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Atomic force microscopy (AFM) is a powerful tool for determining the topography of native biomolecules at subnanometer resolution [1]. In Non Contact mode, Frequency shift (Δf) versus separation (Z) curves lead to a detailed comprehension of the tip-sample interaction providing a quantitative measurement of both force and energy. The intrinsic cantilever properties such as spring constant, Q-factor and tip aspect ratio have to be chosen in relation with the sample surface object of investigation. In this work we present Δf vs Z curves taken with different tips. The high resolution force measurements on soft specimens like APTES functionalized mica (a base substrate to deposit DNA molecules) as returned interaction forces less than 1 nN even by using stiff levers (k ~ 15 N/m) well below the values measured on hard surfaces under the same conditions [2]. All measurements were performed using an Atomic Force Microscope (AFM) operating at room temperature and under Ultra High Vacuum (UHV) conditions in Frequency Modulation (FM). The AFM uses a laser beam deflection method to detect the cantilever deflection. The microscope was completely developed in our laboratory for studying biological samples [3, 4]. The force and energy curves were directly calculated from the original Δf vs Z data by means of the method presented by Sader and Jarvis [5].
A comparison between standard and ultra-sharp tips with different characteristics is then discussed.
References:
[1] F. Schabert, C. Henn, and A. Engel Science 268 92-94 (1995)
[2] M. A. Lantz et al. Science 291 5513, 2580-2583 (2001)
[3] A. Radenovic, E. Bystrenova, L. Libioulle, M. Taborelli, J.A. De Rose and G. Dietler Rev. Sc. Instr. 74 1022 (2003).
[4] S. Tobenas, A. Radenovic, E. Bystrenova, G. Di Santo and G. Dietler J. Journal of Appl. Phys. 45 3B (2005)
[5] J. E. Sader and S. P. Jarvis Applied Physics Letters. 84 10 , 1801-1803 (2004) |