*4.5. Quartz Crystal Microbalance*

Quartz crystal microbalance (QCM) is a sensitive mass sensor which utilises acoustic waves generated by oscillating a piezoelectric, single crystal quartz plate to measure mass changes in the order of nanograms. The association of QCM with dissipation monitoring (QCM-D) allows to also measure the energy loss or dissipation (∆D) of the system [150]. QCM was used to monitor the different stages of the ptBLM formation with cytochrome *c* oxidase from *Rhodobacter sphaeroides* as model protein [151]. The adsorption of cytochrome *c* oxidase at the surface decreased the resonance frequency while increased the dissipation. The Hawkridge group investigated the electrostatic association between cytochrome *c* and cytochrome *c* oxidase immobilised in hBLM with QCM [68,69]. The QCM data revealed the binding of cytochrome *c* and cytochrome *c* oxidase at different ionic strength which was related to the mediated electron transfer. QCM-D was also used to characterise the formation of the cystamine–pyrroloquinoline quinone–thylakoids layers onto SAMs [152].
