Dynamic Atomic Force Microscopy Studies to Characterize Heterogenous Surfaces
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Since the development of the atomic force microscope (AFM) in 1986, it has become a widely used and versatile tool in both the nanoscale imaging of surfaces and in the measurement of many intermolecular and surface forces. In AFM, forces between a probe and the relevant surface are measured through changes in the deflection of a flexible cantilever attached to the probe. For image collection, it is assumed that changes in sample topography affect the probe–sample interaction profile and therefore affect the response of the cantilever. The feedback control required to maintain a constant deflection of the cantilever is then used to reconstruct a three-dimensional image of the surface. In force measurements, the feedback control is disabled, and the deflections of the cantilever are measured as a function of normal probe–sample separation. Often, the interpretation of AFM images is assisted through force measurement results.
While there are currently numerous modes of AFM operation, they can be separated into two major categories—the dc or static method and the dynamic methods. In the dc mode of operation (also known as contact-mode AFM), the static deflection of the cantilever is the measured and controlled signal. The cantilever can be treated as a simple Hookian spring with its deflection proportional to the force acting on the probe. This was the first established AFM mode and is still the most commonly used one for force measurements. Dynamic AFM methods are more recent expansions of the technique and involve forced oscillation of the cantilever and the measurement and control of the dynamic response through quantities such as the cantilever's amplitude of oscillation, its resonance frequency, and its phase lag from the driving signal. Commonly used dynamic methods include noncontact mode (NC) AFM, tapping mode (TM) or intermittent-contact (IC) AFM, frequency modulation (FM) AFM, and force modulation AFM. With the exception of force modulation that uses small-amplitude oscillation of a tip or sample in contact to measure local variations in material viscoelasticity, the other dynamic methods are less-invasive methods of force measurement and imaging. In these methods, the cantilever is oscillated at large amplitudes and at frequencies close to or at its resonance frequency (∼ kHz). The NC and FM modes are operated out of contact with the sample (in the attractive regime). Frequency modulation measures the resonance frequency shift at constant amplitude operation, while NC measures changes in the oscillation amplitude at a fixed driving frequency. In both techniques, probe–sample interactions can be obtained from the frequency shift or from the change in the amplitude of oscillation. Tapping mode operation is similar to NC; however, the cantilever is forced to oscillate at larger amplitudes resulting in intermittent contact with the sample. As a result, the probe–sample interaction potential changes nonlinearly during each oscillation cycle. Because the dynamic quantities measured are averages over the whole oscillation cycle, it is nontrivial to directly relate the observed changes to the probe–sample interaction in TM. This makes the interpretation of both TM images and force measurement results more difficult. A recent review by García and Pérez contains detailed information about the interpretation and application of these dynamic methods in air and vacuum.
Regardless of the complexities, TM is a valuable tool in the gentle and nondestructive imaging of soft and easily deformable surfaces such as biologically relevant surfaces and soft polymer interfaces. Contact loads in TM can be comparable to those in the conventional dc mode; however, it is the reduction of lateral shear of the sample, the brief probe–sample contact time, and the small contact area that enable it to be a less-destructive technique. Some researchers have also hypothesized that soft biological materials undergo hardening under high frequency, making them less susceptible to deformation and damage upon impact. Although TM is now used fairly extensively as an imaging tool, its force measurement capabilities have not been thoroughly exploited especially in nonambient environments appropriate for many surfaces of interest. Potentially, the gentle properties of the TM image process are shared in TM force measurements. Additionally, a dynamic mode of measurement would offer increased sensitivity with the retention of lateral resolution and early detection of the sample before significant indentation takes place. It is these capabilities of TM AFM that we believe will make it a unique tool in the force measurement of heterogeneous biomaterial surfaces. We thus present a review of the common method of modeling TM force measurements in air and extend this method toward TM force measurement in liquid. We will report on the few studies aimed at understanding and improving TM in liquid and will focus on the importance of hydrodynamics in the system. Additionally, we will discuss the application of TM in liquid to the measurement of interactions occurring on biomaterial surfaces.