Biophysics

AFM Uncompromised

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Science  25 Apr 2014:
Vol. 344, Issue 6182, pp. 341
DOI: 10.1126/science.344.6182.341-c

Atomic force microscopy (AFM) is a powerful tool used both for subnanometer imaging and for mechanical probing of molecules. The key measurement in AFM is the deflection of a cantilever, which depends on the force it experiences. AFM is used in single-molecule force spectroscopy to monitor the folding and unfolding of biomolecules. This application requires sensitivity to very small changes in force on short time scales, but also requires long-term force stability. Current AFMs are optimized either for short-term force precision (achieved by using shorter, stiffer cantilevers to reduce hydronamic drag) or for long-term force stability (better performance comes from longer, softer cantilevers). Bull et al. modified a short cantilever with a focused ion beam to achieve excellent short-term precision and long-term stability. AFM cantilevers are typically gold-coated to improve signal intensity, but the gold reduces stability. Removal of the gold except for a protected patch at the end of the cantilever maintained high signal without compromising stability. A protein unfolding assay highlighted the short-term precision, whereas stretching a surface-anchored protein showed sub-pN performance over a force bandwidth of 0.01 to 1000 Hz. Monitoring abrupt unfolding of a protein showed that the cantilever had a temporal response time of about 70 µs. These responsive yet stable cantilevers should benefit diverse AFM studies.

ACS Nano. 10.1021/nn5010588 (2014).

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