IMAGING SOFT SAMPLES WITH THE ATOMIC-FORCE MICROSCOPE - GELATIN IN WATER AND PROPANOL

We have imaged mica coated with thin gelatin films in water, propanol, and mixtures of these two liquids by atomic force microscopy (AFM). T he elastic modulus (Young's modulus) can be tuned from 20 kPa to more than 0.1 GPa depending on the ratio of propanol to water. The resoluti on is best in pure propanol, on the order of 20 nm, and becomes worse for the softer samples. The degradation in resolution can be understoo d by considering the elastic indentation of the gelatin caused by the AFM tip. This indentation becomes larger and thus the contact area bec omes larger the softer the sample is. Therefore this study may be used to estimate the resolution to be expected with an AFM on other soft s amples, such as cells. Nondestructive imaging was possible only by ima ging at forces <1 nN. This was difficult to achieve in contact mode be cause of drift in the zero load deflection of the cantilever, supposed ly caused by temperature drift, but straightforward in tapping mode.