June 24th

Today, we learn how to make DNA slides, how to focus the AFM, and how to scan the DNA samples using an oscillating mode and a contact mode for the cantilever tip. DNA samples in TAE buffer were mounted onto mica sheets put on a cover-glass. The top layer of the mica was removed using scotch-tape to ensure a smooth surface to put the DNA sample in. The sample was incubated by covering the slide with a glass cover for 5 mins. The mica slide was rinsed in ethanol and dried by blowing N2 onto the slide. The slide was mounted onto the stage of the AFM and the red laser beam was adjusted to beam onto the cantilever end that has the Si tip. The DNA sample was first scanned using the tapping mode. No good picture was seen. Next, we decide to scan the DNA sample in the contact mode. The cantilever tip has to be changed to a softer tip made of silicon nitride in order for the tip not to break when it contacts the surface. The force applied was in the picoNewton level. We got a good picture of short DNA sticks which was the desired results. They look like individual E. coli bacterium seen under the electron microscope.