*3.1. Modification of Carbon Nanomaterials with Charged Compounds*

Electrostatic effects play major roles in enzyme-substrate interactions and in DET-type bioelectrocatalysis. For example, it has been found that the electrode potential may affect the adsorption and orientation, and thus, the performance of DET-type bioelectrocatalysis of redox enzymes [50]. In addition, improved O2-reduction reactions have been achieved through the adsorption of *Mv*BOD on negatively charged surfaces, which were produced by the electrooxidation of 4-aminobenzoic acid at the KB [12,47,51], reduced graphene oxide [52], or MgO-templated mesoporous carbon [34] modified electrodes. Specially, the O<sup>2</sup> reduction-current densities produced by *Mv*BOD at 4-aminobenzoic acid functionalized KB modified electrodes is more than 2 times higher than that at pristine KB modified electrodes [12,47] (Figure 5A). The net charge of *Mv*BOD is negative at pH 7, whereas the surface close to the type I copper, the site that is believed to accept electrons from the electrode in the DET reaction case, is positive at pH 7. The electrostatic interaction between the 4-aminobenzoic acid-functionalized carbon surface and the positively charged T1 site is proposed to provide an effective orientation for the DET-type bioelectrocatalysis of *Mv*BOD [47]. Such enhancement can also be found through the adsorption of *Mv*BOD at naphthoate-functionalized CNT-modified electrodes [53], which were prepared using the electrochemical reduction of 6-carboxynaphthalenediazonium or by the in-situ chemical modification of 6-amino-2-naphthoic acid onto CNTs. Similar phenomena have also been reported in *Magnaporthe orizae* BOD (*Mo*BOD), whichillustratedimproved oxygen reduction reactions at 6-carboxynaphthalenediazonium functionalized CNTs [54]. In contrast to *Mv*BOD and *Mo*BOD, BOD from *Bacillus pumilus*(*Bp*BOD) presents a negatively charged T1 site. As expected, the O2-reduction current produced by *Bp*BOD at a positively charged 1-pyrenemethylamine hydrochloride-functionalized CNT electrode is ~0.4 mA cm−<sup>2</sup> , which is much higher than that at a pristine CNT electrode (~0.05 mA cm−<sup>2</sup> ) [55]. Similarly, CueO [56] and *Dv*H2ase [51], which have negatively charged active sites for interfacial electron transfer, show enhanced DET-type bioelectrocatalysis at a p-phenylenediamine-functionalized KB electrode. Consequently, improved DET-type bioelectrocatalysis based on an electrostatic interaction has been applied to the construction of a H2/Air(O2) enzymatic biological fuel cell, which consists of a *MvBOD-*adsorbed 4-aminobenzoic acid-modified KB biocathode and *Dv*H2ase adsorbed p-phenylenediamine-functionalized KB bioanode, with a high-power density of 6.1 mW cm−<sup>2</sup> [51], which is the highest output of DET-type biological fuel cells reported thus far.
