Single-molecule experiment

A single-molecule experiment is an experiment that investigates the properties of individual molecules. Single-molecule studies may be contrasted with measurements on an ensemble or bulk collection of molecules, where the individual behavior of molecules cannot be distinguished, and only average characteristics can be measured. Since many measurement techniques in biology, chemistry and physics are not sensitive enough to observe single molecules, single-molecule fluorescence techniques (that have emerged since the 1990s for probing various processes on the level of individual molecules) caused a lot of excitement, since these supplied many new details on the measured processes that were not accessible in the past. Indeed, since the 1990s, many techniques for probing individual molecules have been developed. A single-molecule experiment is an experiment that investigates the properties of individual molecules. Single-molecule studies may be contrasted with measurements on an ensemble or bulk collection of molecules, where the individual behavior of molecules cannot be distinguished, and only average characteristics can be measured. Since many measurement techniques in biology, chemistry and physics are not sensitive enough to observe single molecules, single-molecule fluorescence techniques (that have emerged since the 1990s for probing various processes on the level of individual molecules) caused a lot of excitement, since these supplied many new details on the measured processes that were not accessible in the past. Indeed, since the 1990s, many techniques for probing individual molecules have been developed. The first single-molecule experiments were patch clamp experiments performed in the 1970s, but these were limited to studying ion channels. Today, systems investigated using single-molecule techniques include the movement of myosin on actin filaments in muscle tissue and the spectroscopic details of individual local environments in solids. Biological polymers' conformations have been measured using atomic force microscopy (AFM). Using force spectroscopy, single molecules (or pairs of interacting molecules), usually polymers, can be mechanically stretched and their elastic response recorded in real time. In the gas phase at ultralow pressures, single-molecule experiments have been around for decades, but in the condensed phase only since 1989 with the work by W. E. Moerner and Lothar Kador. One year later Michel Orrit and Jacky Bernard were able to show also the detection of the absorption of single molecules by their fluorescence. Many techniques have the ability to observe one molecule at a time, most notably mass spectrometry, where single ions are detected. In addition one of the earliest means of detecting single molecules, came about in the field of ion channels with the development of the patch clamp technique by Erwin Neher and Bert Sakmann (who later went on to win the Nobel prize for their seminal contributions). However, the idea of measuring conductance to look at single molecules placed a serious limitation on the kind of systems which could be observed. Fluorescence is a convenient means of observing one molecule at a time, mostly due to the sensitivity of commercial optical detectors, capable of counting single photons. However, spectroscopically, the observation of one molecule requires that the molecule is in an isolated environment and that it emits photons upon excitation, which owing to the technology to detect single photons by use of photomultiplier tubes (PMT) or avalanche photodiodes (APD), enables one to record photon emission events with great sensitivity and time resolution.