*4.1. Quartz Crystal Microbalance*

A quartz crystal microbalance (QCM) is a powerful technique commonly used to measure the adsorption of redox enzymes onto an electrode surface. In general, a QCM instrument operates by measuring the resonance frequency of a piezoelectric quartz chip. According to the Sauerbrey equation [80], the frequency of a piezoelectric quartz chip is typically proportional to the mass on the electrode (assuming a rigid thin film). By coating the piezoelectric quartz chip with carbon nanomaterials, the adsorption behaviors of redox enzymes at the corresponding surfaces can be obtained. Furthermore, considering the large size and structural flexibility of redox enzymes, QCM with dissipation (QCM-D) monitoring to further understand the rigidity of enzyme binding, desorption, reorientation, and conformational changes has been proposed [81]. For example, much greater decreases in frequency and increases in the dissipation factor have been found for the adsorption of *Tv*Lac onto adamantane-functionalized CNT electrodes than onto pristine CNT electrodes [62]. The results indicate higher enzyme loading and stable immobilization of laccase in adamantane-functionalized CNTs than in pristine CNT electrodes. A similar study reported by the same group also showed increased frequency decreases and dissipation factor increases for the adsorption of *Db*H2ase onto adamantane- and anthraquinone-functionalized CNT electrodes [63]. In combination with electrochemical measurements, these studies have concluded that the functionalization of CNTs with adamantane or anthraquinone groups increase the effective adsorption of *Tv*Lac and *Db*H2ase for DET-type bioelectrocatalysis owing to the hydrophobic interaction between the active site pocket of redox enzymes and a functionalized polycyclic modifier. A recent study revealed that upon adsorption of *Tr*Lac onto naphthalene-functionalized CNTs, dissipation increased and resonance frequency decreased rapidly, followed by slow dissipation decrease [82].(Figure 6A) The first rapid dissipation increase and resonance frequency decrease are typical behaviors of protein adsorption, whereas the subsequent slow decrease in dissipation was proposed to be caused by the rearrangement of adsorbed *Tr*Lac, which is important for an electric connection of *Tr*Lac to the naphthalene-functionalized CNTs by cross-comparing the results of an open circuit potential monitoring and dissipation response.
