Redox-mediated tunneling current

The electrochemical detection of redox reactions at the active site of GOD can be followed via electrochemical detection. However, due to the large size of the enzyme, electrons cannot tunnel directly from the active site to the electrode. Molecular mediators such as ferrocene and its derivatives (FcD) have to be co-immobilised in the gel in order to transfer electrons. Ferrocene is then deoxidised at the electrode and the faradaic current is proportional to the amount of glucose (Audebert, 1993 Niu, 2002). 

Under appropriate conditions, the faradaic current may be used to form images of the electrochemical reactivity of a surface. This is known as scanning electrochemical microscopy (SECM), where the transport and heterogeneous redox activity of species within the junction mediate the tip-substrate interaction. This subject has been thoroughly reviewed [43,44], and an excellent paper demonstrating the transition from STM to SECM is available [45]. The possible contribution of confined redox species to resonant tunneling has also been examined [19,46,47]. 

The overall SECM process includes the following consecutive steps (i) mass transfer in the tip/sub-strate gap (the diffusion-limiting current for this step is given by Equation 5.30), (ii) bimolecular ET between the mediator species and the monolayer-bound redox centers (Equation 5.40), and (iii) the electron tunneling through the monolayer (Equation 5.41). The rates of steps (ii) and (iii) are given by Equations 5.42 and 5.43, respectively. 

When the monolayer is blocking (i.e., it contains no redox centers capable of reacting with the mediator species), the mediator regeneration can occur via direct tunneling through the film (Figure 5.7B). The characteristic current for this reaction is... 

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