Surface immobilisation of transition metal substituted Krebs type polyoxometalates, [X2W20M2O70(H2O)6]n− (X = Bi or Sb, M = Co2+ or Cu2+), by the layer by layer technique

A series of transition metal (i.e. Cu2+ and Co2+) substituted Krebs type polyoxometalates (POMs), of the general formula [X2W20M2O70(H2O)6]n−, X = Sb or Bi, M = Co(II) or Cu(II), have been successfully immobilised onto carbon electrode surfaces through the employment of the layer-by-layer (LBL) technique. This involved the construction of alternating anionic POM, [X2W20M2O70(H2O)6]n−, layers and the cationic metallodendrimer, Ru(II)-metallodendrimer as the cationic layers, in addition to a [poly(diallyldimethylammonium chloride)] PDDA base layer. Stable multielectron redox couples associated with the W–O framework, for the Krebs type POMs, and the Ru(III/II) for the metallodendrimer, were clearly observed upon layer construction and redox switching within the pH domain of 2–6.5. The constructed multilayer assemblies exhibited pH dependent redox activity and thin layer behaviour up to 100 mV s−1. The porosity and permeability of the individual multilayer assemblies towards an anionic probe were determined by AC impedance and cyclic voltammetry. The surface morphology of each multilayer was also determined by Atomic Force Microscopy (AFM).