Effects of alternating electric field on the imaging of DNA double-helix structure by atomic force microscope

The effects of alternating electric field on the imaging of DNA double-helix structure were explored by atomic force microscope (AFM). First, the DNA sample was located under an alternating electric field in a fixed direction and dried. Then, AFM was used to obtain the DNA images under different alternating electric fields with the voltage range from 0.5 to 6.0 V and the frequency of 50 kHz. Thus, the DNA double-helix structures with different extensions were observed when the DNA molecules were gradually stretched under different field intensities. The distributions of DNA molecules in solution were random if there were no external forces, and the curved DNA molecules were observed in the AFM image. With the increase in alternating electric voltage (0.5–4.0 V), the DNA structure was shifted from random to oriented conformation and the DNA grooves were further unfolded. While the higher voltage (5.0–6.0 V) resulted in the rupture of DNA chains due to the excessive stretching force. It showed that the optimal voltage was 1.0 V, and the double-helix structure was observed. This method provides an efficient way for monitoring and measuring bio-macromolecules. It may also enable the exploration of the DNA–protein binding and DNA molecular self-assembly processes.