Editorial

Single Molecules and Atoms

The last decade has brought a revolution in theability of chemists to study and manipulate singlemolecules and atoms. Single molecules and atomshave been studied in vacuum, liquids, solids, andon surfaces, both with electron tunneling probes andwith optical methods. Spectroscopy, shape, and evenchemical reactivity have been investigated. Examplesrange from single enzyme molecules to organic dyes,to single isolated atomic ions in a magnetic bottle. Farfrom just a scientific curiosity or just an eleganttextbook example, studies on single molecules andatoms are offering a new perspective on many impor-tant fundamental issues in physics, chemistry, andbiology. In particular, the ability to study singlemembers of a chemical ensemble of molecules isleading to some of the first direct information onspectroscopic and chemical heterogeneity. These stud-ies probe individual members of an ensemble, yieldingdirect information about the distributions of physicalquantities rather than just the first moment of thedistributions.

At the same time, single molecules offer exquisiteprobes in biophysics and materials science for theinvestigation of spatially resolved structure and dy-namics, e.g., the local mobility in a phospholipidbilayer or in a muscle fiber. From an analyticalchemistry perspective, single-molecule spectroscopyrepresents the ultimate in sensitivity. That is, re-search in this area is finding immediate applicationsranging from DNA sequencing to biomolecularsorting.

A driving force in the single-molecule field has beenthe belief that, by simply exploring new regimes, newphysical and chemical effects will be discovered andnew physical insights will be obtained. The field hasbeen punctuated by a series of discoveries that in-volved scientists proposing that a novel single-molecule or -atom study might be possible, followedby the hard work to realize these difficult experimentsin practice. The continually improving technology ofmodern experimental methods, ranging from state-of-the-art lasers and detectors, ultrasensitive fluores-cence detection methods, and scanning microscopyapproaches has been critical in achieving these obser-vations. Many of the central concepts for the entirefield were established in the seminal work in the mid-1980s on the spectroscopy of single isolated ions invacuum. But, the first report of the optical spectros-copy of a single molecule did not appear until 1989,and involved a low-temperature aromatic hydrocarbonpresent as a diluent in a molecular crystal. In theearly 1990s the first reports of single-molecule spec-troscopy at room temperature appeared involvingsamples at surfaces and in liquid solution. Whenelectron rather than optical probes are considered,direct imaging of single molecules and even atoms hasbeen possible for over two decades, through the useof electron microscopy. However, the invention ofscanning tunneling microscopy (STM) in the late1980s allowed for a tremendous improvement inspatial resolution and has produced the ultimate atomimages.

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This special issue includes a selection of some of themost exciting recent directions in single-molecule and-atom investigations. However, due to the limitednumber of articles in an Accounts of Chemical Re-search special issue, it was not possible to thoroughlycover the many important directions in this field. Inparticular, many recent applications of scanning forcemicroscopy and STM to single biomolecules and tosingle molecules at metal interfaces are not included(but have been reviewed recently in several places).

We hope the reader enjoys this “snapshot” of theexciting and rapidly developing field of the spectros-copy and manipulation of single molecules and atoms.

W. E. MoernerGuest Editor

P. F. BarbaraAssociate Editor

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562 Acc. Chem. Res., Vol. 29, No. 12, 1996