Single-molecule experiment

A single-molecule experiment investigates the properties of a single individual molecule that can be isolated or distinguished for the purpose of an experiment or analysis. Single-molecule studies may be contrasted with measurements on an ensemble or bulk collection of molecules, where the individual behaviour can not be distinguished, and only average characteristics can be measured. Although most measurement techniques are not sensitive enough to observe single molecules, single-molecule fluorescence has emerged as a useful tool for probing various processes which cannot be fully understood on the bulk level, such as the movement of myosin on actin filaments in muscle tissue or the details of individual local environments in solids. Another crucial single-molecule technique is single molecule force spectroscopy, where single molecules (or couples of interacting molecules), usually polymers, are mechanically stretched and their elastic response recorded in real time.

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History

In the gas phase at ultralow pressures, single-molecule experiments have been around for decades, but in the condensed phase only in the last 20 years with the groundbreaking work by W. E. Moerner and Michel Orrit and Jacky Bernard has it seen fruition.

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 Neher and Sakmann (who later went on to win the Noble 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 an ideal mean of observing one molecule at a time. 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.

Nanomanipulators such as the atomic force microscope are also suited to single molecule experiments of biological significance, since they work on the same length scale of most biological polymers. Optical tweezers have also been used with success.

Theory

Single molecule fluorescence spectroscopy uses the fluorescence of a molecule to record information pertaining to its environment, structure, and position. The technique affords the ability to obtain information otherwise not available due to ensemble averaging of a bulk material.

Impact

Single-molecule effects

• single-molecule magnet

Single-molecule techniques

• Microscopy
• Fluorescence
• Fluorescence resonance energy transfer
• Force spectroscopy
• Magnetic tweezers
• Optical tweezers
• Raman spectroscopy (Surface Enhanced Raman Spectroscopy)
• SPM imaging
• electron microscopy
• single-molecule spectroscopy