Calibration of T-shaped atomic force microscope cantilevers using the thermal noise method.
Calibration of T-shaped atomic force microscope cantilevers using the thermal noise method.
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DOI：
10.1063/5.0013091
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发表时间：
2020-08
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影响因子：
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通讯作者：
Youngkyu Kim;Nicola Mandriota;Davis Goodnight;O. Sahin
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作者：
Youngkyu Kim;Nicola Mandriota;Davis Goodnight;O. Sahin
原子力显微镜（AFM）中尖端样品相互作用力的测量提供了纳米级分辨率的材料性质信息。扭转-谐波AFM中使用的t形悬臂可以测量快速变化的尖端-样品相互作用力，利用悬臂的扭转（扭转）挠度，由于尖锐尖端的离轴放置。然而，由于挠曲和扭转挠度之间的机械耦合，很难从力-距离曲线中确定挠度灵敏度，因此使用常用的基于热噪声的校准方法对这些悬臂梁进行校准是困难的。在这里，我们通过同时分析弯曲和扭转热噪声谱，以及力-距离曲线测量期间的偏转信号，提出了基于热噪声的t形AFM悬臂梁校准。校准步骤与传统的热噪声方法相同，但计算机执行额外的计算以考虑模式耦合。我们通过确定校准结果对激光光斑在悬臂上的位置的敏感性，对悬臂在悬臂支架中的方向的敏感性，以及通过重复测量来证明校准方法的鲁棒性。我们根据已知的蛋白质（titin的I91结构域）的展开力验证了定量力测量，结果表明六个独立校准的悬臂梁之间的展开力值一致。
The tip-sample interaction force measurements in atomic force microscopy (AFM) provide information about materials' properties with nanoscale resolution. The T-shaped cantilevers used in Torsional-Harmonic AFM allow measuring the rapidly changing tip-sample interaction forces using the torsional (twisting) deflections of the cantilever due to the off-axis placement of the sharp tip. However, it has been difficult to calibrate these cantilevers using the commonly used thermal noise-based calibration method as the mechanical coupling between flexural and torsional deflections makes it challenging to determine the deflection sensitivities from force-distance curves. Here, we present thermal noise-based calibration of these T-shaped AFM cantilevers by simultaneously analyzing flexural and torsional thermal noise spectra, along with deflection signals during a force-distance curve measurement. The calibration steps remain identical to the conventional thermal noise method, but a computer performs additional calculations to account for mode coupling. We demonstrate the robustness of the calibration method by determining the sensitivity of calibration results to the laser spot position on the cantilever, to the orientation of the cantilever in the cantilever holder, and by repeated measurements. We validated the quantitative force measurements against the known unfolding force of a protein, the I91 domain of titin, which resulted in consistent unfolding force values among six independently calibrated cantilevers.