Preparation and Characterization of a Redox Multilayer Film

Feb 26, 2009 - Phone: +39-059-2055027 (R.S.); +358-2-333-6712 (J.L.). Fax: +39-059-373543 (R.S.); +358-2-333-6700 (J.L.). E-mail: renato.seeber@unimor...
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4868

J. Phys. Chem. C 2009, 113, 4868–4874

Preparation and Characterization of a Redox Multilayer Film Containing Au Nanoparticles Fabio Terzi,† Chiara Zanardi,† Barbara Zanfrognini,† Laura Pigani,† Renato Seeber,*,† ¨ a¨ritalo,‡ and Jouko Kankare‡ Jukka Lukkari,*,‡,§ Timo A Department of Chemistry, UniVersity of Modena and Reggio Emilia, Via G. Campi 183, 41100 Modena, Italy, Department of Chemistry, and Turku UniVersity Centre for Materials and Surfaces (MATSURF), UniVersity of Turku, FIN-20014 Turku, Finland ReceiVed: October 23, 2008; ReVised Manuscript ReceiVed: January 22, 2009

Gold nanoparticles encapsulated by negatively charged molecules have been stably anchored at a Au substrate through layer-by-layer deposition technique, employing a redox polyviologen derivative as the cationic counterpart. UV-vis spectroscopy, quartz crystal microbalance, transmission electron microscopy, scanning electron microscopy, atomic force microscopy, and voltammetric measurements have been performed in order to characterize the systems and to give a rationale to the effect of the deposition conditions on the properties of the resulting multilayers. The behavior of two benchmark electroactive species ([Fe(CN)6]4- and [Ru(NH3)6]3+) has been studied on nanoparticle-terminated multilayers. The nanoparticles provide charge percolation through the multilayer and charge transfer with redox species in solution. The results imply that the electrochemical behavior of nanoparticle-containing films is partly dependent on the charge compensation mode within polyelectrolyte multilayers. 1. Introduction Metal nanoparticles (NPs) are emerging as one of the most important themes of basic and applied research in fields such as nanoengineering, nanoelectronics, and nanobiotechnology.1 Remarkable properties are ascribed to NPs by the reduced size. It results in the modification of the optical, electronic, magnetic, and thermal properties with respect to the bulk metals and in the enhancement of the (electro)catalytic properties, thanks to the high percentage of atoms located on surfaces and at edges and vertexes.2 In most of the applications, NPs are required to be stably anchored on surfaces, which often represents the critical step in the development of devices that exploit their properties. Among different procedures proposed for this purpose, the possibility to fix metal NPs on different substrates through a polymeric matrix is quite attractive. On one hand, a suitable organic component may stabilize the catalytic sites from both a mechanical and a functional point of view, and on the other hand, it may contribute on its own to the properties aimed for the composite material as a whole. It may, in fact, possess its own optical, electronic, and catalytic properties. The layer-by-layer (LbL) deposition technique, first developed by Decher,3 represents an effective method for assembling thin films on different substrates. It is based on the electrostatic interaction between positively and negatively charged polyelectrolytes. The advantage of this deposition procedure is the easy formation of a film of strictly controlled thickness depending on the experimental conditions and, in addition, on the nature of the polyelectrolytes and the number of layers deposited. A number of different “building blocks” can be used for as* To whom correspondence should be addressed. Phone: +39-0592055027 (R.S.); +358-2-333-6712 (J.L.). Fax: +39-059-373543 (R.S.); +358-2-333-6700 (J.L.). E-mail: [email protected] (R.S.); [email protected] (J.L.). † University of Modena and Reggio Emilia. ‡ Department of Chemistry, University of Turku. § MATSURF, University of Turku.

sembling multifunctional thin films, which constitutes a further interesting feature of this deposition technique. In recent years, multilayers consisting of conducting4-7 and redox8,9 polymers, enzymes,10,11 carbon nanotubes,12-14 and of different NPs15-18 have been developed, characterized, and employed in applications. Apart from the nature of the components, the properties of the multilayer can be easily modulated through a proper choice of the deposition conditions. In particular, a buffer solution is often preferred for the deposition of pH-sensitive polyelectrolytes in order to control the form of the species in solution. Moreover, the thickness of each individual layer depends on the ionic strength of the polyelectrolyte solution, higher salt concentration generally yielding thicker polyelectrolyte layers.19,20 The charge percolation through multilayers is an essential property of conducting and redox-active multilayers. The electrochemical properties of the multilayers can be described at best by a capillary membrane model, which involves diffusion through open and partially occluded capillaries in the multilayer.21 Polyelectrolyte multilayers prepared using electroinactive materials are supposed to possess neutral capillaries, and the charge of the outermost polyelectrolyte layer modulates the access of anionic and cationic solution-based redox species into the film.22 In general, the thicker the multilayer, the more depressed the electrochemical signal of the species in solution. On the other hand, in multilayers prepared using electroactive materials, the components, i.e., redox or conducting polyelectrolytes, are actually conductive in characteristic potential intervals. Within these intervals, the effective electroactive surface area in multilayers generally increases with the number of layers.4,23-25 Somewhat different conductive material explored for the formation of films through the LbL deposition are NPs. In this particular case, the metal cores should be encapsulated by a charged organic shell. For the deposition of charged NPs, the necessity to avoid aggregation or even coalescence of the metal cores necessitates a low ionic strength in the deposition solution.26,27

10.1021/jp809402j CCC: $40.75  2009 American Chemical Society Published on Web 02/26/2009

Redox Multilayer Film Containing Au Nanoparticles

Figure 1. Structure of the PV derivative.

The NP-containing films are important for electrocatalytic and electroanalytic applications, and therefore, the detailed knowledge of their electrochemical properties is of outmost importance. In pure NP films, the electron transfer takes place by hopping or tunneling between adjacent NPs,28-30 and the film conductivity depends on the length and nature of the encapsulating molecules. Single-electron charging (quantized double layer charging) has been observed in some NP multilayers.31 When metal NPs are imbedded in redox-inactive polyelectrolyte film, the mechanism of charge transfer once more consists of hopping between adjacent NPs, and in some cases quantized double layer charging can be observed.32 NPs improve the conductivity of the film, and even bulk metal conductivity has been reported.33-35 These multilayers exhibit electrocatalytic properties, which are initially enhanced by increasing the number of NP layers but level off in thicker films.36,37 The electrochemical behavior of electroactive species in solution is rather complex on electrodes covered with these multilayers and depends not only on the nature of the outermost layer but also on the composition of the whole multilayer. The capillary membrane model has been used to describe the electrochemical and permeation properties of NP-polyelectrolyte films, too.38 As an example, the electrochemical responses of [Fe(CN)6]4- and [Ru(NH3)6]3+ have been studied on Au NP (encapsulated by mercaptosuccinic acid)/polylysine multilayers. Both the nature of the outermost layer and the electroactive species affect the behavior. The results have been attributed to different charge percolation pathways, involving ionic diffusion through capillaries and pores, and subsequent charge transfer via Au NPs. Relatively few reports have been published on metal NPs imbedded in redox polyelectrolyte multilayers, especially on their electrochemical properties. In multilayers prepared using Os-derivatized poly(vinylimidazole) and poly(acrylic acid), NPs tend to aggregate, though still being electrocatalytically active.39 On the contrary, negatively charged NPs retain their integrity in multilayers prepared with cationic poly(4-vinylpyridine) carrying pendant viologen moieties. However, the results obtained lead the authors to conclude that charge percolation mainly takes place by hopping between adjacent viologen groups and that the NPs do not markedly contribute to the film conductivity.8 In this work, we report the formation of a multilayer consisting of Au NPs encapsulated by citrate anions (Aucitr NPs) and of a polyviologen derivative, namely, poly(pyridinium, 1,1′[1,2-ethanediylbis[imino(2-oxo-2,1-ethanediyl)]]bis-, dichloride), see Figure 1, i.e., a redox-active polycation. NP loading, aggregation/coalescence phenomena, and electrochemical behavior have been studied as a function of the most important experimental parameters, i.e., the ionic strength of the deposition solutions and the contact time with the NP solution. A detailed study of the influence of the deposition conditions for similar polyelectrolytes on the final properties of the multilayers is still absent in the literature. A polyviologen derivative was chosen as the polycation in view of the well-known properties of this class of redox polymers, also employed as redox mediators.40 Among different polyviologen derivatives, linear polyviologens, i.e., those in which the 2,2′-bipyridine units are linked to the

J. Phys. Chem. C, Vol. 113, No. 12, 2009 4869 4,4′-positions through linear alkyl chains, are particularly appealing, thanks to the possibility of modulating the properties of the resulting polymer by properly choosing the length of the alkyl spacer. In particular, the polyviologen derivative used in the present study, hereafter shortly PV, was chosen also in view of the easy formation of the PV2+/PV+• redox couple, responsible for conduction and mediation.40 As to the Aucitr NPs system, citrate was chosen as the encapsulating agent since (i) the relevant Aucitr NPs are watersoluble, negatively charged, and stable for months in aqueous solution, and (ii) it is a very labile encapsulating agent, permitting the metal cores to interact with the external environment and with the chemical species in solution. This is a very important characteristic with respect to many applications, such as in sensors and in (electro)catalytic systems. 2. Experimental Methods 2.1. Chemicals. Sodium 2-mercaptoethanesulfonate, 98% (MESA) and poly(sodium 4-styrene sulfonate) (PSS), Mw ∼ 70 000 g mol-1, 30 wt % solution, were purchased from Aldrich and used as received. All other reagents were of pure or puriss grade and also used as received. All the aqueous solutions were prepared using reverse osmosis water (Millipore, resistivity 18 MΩ/cm). The precursor for the polyviologen, 2-chloro-N-[2-(2-chloroacetylamino)-ethyl]-acetamide, was synthesized according to literature.41 In short, ethylenediamine (2.64 mL; 30.5 mmol) was first dissolved into diethyl ether (50 mL), and then potassium carbonate (8.5 g; 61.6 mmol) was added. The mixture was then cooled down in an ice bath, and a solution of chloroacetylchloride (5 mL; 61.6 mmol) in diethyl ether (100 mL) was added dropwise. The solution was then allowed to warm up to room temperature and stirring was continued overnight, and the product crystallized while the reaction proceeded. Then water (100 mL) was added, and the resulting mixture was filtered. Recrystallization of the solid with ethanol resulted in 2.92 g of the product, yield 45%. 1H NMR (DMSO): 8.28 (2H, t), 4.04 (4H, s), 3.16 (4H, m). Polyviologen was prepared by dissolving 2-chloro-N-[2-(2chloro-acetylamino)-ethyl]-acetamide (2 g; 9.4 mmol) and 4,4′bipyridine (1.466 g; 9.4 mmol) in dimethylformamide (DMF) (20 mL). The solution was stirred in a nitrogen atmosphere at 105 °C for 16 h. The resulting solid material was filtered and rinsed with diethyl ether, giving 4.15 g of the product. The solid material contains one DMF molecule for each bipyridine. The solid product (0.906 g) was then dissolved in water and dialyzed with a dialysis tube (Pierce Snakeskin, Thermo Scientific, U.S.A.; molecular cutoff 3500 Da) against pure water for 3 days. After evaporation and drying in vacuum, 550 mg of the final product was obtained (yield 72.6%). Aucitr NPs were synthesized in the presence of citrate as the encapsulating agent:42 NaAuCl4 · 2H2O was dissolved in water (0.50 L, 1 mM); the solution was heated to 100 °C, and an aqueous solution of sodium citrate, preheated at 90 °C (50 mL, 40 mM), was added. The synthesis was allowed to proceed at 100 °C for 30 min. After the synthesis, the Aucitr NPs were stored in a refrigerator at 4 °C. The metal core mean diameter was calculated to be 14 ( 1 nm on the basis of transmission electron microscope (TEM) images, acquired using a JEOL 2010 instrument, equipped with an energy filter (Gatan Inc.) and an energy-dispersive spectrometer (EDS-INCA system, Oxford Instruments). To this aim, a drop of the solution in which the NPs have been synthesized was deposited onto a Cu/formvar/ carbon grid.

4870 J. Phys. Chem. C, Vol. 113, No. 12, 2009 2.2. Multilayer Buildup. The assembly of the multilayers was performed on different Au substrates, depending on the technique employed for the characterization (see hereafter). Au substrate has been chosen since it constitutes a widely employed surface for supporting organic thin films, thanks to its chemical inertness and reproducibility after a proper cleaning procedure. Moreover, it is well-known that the surface can be easily modified by anchoring a monolayer of thiol molecules. In any case, the metal surface was first modified by immersion in a 1 mM MESA solution for 30 min, to obtain a negatively charged surface. The multilayer was then built by immersing the substrate alternatively in the cationic PV (1.0 mg/mL) polyelectrolyte solution for a constant time of 20 min and in the anionic Aucitr NPs (0.14 mg/mL of Au) solution for 3, 10, or 20 min. The pH of the deposition solutions was adjusted to 7.0 by a phosphate buffer solution (PBS); the concentration of PBS varied, as will be discussed in the following. After every deposition step, the substrate was carefully rinsed with ultrapure water, immersed in a water bath for 1 min, and finally dried with warm air. An alternative multilayer, employed as a reference to test the actual effect of the presence of NPs inside the film, was assembled by substituting PSS for NPs as the anionic polyelectrolyte. In this case, the film was built by alternately immersing the substrate for 20 min in 1 mg/mL PV and for 10 min in 1 mg/mL PSS solutions. In both deposition steps 10 mM PBS was used as the solvent medium. Hereafter, the notations (PV/X)n or PV/X are used for the different multilayers, X being the polyanion layer, consisting either of NPs or PSS; when necessary, n accounts for the number of the deposited PV/X bilayers. 2.3. Multilayer Characterizations. UV-vis spectra were recorded with a Perkin-Elmer Lambda 650 spectrophotometer, working in the wavelength range of 190-900 nm. In this case, a Suprasil quartz slide (Hellma) was first cleaned in a freshly prepared piranha solution (98% H2SO4/30% H2O2 70:30 mixture; Warning! Piranha solution is Very corrosiVe and must be treated with extreme caution; it reacts Violently with organic material and must not be stored in tightly closed Vessels) for 1 h, rinsed thoroughly with reverse osmosis water, and dried; a thin layer of Au (30 nm) was then sputtered on the quartz substrate, constituting the substrate for subsequent LbL deposition. The samples for TEM and EDS analyses were prepared by dipping a Au grid, cleaned in advance with piranha solution and coated by a MESA layer, into the deposition solutions; finally, it was dried in air for at least 10 min. The analyses were performed by cooling the walls of the sample chamber at liquid nitrogen temperature, in order to minimize contamination. Scanning electron microscope (SEM) images were acquired using an ESEM Quanta-200 instrument (FEI Company) on the same films analyzed by UV-vis spectroscopy. Atomic force microscopy (AFM) measurements were performed in air with a Nanoscope 3 D (Veeco) instrument working in tapping mode, using a Si tip (NT-MDT NSG-11) operating at 105 kHz. The samples were prepared similarly to those employed for the UV-vis spectroscopy measurements. We use the root-mean-square roughness, Rrms,43 which is the estimate of the standard deviation of the bidimensional Gaussian-type distribution of heights around the mean value of the collected points, to describe the surface roughness. For both 3 and 10 min deposition times, 10 different Rrms values were computed from 10 different images. Electrochemical tests were performed with an Autolab PGSTAT12 (Ecochemie) potentiostat/galvanostat, under control

Terzi et al. of GPES-dedicated software, in a single-compartment threeelectrode cell, at room temperature, under Ar atmosphere. A 2 mm diameter Au electrode (Metrohm) was used as the substrate for the deposition of the multilayer. It also acted as the working electrode in the electrochemical measurements, complemented by a glassy carbon rod auxiliary electrode (Metrohm) and by an aqueous Ag/AgCl, 3 M KCl reference electrode (Amel). All the potential values given are referred to this reference electrode unless otherwise stated. The cleaning procedure of the Au substrate/electrode consisted of a mechanical cleaning with 1 and 0.3 µm alumina powder; then the substrate was rinsed with ultrapure water in an ultrasonic bath for 5 min before use. Finally, the procedure was completed by cycling the electrode between -0.20 and +1.15 V versus Hg/HgSO4 with 0.1 V s-1 potential scan rate in 0.5 M H2SO4 solution until a reproducible cyclic voltammogram, typical of a clean Au electrode, was obtained. Quartz crystal microbalance (QCM) analyses were performed using a Picobalance from Technobiochip. The substrate consisted of a 10 MHz AT-cut quartz crystal coated by vapordeposited gold, 0.48 cm2 geometric area (Universal Sensors). This substrate was electrochemically cleaned as already described for Au electrodes. Assuming rigid layer behavior the frequency changes ∆f can be converted to mass loadings using the Sauerbrey equation:44

∆f ) -

2f02∆m A√µqFq

Here f0 is the fundamental resonance frequency of the crystal, A is the electrode area, and µq and Fq are the shear modulus and density of quartz, respectively. 3. Results and Discussion A preliminary step of the study consisted in checking the stability of the NP system in the deposition solution. To this aim, UV-vis spectra of Aucitr NP solutions at various PBS concentrations between 10-3 and 10-1 M were recorded, in order to evidence the possible occurrence of metal core aggregation/ coalescence, which is known to take place in solutions at high enough ionic strength.26 A diagnostic tool for checking the occurrence of aggregation or coalescence is the location of the plasmon band.45,46 For the NPs under study, in presence of very weak mutual interactions in aqueous solution, the plasmon band has its maximum at λmax ≈ 520 nm. A decrease of the height of the absorption peak with a concurrent red-shift, which is diagnostic for the NP aggregation, is clearly evident in 0.1 M PBS, i.e., at the ionic strength usually employed for LbL deposition. Similar phenomena are minimized at lower ionic strength, but only at a PBS concentration as low as 1 mM the solution is stable for several hours, which is a necessary condition for achieving a complete multilayer assembly. In view of this result, an unusually low 10 mM concentration of PBS was used also in the deposition of the cationic polyelectrolyte PV layers. In similar PBS concentration no marked aggregation of the NPs occurred. The eventual aggregation of the NPs will be considered and discussed throughout the whole work. Figure 2 shows a typical increase of the mass of the deposit, estimated using the Sauerbrey equation, during the successive immersion of the growing PV/NP multilayer in the cationic PV and anionic NP solutions, respectively. In view of the UV-vis data obtained during the deposition of the subsequent NP layers

Redox Multilayer Film Containing Au Nanoparticles

J. Phys. Chem. C, Vol. 113, No. 12, 2009 4871

Figure 3. (A) Normalized UV-vis spectra for the (PV/NP)6 multilayer obtained with different dipping times in the NP solution; 10 mM PBS for the cationic layer solution; 1 mM PBS for the NP solution. (B) Relevant trends of Amax vs number of layers. In all cases the spectra have been corrected for the absorption of the bare Au substrate. Figure 2. Estimated typical mass/area variation during the deposition of the second PV layer (in 10 mM PBS) (A) and of the subsequent NP layer (in 1 mM PBS) (B) on a growing PV/NP multilayer.

(see below), it seems reasonable to assume similar weight trends also for the subsequent layers. From Figure 2, one can observe that the PV mass increases only negligibly after ca. 20 min dipping, and this time was thus chosen for the PV layer formation. On the contrary, during the NP deposition, the mass increases almost linearly over the whole time interval explored, i.e., over 30 min, which gives the possibility to control the amount of NPs within the multilayer simply by adjusting the deposition time. Different deposition times for the NP layers were used for building (PV/NP)6 multilayers, whose UV-vis spectra are reported in Figure 3A, and a strong influence of the deposition time is clearly evident. In particular, the plasmon band undergoes a red-shift with respect to that recorded in the deposition solution (∆λ ) 40 and 58 nm for 3 and 10 min dipping times, respectively). This effect, already observed for different multilayers containing metal NPs,47 can be ascribed to NP grafted on solid surfaces, which lowers the average distance between adjacent NPs, with consequent coupling of the plasmon oscillations. Moreover, also the slight effect caused by the change of the dielectric constant of the medium surrounding the metal core has to be taken into account. The bathochromic shift becomes more marked upon increasing the deposition time, which is consistent with the increasing amount of NPs deposited, as indicated by the QCM measurements. In the case of even longer deposition times, i.e., 20 min, the spectrum shows two main absorption bands, suggesting that the multilayer is no more homogeneous. Part of the NPs are in close contact with each other, though retaining their nanoparticulate nature, while a significant fraction of the NPs undergo stronger aggregation or even coalescence.47 In conclusion, since strongly aggregated NPs

loose almost completely their remarkable properties and begin to resemble bulk gold, long deposition times should be avoided. Figure 3B shows that, under the described experimental conditions, the overall amount of NPs increases linearly with the number of layers in the PV/NP multilayers, as evidenced by the NP plasmon band intensity. SEM, AFM, and TEM analyses were performed on PV/NP multilayers prepared using the two different immersion times. The tests were performed also in order to understand the electrochemical behavior of the multilayers (see below). SEM measurements showed that the morphology of both multilayers is quite smooth and homogeneous over the whole surface. The dimensions of the SEM images collected (66 µm × 66 µm) were suitable to evaluate systematic morphological features in the submicrometer range. The AFM images complement the SEM results and allow a morphological analysis of the surface at a finer level. The dimension of the images (2 µm × 2 µm) was chosen in order to see the roughness with spatial frequency in the range of reciprocal nanometers to a few tens of reciprocal nanometers. The PV/NP multilayer formed at the longer NP deposition time (see Figure 4A) exhibits an average Rrms roughness of 0.5 nm. On the other hand, the multilayer obtained with 3 min deposition time (see Figure 4B) exhibits a more corrugated morphology, characterized by a significantly higher average Rrms roughness, i.e., ca. 1.3 nm.48 Finally, the AFM measurements confirm that the Au substrate is completely covered by the (PV/NP)6 multilayer, and no evident pinholes are visible. At an even finer level of morphological analysis of the deposit surface, TEM images of the multilayers show the formation of a composite with a high density of NPs. In particular, the interface of the films prepared with the longer deposition time (10 min, see Figure 5A) appears quite smooth and the surface contains densely packed Aucitr NPs. In the case of shorter

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Terzi et al.

Figure 4. AFM images relative to (PV/NP)6 prepared employing different dipping times in NP solution: (A) 10 min and (B) 3 min.

Figure 6. Voltammetric responses of 1 mM [Fe(CN)6]4- in 0.1 M PBS on different (PV/NP)n multilayers: (A) 10 min and (B) 3 min of deposition time for the NPs. (PV/NP) multilayers terminate with a negative NP layer. Potential scan rate was 0.050 V s-1.

Figure 5. TEM images of the film/ambient interface of two (PV/NP)6 multilayers prepared using (A) 10 min and (B) 3 min of NP deposition time. (C) Image on a particularly thin region of the film in panel A; focus on the edge of the film.

deposition time (3 min, see Figure 5B), the interface is significantly more corrugated and the NP density is lower. The AFM and TEM images are in agreement with the UV-vis spectroscopic results, which show that the average distance between the NPs decreases with increasing deposition time. However, in both cases the UV-vis spectra (Figure 3A) indicate that the Aucitr NPs remain individual entities. Interestingly, individual NPs are clearly evident in a few TEM images obtained by focusing the beam at particularly thin border regions of the film. These images (see Figure 5C) enable us to see the individual NPs and to check the average diameter of the NPs imbedded in the organic matrix. In all cases, the Aucitr NPs are homogeneously distributed within the polymer matrix, without any coalescence, and their mean diameter (ca. 14 nm) is equal to that of the individual NPs metal cores in solution. The lack of coalescence is clearly seen by following the growth of the deposit at different stages of multilayer assembly directly

on the TEM grid (the TEM image collected after depositing three bilayers is shown in the Supporting Information). The electrochemical properties of the PV/NP composite, obtained with both 3 and 10 min deposition times of the NP layer, were tested in the negative and positive potential regions with pure PBS solution and with PBS containing the benchmark electroactive species [Fe(CN)6]4- and [Ru(NH3)6]3+. For the (PV/NP)6 multilayer in pure 0.1 M PBS without any electroactive species in solution, a single reversible voltammetric wave of the immobilized PV2+/PV+• couple is observed with the forward cathodic peak located at -0.42 V (see Figure 7). The peak current is linearly dependent on the potential scan rate, typical of a surface-confined redox process, which suggests that the film is thin and the counterion diffusion exhibits negligible influence on the electrochemical response of the viologen moieties. Accordingly, the relatively small difference between the forward and backward peak potentials (ca. 80 mV) attests the low value of the ohmic drop inside the six-bilayer structure. Similar results were obtained for multilayers prepared with different NP deposition times (3 and 10 min). Voltammetric curves recorded at different scan rates for 3 min NP deposition time are reported in the Supporting Information. On PV/NP multilayer prepared using the 10 min NP deposition time, the voltammetric behavior of [Fe(CN)6]4- overlaps that on bare Au (see Figure 6A) and is independent of the number of the bilayers deposited. Since in this potential range polyviologen cannot participate in charge transport, this behavior indicates effective charge percolation throughout the film and suggests the formation of very crowded Aucitr NPs on the whole external surface of the multilayer. When a shorter NP deposition time (3 min) was used, the heights of the voltammetric peaks of the [Fe(CN)6]4-/3- couple increase with the number of bilayers and are greater than on bare Au electrode (see Figure 6B for n ) 2 and 6). Similarly, in this case, effective charge percolation

Redox Multilayer Film Containing Au Nanoparticles

Figure 7. Voltammetric responses of 1 mM [Ru(NH3)6]3+ in 0.1 M PBS on different (PV/NP)n multilayers and of a (PV/NP)6 multilayer in pure 0.1 M PBS. (PV/NP) multilayers terminated with a negative NP layer. Potential scan rate was 0.050 V s-1.

takes place through the multilayer, but the increase of the redox response with the film thickness is more enigmatic. A linear dependence of the peak current on the square root of the potential scan rate on electrodes modified with (PV/NP)6 multilayers indicates a diffusion-controlled oxidation process in both cases. The voltammetric traces recorded at different scan rates can be found in the Supporting Information. Although the individual layers in polyelectrolyte multilayers are generally strongly interpenetrating, the electrochemical properties of the multilayers are sensitive to the charge of the terminating layer. In this work we have used NP-terminated multilayers with negatively charged outer layer, which should favor the redox response of positively charged solution-based redox species. Above we have noted that on certain multilayers the oxidation-reduction currents of an anionic redox couple increase with the film thickness. In order to clarify the behavior of the [Fe(CN)6]4-/3- couple on electrodes covered with PV/ NP multilayers we have studied another redox system based on the [Ru(NH3)6]3+/2+ couple. Both redox couples have similar diffusion coefficients in aqueous solutions.49 On polylysine (pLys)/NP multilayers these couples have previously been found to show a strong terminating layer effect.38 In the paper of Chirea et al., the current peaks due to the cationic redox probe increase with film thickness on NP-terminated (negatively charged) multilayers and decrease on pLys-terminated films. For the anionic probe, very little changes occur on positively terminated films, whereas rapid decay of response takes place on NPterminated multilayers. In the present study, on electrodes modified with (PV/NP)6 multilayers, the cyclic voltammograms of the [Ru(NH3)6]3+/2+ couple in solution are similar than on bare Au electrode and independent of the number of PV/NP bilayers and of the NP deposition time (3 or 10 min) (Figure 7). The voltammetric peaks occur at the same potentials than on bare gold, i.e., at the thermodynamic value, and at considerably more anodic potentials than the reduction of PV. The peakto-peak separation, close to 60 mV, and the linear dependence of the cathodic peak current on the square root of the potential scan rate indicate that the [Ru(NH3)6]3+ reduction is reversible and diffusion-controlled (the voltammetric curves recorded at different scan rates are reported in the Supporting Information). This suggests that metal NPs constitute a conducting pathway throughout the multilayer from the Au substrate to the film/ solution interface and that the outer surface acts as a pseudosolid-gold electrode for the solution species, similar to that in the case of the [Fe(CN)6]4-/3- couple (Figure 6). In the work of Chirea et al. the increased [Ru(NH3)6]3+/2+ redox currents on NP-terminated films were attributed to the electrocatalytic

J. Phys. Chem. C, Vol. 113, No. 12, 2009 4873 effect of the Au NPs.38 Although the encapsulating agent and the nature of the polycation are different in the present work, it is remarkable that we do not notice any current dependence on the number of PV/NP bilayers with [Ru(NH3)6]3+. On the other hand, enhancement of the current is observed for the [Fe(CN)6]4probe species. In addition, in this case, the time used for the NP deposition strongly affects the observed behavior. The AFM and TEM results show that the PV/NP multilayers prepared using the 3 min NP deposition time are much rougher than those prepared using the 10 min deposition time (or the bare gold electrode). However, the increased surface area cannot explain the results because the time scale of voltammetry is much too long for the submicrometer scale surface roughness to have any effect on the diffusion profiles. We have previously shown that anions penetrate into multilayers to different extent and the tendency of incorporation follows the Hofmeister series.50 In addition, multiply charged ions tend to bind more strongly in polyelectrolyte multilayers than the singly charged ones. This suggests a strong binding of the multivalent anion [Fe(CN)6]4- to the polycation backbone as a plausible explanation for the enhancement of the voltammetric response in this case. In multilayers prepared using the 3 min NP deposition time the NP loading is lower than in the films prepared with the 10 min deposition time. As a consequence, after preparation, in the former case not all positive charges in the PV chains are compensated by the negative NPs but, rather, by small counteranions, and the labile nature of the encapsulation may also favor this partially extrinsic charge compensation. During voltammetric experiments, these extrinsically compensated positive charges can be neutralized by [Fe(CN)6]4- anions, and the diffusive flux of these incorporated electroactive species to the interconnected NP network inside the multilayer adds to their diffusive flux from the solution. On the other hand, the positive electroactive probe [Ru(NH3)6]3+ cannot bind to multilayers because there is no extra immobilized negative charge to be compensated. Therefore, no dependence on the NP deposition time can be observed in the redox response of [Ru(NH3)6]3+. The active role of NPs in charge percolation through the PV/ Au NP composite film was supported by control experiments with multilayers prepared using an electroinactive component instead of Au NPs. The preparation of insulating NPs with the same size and surface charge than the Au NPs used in this work is very difficult. Therefore, we have studied the redox behavior of the [Fe(CN)6]4-/3- and [Ru(NH3)6]3+/2+ redox couples on electrodes covered with a PV/PSS multilayer. Substitution of PSS for Au NPs in the multilayer causes a significant change in the electrochemical response after each PSS layer. In case of [Fe(CN)6]4-/3- the redox current decreases and the peak-topeak separation increases as a function of the number of bilayers, demonstrating that the film resistance increases with thickness. After six bilayers the redox signal is completely lost, and the electrode is covered with an insulating, impermeable film (Figure S5 in the Supporting Information). On the other hand, on PV/ PSS multilayers the reduction of [Ru(NH3)6]3+ shifts cathodically to the potential range in which polyviologen is in the mixed-valence state (Figure S6 in the Supporting Information). This enables charge percolation through the film by electronhopping mechanism, typical of redox polymers, and PV can act as a charge mediator between the redox species in solution and the electrode. However, a small voltammetric wave can be seen close to the thermodynamic redox potential of the [Ru(NH3)6]3+/2+ couple. This redox process can be attributed to [Ru(NH3)6]3+ incorporated within the film, similar to

4874 J. Phys. Chem. C, Vol. 113, No. 12, 2009 [Fe(CN)6]4-/3- in PV/NP multilayers. In PV/PSS multilayers the cationic PV takes the role of the bulkier component, while the longer PSS molecules can form loops and tails where charge is compensated by small cations. Similar considerations may also explain the contrasting behavior of the previously reported pLys/NP multilayers compared to the PV/NP films studied in this work.38 The structure and properties of polyelectrolyte multilayers respond in an often unpredictable way when their constituents are changed. Polylysine is a long flexible polymer, whereas the polyviologen derivative synthesized in this work is probably much shorter. On the other hand, the thiol modification of the NPs used in the pLys/NP multilayers is more stable than the citrate encapsulation in this work. 4. Conclusions We have successfully prepared LbL multilayers from cationic redox polyelectrolyte, a polyviologen derivative, and Au NPs encapsulated by negatively charged citrate ions. The Aucitr NPs are stably anchored in the multilayers via electrostatic interactions. The amount of NPs in the film can easily be controlled by their deposition time, and it strongly influences the properties of the multilayer. Very long deposition times (20 min) induce strong aggregation of the NPs, while the shorter times tested, i.e., 10 and 3 min, lead to the incorporation of individual NPs that retain their properties. AFM and TEM images show that the multilayers prepared using the 10 min deposition time for the NPs exhibit lower surface roughness and more crowded NPs at the film/ambient interface than those prepared with the deposition time of 3 min. In the former case, the outer NP layer forms a pseudo-solid-state surface exposed to the solution. Voltammetry of redox species in solution shows that the NPs inside the film enable efficient charge percolation through the film, also in the potential region where PV does not take part in the charge transport. In multilayers prepared using a short (3 min) NP deposition time part of the immobilized positive charge is extrinsically balanced by small anions in solution, which can be exchanged for solution-based electroactive species These results imply that the electrochemical behavior of NP-containing multilayers is partly dependent on the charge compensation mode within polyelectrolyte multilayers, either extrinsic or intrinsic. ¨ . thanks The Academy of Finland Acknowledgment. T.A (Grant No. 111066) for financial support. Centro Interdipartimentale Grandi Strumenti (Universita` di Modena e Reggio Emilia) is acknowledged. Supporting Information Available: TEM image of (PV/ NP)3, scan rate dependence of the electrochemical responses of (PV/NP)6 in pure PBS and in the presence of [Fe(CN)6]4-/3and [Ru(NH3)6]3+/2+ redox couples, and the cyclic voltammograms of [Fe(CN)6]4-/3- and [Ru(NH3)6]3+/2+ on (PV/PSS)n modified electrode. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Nanoscale Materials in Chemistry; Klabunde, K. J., Ed.; John Wiley and Sons: New York, 2001. (2) Bond, G. C.; Louis, C.; Thompson, D. T. Catalysis by Gold; Hutchings, G. J. H., Ed.; Catalytic Science Series; World Scientific Publishing: London, 2006; Vol. 6. (3) Decher, G. Science 1997, 277, 1232. ¨ a¨ritalo, T.; Paukkunen, (4) Lukkari, J.; Saloma¨ki, M.; Viinikanoja, A.; A J.; Kocharova, N.; Kankare, J. J. Am. Chem. Soc. 2001, 123, 6083. (5) Lukkari, J.; Saloma¨ki, M.; Aaritalo, T.; Loikas, K.; Laiho, T.; Kankare, J. Langmuir 2002, 18, 8496.

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