Nonconducting gap

A second, more sophisticated type of devices was fabricated in the following way. First a temporary gold layer, which closed the nonconducting gap between the two chromium electrodes, was evaporated. Subsequently a voided double-gyroid-structured template was prepared on this continuously metal-coated substrate and refilled with PPy. Then the redundant template was dissolved with xylene. Finally the superficial gold layer was etched with a wet gold etchant, which reestablished the gap between the chromium electrodes. 

As expected [34, 35], the current between the anode and the cathode increased over time with cell growth (Fig. 7.4a). Confocal laser scanning microscopy (CLSM) revealed that the cells formed a confluent biofilm that spread across the nonconductive gap (Fig. 7.3b-e). In contrast, there was no growth on the control electrodes that were not connected to the cathode. If only one of the two anodes was connected to the cathode, a biofilm formed only on that side and did not bridge the gap. 

Figure 7.5 Four-probe measurements, (a) Schematic of microbial fuel cell with four gold electrodes, each separated by 50-pm nonconductive gap, serving as an anode (side-view).

Low Conductance in Fumarate-Grown Biofilms When the gold electrodes were not connected to the cathode, but fiimarate was provided in solution as an alternative electron acceptor, biofilms of wild-type and PilA mutant DL-1 strain were formed on the gold surfaces and bridged the nonconductive gap. However, these biofilms exhibited very low conductivity (Fig. 7.9). These results demonstrated that the conductivity of the biofilm depends on the physiological status of the cells. 

S.2 Current-Producing PilA Mutant Biofilms Biofilms of PilA mutant of DL-1 strain of G. sulfurreducens and PilA mutant strain of Geobacter sirdin Speedy produced 10 times less current and did not bridge the nonconducting gap. This is consistent with previous studies that have demonstrated that because pili are electrically conductive [60], they are required for the development of the thick anode biofilms necessary for high-density current production by G. sulfurreducens [35,61]. 

S.3 ShewaneUa oneidensis Biofilm S. oneidensis is reported to possess conductive filaments [63], but S. oneidensis produced biofilms too thin (<10 pm) to bridge the nonconducting gap of our apparatus, consistent with previous electrode smdies [39] and the finding that biofilms of the closely related Shewanella loihica are highly resistive [64],... 

S.4 Aerobic Biofilms Biofilms of well-known biofilm formers such as P. aeruginosa and E. colt grew across the nonconducting gap when their preferred electron acceptor, oxygen, was provided. However, the measured conductance... 

Figure 7.5 Four-probe measurements, (a) Schematic of microbial fuel cell with four gold electrodes, each separated by 50-pm nonconductive gap, serving as an anode (side-view), (b) Schematic of conductivity measurements using four electrodes (side-view). Current is injected through outer two electrodes, and voltage is measured across inner two electrodes, (c) Schematic of four-probe semp used for biofihn and pih measurements (top-view). Inset, AFM image of pili network placed on gold electrodes, (d) Comparison of conductivity measured using two-probe and four-probe methods. Error bars show SD of individual measurements for four biofilms of KN400. Figure adapted from Ref. [4] with permission.

The role of ACh is the same in the transmission of the nerve impulse along the axon and across the synapse. There is only a quantitative difference due to the increased surface at nerve endings, there is less resistance and therefore more flow of current enabling the impulse to cross the nonconducting gap. 

The quantization of the Hall resistance in the FISDW phases is indeed very reminiscent of the quantum Hall effect in the two-dimensional electron gas [136]. There is, however, an important difference between these two phenomena. In both cases the quantization requires a reservoir of nonconducting electronic states. This reservoir is provided either by localized states in the gap between conducting Landau levels or by the electron-hole (spin modulation) condensate for the two-dimensional electron gas and the FISDW of organics, respectively. 

Mechanically deforming the carbon nanotube alters its electric properties. In the example shown in Fig. 5.12, a carbon nanotube was bridged between two electrodes separated by a gap. When the middle of the carbon nanotube was pushed by an AFM tip, the nanotube was bent and its conductivity dropped. The electron conductivity was correlated with the degree of bending, which was measured from the position of the AFM. This process was repeatable. The bending of the carbon nanotube induced the formation of some nonconductive SP orbitals within its structure, althering its conductivity. This behavior is somewhat reminiscent of a switch. 

Most silicate clays and oxides are insulators and semiconductors, types of solids that possess a band gap—a nonallowed region of energy. Eg, between the filled valence band and empty conduction band (see Figure 7.14). This gap prevents electron flow in these minerals, so that they are nonconducting. In the solid, the Fermi... 

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