Plasmon Resonance Tunable by Deaggregation of Gold

[PEO/PAA-AuNP(r)]n multilayer films with the outermost layer of PEO are obtained ...... B. V. Parakhonskiy , M. F. Bedard , T. V. Bukreeva , G. B. Suk...
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J. Phys. Chem. C 2007, 111, 10082-10087

Plasmon Resonance Tunable by Deaggregation of Gold Nanoparticles in Multilayers Conghua Lu, Helmuth Mo1 hwald, and Andreas Fery*,† Interface Department, Max Planck Institute of Colloids and Interfaces, Potsdam, 14424, Germany ReceiVed: March 20, 2007; In Final Form: May 3, 2007

In this paper we report polyelectrolyte multilayer films containing gold nanoparticles (AuNPs) whose spectroscopic properties can be controlled by post-treatment. When building multilayers from hydrogenbonded poly(ethylene oxide) and polyacrylic acid-decorated AuNPs, a commonly observed red-shift of the nanoparticles’ plasmon resonance occurs. For this system, however, this effect can be reversed to a large degree with the film growth and further by post-treatments such as annealing. The effects of water evaporation and film structure changes such as the interdigitation of assembled layers and the induced deaggregation of AuNPs with a uniform distribution in the assembly films have been well investigated.

Introduction Gold nanoparticles (AuNPs), as the most typical noble metallic NPs, have been intensively investigated due to their importance for fundamental studies and applications in the areas of biology, catalysis, and nanotechnology.1 Especially the localized surface plasmon resonance (SPR) of AuNPs, originated from the collective oscillations of surface conduction electrons induced by an electric field (light), has been fully exploited as optical and photonic devices and label-free sensors (or biosensors) with high sensitivities and no photobleaching.1-3 For example, probing the phosphatase activity4 and DNA hybridization1b-e,3 has been performed on a single molecular level through a simple colorimetric and optical detection. It is based on changes in the refractive index of the surrounding medium and the SPR coupling of AuNPs. Here, AuNPs are dispersed in solution or deposited on self-assembed monolayers (SAMs). However, few papers have reported utilizing them to provide information on the internal nanostructures of AuNP-based multilayer films except for their response to the surrounding solvents. Recently, Tsukruk and his co-workers have studied in depth the molecular chain reorganization in the course of elastic deformations of free-standing AuNPs including films with surface-enhanced Raman scattering.5 On the other hand, substantial efforts have been made to modulate the optical properties of as-prepared AuNP solutions/ films by adjusting the SPR-sensitive parameters such as the size, size distribution, shape, interparticle distance, as well as the dielectric constant of the surrounding environment of AuNPs.6-11 For example, AuNP-based multilayer assembly films have been successfully fabricated via dipping layer-by-layer (LbL) selfassembly8,12 or the efficient spin-assisted assembly11,13 and spraying LbL assembly,14 in combination with different driving forces (e.g., electrostatic interaction,11,12a,b,d-g hydrogen bonding,12c and host-guest interaction15). The resulting functional hybrid films with controlled nanostructures and optical properties from individual AuNP, or interlayer (or intralayer) interparticle coupling of AuNPs have been obtained by means of different * Author to whom correspondence should be addressed. Tel: + + 49(0)921-55-2753. Fax: + + 49(0) 921-55-2059. E-mail: [email protected]. † Present address: Department of Physical Chemistry, University of Bayreuth, 95440, Germany.

building blocks including different generations of dendrimers,12f mixed spherical and planar AuNPs,8 and different thicknesses of the polyelectrolyte separation layer inserted into the interlayer of AuNPs.11,12a,16 It is always observed that a red-shift of the SPR absorption band of assembled AuNPs occurs in the assembly process,8,12e,f,14 compared to that of AuNPs in the solution. This red-shift can be mainly explained by the change of the surrounding medium,7,11,12e,16 the decrease in the distance between AuNPs,10a,12f,16 and the formation of aggregated AuNP clusters.1c,12e,14 For fully exploiting the power of gold nanoparticles for sensing applications, control over their plasmon resonance with tailorable red-shifts and peak broadening due to aggregation is of vital importance, which is still a great challenging task. In particular, due to the strong tendency to irreversibly aggregate, a reversible color change is difficult to obtain. In the present paper, we report for the first time that the redshift can be reversed with the film growth and further by posttreatment of the as-prepared films, pointing toward an effective way to control optical characteristics of the film. In this study, films are composed of hydrogen-bonded poly(ethylene oxide) (PEO) and poly(acrylic acid)-capped AuNP (PAA-AuNP). It is believed that the expected self-healing of AuNP-aggregated film nanostructures involved in the above process results in the above optical modulation. It should be noted that the polymeric elastomeric matrix of the hydrogen-bonded PEO/PAA film has been investigated extensively, including its pH/ionic strengthsensitive disintegration and thermal/mechanical properties.17 Experimental Methods Materials. Poly(ethylene oxide) (PEO, MV ∼ 200 000), poly (sodium 4-styrenesulfonate) (PSS, Mw ∼ 100 000), poly(ethylenimine) (PEI, Mw ∼ 25 000), poly(acrylic acid, sodium salt) (PAA, Mw ∼ 30 000), poly(allylamine hydrochloride) (PAH, Mw ∼ 70 000) were purchased from Sigma-Aldrich and used without purification. Preparation of AuNPs capped by PAA was carried out according to refs 6e and 12b with some modifications. Namely, in a 100 mL mixed solution composed of 4 mM PAA and 1 mM AuCl4-, 1 mL of 0.1 M NaBH4 aqueous solution was added with strong magnetic stirring. After an additional 15 min stirring, the mixed solution was kept at 4 °C for later assembly. Here the molar ratio (r) of PAA and

10.1021/jp072214+ CCC: $37.00 © 2007 American Chemical Society Published on Web 06/15/2007

Tunable Plasmon Resonance and AuNP Deaggregation

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Figure 1. Typical UV-vis spectrum evolution of [PEO/PAA-AuNP(4:1)]20 (a) and [PEO/PAA-AuNP(2:1)]20 (b) films in the assembly process on a quartz wafer with the terminating layer of PEO, respectively. The absorbance of the substrate has been subtracted. The relationship of the number of assembly bilayer with the absorbance at the SPR absorption peak (c) and the SPR absorption peak (d) of [PEO/PAA-AuNP(r)]20 film, respectively: r ) 4:1 (I, 9); 16:1 (II, b); 32:1 (III, O); 16:0.25 (IV, 0); 2:1 (V, 2).

Au is 4:1 and the resulting NPs are simply represented as PAAAuNPs(4:1). Similarly, AuNPs capped by PAA with r ) 2:1 [PAA-AuNPs(2:1)] were obtained when the mixed solution was composed of 2 mM PAA and 1 mM AuCl4-. But for other PAAAuNPs with a designed molar ratio of PAA and Au, PAA was directly added to the above as-prepared PAA-AuNP(4:1) solution with or without pre-dilution to obtain PAA-AuNP(16: 0.25), PAA-AuNP(16:1), and PAA-AuNP(32:1) solutions. Before the layer-by-layer assembly, the pH of the assembled solutions of PEO and PAA-capped AuNPs were adjusted to 2.5 with HCl. The un-dissolved PEO was removed by centrifugation at 3000 rpm for 0.5 h. The substrates of silicon wafer as well as glass and quartz slides were cleaned in 5:1:1 (vol %) H2O/ H2O2/NH3 mixed solution following the RCA protocol. Fabrication of PEO/PAA-AuNP Multilayer Films. PEO/ PAA-AuNP multilayer films were fabricated via the typical dipping layer-by-layer (LbL) self-assembly. Namely, the abovetreated substrate was first immersed in 1 mg/mL PEI solution to obtain a PEI-coated substrate and then alternatively immersed into the solutions of 1 mg/mL PSS and 1 mg/mL PAH to obtain a (PSS/PAH)3 film with the outermost layer of PAH. Later, the substrate was alternatively immersed into the solutions of PAAcapped AuNPs (pH ) 2.5) and 1 mg/mL PEO (pH ) 2.5) for 15 min, respectively; 3 min water (pH ) 2.5) washing and subsequent N2 drying were inserted between the assembly procedure. [PEO/PAA-AuNP(r)]n multilayer films with the outermost layer of PEO are obtained on the substrate after simply repeating the above assembly procedure n times. Here, r also represents the molar ratio of PAA and Au in the used assembly solution of PAA-AuNPs. Characterization. Infrared (IR) spectra of the films scratched from the substrate were recorded on a Bruker Equinox 55/S Fourier transform spectrometer equipped with a liquid nitrogen cooled MCT detector for reflection-absorption measurements. UV-vis spectroscopy was carried out on a CARY 50 Conc (Varian) spectrophotometer after completion of each assembly

cycle (the terminating layer of PEO) on a quartz slide. A NanoWizard atomic force microscope (AFM) (JPK Instrument, Berlin) was operated in tapping mode with the silicon probe (NC-W) for AFM images with a typical frequency of 285 kHz. Transmission electron microscopy (TEM) was performed on a Zeiss EM 912 Omega microscope with an accelerating voltage of 120 kV. Results and Discussion The as-prepared AuNPs stabilized with PAA (PAA-AuNPs) are about 5-10 nm in diameter with the typical SPR absorption peak of ∼523 and 532 nm for PAA-AuNPs(4:1) and PAAAuNPs(2:1) in the solution, respectively (Figure S1, Supporting Information). Other PAA-AuNP solutions with r ) 16:0.25, 16: 1, and 32:1 were prepared through directly adding PAA to the as-synthesized PAA-AuNP(4:1) solution, and the sizes of diluted AuNPs almost remained constant according to the TEM images (not shown). Here, the hydrogen bonding between PEO and the capping agent of PAA is used to drive the growth of PEO/ PAA-AuNP multilayer films at pH ) 2.5 because PAA is fully protonated at this pH value.17 It is proved by the monitored IR spectra of PEO/PAA-AuNP films (Figure S2, Supporting Information). Obviously, at pH ) 3, PAA has some ionized carboxylic groups because of the presence of the -CO2- peak at 1549 cm-1 in the assembly film.18 In order to improve the stability of hydrogen-bonding films on the substrate such as a quartz slide, a precursor (PSS/PAH)3 film with the terminating layer of PAH was deposited on the PEI-coated substrate at first, just as described in the experimental section. Figure 1a shows a representative UV-vis spectrum evolution during the film growth of [PEO/PAA-AuNP(4:1)]20 on a quartz wafer with the outermost layer of PEO. With respect to [PEO/ PAA-AuNP(r)]20 films with r ) 16:0.25, 16:1, and 32:1, the spectrum evolution is very similar to that of [PEO/PAA-AuNP(4:1)]20 films. Obviously, the absorption in the visible region is

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Lu et al.

Figure 2. Typical effect of 60 °C annealing on the UV-vis spectra of as-prepared [PEO/PAA-AuNP(4:1)]18 (a, b); (a) is the magnification of the circled part in (b) and the arrows represent the direction of the spectrum change during annealing. (c) shows the corresponding relationship of the resulting SPR peak and the annealing time at different annealing temperatures: 60 (0) and 90 °C (b).

attributed to the typical SPR absorption of AuNPs because the polymeric matrix of PEO/PAA exhibits negligible absorption. The derived relationship of number of assembly bilayers with the absorbance at the SPR peak and the corresponding peak position is also displayed in Figure 1, parts c and d, respectively. It is noted that the real peak positions derived from the spectra are accurate with a precision of 1 nm, but there is sometimes a larger variation (up to 3 nm) with the sample history. The absorbance at the SPR peak is linear to the number of assembly bilayers, indicating a regular deposition of PAA-AuNP on PEO/ PAA-AuNP films in the assembly process, regardless of the used PAA-AuNP assembly solutions with different molar ratios (r) of PAA and Au. The composition of the resulting films such as the fraction of AuNP incorporated into the films has also been manipulated (Figure 1c). As expected, utilization of a PAAAuNP assembly solution with a lower molar ratio of PAA and Au leads to a film with a higher content of AuNP, agreeing with the color change of the resulting composite films from light to deep red colors (Figure S3a-d, Supporting Information). We think that the added PAA molecules in the assembly solution are in a free state or attached on the surface of AuNPs. These two kinds of PAA molecules will compete with the interaction of PEO, resulting in the decrease of AuNPs deposited in the film if more PAA is added to the solution. Furthermore, at the beginning of several assembly bilayers (typical 2-4), there exists the familiar red-shift of the SPR peak of AuNPs in the assembled films, compared to that of PAA-AuNPs in the solution (Figure 1, parts a and d). This kind of red-shift has been well documented including the change of refractive index of the surrounding environment,7,11,12e,16 the distance between AuNPs,10a,12f,16 and the size of aggregated AuNP clusters.1c,12e,14 It is supported by the fact that a higher molar ratio (r) of PAA and Au in the assembly solution results in a smaller red-shift because of a lower coverage of AuNPs incorporated with a bigger distance of interparticles and better distribution in the assembled film. For example, the red-shift for PEO/PAA-AuNP films with r ) 16:0.25 (Figure 1d, IV) and r ) 4:1 (Figure 1d, I) corresponds to ∼12 and 50 nm, respectively. With the further growth of the multilayer film, however, the SPR maximum absorption of AuNPs increasingly moves toward the short wavelength (simply named as the blue-shift) until the bilayer number arrives at 12-14 and then stays constant (Figure 1d, I-IV). For the PEO/PAA-AuNP films with r ) 16:0.25, 32:1, 16:1, and 4:1, the corresponding shift toward the short wavelength is ∼7, 11, 16, and 24 nm, respectively. This blue-shift with the film growth is consistent with the observed trend in the case of infiltration of AuNPs into as-prepared polyelectrolyte

multilayers (PEMs) with an increasing number of bilayers (i.e., film thickness).16 As for the dark-red multilayer film of PEO/PAA-AuNP(2:1) (Figure S3e, Supporting Information), two broad absorption bands appear (at ∼565 and 765 nm) during the film buildup (Figure 1b), obviously different from the above-mentioned films with only one absorption band in the visible region. For the first one, the same red-shift of the SPR absorption exists at the beginning of 6-8 bilayers and then remains almost constant with ∼2 nm fluctuation at ∼562 nm in the following assembly, whereas the second peak is more and more pronounced with the film growth. The whole two-absorption-band evolution is similar to the result of AuNPs deposited on SAMs with prolonged adsorption time,1a,7,11 and it is much more similar to the case of densely packed AuNP layers embedded in PEMs.11 Therefore, the absorption band at ∼565 nm originates from the SPR peak of an individual AuNP, while the second broad absorption band at ∼765 nm cannot be assigned to the SPR peak of AuNPs induced by intralayer interparticle coupling because of its typical absorption band falling in the range of 600-660 nm.1a,7,11 It is more likely to result from the interlayer interparticle coupling which has been observed in the region of 780-800 nm in the well-designed multilayer film with AuNP layers separated by a suitable thickness of PEMs.11 This means that the PEO/PAA-AuNP(2:1) film has a more stratified structure than PEO/PAA-AuNP(r) films with other r values, which will be involved in the following discussion. We investigate the effect of annealing on the optical properties of the as-prepared [PEO/PAA-AuNP(4:1)]18 film (Figure 2). At 60 °C, the SPR peak increasingly shifts from the original 547 nm to the final 534 nm (i.e., ∼13 nm blue-shift) with an increase in the heating time. Meanwhile, the absorbance at the SPR peak also increases during heating. Indeed, from the inset without amplification shown in Figure 2a, it is seen that the integrated intensity of the SPR peak decreases in the annealing process (Figure 2b). At higher-temperature annealing, such as at 90 °C, there exists a ∼17 nm blue-shift with a higher rate (Figure 2c). As for the multilayer films of PEO/PAA-AuNP with r ) 16: 0.25, 16:1, and 32:1, they have a similar tendency with a smaller blue-shift (typically ∼3 nm) under the same conditions. IR spectra of the annealed films show that no new chemical bonds have been formed in the film below 110 °C annealing, while at 165 °C, the hydrogen bonding of PAA-PAA can be converted into the covalent anhydride bond due to the appearance of typical anhydride absorption at about 1800 cm-1 coupling with the shift of the CdO peak toward a higher frequency (Figure S2, Supporting Information).17c,18 It is noted that longer annealing,

Tunable Plasmon Resonance and AuNP Deaggregation

Figure 3. Effect of 90 °C annealing on the UV-vis spectra of asprepared [PEO/PAA-AuNP(2:1)]18 film. The inset shows the relationship of the absorbance at 765 and 1000 nm with annealing time, respectively.

especially at a higher temperature such as at 165 °C, will lead to the formation of larger AuNP clusters because the SPR peak again moves toward a long wavelength. But in the case of [PEO/ PAA-AuNP(2:1)]18 multilayer films, there exists a different spectrum evolution during annealing under the same conditions (Figure 3). For the first absorption band at ∼565 nm, there is a similar main tendency with a smaller blue-shift during annealing, just as shown in Figure 2a; however, for the second broad peak at ∼765 nm, the absorbance continuously decreases with annealing until the peak disappears. For the part between 850 and 1100 nm, the absorbance decreases at the beginning of 1 h annealing and then increases again with prolonged annealing time, just as indicated by the solid black arrows in Figure 3. These distinct changes are more obvious to see through the relationship of the absorbance at 765 and 1000 nm with the annealing time (inset, Figure 3). The surface morphologies of the films before and after 90 °C annealing were further studied with AFM (Figure 4). In our case, an AFM phase image is better and clearer than the corresponding AFM height image to record the underlayer

J. Phys. Chem. C, Vol. 111, No. 27, 2007 10085 AuNPs because the PEO layer is the outermost one. Obviously, AuNPs with a smaller diameter are more uniformly distributed in the annealed films of [PEO/PAA-AuNP(4:1)]18 (Figure 4b) and [PEO/PAA-AuNP(2:1)]18 (Figure 4d), compared to those that have some aggregations and nonuniform distribution of AuNPs to some degree before annealing (Figure 4a,c). The annealing-induced film nanostructure changes with deaggregation, and ordered layout of AuNPs are believed to be related to the blue-shift of the SPR peak. Additionally, in comparison with the [PEO/PAA-AuNP(4:1)]18 film (Figure 4a), the underlayer of AuNPs below the PEO layer in [PEO/PAA-AuNP(2:1)]18 film is more difficult to discern before heating (Figure 4c), implying that the former has a more stratified structure which has been deduced from the SPR peak at ∼765 nm (Figure 1b). This kind of blue-shift toward low wavelengths also occurs when the films are simply kept at room temperature in air or pH ) 2.5 solution (Figure 5). Compared with the above annealing, the blue-shift is far slower in the current case. For example, in air, about 18 days are needed to reach an equilibrium state with the final SPR peak of 532 nm. When the film is kept in a pH ) 2.5 solution, it needs ∼4 days to reach an equilibrium state where the final peak is at ∼542 nm with a smaller blueshift (∼6 nm). Even for the PEO/PAA-AuNP(2:1) film, which is kept at room temperature for a longer time such as 1 month, the broad absorption band at 765 nm also disappears, just as occurred in the 90 °C annealing film. These results indicate that the blue-shift of the SPR peak of AuNPs arises from the same internal structure changes during the post-annealing and waiting in air. The above novel well-tailored optical properties of AuNPs can be used to analyze the internal structure change of multilayer assembly films. It is accepted that a two-step process is involved in multilayer formation with a fast adsorption occurring within seconds to minutes, and a much slower reorganization process, which can occur as slowly as over months.19 Especially in the current short deposition time, the average thickness of one PEO/ PAA-AuNP bilayer can reach ∼25 nm, far thicker than that of a typical bilayer of PEs and PE/inorganic NPs. In our case, it is expected that two kinds of hydrogen bonding exist in the

Figure 4. (1 × 1 µm2) AFM phase images of as-prepared [PEO/PAA-AuNP(4:1)]18 (a, b) and [PEO/PAA-AuNP(2:1)]18 (c, d) before (a, c) and after (b, d) 90 °C annealing for 6 h.

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Lu et al. some assembly systems including inorganic NPs involved and have also been investigated with Fo¨rster energy transfer22 and small-angle neutron reflectivity, and so forth.23 As a result, the surrounding medium of AuNPs has been changed and the aggregated AuNPs have been disaggregated and distributed more uniformly (Scheme 1). This leads to the blue-shift of the SPR peak of AuNPs (Figure 1) and the disappearance of the absorption peak at ∼765 nm because of the formation of an intermixed structure of the PEO/PAA-Au(2:1) film (Figure 3). Naturally, in the post-treatment process, a higher temperature speeds up the mobility of polymer chains and the self-healing process and leads to a fast blue-shift, just as shown in Figures 2 and 3. Similarly, in aqueous solution, water-induced swelling of the film and water’s plasticization effect also result in a faster blue-shift than in air (Figure 5). We further find that if the film is stored in a desiccator at room temperature, the SPR peak can remain constant for a long time. It implies that it is possible to freeze the optical properties of the AuNP-based films at a designed stage. It is worthy of pointing out that unfortunately, the 90 °C annealing-induced exchange of hydrogen bonding between PEO-PAA and PAAPAA has not been obviously reflected by the IR spectrum change at ∼1726 and ∼1700 cm-1 (Figure S2, Supporting Information). We think it may be due to the formed composite film without 1:1 stoichiometry (PAA is far more than PEO). Indeed, it has been observed that the purely polymeric assembly film of PEO/PAA fabricated at pH ) 2.25 is composed of 30 mol % PEO.17c Of course, the complicated effect of adsorbed water in the assembly films should be considered because the hydrogenbonded PEO/PAA-AuNP multilayer films are in a hydrated state. In our case, we can simply and roughly estimate the effect of water evaporation on the dielectric constant of the surrounding medium and the SPR peak of AuNP, according to the following equation and the theoretical data shown in Figure 9b of ref 7: where , y, and n represent the dielectric constant, the molar 12b,19-23

Figure 5. The SPR peak evolution of as-prepared [PEO/PAA-AuNP(4:1)]15 films kept at room temperature in air (9) or in pH ) 2.5 aqueous solution (b), respectively.

SCHEME 1: Schematic Illustration of the Internal Nanostructure Change of PEO/PAA-AuNP Multilayer Films during the Layer-by-Layer Film Buildup and Subsequent Post-treatments (e.g., annealing)a

a The displacement of hydrogen bonding of PAA-PAA with energyfavorable hydrogen bonding of PEO-PAA leads to the formation of a more stable film with interpenetrated nanostructures.

PEO/PAA-AuNP multilayer films: hydrogen bonding of PEOPAA and PAA-PAA (i.e, intermolecular dimers of PAA, typically in a lower pH solution17,18a). It is supported by the monitored IR spectra of the assembly film (Figure S2, Supporting Information). The higher-frequency band (∼1726 cm-1) beside the broad and asymmetry band at ∼1700 cm-1 of the non-annealing film may be ascribed to the hydrogen-bonded CdO due to the hydrogen bonding of PEO and PAA, while the band at ∼1700 cm-1 is the characteristic hydrogen-bonded CdO of dimers, which is similar to the result of PEO/poly(methacrylic acid) blends.18a The PAA-PAA interaction will lead to the formation of aggregated AuNPs to some degree (Scheme 1), especially in the case of AuNPs without enough stabilizing agent of PAA on their surface (Figure 4c). As a result, it is observed that a relatively stratified film structure occurs in the PEO/PAA-AuNP(2:1) film, in agreement with the derived results of AFM phase images (Figure 4c) and UV-vis spectra (Figure 1b). In addition to this, the formation of agglomerates and islands is also well related to the expected mechanism that PE molecules not only pull NPs toward the surface but also bridge NPs both in solution and on the surface.20 With the increase in residence time, the hydrogen bonding of PAA-PAA will be automatically and increasingly converted into the more stable and energetically favored PEO-PAA interaction. It is indicative of the presence of layer interpenetration between neighboring layers of PEO and PAA-capped AuNPs (Scheme 1). These interdigitated nanostructures have been observed in

∆ ) 2 - 1 ) [ynpolymer + (1 - y)nair]2 - [ynpolymer + (1 - y)nwater]2 ratio of polymer, and the refractive index of the surrounding medium of AuNPs, respectively. Here, npolymer ≈ 1.5, nwater ) 1.33, and nair ) 1. If y ) 0.5 (i.e., the equal molar ratio of polymer and water around AuNPs), then ∆ ≈ 0.44 and the blue-shift is estimated to be ∼10 nm, which is still smaller than the experiment value of ∼17 nm. In addition, we have not considered the effect of the decrease of the film thickness (∼20%, estimated by AFM cross-sectional measurement) and the occupation of the left volume with polymer (PAA, or PEO, or both) induced by the evaporation of the adsorbed water from the vicinity of AuNPs. On the contrary, they will result in a red-shift of the SPR peak. Furthermore, it is found that when the annealed film was kept in the pH ) 2.5 solution for 20 days, the maximum absorption moved from ∼532 nm to the final ∼537 nm, but could not return to the original ∼547 nm of the assembly film without any post-treatment. This result strongly suggests that the above-observed blue-shift of AuNPs is not mainly originated from the evaporation of the adsorbed water in the assembled film. Conclusions In summary, during the growth of elastic multilayer films of hydrogen-bonded PEO/PAA-AuNP, the red-shift of the SPR peak of AuNPs is observed at the first of several numbers’

Tunable Plasmon Resonance and AuNP Deaggregation bilayers and then continuously moves toward short wavelengths with the following film buildup. This kind of blue-shift can be further modulated by post-treatments of as-prepared multilayer films, through annealing or just storing at room temperature in air or in an acid aqueous solution. It is found that the evolution of the optical properties of AuNPs results from the removal of water from the hydrated multilayer films and, more importantly, from the film nanostructure changes (e.g., the automatic interdigitation of PEO and PAA and the induced deaggregation of AuNPs with a uniform distribution). This kind of selfdeaggregation property of AuNPs originated from formation of the energy-favorable interpenetrated multilayer structures greatly improves our understanding of how to control the microstructures of composite films with tuned optical properties, which is very important for their applications in sensing and catalysis.1e,13,16a,24 On the other hand, our results also show the great potential of surface plasmonic properties in detection of complicated film structures. Acknowledgment. C.L. is grateful to the Alexander von Humboldt Foundation for a research fellowship. C.L. also thanks Rona Pitschke, Dr. Laemthong Chuenchom, and Prof. Shuxue Zhou for their help in the measurements of TEM images and IR spectra, respectively. Supporting Information Available: TEM image and UV spectra of as-prepared PAA-capped AuNPs (Figure S1), IR spectra (Figure S2), and optical microscopy images (Figure S3) of as-prepared PEO/PAA-AuNP multilayer films. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) 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. (b) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078. (c) Storhoff, J. J.; Lazarides, A. A.; Mucic, R. C.; Mirkin, C. A.; Letsinger, R. L.; Schatz, G. C. J. Am. Chem. Soc. 2000, 122, 4640. (d) Hutter, E.; Fendler, J. H. AdV. Mater. 2004, 16, 1685. (e) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (2) Lu, Y.; Yin, Y.; Li, Z.; Xia, Y. Nano Lett. 2002, 2, 785. (3) (a) Reinhard, B. M.; Siu, M.; Agarwal, H.; Alivisatos, A. P.; Liphardt, J. Nano Lett. 2005, 5, 2246. (b) So¨nnichsen, C.; Reinhard, B. M.; Liphardt, J.; Alivisatos, A. P. Nat. Biotech. 2005, 23, 741. (4) Choi, Y.; Ho, N.-H.; Tung, C.-H. Angew. Chem., Int. Ed. 2007, 46, 707. (5) Jiang, C.; Lio, W. Y.; Tsukruk, V. V. Phys. ReV. Lett. 2005, 95, 115503. (6) (a) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 4212. (b) Jana, N. R.; Gearheart, L.; Murphy, C. J. Langmuir 2001, 17, 6782. (c)

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