EDITORIAL
Near Field Scanning Optical Microscopy Current problems in analytical chemis try often require ultramicroanalysis (mi croscopy) in addition to traditional spec troscopic and trace methods. Witness the growing interest in cell biology. It has fostered an expansion of the tools avail able to the analytical microscopist. The need for high spatially resolved informa tion is obvious: Chemical transport in cells occurs on a submicrometer scale. Current techniques (i.e., EM and SIMS) are destructive and cannot be directly applied to living systems. Our old friend light microscopy in its polarized, fluores cence, and confocal incarnations is quite useful, although it is limited to a spatial resolution of ~0.1 μπι. There is a real need for a nondestructive superresolution technique that can be applied to a living system and that is capable of pro viding morphological and spectroscopic information. This need is currently be ing addressed by near field scanning op tical microscopy (NSOM) (1,2). NSOM, a superresolution imaging technique, relies on the particle nature of light. Light is passed through an opaque aperture of a diameter, D, less than the wavelength of the incident radi ation. Within a short distance of the ap erture—the near field—the radiation re mains collimated. Thus spatial informa tion, having spatial resolution D, is obtained if the sample is brought within the near field and scanned with respect to the source. By manipulating the source wavelength and the detector as sembly, a wide variety of spatially re solved spectroscopies can be realized. Lewis and co-workers (i) are currently
studying the feasibility of NSOM in the fluorescence mode as applied to biologi cal systems. Using visible laser radiation as a source, a conservative estimate of spatial resolution is 500 À, which should yield information on cell and organelle membranes and DNA binding in chromosomes, to name only a few. Because this information is chemical in nature, a new chemical microscopy is in hand. It is most interesting to note that although NSOM is technologically new, the concept is more than 30 years old. O'Keefe originally proposed the idea in a letter to the Journal of the Optical Society of America (3). He proposed it as "a concept illustrating a method by which it might conceivably be possible to go beyond the resolving power of l i g h t . . . . The realization of this proposal is rather r e m o t e . . . . " Technology has taken some 34 years to catch up with O'Keefe's vision. This should remind us of light microscopy's staying power as an analytical technique; through the marriage of physics and analytical chemistry, it will continue to grow and thrive. References (1) Betzig, E.; Lewis, Α.; Harootunian, Α.; Isaac son, M.; Kratschmer, E. Biophys. J. 1986, 49, 269. (2) Betzig, E.; Isaacson, M.; Barshatzky, H.; Lewis, Α.; Lin, K. Proc. SPIE—Int. Soc. Opt. Eng. 1988, 897 (Scanning Microsc. Technol. Appl.), 91. (3) O'Keefe, J. A. J. Opt. Soc. Am. 1956,46, 359.
ANALYTICAL CHEMISTRY, VOL. 61, NO. 19, OCTOBER 1, 1989 · 1075 A
