Optical Probing of Single Molecules: Examples from Physics, Chemistry, and Biophysics

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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