Abstract
A procedure for direct functionalization of silicon nitride atomic force microscope (AFM) probes with 3-aminopropyltriethoxy silane (APTES) was developed. The probes were modified in vapors of APTES at ambient conditions (AP-tips). The functionalization of tips was monitored by fluorescent microscopy and the force measurements with AFM. AP-tips were derivatized with a non-fluorescent quinoline analog (FQ) which reacts with primary amines to form a strongly fluorescent isoindole. AP-tip fluorescence was significantly higher than control probes. Amino modified tips manifest a strong adhesion to freshly cleaved mica and this behavior is qualitatively similar to the interaction of unmodified silicone nitride probes with AP-mica. The adhesion of AP-tips to mica or AP-mica to unmodified tips decreases as ionic strength increases indicating a dominant electrostatic component of the adhesion effect. This interpretation is supported by the experiments at different pH and the reaction of AP-tips with glutaraldehyde. The adhesion effect is very small at basic pH due to the deprotonation of amino groups. The reaction of immobilized amines by the treatment of the tips with glutaraldehyde decreases the tip-surface interaction dramatically. Like AP-mica, AP-trips are stable during storage and thus can be used for additional derivation steps.