In microbial organisms like bacteria, pili (singular: pilus) are filament-like appendages that are nanometers in diameter and microns long. The sizes and structures of the different types of pili found in nature are adapted to serve one of many distinct functions for the organism from which they come. The pili expressed by the bacterium Geobacter sulfurreducens act as electrically conductive nanowires that provide conduits for electrons to leave the cell during its respiratory cycle. Biological experiments have suggested that long range electron transfer across micron distances may proceed along the protein matrix, rather than by metalcofactors (metal atoms bound to the protein). Protein conductivity across such distances would require a novel transport mechanism. In an effort to elucidate this mechanism, our lab has used two electronically sensitive scanning probe techniques: Scanning Tunneling Microscopy (STM) and Conductive Probe Atomic Force Microscopy (CP-AFM).I employed the high resolution imaging and electronic sensitivity of STM to resolve the molecular sub-structure and local electronic density of states (LDOS) at different points above pili from purified preparations, deposited onto a conducting substrate. The significantand stable tunneling currents achieved for biologically relevant voltages, in the absence of metal cofactors, demonstrated conduction between tip and substrate via the protein matrix.We observed periodicity of roughly 10 nm and 2.5 nm in topographs of the pili. In our acquisition of LDOS, we observed gap-like asymmetric energy spectra that were dependent upon the location of the tip above the pilus, suggestive of easier current flow out of oneside of the cylindrical pilus and into the opposite side. Voltage-dependent STM imaging, which also contains information about the LDOS at each pixel, was consistent with this interpretation. The asymmetry in spectra observed on one pilus edge had a slightly larger magnitude than the other edge, by a factor of 1.3. The width of the gap-like feature was roughly 1 V.For direct observation of the long range electron transport, we developed a method whereby a conductive AFM tip measured current flowing to a surface electrode via the longitudinal axis of a deposited pilus. These samples also lacked embedded metal atoms. We achieved a proof-of-principle measurement of conductivity across a 200 nm distance. The upper bound of the resistance at this distance was 40 megaohms, suggesting a preliminary upper bound on the longitudinal resistivity of 0.4 Ohm-cm, comparable to that of the basal plan of graphite. I will place these results in context with continuing work in our lab to optimize the protocols for reproducible deposition quality on the electrodes.
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