Minimizing Tip-Sample Contact Force in Automated Atomic Force Microscope Based Force Spectroscopy

Abstract

In atomic force microscope based force spectroscopy, it is often necessary to minimize the tip-sample contact force. While it is possible to control the contact force using force feedback, this method is susceptible to sensor drift and is often under-utilized due to the noise associated with the feedback process. Here we present a method to control the tip-sample contact force for repeated pulling cycles without relying on force feedback or tedious user-controlled z-stage step increments. The custom pulling program uses the data recorded during the previous retraction cycle to automatically reposition the sample surface to account for changes in topography and system drift. Using this method we were able to complete 250 automated pulling cycles, 76% of which had evidence of tip-sample contact. Of those pulling cycles with tip-sample contact, the average contact force was 83 pN, with the maximum contact force not exceeding 292 pN.

Specific Binding at the Cellulose Binding Module–Cellulose Interface Observed by Force Spectroscopy

“…To assess the mechanical strength of these interactions in comparison to the noncovalent coordination of the His 6 tag and the NTA–Ni­(II) complex, a model peptide with the primary sequence CGWG­GHHH­HHH was synthesized using microwave-assisted solid-phase peptide synthesis. The peptide cysteine residue was used to covalently link the peptide to a maleimide-terminated silicon surface using chemistry similar to that described above and elsewhere. ,− An AFM cantilever bearing a Ni­(II)-loaded NTA monolayer was gently brought into contact with the peptide surface; upon retraction, the deflection of the cantilever yielded a force vs distance plot characteristic of force-induced rupture of the His 6 –Ni­(II)–NTA interaction. We repeated the cycle 250 times and plotted the frequency of observed rupture forces and lengths (Figure ).…”

“…The peptide cysteine residue was used to covalently link the peptide to a maleimide-terminated silicon surface using chemistry similar to that described above and elsewhere. 22,35−38 An AFM cantilever bearing a Ni(II)loaded NTA monolayer was gently 39 brought into contact with the peptide surface; upon retraction, the deflection of the cantilever yielded a force vs distance plot characteristic of forceinduced rupture of the His 6 −Ni(II)−NTA interaction. We repeated the cycle 250 times and plotted the frequency of observed rupture forces and lengths (Figure 2).…”

“…The unbinding experiments were performed as outlined by Bowers et al Automated pulling experiments were carried out on a custom-built three-axis AFM composed of a MultiMode head (Digital Instruments, Santa Barbara, CA) mounted on xy - and z -positioning stages (Physik Instrumente, Auburn, MA). , At least 250 pulls were generated over four random locations on the surface and the data collected using an automated program. , The sample surface was brought into contact with the tip, held for a dwell time of 1 s, and then retracted at a constant velocity of 200 nm s –1 . The force loading rate (383 ± 183 pN s –1 ) was determined from the cantilever spring constant (120 pN nm –1 ) provided by the manufacturer and the slope on the force curve before an unbinding event; see the Supporting Information of Bowers et al As per our prior findings, pulls with contact forces in excess of 350 pN or below 30 pN were removed for analysis.…”

“…The unbinding experiments were performed as outlined by Bowers et al 33 Automated pulling experiments were carried out on a custom-built three-axis AFM composed of a MultiMode head (Digital Instruments, Santa Barbara, CA) mounted on xy-and z-positioning stages (Physik Instrumente, Auburn, MA). 39,47 At least 250 pulls were generated over four random locations on the surface and the data collected using an automated program. 39,47 The sample surface was brought into contact with the tip, held for a dwell time of 1 s, and then retracted at a constant velocity of 200 nm s −1 .…”

“…39,47 At least 250 pulls were generated over four random locations on the surface and the data collected using an automated program. 39,47 The sample surface was brought into contact with the tip, held for a dwell time of 1 s, and then retracted at a constant velocity of 200 nm s −1 . The force loading rate (383 ± 183 pN s −1 ) was determined from the cantilever spring constant (120 pN nm −1 ) provided by the manufacturer and the slope on the force curve before an unbinding event; see the Supporting Information of Bowers et al 33 As per our prior findings, pulls with contact forces in excess of 350 pN or below 30 pN were removed for analysis.…”