Quantitative Mechanical Mapping of Biomolecules and Cells in Fluid
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1261-U01-05
Quantitative Mechanical Mapping of Biomolecules and Cells in Fluid Chanmin Su, Shuiqing Hu, Yan Hu, Natalia Erina and Andrea Slade Veeco Instruments Inc., 112 Robin Hill Road, Santa Barbara, CA 93117, U.S.A
ABSTRACT Though atomic force microscopy (AFM) interrogates biological materials through mechanical interactions, achieving quantitative mechanical information such as modulus and adhesion at high resolution has been a challenging task. A technology for nanometer scale mechanical property mapping, peak force tapping (PFT), was developed to achieve high resolution imaging and quantitative mechanical measurements simultaneously. PFT controls instantaneous interaction force and record force spectroscopy at each pixel to calculate mechanical properties. A feedback loop maintains a constant peak force, a local maximum point in the force spectroscopy, at the level of Pico Newtons throughout the imaging process. Such high precision force controls enable application of ultra-sharp probe to image biological samples in vitro and accomplish molecular resolution in protein membranes. More importantly a full suite of mechanical properties, modulus, adhesion, energy dissipation and deformation are mapped concurrent with topographic imaging. To calculate nanomechanical properties reliably cantilever spring constant and tip shape were calibrated systematically. A method to accurately determine cantilever spring constant, capable of wafer scale cantilever calibration, was developed and tested against traceable force methods. With the knowledge of tip shape, derived from morphological dilation method using a reference sample, mechanical properties measured at the nanometer scale was compared with bench mark materials ranging from 0.7 MPa to 70 GPa. The same method was also applied to OmpG membranes, Lambda DNA strings, as well as live cells. Adhesion force mapping was also applied to detection of biotin/avidin un-bonding events using functionalized probes. The limitation of the measurement accuracy in biology samples will be discussed. INTRODUCTION Since the pioneering work of Quate et al1, AFM has been growing into a major imaging tool for nanometer scale material study. Compare with other high resolution imaging tools, AFM has the advantage of in vitro measurements in buffer solutions and at various temperatures. It was applied to biological materials at the very start of the technology2. Many original work of high resolution AFM led to ground breaking studies such as molecular rotor3, native membrane structure4 and many other examples. In addition to using AFM primarily as a topographic imaging tool the mechanical interactions also provide rich information of protein-protein, ligand-receptor interactions. These interactions were usually measured by force curve or force spectroscopy. The signature of the interactions is reflected by the adhesion forces due to specific binding5. The high resolution imaging and force spectroscopy generally evolve as two different branches with emphasis in imaging and mechanical proper
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