4.4.1. Electron Microscopy

Electron microscopy (EM) techniques are available to investigate membrane proteins (typically by cryo transmission electron microscopy, TEM or cryo-EM) and electrode structures (typically by scanning electron microscopy, SEM) at various scales. However, EM has not been widely used to study membrane proteins immobilised on electrodes as TEM and SEM are based on different modes of observation [135]. In 2015, Monsalve et al. [136] used SEM and TEM to characterise the morphology and size distribution of gold nanoparticles on a gold electrode used for direct absorption of *Aquifex aeolicus* [NiFe] membrane-bound hydrogenase surrounded by DDM detergent.
