Direct observation of bioelectrochemical processes by scanning tunneling microscopy

We have imaged DNA deposits that have been reacted onto a gold electrode. Our scanning tunneling microscope (STM) is operated in a three electrode cell using insulated tips. Samples are deposited at a controlled potential relative to an Ag/AgCl reference electrode. Many different adsorption geometries are observed, different substrate potentials resulting in characteristic adsorption patterns. Here, we report initial results for negative electrodes. At — 2.3 V (Ag/ AgCl), the DNA appears to attach to the substrate end on. As the potential is lowered towards zero volts, the DNA attaches side on, forming aggregates of decreasing complexity with decreasing substrate potential. Isolated molecules are observed near — 1 V (Ag/AgCl). Below this potential, coverage is much less dense. Those molecules that adsorb do so in aggregates, which may bind in alignment with the underlying atomic structure of the substrate.