2.2.2. Microscopy

Electrochemical scanning probe microscopies (SPMs), especially STM and atomic force microscopy (AFM), have been extensively used to provide structural SAM properties at the single-molecule level. The principle of STM is based on the quantum tunneling effect. A bias voltage is applied across the metal support and the extremely sharp STM probe, generating tunneling currents, which are transformed to high-resolution conductivity images when the STM tip is scanned across the SAM-modified metal support. Zhang and associates have reported extensive in situ STM investigations using a wide range of alkanethiols on single-crystal gold [6,64–66]. *In situ* STM combined with electrochemical control can also directly map real-time SAM dynamics and structural features. *In situ* AFM has also attracted attention in bioelectrochemistry [67]. AFM records complex forces, mapping the structural information by allowing the tip to directly contact the redox protein/enzyme molecules. High-resolution STM has been employed to record the SAM thickness and molecular orientations. Contact angle (CA) measurement is a simple and straightforward method for monitoring the hydrophobic/hydrophilic properties of SAMs [1,5].
