Scanning tunneling microscopy and atomic force microscopy studies of biomaterials at a liquid–solid interface

Abstract

We have compared the scanning tunneling microscopy (STM) and atomic force microscopy (AFM) using a clean gold surface under electrochemical potential control, finding that the STM usually yields higher resolution, although this may be a consequence of better production methods for STM tips. New methods for tethering DNA molecules to substrates have yielded many excellent AFM images of DNA, even under water. The highest resolution has been obtained with specially sharpened tips, but recent work suggests that this may not be necessary. Control of contamination, and tip–sample interactions (i.e., adhesion) are also important factors. We describe a scheme of magnetic control of AFM cantilevers which should overcome the mechanical instability that places a lower limit on contact force. This should permit active control of interaction forces with the instrument operated in water. We illustrate local force measurements made under water with examples of the measurements of the microelastic properties of bone and of the electrostatic interactions between pairs of colloidal particles. The STM has been used to image small DNA oligomers on a gold electrode in water at high resolution. STM images of DNA bases show that the purines form stacked aggregates with the same base-stacked structure as found in DNA. There is evidence of a connection between the macroscopically measured electrochemical properties of thymine and its STM contrast. Such electronic sensitivity might be useful for chemical identification of molecules.