Transistor-like Behavior of Single Metalloprotein Junctions

Single protein junctions consisting of azurin bridged between a gold substrate and the probe of an electrochemical tunneling microscope (ECSTM) have been obtained by two independent methods that allowed statistical analysis over a large number of measured junctions. Conductance measurements yield (7.3 ± 1.5) × 10^–6<i>G</i>₀ in agreement with reported estimates using other techniques. Redox gating of the protein with an on/off ratio of 20 was demonstrated and constitutes a proof-of-principle of a single redox protein field-effect transistor

Controlling Formation of Single-Molecule Junctions by Electrochemical Reduction of Diazonium Terminal Groups

We report controlling the formation of single-molecule junctions by means of electrochemically reducing two axialdiazonium terminal groups on a molecule, thereby producing direct Au–C covalent bonds <i>in situ</i> between the molecule and gold electrodes. We report a yield enhancement in molecular junction formation as the electrochemical potential of both junction electrodes approach the reduction potential of the diazonium terminal groups. Step length analysis shows that the molecular junction is significantly more stable, and can be pulled over a longer distance than a comparable junction created with amine anchoring bonds. The stability of the junction is explained by the calculated lower binding energy associated with the direct Au–C bond compared with the Au–N bond

Current–Voltage Characteristics and Transition Voltage Spectroscopy of Individual Redox Proteins

Understanding how molecular conductance depends on voltage is essential for characterizing molecular electronics devices. We reproducibly measured current–voltage characteristics of individual redox-active proteins by scanning tunneling microscopy under potentiostatic control in both tunneling and wired configurations. From these results, transition voltage spectroscopy (TVS) data for individual redox molecules can be calculated and analyzed statistically, adding a new dimension to conductance measurements. The transition voltage (TV) is discussed in terms of the two-step electron transfer (ET) mechanism. Azurin displays the lowest TV measured to date (0.4 V), consistent with the previously reported distance decay factor. This low TV may be advantageous for fabricating and operating molecular electronic devices for different applications. Our measurements show that TVS is a helpful tool for single-molecule ET measurements and suggest a mechanism for gating of ET between partner redox proteins

Single Molecular Switches: Electrochemical Gating of a Single Anthraquinone-Based Norbornylogous Bridge Molecule

Herein we report the electrochemical gating of a single anthraquinone-based molecule bridged between two gold electrodes using the STM break-junction technique. Once a molecule is trapped between the STM gold tip and the gold substrate, the potential is swept in order to alternate between the oxidized anthraquinone (AQ) and the reduced hydroanthraquinone (H₂AQ) forms. It is shown that the conductance increases about an order of magnitude with a net conversion from the oxidized AQ form to the reduced H₂AQ form. The results obtained from sweeping the potential (dynamic approach) on a single molecule are compared to those obtained from measuring the conductance at several fixed potentials (static approach). By comparing the static and dynamic approach, qualitative information about the kinetics of the redox conversion was achieved. The threshold potential of the conductance enhancement was found to shift to more negative potentials when the potential is swept at a single molecule. This shift is attributed to a slow redox conversion between the AQ and the H₂AQ forms. The hypothesis, of slow redox kinetics being responsible for the observed differences in the single-molecule conductance studies, was supported by electron transfer kinetics studies of bulk self-assembled monolayers using both cyclic voltammetry at different sweeping rates and electrochemical impedance spectroscopy

Highly Conductive Single-Molecule Wires with Controlled Orientation by Coordination of Metalloporphyrins

Porphyrin-based molecular wires are promising candidates for nanoelectronic and photovoltaic devices due to the porphyrin chemical stability and unique optoelectronic properties. An important aim toward exploiting single porphyrin molecules in nanoscale devices is to possess the ability to control the electrical pathways across them. Herein, we demonstrate a method to build single-molecule wires with metalloporphyrins via their central metal ion by chemically modifying both an STM tip and surface electrodes with pyridin-4-yl-methanethiol, a molecule that has strong affinity for coordination with the metal ion of the porphyrin. The new flat configuration resulted in single-molecule junctions of exceedingly high lifetime and of conductance 3 orders of magnitude larger than that obtained previously for similar porphyrin molecules but wired from either end of the porphyrin ring. This work presents a new concept of building highly efficient single-molecule electrical contacts by exploiting metal coordination chemistry

Metal-Controlled Magnetoresistance at Room Temperature in Single‑Molecule Devices

The appropriate choice of the transition metal complex and metal surface electronic structure opens the possibility to control the spin of the charge carriers through the resulting hybrid molecule/metal <i>spinterface</i> in a single-molecule electrical contact at room temperature. The single-molecule conductance of a Au/molecule/Ni junction can be switched by flipping the magnetization direction of the ferromagnetic electrode. The requirements of the molecule include not just the presence of unpaired electrons: the electronic configuration of the metal center has to provide occupied or empty orbitals that strongly interact with the junction metal electrodes and that are close in energy to their Fermi levels for one of the electronic spins only. The key ingredient for the metal surface is to provide an efficient <i>spin texture</i> induced by the spin–orbit coupling in the topological surface states that results in an efficient spin-dependent interaction with the orbitals of the molecule. The strong magnetoresistance effect found in this kind of single-molecule wire opens a new approach for the design of room-temperature nanoscale devices based on spin-polarized currents controlled at molecular level

Metal-Controlled Magnetoresistance at Room Temperature in Single‑Molecule Devices

The appropriate choice of the transition metal complex and metal surface electronic structure opens the possibility to control the spin of the charge carriers through the resulting hybrid molecule/metal <i>spinterface</i> in a single-molecule electrical contact at room temperature. The single-molecule conductance of a Au/molecule/Ni junction can be switched by flipping the magnetization direction of the ferromagnetic electrode. The requirements of the molecule include not just the presence of unpaired electrons: the electronic configuration of the metal center has to provide occupied or empty orbitals that strongly interact with the junction metal electrodes and that are close in energy to their Fermi levels for one of the electronic spins only. The key ingredient for the metal surface is to provide an efficient <i>spin texture</i> induced by the spin–orbit coupling in the topological surface states that results in an efficient spin-dependent interaction with the orbitals of the molecule. The strong magnetoresistance effect found in this kind of single-molecule wire opens a new approach for the design of room-temperature nanoscale devices based on spin-polarized currents controlled at molecular level

Ambipolar Transport in an Electrochemically Gated Single-Molecule Field-Effect Transistor

Charge transport is studied in single-molecule junctions formed with a 1,7-pyrrolidine-substituted 3,4,9,10-perylenetetracarboxylic diimide (PTCDI) molecular block using an electrochemical gate. Compared to an unsubstituted-PTCDI block, spectroscopic and electrochemical measurements indicate a reduction in the highest occupied (HOMO)–lowest unoccupied (LUMO) molecular orbital energy gap associated with the electron donor character of the substituents. The small HOMO–LUMO energy gap allows for switching between electron- and hole-dominated charge transports as a function of gate voltage, thus demonstrating a single-molecule ambipolar field-effect transistor. Both the unsubstituted and substituted molecules display similar n-type behaviors, indicating that they share the same n-type conduction mechanism. However, the substituted-PTCDI block shows a peak in the source–drain current <i>vs</i> gate voltage characteristics for the p-type transport, which is attributed to a two-step incoherent transport <i>via</i> the HOMO of the molecule

Bioengineering a Single-Protein Junction

Bioelectronics moves toward designing nanoscale electronic platforms that allow <i>in vivo</i> determinations. Such devices require interfacing complex biomolecular moieties as the sensing units to an electronic platform for signal transduction. Inevitably, a systematic design goes through a bottom-up understanding of the structurally related electrical signatures of the biomolecular circuit, which will ultimately lead us to tailor its electrical properties. Toward this aim, we show here the first example of bioengineered charge transport in a single-protein electrical contact. The results reveal that a single point-site mutation at the docking hydrophobic patch of a Cu-azurin causes minor structural distortion of the protein blue Cu site and a dramatic change in the charge transport regime of the single-protein contact, which goes from the classical Cu-mediated two-step transport in this system to a direct coherent tunneling. Our extensive spectroscopic studies and molecular-dynamics simulations show that the proteins’ folding structures are preserved in the single-protein junction. The DFT-computed frontier orbital of the relevant protein segments suggests that the Cu center participation in each protein variant accounts for the different observed charge transport behavior. This work is a direct evidence of charge transport control in a protein backbone through external mutagenesis and a unique nanoscale platform to study structurally related biological electron transfer

Role of Ring <i>Ortho</i> Substituents on the Configuration of Carotenoid Polyene Chains

The 9-(<i>Z</i>)-configuration was exclusively obtained in the carotenoid polyene chain irrespective of olefination and disconnection methods for terminal <i>ortho</i>-unsubstituted benzene rings. The 2,6-dimethyl substituents in the terminal rings secure an all-(<i>E</i>)-polyene structure. The single molecular conductance of the pure 9-(<i>Z</i>)-carotene was measured for the first time to be 1.53 × 10^–4 ± 6.37 × 10^–5G₀, whose value was 47% that of the all-(<i>E</i>)-carotene ((3.23 × 10^–4) ± (1.23 × 10^–4) G₀)