A tapping-mode AFM study of the compression of grafted poly(ethylene glycol) chains

We investigate the measurement of forces arising from the compression of surface-grafted poly( ethylene glycol), PEG, chains in aqueous solutions using sharp, unmodified tips in Tapping-Mode atomic force microscopy (TM-AFM). PEG chains were grafted to silica by reaction with preadsorbed poly( ethylenimine) (PEI), affording control of grafting density by reaction conditions. Numerical simulation of the cantilever oscillation, accounting for distance-dependent damping, was used to quantify the effective forces measured in TM-AFM. The decay length of the steric force was compared to those obtained using conventional AFM, and a good correlation was found between the two methods using PEG chains in both a mushroom and brush conformation. A significant disparity in the magnitude of the steric force obtained by conventional AFM force measurements and TM-AFM was observed, and this was attributed to a deeper compression/penetration of the polymer-coated surface by the tip in the case of TM-AFM.