*3.1. Unmodified Electrode*

Detergent solubilised membrane proteins can be absorbed directly on carbon, metal or semi-conductor electrodes in an approach known as protein film electrochemistry (Figure 2a) [33]. A frequently used electrode material is pyrolytic graphite "edge" (PGE) which has a rough, negatively charged surface, which can be tailored with polycations such as polymixin B or poly-l-lysine if required. Respiratory nitrate reductase (NarGHI) isolated from *Paracoccus pantotrophus* in *n*-dodecyl β-D-maltoside (DDM) buffer was adsorbed onto PGE electrodes and showed direct electron transfer (DET) with the electrode [34]. The catalytic activity of NarGHI from *E. coli* was studied over a wide pH range (5 < pH < 9), nitrate concentrations and in the presence of inhibitor [35]. Similarly, purple bacteria RC-LH I was solubilised in 0.1% lauryldimethylamine N-oxide (LDAO) buffer and immobilised directly on a bare Au electrode [36]. RC-LH I did not exhibit DET, but showed photocurrents (~64 nA/cm<sup>2</sup> ) when ubiquinone-0 and cytochrome *c* were present in solution, indicating that electron transfer needs to be mediated by small redox compounds in this system (mediated electron transfer, MET). Like water-soluble proteins, membrane proteins on bare electrodes often lack a specific orientation and this can impede efficient electron transfer between electrode and proteins. Making use of the amphiphilic nature of membrane proteins, a densely packed protein monolayer with defined orientation can be formed on electrode surfaces by Langmuir−Blodgett deposition and this was shown to increase the electron transfer efficiency [37–41]. An example of this strategy can be found in Kamran et al. where detergent solubilised-RC-LH I was first pre-assembled in a monolayer at a water-air interface and then transferred onto a gold-coated electrode, reaching photocurrent values of ~45 µA/cm<sup>2</sup> [37].
