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Tutorials - Scanning Near-Field Optical Microscopy (SNOM)
What is Scanning Near-Field Optical Microscopy?
SNOM is a powerful microscopy technique that overcomes the Abbé diffraction limit
Scanning near-field optical microscopy (SNOM) is one of the family of scanning probe microscope techniques, which use a small probe to scan across the surface and build up an image line by line. The difference between a SNOM and other probe microscopes is that optical information is collected from the sample. A sharp micromachined tip is used that works like a nano light source.
The light source illuminates only a tiny part of the sample at any time
The microscope works in the "near-field" because the distance between the probe and the sample surface is so much smaller than the wavelength of the light. This means that high resolution optical information is available that is not bound by the far-field diffraction limits. The fundamental advantage is that the user gets a molecularly resolved optical and 3D topographic image simultaneously. Thus the optical and topographical features can easily be compared.
Different types of SNOM probes can be usedDifferent sorts of SNOM probes are available, which use different types of control mechanisms to scan the surface. One option is to use cantilevered probes, such as the ones that are used for AFM. This has the advantage that the SNOM instrument is capable of all the normal AFM scanning modes. The scanning and topography measurement then works using the same principles as for a standard AFM.
Many optical modes are available for SNOM microscopes
SNOM can be used in many of the modes of conventional optical microscopy, such as reflection, transmission, scattered light or polarization contrast. The tip can even be used for optical nanolithography. SNOM fluorescence imaging has a great potential for life science applications. It can be used to see single molecules in the topography and fluorescence images, or to locate fluorescently labelled regions on cell membrane surfaces, for example.
SNOM has been applied to a full range of biological samples
SNOM imaging has been applied to many kinds of biological samples, from single fluorescently labelled molecules such as DNA or proteins, up to whole cells and chromosomes. SNOM is particularly suited to labelling cell surface membrane proteins, since the illumination depth is limited to tens of nanometers. This allows good discrimination between proteins at the cell membrane and those in the cytoplasm. Fluorescent labelling of chromosomes is also common, using FISH or other techniques to label particular areas, which can then be identified in the topography images.