Imaging deoxyribose nucleic acid molecules on a metal surface under water by scanning tunneling microscopy

We have investigated the mechanism of a scanning tunneling microscope operated in air and water by measuring current-distance (I_t~s) characteristics and making spatial maps of dl_t /ds, for Pt and W tips and gold substrates. Although the I_t-s curves are characteristic of tunneling, their interpretation in terms of work functions would require values between a few tenths of and a few thousandths of the vacuum values. These values do not depend on the metals or the tunneling medium. We conclude that the tip and substrate are always in contact via an insulating adsorbate layer. We prepare flat, clean gold surfaces onto which we can form stable aggregates of DNA molecules under a buffer solution in a small electrochemical cell by a cycle of plating followed by stripping. The aggregates are imaged both as depressions in a constant-current topology map, and dramatic changes on a dl_t /ds map. 20-A corrugations are clearly visible in dense aggregates of high molecular weight DNA which appear to pack with liquid-crystalline order. On occasion the path of an individual molecule can be followed.