Scanning transmission x-ray microscopy of cultured cells

Abstract

Soft x-rays provide an interesting possibility for imaging thick specimens at resolution better than that of the light microscope. Because of the way these x-rays interact with matter, a transmission image is formed essentially entirely by absorption, and suffers negligible blurring due to scattering. The only serious effect of sample thickness is to attenuate the x-ray beam. At x-ray wavelengths between 2.3 and 4.4 nm, the absorption is particularly appropriate for examining thick biological specimens. In this region water is weakly absorbing while carbon is strongly absorbing. As a result, absorption by a whole live cell is dominated by biological molecules. The magnitude of the absorption is such that cells up to ten microns thick can be imaged. Finally, these x-rays can traverse up to a millimater of air without serious attenuation. Transmission microscopy of live cells in air is thus possible.The technology for producing high resolution x-ray images has only recently become available. Zone-plate focussing has been perfected to the point where an x-ray beam spot 40 nm in diameter can be formed. This spot can be raster-scanned over a specimen and the transmitted x-rays detected, to form a scanning transmission x-ray microscope (STXM) image. The spot size, and resolution, are expected to improve to about 20 nm in the next few years. A very intense source of nearly monochromatic soft x-rays is also needed, at present only available at synchrotron light sources. We are working with a group from SUNY Stony Brook, IBM Watson Laboratories, and the Center for X-Ray Optics who have just finished building a microscope at the National Synchrotron Light Source. Two other microscopes are now being built, at Daresbury in England, and in Berlin.