Imaging isolated strands of DNA molecules by atomic force microscopy

We have employed an atomic force microscope (AFM) to image in air isolated strands of pBS+ plasmid DNA adsorbed onto freshly cleaved mica. At a DNA concentration below 0.3 micrograms/ml isolated strands of the plasmid DNA are usually seen, while for concentrations higher than 3 micrograms/ml a uniform coverage of interconnected DNA strands was observed. We found that the contrast and the width of DNA were dependent upon humidity. When the relative humidity exceeds 60%, negative contrast images with strand widths 20 times the width of DNA are found, while positive contrast images with 7 to 10 times the width of DNA are found when the humidity is below 30%. By placing the AFM in an environment where the humidity could be controlled, we were able to switch between positive and negative contrasts.