Polyelectrolyte Multilayer Films as Templates for the In Situ

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J. Phys. Chem. C 2009, 113, 18020–18026

Polyelectrolyte Multilayer Films as Templates for the In Situ Photochemical Synthesis of Silver Nanoparticles Emina Veletanlic and M. Cynthia Goh* Department of Chemistry, UniVersity of Toronto, 80 Saint George Street, Toronto, Ontario M5S 3H6, Canada ReceiVed: March 24, 2009; ReVised Manuscript ReceiVed: August 3, 2009

Multilayer films formed by the sequential adsorption of polyelectrolytes (PEs) from aqueous solution were used as templates for the in situ growth of silver nanoparticles. When films consisting of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) or PAA and poly(ethyleneimine) (PEI) are immersed in AgNO3 solution, functional groups within the film bind silver cations from solution. Bound Ag ions were photochemically reduced to metal nanoparticles upon exposure to UV radiation. The number of available binding groups and binding capacity of multilayer films were shown to be strongly dependent on multilayer processing conditions, including chemical structure of PEs, multilayer assembly pH, as well as the AgNO3 solution concentration and time interval used for Ag+ binding. These parameters were studied using a combinatorial approach to multilayer analysis, which provides a means to investigate in parallel the variables that affect particle synthesis. By using UV-vis spectroscopy and scanning transmission electron microscopy (STEM), the optimal conditions were determined to generate composite films containing small (3-5 nm) isolated spherical nanoparticles that are distributed randomly throughout the PE matrix. A striking discovery was the sensitivity of Ag+ binding to the structure of deposited PEs. Films consisting of branched PEI and PAA assembled at specific pH values have displayed a remarkably high affinity for metal cations relative to PAH/PAA films assembled at the same conditions. 1. Introduction Methods for generating metal nanoparticles are diverse and can range from the classical chemical reduction of metal salts in solution1,2 or growth of particles in microemulsions3,4 to fabrication via laser ablation5,6 or thermal decomposition of appropriate precursors.7,8 Another approach employed more recently is the synthesis of particles inside multilayer thin films prepared by the sequential adsorption of weak polyelectrolytes (PEs) via layer-by-layer (LbL) assembly. These films can be enriched in various functional groups (e.g., carboxylic acids) that may be used as templates for the in situ growth of inorganic nanoparticles. Analogous to biomineralization processes, metallic or semiconducting particles can be induced to nucleate and grow in these organic matrices.9-15 The process consists of immersing films into aqueous metal salt solutions that allow functional groups within the film to bind metal cations from solution. The formation of nanoparticles is induced with the appropriate physical, chemical, or photochemical treatment in which metal ions are reduced resulting in a polymer/metal nanoparticle composite system. Although many of the previous studies have focused on the production of silver nanoparticles from silver salts, it is reasonable to believe that, in theory, there should be no restrictions on the choice of metal salt precursors as long as they are compatible with aqueous solution chemistry and can be bound by functional groups in the film. LbL films, which effectively act as tiny reactors for generating nanoparticles, are very versatile since the composition and number of functional groups within a multilayer assembly are highly tunable with variations in assembly conditions including the chemical structure of chosen PEs and deposition pH.16 The porous nature of polyelectrolyte multilayer (PEM) films allows * To whom correspondence should be addressed. Phone: (416) 978-6254. Fax: (416) 978-4526. E-mail: [email protected].

particles to grow, while surrounding polymer chains limit particle size and prevent aggregation. Because nanoparticles exhibit exceptional size-dependent physical and chemical properties,17-19 such an inorganic-organic mixture could surpass by far the performance of the individual components and result in numerous scientifically or commercially interesting properties by providing enhancements in, for example, dielectric,20 biosensing,21 or electrochromic22 functionality. An alternative to fabricating such polymer/nanoparticle composites is by introduction of presynthesized nanoparticles into films with oppositely charged PEs via LbL assembly, but this approach creates stratified composites with defined particle layers.23-27 In situ synthesis generates particles that are well dispersed in the film and also affords better control over concentration because the metal content can be tuned in postassembly processing by subjecting the film to multiple ion binding and reduction cycles. Rubner’s group has explored the synthesis of nanoparticles in films composed of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH).9,10 In an example of their work, Ag+ and Pb2+ were bound to COOH repeat units by ion exchange followed by exposure to H2(g) and H2S(g) atmospheres to generate Ag and PbS nanoparticles, respectively. Particle concentration and size were affected by the PE deposition pH and the number of exchange/reduction cycles. Higher Ag weight fractions were attained at lower assembly pH values while particles were observed to be larger with an increasing number of silver binding and reduction cycles. Amine functional groups could prove particularly useful for binding of anions that cannot be bound by COOH moieties, but these groups have thus far only been derived indirectly from PAH/PAA and PAH/poly(styrene sulfonate) (PSS) films which undergo molecular rearrangements with postassembly treatment

10.1021/jp902671e CCC: $40.75  2009 American Chemical Society Published on Web 09/18/2009

In Situ Photochemical Synthesis of Silver Nanoparticles steps and generate free amines in the process.12 Poly(ethyleneimine) (PEI) offers an advantage over PAH because its various amine groups allow the formation of complexes with a wide variety of transition metal cations.28-30 Although films consisting of linear PEI and PAA have been investigated for the unusual porosity transitions that occur with postassembly pH treatments31 and relatively high ionic conductivities achievable at room temperature,32 films assembled either from linear or branched PEI remain largely unexplored as reactors for metallic nanoparticles.11 In general, slight variations in assembly conditions can radically affect the properties of weak PE matrices. The number of Ag+-binding functional groups is particularly sensitive to the pH of deposition solutions,16 but because ions are bound from a metal salt solution, Ag+ solution concentration and binding time are also important. Therefore, in this paper, we take advantage of the combinatorial approach using standard microtiter plates, which permits the simultaneous investigation of a large number of variables that affect in situ particle growth,33 to examine and compare the effects of the silver salt solution concentration and immersion time on the synthesis of nanoparticles in PEMs composed of PAH/PAA and PEI/PAA; these parameters have not yet been systematically investigated. Silver ions were photochemically reduced in UV light to yield polymer/ Ag particle composites. We have characterized the size and distribution of particles throughout the film thickness using scanning transmission electron microscopy (STEM). Photochemical reduction provides several advantages in that it does not cause deformation of the substrate often generated by heatinduced stresses during thermal treatment, avoids film decomposition or delamination that may occur when films are immersed into solutions containing strong reducing agents, and finally, provides an opportunity to selectively expose regions of the film to UV light to create patterned composite systems for specific applications. 2. Experimental Methods Chemicals and Materials. PAA (MW ) 1 250 000 g/mol), PAH (MW ) 60 000 g/mol), and silver nitrate (AgNO3, min. 99.8%) were purchased from Sigma Aldrich Chemical Co. Branched PEI (MW ) 750 000 g/mol) and linear PEI (MW ) 250 000 g/mol) were acquired from Polysciences, Inc. Deionized water was used in all experiments (Barnstead, resistivity 18 MΩ · cm). All chemicals were used as received. Combinatorial experiments were performed in 96-well microtiter plates (Sarsted). Copper grids were purchased from Ted Pella, Inc. Multilayer Film Preparation. For combinatorial studies, four-bilayer films of PAH/PAA or PEI/PAA were prepared in 96-well polystyrene microtiter plates, which were thoroughly rinsed prior to deposition. All polymer solutions were prepared with deionized water, diluted to 1 mg/mL and were pH adjusted with either NaOH or HCl. All solution pH values were measured with pH paper (Fisher Scientific). For each adsorption step, a total solution volume of 100 µL was added to individual wells with a multichannel pipettor. Film deposition began with the polycation (i.e., with PAH or PEI next to substrate). PE solutions were left to adsorb for 10 min while being simultaneously shaken with an orbital shaker. All microtiter wells were rinsed five times after each adsorption step with a stream of deionized water from a wash bottle. These multilayer films constitute the base layers for subsequent soaking in metal salt solutions. The parameter space was studied in a combinatorial fashion. AgNO3 solutions with concentrations ranging from 0.001 to 1.0 M (pH 5.8) were prepared to test the effects of concentration and time

J. Phys. Chem. C, Vol. 113, No. 42, 2009 18021 on nanoparticle formation. AgNO3 solution (60 µL) was added to individual wells with a multichannel pipettor for times between 5 min and 3 h. Plates were rinsed, left to dry in air, and then exposed to UV radiation for 90 min (germicidal lamps, 254 nm, 6.9 W, The American Ultraviolet Company; distance from lamp 16 cm). We adopt the notation used previously to describe the films.10 Multilayer films are denoted by (PAHX/ PAAY)i, where X is the polycation solution pH, Y is the polyanion solution pH, and i is the number of bilayers. The following films were prepared for combinatorial experiments: (PAH2.3/PAA2.5)4, (PAH4.2/PAA4.2)4, (PAH7.5/PAA3.7)4, (PAH6.4/ PAA6.5)4, (PEI2.2/PAA2.5)4, (PEI4.2/PAA4.2)4, (PEI7.3/PAA3.7)4, and (PEI6.8/PAA6.5)4. Only one type of multilayer film was prepared per microtiter plate, and because combinations of times and concentrations were duplicated in wells, a total of 48 different conditions were examined in parallel for each film. Characterization. UV-vis spectra of PEMs were studied using a Safire2 microplate reader (Tecan US, Inc.). For STEM imaging, 10-bilayer films were prepared on polystyrene substrates (10 min dipping in PE solutions with 2 × 1 min rinses with deionized water) using a home-built automated dipping robot. Polystyrene substrates were cut from disposable Petri dishes prior to deposition and sonicated in deionized water. Film (75 nm thick) cross sections were then cut using an ultramicrotome (Leica Ultracut UCT) with a diamond knife at room temperature and picked up with copper grids. STEM was performed on these sections with a Hitachi HD-2000 at 200 kV operated in the light-field mode. Particle size and film thickness were measured using ImageTool image processing and analysis software (The University of Texas Health Science Center, San Antonio, TX). 3. Results and Discussion When PAH/PAA or PEI/PAA multilayer films are immersed in a silver salt solution, repeating COOH or amine functionalities in the film bind Ag+ ions from solution. One of the objectives of our experiments was to systematically investigate the effects of Ag+ concentration and immersion time in solution on the photochemical synthesis of nanoparticles in different LbL assemblies. We have performed experiments in microtiter plates on multilayer films with AgNO3 solutions ranging in concentration from 0.001 to 1.0 M. Bound Ag+ ions were photochemically reduced to zerovalent metal nanoparticles upon exposure of films to UV radiation. Ag nanoparticles display a characteristic surface plasmon resonance (SPR) absorbance that arises due to the collective oscillations of conduction electrons in the metal. Because the typical Ag plasmon absorbance spans the visible energy spectrum, UV-vis spectroscopy is a convenient method of tracking particle formation. UV-vis spectra were collected after irradiation; the resulting spectral intensities provide an indirect measure of the amount of Ag+ absorbed from solution and allow us to investigate variations in Ag+ solution concentration and dipping time. Since the PEs used for assembly have no absorbance peaks in the region of interest, measurements were conducted and reported in terms of the Ag plasmon peak. The combinatorial approach provides a high-throughput means to simultaneously investigate dozens of different combinations of concentration and binding time. Because only one type of multilayer film was prepared per microtiter plate, dozens of such combinations could be examined in parallel for each film. We have examined UV-vis properties of Ag/polymer composites prepared from PEM films that were assembled at a variety of deposition pH values to give a rich selection of film architectures and fractions of functional groups for Ag+ binding from solution.

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Veletanlic and Goh be absorbed into the film for attaining the critical cluster radius needed for particle growth. A surprising finding that emerged from our study was that the PEI/PAA system exhibited considerably higher absorbance values at all concentrations (Figure 1c). PEI is appealing because its amine groups form complexes with transition metal cations and is thus thought to have higher ion binding capacity.28-30 Composite films, which were prepared from AgNO3 solutions of higher concentration, generally had higher absorbance at λmax after irradiation but using a concentration of 0.15 M offered sufficiently high Ag+ binding for producing particles. The films in Figure 1c showed a pronounced increase in absorbance between 0.001 and 0.15 M followed by a comparatively modest rise between 0.30 and 1.0 M. Because this trend was the same regardless of multilayer deposition pH or composition, we speculate that Ag+ binding proceeds in a similar manner. At low salt solution concentration, Ag+ ions may bind to available functional sites via ion exchange with acid protons, or via formation of silver-amino complexes in the film. However, it is well known that multilayer films swell in salt solutions, especially at high salt concentrations.37 LbL assembled films are held together by electrostatic interactions between oppositely charged polymer segments that are easily disrupted by external salt ions. At high AgNO3 concentrations, paired polymer segments are forced apart providing additional sites for Ag+ ions according to + + Ρol-Ρol(multilayer) + Αg(aq) + ΝΟ3(aq) T

Figure 1. (a) UV-vis absorbance of four-bilayer films of (a) PAH7.5/ PAA3.7 and (b) PEI7.3/PAA3.7. Spectra were collected for films that were prepared by addition of AgNO3 solutions to microtiter wells for 15 min, followed by exposure of rinsed and dried films to UV-radiation. The arrows indicate that absorbance intensities increase for films prepared from solutions with higher salt concentration. (c) Spectral values plotted at 448 nm as a function of AgNO3 concentration for PEI7.3/PAA3.7 (b) and PAH7.5/PAA3.7 (O).

Effect of Ag+ Concentration. Metal salt solutions of different concentrations were added to microtiter wells primed with multilayer films and for the particular sample set in Figure 1, films were soaked in AgNO3 solutions for 15 min followed by rinsing, drying and exposure to UV radiation. Figure 1 shows UV-vis data for two types of films; PEI7.3/PAA3.7 was chosen because it exhibited surprisingly high UV-vis values while the spectra of the PAH7.5/PAA3.7 system were representative of other multilayer films and serve as a comparison. (Results for all other films are shown in Figure S1 in Supporting Information). Characteristic absorbance spectra of Ag nanoparticles showing a peak at ∼448 nm (λmax) were only observed for AgNO3 concentrations g0.050 M. The number, width, and position of SPRs are greatly dependent on particle morphology and single resonance peaks such as those observed here are typically exhibited by spherical metal particles.34 Peaks in the near-IR region that are usually attributed to the increase in particle size due to aggregation were noticeably absent suggesting that particles remained isolated when higher AgNO3 concentrations were used for silver binding.35 No SPR peaks were observed in any of the eight composite films that were immersed in 0.001 and 0.010 M AgNO3. For ion exchange processes, metal ion binding generally depends on several parameters including structural characteristics of the solid matrix, interactions between diffusing ions and the solid matrix, and the chemical potential of ions involved.36 The incorporation of Ag+ into the film may therefore be driven by the chemical potential of the silver salt in solution, and at very low solution concentrations, insufficient amounts of Ag+ may

+ + Ρol+ΝΟ3(multilayer) Ρol-Αg(multilayer)

where Pol( represents positively or negatively charged polymer units in the PEM assembly. In some cases, the swelling may be substantial enough to trigger multilayer decomposition or delamination, and in fact, we did observe instability in some of our multilayer films immediately after immersion in solutions of high concentration. Although data have only been presented for films which were immersed in AgNO3 for 15 min, similar concentration-dependent increases in absorbance intensities were observed for longer dipping times (Figure S2 in Supporting Information). Effects of Binding Time. Various AgNO3 solutions were added to microtiter plates primed with multilayer films for time intervals between 5 min and 3 h. Figure 2 shows trends in absorbance monitored at 448 nm for all AgNO3 concentrations and most plots showed a noticeable rise in absorbance between 5 and 30 min followed by modest growth between 30 min and 3 h, but it is clear that the PEI7.3/PAA3.7 films had the highest intensities. In one previous study, Wang et al. had immersed PE films in silver acetate solutions for ∼30 h, followed by lengthy reduction in a hydrogen atmosphere for 30 h to produce nanoparticles.10 However, our results show that significantly shorter immersion times are possible and that an adequate amount of Ag+ for particle growth could be absorbed in ∼30 min. Figure 3 provides more information about SPR results for two selected films and the data obtained by monitoring absorbance at 448 nm support the conclusions drawn from previous figures. When AgNO3 solutions of at least 0.050 M were used to prepare Ag/polymer composite films, the ‘saturation’ point in absorbance at λmax was reached in ∼30 min. UV-vis data also appeared to be more strongly dependent on the concentration of the external salt solution rather than time allotted for Ag+ binding from solution. Electron Microscopy Characterization. We have performed STEM analysis on film cross sections in order to gain insight

In Situ Photochemical Synthesis of Silver Nanoparticles

Figure 2. Absorbance at 448 nm (∼λmax) for four-bilayer films dipped in AgNO3 solutions of different concentrations between 5 min and 3 h.

into the size and distribution of Ag nanoparticles. Cross-sectional images in Figure 4 show randomly dispersed spherical nanoparticles in films assembled at various pH values. AgNO3 concentration of 0.75 M and binding time of 2 h were selected on the basis of combinatorial experiments to ensure completion of the exchange process. Average nanoparticle sizes were obtained graphically using image processing and analysis software, and thus, it is important to emphasize that the histograms will not be used for rigorous quantitative analysis. Particles were not restricted to single layers. It is well known that PEM films consist of an interpenetrated network of polycation and polyanion chains and functional groups that participate in Ag+ binding are thus randomly distributed throughout the film thickness resulting in equally well-dispersed nanoparticles after nucleation and growth. In order to understand and explain the Ag+-binding capacity of the PE systems studied here, identifying the factors that govern the total number of functional groups is extremely important. From a fundamental point of view, PAH/PAA films form a complex polymer system whose properties, including film thickness,16 porosity,38 and dielectric properties39 are extremely sensitive to deposition conditions. For instance, the film thickness undergoes drastic changes as pH values of PE solutions are increased from 2.5 to 9.0.40 PE conformation and degree of ionization depend on the pH of the solution, and the state in which the polymer adsorbs (i.e., extended chains versus globules) can greatly affect film morphology and the number of available COOH groups, which is particularly important for the Ag+-proton exchange. The pKa of PAA, defined as the pH at which the degree of ionization is 50%, is ∼6.5.16

J. Phys. Chem. C, Vol. 113, No. 42, 2009 18023 Figure 4a shows a population of densely packed nanoparticles with an average diameter of 3.4 nm. PAH2.3/PAA2.5 multilayers form moderately thick films with a low internal degree of ionization and a high density (up to 70%) of COOH functional groups that are available for Ag+ binding because many unprotonated PAA chains are incorporated during assembly.16,41 Because of the high number of COOH groups, the Ag volume fraction in these multilayer films is thought to be higher relative to films assembled at other pH combinations. When PAH and PAA solutions were adjusted to pH 4.2, thicker films were obtained because more PAH chains must be adsorbed to compensate the increased charge density of PAA (Figure 4b). A relatively large number (∼40%) of COOH groups remain in the film, although lower than in the previous case.16,41 The extent of layer interpenetration in PAH/PAA films assembled at pH 7.5/3.7 is poor and surface layers are enriched in the polymer found on the surface. Previous studies have shown that PAH7.5/ PAA3.7 form thick bilayer films with a high density of COO--NH3+ ion pairs and because there are very few functional groups in the film, its properties with respect to the Ag+-proton exchange are dominated by the surface layer.42 Only a PAA-terminated film has residual nonionized groups that permit Ag+ binding. The top region of the PAA-terminated film in Figure 4c is heavily populated with nanoparticles, while the remainder of the film has a sparse particle distribution. The arrow indicates the depth in the film at which particles were still observed and this was reported as the film thickness. The PAH6.4/PAA6.5 films contain virtually no COOH functional groups and no nanoparticles were observed in microtiter plate experiments regardless of AgNO3 solution concentration; all bilayers remained colorless following photochemical reduction.42 This is in agreement with the results of Yoo et al., which showed that the adsorption of a positively charge dye, methylene blue, is dependent on the number of nonionized COOH groups and could be prevented when the pH of deposition solutions was adjusted to 6.5.41 Estimated average thicknesses for 10-bilayer films of PAH2.3/PAA2.5 and PAH7.5/PAA3.7 after a single Ag loading cycle were 49 and 139 nm, respectively. Greater film thicknesses were also observed with increasing assembly pH for PEI/PAA films (Figure 4d-f): 73 nm at pH 2.2/2.5, 83 nm at pH 4.2/4.2, and 335 nm at pH 7.3/3.7. PEI and PAH exhibit similar protonation behavior. The fraction of ionized groups increases at low pH, and it may not be unreasonable to assume that many of the arguments regarding film morphology and number of nonionized COOH groups in PAH films are also valid for PEI-containing films, but PEI7.3/ PAA3.7 films shown in Figure 4f are considerably thicker due to the high sorption of Ag+. In this study, we have assembled PEI/PAA multilayers from deposition solutions at various pH values to determine the most favorable conditions for Ag+ binding. In terms of UV-vis absorbance intensities, PEI/PAA multilayers assembled at pH values 2.2/2.5 and 4.2/4.2 are similar to PAH/PAA films assembled at nearly the same conditions. However, although PAH is nearly 100% protonated within the pH range investigated in this study, PEI protonation varies from ∼70-75% at pH 2.2 to 30-35% at pH 7.3.43 When solution pH of PEI and PAA is adjusted to 7.3 and 3.7, respectively, the degree of ionization is low for both polymers and PE chains assume a more globular conformation in solution because repulsive interactions between chain segments are now minimized. Adsorbed PEs therefore tend to adhere to surfaces in form of loops and tails instead of flat chains resulting in thicker multilayer assemblies.

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Veletanlic and Goh

Figure 3. Absorbance at 448 nm for PEI7.3/PAA3.7 and PAH7.5/PAA3.7 films as a function of binding time in (a) and (c) and AgNO3 solution concentration in (b) and (d). Note the difference in the scale of the y axis between the films. In (a) and (c) absorbance was studied at various AgNO3 concentrations, 0.001 M (b), 0.010 M (O), 0.050 M ([), 0.15 M (]), 0.30 M (1), 0.50 M (3), 0.75 M (9), 1.0 M (0). In (b) and (d), spectral values were collected for films which were exposed to AgNO3 solutions for different time intervals, 5 min (0), 15 min (left-pointing triangle), 30 min (`), 1 h (0), 2 h (2), 3 h (4).

Figure 4. Cross-sectional STEM images of PAH/PAA and PEI/PAA multilayer films assembled at a variety of pH values with corresponding particle histograms. Nanocomposites were prepared by immersing 10-bilayer films in 0.75 M AgNO3 for 2 h. Rinsed and dried films were subsequently exposed to UV light for 90 min. Average thicknesses were estimated from STEM images: (a) 49, (b) 91, (c) 139, (d) 73, (e) 83, and (f) 335 nm. Average particle diameters and standard deviations based on the measurement of 100 particles are (a) 3.4 ( 1.5, (b) 5.1 ( 1.2, (c) 4.6 ( 0.9, (d) 3.8 ( 0.7, (e) 3.8 ( 1.8, and (f) 5.2 ( 1.1 nm. Vertical axes in histograms represent particle count.

These PEI/PAA films showed a surprisingly high Ag+-binding capacity, and we speculate that PEI provides an advantage over PAH because its functional groups form metal ion-amino complexes with transition metal cations. The commercially available branched PEI polymer used in this study contains three different types of amine groups in roughly equal proportion: 34% primary groups at chain ends, 35% secondary groups in the main chain, and 31% tertiary amine branch points.44 Unlike linear PEI, which consists of only secondary amines and thus has distinct protonation values at approximately pH 6 and 10, amine groups in branched PEI display varying affinities for protons.45,46 Under conditions for which high silver binding is achieved in this work, the number of charged and uncharged amines may be most favorable to allow the assembly of PEI with PAA via electrostatic adsorption from solution and at the

same time provide good Ag+ binding when immersed in AgNO3 solution. A visual comparison of films fabricated from linear or branched PEI with PAA at pH 7.3/3.7 demonstrates the difference in binding capacity after a single ion exchange and irradiation cycle (Figure 5). The films were assembled on a clear polystyrene substrate. Upon UV irradiation, the film composed of linear PEI (Figure 5a) had an amber color characteristic of silver nanoparticles that absorb in the visible region of the energy spectrum. The film assembled from branched PEI (Figure 5b) had a metallic appearance that arises from higher Ag+ binding and a dense particle population in the film. We speculate that tertiary amines that are abundant in branched PEI but are absent in PAH or linear PEI play an important role in achieving such high silver binding; the loading appears to be highly sensitive to the structure of PE building blocks.

In Situ Photochemical Synthesis of Silver Nanoparticles

J. Phys. Chem. C, Vol. 113, No. 42, 2009 18025 and PAA have been found particularly interesting because of their remarkably high silver binding capacity, not attainable with PAH/PAA films or even films assembled from linear PEI and PAA at the same conditions. The chemical structure of PEs plays a crucial role in forming complexes with Ag+ and the internal molecular arrangement of functional groups in PEI/PAA multilayers dramatically influences the binding of metal cations.

Figure 5. Photographs of 10-bilayer films of linear PEI7.3/PAA3.7 (a) and branched PEI7.3/PAA3.7 (b) after a single Ag exchange and reduction cycle. PE films were prepared on polystyrene substrates and immersed in 0.75 M AgNO3 solution for 2 h.

Efforts have been directed toward finding simple and inexpensive synthetic methods that provide control over particle shape and size distribution. In a typical solution synthesis, particles nucleate and grow but tend to aggregate into precipitates. In that case, polymers can act as stabilizers by providing a more rigid environment to prevent coalescence and control particle size. Similarly, in PE films, the dense solid matrix limits particle growth resulting in much smaller nanoparticles. In fact, it is so effective that particle aggregation is prevented even after six Ag loading steps (results not shown here). It was suggested that the dense molecular architecture at low assembly pH contributes to smaller particles while at higher pH, particle growth is more facile due to a more porous structure of PE multilayers.47 On the basis of our results, PEM films are deemed excellent reactors for the synthesis of small Ag nanoparticles. Unlike multilayer films prepared from strong PEs in which most repeat units are partnered up with oppositely charged segments, weak PE films have a tunable fraction of functional groups available for binding of Ag+ ions from aqueous solution, and in comparison to films obtained by casting or spin coating, in situ synthesis provides well-dispersed particles without aggregation. The combinatorial approach has also proven to be an effective tool for optimizing parameters that affect particle synthesis and which had not been previously investigated. This technique has given us a considerable set of data for finding the best conditions for generating films with a high affinity for metal ions and is, in fact, ideally suited to the analysis of properties that can be studied using absorbance or fluorescence spectroscopy. 5. Conclusions We have demonstrated that LbL assembled PAH/PAA and PEI/PAA films can be used as templates for the in situ synthesis of Ag nanoparticles. The film thickness, morphology, and total number of Ag+-binding groups were highly tunable and particularly sensitive to assembly pH of the PEM assembly. Using multilayer films assembled from a range of solution pH values to give a wide selection of film architectures, we have shown that Ag+ concentration and binding time are important factors in the preparation of composite systems. Typical spectra of Ag nanoparticles were observed for AgNO3 concentrations g0.050 M with λmax around 448 nm. For solutions with an adequately high metal salt concentration, sufficient amounts of Ag+ were incorporated into films in less than 30 min resulting in the formation of small (3-5 nm) isolated spherical nanoparticles distributed randomly throughout the PE matrix as confirmed by STEM analysis. The combinatorial approach is valuable in studying the parameters that affect particle formation because the simultaneous collection of UV-vis data for a large number of films provides a high-throughput approach for analyzing composite systems. Films consisting of branched PEI

Acknowledgment. The authors acknowledge the financial support received from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Ontario Graduate Scholarships in Science and Technology (OGSST) program. The authors also thank Ilya Gourevich from the University of Toronto’s Centre for Nanostructure Imaging for his assistance with electron microscopy imaging and sample preparation. Supporting Information Available: Additional UV-vis absorbance spectra derived from combinatorial experiments. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Lee, Y.; Choi, J.-r.; Lee, K. J.; Stott, N. E.; Kim, D. Nanotechnology 2008, 19, 415604. (2) Huang, K.-C.; Ehrman, S. H. Langmuir 2007, 23, 1419–1426. (3) Niemann, B.; Veit, P.; Sundmacher, K. Langmuir 2008, 24, 4320– 4328. (4) Bellusci, M.; Canepari, S.; Ennas, G.; La Barbera, A.; Padella, F.; Santini, A.; Scano, A.; Seralessandri, L.; Varsano, F. J. Am. Ceram. Soc. 2007, 90, 3977–3983. (5) Amoruso, S.; Bruzzese, R.; Wang, X.; Nedialkov, N. N.; Atanasov, P. A. Nanotechnology 2007, 18, 145612. (6) Suzuki, K.; Kageyama, K.; Takagi, H.; Sakabe, Y.; Takeuchi, K. J. Am. Ceram. Soc. 2008, 91, 1721–1724. (7) Peng, S.; Wang, C.; Xie, J; Sun, S. J. Am. Chem. Soc. 2006, 128, 10676–10677. (8) Baskoutas, S.; Giabouranis, P.; Yannopoulos, S. N.; Dracopoulos, V.; Toth, L.; Chrissanthopoulos, A.; Bouropoulos, N. Thin Solid Films 2007, 515, 8461–8464. (9) Joly, S.; Kane, R.; Radzilowski, L.; Wang, T.; Wu, A.; Cohen, R. E.; Thomas, E. L.; Rubner, M. F. Langmuir 2000, 16, 1354–1359. (10) Wang, T. C.; Rubner, M. F.; Cohen, R. E. Langmuir 2002, 18, 3370–3375. (11) Dai, J.; Bruening, M. L. Nano Lett. 2002, 2, 497–501. (12) Chia, K.-K.; Cohen, R. E; Rubner, M. F. Chem. Mater. 2008, 20, 6756–6763. (13) Wang, C.; Wang, E.; Lan, Y.; Li, Q.; Mao, B.; Tian, C. Thin Solid Films 2008, 516, 6058–6062. (14) Vago, M.; Tagliazucchi, M.; Williams, F. J.; Calvo, E. J. Chem. Commun. 2008, 5746–5748. (15) Ma, H.; Yang, G.; Yu, L.; Zhang, P. Surf. Coat. Technol. 2008, 202, 5799–5803. (16) Choi, J.; Rubner, M. F. Macromolecules 2005, 38, 116–124. (17) Biju, V.; Itoh, T.; Anas, A.; Sujith, A.; Ishikawa, M. Anal. Bioanal. Chem. 2008, 391, 2469–2495. (18) Moores, A.; Goettman, F. New J. Chem. 2006, 30, 1121–1132. (19) Haick, H. J. Phys. D: Appl. Phys. 2007, 40, 7173–7186. (20) Lu, J.; Moon, K.-S.; Wong, C. P. J. Mater. Chem. 2008, 18, 4821– 4826. (21) Pieczonka, N. P. W.; Goulet, P. J. G.; Aroca, R. F. J. Am. Chem. Soc. 2006, 128, 12626–12627. (22) Namboothiry, M. A. G.; Zimmerman, T.; Coldren, F. M.; Liu, J.; Kim, K.; Carroll, D. L. Synth. Met. 2007, 157, 580–584. (23) Geraud, E.; Mo¨hwald, H.; Shchukin, D. G. Chem. Mater. 2008, 20, 5139–5145. (24) Zhang, F.; Srinivasan, M. P. J. Colloid Interface Sci. 2008, 319, 450–456. (25) Sahu, S.; Majee, S. K.; Pal, A. J. Appl. Phys. Lett. 2007, 91, 143108. (26) Liu, X.; Wang, J.; Zhang, J.; Liu, B.; Zhou, J.; Yang, S. Thin Solid Films 2007, 515, 7870–7875. (27) Estephan, Z. G.; Alawieh, L.; Halaoui, L. I. J. Phys. Chem. C 2007, 111, 8060–8068. (28) Belfiore, L. A.; Indra, E. M. J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 552–561. (29) Rivas, B. L.; Seguel, G. V.; Geckeler, K. E. Macromol. Mater. Eng. 1996, 238, 1–10.

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