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Stable immobilization and reversible electrochemistry of cytochrome c in a transparent indium tin oxide film with a well‐defined mesoporosity (mpITO) is demonstrated. The transparency and good conductivity, in combination with the large surface area of mpITO, allow the incorporation of a high amount of electroactive biomolecules and their electrochemical and spectroscopic investigation. UV/Vis and resonance Raman spectroscopy, in combination with direct protein voltammetry, are employed for the characterization of cytochrome c immobilized in the mpITO and reveal no perturbation of the structural integrity of the redox protein. The potential of this modified material as a biosensor for detection of superoxide anions is also demonstrated.

The preparation of porous, i.e., high surface area electrodes from transparent conducting oxides, is a valuable goal in materials chemistry as such electrodes can enable further development of optoelectronic, electrocatalytic, or bioelectronic devices. In this work the first tin-rich mesoporous indium tin oxide is prepared using the molecular heterobimetallic single-source precursor, indium tin tris-tert-butoxide, together with an appropriate structure-directing template, yielding materials with high surface areas and tailorable pore size. The resulting mesoporous tin-rich ITO films show a high and durable electrical conductivity and transparency, making them interesting materials for hosting electroactive biomolecules such as proteins. In fact, its unique performance in bioelectronic applications has been demonstrated by immobilization of high amounts of cytochrome c into the mesoporous film which undergo redox processes directly with the conductive electrode material.

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1Introduction Molybdenum (Mo) containing enzymes have been associated with the catalysis of important reactions in the metabolism of smallm oleculess uch as carbon nitrogen, and sulfur [1a, b].T he sulfite oxidizing enzymes sulfite oxidase (SO) and sulfite dehydrogenase (SDH) catalyze the oxidation of sulfite to sulfate accompaniedw ith the release of two electrons and two protons according to the following equation:All sulfite oxidizing enzymes share the same catalytic Mo cofactor (Moco) containingu nit. But the number and type of additional redoxc enters differb etween enzymes from different classes of organisms, which has an impact on the type of electrona cceptor the enzyme can use [2a, b].O nly in the vertebrate SO ah eme b binding domaini sc ovalently linkedt ot he core structure of the enzyme.T he human sulfite oxidase (hSO) shows al arge structural homology to the enzyme from chicken liver (cSO) which is ad imeric enzyme where each subunit of the homodimer consists of an N-terminal heme b binding domain( SO-HD), followed by ap olypeptide tetherc onnectingt he heme domain to the Moco-containing domain (SO-MD) and ad imerization domain at the C-terminus [3].Thes ingle steps of the SO catalyzed reaction involve the two-electron oxidationo fs ulfite to sulfate at the Moco center, where Mo VI is reduced to Mo IV .I nv ertebrate SO the Mo VI regeneration is naturally coupled to the reduction of two equivalents of soluble cytochrome c which proceeds in two subsequent intra-and intermolecular singlee lectron transfer steps via the heme b5 containing site [4a, b, c].T hese steps involve ar epositioning of the domains to reduce electron transfer distances permitting electron transfer from Mo to Fe of SO-HD and to cytochrome c [5a, b].Areaction with soluble [6a, b] and polymerb ound redoxm ediators is also possible [7a, b]. Direct unmediated electron transfer from the reduced enzyme to electrodes proceedsv ia the hemed omain [8]. This process was most effective when the enzymei si mmobilized on electrode surfaces modified with positive charged SAMs facilitating the oriented assembly of SO electron transfer and bioelectrocatalysis [9a, b].Abstract:W er eport efficient bioelectrocatalytic sulfite oxidation by human sulfite oxidase (hSO) immobilizedo n ag old nanoparticle (AuNP)m odified gold electrode.T he AuNP were synthesized in aqueous phase by using branched polyethyleneimine (PEI) as reducing as well as stabilizing agent. Golde lectrodes were modified by as elf assembled monolayer of dithio-bis(N-hydroxysuccinimidyl propionate) (DTSP) onto which the NP and hSO were immobilized. Cyclic voltammetry of the hSO modified electrode in the absence of substrater evealedaquasi-reversible direct electrochemical reactiono ft he heme domaino fhSO with fast electron transfer rate.T he electron transferr ate constant of k s = 32 s À1 and the formal potential E 0 ' = À0.155 Vv s. Ag/AgCl/1 MK Cl were estimated. Comparatives tudies with nanoparticles of BaSO 4 indicate the importance of the NP conductivity for charge transfer an...