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Live-Cell Imaging:

2. Near-Field Optical Microscopy (NSOM)

    We have built a custom designed NSOM system that employs a fiber-optic based cantilever tip that allows simultaneous probing of surface morphology using tapping-mode atomic force microscopy (see schematic below)

Figure 1. Schematic of combined atomic force microscopy and near-field scanning optical microscopy

I. Introduction to AFM/NSOM Instrumentation
II. AFM/NSOM Imaging of red blood cells
III. AFM/NSOM Imaging of cancer cells

I. Introduction to AFM/NSOM Instrumentation

    To complement studies using conventional optical microscopy techniques and to obtain topographic and optical images of cells with nanoscale resolution, we developed and employ a unique optical microscopy setup that combines tapping mode atomic force microscopy (AFM) and near-field scanning optical microscopy (NSOM) as schematically shown in figure 1.

    The setup works in a manner similar to a normal force tapping mode AFM, except that the conventional tip is replaced by a tapered optical fiber with sub-wavelength aperture ~100nm in diameter at the end. Light excitation is delivered through the fiber. The diffraction through the subwavelength aperture creates a near field at the end of tip which excites the almost only fluorofores on the surface of the cell. Therefore simultaneously 3D morphological information and surface sensitive fluorescence imaging can be obtained. Since the resolution the NSOM is decided by the diameter of the aperture, the diffraction limit of optical microscopy is broken. Nanoscale distribution of molecules thus can be obtained.

Below we show a couple illustrative examples of cell image using AFM/NSOM.

II. Imaging of red blood cells

The optical and topographic images obtained simultaneously for red blood cells are shown below:



III. Imaging of cancer cells

    Applications of quantum dots as biological probes in conjunction with nano-scale detection can be extended towards other novel and medically significant uses such as the early detection of cancer which is currently another area of our focus. We are working in collaboration with Prof. Richard Cote, Dr. Ram Dattar and Dr. Deborah Hawes's laboratory in the department of Pathology at the USC Health Sciences Campus to work towards establishing new paradigms that would enable early detection of cancer through the use of Nanotechnology. Our approach is to obtain simultaneous topographical and optical information from cancerous versus normal cells using the near-field optical microscope to be able to distinguish any structural and/or optical differences at the nano-scale. The figure below shows simultaneously obtained topographic and fluorescence optical images of breast cancer cells SK-BR-3 and MDA-MB-231. The cells are labeled with CdSe/ZnS (600nm emission) quantum dot targeting Her2/neu receptors on the cell surface. Images are obtained using a tip of 100nm diameter aperture. The resolution of the NSOM images, as indicated by the smallest features on it, is ~150nm. Note that the fluorescence NSOM image of both SKBR3 and MDA cell is marked by bright areas of ~500nm diameter. This indicates Her2/neu receptors on the SKBR3 cell surface are not distributed evenly but instead localized in clusters. . The only difference is that the number of such clusters on MDA is much smaller than on SKBR3 cell, which is understandable given the total lower number of Her2/neu receptor on MDA (2x104/cell) than on SKBR3 (1x106/cell).

 

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