Synthesis, characterization and fabrication on a glassy carbon electrode of a tetra-iron substituted sandwich-type pentadecatungstodiarsenate heteropolyanionElectronic supplementary information (ESI) available: model of layer formation, CV of 4-ABA on a GCE and of QPVP-Os/4-ABA/GCE in pH 4.7 buffer, plots of Ed vs. log (i/id – i) and XRD of the title compound. See http://www.rsc.org/suppdata/nj/b2/b205766m/