Scanning probe acceleration microscopy (SPAM) in fluids: Mapping mechanical properties of surfaces at the nanoscale

Abstract

One of the major thrusts in proximal probe techniques is combination of imaging capabilities with simultaneous measurements of physical properties. In tapping mode atomic force microscopy (TMAFM), the most straightforward way to accomplish this goal is to reconstruct the time-resolved force interaction between the tip and surface. These tip-sample forces can be used to detect interactions (e.g., binding sites) and map material properties with nanoscale spatial resolution. Here, we describe a previously unreported approach, which we refer to as scanning probe acceleration microscopy (SPAM), in which the TMAFM cantilever acts as an accelerometer to extract tip-sample forces during imaging. This method utilizes the second derivative of the deflection signal to recover the tip acceleration trajectory. The challenge in such an approach is that with real, noisy data, the second derivative of the signal is strongly dominated by the noise. This problem is solved by taking advantage of the fact that most of the information about the deflection trajectory is contained in the higher harmonics, making it possible to filter the signal by ''comb'' filtering, i.e., by taking its Fourier transform and inverting it while selectively retaining only the intensities at integer harmonic frequencies. Such a comb filtering method works particularly well in fluid TMAFM because of the highly distorted character of the deflection signal. Numerical simulations and in situ TMAFM experiments on supported lipid bilayer patches on mica are reported to demonstrate the validity of this approach.atomic force microscopy ͉ Fourier transform atomic force microscopy ͉ nanoaccelerometry

Origins of phase contrast in the atomic force microscope in liquids

“…This theory forms the bedrock upon which phase-contrast imaging is currently based, at least under ambient and vacuum conditions. dAFM is now a well-known and broadly extended technique for nanoscale imaging and force spectroscopy in the biology community (7)(8)(9)(10). Because the natural medium for the study of biological samples is liquid, it is of fundamental importance to develop a proper description of the different working modes of dAFM when the probe and sample are immersed in liquids.…”

Frequency response of an atomic force microscope in liquids and air: Magnetic versus acoustic excitation

“…1͑a͒ and 1͑b͔͒. Legleiter et al 17 and Jan et al 18 have also studied some theoretical aspects of this problem.…”

“…[25][26][27][28] Despite its apparent simplicity, point-mass models render a qualitative and semiquantitative comparison with experiments. 14,17,18 The deflection of an AFM microcantilever driven by a magnetic force in fluids and far from the sample surface, can be described by 28 z͑t͒…”

“…14 Recently, several experiments and simulations have pointed out the existence of remarkable differences in the dynamic behavior of AFM microcantilevers in liquids depending on the excitation mode. [15][16][17][18][19] Experiments performed by Volkov et al 16 showed that the resonance curve of a fluiddriven cantilever tends at high frequencies to a finite value of the oscillation amplitude. This is at variance with the observed behavior of magnetically excited cantilevers that show resonance curves with negligible oscillation amplitudes at high frequencies 10,13 ͓see also Figs.…”

Simultaneous imaging of surface and magnetic forces

“…Recently multifrequency AFM modes have emerged which provide more measurement channels while scanning 6 , allowing single-pass imaging of magnetic forces 7,8 . Some of these methods have demonstrated the ability to rapidly capture the entire force-distance curve [9][10][11][12][13][14][15] allowing for simultaneous imaging of topography and material properties. In this letter we perform Intermodulation AFM (ImAFM) 16 with a magnetically coated tip and analyze the data so as to separate the long-range magnetic force from near-surface forces, thus obtaining simultaneous imaging of topography, mechanical and magnetic properties in one scan, with a standard cantilever, at a typical scan speed for dynamic AFM.…”