Single molecules

At first an experimental challenge, the ability to conduct experiments with single molecules has been strongly connected with progress in experimental techniques and instrumentation. Scanning probe techniques, such as the scanning tunneling microscope or the atomic force microscope (AFM), use sharp tips in close proximity (10−9 m) to a sample to measure tunneling currents or weak mechanical forces that, in turn, allow generation of a real-space “image” of a single atom or molecule (1). In another approach, macromolecules can be clamped between an AFM tip and a substrate to determine the forces needed to stretch a single polymer chain. Similar experiments are feasible by use of optical tweezers, where a macromolecule is attached to a tiny bead and a substrate. The light force acting on the bead can be used to translate it against a force generated by the macromolecule (2). In the optical domain, the fluorescence emission of single molecules (or more general single fluorophores) in some condensed-phase environments can be imaged by advanced optical microscopies, such as scanning confocal microscopy or near-field scanning optical microscopy (3). At low temperatures, single fluorophores can also be isolated by a frequency selective technique that uses the fact that the sharp optical transition frequencies of dopant molecules are different because of imperfections of the environment (4). These optical techniques allow for detailed spectroscopic investigations at the single-molecule level, taking advantage of spectral, time-resolved, and polarization information.