*3.2. Modification of Carbon Nanomaterials with Polycyclic Compounds*

Contrary to the charged surface of redox enzymes, the active site of redox enzymes is usually surrounded by polypeptides to form a substrate-binding "pocket" that is often non-charged and hydrophobic. Based on this knowledge, by modifying an electrode surface with polycyclic aromatic compounds, the hydrophobic interactions (π-π stacking) between the active center "pocket" of the enzyme and the modifier may induce a favorable orientation. Anthracene and its derivatives are the most common groups that are modified into carbon nanomaterials for the oriented immobilization of redox enzymes. The concept was first proposed by Armstrong and his coworkers, who reported an improved O2-reduction reaction by the adsorption of laccase at the anthracene-functionalized pyrolytic graphite "edge" electrode [57]. This approach has been further developed for the orientated-immobilization of laccase at CNTs functionalized with anthracene-2-carbonyl chloride [58] or 1-(2-anthraquinonylaminomethyl) pyrene [59]. For example, the optimized O2-reduction current density produced by *Tv*Lac at an anthracene-2-carbonyl chloride functionalized CNT electrode is 140 µA cm−<sup>2</sup> , while no obvious catalytic current can be observed by *Tv*Lac at a pristine CNT electrode [58]. In these studies, polycyclic anthracene groups tailored to carbon nanomaterials were expected to insert the hydrophobic substrate-binding "pocket" to control the orientation of redox enzymes for an efficient interfacial electron transfer. Interestingly, CNTs modified by 1-[bis(2-anthraquinonyl)aminomethyl] pyrene (PyrAQ2) with two anthraquinone groups showed greater efficiency for improvement of DET-type bioelectrocatalysis of laccase than that modified by 1-(2-anthraquinonylaminomethyl) pyrene (PyrAQ) with only one anthraquinone group [60]. The O2-reduction current density produced by *Tv*Lac at PyrAQ2-functionalized CNT electrodes reached to 0.9 mA cm−<sup>2</sup> , which is much higher than that at PyrAQ-functionalized CNT electrodes (0.35 mA cm−<sup>2</sup> ) [60]. The increased anthraquinone groups seem to increase the probability of laccases with a controlled orientation. In addition to anthracene, surfaces functionalized by pyrene [61] and adamantane [62] have been found to be efficient in controlling the orientation of Lac for the improvement of DET-type bioelectrocatalysis. (Figure 5B) Furthermore, such improved DET-type bioelectrocatalysis using a similar approach has also been reported when using FDH in anthracene-functionalized CNTs [63] and *Desulfomicrobium baculatum* H2ase (*Db*H2ase) in anthraquinone- or adamantane-functionalized CNTs [64].
