A single-molecule stretching method for lateral and normal AFM lever calibration
A novel method for quantitative lateral force measurement (LFM) calibration has been developed. Using a single-molecule spectroscopy approach it is possible to calibrate the AFM levers for both lateral and normal spring constants with a single image scan. Moreover, our method does not involve tip modifications. Dextran molecules were chosen for testing our calibration procedure due to their characteristic plateau feature in the force-elongation curve which enables an easy identification of single-molecule stretching events. Using a non-standard (tilted) geometry of AFM scanning, it is possible to observe different components of the stretching force on both normal and lateral force signals. These signals can be further compared to the values obtained by standard (normal) spectroscopic measurements. The values of the normal spring constant obtained with our method are in good agreement with the results obtained from the method exploiting the energy equipartition theorem. The statistical analysis shows that the approach proposed in our paper gives reproducible results of the lateral sensitivity with a relative standard deviation less than 15%.