Stanislav Tsoi

Long-range electron transport in Geobacter sulfurreducens biofilms is redox gradient-driven

Geobacter spp. can acquire energy by coupling intracellular oxidation of organic matter with extracellular electron transfer to an anode (an electrode poised at a metabolically oxidizing potential), forming a biofilm extending many cell lengths away from the anode surface. It has been proposed that long-range electron transport in such biofilms occurs through a network of bound redox cofactors, thought to involve extracellular matrix c-type cytochromes, as occurs for polymers containing discrete redox moieties. Here, we report measurements of electron transport in actively respiring Geobacter sulfurreducens wild type biofilms using interdigitated microelectrode arrays. Measurements when one electrode is used as an anode and the other electrode is used to monitor redox status of the biofilm 15 μm away indicate the presence of an intrabiofilm redox gradient, in which the concentration of electrons residing within the proposed redox cofactor network is higher farther from the anode surface. The magnitude of the redox gradient seems to correlate with current, which is consistent with electron transport from cells in the biofilm to the anode, where electrons effectively diffuse from areas of high to low concentration, hopping between redox cofactors. Comparison with gate measurements, when one electrode is used as an electron source and the other electrode is used as an electron drain, suggests that there are multiple types of redox cofactors in Geobacter biofilms spanning a range in oxidation potential that can engage in electron transport. The majority of these redox cofactors, however, seem to have oxidation potentials too negative to be involved in electron transport when acetate is the electron source.

Electrochemically controlled conductance switching in a single molecule: quinone-modified oligo(phenylene vinylene).

Reversible conductance switching in single quinone-oligo(phenylene vinylene) (Q-OPV) molecules was demonstrated using electrochemical STM. The switching was achieved by application of electrochemical potential to the substrate supporting the molecule. The ratio of conductances between the high- and low-conductivity states is over 40. The high-conductivity state is ascribed to strong electron delocalization of the fully conjugated hydroquinone-OPV structure, whereas the low-conductivity state is characterized by disruption of electron delocalization in the quinone-OPV structure.

Increasing Efficiency of Photoelectronic Conversion by Encapsulation of Photosynthetic Reaction Center Proteins in Arrayed Carbon Nanotube Electrode

The construction of efficient light energy converting (photovoltaic and photoelectronic) devices is a current and great challenge in science and technology and one that will have important economic consequences. Here we show that the efficiency of these devices can be improved by the utilization of a new type of nano-organized material having photosynthetic reaction center proteins encapsulated inside carbon nanotube arrayed electrodes. In this work, a generically engineered bacterial photosynthetic reaction center protein with specifically synthesized organic molecular linkers were encapsulated inside carbon nanotubes and bound to the inner tube walls in unidirectional orientation. The results show that the photosynthetic proteins encapsulated inside carbon nanotubes are photochemically active and exhibit considerable improvement in the rate of electron transfer and the photocurrent density compared to the material constructed from the same components in traditional lamella configuration.

Thermally activated long range electron transport in living biofilms.

Microbial biofilms grown utilizing electrodes as metabolic electron acceptors or donors are a new class of biomaterials with distinct electronic properties. Here we report that electron transport through living electrode-grown Geobacter sulfurreducens biofilms is a thermally activated process with incoherent redox conductivity. The temperature dependency of this process is consistent with electron-transfer reactions involving hemes of c-type cytochromes known to play important roles in G. sulfurreducens extracellular electron transport. While incoherent redox conductivity is ubiquitous in biological systems at molecular-length scales, it is unprecedented over distances it appears to occur through living G. sulfurreducens biofilms, which can exceed 100 microns in thickness.

Measuring conductivity of living Geobacter sulfurreducens biofilms

To the Editor — Certain microorganisms can use an electrode as a metabolic electron acceptor or donor by means of extracellular electron transport (EET) processes1,2. Such microorganisms are studied as potential catalysts for electrode reactions such as the electrosynthesis of fuel precursors from reduction of CO2 using renewable sources of electricity3. The appeal of microbial electrode catalysts is that they self-assemble and self-heal, and the prospect of optimizing their catalytic properties (for example, reaction product and yield) through molecular engineering. In addition to enabling electron transport across a microbial/electrode interface, EET processes can often facilitate long-distance electron transport, resulting in the formation of multi-cell-thick electrode-grown biofilms, which are electrically conductive and can exceed 100 μm thickness. Such biofilms challenge the notion that biological electron transport is limited to molecular length scales. The fundamental mechanism of EET underlying biofilm conductivity has implications across many disciplines and is yet unresolved. Malvankar et al. reported that living electrode-grown biofilms comprising Geobacter sulfurreducens, a well-studied long-distance EET-capable microorganism, possess metallic-like conductivity similar to that of organic semiconductors4, a property that would make these biofilms unique among all biological materials. Electrochemical gating measurements were performed in a manner similar to that used to study conducting polymer films in electrolytic solutions5. Based on the resulting conductivity versus gate potential response, the authors proposed that living electrodegrown G. sulfurreducens biofilms are metallic-like conductors. When performing our own electrical electrochemical gating measurements of living electrode-grown G. sulfurreducens biofilms, we obtained a distinctly different conductivity versus gate potential response — one consistent with redox conductivity, similar to that of redox polymers6 and not consistent with metalliclike conductivity (Fig. 1 and Supplementary Fig. 1)5. Furthermore, it was recently demonstrated that conductivity of living electrode-grown G. sulfurreducens biofilms decreases with decreasing temperature in a manner that is also consistent with redox conductivity and not with metallic-like conductivity1. And it was also recently demonstrated that conductivity of these biofilms examined in air increases with decreasing temperature when the ambient water content is kept constant, and decreases with decreasing temperature when the ambient relative humidity is kept constant2. Again, both dependencies are consistent with redox conductivity and not with metallic-like conductivity2. Redox conductivity is ubiquitous in biological systems at molecular length scales, but is without precedence for distances over which electron transport appears to occur through electrode-grown G. sulfurreducens biofilms. The different conductivity versus gate potential response we obtained for living electrode-grown G. sulfurreducens biofilms prompted us to undertake a direct comparison of electrochemical gating measurements performed using our methods and measurements we replicated using the methods of Malvankar and colleagues. In this comparison, electrochemical gating measurements were performed on living G. sulfurreducens biofilms (Fig. 1 and Supplementary Fig. 1) as well as on two well-known conducting polymers: electropolymerized polyaniline, a known organic semiconductor5 (referred to here as PANI) (Fig. 2 and Supplementary Fig. 2); and poly(Nvinylimidazole [Os(bipyridine)2Cl]), a known redox conductor7 (referred to here as PVI-Os(bipy)2Cl) (Fig. 2 and Supplementary Fig. 3). Following Malvankar and colleagues’ approach, our biofilm electrochemical gating measurements were performed under physiologically relevant conditions in an aqueous electrolyte medium using gold source and drain electrodes patterned on a glass surface. Biofilms were grown that extended across the gap separating the electrodes, electrically connecting the source and drain. Different potentials were applied to the electrodes (ES and ED), generating a source–drain current (ISD) through the biofilm between the electrodes. In the limit of sufficiently small source–drain voltage, VSD = ED – ES ≤ 0.05 V (ref. 1), Ohm’s law applies such that:

Van der Waals screening by single-layer graphene and molybdenum disulfide

A sharp tip of atomic force microscope is employed to probe van der Waals forces of a silicon oxide substrate with adhered graphene. Experimental results obtained in the range of distances from 3 to 20 nm indicate that single-, double-, and triple-layer graphenes screen the van der Waals forces of the substrate. Fluorination of graphene, which makes it electrically insulating, lifts the screening in the single-layer graphene. The van der Waals force from graphene determined per layer decreases with the number of layers. In addition, increased hole doping of graphene increases the force. Finally, we also demonstrate screening of the van der Waals forces of the silicon oxide substrate by single- and double-layer molybdenum disulfide.

Robust reduction of graphene fluoride using an electrostatically biased scanning probe

AbstractWe report a novel and easily accessible method to chemically reduce graphene fluoride (GF) sheets with nanoscopic precision using high electrostatic fields generated between an atomic force microscope (AFM) tip and the GF substrate. Reduction of fluorine by the electric field produces graphene nanoribbons (GNR) with a width of 105-1,800 nm with sheet resistivity drastically decreased from >1 TΩ·sq.−1 (GF) down to 46 kΩ·sq.−1 (GNR). Fluorine reduction also changes the topography, friction, and work function of the GF. Kelvin probe force microscopy measurements indicate that the work function of GF is 180–280 meV greater than that of graphene. The reduction process was optimized by varying the AFM probe velocity between 1.2 μm·s−1 and 12 μm·s−1 and the bias voltage applied to the sample between −8 and −12 V. The electrostatic field required to remove fluorine from carbon is ∼1.6 V·nm−1. Reduction of the fluorine may be due to the softening of the C-F bond in this intense field or to the accumulation and hydrolysis of adventitious water into a meniscus.

Experimental demonstration of the optical lattice resonance in arrays of Si nanoresonators

Optical resonances of crystalline Si nanopillar arrays on a Si substrate are studied using optical reflectivity and Raman spectroscopy. When the nanopillars are arranged in a two-dimensional lattice, a collective resonance is observed in the reflection spectra which is absent for randomly distributed nanopillars. The resonance is due to coherent oscillations in nanopillars, can be tuned spectrally by the nanopillar diameter and lattice period, and strongly suppresses reflection from the Si surface. Raman scattering demonstrates that the reduced reflectivity is accompanied by increased electromagnetic field confined in Si, thus suggesting potential application of the lattice resonance in surface enhanced spectroscopy and thin film solar cells.

Molecular conductance switching via controlled alteration of electron delocalization: Quinone-modified oligo(phenylenevinylene)

Reversible conductance switching in single quinone-modified oligo(phenylenevinylene)s (OPV) was studied using electrochemical scanning tunnel microscopy. The switching was achieved through electrochemical oxidation/reduction in the quinone moiety of the molecule. The strong electron delocalization of the reduced hydroquinone-OPV structure resulted in the high-conductance state, whereas the weaker delocalization of the oxidized quinone-OPV was responsible for the low-conductance state. The ratio of the conductances was measured to be in excess of 40.

Observation of two discrete conductivity states in quinone-oligo(phenylene vinylene)

The single-molecule conductivity of quinone-oligo(phenylene vinylene) (Q-OPV) attached to a gold substrate was studied using electrochemical scanning tunnelling microscopy. The results show that the molecule has two discrete conductivity states: a low-conductivity state, when it is oxidized, and a high-conductivity state, when reduced. The electron transport through the molecule in both states occurs via coherent tunnelling. The molecular conductivity in either oxidation state is independent from the electrochemical gate potential; however, the gate potential can be used to switch the oxidation state of the molecule. Numerical calculations suggest that the highest occupied molecular orbital (HOMO) of Q-OPV controls tunnelling through the molecule and that the independence of conductivity from the electrochemical gate in either oxidation state originates from strong penetration of HOMO into the substrate. In addition, the greater delocalization of HOMO in the reduced state than in the oxidized state explains the greater conductivity of Q-OPV in the former than in the latter.