Monolayers of molecules, which retain their function in the adsorbed state on solid surfaces, are important
in materials science, analytical detection, and other technology approaching the nanoscale. Molecular
monolayers, including layers of functional biological macromolecules, offer new insight in electronic properties
and stochastic single-molecule features and can be probed by new methods which approach the single-molecule
level. One of these is in situ scanning tunneling microscopy (STM) in which single-molecule electronic
properties directly in aqueous solution are probed. In situ STM combined with physical electrochemistry,
single-crystal electrodes, and spectroscopic methods is now a new dimension in interfacial bioelectrochemistry.
We overview first some approaches to spectroscopic single-molecule imaging, including fluorescence
spectroscopy, chemical reaction dynamics, atomic force microscopy, and electrochemical single-electron
transfer. We then focus on in situ STM. In addition to high structural resolution, in situ STM offers a single-molecule spectroscopic perspective. This emerges most clearly when adsorbate molecules contain accessible
redox levels, and the tunneling current decomposes into successive single-molecule interfacial electron transfer
(ET) steps. Theories of electrochemical ET and in situ STM of redox molecules as well as specific cases are
addressed. Two-step in situ STM represents different molecular mechanisms and even new ET phenomena,
related to coherent many-electron transfer. A number of systems are noted to accord with these views. The
discussion is concluded by attention to one of the still very few redox proteins addressed by in situ STM, the
blue copper protein Pseudomonas
aeruginosa azurin. Use of comprehensive electrochemical techniques has
ascertained that well-defined protein monolayers in two opposite orientations can be formed and interfacial
tunneling patterns disclosed. P. aeruginosa azurin emerges as by far the most convincing case where in situ
STM of functional metalloproteins to single-molecule resolution has been achieved. This comprehensive
approach holds promise for broader use of in situ STM as a single-molecule spectroscopy of metalloproteins
and illuminates prerequisites and limitations of in situ STM of biological macromolecules.