AFM Imaging of Water, Cells and Tissues
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AFM Imaging of Water, Cells and Tissues Xiaodong Li Department of Mechanical Engineering and University NanoCenter, University of South Carolina, 300 Main Street, Columbia, SC 29208, U.S.A. e-mail: [email protected] ABSTRACT This paper presents the results of several AFM case studies of water, tumor cells, pit cells, and type-I collagen samples. The AFM imaging procedures, surface structural characterization capabilities such as contact mode, tapping mode, and friction mode are discussed. The difference in surface morphology between the pit cell submerged in liquid and the cell in air was observed. The AFM tapping mode phase imaging technique was used to study the crosslinks in the type-I collagen. The calibration method for accurately measuring the AFM cantilever spring constant with the help of a nanoindenter is also presented. INTRODUCTION The atomic force microscope (AFM) has emerged as a powerful tool for studying the structure and function of cells and tissues. The extreme resolution of the AFM allows one to study both static and dynamic situations in live cells and tissues [1-4]. For example, the AFM has been used to describe the exit of pox virus from live monkey cells [5], reveal ongoing intracellular filament dynamics underneath the cell membrane [6,7], image cytoskeletal structures in live cells [6], and detect the fibril orientation in collagen materials [8]. Although many research papers have been published in this area, experimental details and calibration procedures have not been well documented. Many results have been reported on dry samples or semidry samples. Biological samples are, in general, wet or submerged in liquid. Dry samples may not represent the real surface morphology and structure of live samples. Direct imaging the live samples in wet condition or in liquid is greatly needed. To image these samples with the AFM remains a great challenge. The interaction between the AFM tip and sample surface includes van der Waals, capillary, electrostatic, elastic, brush, binding, and molecule extension. A careful calibration and an in-depth understanding of experimental procedures, which have been largely ignored, will help in improve the AFM measurement accuracy and avoid obtaining artificial images. The objectives of this research are to show the AFM imaging procedures, to introduce the AFM surface structural characterization capabilities such as contact mode, tapping mode, and friction mode, and to establish calibration method for accurately measuring the AFM cantilever spring constant. EXPERIEMNTAL DETAILS A Veeco Dimension 3100 AFM (Veeco Metrology Group) was used in this study. Water, tumor cells, pit cells, and type-I collagen were imaged using the AFM contact mode and tapping mode. Some samples were submerged in fluid. To image an object submerged in liquid or a wet sample, a liquid holder is needed to protect the AFM piezo scanner, making the AFM
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operation more complicated. The AFM used in this study is equipped with an optical microscope which can be used to locate a cell
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