Redox Switching of Polyoxometalate–Methylene Blue-Based Layer

*Fax: +353 42 933 1163; Tel: +353 42 937 4579; E-mail: [email protected]. ... Shahzad Imar , Fathima Laffir , Gordon Armstrong , and Timothy McCorm...
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Redox Switching of Polyoxometalate−Methylene Blue-Based Layer-by-Layer Films Nargis Anwar,† Mikhail Vagin,† Rashda Naseer,† Shahzad Imar,† Masooma Ibrahim,‡ Sib Sankar Mal,‡ Ulrich Kortz,‡ Fathima Laffir,§ and Timothy McCormac*,† †

Electrochemistry Research Group, Department of Applied Science, Dundalk Institute of Technology, Dublin Road, Dundalk, County Louth, Ireland ‡ School of Engineering and Science, Jacobs University, P.O. Box 750561, 28725 Bremen, Germany § Materials and Surface Science Institute, University of Limerick, Limerick, Ireland ABSTRACT: Iron-substituted crown-type polyoxometalate (POM) [P8W48O184Fe16(OH)28(H2O)4]20− has been successfully immobilized onto glassy carbon electrode surfaces by means of the layerby-layer (LBL) technique employing the cationic redox active dye, methylene blue (MB). The constructed multilayers exhibit pHdependent redox activity for both the anionic POM and the cationic dye moieties, which is in good agreement with their solution behavior. The films have been characterized by alternating current impedance, atomic force microscopy, and X-ray photoelectron spectroscopy, whereby the nature of the outer layer within the assemblies was found to have an effect upon the film’s behavior. Preliminary investigations show that the POM dye-based films show electrocatalytic ability toward the reduction of hydrogen peroxide, however, only when there is an outer anionic POM layer.

1. INTRODUCTION Polyoxometalates (POMs) are inorganic metal−oxygen clusters that display great diversity in both their structure and composition.1,2 Their properties enable them to be employed across a wide domain, including material science, medicine, catalysis, biotechnology, and nanotechnology.3−12 What is of general interest when considering these application domains is the ability to surface-immobilize these POMs onto a variety of surfaces whereby their inherent redox and photophysical properties are maintained. The various techniques utilized to date for surface attachment of POMs include self-assembled monolayers (SAMs), Langmuir−Blodgett and sol−gel films, electrodeposition, entrapment into conducting polymer films, and the layer-by-layer (LBL) self-assembly method.13−24 Electrostatic attractions and van der Waals forces are considered to be involved during the growth of the such LBL layers.25 Utilizing the electrostatic attraction between oppositely charged species,26 the LBL method is a great tool of immobilization for the construction of organized multilayer assemblies. Iler was the first to discover the method in 1966,27 and it was not until 1991 that this work was rediscovered through the work of Decher and Hong.28 The LBL method is both simple and efficient with functional supramolecular systems being easily fabricated on various surfaces by controlling the composition, thickness, and orientation of each layer at the molecular level within the assembly. These structures show good mechanical and chemical stability, which make them attractive for sensing and electronic applications. The possibility to adopt different sizes and shapes of the © 2012 American Chemical Society

substrate is also another advantage of the LBL technique.26,29−31 A wide range of POMs have been surface-attached through the LBL technique, e.g., Wells−Dawson-type [P2W18O62]6−, Keggin-type [α-SiW12O40]4−, transition metal-substituted Krebs-type POMs [X2W20M2O70(H2O)6]n−, where (X = Bi or Sb, M = Co 2+ or Cu 2+ ), and sandwich-type POMs [Co4(H2O)2(PW9O34)2]10−.6,32−34 A number of substrates have also been employed, such as glassy carbon,6,30,32,33,35−37 highly ordered pyrolytic graphite,37 mercury, platinum, gold,36 quartz,26,30,32,33,35,38 indium tin oxide (ITO),34,37−39 goldcoated quartz,37,39 silicon,38,39 and mica substrate.39 A variety of cationic moieties have been incorporated into these POMbased multilayers systems, such as, ruthenium(II) polypyridyl complexes, 38,40 conducting 41,42 and redox active polymers,30,36,43 metallodendrimers,35 metalloporphyrins,44 polyelectrolytes,29,32,33,39,45−47 cationic surfactants,40 dye molecules,36 and various multiply charged cations.6,37,44,47 Two methods are generally used to construct the multilayer assemblies onto a modified surface. The first one is immersion growth, e.g., alternately dipping a solid substrate into two solutions of oppositely charged modifiers.30,36,41−43 Electrochemical growth involves alternate cyclic potential sweeps of the substrate being performed in a solution of oppositely charged species.36,44 Cyclic voltammetry,6,30,35,36,39 UV/visible Received: January 27, 2012 Revised: February 21, 2012 Published: February 22, 2012 5480

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(UV−vis) spectroscopy,6,26,30,32−35,37−39 and impedance spectroscopy30 were employed to look at the growth of multilayer assemblies onto substrates. The characterization of multilayers was performed by electrochemical quartz crystal microgravimetry (EQCM), 37 scanning electron microscopy (SEM),26 atomic force microscopy (AFM),32,33,35,38,39 X-ray photoelectron spectroscopy (XPS),38,39 fluorescence spectroscopy,26 Fourier transform infrared (FTIR) spectroscopy, and electron spin resonance (ESR) techniques.39 Different polycations and polyanions have also been adhered to the electrode surfaces by the LBL technique using a range of ionic dyes.45−49 The combination of organic dye moieties, which possess high molar extinction coefficients and absorb visible light readily, with redox active POMs, which are poor absorbers of visible light, can yield organic− inorganic hybrid materials that possess unique photocatalytic properties. However, only a few examples are reported for the LBL immobilization of POMs with dye molecules.26,50 The LBL technique is employed for the electrode surface attachment of metal nanoparticles, which can be used for detailed studies of the kinetics of electron transfer to them.51 The recently reported iron-substituted POM of crown-type [P8W48O184Fe16(OH)28(H2O)4]20− (P8W48Fe16),52 shown in Figure 1, possesses 16 equivalent iron centers, which are

employment of the LBL technique. It is seen that the characteristic redox behavior of both the POM and the MB have been maintained within the solid state. The results for the electrochemical and surface properties of these nanostructured POM-based layers is discussed in detail. This contribution represents the first time successful immobilization of [P8W48O184Fe16(OH)28(H2O)4]20−.

2. EXPERIMENTAL SECTION 2.1. Materials. The hydrated potassium-lithium salt of the crowntype 48-tungsto-8-phosphate Li 4 K 16 [P 8 W 48 O 184 Fe 16 (OH) 28 (H2O)4]·66H2O·2KCl (LiK- P8W48Fe16) was synthesized according to the literature.53 An 8% solution of poly(diallyldimethylammonium chloride) (PDDA, MW 20000) was prepared from stock. All other chemicals were of reagent grade, purchased from Aldrich, and were used as received unless otherwise stated. Alumina powders of sizes 0.05, 0.3, and 1.0 μm were received from CH Instruments. Water was purified using a Milli-Q water purification system. 2.2.1.Apparatus and Procedures. 2.2.1. Electrochemical Measurements. All electrochemical experiments were performed with a CHI660 electrochemical workstation employing a conventional three-electrode electrochemical cell. A GCE (3 mm diameter, surface area 0.0707 cm2), a platinum wire as the auxiliary electrode, and silver/silver chloride as the reference electrode (3 M KCl) in aqueous media were employed for all electrochemical measurements unless stated otherwise. The working electrode was successively polished with 1.0, 0.3, and 0.05 μm alumina powders and sonicated in water for 10 min after each polishing step. Finally, the electrode was washed with ethanol and then dried with a high-purity nitrogen stream immediately before use. Solutions were degassed for at least 20 min with high-purity nitrogen and kept under a blanket of nitrogen during all electrochemical experiments. The following electrolytes were used for the electrochemical experiments involving P8W48Fe16: 0.5 M Li2SO4 (pH 2.0), 1 M LiCl (pH 1.0−3.0), and 1 M CH3COOLi (pH 3.5−7.0). The pH adjustment was done with 0.5 M H2SO4, 1 M HCl, and 1 M CH3COOH, respectively. 2.2.2. Construction of Multilayer Assemblies of POM and MB. A clean GCE was immersed in the 8% (v/v) PDDA solution for 1 h for initial surface modification (step 1). The electrode was then rinsed thoroughly with deionized water and dipped in a 0.25 mM solution of LiK- P8W48Fe16 in pH 2 buffer solution (0.5 M Li2SO4, 0.5 M H2SO4) for 20 min (step 2). The electrode was then again rinsed thoroughly with deionized water and dried with high-purity nitrogen. This process resulted in a bilayer composed of both an underlying PDDA layer and an outer POM layer. The bilayer (PDDA/POM) modified electrode was then soaked for 20 min in a 0.02 mM aqueous MB solution for 20 min (step 3). The electrode was then again rinsed thoroughly with deionized water and dried with high-purity nitrogen. This resulted in a PDDA/POM/MB trilayer configuration on the underlying carbon electrode. To build the desired number of layers onto the electrode surface, steps 2 and 3 were repeated the required number of times. The terminal outer layer was chosen either to be an anionic POM or a cationic MB layer. 2.2.3. Atomic Force Microscopy. LBL films were formed ITO slides. AFM imaging was recorded with a Digital Instruments Nanoscope III with tapping mode using Si 3N4 cantilever tips. The spring constant of these pyramidal shape tips was between 12 and 103 N/m, and the size ranged between 3.6 and 5.6 mm. The images were analyzed using Nanotech Electronica WSxM image software. 2.2.4. Electrochemical Impedance Spectroscopy (EIS). EIS was carried out in 10 mM potassium ferricyanide and 10 mM potassium ferrocyanide solution in 0.1 M KCl at a potential of +230 mV (versus Ag/AgCl) from 0.1 to 106 Hz employing a voltage amplitude of 5 mV. The measurement solution was freshly prepared and constantly degassed with nitrogen.

Figure 1. Combined polyhedral/ball-and-stick representation of [P8W48O184Fe16(OH)28(H2O)4]20−. Color code: Fe (brown), O (red), PO4 tetrahedra (pink), WO6 octahedra (teal).

characterized by a single multielectron wave of simultaneous reduction in solution and with external and substitution-labile coordination positions. The fast kinetics associated with the electrochemical reaction and the multimetal substitutions are the two main points, which make this POM an attractive candidate for electrocatalytic applications. The present work focuses on the immobilization of P8W48Fe16 and the cationic redox dye methylene blue (MB) onto a glassy carbon electrode (GCE) surface through the 5481

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2.2.5. X-ray Photoelectron Spectroscopy. LBL films were formed on ITO slides. Analysis was performed in a Kratos AXIS 165 spectrometer using monochromatic Al Kα radiation of energy 1486.6 eV, 150 W (10 mA, 15 kV). The pass energy was 160 eV for survey spectra and 20 eV for narrow regions. In the near-surface region, the atomic concentrations of the chemical elements were evaluated after subtraction of a Shirley-type background by considering the corresponding Scofield atomic sensitivity factors. Core level binding energies were determined using the C 1s peak at 284.8 eV as the charge reference. The standard deviation of the peak position associated with the calibration procedure was ±0.05 eV.

composed of a pH-dependent bielectronic redox process, with an E1/2 of +0.135 V with this formal potential undergoing a cathodic shift of 60 mV/pH unit with an increase in pH, which corresponds to the involvement of two protons.50 The redox behavior of both the P8W48Fe16 and the MB reported herein agrees with the literature.57,59 3.2. LBL Films of P8W48Fe16. Cyclic voltammetry was utilized to monitor the growth of the multilayer assemblies based upon PDDA, P8W48Fe16, and MB after the deposition of each molecular layer, with Figure 3 representing such growth. Figure 3A shows the resulting cyclic voltammograms of the multilayer assembly after the deposition of each POM anionic layer, whereas Figure 3B shows the resulting cyclic voltammograms of the multilayer assembly after the deposition of each MB cationic layer. What is readily seen in both panels is that as

3. RESULTS AND DISCUSSION The electrochemical behavior of both P8W48Fe16 and MB were studied in solution and after LBL immobilization onto GCE surfaces. The growth of the POM-based films was monitored by cyclic voltammetry. The constructed POM assemblies were then characterized through the employment of electrochemical techniques, UV−vis spectroscopy, AFM, and XPS. 3.1. Solution Electrochemistry of P8W48Fe16. Figure 2 represents the cyclic voltammograms obtained at a GCE for the

Figure 2. pH effect of POM redox activity in solution. Cyclic voltammograms were recorded at GCE for a LiK- P8W48Fe16 solution (0.04 mM) in pH 2 buffer (A) and pH 4.5 buffer (B); scan rate 10 mV s−1.

redox chemistry of P8W48Fe16 at both pH 2 and pH 4.5. The reduction peak observed at −0.21 V in curve A (pH 2) represents the simultaneous one-electron reduction of the 16 iron(III) centers in P8W48Fe16.53 The corresponding reoxidation peak for these iron centers appears at +0.588 V. The next two reduction processes (I and II), which are observed at −0.37 V and −0.54 V, represent two consecutive eight-electron redox-processes associated with the POM’s tungsten-oxo (W−O) framework. It is well-known that POMs exhibit pHdependent redox processes both in solution and when surface immobilized.54−56 McCormac et al. have previously shown that the metal ion-substituted Wells−Dawson POMs possess pHdependent single and multiple electron redox processes.8 The cathodic shift in the measured redox potentials for these tungsten-oxo processes with an increase in pH (curve B at Figure 2) reveals the involvement of protons in the aforementioned redox processes. The potential shifts exceed the value of 59 mV/pH, which is typical for the POMs redox processes with the same numbers of both protons and electrons being involved.57 The redox behavior of the dye, MB, at pH 2 is

Figure 3. Chemical switching of P8W48Fe16−MB LBL film. Consecutive cyclic voltammograms of LBL film recorded after eight “POM” deposition steps (A) and after eight “MB” deposition steps (B). 0.5 M Li2SO4, 0.5 M H2SO4, pH 2, scan rate 10 mVs−1. (C) The dependences of peak charges (□ - MB reduction peak; ■ - W−O II oxidation peak) on the number of layers. Inset: LBL film growth onto PDDA-modified ITO slides monitored by UV−vis spectroscopy after MB steps of different assembly numbers. 5482

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P 8 W 48 Fe 16 , the E 1/2 values for the first and second W−O-based redox processes shift from −0.313 V and −0.512 V, in solution, to −0.262 V (ΔEp 10 mV) and −0.543 V (ΔEp 7 mV), respectively in the immobilized state with an outer POM layer. When the outer layer is cationic in nature, these two W−O redox processes shift to −0.300 V (ΔEp 86 mV) and −0.553 V (ΔEp 19 mV). It can be seen in the latter case that the peak-topeak separations have increased in value, which would reflect the difficulty of charge balancing protons from the contacting electrolyte passing into the film through the outer cationic layer upon redox switching through the POM’s W−O-based processes. A similar control of electrochemical properties of LBL film-modified electrodes due to the nature of the terminal layer has been observed with Pt nanoparticle-based deposits.51 Figure 4A,B illustrates the resulting cyclic voltammograms for the P8W48Fe16−MB multilayer films as a function of scan

the number of layers deposited onto the electrode surface increases the currents associated with the redox chemistry of both the P8W48Fe16 and MB moieties, thus indicating the successful inclusion of both these moieties into the multilayer system. In addition, when the MB layer is the outer layer the redox activity for the POMs 16 Fe(III) centers is not apparent. The reason for this remains unclear at present. Figure 3C then represents the dependence of peak charges for the POM’s W−O second oxidation wave and for the MB reduction wave on the numbers of deposited layers within the LBL assembly. Figure 3C shows a decrease in the redox peak currents and associated charge related to the second W−O redox process for the P8W48Fe16 POM after the deposition of each MB cationic layer, which is also accompanied by an increase in the capacitive currents. This commences after the deposition of the fifth cationic MB layer and becomes more pronounced as more layers are deposited on the LBL assembly. However, for the MB reduction wave, as the numbers of layers is increased, there is a gradual increase in the redox process’s charge with no associated decrease upon the addition of each POM layer. However by the deposition of the 10th POM layer and thereafter, there is a continual decrease in the associated charge for the MB reduction process upon the addition of each POM layer. The role of both anionic and cationic species moving into/ out of the film, from the contacting electrolyte, upon redox switching of the film, must play a role in this observed film behavior. In addition, the presence of an outer cationic MB layer must hinder the passage of protons from the supporting electrolyte into the film upon the redox switching of the film through the POM’s W−O based processes. In addition, the difference in the porosity of the film when there is either an outer anionic or cationic layer would play a role in the observed electrochemical behavior of the film. There is a continuous growth in the multilayer’s surface coverage from 0.003 nmol cm2 for the second POM layer to 0.3 nmol cm2 for 16th POM layer, and from 0.015 nmol cm2 for the first MB layer to 0.6 nmol cm2 for the 15th MB layer. The growth of the LBL films was also monitored by UV−vis spectroscopy as shown in Figure 3D. MB exhibits a blue shift (Soret band) after deposition by changing the λmax from 664 nm for the monomer to 605 nm for eight layers of MB. Generally, dyes form aggregates with anionic species while being modified onto substrates. According to the literature, MB belongs to phenothiazine dyes, and the H-aggregates of phenothiazine systems are formed through π−π staking, with other factors also influencing the formation, such as molecular structures and templating reagents.59 As previously observed,59 it is proposed here that H-aggregates between the MB and POM are formed because of the rigidity of the POM, which could fit inside the MB packing for stability onto the substrate. When the LBL film is exposed to sun light for 48 h, no changes in the absorbance spectrum are observed; in addition, exposure of the LBL film-modified ITO slide to temperatures up to 433K did not lead to spectral changes, thus showing the inherent stability of the P8W48Fe16−MB multilayer films. Upon immobilization within the multilayer assembly, there were subtle changes in the redox behavior of both the MB and P8W48Fe16 moieties as compared to their solution behavior. The E1/2 value for the MB redox process shifts from +0.133 V in solution to +0.091 V and +0.103 V within the multilayer assembly when the outer layer is either anionic or cationic in nature, respectively. In terms of the redox activity for

Figure 4. Scan rate studies of P8W48Fe16−MB LBL films composed of 16 layers with an outer POM layer (A) and composed of 17 layers with an outer MB layer (B). Inset: the scan rate dependences for the first (I) W−O anodic wave at the LBL film with an outer POM layer (□) and an outer MB layer (■). 0.5 M Li2SO4, 0.5 M H2SO4, pH 2.

rate, for films with an outer POM or MB layer, respectively. The redox processes associated with the POM’s second W−O redox process and MB showed thin layer behavior for up to 1 V/s when the outer layer was cationic in nature. However, the second W−O redox process showed thin layer behavior up to 100 mV/s when the outer layer was anionic in nature (Inset of Figure 4B). The stability of LBL films was investigated by redox switching the film through the various redox processes and monitoring the associated change in redox peak currents at pH 2. For multilayers composed of an outer P8W48Fe16 layer, the 5483

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Fe(CN)63−/Fe(CN)64− redox system, that is, +0.27 V. Figure 6A shows the impedance spectra, as a Nyquist plot, obtained during the various stages of the deposition of the multilayer

current associated with the second W−O (II) POM-based redox process increased by approximately 50% after 500 redox cycles, which could indicate film saturation with ions injected into the film upon cycling. However, the redox activity associated with the POM’s Fe(III) centers was found to be unstable upon redox cycling at pH 2. 3.2.1. Effect of Solution pH. It is well known that the redox processes associated with the W−O frameworks of POMs are pH-dependent in nature, either in the solution or in the immobilized state.3,6,8,12 Figure 5 exhibits the effect of the

Figure 5. pH effect upon layer’s redox activity. Cyclic voltammograms were recorded at an electrode modified with an LBL film composed of 16 layers with an outer POM layer (A,B) and composed of 17 layers with an outer MB layer (C,D) in pH 2 (A,C; 0.5 M Li2SO4, 0.5 M H2SO4) and in pH 5 (B,D; 1 M CH3COOLi, 1 M CH3COOH) buffers; scan rate 10 mVs−1.

solution pH upon the redox activity of the P8W48Fe16−MB multilayer films when the outer layer is anionic (Figures 5A,B) or when it is cationic (Figures 5C and D) in nature. What is observed is that, as the pH is made more alkaline, there is a cathodic shift in the E1/2 values for the POM’s W−O processes and the MB redox processes. In addition, for layers composed of an outer POM layer there appears to be no shift in the 16 electron oxidation of the Fe (II) centers within the POM. The shifts observed for the POM’s second W−O redox process were 78.8(±3.4) and 76(±2.54) mV pH−1 for films composed of an outer POM or MB layer, respectively. The shifts observed for the bielectronic redox process of MB were 84.3(±3.97) and 71.4(±2.99) mV pH−1, respectively. 3.2.2. Electrochemical Impedance Spectroscopy. EIS studies have been carried at the various layer depositions during the growth of the P8W48Fe16−MB multilayer films. As detailed in the Experimental Section, the redox probe, Potassium ferri/ferrocyanide, was employed, with the applied experimental potential being set at the formal potential of the

Figure 6. Chemical switching of the P8W48Fe16−MB LBL film monitored by impedance spectroscopy. (A) Nyquist plot of impedance spectra of electrode modified with P8W48Fe16−MB LBL films (■1 spectrum of blank GCE; □2 - spectrum of PDDA-modified electrode; ▲3, ●5 and ■7 - spectra of modified electrode after POM deposition steps; ○4, □6, and △8 - spectra of modified electrode after MB deposition steps) in ferro/ferricyanide solution (10 mM K3[Fe(CN)6], 10 mM K4[Fe(CN)6], 0.1 M KCl); 10 mV amplitude, 230 mV potential of measurement. (B,C,D) The dependencies of fitted values of Randles circuit elements (double layer capacitance, charge transfer resistance and Warburg impedance) on the numbers of assembly layers. 5484

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assembly. What is observed is that the values of the charge transfer resistance RCT, which can be roughly estimated as the diameter of a semicircle at the kinetically controlled region, repeatedly increase after the deposition of each of the P8W48Fe16 anionic layers but decrease after the deposition of the cationic MB layers. The interpretation of impedance data has been carried out through the employment of an equivalent Randles circuit, which consists of double-layer capacitance in series with a solution resistance and in parallel with a diffusional branch, i.e., Warburg impedance and charge transfer resistance. The constant phase element was introduced instead of the doublelayer capacitance, which illustrates the nonuniform distribution of the capacitance over the electrode surface. Fitting of the resulting data revealed the switching behavior of the double layer capacitance Cdl (Figure 6B). It can be seen that the modification of the blank GCE with the first layer of PDDA led to a 60% decrease in Cdl. The adsorption of P8W48Fe16 polyanion at an even layer number led to subsequent increases in both Cdl and RCT (Figure 6C). This probably reflects the more compact “less porous” nature of the highly charged P8W48Fe16 POM layers. The decrease of RCT values after the deposition of the cationic MB layers can be a result of attractive interactions between the positively charged MB molecules at the outer layer of the LBL film-modified electrode and negatively charged redox probe molecules, which enhance the probe diffusion through the film to the underlying electrode surface. The effect of the increase in Cdl diminishes for higher layer numbers, which illustrates the building of the LBL film and the lower impact on the double layer at electrode/solution interface. The change in RCT between the two deposition steps is enhanced with increasing layer number, which influences the redox probe diffusion through the film. The Warburg impedance (Figure 6D), which represents the thickening of diffusion through the film, increases sharply after the first layer deposition and then sequentially decreases. This effect is probably due to the enhancement of active surface available for charge transfer from/to the redox probe. In terms of impedance measurements, the film exhibited good stability. 3.2.3. Film Permeability. The porosity of the P8W48Fe16− MB multilayer films was investigated by studying the effect of a redox probe, namely, [Fe(CN)6]3−/4−, upon the voltammetry of the multilayer assembly. The fate of the probe when in contact with the multilayer assembly can be one of the following: the probe can diffuse through the assembly and undergo reaction at the underlying electrode surface, or can undergo redox reaction at the film/solution interface by mediated electron transfer by redox sites present within the LBL film.6 The anionic probe, [Fe(CN)6]3−/4−, exhibits a monoelectronic redox process with an E1/2 of +0.21 V at pH 2. Figure 7 presents the voltammetric responses of the ferri/ferrocyanide couple obtained at a blank GCE (Figure 7A, dashed line) and at electrodes modified with two LBL films of different thicknesses. For the two films composed of four bilayers, with either an outer POM (Figure 7A) or an outer MB (Figure 7B), a degree of film porosity is apparent as the [Fe(CN)6]3−/4− is able to diffuse through the multilayer assembly and react at the underlying electrode surface albeit with reduced peak currents as opposed to the [Fe(CN)6]3−/4− at the bare electrode surface. However, as seen in Figure 7C,D, for multilayer films composed of eight bilayers, with either an outer POM or cationic MB layer, there is an

Figure 7. Permeability of films by anionic redox probe. (A) LBL film composed of eight layers with an outer POM layer; (B) LBL film composed of nine layers with an outer MB layer; (C) LBL film composed of 16 layers with an outer POM layer; (D) LBL film of 17 layers with an outer MB layer. Cyclic voltammograms were recorded at pH 2 buffer (0.5 M Li2SO4, 0.5 M H2SO4) before (thin line) and after (thick line) addition of 1 mM K3[Fe(CN)6]. Dashed line voltammogram at blank GCE.

absence of redox chemistry for [Fe(CN)6]3−/4−, thus indicating the lack of film porosity at this film thickness. 3.2.4. AFM Imaging and XPS. AFM imaging of the P8W48Fe16−MB LBL film at ITO glass slides was performed to find out the topography of the deposits. Figure 8 presents the AFM images of a blank ITO slide (Figure 8A) and slides after the first (PDDA, Figure 8B), second (first POM step, Figure 8C), and 17th layers (Figure 8D) of LBL assembly. Root-mean-square surface roughness parameters were 22.6, 23.6, 10.4, and 4.5 nm, respectively. Larger features from the topography of the ITO substrate were also seen in the images obtained from glass slide modified with PDDA, along with a more globular structure suggestive of a polymer film. Further changes in surface topography were observed for the first POM layer and multilayer films. The deposition of first POM layer led to sufficient decrease of surface roughness. Increasing the number of deposited layers resulted in further reduced film defects. The topography of samples B and D featured globular structures. Little phase contrast was seen for sample D, which illustrates a homogeneous surface of deposit within the areas of interest imaged. The values of surface roughness reported previously for the similar systems were in comparison to what we got here. Quartz slides deposited with the layer by layer assembly of [Eu(SiW10VO39)2]15− and polyethyleneimine displayed a surface roughness of 2.4 nm.58 Also the Fluorinedoped tin oxide thin films deposited by chemical vapor deposition showed surface roughness values between 5 and 35 nm.59 XPS analysis of the P8W48Fe16−MB LBL film showed the presence of N (4.8%), C (50.1%), O (33.5%), W (7.6%), and Fe (1%). 3.2.5. Preliminary Electrocatalytic Properties of LBL Films. POMs have previously been employed for the reduction of hydrogen peroxide.3,60,61 The ability of the P8W48Fe16−MB LBL films, when the outer layer is either anionic or cationic, to electrocatalytically reduce hydrogen peroxide has been investigated. Figure 9A represents the voltammetric responses 5485

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Figure 8. AFM images of ITO glass slides during different stages of layer construction. (A) blank slide, (B) slide modified with initial PDDA layer, (C) slide after subsequent first POM deposition step and (D) slide after 16th POM layer deposition.

outer layer of the multilayer assembly is the MB cationic moiety, then no electrocatalytic effect is observed, as seen in Figure 9B. These preliminary results support our findings detailed in previous sections, where the nature of the outer layer has an effect on the multilayer’s properties.

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CONCLUSIONS Stable and reproducible multilayer films composed of the ironsubstituted POM Li4K16[P8W48O184Fe16(OH)28(H2O)4]·66H2O·2KCl and MB have been deposited on carbon and ITO electrode surfaces through the employment of the LBL technique. It was found that such layers exhibited the expected pH-dependent redox activity associated with both the POM and MB species. The films were found to exhibit thin layer behavior up to 100 mV s−1 when the outer layer was anionic in nature, and 1 V s−1 when the outer layer was cationic. Through the employment of AC impedance and cyclic voltammetry, it was found that the redox switching and permeability of the constructed layers was dependent on both the layer thickness and the nature of the outermost layer. Preliminary investigations showed that the films exhibited the ability to electrocatalytically reduce hydrogen peroxide only when the outer layer was anionic in nature.

Figure 9. Cyclic voltammograms obtained for electrode-modified P8W48Fe16−MB films upon the addition of hydrogen peroxide. P8W48Fe16−MB films composed of 12 layers with an outer POM layer (A) and 13 layers with an outer MB layer (B) in the absence (solid lines) and after addition of hydrogen peroxide (0.2 mM and 0.8 mM, dashed lines). 1 M H2SO4; scan rate 10 mV/s.

of P8W48Fe16−MB LBL film-modified electrodes composed of eight bilayers with an outer POM layer in the absence and presence of both 0.2 and 0.8 mM hydrogen peroxide. What is clearly observed is an increase in the reduction currents associated with the POM’s tungsten-oxo redox processes upon successive additions of H2O2. This indicates that it is the multiply reduced form of the POM, which catalyzes the reduction of the added H2O2. The measured catalytic currents were found to be linear up to 3 mM. Interestingly when the

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AUTHOR INFORMATION

Corresponding Author

*Fax: +353 42 933 1163; Tel: +353 42 937 4579; E-mail: tim. [email protected]. 5486

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Notes

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The authors declare no competing financial interest.

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