W. E. Moerner

Current Research Interests

Most biophysical or chemical experiments in condensed matter measure the average behavior of a huge number, N, of molecules, where N may range from millions to billions to Avogadro's Number. At the same time, most theoretical models are intended to describe the behavior of a single molecule interacting with its surroundings, and averaging over the number of molecules N is normally required to compute an observable. Using precision laser spectroscopic techniques, we have been detecting and probing the detailed properties of individual impurity molecules hidden deep inside a cell, in a protein, or even in a liquid, i.e., the ultimate limit of N=1. This was first done in the Moerner Lab in 1989, and has since expanded dramatically to include many groups around the world. A key reason for doing this is to explore heterogeneity that is normally obscured by ensemble averaging.

Studying one individual molecule in a solid means we are working with an extremely small number of moles of material. You might be aware that the international standards organization, IUPAC, has defined several new prefixes: zepto- for 1E-21, and yocto- for 1E-24. Thus 1 molecule is equivalent to 1.66 yoctomoles. But we think this is unwieldy. Thus we define a new prefix guaca- so that (with apologies to Prof. Avogadro)

1 guacamole = 1 / ( Avocado's Number) of moles.

More seriously, it is worth recalling that each molecule we are probing is only 1 or 2 nanometers in size. This means that when we use a laser to select one probe molecule, we can sense details of the immediate local environment of a truly nanoscopic probe.

To achieve this extreme reduction of the concentration and reach the single-molecule level, we use either (a) extremely low concentrations and diffraction-limited confocal, TIRF, or far-field microscopy, or (b) near field optical excitation to pump sample volumes much smaller than the diffraction limit, or (c) superresolution imaging by single-molecule active control. By studying a large number of individual molecules one at a time, we are able not only to observe how the usual ensemble average behavior is formed, but also to see unexpected, surprising behavior normally hidden by the usual ensemble averaging.

The phenomena under study include protein localization patternd in bacteria, chaperonin proteins, and new fluorophores for active-control superresolution imaging. By dispersing the emitted light, even the vibrational mode spectrum of a single molecule may be measured! By measuring correlations in the emitted photon stream, fast dynamics including environmental fluctuations, or the purely quantum-mechanical behavior termed photon antibunching may be probed. In biomolecules, we observe fascinating differences in behavior due to conformational states, local environments, or enzymatic cycle, all of which are obscured in large N experiments.

Importantly, a single molecule can be viewed as a probe of its immediate local nanoenvironment on the scale on the order of the molecular size (~1 nm). Because single molecules are nanoscale emitters, when active control is used to turn molecules on and off, it is possible to build up a super-resolution image of the sample, far beyond the optical diffraction limit, typically on the 40 nm scale. Several advanced optical techniques for obtaining thee-dimensional information from single-molecule photoswitching are under study, and we apply these methods to imaging a variety of cellular structures in bacteria and in mammalian cells and to tracking of RNA in living yeast.