Scott Calabrese Barton

Transparent and catalytic carbon nanotube films

We report on the synthesis of thin, transparent, and highly catalytic carbon nanotube films. Nanotubes catalyze the reduction of triiodide, a reaction that is important for the dye-sensitized solar cell, with a charge-transfer resistance as measured by electrochemical impedance spectroscopy that decreases with increasing film thickness. Moreover, the catalytic activity can be significantly enhanced by exposing the nanotubes to ozone in order to introduce defects. Ozone-treated, defective nanotube films could serve as catalytic, transparent, and conducting electrodes for the dye-sensitized solar cell. Other possible applications include batteries, fuel cells, and electroanalytical devices.

Kinetics of Redox Polymer-Mediated Enzyme Electrodes

Oxygen-reducing enzyme electrodes are prepared from laccase of Trametes versicolor and a series of osmium-based redox polymer mediators covering a range of redox potentials from 0.11 to 0.85 V. Experimentally obtained current density generated by the film electrodes is analyzed using a one-dimensional numerical model to obtain kinetic parameters. The bimolecular rate constant for mediation is found to vary with mediator redox potential from 250 s(-1) M(-1) when mediator and enzyme are close in redox potential to 9.4 x 10(4) s(-1) M(-1) when the redox potential difference is large. The value of the bimolecular rate constant for the simultaneously occurring laccase-oxygen reaction is found to be 2.4 x 10(5) s(-1) M(-1). The relationship between mediator-enzyme overpotential and bimolecular rate constant is used to determine the optimum mediator redox potential for maximum power output of a hypothetical biofuel cell with a planar cathode and a reversible hydrogen anode. For laccase of T. versicolor (E(e)(0) = 0.82), the optimum mediator potential is 0.66 V (SHE), and a molecular structure is presented to achieve this result.

Substrate channelling as an approach to cascade reactions

Millions of years of evolution have produced biological systems capable of efficient one-pot multi-step catalysis. The underlying mechanisms that facilitate these reaction processes are increasingly providing inspiration in synthetic chemistry. Substrate channelling, where intermediates between enzymatic steps are not in equilibrium with the bulk solution, enables increased efficiencies and yields in reaction and diffusion processes. Here, we review different mechanisms of substrate channelling found in nature and provide an overview of the analytical methods used to quantify these effects. The incorporation of substrate channelling into synthetic cascades is a rapidly developing concept, and recent examples of the fabrication of cascades with controlled diffusion and flux of intermediates are presented.

Bioelectrocatalytic hydrogels from electron-conducting metallopolypeptides coassembled with bifunctional enzymatic building blocks

Here, we present two bifunctional protein building blocks that coassemble to form a bioelectrocatalytic hydrogel that catalyzes the reduction of dioxygen to water. One building block, a metallopolypeptide based on a previously designed triblock polypeptide, is electron-conducting. A second building block is a chimera of artificial α-helical leucine zipper and random coil domains fused to a polyphenol oxidase, small laccase (SLAC). The metallopolypeptide has a helix–random-helix secondary structure and forms a hydrogel via tetrameric coiled coils. The helical and random domains are identical to those fused to the polyphenol oxidase. Electron-conducting functionality is derived from the divalent attachment of an osmium bis-bipyrdine complex to histidine residues within the peptide. Attachment of the osmium moiety is demonstrated by mass spectroscopy (MS-MALDI-TOF) and cyclic voltammetry. The structure and function of the α-helical domains are confirmed by circular dichroism spectroscopy and by rheological measurements. The metallopolypeptide shows the ability to make electrical contact to a solid-state electrode and to the redox centers of modified SLAC. Neat samples of the modified SLAC form hydrogels, indicating that the fused α-helical domain functions as a physical cross-linker. The fusion does not disrupt dimer formation, a necessity for catalytic activity. Mixtures of the two building blocks coassemble to form a continuous supramolecular hydrogel that, when polarized, generates a catalytic current in the presence of oxygen. The specific application of the system is a biofuel cell cathode, but this protein-engineering approach to advanced functional hydrogel design is general and broadly applicable to biocatalytic, biosensing, and tissue-engineering applications.

Mediated Enzyme Electrodes with Combined Micro- and Nanoscale Supports

We report the electrochemical performance of a redox polymer-mediated glucose anode catalyzed by glucose oxidase and sup-ported on a multiscale carbon material. The support is composed of carbon paper upon which is grown multiwall nanotubes bychemical vapor deposition combined with ohmic heating of the carbon paper. The material possessed 100-fold higher surface areaand demonstrated tenfold higher electrochemical performance, compared to bare carbon paper. Maximum performance is limitedby biocatalyst and reactant transport in the micro- and nanoscale pores of the support material.© 2007 The Electrochemical Society. DOI: 10.1149/1.2712049 All rights reserved.Manuscript submitted November 8, 2006; revised manuscript received January 4, 2007. Available electronically March 6, 2007.

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.

A Methanol Sensor for Portable Direct Methanol Fuel Cells

An aqueous methanol sensor for portable direct methanol fuel cell applications is demonstrated. The design is based on current output limited by methanol diffusion through a Nafion 117 perfluorosulfonic acid membrane. Steady-state polarization measurements demonstrate sensitivity to concentrations of 0 to 4 M over a temperature range of 40 to 80C. Furthermore, a correlation that is first order in concentration and temperature is demonstrated for concentrations of 0 to 3 M, with an accuracy of {+-}0.1 M. Measurements of transient response to step concentration change indicate a response time of about 10 to 50 s, depending primarily on temperature.

Impact of transition metal on nitrogen retention and activity of iron–nitrogen–carbon oxygen reduction catalysts

Iron based nitrogen doped carbon (FeNC) catalysts are synthesized by high-pressure pyrolysis of carbon and melamine with varying amounts of iron acetate in a closed, constant-volume reactor. The optimum nominal amount of Fe (1.2 wt%) in FeNC catalysts is established through oxygen reduction reaction (ORR) polarization. Since the quantity of iron used in FeNCs is very small, the amount of Fe retained in FeNC catalysts after leaching is determined by UV-VIS spectroscopy. As nitrogen is considered to be a component of active sites, the amount of bulk and surface nitrogen retention in FeNC catalysts are measured using elemental analysis and X-ray photoelectron spectroscopy, respectively. It is found that increasing nominal Fe content in FeNC catalysts leads to a decreased level of nitrogen retention. Thermogravimetric analysis demonstrates that increasing nominal Fe content leads to increased weight loss during pyrolysis, particularly at high temperatures. Catalysts are also prepared in the absence of iron source, and with iron removed by washing with hot aqua regia post-pyrolysis. FeNC catalysts prepared with no Fe show high retained nitrogen content but poor ORR activity, and aqua regia washed catalysts demonstrate similar activity to Fe-free catalysts, indicating that Fe is an active site component.

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:

Mediated Biocatalytic Cathode for Direct Methanol Membrane-Electrode Assemblies

A membrane-electrode assembly (MEA) incorporating a biocatalytic cathode with a conventional, platinum-based anode is demonstrated in operation with hydrogen and methanol fuels. The biocatalytic cathode comprised the enzyme laccase and a redox mediator immobilized within a polymer hydrogel on a carbon-fiber paper support. The cell demonstrated an open circuit potential (OCP) of 1.1 V and a maximum current density of 6 mA/cm 2 when supplied with hydrogen and an air-saturated citrate buffer solution at pH 4 and 40°C. With 10 M methanol fuel, the OCP was 0.8 V and maximum current density was 4 mA/cm 2 . The tolerance of the laccase cathode to the presence of methanol was demonstrated by polarization of the MEA in the presence of methanol feed concentrations up to 10 M. A 6% increase in current density at 0.2 V cell potential was found for 10 M methanol as compared to 1 M methanol.