Quantification of viscoelasticity using dynamic Atomic Force Microscopy in liquid environment

We have developed a method to quantify the dissipation and stiffness (viscoelasticity) of nanoscale interactions in aqueous environment. Experiments were performed using two cantilever-deflection detection schemes and displacement-detection using a fibre interferometer-based home-built setup. Cantilever is excited using two mechanisms: base and tip excitation. A single protein molecule (Titin-I27), tethered between cantilever-tip and substrate, is pulled with constant speed (~ 20 nm/s). Cantilever-tip is driven with low frequency (~ 133 Hz), amplitude (~ 1Å) in the experiment, and phase and amplitude are recorded. We analyzed the data using existing theoretical models and also a model proposed by us. Pairing different excitation and detection schemes; and analyzing the data with different models, a robust study of mechanical properties of I27 molecule was made. Our observed results show inconsistencies with previous studies. We believe it is due to the inappropriate selection of experimental parameters and theoretical models. This leads artefacts in the results. We propose the appropriate ways of performing measurements and theoretical model for data analysis which accurately quantify the dissipation and stiffness of nanoscale interactions in aqueous environment.