Macroporous indium tin oxide electrode layers as conducting substrates for immobilization of bulky electroactive guests

Macroporous indium tin oxide (ITO) electrodes with a defined uniform pore size were prepared via direct co-assembly of ultra-small indium tin hydroxide (nano-ITOH) nanoparticles and poly(methyl methacrylate) (PMMA) beads. The use of nano-ITOH nanoparticles enables a facile large-scale fabrication of homogeneous crack-free coatings with good adhesion to the substrate, good optical quality and tunable thickness, which easily transform at 400 °C into crystalline ITO with similar morphology. Macroporous ITO films exhibit reasonably high electric conductivity of 4.0 ± 0.3 S cm−1 and open interconnected pores with a uniform size of ca. 300 nm, which makes them suitable conducting platforms for immobilization of bulky redox species or for deposition of functional electroactive layers. Deposition of functional semiconducting layers on the walls of the porous ITO scaffold was shown for titanium dioxide, which penetrates the ITO framework as shown by transmission electron microscopy (TEM) analysis of a cross-section. The obtained layers were used as conducting substrates for the immobilization of the heme proteins cytochrome c and hemoglobin, which demonstrate a direct electron transfer to the macroporous ITO electrode. Surface coverage of cytochrome c adsorbed on the macroporous ITO electrode is more than 12 times higher than on a planar one, reaching 400 pmol cm−2 for a film of ca. 360 nm in thickness. Hemoglobin adsorbed on the macroporous ITO electrodes also demonstrates a noticeably high surface coverage of ca. 160 ± 20 pmol cm−2, which is roughly 7–10 times higher than the theoretical value for monolayer coverage.