Hybrid Wiring of the Rhodobacter sphaeroides Reaction Center for Applications in Bio-photoelectrochemical Solar Cells

The growing demand for nonfossil fuel-based energy production has drawn attention to the utilization of natural proteins such as photosynthetic reaction center (RC) protein complexes to harvest solar energy. The current study reports on an immobilization method to bind the wild type Rhodobacter sphaeroides RC from the primary donor side onto a Au electrode using an immobilized cytochrome <i>c</i> (cyt <i>c</i>) protein via a docking mechanism. The new structure has been assembled on a Au electrode by layer-by-layer deposition of a carboxylic acid-terminated alkanethiol (HOOC (CH₂)₅S) self-assembled monolayer (SAM), and layers of cyt <i>c</i> and RC. The Au|SAM|cyt <i>c</i>|RC working electrode was applied in a three-probe electrochemical cell where a peak cathodic photocurrent density of 0.5 μA cm^–2 was achieved. Further electrochemical study of the Au|SAM|cyt <i>c</i>|RC structure demonstrated ∼70% RC surface coverage. To understand the limitations in the electron transfer through the linker structure, a detailed energy study of the SAM and cyt <i>c</i> was performed using photochronoamperometry, ellipsometry, photoemission spectroscopy, and cyclic voltammetry (CV). Using a simple rectangle energy barrier model, it was found that the electrode work function and the large barrier of the SAM are accountable for the low conductance in the devised linker structure

Interface Formation Between ZnO Nanorod Arrays and Polymers (PCBM and P3HT) for Organic Solar Cells

We investigated the interface formation between a ZnO nanorod array and active layers of [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) and poly­[3-hexylthiophene] (P3HT) in organic solar cells (OSC). We measured the interfacial electronic structures with in situ photoemission spectroscopy combined with an electrospray deposition system. Different interfacial electronic structures were observed on the ZnO nanorod array, which were compared to those of a two-dimensional ZnO film. Comparing the interfacial orbital line-ups of the active layers on the nanorod array and the film, PCBM shows Fermi level pinning behavior, but P3HT does not. These induce nearly identical orbital line-ups at the interfaces of PCBM/film and PCBM/nanorod but different line-ups at the interfaces of P3HT/film and P3HT/nanorod. These differences are understood with the integer charge transfer model with the different thresholds of Fermi level pinning of PCBM and P3HT. These results give insight into the design not only of OSCs but also of any organic electronic devices with nanostructures: changes in electronic structure due to the nanostructure formation should be considered thoroughly

Charge Transfer through Modified Peptide Nucleic Acids

We studied the charge transfer properties of bipyridine-modified peptide nucleic acid (PNA) in the absence and presence of Zn­(II). Characterization of the PNA in solution showed that Zn­(II) interacts with the bipyridine ligands, but the stability of the duplexes was not affected significantly by the binding of Zn­(II). The charge transfer properties of these molecules were examined by electrochemistry for self-assembled monolayers of ferrocene-terminated PNAs and by conductive probe atomic force microscopy for cysteine-terminated PNAs. Both electrochemical and single molecular studies showed that the bipyridine modification and Zn­(II) binding do not affect significantly the charge transfer of the PNA duplexes

The Role of Gold-Adsorbed Photosynthetic Reaction Centers and Redox Mediators in the Charge Transfer and Photocurrent Generation in a Bio-Photoelectrochemical Cell

Bacterial photosynthetic reaction centers (RCs) are promising materials for solar energy harvesting, due to their high quantum efficiency. A simple approach for making a photovoltaic device is to apply solubilized RCs and charge carrier mediators to the electrolyte of an electrochemical cell. However, the adsorption of analytes on the electrodes can affect the charge transfer from RCs to the electrodes. In this work, photovoltaic devices were fabricated incorporating RCs from purple bacteria, ubiquinone-10 (Q2), and cytochrome c (Cyt c) (the latter two species acting as redox mediators). The adsorption of each of these three species on the gold working electrode was investigated, and the roles of adsorbed species in the photocurrent generation and the cycle of charge transfer were studied by a series of photochronoamperometric, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and cyclic voltammetry (CV) tests. It was shown that both redox mediators were required for photocurrent generation; hence, the RC itself is likely unable to inject electrons into the gold electrode directly. The reverse redox reactions of mediators at the electrodes generates electrical current. Cyclic voltammograms for the RC-exposed gold electrode revealed a redox couple due to the adsorbed RC at ∼ +0.5 V (vs NHE), which confirmed that the RC was still redox active, upon adsorption to the gold. Photochronoamperometric studies also indicated that RCs adsorb, and are strongly bound to the surface of the gold, retaining functionality and contributing significantly to the process of photocurrent generation. Similar experiments showed the adsorption of Q2 and Cyt c on unmodified gold surfaces. It was indicated by the photochronoamperometric tests that the photocurrent derives from Q2-mediated charge transfer between the RCs and the gold electrode, while solubilized Cyt c mediates charge transfer between the P-side of adsorbed RC and the Pt counter electrode. Also, the stability of the adsorbed RCs and mediators was evaluated by measuring the photocurrent response over a period of 1 week. It is found that ∼46% of the adsorbed RCs remain active after a week under aerobic conditions. A significantly extended lifetime is expected by removing oxygen from the electrolyte and sealing the device