Molecular voids formed from effective attraction in submonolayer DNA deposited on Au(111)

The development of DNA-based biosensors requires a deep understanding of how DNA molecules adsorb and organize on solid state surfaces as well as the electronic properties of individual and aggregates of DNA molecules. Using scanning tunneling microscopy (STM) and atomic force microscopy (AFM), we have successfully characterized an attractive force driven molecular void formation for DNA chemically adsorbed on Au(111) as a function of strand length and deposition conditions. Here we report the observation of these void structures formed on the Au(111) surface by adsorption of both 45 and 90 base pair long, thiolated double-stranded DNA. We found that the average void diameter decreases when increasing the number of base pairs exposed to the surface. The critical determinant in the molecular void formation is the total charge delivered to the surface via the adsorption of the DNA strands and the related counterions, which can ultimately be quantified by the number of base pairs in each adsorbed DNA molecule. Complementary measurements involving STM and AFM suggest that an intact Au(111) surface area is preserved inside the void and is surrounded by a submonolayer of DNA molecules adsorbed on the surface. The discussion of the possible mechanisms for the void formation implies an effective attraction between the DNA molecules.