Thin mesoporous nanocomposite films of SnO2-poly(diallyidimethylammonium chloride) are formed in a layer-by-layer deposition process from SnO2 nanoparticles (nominal 15 nm diameter) and poly(diallyidimethylammonium chloride) binder. Atomic force imaging and electron microscopy suggest that each deposition cycle is adding ca. 10 nm of SnO2-poly(diallyidimethammonium chloride) to the film thickness. Data from cyclic voltammetry suggest that SnO2-poly(diallyldimethylammonium chloride) films are stable and n-type semiconducting but actually electrically sufficiently conducting (similar to mesoporous SnO2) over a wide range of applied potentials.The immobilization of redox proteins into the mesoporous structure is attempted for the heme proteins methemoglobin (bovine) and cytochrome P450(cam) (CYP101). Very similar voltammetric responses for these systems indicate denaturation in the presence of polyelectrolyte or the charged SnO2 surface. In addition, it is shown that methemoglobin and cytochrome P450(cam) are even demetallated to give Fe(III/II) immobilized in electrochemically active form within the SnO2-poly(diallyldimethylammonium chloride) film. Most of this Fe(III/II) can be sequestered and identified in a test reaction with ethylenediaminetetraacetate (EDTA). (C) 2007 Elsevier B.V. All rights reserved.