Electrochemical current rectification at bio-functionalized electrodes Yaqing Liu, Andreas Offenhäusser, Dirk Mayer ⁎ Institute of Bio-and Nanosystems (IBN 2), Research Center Juelich, 52425, Germany JARA-Fundamentals of Future Information Technology, Germany abstract article info Article history: Received 29 April 2009 Received in revised form 26 June 2009 Accepted 30 June 2009 Available online 8 July 2009 Keywords: Bio-diode MP-11 Gold electrode Bioelectrochemical current rectification Bio-ECR In the present paper, we demonstrate the electrochemical rectification of a redox current which is transferred between redox probes (ferricyanide) in solution and a gold electrode functionalized with the biomolecular redox mediator microperoxidase-11 (MP-11). MP-11 is the redox active, heme-containing domain of the biological electron shuttle cytochrome c (cyt c). In our system, a unidirectional current develops due to selective electron transport from the bio-functionalized electrode to ferricyanide such that MP-11 controls the read-out of our coupled redox system. The electrode was functionalized by adding a monolayer of undecanethiol (UDT) to promote the physisorption of MP-11 and inhibit the direct electron transfer between redox probe and electrode. The relative position of redox donator, mediator, and acceptor equilibrium potentials defines the charge transport and a potential-dependent electrochemical current rectification. The results of our investigations demonstrate that functional building blocks of proteins can be reassembled into new conceptual devices with operation modes deviating from their native function, which could prove highly useful in future design of biosensors and bioelectronic systems. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Within the last decade, protein electrochemistry at functionalized metal electrodes has become important not only for fundamental studies of charge transfer properties of redox proteins but also for fostering the development of sensitive biosensor and bioelectronic devices. Inspired by biological systems such as photosynthetic and redoxcatalytical reactions, different kinds of bioelectronic devices have been designed and realized, including biosensors, biofuel cells, and bioactuators [1–3]. Recently, switchable and tunable bioelectrocatalytic systems based on electrically wired enzymes were developed to mimic logic operations [4]. Current rectifiers can be combined to logic gates and are crucial elements for assembling of higher-order bioelectronic components [5]. One major challenge in this field of research is to control the transport of charge across the bio-inorganic interface. With the motivation to pave the way for the development of new, functional bioelectronic devices, we investigated the electrochemical current rectification mediated by naturally-occurring, electroactive biomolecules at the solid–liquid interface. Electrochemical current rectifiers (ECR) create a unidirectional current in solution by facilitating transfer of electrons from a redox probe to an electrode under potential control while simultaneously blocking the reverse electron transfer. Murray et al. reported the development of the first ECRs [6–10]. In their experiments, the electrodes were modified with polymers containing electron-transfer mediators, which permit a unidirectional current flow between electrode and probe molecules. An essential prerequisite for the proper performance of these ECRs was that the thick polymer film suppressed the direct electron transfer of electroactive species at electrode [11]. Various ECRs have since been realized based on synthetic redox active molecules or polymers [12– 18]. Recently, Azzaroni et al. reported the fabrication of a biodiode by means of ferrocene-labeled streptavidin which facilitated unidirec- tional current flow [19]. The streptavidin molecules fulfilled two essential tasks of an ECR (i) inhibiting the direct charge transfer between electrode and redox probes and (ii) acting as binding matrix for the tunable redox mediator ferrocene. Their results demonstrated that biomolecules could be tailored and incorporated into supramo- lecular bioassemblies thereby providing a means to perform electro- nic operations. Many biological processes in nature are controlled by unidirec- tional transport of ions and electrons across interfaces. Our goal is to use evolutionarily-optimized electroactive proteins as biomachineries for the realization of conceptual bioinorganic devices like biodiodes. In our experiments, electrochemical rectification was realized by means of the biomolecular electron transfer mediator microperoxidase-11. MP-11 is the heme-binding unit of cytochrome c, which acts as electron shuttle in systems such as the respiratory chain. This redox active amino acid fragment was extracted from cyt c by a selective digest and reassembled as functional building block into a bio- inorganic architecture which facilitates the control of electron transfer over the solid liquid interface. We showed in our experiments that MP-11 restricts the initially bidirectional redox reaction of ferri/ ferrocyanide to unidirectional cathodic charge transfer. Such current rectifying bio-inorganic hybrids can be considered as biological Bioelectrochemistry 77 (2010) 89–93 ⁎ Corresponding author. Institute of Bio-and Nanosystems (IBN 2), Research Center Juelich, 52425, Germany. Tel.: +49 2461 61 4023; fax: +49 2461 61 8733. E-mail address: dirk.mayer@fz-juelich.de (D. Mayer). 1567-5394/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.bioelechem.2009.06.015 Contents lists available at ScienceDirect Bioelectrochemistry journal homepage: www.elsevier.com/locate/bioelechem