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) [P 8 W 48 O 184 Fe 16 (OH) 28 (H 2 O) 4 ] 20- has been successfully immobi- lized onto glassy carbon electrode surfaces by means of the layer- by-layer (LBL) technique employing the cationic redox active dye, methylene blue (MB). The constructed multilayers exhibit pH- dependent 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 oppo- sitely 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 substrate is also another advantage of the LBL techni- que. 26,29-31 A wide range of POMs have been surface-attached through the LBL technique, e.g., Wells-Dawson-type [P 2 W 18 O 62 ] 6- , Keggin-type [α-SiW 12 O 40 ] 4- , transition metal-substituted Krebs-type POMs [X 2 W 20 M 2 O 70 (H 2 O) 6 ] n- , where (X = Bi or Sb, M = Co 2+ or Cu 2+ ), and sandwich-type POMs [Co 4 (H 2 O) 2 (PW 9 O 34 ) 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 gold- coated quartz, 37,39 silicon, 38,39 and mica substrate. 39 A variety of cationic moieties have been incorporated into these POM- based multilayers systems, such as, ruthenium(II) polypyridyl complexes, 38,40 conducting 41,42 and redox active poly- mers, 30,36,43 metallodendrimers, 35 metalloporphyrins, 44 poly- electrolytes, 29,32,33,39,45-47 cationic surfactants, 40 dye mole- cules, 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 Electro- chemical 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 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 5480 dx.doi.org/10.1021/la3004068 | Langmuir 2012, 28, 5480-5488