Molecular Architecture: Construction of Self-Assembled Organophosphonate Duplexes and Their Electrochemical Characterization

Self-assembled monolayers of phosphonates (SAMPs) of 11-hydroxyundecylphosphonic acid, 2,6-diphosphonoanthracene, 9,10-diphenyl-2,6-diphosphonoanthracene, and 10,10′-diphosphono-9,9′-bianthracene and a novel self-assembled organophosphonate duplex ensemble were synthesized on nanometer-thick SiO₂-coated, highly doped silicon electrodes. The duplex ensemble was synthesized by first treating the SAMP prepared from an aromatic diphosphonic acid to form a titanium complex-terminated one; this was followed by addition of a second equivalent of the aromatic diphosphonic acid. SAMP homogeneity, roughness, and thickness were evaluated by AFM; SAMP film thickness and the structural contributions of each unit in the duplex were measured by X-ray reflection (XRR). The duplex was compared with the aliphatic and aromatic monolayer SAMPs to determine the effect of stacking on electrochemical properties; these were measured by impedance spectroscopy using aqueous electrolytes in the frequency range 20 Hz to 100 kHz, and data were analyzed using resistance–capacitance network based equivalent circuits. For the 11-hydroxyundecylphosphonate SAMP, <i>C</i>_SAMP = 2.6 ± 0.2 μF/cm², consistent with its measured layer thickness (ca. 1.1 nm). For the anthracene-based SAMPs, <i>C</i>_SAMP = 6–10 μF/cm², which is attributed primarily to a higher effective dielectric constant for the aromatic moieties (ε = 5–10) compared to the aliphatic one; impedance spectroscopy measured the additional capacitance of the second aromatic monolayer in the duplex (2ndSAMP) to be <i>C</i>_Ti/2ndSAMP = 6.8 ± 0.7 μF/cm², in series with the first

Photocurrent Generation in Diamond Electrodes Modified with Reaction Centers

Photoactive reaction centers (RCs) are protein complexes in bacteria able to convert sunlight into other forms of energy with a high quantum yield. The photostimulation of immobilized RCs on inorganic electrodes result in the generation of photocurrent that is of interest for biosolar cell applications. This paper reports on the use of novel electrodes based on functional conductive nanocrystalline diamond onto which bacterial RCs are immobilized. A three-dimensional conductive polymer scaffold grafted to the diamond electrodes enables efficient entrapment of photoreactive proteins. The electron transfer in these functional diamond electrodes is optimized through the use of a ferrocene-based electron mediator, which provides significant advantages such as a rapid electron transfer as well as high generated photocurrent. A detailed discussion of the generated photocurrent as a function of time, bias voltage, and mediators in solution unveils the mechanisms limiting the electron transfer in these functional electrodes. This work featuring diamond-based electrodes in biophotovoltaics offers general guidelines that can serve to improve the performance of similar devices based on different materials and geometries

Emergence of Photoswitchable States in a Graphene–Azobenzene–Au Platform

The perfect transmission of charge carriers through potential barriers in graphene (Klein tunneling) is a direct consequence of the Dirac equation that governs the low-energy carrier dynamics. As a result, localized states do not exist in unpatterned graphene, but quasibound states <i>can</i> occur for potentials with closed integrable dynamics. Here, we report the observation of resonance states in photoswitchable self-assembled molecular­(SAM)-graphene hybrid. Conductive AFM measurements performed at room temperature reveal strong current resonances, the strength of which can be reversibly gated <i>on</i>- and <i>off</i>- by optically switching the molecular conformation of the mSAM. Comparisons of the voltage separation between current resonances (∼70–120 mV) with solutions of the Dirac equation indicate that the radius of the gating potential is ∼7 ± 2 nm with a strength ≥0.5 eV. Our results and methods might provide a route toward <i>optically programmable</i> carrier dynamics and transport in graphene nanomaterials