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Sensors 2015, 15(8), 18381-18401; doi:10.3390/s150818381

Tip Effect of the Tapping Mode of Atomic Force Microscope in Viscous Fluid Environments

Department of Civil and Environmental Engineering, National University of Kaohsiung, No. 700, Kaohsiung University Rd., Nanzih District, 81148 Kaohsiung, Taiwan
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Author to whom correspondence should be addressed.
Academic Editor: Vittorio M.N. Passaro
Received: 23 March 2015 / Revised: 2 July 2015 / Accepted: 14 July 2015 / Published: 28 July 2015
(This article belongs to the Section Physical Sensors)
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Abstract

Atomic force microscope with applicable types of operation in a liquid environment is widely used to scan the contours of biological specimens. The contact mode of operation allows a tip to touch a specimen directly but sometimes it damages the specimen; thus, a tapping mode of operation may replace the contact mode. The tapping mode triggers the cantilever of the microscope approximately at resonance frequencies, and so the tip periodically knocks the specimen. It is well known that the cantilever induces extra liquid pressure that leads to drift in the resonance frequency. Studies have noted that the heights of protein surfaces measured via the tapping mode of an atomic force microscope are ~25% smaller than those measured by other methods. This discrepancy may be attributable to the induced superficial hydrodynamic pressure, which is worth investigating. In this paper, we introduce a semi-analytical method to analyze the pressure distribution of various tip geometries. According to our analysis, the maximum hydrodynamic pressure on the specimen caused by a cone-shaped tip is ~0.5 Pa, which can, for example, pre-deform a cell by several nanometers in compression before the tip taps it. Moreover, the pressure calculated on the surface of the specimen is 20 times larger than the pressure without considering the tip effect; these results have not been motioned in other papers. Dominating factors, such as surface heights of protein surface, mechanical stiffness of protein increasing with loading velocity, and radius of tip affecting the local pressure of specimen, are also addressed in this study. View Full-Text
Keywords: atomic force microscope; pressure; vorticity; semi-analytical method atomic force microscope; pressure; vorticity; semi-analytical method
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Shih, H.-J.; Shih, P.-J. Tip Effect of the Tapping Mode of Atomic Force Microscope in Viscous Fluid Environments. Sensors 2015, 15, 18381-18401.

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