Nanoparticle Films Containing

film growth depends on the polymer solution pH and other details of the solution exposures. Growth ... Xia, C.; Park, M.; Advincula, R. Langmuir 2001,...
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Layer-by-Layer Growth of Polymer/Nanoparticle Films Containing Monolayer-Protected Gold Clusters Jocelyn F. Hicks, Young Seok-Shon, and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina CB-3290, Chapel Hill, North Carolina 27599-3290 Received October 10, 2001. In Final Form: December 19, 2001 Multilayer films of nanoparticles were grown in a systematic and controlled manner layer-by-layer by alternating exposures of suitably functionalized substrates (glass, Au) to either poly(allylamine) and carboxylic acid-functionalized nanoparticles or to poly(styrene sulfonate) and arylamine-functionalized nanoparticles. Electrostatic interactions comprise the dominant film growth factors. The rate of multilayer film growth depends on the polymer solution pH and other details of the solution exposures. Growth was followed by spectrophotometry of the Au nanoparticle cores, voltammetry of the Au core double layer charging, and film mass (quartz crystal microbalance). The first example is reported of quantized double layer charging of the Au cores in a layer-by-layer film that is composed of monolayer-protected clusters and a polyelectrolyte.

Introduction Combining the versatility of polymeric materials with materials such as quantum dots,1 fullerenes,2 redox molecules,3 metals,4 and metal nanoparticles5 offers the potential for applications in the semiconductor, photovoltaic, and molecular electronics fields. The recent literature contains a number of reports1-6 describing combinations of polymers with nanoscopic and microscopic materials to form thin-film multilayer composites. Many of the composite thin-film materials have been created using a simple layer-by-layer method popularized by Decher,7 who exposed a substrate surface alternately to solutions of cationic and anionic polymers to create a multilayer film. The ease with which multilayer films of diverse substances can be grown using the layer-by-layer approach and related methods is remarkable, as is the sensitivity6,8-10 of the growth and properties of the multilayer films to chemical factors such as polyelectrolyte concentration and the presence of electrolyte, pH, and hydrogen-bonding and covalent-bonding groupings. Some (1) Mamedov, A. A.; Belov, A.; Giersig, M.; Mamedova, N. N.; Kotov, N. A. J. Am. Chem. Soc. 2001, 123, 7738. (2) Ramos, A. M.; Rispens, M. T.; van Duren, J. K. J.; Hummelen, J. C.; Janssen, R. A. J. J. Am. Chem. Soc. 2001, 23 (27), 6714-6715. (3) Abruna, H. D.; Denisevich, P.; Umana, M.; Meyer, T. J.; Murray, R. W. J. Am. Chem. Soc. 1981, 103, 1. (4) (a) Kovtyukhova, N. I.; Ollivier, P. J.; Chizhik, S.; Dubravin, A.; Buzaneva, E.; Gorchinskiy, A. D.; Marchenko, A.; Smirnova, N. Thin Solid Films 1999, 337, 166. (b) Kovtyukhova, N. I.; Gorchinskiy, A. D.; Waraksa, C. C. Mater. Sci. Eng. 2000, 70, 424. (c) Cassagneau, T.; Fendler, J. H.; Mallouk, T. E. Langmuir 2000, 16, 241. (d) Kovtyukhova, N. I.; Martin, B. R.; Mbindyo, J. K. N.; Smith, P. A.; Razavi, B.; Mayer, T. S.; Mallouk, T. E. J. Phys. Chem. B 2001, 105, 8762. (5) (a) Gittins, D. I.; Caruso, F. J. Phys. Chem. B 2001, 105, 6846. (b) Boal, A.; Ilhan, F.; DeRouchey, J.; Russell, T.; Rotello, V. Nature 2000, 404, 746. (c) Marinakos, S. M.; Novak, J. P.; Brousseau, L. C., III.; House, A. B.; Edeki, E. M.; Feldhaus, J. C.; Feldheim, D. L. J. Am. Chem. Soc. 1999, 121, 8518. (d) Marinakos, S. M.; Schlutz, D. A.; Feldheim, D. L. Adv. Mater. 1999, 11, 34. (e) Youk, J. H.; Locklin, J.; Xia, C.; Park, M.; Advincula, R. Langmuir 2001, 17, 4681. (6) Fendler, J. H. Chem. Mater. 2001, 13 (10), 3196-3210 and references therein. (7) Decher, G. Science 1997, 277, 1232. (8) Fery, A.; Scholer, B.; Cassagneau, T.; Caruso, F. Langmuir 2001, 17 (13), 3779-3783. (9) (a) Zalewska, A.; Stygar, J.; Ciszewska, E.; Wiktorko, M.; Wieczorek, W. J. Phys. Chem. B 2001, 105, 5847. (b) Yoshikawa, Y.; Matsuoka, H.; Ise, N. Br. Polym. J. 1986, 18, 242. (c) Yoo, D.; Shiratori, S.; Rubner, M. F. Macromolecules 1998, 31, 4309. (d) Shiratori, S.; Rubner, M. F. Macromolecules, 2000, 33, 4213.

of the dependence relates to the stability of the layers to rinsing steps, others to the surface charge of the successive component layers, and doubtless others to the relationship between the polyelectrolyte structure and charge density and the means by which the polyelectrolytes become incorporated into the multilayer film. The nanoscopic components of multilayer films have included semiconductor nanoparticles and quantum dots;1,11,12 magnetic nanoparticles;13 graphite oxide nanosheets;12 zeolite particles;14 and metal nanoparticles,5,12,15-18 including multilayer films composed of oppositely charged nanoparticles.18 The latter are the most relevant to the present work, which deals with nanoparticles with functionalized monolayers around metal cores, where we apply layer-by-layer assembly to deposit, on glass and Au electrodes (both naked and precoated with a self-assembled monolayer), two new kinds of multilayer films, both composed of alternating polymers and charged monolayerprotected clusters (MPCs) of gold.19 The components of these multilayer films are summarized in Scheme 1. One multilayer film was generated by alternately exposing a surface to a solution of poly(allylamine hydrochloride) (PAH) and another solution of a MPC (10) (a) Dubas, S. T.; Farhat, T. R.; Schlenoff, J. B. J. Am. Chem. Soc. 2001, 123, 5368. (b) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153. (c) Farhat, T.; Yassin, G.; Dubas, S. T.; Schlenoff, J. B. Langmuir 1999, 15, 6621. (d) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626. (e) Ostrander, J. W.; Mamedov, A. A.; Kotov, N. A. J. Am. Chem. Soc. 2001, 123, 1101. (11) (a) Sun, Y.; Hao, E.; Zhang, X.; Yang, B.; Shen, J.; Chi, L.; Fuchs, H. Langmuir 1997, 15, 5168. (b) Heeger, A. L. J. Phys. Chem. B 2001, 105, 8475. (12) Cassagneau, T.; Fendler, J. H. J. Phys. Chem. 1999, 103, 1789. (13) (a) Mamedov, A. A.; Kotov, N. A. Langmuir 2000, 16, 5530. (b) Mamedov, A. A.; Ostrander, J.; Aliev, F.; Kotov, N. A. Langmuir 2000, 16, 3941. (14) (a) Rhodes, K. H.; Davis, S. A.; Caruso, F.; Zhang, B.; Mann, S. Chem. Mater. 2000, 12, 2832. (b) Lee, G. S.; Lee, Y. J.; Yoon, K. B. J. Am. Chem. Soc. 2001, 123 (40), 9769-9779. (15) Hao, E.; Lian, T. Chem. Mater. 2000, 13, 3392. (16) (a) Auer, F.; Scotti, M.; Ulman, A.; Jordan, R.; Sellergren, B.; Garno, J.; Lui, G. Y. Langmuir 2000, 16, 7554. (b) Pastoriza, I.; Koktysh, D. S.; Mamedov, A. A.; Giersig. M.; Kotov, N., A.; Liz-Marzan, L. M. Langmuir 2000, 16, 2731. (17) Feldheim, D. L.; Grabar, K. C.; Natan, M. J.; Mallouk, T. E. J. Am Chem. Soc. 1996, 118, 7640. (18) Cliffel, D. E.; Zamborini, F. P.; Murray, R. W. Langmuir 2000, 16, 9699. (19) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27.

10.1021/la0156255 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/08/2002

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Scheme 1. Cartoon of Polymers, Poly(allyamine hydrochloride) (PAH) and Poly(sodium 4-styrene sulfonate) (PSS) MPCs, and Exchange Ligands, Mercaptoundecanoic Acid (MUA) and 4-Mercaptophenylamine (ATH), Used in Building MPC/Polymer Multilayers

Scheme 2. Cartoon Depicting Layer-by-layer Growth of Polymer/Nanoparticle Film on Functionalized Glass

structures cannot be regarded as static assemblies, and the cartoon additionally suggests considerable layer disorder in the multilayer. Experimental Section 20

having a mixed monolayer of hexanethiolate and mercaptoundecanoic acid (MUA) ligands. This kind of multilayer film will be abbreviated PAH/MUA. The second multilayer film was generated by alternately exposing the surface to a solution of polystyrene sulfonic acid and a solution of a MPC having a mixed monolayer21 of hexanethiolate and aminothiophenol (ATH) ligands. This kind of multilayer film will be abbreviated PSS/ATH. In PAH/MUA films, the nanoparticles are neutral or anionic; in PSS/ATH films, they are cationic. Results of varying the pH of the reagent solutions suggest that the multilayer-building driving forces are combinations of electrostatic and acid-base chemistry. Growth of the multilayer films was followed by optical absorbance of films grown on glass slides, by mass changes of films on Au by a quartz crystal microbalance (QCM), and by properties of films on Au electrodes as permeation barriers. Additionally, solutions of MPCs with mixed monolayers20,21 of hexanethiolate and mercaptoundecanoic acid (MUA) ligands are known to exhibit the singleelectron charging phenomenon known as quantized double layer (QDL) charging.22 The QDL arises because the monolayer-coated nanoparticles have subattofarad double layer capacitances in electrolyte solutions. Such mixedmonolayer MPCs continue to exhibit quantized double layer charging when linked together into multilayer films by means of metal ion coordination bridges20,23 and, as shown here, when incorporated into layer-by-layer PAH/ MUA films. Quantized double layer charging has not heretofore been reported in films grown by layering a MPC with a polyelectrolyte, and its presence suggests a substantial microscopic mobility of both electrolyte ions and nanoparticles within the film matrix (vide infra). Scheme 2 depicts a cartoon of a polymer/nanoparticle film grown on a glass slide. The QDL result implies that such (20) (a) Zamborini, F. P.; Hicks, J. F.; Murray, R. W. J. Am Chem. Soc. 2000, 122, 4515. (b) Chen, S. J. Phys. Chem. B 2000, 104, 663. (21) Shon, Y. S.; Murray, R. W. University of North Carolina, Chapel Hill, North Carolina. Unpublished results, 2001. (22) Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J.; Whetten, R. L.; Bigioni, T. P.; Guthrie, D. K.; First, P. N. J. Am. Chem. Soc. 1997, 119, 9279. (23) (a) Hicks, J. F.; Zamborini, F. P.; Murray, R. W. J. Am Chem. Soc. 2001, 123, 7048. (b) Hicks, J. F.; Zamborini, F. P.; Murray, R. W., manuscript in preparation.

Chemicals. The polymers poly(allylamine hydrochloride) (PAH, MW ca. 70 000) and poly(sodium 4-styrene sulfonate) (PSS, MW ca. 70 000) were purchased from Aldrich and used as received. Ten milligrams of each polymer was dissolved in 10 mL of Nanopure water to yield ca. 14 µM solution concentrations. The polymer solution pH values were adjusted to the desired values with 0.2 M NaOH and HCl solutions. MPC Synthesis. MPCs (monolayer-protected clusters, average diameter core 1.6 nm, average MW 34 660) with monolayers composed of hexanethiolate (C6) ligands were synthesized using a modified Brust24 method, the details of which are described in previous publications.25 The mercaptoundecanoic acid (MUA) and 4-aminothiolphenol (ATH) ligands were incorporated into the monolayer shell, after the initial synthesis and cluster cleanup, using “place-exchange” reactions such as those previously published.26 Briefly, the prepared C6 MPC is stirred in a solution of the desired thiol in THF for ca. 4 days, during which ligand exchange occurs between the MPC thiolates and the thiols in solution; the latter can be found26 as solution thiols following place exchange. The resulting mixed monolayers of the product MPCs were both determined via NMR spectroscopy to contain approximately 33 C6 ligands and 20 MUA or ATH ligands per MPC core, on average. The NMR determination involves quantitatively desorbing the ligands from the MPC core as disulfides by the addition of a crystal of iodine to the MPC solution.26 The ca. 30 µM MPC solutions used in the buildup of MPC/ polymer multilayers were made by dissolving 10 mg of each MPC in 10 mL of pure ethanol (Aldrich). The solutions were filtered through a Nalgene 0.2-µm-pore syringe filter to remove any MPCs not completely soluble in the ethanol. Although both kinds of MPCs contain mixed monolayers, they are, for simplicity, abbreviated as MUA MPCs and ATH MPCs, and the corresponding multilayers made from them as PAH/ MUA and PSS/ATH films, respectively (see Scheme 1). Spectroscopy. UV/vis spectra of films were taken by preparing the films on glass slides. The glass slides were functionalized with a bonding layer of 3-mercaptopropyl trimethoxy silane, which, in a solution of mixed-monolayer MPC, will bind a layer of nanoparticles by place exchange of the surface-attached thiol into the MPC monolayer. (24) Brust, M.; Walker, M.; Bethell, D.; Schriffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (25) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (26) (a) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175. (b) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906.

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The silanization procedure starts with glass microscope slides that have been cleaned in a piranha solution (2:1 sulfuric acid/ hydrogen peroxide), rinsed thoroughly with soapy water and then distilled water, and placed in 100 mL of isopropyl alcohol solution containing 1 mL of the 3-mercapto siloxane and 1 mL of Nanopure water. After being heated for 30 min, the glass slides are rinsed with ethanol and allowed to dry in a 100 °C oven for at least 30 min. The glass slides are then soaked in the desired MPC solution for at least 24 h to exchange some of the MPC thiolate ligands for those on the glass surface. This results in submonolayer coverage of the MPC on the glass surface. MPC/polymer multilayers were built up on the glass slide by alternately dipping the slide into the respective polymer and MPC solutions. The procedure is as follows: (1) After bonding a submonolayer of MPC as noted above, (2) the slide was removed from the MPC solution, rinsed thoroughly with ethanol and then with water, and placed in the desired polymer solution for 10 min. (3) The slide was removed from the polymer solution and rinsed thoroughly with water and then ethanol before being placed back into the MPC solution for 10 min. Steps 2 and 3 constitute a “dipping cycle” and were repeated to grow multilayer films on the glass slide. After each dipping cycle (completion of step 3), the slide was dried under nitrogen before its 300-900 nm spectrum was measured, using a silanized glass slide as a reference, in a Unicam UV/vis spectrometer. A four-step variant of the above procedure will be explained later. Electrochemistry. Electrochemical measurements were carried out in 0.1 M tetrabutylammonium hexafluorophosphate (Bu4N+PF6-) in dichloromethane (CH2Cl2) solutions at a 1.6mm-diameter Au disk working electrode. The working electrode was polished prior to each experiment with 0.25-µm diamond paste (Buehler), rinsed copiously with distilled water, and potential cycled in sulfuric acid for 3-5 min.27 The clean Au electrode surface was exposed to a 2 mM ethanolic solution of the desired thiol (MUA or ATH) for g24 h to prepare a self-assembled monolayer (SAM) upon which the MPC/polymer multilayers would be grown. The polymer-then-MPC solutions used to build up the multilayers are the same as those used in the spectrophotometry experiments (see above) in regard to both concentrations and polymer solution pH values. Potentials are reported vs a Ag/0.1 M AgNO3 reference electrode in acetonitrile. A Pt flag served as the counter electrode. QCM. Quartz crystal microbalance (QCM) experiments were carried out on a polished 5-MHz gold-titanium AT cut crystal (Maxtek, Inc). The oscillation frequency was generated using a home-built oscillator, and the frequency of the crystal was counted using an HP 53131A universal counter. The QCM was powered by an HP E3616A DC power supply. The frequency counter was interfaced to a 486 TMC PC, and a Labview program (designed in-house) monitored the frequency change versus time. Prior to use, the gold surface was modified with a MUA self-assembled monolayer. PAH/MUA multilayers were grown using the same procedures as described above. PSS/ATH multilayers were not studied by QCM.

Results and Discussion Spectrophotometry and pH Effects in Layer-byLayer Growth. The average diameter (transmission electron microscopy) of the hexanethiolate-coated MPCs from which the two, mixed-monolayer MPCs were prepared is 1.6 nm. The 520-nm surface plasmon resonance band for monolayer-protected Au nanoparticles25,28 of this small size, and for ultrasmall Au particles prepared radiolytically,29 is very faint. The surface plasmon band rides atop the strongly absorbing30 interband transitions of the Au cores. The polymer absorbance in comparison (27) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 2001. (28) (a) Vezmar, I.; Alvarez, M. M.; Khoury, J. T.; Salisbury, B. E.; Whetten, R. L. Z. Phys. D 1997, 40, 147. (b) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (29) Henglein, A. Langmuir 1999, 15, 6738. (30) Mulvaney, P. Langmuir 1999, 12, 788.

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Figure 1. UV/vis spectra showing layer-by-layer growth of polymer/MPC films on thiol-functionalized glass slides. (A) PAH/ MUA multilayer film formed by alternately exposing the slide to a pH 9.2 solution of poly(allylamine hydrochloride) (PAH) and another solution of a MPC with a mixed monolayer of hexanethiolate and mercaptoundecanoic acid (MUA) ligands. (B) PSS/ATH multilayer film formed by alternately exposing the slide to a pH 1.4 solution of poly(styrene sulfonic acid) and another solution of a MPC with a mixed monolayer of hexanethiolate and 4-mercaptophenylamine (ATH) ligands. (The hiccups at ca. 400 and 580 are spectrophotometer artifacts.)

should be minor, and it is expected that the spectra of multilayer films will be dominated by their MPC content. Figure 1 shows spectra of the two studied kinds of layerby-layer films (PAH/MUA and PSS/ATH, Scheme 1) as they are grown by alternating exposure to the polymer and MPC solutions. In both cases, the 520-nm surface plasmon resonance band gradually becomes more evident as successive layers are added to the film. We and others have previously observed15,24,31 plasmon band enhancements when MPC cores are induced to approach one another through interactions between their monolayer functionalities. In Figure 1, the plasmon band enhancement for the PSS/ATH multilayer (Figure 1B) is much more pronounced than that for the PAH/MUA multilayer. The implication of the greater enhancement is that the nanoparticle cores experience a relatively stronger intercore surface electronic interaction in the PSS/ATH film, due either to a greater proximity to one another (either in a static or in an average dynamic sense) or to the intervening aromatic medium more effectively transmitting the electronic interaction. We are unable to distinguish between these possibilities. The energies for the surface plasmon peaks in Figure 1 shift very little as the layers are grown, and the intensities of the peaks increase. This is in accord with a previous study32 showing that the plasmon band energy (31) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 7081.

Layer-by-Layer Growth of Polymer/Nanoparticle Films

Figure 2. Plot of 520-nm absorbance of a multilayer film grown on a glass slide as a function of the pH of the polymer solution. (A) PAH/MUA system, pH as shown in the figure. (B) PSS/ATH system, pH as shown in the figure, except for lowest curve (1.4) obtained with a four-step dipping procedure; see text for details.

of MPCs is strongly affected by the core-shell volume ratio but only modestly so by the surrounding solvent dielectric. The spectra in Figure 1 display roughly uniform increases in absorbance, at least for the first several dipping cycles, suggesting that each dipping cycle deposits roughly the same quantity of nanoparticle. Using an experimentally determined molar absorptivity,33 the average 0.026 ( 0.007 increment in absorbance over the first 10 dipping cycles indicates that (7.5 ( 2.1) × 10-11 mol/cm2 of MPCs is incorporated into each kind of layerby-layer film on each dipping cycle. (This result accounts for film growth on both sides of the glass slide.) Assuming that a monolayer33 of MPCs corresponds to 2 ×10-11 mol/ cm2, ca. 3.7 ( 1.3 monolayers of MPC are added per dipping cycle in both multilayer systems. The pH values of the polymer solutions in Figure 1A and B were 9.2 and 1.4, respectively. The film growth rates at these particular pH values (with successive dipping cycles) were found to be the same for the PAH/ MUA and PSS/ATH multilayer systems. The film growth rates, in fact, are strongly pH-dependent, as shown in Figure 2. When the pH of the poly(allylamine) solution is lowered, the PAH/MUA multilayer accumulates more gradually with successive dipping cycles (Figure 2A). The opposite is true of the PSS/ATH multilayer: the multilayer film grows more rapidly at lower poly(styrene sulfonate) polymer solution pH values (Figure 2B). (32) Templeton, A. C.; Mulvaney, P.; Pietron, J. J.; Murray, R. W. J. Phys. Chem. B 2000, 104, 564. (33) The (520-nm) molar absorptivity () of MPCs in dilute solution and the quantity of MPCs in a single monolayer (Γ) have been determined34 as 3.7 × 105 M-1 cm-1 and 2 × 10-11 mol/cm2, respectively. The number of monolayers deposited in each dipping cycle can be calculated from changes in absorbance by A ) 103Γ.

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We have not conducted an extensive examination of the several factors that might influence formation of the layer-by-layer film, such as the polymer, MPC, and electrolyte concentrations. The following observation does show that film growth can be quite sensitive to the solution exposure protocol. The lower (pH 1.4) curve in Figure 2 shows the result of an alternative (four-step) dipping procedure for the PSS/ATH system, where the film was explicitly acidified in separate dipping steps between the ATH MPC and PSS dipping steps. The four-dip procedure was as follows: (1) After being removed from the MPC solution (24 h exposure), the glass slide was rinsed with ethanol and water and placed in a pH ∼1.4 acid solution for 5 min. (2) The slide was removed, rinsed with water, and placed in a pH 6.7 PSS polymer solution for 10 min. (3) The slide was then removed from the polymer solution, rinsed with water, and returned to the pH ∼1.4 acid solution for 5 min. (4) The glass slide was then removed from the acid solution, rinsed with water and ethanol, and placed in the MPC solution for 10 min. Steps 1-4 were repeated to build a multilayer film on the glass slide. This procedure produced a uniform film growth rate (per four-step dipping cycle) that added ca. 0.6 ( 0.3 monolayer of nanoparticles per dipping cycle. This growth rate is slower than that of pH 6.7 PSS (without the separate pH 1.4 acidifications) and is much slower than the growth rate of 3.5 MPC monolayers per cycle achieved in the simpler procedure of Figure 2B (top curve) in which the PSS solution was pH 1.4. In interpreting the acidity effects depicted in Figure 2, several points should be kept in mind. First, the fact that the MPCs used in the Figure 2 experiments included ca. 20 MUA and ATH ligands per MPC ensures that, under basic and acidic conditions, respectively, the nanoparticles will be polyanionic and polycationic, respectively. Second, although soluble in ethanol, the ATH MPC is poorly soluble in water except at low pH, where it is protonated. The MUA MPC is also poorly soluble in water, but its solubility increases at higher pH where carboxylates are formed. Third, the PSS polymer is a strong acid and can be considered polyanionic at all of the pH values employed here. The PAH polymer is polycationic at low pH and remains partly cationic (pKa ≈ 9.4) at the highest pH (9.2) employed. (The pKa’s of individual amine sites on the PAH polyelectrolyte, of course, exhibit a dispersity,9b and the values cited above are only estimates of how protonation changes with pH.) Fourth, one can expect that the amines of the ATH MPCs in a PSS/ATH multilayer will be protonated and polycationic following exposure of the film to a pH 1.4 acid solution but less completely so following exposure to a pH 6.7 PSS solution. Also, for a PAH/MUA multilayer, one can expect that the MUA MPCs in the film will be deprotonated and polyanionic following exposure of the film to a pH 9.2 PAH solution but more neutral following exposure to a pH 3.6 PAH solution. Fifth, no attempt was made to buffer the MPC dipping solutions, so the interfacial acidity of a multilayer film exposed to a MPC solution is likely to be influenced by the pH of the preceding polymer solution dipping exposure. Sixth, the incorporation of multilayers of MPCs in a single dipping cycle (i.e., pH 9.2 in Figure 2A and pH 1.4 in Figure 2B) must reflect a polymer surface that is not smooth but rather has a substantial roughness, or an ability to loop chains into the MPC layer, or a permeability to incoming MPCs. This latter behavior is somewhat analogous to multilayer MPC growth where metal ions are used to form coordinative bridges;20 the metal ions are judged to be capable of migrating into the growing MPC multilayer.

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Considering the preceding points, we seek an understanding of (a) the source of the pH dependence of multilayer growth seen in Figure 2A,B and (b) the reason for the strong dependence of the results in Figure 2B on the solution exposure protocol. We believe that issue (a) is a consequence of the fifth point, namely, that the acidity or basicity retained in the multilayer films can alter the state of protonation of the MPCs at the interface where they are being incorporated into the films. Thus, the acidity retained in a PSS/ATH film after exposure to a pH 1.4 PSS solution promotes protonation of incoming ATH MPCs and consequent electrostatic binding to the polyanionic PSS layer. When the PSS solution has a higher pH (i.e., 6.7), this retained acidity effect is lessened, and fewer protonated ATH MPC nanoparticles become incorporated into the PSS/ATH multilayer film. In regard to the PAH/ MUA system, a similar argument applies, namely, that the basicity retained in a PAH/MUA film after exposure to a pH 9.2 PAH solution promotes deprotonation of incoming MUA MPCs and consequent electrostatic binding to the polycationic PAH layer. When the PAH solution has a lower pH (i.e., 3.6), the MUA MPCs are less ionized, whereas the PAH component of the film is strongly cationic. It can be surmised that hydrogen-bonding effects then become more important; the role of hydrogen bonding in a weak acid-weak base layer-by-layer film has been reported by Hao.15 Previous reports9c,d have outlined the pH dependence of amounts of polyelectrolyte attached in layered films as the “titration” of one polyelectrolyte by the other. Therefore, varying the charge of polyelectrolyte already incorporated into the film requires more or less of the incoming counterionic polyelectrolyte to neutralize the existing surface charge, which is a source of the pH dependence of the film thickness. A similar picture applies to the MPC/ polymer films, with the additional nuances noted above. As for issue (b), the essential difference in the results in Figure 2B (top curve vs lower curve) is the exposure of a PSS/ATH multilayer (containing multiple monolayers from the previous dipping cycle exposure to the pH 6.7 ATH MPC solution), in the lower curve, to a pH 1.4 aqueous bath containing no PSS polymer. This causes the quantity of MPCs in the resulting film to be decreased to less than 1 monolayer. The most straightforward explanation of this result is that a large fraction of the ATH MPCs becomes solubilized in the pH 1.4 aqueous bath. In summary of the above discussion, the factors that promote layer-by-layer growth in the present system are primarily electrostatic interactions. Hydrogen bonding is seen to play a less dominant role, and the pH-dependent solubilization of one of the layer components (the ATH MPC) is also important. Concealed within these several effects might also be hydrophobic interactions between the hexanethiolate part of the MPC monolayers and the organic polymer. Electrochemistry. Double layer charging of a metallic nanoparticle refers to an alteration of the metallic core free-electron population, accompanied by the formation of an ionic space charge in the electrolyte solution around the nanoparticle. Such charging has been studied electrochemically for a variety of nanoparticles35 including MPCs.36 Also, for very small, reasonably monodisperse cores and low-dielectric monolayer MPCs, the double layer (34) (a) Hicks, J. F.; Patel, N.; Murray, R. W. University of North Carolina, Chapel Hill, North Carolina. Unpublished results, 2000. (b) Cliffel, D. E.; Murray, R. W. University of North Carolina, Chapel Hill, North Carolina. Unpublished results, 2000. (35) Ung, T.; Giersig, M.; Dunstan, D.; Mulvaney, P. Langmuir 1997, 13, 1773.

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capacitance per nanoparticle is sufficiently small (subattofarad) that palpably large changes in the nanoparticle potential result from single-electron transfers to/from the MPC core.22,37 We term this size-dependent property quantized double layer (QDL) capacitance, and it is evidenced by multiple, regularly spaced current peaks in the voltammetry of MPC solutions37 and films.20 If the MPCs are not monodisperse, the QDL patterns are blurred in the mixture, and only a smooth current continuum is seen. QDL peaks have been observed23 in the voltammetry of multilayer films composed of MUA-functionalized MPCs similar to those used in this study. Those MPC multilayers were formed by using metal ion/carboxylate coordinative bridges in a layer-by-layer assembly, in contrast to the present structures, which are alternating MPCs and polyelectrolytes. In the present work, the multiple-currentpeak voltammetric signature of the QDL phenomenon can be observed when using the poly(allylamine) polymer in the layer-by-layer PAH/MUA system,38 as shown in the cyclic voltammetry (CV) and differential pulse voltammetry (DPV) traces in Figure 3. The voltammetric responses are qualitatively similar to those observed in electrolyte solutions of the same MUA MPCs and are spaced approximately the same distance apart on the potential axis, indicating a very similar double layer capacitance. The capacitance per MPC in the film is 0.61 aF as determined from a plot37a (Figure 3 inset) of the QDL charging peak potentials versus the charge state of the MPC core (relative to EPZC at ca. -0.2V). The corresponding capacitance for the dissolved MPC is 0.58 aF in the same electrolyte solution. (Changes in nanoparticle capacitance with environment in films can occur, as, for example,39 in the solid-state single-electron charging of a gold nanoparticle in a biopolymer film.) That QDL peaks are seen in the multilayer PAH/MUA film’s voltammetry in Figure 3 (top) means that, at low potential scan rates, electronic charge can be passed through the MPC layers rapidly enough to maintain the Fermi level of the MPCs in the entire film in nearequilibrium with that of the electrode. (This is not the case at fast scan rates; see below.) The increase in peak current with dipping cycle, at a potential sweep rate of 10 mV/s, is linear up to four dipping cycles (each dipping cycle attaches ca. 3.5 MPC monolayers). This means that the double layers of roughly 14 monolayers of MPC cores are being charged in the cyclic voltammogram of the thickest film in Figure 3 (top). This fast charging of multilayers of MPCs was also seen in metal-ion-linked (via carboxylate metal ion chemistry) multilayer films.20,23 The present experiments take the observation a step further by demonstrating concerted QDL charging of multiple MPCs within a polymer matrix. We have not attempted the quantitative measurements of the rates of core-core electron hopping that we recently reported for the other case.23 The observation of electronic charge transport through the PAH/MUA film relies upon electrolyte ion mobility (36) Green, S. J.; Pietron, J. J.; Stokes, J. J.; Hostetler, M. J.; Vu, H.; Wuelfing, W. P.; Murray, R. W. Langmuir 1998, 14, 5613. (37) (a) Chen, S.; Murray, R. W.; Feldberg, S. W. J. Phys. Chem. B. 1998, 102, 9898. (b) Chen, S.; Murray, R. W. J. Phys. Chem. B. 1999, 103, 9996. (c) Chen. S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098. (d) Hicks, J. F.; Templeton, A. C.; Chen, S.; Sheran, K. M.; Jasti, R.; Murray, R. W.; Debord, J.; Schaaff, T. G.; Whetten, R. L. Anal. Chem. 1999, 71, 3703. (38) The film preparation is slightly different; see the figure legend. (39) Berven, C. A.; Clarke, L.; Mooster, J. L.; Wybourne, M. N.; Hutchison, J. E. Adv. Mater. 2001, 13, 109.

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Figure 3. Quantized double layer charging voltammetry of PAH/MUA film in 0.1 M Bu4NPF6 in CH2Cl2, potential vs Ag/ AgNO3 reference, 0.02-cm2 Au working electrode area, Pt flag counter electrode. (Top) Cyclic voltammetry (potential scan rate 10 mV/s) of films prepared with 2, 3, and 4 polymer/MPC dipping cycles. (Bottom) Differential pulse voltammetry (potential scan rate 50 mV/s) of a PAH/MUA film prepared using 11/2 dipping cycles (one cycle plus an extra final dip in the PAH solution). Potentials of QDL charging peaks (labeled with asterisks) are plotted in the inset vs the MPC core charge state; the slope gives a capacitance of 0.61 aF/MPC.

within the film. When an electrode potential is applied and electronic charge is passed between the electrode and the film, countervailing electrolyte ingress or egress occurs so that local electroneutrality is maintained within the film. The double layer charging of each nanoparticle (which generates an ionic space charge balancing the electronic charge on the core) is thus, at equilibrium, more or less independent of the other nanoparticles. Second, the observation of electronic charge transport shows that the nanoparticles have some degree of short range mobility, i.e., that the film’s structure is not static but somewhat fluid. Electron transport through the film to charge the nanoparticle cores is a bimolecular process in which electrons hop between nearby nanoparticles. Relatively fast electron transport then relies on thermal motions of nanoparticles that bring them close together at a reasonably fast frequency. If the films were static and the nanoparticles lacked microscopic mobility, the intervening polyelectrolyte structure would act as a tunneling barrier that retards the rate of electron hopping and transport. That is, there is a dual mobility of electrolyte ions and nanoparticles within the film, whose layered structure cannot be regarded as being static. Third, the observation of electronic charge transport as a QDL process, in which the MPC core charges are uniformly incremented by singleelectron changes producing discrete current peaks, is possible only when the MPCs are reasonably monodisperse, as noted above. There are major differences in the double layer charging properties of the PAH/MUA and PSS/ATH layer-by-layer

Figure 4. (A) Electrochemistry resulting from films prepared using five dipping cycles: PAH film (solid line) and PSS film (dashed line) at 1 V/s, 0.1 M Bu4NPF6 in CH2Cl2, potential vs Ag/AgNO3 reference, 0.02-cm2 working electrode area, Pt flag counter electrode. (B) Plot of current at 0 mV vs Ag/AgNO3 vs potential scan rate for PSS film after 1 (b), 5 (9), and 10 (2) dipping cycles. (C) Plot of current at 0 mV vs Ag/AgNO3 vs potential scan rate for PAH film after one dipping cycle.

films. The fine structure of QDL charging is not seen in PSS/ATH multilayer films. (The core size monodispersity in the MPC sample40 might have been inadequate.) Instead, the PSS/ATH multilayer shows a featureless double layer current envelope (Figure 4A, dashed line). However, the rate at which double layer charge can be transported through the PSS/ATH multilayer is quite high relative to the rate for the PAH/MUA multilayer. If the two multilayers are grown to approximately equal thicknesses (containing ca. 3.5, 15, and 35 MPC monolayers, estimated from the UV/vis spectra), their voltammetry observed in the same electrolyte solution at potential scan rates varying from 50 mV/s to 25 V/s shows that the PSS system charges much more rapidly than the PAH system. Figure 4A shows that, at 1 V/s, the PSS/ATH multilayer displays a classical rectangular charging envelope,27 in contrast to the distorted and tilted current-voltage plot for the PAH/MUA system. Also, currents grow linearly (40) MPC samples must be reasonably monodisperse in core size to exhibit QDL charging of the MPC core.

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Figure 5. Cyclic voltammetry (50 mV/s) of a 2 mM ferrocene solution at a bare electrode (solid line), in contact with a PAH multilayer film (dashed line), and in contact with a PSS multilayer film (thin line) in 0.1 M Bu4NPF6 in CH2Cl2, potential vs Ag/AgNO3 reference, 0.02-cm2 Au working electrode area, Pt flag counter electrode. Both films contain ca. 17 MPC monolayers, attached in five dipping cycles,41 with each cycle attaching ca. 3.5 MPC monolayers and the final dip being in MPC solution, leaving MPC at the film-solution interface.

with increasing potential scan rates (as expected for capacitance charging) and for thicker films with the PSS/ ATH system (Figure 4B) as compared to the PAH/MUA system (Figure 4C), where the current folds over at relatively low potential scan rates even for a film containing only 3.5 MPC monolayers. The voltammetry of ferrocene (Figure 5) further illustrates the difference in the rate of electron transport through PAH/MUA and PSS/ATH films that were grown to contain approximately the same amounts of MPC (estimated from the UV/vis spectra). Compared to a naked electrode (solid curve), the ferrocene voltammetry on the PSS/ATH multilayer (dotted curve) is slightly affected by the film: the peak potential splitting is ca. 100 mV relative to the ideal reversible 59 mV, and the current is slightly depressed. The PAH/MUA film, on the other hand, substantially shuts off the ferrocene voltammetry (dashed curve). The results in Figures 4 and 5 are consistent with PSS/ ATH multilayers having, relative to the PAH/MUA multilayers, (a) a much higher mobility of electrolyte ions moving through the film (necessary for electroneutrality in double layer charging and in the ferrocene voltammetry) and (b) a greater facility of the MPC-MPC contacts that are necessary to transport electronic charge from the electrode through the film by electrons hopping from MPC core to MPC core. In other words, the PSS/ATH multilayer is ionically more conductive; has a much more open structure; and, as a layer-by-layer film, probably has a greater residual content of charge-compensating counterions than does the PAH/MUA film. Quartz Crystal Microbalance. Growth of the PAH/ MUA multilayer was also tracked in a quartz crystal (41) The dipping procedure is slightly altered from that used to build films on the glass slide. A MUA or ATH SAM is first formed on the electrode surface; from there, the dipping procedure proceeds as follows: (1) The electrode is rinsed with ethanol and then water and placed in the desired polymer solution for 10 min. (2) The electrode is removed, rinsed with water and ethanol, and placed in the appropriate MPC solution for 10 min. Steps 1 and 2 constitute a dipping cycle and are repeated to build a multilayer film on the electrode surface. The final dip is always in a MPC solution.

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Figure 6. QCM results. Plot of decrease in resonant frequency with number of dips for PAH MPC. The pH of the polymer solution was 9.2.

microbalance (QCM) experiment. The change in QCM frequency with each dipping cycle41 is linear, as shown in Figure 6. According to the Sauerbrey equation, such frequency changes are proportional to the change in mass on the crystal surface

∆f ) -2fo2∆m/A(µqFq)1/2

(1)

where fo is the crystal oscillation frequency, ∆m is the change in mass, A is the area of the quartz crystal being modified (1.33 cm2), Fq is the density of quartz, and µq is the shear modulus. The average change in mass was 3.4 × 10-6 g per dipping cycle; using a MPC monolayer mass of 6.9 × 10-7 g/cm2, ca. 3.5 monolayers of MPC nanoparticles were incorporated into the PAH/MUA film per dipping cycle. This result agrees exactly with the spectrophotometric value estimated above under the same experimental conditions, i.e., polymer solution pH of 9.2. The mass of the polymer film is ignored in the above calculation because of the huge difference in mass/charge ratio between the MPC and the poly(allylamine) polymer. Conclusions We have shown that nanoparticles with dimensions (cores of 1.6 nm) more comparable to polymeric repeat unit dimensions than previously observed can be combined with counterionic polymers in layer-by-layer assembly. Both cationic/anionic and anionic/cationic variations of polymers and nanoparticles are demonstrated. Notable aspects of the assembly include multiple monolayers of nanoparticles being incorporated during individual exposure cycles, implying looping/entanglement of charged polymer chains with charged nanoparticles. Also, quantized double layer charging in a layer-by-layer assembled film composed of alternating layers of nanoparticles and polyelectrolyte is demonstrated for the first time. The results expand the range of conceivable layered film growth procedures to a smaller dimensional scale than heretofore recognized. Acknowledgment. This research was supported in part by grants from the National Science Foundation and the Office of Naval Research. J.F.H. thanks the Lord Corporation (Cary, NC) for fellowship support. LA0156255