Point-projection imaging of macromolecular contours
A point-projection microscope is described which uses an electric field of several volts per nanometre to generate ions from a layer of benzene condensed in high vacuum on to the apex of a cryogenically cooled field-emitter tip. Clusters of ferritin (a spherical protein molecule) embedded in the layer, have been imaged by exposing their contours during controlled field-desorption. Image contrast is high. A magnification of 2 times 105 has been achieved at a spatial resolution estimated to be better than 3 nm. Three-dimensional reconstructions of cluster morphology from a series of molecular contours are in qualitative agreement with transmission-electron micrographs of the ferritin covered emitter apex. Reproducibility of the cluster contours, and TEM images of the apex before and after imaging suggest that the imaging procedure is nondestructive.