly at different steps: (A) APhS-modifed substrate; (B) AuNP layer; (C) APAA layer; (D) the second AuNP layer.

of particles, whose surface topography is shown in Figure 5B, where agglomeration of AuNPs can be clearly seen. In the next step, the assembly was exposed to a 5 mM solution of APAA in DMAc, and a layer of polymer was anchored on the surface of the AuNPs. This adsorption resulted in a noticeable smearing of the AuNP features and the appearance of small aggregates which are formed by random coils of the polymer chains (Figure 5C). These aggregates are typically observed in multilayered ultrathin films fabricated by LBL assembly between two different

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Figure 7. N1s core-level XPS spectra at various steps of the AuNP/APAA assembly: (A) the APhS-modified substrate; (B) the first AuNP layer; (C) the first APAA layer; (D) the seventh AuNP layer; (E) the seventh AuNP layer after thermal curing at 200 °C; (F) the Au4f core-level spectrum for the first AuNP layer.

polymers,23-24 or between a polymer and nanoparticles or other formats of building blocks.25-26 When the assembly obtained above (APAA terminated) was subsequently treated with the solution of AuNPs, a new layer of particles were adsorbed; these particles were less densely populated compared with those on the APhS-modified substrate, and there is no significant agglomeration occurring between the particles, as shown in Figure 5D. One possible reason for this difference is that the binding sites (amine groups) provided by APAA chains or aggregates are more sparsely populated than those from the amine-terminated substrate (the binding mech(23) Breit, M.; Gao, M.; von Plessen, G.; Lemmer, U.; Feldmann, J.; Cundiff, S. T. J. Chem. Phys 2002, 117, 3956. (24) Viinikanoja, A.; Areva, S.; Kocharova, N.; Aaritalo, T.; Vuorinen, M.; Savunen, A.; Kankare, J.; Lukkari, J. Langmuir 2006, 22, 6078. (25) Zhao, S.; Zhang, K.; An, J.; Sun, Y.; Sun, C. Mater Lett. 2006, 60, 1215. (26) Mwaura, J. K.; Pinto, M. R.; Witker, D.; Ananthakrishnan, N.; Schanze, K. S.; Reynolds, J. R. Langmuir 2005, 21, 10119.

anism will be elaborated in later paragraphs). A similar influence of binding sites on the immobilization of AuNPs was reported by Yonezawa et al.,27 who found that the particles are packed less densely on poly (ethyleneimine) substrate than on a uniform ammonium layer due to the smaller abundance and uniformity of binding sites of the latter. Figure 5E shows the surface feature of the seventh AuNP layer. X-ray photoelectron spectroscopy (XPS) wide scans (Figure 6) were performed to examine the stepwise buildup of the composite film. The APhS-modified substrate showed a clear signal for nitrogen at the binding energy of near 398 eV (spectrum A). When AuNPs were immobilized, the nitrogen signal nearly disappeared (spectrum B), indicating that the amine substrate was almost fully covered with AuNPs and therefore almost inaccessible to XPS detection; meanwhile, a strong gold signal appeared at around 86 eV. After adsorption of APAA onto the (27) Yonezawa, T.; Onoue, S.; Kunitake, T. AdV. Mater. 1998, 10, 414.

Gold-Nanoparticle/Polyimide Composite Thin Film

Figure 8. UV-vis absorption spectra of the APAA/AuNP at various steps. Curve 1 corresponds to the AuNP layer on the APhS-modified substrate; curves 2-7 correspond to AuNP layers immobilized on APAA.

AuNP layer, the gold signal became a bit weaker, accompanied by the appearance of a fluorine signal from APAA and a nitrogen signal stronger than that in the previous step (spectrum C). When the second layer of AuNPs was adsorbed, the gold signal became stronger again (spectrum D), but not as strong as that for the first AuNP layer, probably due to the lower density of the second layer. Presented in Figure 7 are the N1s core-level XPS spectra following different steps in the composite film construction. Spectrum A was obtained from the APhS-treated substrate, in which the single-component peak (at 398.8 eV) is attributed to the primary amine. Upon AuNP adsorption, the nitrogen peak became weakened (also see the wide-scan spectrum in Figure 6) and can be curve-fitted into a single component with a similar binding energy as the primary amine (spectrum B). We assign this single peak to amines coordinated with AuNPs and this assignment agrees with reported nitrogen binding energies for amine-gold interaction. Kumar et al.28 reported a binding energy of 399.3 eV for nitrogen from laurylamine bound to the surface of AuNPs. Manna et al.29 synthesized AuNPs encapsulated by the fourth-generation poly (amido amine) dendrimer, PAMAM, and found the binding energy of nitrogen (from PAMAM amine) was 399.2 eV; they concluded that the Au-amine interaction was strong but did not further specify the nature of the interaction. When APAA was assembled on to the AuNP layer, the nitrogen peak (spectrum C) was curve-fitted into three components at 398.6, 399.5, and 401.2 eV, assignable to amine (both free and coordinated to gold), amide (from APAA), and protonated amine or ammonium (also from APAA), respectively;21 the amine component was predominant over the other two. This suggests that the binding between APAA and AuNP is based on both metal-ligand coordination (between metallic gold atom and amine) and electrostatic interaction (between the ammonium and the negative charges on the surface of the particles), the former dominating over the latter. After the seventh layer of AuNP was assembled, the nitrogen peak (spectrum D) could also be fitted into three components, whose binding energies and assignments are the same as in spectrum C. Subsequent to the (28) Kumar, A.; Mandal, S.; Selvakannan, P. R.; Pasricha, R.; Mandale, A. B.; Sastry, M. Langmuir 2003, 19, 6277. (29) Manna, A.; Imae, T.; Aoi, K.; Okada, M.; Yogo, T. Chem. Mater 2001, 13, 1674.

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Figure 9. Absorbance of the AuNP/APAA assembly at 520 nm as a function of the number of AuNP layers.

seventh AuNP layer, the sample was thermally cured, and the imide functionality was detected by XPS (the peak component at 400.1 eV in spectrum E),22 though imidiation was incomplete. The core-level Au4f spectrum of AuNPs after being immobilized on APhS layer is shown as spectrum F in Figure 7. The two peak components at 83.9 eV (4f 5/2) and 87.6 eV (4f 7/2) confirm the metallic state of the particle, and are consistent with those for the AuNPs encapsulated by primary amines reported by Leff et al.30 and Kumar et al.;28 the former showed that the interaction between gold and amine was weak covalent bonding and the latter found both electrostatic interaction and complexation existed. Thus, the Au4f spectrum further corroborates the inference that coordination is responsible for the binding between AuNPs and amine in the composite assembly. The LBL assembly was also studied by UV-visible spectroscopy and the spectra are shown in Figure 8. When AuNPs were immobilized on the APhS-modified substrate, two plasmon bands are observed in curve 1: one at ca. 520 nm, which is assigned to the plasmon resonance of individual particles, and the other at ca. 700 nm, associated with a collective surface plasmon oscillation due to particle aggregation.31,16 Subsequent layers of AuNP, which were added on APAA, all gave spectra featuring a plasmon band at ca. 520 nm and a shoulder at ca. 700 nm (see curves 2-7). The presence of a shoulder, instead of a full band at 700 nm as in the case of the first layer, indicates a lower density of particles as compared with the first layer, and this density remained virtually stable starting from the second layer based on the similarity of shoulders in curves 2-7. Our observations herein are markedly different from those in the work of Ung et al., who made an electrostatic LBL assembly using bare AuNPs and polydiallyl dimethylammonium (PDDA) chloride, and found a progressively stronger absorption band at ca. 700 nm with increasing number of deposition cycles due to the increased density of particles and thus increased dipole coupling between particles.32 The difference may also suggest that the AuNP/ APAA binding mechanism is different from that for AuNP/PDDA: the former is dominated by coordination while the latter is purely electrostatic, because APAA is a weak acid, (30) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (31) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629. (32) Ung, T.; Liz-Marza´n, L. M.; Mulvaney, P. J. Phys. Chem. B. 2001, 105, 3441.

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instead of an ammonium salt in its DMAc solution. In addition to the two plasmon bands for AuNPs, one absorption band at around 390 nm appeared in all the spectra, which may originate from π-π* transition of the aniline moieties (benzene ring coupled with amine, from APhS and APAA); such an absorption band has been reported for similar moieties.33 Figure 9 shows the dependence of absorbance of the AuNP/ APAA assembly at 520 nm on the number of assembled AuNP layers. It reveals a nonlinear LBL film buildup behavior: the first and second layers contributed more significantly to the film growth compared with the subsequent layers. The first AuNP layer (built on the APhS modified substrate) is dense and shows significant particle aggregation and resulted in high absorbance; the second layer should also possess reasonable coverage in view of its underlying APAA, which is directly supported by the dense first AuNP layer, and thus created comparable absorbance as the first layer. However, the particle density and degree of aggregation in the second layer will be lower than that in the first, which may lead to less abundant APAA assembled onto the second AuNP layer, and consequently less abundant AuNPs in the third layer. The growth of assembly became stable and approximately linear starting from the second adsorption of AuNPs probably because the influence of the large density of the first AuNP layer became weakened and in each addition step the particles had almost the same underlying APAA substrate. The assembled multilayer film showed good stability toward thermal treatment and solvent etch. As detailed in the experimental section, the film fabrication process involved repeated 10 min (33) (a) Hartmann, V.; Losche, M.; Mello, S. V.; Oliveira, O. N. Mater. Sci. Eng., C 1999, 8-9, 425. (b) Sivakumar, C.; Gopalan, A.; Vasudevan, T.; Wen, T. C. Synth. Met. 2002, 126, 123. (c) Liou, G. S.; Hsiao, S. H.; Chen, W. C.; Yen, H. J. Macromolecules 2006, 39, 6036.

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bakes at 120 °C (for water removal after AuNP adsorption), repeated 1 h DMAc soaks (for APAA adsorption) and a final imidization step at 200 °C for 2 h. These treatments are evidently harsher than those in most reported methods of building multilayer AuNP composites;1-5 however, the film could withstand such treatment and exhibited stepwise growth as shown by XPS results and UV-vis spectroscopy.

Conclusions We have described a method for making multilayered AuNP/ PI composite films using LBL assembly. In-house synthesized AuNP (naked, without stabilizing agent) and APAA (bearing amino pendant groups) were used for the film fabrication. The binding force between the two building blocks was shown to be a combined effect of metal-ligand coordination and electrostatic interaction, with the former being predominant. The stepwise growth of the film was confirmed by UV-vis spectroscopy, and the degree of particle aggregation was found to be lower starting from the second layer as compared with the first; this behavior is markedly different from the assembly based on purely electrostatic attraction. The inherent robustness of PI and the coordination-based interlayer binding force may ensure a good stability of the resultant film. The approach described herein can be easily extended to the construction of multilayered composite films containing other nanoparticles such as silver, palladium, copper, and so on. Acknowledgment. The authors thank the National University of Singapore for providing financial support for this project and a scholarship for Fengxiang Zhang. LA0635045