Molecular machines working on surfaces and at interfaces

In the past ten years a great variety of artificial molecular machines have been constructed, and very interesting concepts for controlling molecular-level movements by external inputs have been developed. Most of the studies, however, have been performed in solution, where the investigated systems contain a huge number of molecules which behave independently from one another because they cannot be addressed individually. Before such systems can find applications in many fields of technology, they must be interfaced with the macroscopic world by ordering them in some way so that they can behave coherently and can be addressed in space. The problem of obtaining ordered arrays of molecular machines can be addressed by a variety of techniques, which include deposition on surfaces, incorporation into polymers, organization at interfaces, and immobilization in membranes or porous materials. In the last few years, the development of scanning-probe techniques has also enabled direct observation and manipulation of single molecular-machine molecules on surfaces. Techniques of this kind have opened novel routes to the study of molecular machines, and have also contributed to better understanding the differences between movement at the macroscopic and molecular levels. This paper reviews some recent achievements in the field of molecular machines working on surfaces and at interfaces, as single molecules or ordered arrays. Hybrid natural-artificial machines are also discussed, and the working mechanism of some natural machines is illustrated for the purpose of comparison.