Magnetic and acoustic tapping mode microscopy of liquid phase phospholipid bilayers and DNA molecules

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Abstract
We have constructed a fluid cell for an atomic force microscope that operates in tapping mode using either an oscillating piezo or magnetic drive. This fluid cell allows direct comparison of the image quality using the two drive mechanisms over identical areas of a sample without fluid or cantilever exchange. We found that the magnetically driven cantilever’s tuning curve was very similar to the thermal noise power spectrum, allowing an accurate determination of the cantilever resonance frequency. This is in contrast to the piezo driven tuning curve, which contained a number of peaks that appeared to be a convolution of the true cantilever resonance with the complicated acoustic spectrum of the fluid cell. We imaged two biologically relevant samples: DNA molecules and liquid phase phospholipid bilayers. For both samples, we found that the image quality, as measured by feature height, lateral resolution, and image stability, was independent of the drive method. This suggests that, despite the apparent differences in the frequency response, the physical motion of the cantilever tip, when it is driven near its resonance frequency, is the same for both driving mechanisms.