Imaging of metal-coated biological samples by scanning tunneling microscopy

A method for imaging biological samples by scanning tunneling microscopy (STM) is presented. There are two main difficulties in imaging biological samples by STM: (1) the low conductivity of biological material and (2) finding a method of reliably depositing the sample on a flat conducting surface. The first of these difficulties was solved by coating the samples with a thin film of platinum-carbon. The deposition problem was solved by a method similar to a procedure used to deposit biological molecules onto field ion microscope (FIM) tips. STM images of bacteriophage T7 and filamentous phage fd are shown. The substrate on which the samples were absorbed was atomically flat gold. The images do not show molecular detail due to the metal coating, but the gross dimensions and morphology are correct for each type of virus. Also, the surface density of virus particles increases and decreases in the way expected when the conditions of deposition are changed. These methods allow reliable and reproducible STM imaging of biological samples.