Biological single molecule applications and advanced biosensing

1.1. Macroscopic versus microscopic measurements in biology Macroscopic experiments yield time and population averages of the individual characteristics of each molecule. At the level of the individual molecules, the picture is quite different: individual molecules are found in states far from the mean population, and their instantaneous dynamics are seemingly random. Whenever unusual states or the rapid, random motions of a molecule are important, the macroscopic picture fails, and a microscopic description becomes necessary. Single-molecule experiments differ from macroscopic measurements in two fundamental ways: first, in the importance of the fluctuations in both the system and in the measuring instrument, and second, in the relative importance of force and displacement as variables under experimental control and subject to direct experimental measurements. In single-molecule experiments, the crucial parts of the measuring instruments themselves are small and subject to the same fluctuations as the system under study. Single-molecule experiments thus, give access to some of the microscopic dynamics that are hidden in the macroscopic experiments.