Adhesion forces in AFM of redox responsive polymer grafts: Effects of tip hydrophilicity

The adherence between silicon nitride AFM tips and redox-active poly(ferrocenylsilanes) (PFS) grafts ongold was investigated by electrochemical AFM force spectroscopy. Before the adhesion measurementssilicon nitride AFM probes were cleaned with organic solvents (acetone and ethanol) or piranha solution.Interestingly, these different AFM tip cleaning procedures drastically affected the observed adhesionforces. Water contact angle measurements on the corresponding AFM probe chips showed that piranhatreatment resulted in a significant increase of AFM probe chip surface hydrophilicity compared to theorganic solvent treatment. Obviously this hydrophilicity change caused drastic, even opposite changesin the tip-PFS adhesive force measurement upon electrode potential change to reversibly oxidize andreduce the PFS grafts. Our findings are of pivotal importance for AFM tip adhesion measurements utilizingstandard silicon nitride AFM tips. Probe hydrophilicity must be carefully taken into consideration and controlled.