Kevin D. McCarthy

Extracellular electron transfer via microbial nanowires

Microbes that can transfer electrons to extracellular electron acceptors, such as Fe(iii) oxides, are important in organic matter degradation and nutrient cycling in soils and sediments. Previous investigations on electron transfer to Fe(iii) have focused on the role of outer-membrane c-type cytochromes. However, some Fe(iii) reducers lack c-cytochromes. Geobacter species, which are the predominant Fe(iii) reducers in many environments, must directly contact Fe(iii) oxides to reduce them, and produce monolateral pili that were proposed, on the basis of the role of pili in other organisms, to aid in establishing contact with the Fe(iii) oxides. Here we report that a pilus-deficient mutant of Geobacter sulfurreducens could not reduce Fe(iii) oxides but could attach to them. Conducting-probe atomic force microscopy revealed that the pili were highly conductive. These results indicate that the pili of G. sulfurreducens might serve as biological nanowires, transferring electrons from the cell surface to the surface of Fe(iii) oxides. Electron transfer through pili indicates possibilities for other unique cell-surface and cell–cell interactions, and for bioengineering of novel conductive materials.

Electricity generation by Geobacter sulfurreducens attached to gold electrodes

The versatility of gold for electrode manufacture suggests that it could be an ideal material for some microbial fuel cell applications. However, previous studies have suggested that microorganisms that readily transfer electrons to graphite do not transfer electrons to gold. Investigations with Geobacter sulfurreducens demonstrated that it could grow on gold anodes producing current nearly as effectively as with graphite anodes. Current production was associated with the development of G. sulfurreducens biofilms up to 40 microm thick. No current was produced if pilA, the gene for the structural protein of the conductive pili of G. sulfurreducens, was deleted. The finding that gold is a suitable anode material for microbial fuel cells offers expanded possibilities for the construction of microbial fuel cells and the electrochemical analysis of microbe-electrode interactions.

Single-molecule chiroptical spectroscopy: Fluorescence excitation of individual helicene molecules in polymer-supported thin-films.

We present results of fluorescence excitation circular dichroism studies of the chiroptical response of single (bridged triarylamine) helicene molecules immobilized at a polymer interface. We extract directly dissymmetry parameters, and corresponding probability distributions, associated with the single-molecule fluorescence excitation associated with modulation of a circular polarized excitation field for three different excitation wavelengths (405, 440, 457 nm) showing circular dichroism in bulk films. The observed single molecule chiroptical response is anomalously large in comparison with the results of time-dependent density functional calculations, and the observed defocused emission patterns seem to indicate a higher multipole nature to the transition probed. Our results provide new insights into chiroptical properties of chiral fluorophores that are hidden under the extensive averaging associated with conventional chiroptical probes.

Linear Dipole Behavior in Single CdSe-Oligo(phenylene vinylene) Nanostructures

We report on linearly polarized absorption and emission from individual (4.3 nm) CdSe quantum dots whose surfaces are coordinated with monodisperse oligo-phenylene vinylene ligands. Shown previously to suppress quantum dot blinking, we demonstrate here that the electronic interaction of photoexcited ligands with the quantum dot core is manifested as a strong polarization anisotropy in absorption (M = 0.5), as well as distinct linear dipole emission patterns from the quantum dot core. Further, there is a correlation between the quantum dot emission moment and polarization orientation corresponding to the absorption maxima that is manifested as fluctuations in emission moment orientation in the X-Y plane. The observed polarization effects can be switched off by tuning the excitation away from the ligand absorption band. We propose a mechanism based on exciton dissociation from the photoexcited ligand, followed by the pinning of electrons at the quantum dot surface. The resulting Stark interaction is sufficiently strong to break the 2D degeneracy of the emission moment within the dot, and may therefore account for the linear dipole emission character.

Directional Absorption and Emission in Hybrid Polymer-Quantum Dot Nanostructures Studied by Scanning Probe and Fluorescence Microscopy

The optical properties of hybrid organic/quantum dot nanostructures, probed at the single molecule limit by scanning probe and fluorescence microscopy, reveal novel and highly directional absorption and emission characteristics, ideal for polarization-based switching applications.

Diffusive Coordinate Model for Blinking Suppression and Intensity Fluctuations in CdSe-OPV Nanocrystals

We describe here numerical simulation of a modified Frantsuzov-Marcus diffusive coordinate (DC) model[1] which yields blinking suppression and low frequency fluctuations as observed[2] in Oligo-(phenylene vinylene) (OPV) coated CdSe quantum dots.

Periodic Intensity Fluctuations in Functionalized Semiconductor Quantum Dots: Correlation with Ligand Coverage

Fluorescence microscopy has been used in conjunction with atomic force microscopy to study size-correlated emission properties of single oligo-phenylene vinylene-functionalized CdSe nanocrystals, which reveals size-dependent intensity fluctuations on time scales of 10-60 seconds.