One of the unique features of the atomic force microscope (AFM)
is its capacity to measure interactions between tip and sample with high
sensitivity and unparalleled spatial resolution. Since the development of
methods for the functionalization of the tips, the versatility of the AFM has
been expanded to experiments where specific molecular interactions are
measured. For illustration, we present measurements of the interaction between
complementary strands of DNA. A necessary prerequisite for the quantitative
analysis of the interaction force is knowledge of the spring constant of the
cantilevers. Here, we compare different techniques that allow for the in situ
measurement of the absolute value of the spring constant of cantilevers.