Instrumental drift in atomic force microscopy (AFM) remains a critical issue that limits the precision and duration of experiments. Previously, we developed an active optical stabilization technique to improve tip-sample stability at ambient conditions. However, force drift also occurs via uncontrolled deflection of the zero-force position of the cantilever. We found that the primary source of force drift in liquid for a popular class of soft cantilevers is their gold coating, even though they are coated on both sides to minimize drift. While removing the gold led to ~10-fold reduction in reflected light, we nonetheless improved the force stability of bioAFM by a factor of 10, achieving a sub-pN force precision over a broad bandwidth (0.01–10 Hz) just 30 minutes after loading. We subsequently extended AFM's sub-pN bandwidth by a factor of ~50 to span five decades of bandwidth (Δf ≈ 0.01–1,000 Hz by developing an efficient process to modify a short (L = 40 μm) commercially available cantilever (BioLever Mini) with a focused ion beam (FIB). Measurements of mechanically stretching individual proteins showed improved force precision coupled with state-of-the-art force stability and no significant loss in temporal resolution compared to the stiffer, unmodified cantilever. Importantly, these cantilevers were robust and were reused for SFMS over multiple days. Hence, we expect these responsive, yet stable, cantilevers to broadly benefit diverse AFM-based studies.
JILA, NIST & Univ. of Colorado, Boulder, CO
Dept. of Molecular, Cellular, and Developmental Biology, Univ. of Colorado, Boulder, CO