Redox Switching of Polyoxometalate–Methylene Blue-Based Layer-by-Layer Films

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 immobilized 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.

Nitrate and Nitrite Electrocatalytic Reduction at Layer-by-Layer Films Composed of Dawson-type Heteropolyanions Mono-substituted with Transitional Metal Ions and Silver Nanoparticles

“…In addition, the first reduction peak current of the W-O I redox process is higher than that observed for other HPAs, which reflects the contribution from reduction of Cu 2+ -center. The presence of the Fe 3+ -center ( Interestingly the redox processes associated with the Cu 2+ -and Fe 3+ metal centres in the HPA moieties disappear upon layer formation, which is in agreement with literature data for mono-substituted HPAs [58,66,67] and represents the phenomenon of electrostatic interaction between the HPA and the encapsulating multilayer film.…”

“…Instead of the hopping conductivity observed in HPA-based LBL films assembled with cathionic insulators and characterized with the decay of electronic communication rate with the increase of number of layers [66,73,74], the AgNPbased LBL films showed the retention of communication rate with the assembly [58] probably due to the maintenance of metallic conductivity within the electrocatalytic film, which might be the main reason of NRR electrocatalysis.…”

“…The equivalent circuit, which consists of a double layer capacitance in series with a solution resistance and in parallel with a branch of diffusion, i.e. charge transfer resistance and Warburg impedance [66], has been applied for the interpretation of the experimental data. Fitting of the experimental data confirmed reproducible switching behaviour of the charge transfer resistance with the outer layers (Fig.…”

“…Thirdly, in contrast to organic cations (e.g. Methylene Blue) [66], the strong electrostatic interactions between the LBL assembly blocks can result in non-uniform and defective films.…”

Electrocatalysis by crown-type polyoxometalates multi-substituted by transition metal ions; Comparative study

“…The interpretation of the impedance data has been carried out employing the Randles equivalent circuit [27], which consists of a double layer capacitance in series with solution resistance and in parallel with a diffusion branch, i.e. Warburg impedance in series with a charge transfer resistance.…”