Imaging Single Enzyme Molecules under In Situ Conditions

摘要

The electrochemical investigation of redox enzymes has become a broad field of research because the redox properties of the enzymes make them candidates for biosensors and bioelectronic nanodevices. The degree of surface coverage, the surface orientation of the enzymes, that is, the position of the active site, and the activity of a single enzyme are still a question of dispute; this data can not be extracted from integral electrochemical measurements, such as cyclic vol-tammetry (CV). It has been shown, however, that scanning probe techniques, especially the electrochemical scanning tunneling microscopy (EC-STM), can reveal the structure and reactivity of enzymes down to a single-molecule level, although some experimental problems remain. To avoid any damage to the protein structure only small tunneling currents I_T can be applied. Therefore, scanning electrochemical potential microscopy (SECPM), a technique which measures the potential at zero current (I=0) may be advantageous. The hardware is similar to an EC-STM, however the tip, is used as a potential sensor. The potential difference between the tip and the applied potential at the working electrode is measured with a high-impedance potential amplifier and serves as a feedback signal in the x-y scanning mode. In addition, SECPM also offers the possibility to map the potential distribution of the interface in the x-z direction, that is, perpendicular to the electrode surface. According to the Gouy-Chapman-Stern theory, at the electrode/electrolyte interface of every electrode there is an electrochemical double layer (EDL), the potential of which decreases with the distance from the electrode surface.