Optical Probing of Single Molecules: Examples from Physics, Chemistry, and Biophysics
W. E. Moerner
Department of Chemistry, Stanford University
11.30am Thursday 27 November 2003, EN413, Engineering Building, Swinburne
It has now been more than ten years since the first optical detection and spectroscopy of a single
molecule in a solid (Phys. Rev. Lett. 62, 2535 (1989)). The interest in optical probing of individual molecules
continues to expand, driven by the lifting of ensemble averaging to expose hidden heterogeneity and by the ability to acquire
local information on structure and dynamics in complex systems in a weakly perturbative manner. The early years concentrated
on high-resolution studies of aromatic hydrocarbon molecules like pentacene in organic crystals like p-terphenyl at liquid helium
temperatures. Physical effects observed included spectral diffusion, magnetic resonance of a single molecular spin, and
single-molecule vibrational structure. In the mid-90's, much of the effort in the field moved to room temperature, which
allowed explorations of a wide array of biomolecules such as green fluorescent protein, local concentration sensors,
enzymes performing catalysis, and many others. Recently, some groups have begun to explore the behavior of individual
fluorescent molecules in the challenging environment of living cells. By extrinsic labeling of an antigenic peptide, we have
completed a detailed study of the diffusion of single copies of major histocompatibility complexes of type II (MHCII) in the
membranes of CHO cells. The results from this study bear on fundamental properties of the cell membrane, in particular on the
presence of significant confinement restricting the motion of the MHCII transmembrane proteins and the role of cholesterol.
Finally, in the area of quantum optics, the high stability of some aromatic hydrocarbons in crystals has allowed the creation
of a novel room-temperature light source of single photons on demand based on a single-molecule emitter.
For each pump pulse, one and only one photon is emitted with high probability, making this light source potentially useful
for quantum optical communication systems.
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